Diversity Techniques in IEEE 802.11g WLAN
Sonia Sharmin Islam (Student No. 9906044) Ayon Quayum (Student No. 9906051) Md. Ataul Goni Osmani (Student No. 8706172)
Bachelor of Science in Electrical and Electronics Engineering
Department of Electrical and Electronic Engineering,
Bangladesh University of Engineering and Technology, Dhaka-1000 March, 2005
Contents
Certification
x
Declaration
xii
Abstract
xiii
Acknowledgement
xiv
1 Introduction
1
1.1
A Short History of WLAN . . . . . . . . . . . . . . . . . . . . . . .
2
1.2
Unlicensed Band . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
1.3
Challenges With Unlicensed Band . . . . . . . . . . . . . . . . . . .
4
1.4
Diversity Techniques in WLAN . . . . . . . . . . . . . . . . . . . .
5
1.5
Overview of the dissertation . . . . . . . . . . . . . . . . . . . . . .
6
2 OFDM Principle 2.1
2.2
7
An Introduction To OFDM . . . . . . . . . . . . . . . . . . . . . .
7
2.1.1
OFDM Basics . . . . . . . . . . . . . . . . . . . . . . . . . .
7
2.1.2
FDM & OFDM: An Analogical Interpretation . . . . . . . .
8
Orthogonality: An Essential Term In OFDM . . . . . . . . . . . . .
9
i
ii
CONTENTS 2.2.1
Orthogonality In General . . . . . . . . . . . . . . . . . . . .
9
2.2.2
Frequency Domain Orthogonality . . . . . . . . . . . . . . . 10
2.3
Generation Of Subcarriers Using The IFFT . . . . . . . . . . . . . 12
2.4
OFDM Generation & Reception . . . . . . . . . . . . . . . . . . . . 14
2.5
2.4.1
Brief Discussion On FFT & IFFT . . . . . . . . . . . . . . . 14
2.4.2
Basic Block Of OFDM Transceiver . . . . . . . . . . . . . . 15
2.4.3
Serial To Parallel Conversion
2.4.4
Subcarrier Modulation . . . . . . . . . . . . . . . . . . . . . 17
2.4.5
Frequency To Time Domain Conversion
2.4.6
RF Modulation . . . . . . . . . . . . . . . . . . . . . . . . . 20
. . . . . . . . . . . . . . . . . 16
. . . . . . . . . . . 19
Properties of OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.5.1
Spectrum & Performance . . . . . . . . . . . . . . . . . . . . 21
2.5.2
Bit Error Performance . . . . . . . . . . . . . . . . . . . . . 22
2.5.3
Peak to Average Power Ratio(PAPR) . . . . . . . . . . . . . 22
2.5.4
Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.5.5
Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.6
Parameters of Real OFDM . . . . . . . . . . . . . . . . . . . . . . . 23
2.7
Guard Period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.7.1
Protection Against ISI . . . . . . . . . . . . . . . . . . . . . 23
2.7.2
Protection Against ICI . . . . . . . . . . . . . . . . . . . . . 24
2.7.3
Insertion of a Guard period: . . . . . . . . . . . . . . . . . . 25
2.8
Additive White Gaussian Noise: Effect on OFDM . . . . . . . . . . 26
2.9
Modulation Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.10 OFDM & Single Carrier Transmission :A Comparative Discussion . 28 2.10.1 Similarity in Performance . . . . . . . . . . . . . . . . . . . 28 2.10.2 Distinction in Performance . . . . . . . . . . . . . . . . . . . 29
CONTENTS
iii
2.11 OFDM: Advantages & Disadvantages . . . . . . . . . . . . . . . . . 31 2.11.1 Privileges of OFDM . . . . . . . . . . . . . . . . . . . . . . 31 2.11.2 OFDM System: Drawbacks . . . . . . . . . . . . . . . . . . 32 3 An Introduction To IEEE Standard: 802.11g 3.1
3.2
33
WLAN Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.1.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.2
IEEE 802.11 Specifications . . . . . . . . . . . . . . . . . . . 34
802.11g: A Chronological Change in WLAN Standard . . . . . . . . 34 3.2.1
IEEE 802.11 Background . . . . . . . . . . . . . . . . . . . . 34
3.2.2
IEEE 802.11b Background . . . . . . . . . . . . . . . . . . . 35
3.2.3
IEEE 802.11a Background . . . . . . . . . . . . . . . . . . . 36
3.2.4
IEEE 802.11g Background . . . . . . . . . . . . . . . . . . . 37
3.3
IEEE 802.11: An Overview . . . . . . . . . . . . . . . . . . . . . . . 39
3.4
Requirements of IEEE 802.11 . . . . . . . . . . . . . . . . . . . . . 39
3.5
Reference Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.6
Layered Protocol Architecture . . . . . . . . . . . . . . . . . . . . . 41
3.7
Introduction to MAC Sublayer . . . . . . . . . . . . . . . . . . . . . 42
3.8
3.7.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.7.2
The Access Mechanisms . . . . . . . . . . . . . . . . . . . . 43
3.7.3
General MAC Frame Format . . . . . . . . . . . . . . . . . . 45
3.7.4
Control Field in MAC Frames . . . . . . . . . . . . . . . . . 46
3.7.5
MAC Management Sublayer . . . . . . . . . . . . . . . . . . 46
The PHY Layer: A Glance . . . . . . . . . . . . . . . . . . . . . . . 48 3.8.1
FHSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.8.2
DSSS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
iv
CONTENTS 3.8.3 3.9
DFIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
802.11g: A Natural Demand in Wireless Communication . . . . . . 52 3.9.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
3.9.2
Data rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.9.3
The PHY Layer . . . . . . . . . . . . . . . . . . . . . . . . . 53
3.9.4
The MAC Layer . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.9.5
Characteristics of 802.11g . . . . . . . . . . . . . . . . . . . 55
3.10 Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4 Diversity Techniques
60
4.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
4.2
Fundamental Concepts of Diversity . . . . . . . . . . . . . . . . . . 60
4.3
4.4
4.2.1
Space Diversity . . . . . . . . . . . . . . . . . . . . . . . . . 61
4.2.2
Frequency Diversity . . . . . . . . . . . . . . . . . . . . . . . 63
4.2.3
Time Diversity . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.2.4
Multipath Diversity . . . . . . . . . . . . . . . . . . . . . . . 64
4.2.5
Polarization Diversity . . . . . . . . . . . . . . . . . . . . . . 65
4.2.6
Pattern Diversity . . . . . . . . . . . . . . . . . . . . . . . . 65
4.2.7
Angle Diversity . . . . . . . . . . . . . . . . . . . . . . . . . 66
Different Techniques for transmit diversity . . . . . . . . . . . . . . 66 4.3.1
Alamouti Scheme . . . . . . . . . . . . . . . . . . . . . . . . 67
4.3.2
Cyclic Delay Diversity . . . . . . . . . . . . . . . . . . . . . 69
Receive Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 4.4.1
Receive Antenna Array . . . . . . . . . . . . . . . . . . . . . 72
4.4.2
Channel Estimation . . . . . . . . . . . . . . . . . . . . . . . 77
4.4.3
Decision Making . . . . . . . . . . . . . . . . . . . . . . . . 79
CONTENTS
v
5 Simulation and Result
81
5.1
Transmitter Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
5.2
Channel Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.3
Receiver Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.4
Weight Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.5
Linear Combination of Weighted Signal . . . . . . . . . . . . . . . . 85
5.6
Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.6.1
Performance Comparison by varying Receiving Antenna . . 86
5.6.2
Performance Comparison by varying the number of receiver antenna with 64 and 16 QAM . . . . . . . . . . . . . . . . . 87
5.6.3
Some Test Cases . . . . . . . . . . . . . . . . . . . . . . . . 87
6 Conclusion
94
6.1
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
6.2
Contribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
6.3
Future Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
A Least Mean Square (LMS) Method
97
B Recursive Least-Squares (RLS) Method
98
C IQ Diagrams
100
Glossary
103
List of Tables 2.1
Parameters of Real OFDM . . . . . . . . . . . . . . . . . . . . . . . 24
3.1
802.11g Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3.2
Compatibility in 802.11g . . . . . . . . . . . . . . . . . . . . . . . . 58
4.1
Transmitted signal in Alamouti Scheme . . . . . . . . . . . . . . . . 67
5.1
Different Parameters of the transmitter, according to the IEEE802.11g Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.2
Coding Rate and Different Modulation Schemes for 802.11g PHY Layer with Allowed Constellation error . . . . . . . . . . . . . . . . 83
5.3
Vector Channel Model Parameters . . . . . . . . . . . . . . . . . . . 83
vi
List of Figures 1.1
Frequency range of unlicensed bands . . . . . . . . . . . . . . . . .
3
2.1
Area under sine wave over one period is zero . . . . . . . . . . . . .
9
2.2
Time domain construction of an OFDM signal . . . . . . . . . . . . 11
2.3
Frequency response of a 5 tone OFDM signal . . . . . . . . . . . . . 11
2.4
4 Sub carriers of an OFDM symbol . . . . . . . . . . . . . . . . . . 13
2.5
Block diagram of a basic OFDM transceiver . . . . . . . . . . . . . 15
2.6
Constellation of 16-QAM
2.7
IQ diagram of 16-QAM signal with noise . . . . . . . . . . . . . . . 18
2.8
IFFT stage of OFDM generation . . . . . . . . . . . . . . . . . . . 19
2.9
RF Modulation of Complex Base Band Signal (Analog Techniques)
20
2.10 RF Modulation of Complex Base Band Signal (Digital Techniques)
21
. . . . . . . . . . . . . . . . . . . . . . . 17
2.11 Adding Guard Period to OFDM Symbol . . . . . . . . . . . . . . . 25 3.1
Ad Hoc Network and Infrastructure Network . . . . . . . . . . . . . 40
3.2
Protocol Entities for the IEEE 802.11 . . . . . . . . . . . . . . . . . 42
3.3
Primary operation of the CSMA/CA in the IEEE 802.11 . . . . . . 43
3.4
General MAC frame format of the IEEE 802.11 . . . . . . . . . . . 45
3.5
PLCP Frame for the FHSS of the IEEE 802.11 . . . . . . . . . . . . 49 vii
viii
LIST OF FIGURES
3.6
Overlapping frequency bands for the DSSS in the IEEE802.11 . . . 50
3.7
PLCP Frame for the DSSS of the IEEE 802.11 . . . . . . . . . . . . 51
3.8
PLCP Frame for the DFIR of the IEEE 802.11 . . . . . . . . . . . . 51
4.1
Space Diversity (MIMO) . . . . . . . . . . . . . . . . . . . . . . . . 62
4.2
Frequency Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
4.3
Time Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4
Multipath Diversity . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
4.5
Alamouti Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
4.6
Cyclic Delay Diversity Scheme . . . . . . . . . . . . . . . . . . . . . 70
4.7
CDD Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.8
Uniform Linear Array System . . . . . . . . . . . . . . . . . . . . . 73
4.9
Circular Array System . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.10 Rectangular Planar Array System . . . . . . . . . . . . . . . . . . . 76 4.11 Hexagonal Planar Array System . . . . . . . . . . . . . . . . . . . . 77 4.12 Hexagonal Array Geometry . . . . . . . . . . . . . . . . . . . . . . 77 5.1
Transmitter and Receiver Model . . . . . . . . . . . . . . . . . . . . 84
5.2
Performance comparison for different receiving antenna in AWGN channel
5.3
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Performance comparison for different receiving antenna in vector channel
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.4
Performance for different receive antennas In 16 QAM and 64 QAM 88
5.5
Transmitting signal for 64 QAM . . . . . . . . . . . . . . . . . . . . 88
5.6
Signals in different receive antennas for 2 antenna . . . . . . . . . . 89
5.7
Signals in different receive antennas for 4 antenna . . . . . . . . . . 89
5.8
Signals in different receive antennas for 8 antenna . . . . . . . . . . 90
LIST OF FIGURES 5.9
ix
Weighted output signal for single receive antenna . . . . . . . . . . 90
5.10 Weighted output signal for 2 receive antenna . . . . . . . . . . . . . 91 5.11 Weighted output signal for 4 receive antenna . . . . . . . . . . . . . 91 5.12 Weighted output signal for 8 receive antenna . . . . . . . . . . . . . 92 5.13 Weighted output signal for 16 receive antenna . . . . . . . . . . . . 92 5.14 Weighted output signal for 32 receive antenna . . . . . . . . . . . . 93 C.1 IQ Diagram of BPSK (left) and QPSK (right) . . . . . . . . . . . . 100 C.2 IQ Diagram of 8-QAM (left) and 16-QAM (right) . . . . . . . . . . 101 C.3 IQ Diagram of 32-QAM (left) and 64-QAM (right) . . . . . . . . . 101 C.4 IQ Diagram of 128-QAM (left) and 256-QAM (right) . . . . . . . . 102 C.5 IQ Diagram of 512-QAM (left) and 1024-QAM (right) . . . . . . . . 102
Certification The thesis titled “Diversity Techniques in IEEE 802.11g WLAN”submitted by Sonia Sharmin Islam (9906044), Ayon Quayum (9906051) and Md. Ataul Goni Osmani (8706172), session
, has been accepted satisfactory in partial fulfillment
of the requirement for the degree of Bachelor of Science in Electrical and Electronics Engineering on
,2005.
Supervisor
Dr. M. Nazrul Islam Associate Professor, Department of Electrical and Electronics Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh.
Co-Supervisor
Dr. Fakhrul Alam Assistant Professor, Institute of Information and Mathematical Science, Massey University, Auckland, Newzealand.
x
Declaration It is hereby declared that this thesis or any part of it has not been submitted elsewhere for the award of any degree of diploma. Signature of Candidates
Sonia Sharmin Islam (Student No. 9906044)
Ayon Quayum (Student No. 9906051)
Md. Ataul Goni Osmani (Student No. 8706172) xi
Abstract Today 802.11, an IEEE family of standards for WLAN, is attracting attention of worldwide industry. Since its formation in 1999 it has extended to 802.11a, 802.11b and now 802.11g. Among these standards 802.11g is the most attracting for its backward compatibility and ability to transfer data at a very high rate (upto 54 Mbps). There is a constant number of growing consumer products that uses ISM band for WLAN system. And these services are extending itself from handheld PDA to digital watch, from laptop computers to mobile phones. Today the main challenge is to improve the performance of WLAN system within these standards. In this thesis we have simulate a single input multiple output (SIMO) environment in 802.11g system. Simulation results show that space diversity improves the performance of system. LMS algorithm is used to estimate weight for different receive antennas.
xii
Acknowledgement We would like to express our gratitude to Dr. Nazrul Islam, Associate Professor of the department of electrical and electronic engineering, BUET, for his unceasing support and encouragement to us. He helped and directed us with his knowledge during the research without which it would be impossible to complete the work. Dr. Fakhrul Alam guided us and shared his knowledge throughout the project. He has introduced us to the exquisite world of wireless communication. During the research he kindly shared with us his recent research materials. It would be beyond our reach to properly express our gratitude to him. We are also thankful to Mursalin Habib, Anindita Das Talukdar and Mashaeikh Hossian for their help throughout the research with their findings and sharing of knowledge, for their help in simulation and to prepare this dissertation. And finally, we are grateful to our parents for their unconditional patience and love. We could never have finished the work without their support. Dhaka,
Sonia Sharmin Islam
Bangladesh
Ayon Quayum
March 2005
Md. Ataul Goni Osmani xiii
xiv
Chapter 0. Acknowledgement
Chapter 1
Introduction
It was scarcely a hundred years ago when Sir Jagadish Chandra Bose has experienced the marvel of wireless communication in his small laboratory. Yet the real extent of wireless communication was beyond anyone’s dream in the beginning. It experienced expansion boom in last two decades. Today it is encompassing the communication field with unprecedent rapidness. Wired communication is becoming behind the scene technology. In the frontier wireless communication becoming more and more predominant. Local Area Network (LAN) is not out of its influence. 802.11 is today’s predominant standard for LAN. And it’s rise is unavoidable as it gives us freedom to move without any cumbersome arrangement of wire and accessories. This accounts for the growing popularity of Wireless Local Area Network ( WLAN). 1
2
1.1
Chapter 1. Introduction
A Short History of WLAN
Ethernet became the predominant LAN technology in the wired world. Defined by the Institute of Electrical and Electronic Engineers (IEEE) with the 802.3 standard, it has provided an evolving, high-speed, widely available and interoperability networking standard. Ethernet originally provided 10 megabit per second (Mbps) transfer rates. Later it included the 155Mbps transfer rates required for network backbones and bandwidth intensive applications.The open IEEE 802.3 standard resulted in a wide range of suppliers, products and price points for Ethernet users. Ethernet standards guarantee interoperability, enabling users to select products from different vendors, while the standard ensures that they will work together. The first wireless LAN technologies operated in the 900MHz band and were low speed (1-2Mbps), proprietary offerings. Despite these shortcomings, the freedom and flexibility of wireless allowed these early products to find there way into vertical markets like retail and warehousing where mobile workforces used hand-held devices for inventory management and data collection. In 1991 realizing that in order for wireless LANs to gain broad market acceptance, to govern wireless LAN technology Aironet pushed with other wireless makers for standards. Around 1992, wireless LAN makers began developing products operating in the unlicensed 2.4 GHz frequency band. This opened two additional vertical markets. Healthcare, with a highly mobile workforce, began using portable computers to access patient information. And as computers made there way into the classrooms, educational institutions began installing wireless networks to avoid the high cost
1.2. Unlicensed Band
3
of wiring buildings. In June, 1997 the IEEE released the 802.11 standard for wireless local area networking [7]. IEEE 802.11 standard supports transmission in infrared light and two types of radio transmission within the unlicensed 2.4GHz frequency band: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). Today several 802.11 standards exist. 802.11b is an expansion of the standard that allows transmission speeds of up to 11Mbps. The newer 802.11a and 802.11g versions support speeds as high as 54Mbps.
1.2
Unlicensed Band
Figure 1.1: Frequency range of unlicensed bands
With the rise of wireless LAN question arises about its operating band. Because all commercial bands are used and highly priced by the government. Individual users would not be able to support licensed band. So the solution is found in unlicensed band. The unlicensed bands are identified by the International Telecommunications Union (ITU) Radio Regulations as an ISM (Industrial, Scientific and Medical) band. This means that, within the specified band, there are also devices
4
Chapter 1. Introduction
which are not radio communication devices, but do affect the quality and usefulness of the spectrum. Examples of such devices are video/audio transmitters for entertainment and surveillance, broadcast links for highpower FM television, microwave ovens and military radio systems. The single most attractive aspect of the ISM band is that it is free. So it is most suitable for WLAN technology. And till now WLAN industry is growing exploiting ISM band. Below is a summary showing some of the technologies between 2400 and 2483.5 MHz, 4.90 GHz and 5.90 GHz as and between 868 and 928 MHz • Wi-Fi (IEEE 802.11b/g/a) • Bluetooth wireless technology (IEEE 802.15.1) • WiMedia (IEEE 802.15.3) • Zigbee (IEEE 802.15.4)
1.3
Challenges With Unlicensed Band
Unlicensed band comes with mixed blessings. It is free and suitable for WLAN technology. At the same time it is too much crowded. As a result it is often becomes impossible to achieve expected performance with ISM band. Another problem is that this technology is used extensively in urban area. Where noisy environment and crowed surroundings often reduces the performance of WLAN to great extent. And at the same time the market demand is always high for more bandwidth and faster connections. So it becomes a high priority to improve the performance of WLAN systems.
1.4. Diversity Techniques in WLAN
5
There are many techniques to improve the performance of WLAN systems. To combat noise generated from other devices use of smart antenna to beamform is a good choice. To mitigate the effects of other noises and environments diversity is an excellent technology. Recently diversity techniques are extensively researched. They offer a very stable and high data rate even in the noisy environment.
1.4
Diversity Techniques in WLAN
The use of antenna arrays at a small wireless terminal (SWT) has been viewed unfavorably because of technical difficulties. The SWT devices include handset in cellular systems, wireless local area networks (WLAN) access points and user terminals, PDA, Bluetooth access points and terminals and indoor wireless fixed point to point systems. The new generation of high-speed, low power digital signal processors and miniaturized RF components facilitate computationally complex operations at the handheld terminal and supporting multiple RF chains for array processing. Use of smart antenna at the handset and performance improvement has been reported in [MOS00] and [DIE01]. More sophisticated diversity algorithms or beamforming techniques have not been implemented so far in SWTs. A multiple input multiple output (MIMO) system that provides higher order of diversity benefits and throughput enhancement. The spacing of the antennas must be large enough (within practical limits) to yield uncorrelated signals at the antenna branches. In recent researches it was shown that a very small distance which is a fraction of the wavelength may able to produce uncorrelated channel response in case of multipath diversity. In OFDM base technology it is possible to exploit this feature. The small interelement spacing is adequate for both diversity and
6
Chapter 1. Introduction
beamforming operation in OFDM system.
1.5
Overview of the dissertation
The chapters in this dissertation are organized to provide an understanding in OFDM based WLAN system in particular 802.11g system. Chapter 2 provides the basic concept of OFDM systems. Theoretical and practical systems are discussed. Chapter 3 gives a short overview on 802.11g standard. MAC and PHY layer of 802.11g are discussed. Detail discussion can be found in IEEE standard documents. Chapter 4 describes the different techniques of diversity. Receive diversity is discussed in detail. Chapter 5 shows the simulation results for SIMO environment maintaining 802.11g standard. The transmitter and receiver models are also discussed. Chapter 6 concludes the dissertation with a brief overview of research accomplishment and discusses future research directions.
Chapter 2
OFDM Principle
2.1
2.1.1
An Introduction To OFDM
OFDM Basics
OFDM stands for Orthogonal frequency Division Multiplexing. It is a special case of multicarrier transmission, where a single datastream is transmitted over a number of lower rate subcarriers. OFDM is a combination of modulation & multiplexing. Multiplexing generally refers to independent signals, those produced by different sources. In OFDM, multiplexing is applied to independent signals but these independent signals are a sub-set of the one main signal. The signal itself is first split into independent channels, modulated by data & then re-multiplexed to create the ofdm carrier. 7
8
2.1.2
Chapter 2. OFDM Principle
FDM & OFDM: An Analogical Interpretation
OFDM is different from FDM in several ways. In conventional broadcasting each radio station transmits on a different frequency, effectively using FDM to maintain a separation between the stations. There is no synchronization between each of these stations. With an OFDM transmission, the information signals from multiple stations is combined into a single multiplexed stream of data. This data is then transmitted using an OFDM ensemble that is made up from a dense packing of many subcarriers. All the subcarriers within the OFDM signal are time & frequency synchronized to each other, allowing the interference between subcarriers to be carefully controlled.
With FDM, the transmission signals need to have a large frequency guard-band between channels to prevent interference. This lowers the overall spectral efficiency. With OFDM, the orthogonal packing of the subcarriers greatly reduces this guard-band, improving the spectral efficiency.
Again, each of the carriers in a FDM transmission can use an analogue or digital modulation scheme. there is no synchronization between the transmission & so one station could transmit using PM & another using PSK. In a single OFDM transmission all the subcarriers are synchronized to each other, restricting digital modulation scheme.
2.2. Orthogonality: An Essential Term In OFDM
2.2
9
Orthogonality: An Essential Term In OFDM
2.2.1
Orthogonality In General
Signals are orthogonal if they are mutually independent of each other. Orthogonality is a property that allows multiple information signals to be transmitted perfectly over a common channel & detected without interference. To define orthogonality, let us take a sine wave of frequency m & multiply it by a sinusoid, either sine or cosine, of frequency n, where both m & n are integers. The area or integral under this product is given by,
Figure 2.1: Area under sine wave over one period is zero
f (t) = sin mωt × sin nωt By the simple trigonometric relationship, this is equal to sum of a two sinusoids
10
Chapter 2. OFDM Principle
of frequencies (n − m) & (n + m). = 21 cos(m − n) − 12 cos(m + n) These two components are each a sinusoid, so the integral is equal to zero over one period. Z2π
1 cos(m − n)ωt − 2
Z2π
1 cos(m + n)ωt = 0 − 0 = 0 2
0
0
So, when we multiply a sinusoid of frequency n by frequency m/n, the area under the product is zero. In general for all integers of frequency n & m, cos mx, cos nx, sin mx & sin nx are all orthogonal to each other. These frequencies are called harmonics. OFDM achieves orthogonality in the frequency domain by allocating each of the separate information signals onto different subcarriers. OFDM signals are made up from a sum of sinusoids, with each corresponding to a subcarrier. The baseband frequency of each subcarrier is chosen to be an integer multiple of the inverse of the symbol time, resulting in all subcarriers having an integer number of cycles per symbol. as a consequence the subcarriers are orthogonal to each other.
2.2.2
Frequency Domain Orthogonality
In the frequency domain, each OFDM subcarrier has a sinc,
sin(x) x
frequency re-
sponse.This is a result of the symbol time corresponding to the inverse of the carrier spacing.
2.2. Orthogonality: An Essential Term In OFDM
Figure 2.2: Time domain construction of an OFDM signal
Figure 2.3: Frequency response of a 5 tone OFDM signal
11
12
Chapter 2. OFDM Principle
As far as the receiver is concerned each OFDM symbol transmitted for a fixed time (TF F T ) with no tapering at the ends of the symbol. This symbol time corresponds to the inverse of the subcarrier spacing of
1 TF F T
Hz. This rectangular waveform
in the time domain results in a sinc frequency response in the frequency domain. The sinc shape has a narrow main lobe, with many side-lobes that decay slowly with the magnitude of the frequency difference away from the center [17]. Each carrier has a peak at the center frequency & nulls evenly spaced with a frequency gap equal to the carrier spacing. The orthogonal nature of the transmission is a result of the peak of each subcarrier corresponding to the nulls of all other subcarriers. When this signal detected using a Discrete Fourier Transform (DFT) the spectrum is not continuous. If the DFT is time synchronized, the frequency samples of the DFT correspond to just the peaks of the subcarriers & thus the overlapping frequency region between subcarriers does not affect the receiver. the measured peaks correspond to the nulls for all other subcarriers, resulting in orthogonality between the subcarriers.
2.3
Generation Of Subcarriers Using The IFFT
An OFDM signal consists of a sum of subcarriers that are modulated by using Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM). If di are the complex QAM symbols, Ns is the number of subcarriers, T the symbol duration, and fc the carrier frequency, then one OFDM symbol starting at t=ts can be written as Ns −1 2 X Ns i + 0.5 di+ (t − ts ) exp j2π fc − , ts ≤ t ≤ ts + T s(t) = Re −Ns 2 T i=
2
2.3. Generation Of Subcarriers Using The IFFT
13 (2.1)
s(t) = 0, t < ts ∧ t > ts + T But in the literature, often the equivalent complex baseband notation is used, which is given below. Ns −1 2
s(t) =
X i=− N2s
i Ns exp j2π (t − ts , ts ≤ t ≤ ts + T di + 2 T
(2.2)
s(t) = 0, t < ts ∧ t > ts + T In this representation, the real & imaginary parts correspond to the in-phase & quadrature parts of the OFDM signal, which have to multiplied by a cosine & sine of the desired carrier frequency to produce the final OFDM signal. Let us take an example. in 2.4 four subcarriers from one OFDM signal is shown.
Figure 2.4: 4 Sub carriers of an OFDM symbol Here, all subcarriers have the same amplitude & phase, but practically the amplitudes & phases may be modulated differently for each subcarrier. each subcarrier
14
Chapter 2. OFDM Principle
has exactly an integer number of cycles in the interval T, and the number of cycles between adjacent subcarriers differs by exactly one. This property accounts for orthogonality between the subcarriers. If the j t h subcarrier from 2.2 is demodulated by down converting the signal with a frequency of
j T
& then integrating the
signal over T seconds, the result is written as described below. TR +ts
Ns
exp
ts Ns −1 2
=
P
i=− N2s
−j2π Tj
2P−1 (t − ts ) di+ Ns exp j2π 2i (t − ts ) dt i=− N2s
2
di+ Ns exp j2π Ti (t − ts ) dt
(2.3)
2
= di+ Ns T 2
By looking at the intermediate result, it can be seen that a complex carrier is integrated over T seconds. For the demodulated subcarrier j, this integration gives the desired output dj+ Ns , which is the QAM value for that particular subcarrier. 2
2.4 2.4.1
OFDM Generation & Reception Brief Discussion On FFT & IFFT
We can use IFFT to produce a time domain signal. We should pretend that the input bits are not the time domain representations but are frequency amplitudes. In this way, we can create an output by using IFFT & the output signal will be a time domain signal.[21] The IFFT is a mathematical concept & does not care about what goes in & what goes out. As long as what goes in is amplitudes of some sinusoids, the IFFT will
2.4. OFDM Generation & Reception
15
crunch these numbers to produce a correct time domain result. Both IFFT & FFT will produce the identical results on the same input.
2.4.2
Basic Block Of OFDM Transceiver
OFDM signals are commonly generated digitally due to the difficulty in creating large banks of phase lock oscillators & receivers in the analog domain.
Figure 2.5: Block diagram of a basic OFDM transceiver The transmitter section converts digital data to be transmitted, into a mapping of subcarrier amplitude & phase. It then transforms this spectral representation of the data into the time domain using IFFT. To transmit the OFDM signal, the calculated time domain signal is then mixed up to the required frequency [4]. The receiver performs the reverse operation of the transmitter. It mixes the RF signal to base band for processing & then use FFT to analyze the signal in the
16
Chapter 2. OFDM Principle
frequency domain. The amplitude & phase of subcarriers is then picked out & converted back to digital data.[30]
2.4.3
Serial To Parallel Conversion
Data to be transmitted is typically in the form of a serial data stream. In OFDM, each symbol typically transmits 40-4000 bits, & so a serial to parallel conversion stage is needed to convert the input serial bit stream to the data to be transmitted in each OFDM symbol. The data allocated to each symbol depends on the modulation scheme used & the number of subcarriers. For a subcarrier modulation of 16-QAM, each subcarrier carries 4 bits of data, & so for a transmission using 100 subcarriers the number of bits per symbol would be 400. The modulation scheme used on each subcarrier can vary & so the number of bits per subcarrier also varies. As a result, the serial to parallel conversion stage involves filling the data payload for each subcarrier. At the receiver, the reverse process takes place with the data from the subcarriers being converted back to the original serial data stream. When an OFDM transmission occurs in a multipath radio environment, frequency selective fading can result in groups of subcarriers being heavily attenuated, which in turn can result in bit errors. Most Forward Error Correction(FEC) schemes tend to work more effectively if the errors are spread evenly, rather than in large clusters. So to improve the performance most systems employ data scrambling which is implemented by randomising the subcarrier allocation of each sequential data bit. At the receiver, the reverse scrambling is used to decode the signal.
2.4. OFDM Generation & Reception
2.4.4
17
Subcarrier Modulation
Once each subcarrier has been allocated bits for transmission, they are mapped using a modulation scheme to a subcarrier amplitude & phase, which is represented by a complex In-phase & Quadrature-phase vector.
Figure 2.6: Constellation of 16-QAM
A 16-QAM scheme maps 4 bits for each symbol. Each combination of the 4 bits of data corresponds to a unique IQ vector. A large number of modulation schemes are available allowing the number of bits transmitted per carrier per symbol to be varied. In the receiver, mapping the received IQ vector back to the data word performs
18
Chapter 2. OFDM Principle
subcarrier demodulation. During transmission, noise & distortion becomes added to the signal due to thermal noise, signal power reduction & imperfect channel equalization.
Figure 2.7: IQ diagram of 16-QAM signal with noise
Here, each of the IQ points is blurred in location due to the channel noise. For each received IQ vector the receiver has to estimate the most likely original transmission vector. This is achieved by finding by finding the transmission vector that is closest to the received vector.
2.4. OFDM Generation & Reception
2.4.5
19
Frequency To Time Domain Conversion
After the subcarrier modulation stage each of the data subcarriers is set to an amplitude & phase based on the data being sent & the modulation scheme; all unused subcarriers are set to zero. This sets up the OFDM signal in the frequency domain. An IFFT is then used to convert this signal to the time domain, allowing it to be transmitted.
Figure 2.8: IFFT stage of OFDM generation
20
Chapter 2. OFDM Principle
In the frequency domain, before applying the IFFT, each of the discrete samples of the IFFT corresponds to an individual subcarrier. Most of the subcarriers are modulated with data. The outer subcarriers are unmodulated & set to zero amplitude.
2.4.6
RF Modulation
The output of the OFDM modulator generates a base band signal, which must be mixed up to the required transmission frequency. This can be implemented using analog techniques or using a Digital Up Converter.
Figure 2.9: RF Modulation of Complex Base Band Signal (Analog Techniques) Both techniques perform the same operation. The performance of the digital modulation will tend to be more accurate due to improved matching between the processing of the I & Q channels & the phase accuracy of the digital IQ
2.5. Properties of OFDM
21
Figure 2.10: RF Modulation of Complex Base Band Signal (Digital Techniques) modulator.
2.5 2.5.1
Properties of OFDM Spectrum & Performance
Unshaped QPSK signal produces a spectrum such that its bandwidth is equal to (1 + α)Rs .In OFDM, the adjacent carriers can overlap. The addition of two carriers, allows transmitting 3Rs over a bandwidth of -2Rs to 2Rs or total of 4Ts . This gives a bandwidth efficiency of for 5 carriers.
4 3
Hz for 3 carriers &
6 5
22
Chapter 2. OFDM Principle
as more & more carriers are added, the bandwidth approaches, N +1 bitsperHz. N So, larger the number of carriers, the better.
2.5.2
Bit Error Performance
The BER of an OFDM is only exemplary in a fading environment. OFDM signal due to its amplitude variation does not behave well in a non-linear channel such as created by high power amplifiers on board satellites. Using OFDM for a satellite would require a fairly large backoff, on the order of 3dB, so there must be some other compelling reason for its use such as when the signal is to be used for a moving user.
2.5.3
Peak to Average Power Ratio(PAPR)
If a signal is a sum of N signals each of maximum amplitude equal to 1V, then it is conceivable that we could get a maximum amplitude of N that is all N signals add at a moment at their maximum points. The PAPR is defined as, R=
| x(t) |2 Pavg
For an OFDM signal, that has 128 carriers, each with normalized power of 1 watt, then the maximum PAPR can be as large as log(128) or 21dB. This is at the instant when all 128 carriers combine at their maximum points. The RMS PAPR will be around half this number or 10-12dB. This same PAPR is seen in CDMA signal as well.
2.6. Parameters of Real OFDM
23
The large amplitude variation increases in-band noise & increases the BER when the signal has to go through amplifier non-linearities. Large backoff is required in such cases. This makes use of OFDM just as problematic as multi-carrier FDM in high power amplifier applications.
2.5.4
Synchronization
Tight synchronization is needed. Often pilot tones are served in the subcarrier space. These are used to lock on phase & to equalize the channel.
2.5.5
Coding
The subcarriers are typically coded with Convolutional coding prior to going through IFFT. The coded version of OFDM is called COFDM or Coded OFDM.
2.6
Parameters of Real OFDM
2.7
Guard Period
2.7.1
Protection Against ISI
For a given system bandwidth the symbol rate for an OFDM signal is much lower than a single carrier transmission scheme. For a single carrier BPSK modulation, the symbol rate corresponds to the bit rate of the transmission. For OFDM, the system bandwidth is broken up into Nc subcarriers, resulting in a symbol rate that
24
Chapter 2. OFDM Principle
is Nc times lower than the single carrier transmission. This low symbol rate makes OFDM naturally resistant to effects of Inter-Symbol Interference(ISI) caused by multipath propagation, which is caused by the radio transmission signal reflecting off objects in the propagation environment. The effect of ISI on an OFDM signal can be further improved by the addition of a guard period to the start of each symbol.
2.7.2
Protection Against ICI
The guard period could consist of no signal at all. In that case, the problem of Intercarrier Interference (ICI) would arise. ICI is crosstalk between different subcarriers, which means they are no longer orthogonal. let, a subcarrier 1 & a delayed subcarrier 2 are shown. When an OFDM receiver tries to demodulate the first subcarrier, it will encounter some interference from the
P arameters
Specif ications
Data rates
6 Mbps to 48 Mbps
Modulation
BPSK, QPSK, 16 QAM & 64 QAM
Coding
Convolutional concatenated with Reed Solomon
FFT size
64 with 54 subcarriers uses, 48 for data & 4 for pilots
Subcarrier Frequency Spacing
20 MHz divided by 64 carriers or 0.3125 MHz
FFT period
1 ∇f
=3.2 µ sec
Guard duration
One quarter of symbol time, 0.8 µ sec
Symbol time
4 µ sec
Table 2.1: Parameters of Real OFDM
2.7. Guard Period
25
second subcarrier, because within the FFT interval, there is no integer number of cycles difference between subcarrier 1 & 2. At the same time, there will be crosstalk from the first to the second subcarrier for the same reason [11]. To eliminate ICI, the OFDM symbol is cyclically extended in the guard period. This ensures that delayed replicas of the OFDM symbol always have an integer number of cycles within the FFT interval, as long as the delay is smaller than the guard period. As a result, multipath signals with delays smaller than the guard period cannot cause ICI.
2.7.3
Insertion of a Guard period:
Figure 2.11: Adding Guard Period to OFDM Symbol The total length of the symbol is Ts = TG + TF F T , where Ts is the total length of symbol in samples , TG is the length of the guard period in samples, & TF F T is the size of the IFFT used to generate the OFDM signal.
26
Chapter 2. OFDM Principle
In addition to protecting the OFDM from ISI, the guard period also provides protection against time-offset errors in the receiver.
2.8
Additive White Gaussian Noise: Effect on OFDM
Noise exists in all communications systems operating over an analog physical channel, such as radio. The main sources are thermal background noise, electrical noise in the receiver amplifiers, & inter-cellular interference. In addition to this noise can also be generated internally to the communications system as a result of Inter-Symbol Interference, Inter-Carrier Interference & Inter-Modulation Distortion. These sources of noise decrease the Signal to Noise Ratio(SNR) & thus limiting the spectral efficiency of the system. Noise is the main detrimental effect in most radio communication systems. Most types of noise present in radio communication systems can be modelled accurately using Additive White Gaussian Noise(AWGN). This noise has a uniform spectral density & a Gaussian distribution in amplitude. Thermal & electrical noise from amplification, primarily have white Gaussian noise properties, allowing them to be modeled accurately with AWGN. Also most other noise sources have AWGN properties due to the transmission being OFDM. OFDM signals have a flat spectral density & a Gaussian amplitude distribution provided that the number of carriers is large, because of this the inter-cellular interference from other OFDM systems have AWGN properties. For the same reason ICI, ISI, & IMD also have AWGN properties for OFDM signals.
2.9. Modulation Schemes
2.9
27
Modulation Schemes
Digital data is transferred in an OFDM link by using a modulation scheme on each subcarrier. A modulation scheme is a mapping of data words to a real(In phase) & imaginary(Quadrature) constellation, also known as an IQ constellation. A 256QAM has 256 IQ points in the constellation, constructed in a square with 16 evenly spaced columns in the real axis & 16 rows in the imaginary axis. the number of bits that can be transferred using a single symbol corresponds to log2 (M ), where m is the number of points in the constellation, thus 256-QAM transfers 8 bits per symbol. each data word is mapped to one unique IQ location in the constellation. p The resulting complex vector I + j.Q, corresponds to an amplitude of I 2 + Q2 √ & a phase of < (I + j.Q), where j = −1. Increasing the number of points in the constellation does not change the bandwidth of the transmission, thus using a modulation scheme with a large number of constellation points, allows for improved spectral efficiency [16]. 256-QAM has a spectral efficiency of 8 b/s/Hz, compared with only 1 b/s/Hz for BPSK. The greater the number of points in the modulation constellation, the harder they are to resolve at the receiver. As the IQ locations become spaced closer together, it only requires a small amount of noise to cause errors in the transmission. This results in a direct trade off between noise tolerance & the spectral efficiency of the modulation scheme & was summarized by Shannon’s Information Theory, which states that the maximum capacity of a channel of bandwidth W, with a signal power of S, perturbed by white noise of average power N, is given by C = W log2 (1 +
S ) N
The spectral efficiency of a channel is a measure of the number of bits transferred
28
Chapter 2. OFDM Principle
per second for each Hz of bandwidth & thus the spectral efficiency SE is given by, SE =
C S = log2 (1 + ) W N
where both the signal & noise is linear scale & the spectral efficiency is measured in b/s/Hz.
2.10
OFDM & Single Carrier Transmission :A Comparative Discussion
2.10.1
Similarity in Performance
The BER of an OFDM system is dependent on several factors, such as the modulation schemes used, the amount of multipath, & the level of noise in the signal. However, the performance of OFDM with just AWGN is exactly the same as that of a single carrier coherent transmission using the same modulation scheme. A single OFDM subcarrier is exactly the same as a single carrier transmission that is a quadrature modulated with no band pass filtering. the transmitted amplitude & phase is held constant over the period of the symbol & is set based on the modulation scheme & the transmitted data. This transmitted vector is then updated at the start of each symbol. This results in a sinc frequency response, which is required response for OFDM. The optimal receiver for such a single carrier transmission is to use a coherent matched receiver, which can be implemented by mixing the signal to DC using an
2.10. OFDM & Single Carrier Transmission :A Comparative Discussion
29
IQ mixer. This results in an IQ output that describes the amplitude & phase of the received modulated carrier. The amplitude & phase of the transmitted signal is constant over the symbol period, & so the optimal method of removing the most noise from the signal is to use an integrate-and-dump filter. This filter averages the received IQ vector over the entire symbol, then performs IQ demodulation on average. The demodulation of an OFDM signal is performed in exactly the same manner. In the receiver a FFT is used to estimate the amplitude & phase of each subcarrier. The FFT operation is exactly equivalent to IQ mixing each of the subcarriers to DC then applying an integrate-and-dump over the number of samples in the FFT. So the FFT performs the same operation as the matched receiver for the single carrier transmission, except for a bank of subcarriers. So, we can conclude that in AWGN, OFDM will have the same performance as a single carrier transmission with no band limiting.
2.10.2
Distinction in Performance
Most propagation environments suffer from the effects of multipath propagation. For a given fixed transmission bandwidth, the symbol rate for a single carrier transmission is very high, where as for an OFDM signal it is N times lower, where N is the number of subcarriers used. This lower symbol rate results in a lowering of the ISI. In addition to lowering of the symbol rate, OFDM systems can also use a guard period at the start of each symbol. This guard period removes any ISI shorter than its length. if the guard period is sufficiently long, then all the ISI can be removed.
30
Chapter 2. OFDM Principle
Multipath propagation results in frequency selective fading that leads to fading of individual subcarriers. Most OFDM systems use Forward Error Correction to compensate for the subcarriers that suffer from severe fading. the additional spectral efficiency of those subcarriers that have a SNR greater than the average tends to compensate for subcarriers that are subjected to fading. As a result of this the performance of such an OFDM system in a multipath environment is similar to its performance in an AWGN channel. The performance of the OFDM system will be primarily determined by the noise seen at the receiver. However, the performance of a single carrier transmission will degrade rapidly in the presence of multipath. Again one of the main reasons to use OFDM is its ability to deal with large delay spreads with a reasonable implementation complexity. In a single carrier system, the implementation complexity is dominated by equalization, which is necessary when the delay spread is larger than about 10% of the symbol duration. OFDM does not require equalizer. Another complexity advantage of OFDM is the fact that the FFT does not really require full multiplications, but rather phase rotations, which can be efficiently implemented by the CORDIC algorithm . Because phase rotations do not change the amplitude, they do not increase the dynamic range of the signals, which simplifies the fixed point design. For the difference in complexity, OFDM has another advantage over single carrier systems with equalizers. For the latter systems, the performance degrades abruptly if the delay spread exceeds the value for which the equalizer is designed. Because of error propagation, the raw bit error probability increases so quickly that introducing lower rate coding or a lower constellation size does not significantly
2.11. OFDM: Advantages & Disadvantages
31
improve the delay spread robustness. For OFDM, there are no such nonlinear effects as error propagation, & coding & lower constellation sizes can be employed to provide fallback rates that are significantly more robust against delay spread. This is an important consideration, as it enhances the coverage area & avoids the situation the situation that users in bad spots cannot get any connection at all.
2.11
OFDM: Advantages & Disadvantages
2.11.1
Privileges of OFDM
• OFDM is an efficient way to deal with multipath; for a given delay spread, the implementation complexity is significantly lower than that of a single carrier system with an equalizer. • In relatively slow time-varying channels, it is possible to significantly enhance the capacity by adapting the data rate per subcarrier according to the signalto-noise ratio of that particular subcarrier. • OFDM is robust against narrowband interference, because such interference affects only a small percentage of the subcarriers. • OFDM makes single-frequency networks possible, which is especially attractive for broadcasting applications.
32
Chapter 2. OFDM Principle
2.11.2
OFDM System: Drawbacks
• OFDM is more sensitive to frequency offset & phase noise. • OFDM has a relatively large peak-to-average power ratio, which tends to reduce the power efficiency of the RF amplifier.
Chapter 3 An Introduction To IEEE Standard: 802.11g
3.1
3.1.1
WLAN Standards
Introduction
The foundation of mainstream WLAN products began with the original 802.11 standard developed in 1997 by the Institute of Electrical & Electronics Engineers (IEEE). That base standard continues to be enhanced through document additions that are designated by a letter following the 802.11 name, such as 802.11b, 802.11a or 802.11g. The letter suffix represents the task group that defines the extension to the standard. These enhancements bring increases in data rate & functionality leading to rapid progression of the WLAN market. 33
34
Chapter 3. An Introduction To IEEE Standard: 802.11g
3.1.2
IEEE 802.11 Specifications
Characteristics
802.11b
802.11a
802.11g
Standard approved
July 1999
July 1999
June 2003
Maximum data rate
11 Mbps
54 Mbps
54 Mbps
Modulation
CCK
OFDM
CCK & OFDM
Data rates
1,2,5.5,11 Mbps
6,9,12,18,
CCK:1,2,5.5,11 Mbps
24,36,48,54 Mbps
OFDM:6,9,12,18,24, 36,48,54 Mbps
Frequencies
2.4-2.497 GHz
5.15-5.35 GHz
2.4-2.497 GHz
5.425-5.675 GHz 5.725-5.875 GHz Table 3.1: 802.11g Specifications
3.2
802.11g: A Chronological Change in WLAN Standard
3.2.1
IEEE 802.11 Background
IEEE 802.11 denotes a set of Wireless LAN standards developed by working group 11 of IEEE 802. Better known in the industry as “802.11 Legacy” to avoid confusion, 802.11 is just the original version or base set of the Wireless LAN working group 11. Although 802.11 Legacy products are nonexistent today, the IEEE 802.11 standard is the foundation for the 802.11 “family”. The 802.11 “family” currently includes three separate protocols that focus on encoding (a, b, g); other
3.2. 802.11g: A Chronological Change in WLAN Standard
35
standards in the family (c-f, h-j, n) are service enhancement and extensions, or corrections to previous specifications [5]. The 802.11 specification utilizes frequencies used are in the microwave range and are subject to minimal governmental regulation. The 5 GHz transmission rate stated in this specification in the spectrum referred to as U-NII (Unlicensed National Information Infrastructure) in the United States. The U-NII spectrum is located at 5.15 - 5.35 GHz and 5.725 - 5.825 GHz [7].
3.2.2
IEEE 802.11b Background
The 802.11b was the first IEEE standard in wireless technology to become commonly accepted by wireless manufacturers. IEEE ratified the standard in 1999 and soon after, the public began to see these wireless products on the market. Since its induction, 802.11b wireless bases, or “hotspots” have been installed everywhere, from coffee shops to universities and corporations. IEEE 802.11b has a range of about 100 to 150 feet using a low-gain omnidirectional antenna. This type of antenna is typical hardware using the 802.11b standard. 802.11b has a maximum throughput of 11 Mbps for data transfer. However, a significant percentage of this bandwidth is used for communications overhead such as header information and security used in wireless transmission. So the actual data rate tends to be about 5.5 Mbps [8]. The 802.11b standard uses high rate direct-sequence spread spectrum (HR-DSSS). HR-DSSS is a signal structuring technique that utilizes a digital code sequence having a chip rate much higher than the information signal bit rate. Each information bit of a digital signal is transmitted as a pseudorandom sequence of chips
36
Chapter 3. An Introduction To IEEE Standard: 802.11g
to resemble white noise. The 802.11b standard slices up the spectrum into 14 overlapping, staggered channels of 22 MHz each. Three or four channels may be used simultaneously in the same area with little or no overlap. Complementary Code Keying (CCK) is the modulation type used for 802.11b which allows higher data speeds and is less susceptible to multipath-propagation interference.
3.2.3
IEEE 802.11a Background
The 802.11a standard, like 802.11b, was ratified in 1999. However, product using this standard didn’t start shipping until 2001 due to the wide acceptance of the IEEE 802.11b standard. The 802.11a standard uses the 5 GHz frequency band, and operates at a raw speed of 54 Mbps. At 5 GHz, 802.11a has an effective range of only 25 to 75 feet indoors. Due to the power limitations placed upon the industry by the FCC, manufacturers cannot make up the loss in signal strength due to attenuation [9]. The IEEE 802.11a standard uses orthogonal frequency division modulation (OFDM). OFDM is also called orthogonal frequency division multiplexing and sometimes referred to as discrete multitone modulation (DMT). OFDM is a modulation technique for modulating or encoding digital information into an analog carrier electromagnetic signal, a radio wave. OFDM modulation for transmission and demodulation for reception, are typically implemented using digital filter banks generally using the Fast Fourier Transform (FFT). 802.11a has not seen wide adoption due to the high adoption rate of 802.11b, and its inherent range limitations. Most uses of 802.11a have been in businesses applications where high rate data sharing is a necessity. The higher cost of 802.11a
3.2. 802.11g: A Chronological Change in WLAN Standard
37
at about $150 per access point odes not appeal to the average consumer because with the limited range multiple access points may be required to cover a desired area. Manufacturers have slowed in producing new wireless products using the 802.11a due to this lack of interest and have began producing dual band access point that use both 802.11a and 802.11g.
3.2.4
IEEE 802.11g Background
IEEE started the “g” taskforce to address the shortcomings of both IEEE 802.11b and 802.11a and design a better system. In December 2002 manufacturers began to release into the market wireless products based on the IEEE 802.11g standard DRAFT. Some of these pre-standard 802.11g-based products were notorious for diminishing the performance of 802.11b devices operating on 802.11g-based networks. This equipment prioritized 802.11g traffic over 802.11b traffic in mixed WLAN environments. The IEEE 802.11g standard was finally ratified on June13, 2003, and the Wi-Fi Alliance announced its first official certification of 802.11gbased products on July 8, 2003. The IEEE 802.11g standard, like the 802.11b standard, operates in the 2.4 GHz band. This new standard has the ability to send data at 54 Mbps raw or about 24.7 Mbps. This is the same maximum throughput as 802.11a standard. Like the IEEE 802.11a standard, IEEE 802.11g uses rate adaptively so that bandwidth is allocated to whichever stream needs it most. IEEE 802.11g standard incorporates reduction the transmission speed to 48, 36, 24, 12, then 6 Mbps if needed [10]. Extensions have also been made to the IEEE 802.11g standard in order to increase speed to 108 Mbps. However, like with 802.11b these extensions are solely pro-
38
Chapter 3. An Introduction To IEEE Standard: 802.11g
prietary and not endorsed by the IEEE. Many companies call enhanced versions ”802.11g+”. Companies have once again used packet bursting to achieve these new data rates.
The IEEE 802.11g standard uses both OFDM and CCK modulation techniques. OFDM is used to encoding the digital information for higher data rates and CCK is used for modulating the data when used to communicate with 802.11b.
One of the advantages of 802.11g is that it is fully backwards compatible with 802.11b. Unlike many areas of technology, where backward compatibility is left up to the manufacturer, IEEE has made it mandatory for wireless products using the 802.11g standard be compatible with it 2.4 GHz predecessor. This has made it easy for home and business users to upgrade gradually without an overwhelming cost.
Similar to IEEE 802.11b, 802.11g has a range of about 100 to 150 feet indoors using a low-gain omnidirectional antenna. However, IEEE 802.11g standard handles the inevitable signal reflection better than both 802.11b and 802.11a. Radio signals bounce off of everything at different angles and speeds. A wireless receiver must be able to reconcile the different reflections of the same signal that arrive at slightly different times into a single set of data.
Even though products using the IEEE 802.11g are fairly new, like the rest of technology, prices are plummeting. A wireless base station can be found at your local retailer for approximately $50.
3.3. IEEE 802.11: An Overview
3.3
39
IEEE 802.11: An Overview
The IEEE 802.11 standard provides for a general PHY and MAC layer specification that can accommodate any connectionless applications whose transport & network layers accommodate the IEEE 802.11 MAC layer. Today, the TCP/IP is the dominant transport/network layer protocol hosting all popular connectionless applications such as Web access, e-mail, FTP, or telnet, and it works overall MAC layers of the LANs, including IEEE 802.11. Therefore, the IEEE 802.11 standard does not need to specify the services. However, IEEE 802.11 provides for local & privately owned WLANs with a number of competing solutions.
3.4
Requirements of IEEE 802.11
The number of participants in the IEEE 802.11 standards soon exceeded a hundred suggesting a number of alternative solutions. the finalized set of requirements, which did not came about easily, indicated that the standard should provide • Single MAC to support multiple PHY layers • Mechanisms to allow multiple overlapping networks in the same area • Provisions to handle the interference from other ISM band radios & microwave ovens • Mechanism to handle ”hidden terminals” • Options to supporttime-bounded services • Provisions to handle privacy & access control
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Chapter 3. An Introduction To IEEE Standard: 802.11g
In addition it was decided that the standard would not be concerned with licensed band operations. These requirements set the overall direction of the standard in adopting different alternatives [28].
3.5
Reference Architecture
The connectionless IEEE 802.11 local network defines two topologies & several terminologies to start the standardization process
Figure 3.1: Ad Hoc Network and Infrastructure Network The above figure illustrates the infrastructure & ad hoc topologies that are the two configurations that the IEEE 802.11 standard considers. In the infrastructure configuration, wireless terminals are connected to a backbone network through APs. In the ad hoc configuration, terminals communicate in a peer-to-peer basis.
3.6. Layered Protocol Architecture
41
In IEEE 802.11 terminology, the AP provides access to distribution services through the wireless medium. the Basic Service Area (BSA) is the coverage area of one access point. the Basic Service Set (BSS) is a set of stations controlled by one access point. The Distribution System (DS) is the fixed infrastructure used to connect a set of BSS to create an Extended Service Set (ESS). IEEE 802.11 also defines portal(s) as the logical point(s) at which non-802.11 packets enter an ESS.
3.6
Layered Protocol Architecture
The protocol stack of the IEEE 802.11 standard is depicted according to the figure. The traditional simple MAC and PHY layers definition in the IEEE 802.11 substandard are broken into other sub layers to make the specification process easier. The MAC layer is divided into MAC sub layer and MAC management sub layer entities [20]. The MAC sub layer is responsible for access mechanism and fragmentation and reassembly of the packets. The MAC layer management sub layer is responsible for roaming in ESS, power management, and association, dissociation, and re association processes for registration connection management. The PHY layer is divided into three sub layers: PHY layer convergence protocol (PLCP), PHY medium dependent protocol, and the PHY layer management sub layer. The PLCP is responsible for carrier sensing assessment and forming packets for different PHY layers. The PMD sub layer specifies the modulations and coding technique for signaling with the medium, and PHY layer management decides on channel tuning to different options for each PHY layer .In addition IEEE 802.11 specifies a sta-
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Chapter 3. An Introduction To IEEE Standard: 802.11g
Figure 3.2: Protocol Entities for the IEEE 802.11
tion management sub layer that is responsible for coordination of the interactions between MAC and PHY layer.
3.7
3.7.1
Introduction to MAC Sublayer
Introduction
The overall MAC layer responsibilities are divided between MAC sublayer & MAC layer management sublayer. The major responsibilities of the MAC sublayer are to define the access mechanisms & packet formats.The MAC management sublayer defines roaming support in the ESS, power management & security.
3.7. Introduction to MAC Sublayer
3.7.2
43
The Access Mechanisms
The IEEE 802.11 specifies three access mechanisms that support both contention & contention-free access.
The Contention Mechanism The contention mechanism is supported by CSMA/CA protocol. In CSMA/CA, as soon as the MAC has a packet transmit, it senses the channel to see if the channel is available both physically & virtually. If the channel is virtually busy because a NAV signal is turned on, the operation is delayed until the NAV signal has disappeared. When the channel is virtually available, the MAC layer senses the PHY condition of the channel [20].
Figure 3.3: Primary operation of the CSMA/CA in the IEEE 802.11 If the channel is idle, the terminal waits for a DIFS period & transmits the data. If the channel is sensed busy, MAC runs a random random number generator to set a backoff clock. During the transmission of the packet & its associated DIFS, contention is differed but sensing continues. When the channel is available, a contention window starts in which all terminals having packets for transmission run down their backoff clocks. The first terminal that expires its clock starts
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Chapter 3. An Introduction To IEEE Standard: 802.11g
transmission. Other terminals sense the new transmission & freeze their clock to be restarted after the completion of the current transmission in the next contention period.
The Contention-free Mechanism There are two cases for contention-free transmission. the first case is the RTS/CTS & the second case is the implementation of the PCF for time-bounded information.
The RTS/CTS Mechanism In the RTS/CTS mechanism, a terminal ready for transmission sends a short RTS packet identifying the source address, destination address, & the length of the data to be transmitted. The destination station will respond with a CTS packet after a SIFS period. The source terminal receives the CTS & sends the data after another SIFS. The destination terminal sends an ACK after another SIFS period. Other terminals hearing RTS/CTS that is not addressed to them will go to the virtual carrier-sensing mode for the entire period identified in the RTS/CTS communication, by setting their NAV signal on . Therefore, the source terminal sends its packet with no contention. After completion of the transmission, the destination terminal sends an acknowledgement packet, & the NAV signal is terminated, opening the contention for other users.
The PCF Mechanism The PCF mechanism is built on top of the DCF using CSMA support contention-free time bounded & asynchronous transmission operations.
3.7. Introduction to MAC Sublayer
45
In the PCF operation, only available for infrastructure networks, the AP takes charge of the operation to provide the service to all terminals involved. The AP stops all other terminals & polls other stations in a semiperiodic manner. The AP organizes a periodical contention-free period for the time-bounded information. It coordinates time-bounded data to be transmitted at the beginning of each CFP, & during those periods it arranges an NAV signal for all other terminals.
3.7.3
General MAC Frame Format
Figure 3.4: General MAC frame format of the IEEE 802.11 The general MAC frame format starts with the frame control field. This field carries instructions on the nature of the packet. It distinguishes data from control & management frame & specifies the type of control or management signaling that the packet is meant to do.
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Chapter 3. An Introduction To IEEE Standard: 802.11g
The duration/ID field is used to identify the length of the fragmented packets to follow. the four address fields identify the source, destination, & Aps that they are connected. The sequence control is used for fragmentation numbering to control the sequencing. The sequence control & duration /ID are only available in the 802.11 to support fragmentation & the resembly feature of the MAC protocol.
3.7.4
Control Field in MAC Frames
IEEE 802.11 is a wireless network that needs to have control & management signaling to handle registration process, mobility management, & security. to implement these features, the frame format should accommodate a number of instructing packets.The capability of implementing these instructions is embedded in the control field of the MAC frames.
3.7.5
MAC Management Sublayer
The MAC management sublayer handles establishment of communications between stations & APs. This layer handles the mechanisms required for a mobile environment.
Registration The beacon is a management frame that is transmitted quasi-periodically by the AP to establish the timing synchronization function. It contains information such as the BSS-ID, timestamp, traffic indication map, power management, & roaming.
3.7. Introduction to MAC Sublayer
47
RSS measurements are made on the beacon message. The beacon is used to identify the AP, the network, & so on.
Handoff There are three mobility types in IEEE 802.11. The ”’no transition” type implies that the MS is static or moving within a BSA. A ”BSS transition” indicates that the MS moves from one BSS to another within the same ESS. The most general form of mobility is ”ESS transition” when the MS moves from one BSS to another BSS that is part of a new ESS.
Power Management The power conservation problem in WLANs is that stations receive data in bursts but remain in an idle receive state constantly, which dominates the LAN adaptor power consumption. The IEEE 802.11 solution is to put the MS in sleeping mode, buffer the data at AP, & send the data when the MS is awakened. Compared with continuous power control in cellular telephones, this is a solution tailored for bursty data applications.
Security There are provisions for authentication & privacy in IEEE 802.11. There are two types of authentication schemes in IEEE 802.11. The open system authentication is the default. Here the request frame sends the authentication algorithm ID for ”open system”. The response frame sends the results of request. The shared key authentication provides a greater amount of security. The request frame sends the
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Chapter 3. An Introduction To IEEE Standard: 802.11g
authentication frame ID for the ”shared key” using a 40-bit secret code that is shared between itself & the AP. the second station sends a challenge text. The first station sends the encrypted challenged text as the response. The second station sends the authentication result.
3.8
The PHY Layer: A Glance
Three physical media are defined in the original 802.11 standard 1. FHSS operating in the 2.4 GHz ISM band at data rates of 1 & 2 Mbps 2. DSSS operating in the 2.4 GHz ISM band at data rates of 1 & 2 Mbps 3. Infrared at 1 & 2 Mbps operating at a wavelength between 850 and 950 nm. When the MAC protocol data units (MPDU) arrive to the PLCP layer, a header is attached that is designed specifically for the PMD of the choice for transmission. The PLCP packet is then transmitted by PMD according to the specification of the signaling techniques. In the original IEEE802.11, there are three choices of FHSS, DSSS, and DFIR for PMD transmission and therefore IEEE802.11 defines three PLCP packet formats to prepare the MPDU for transmission.
3.8.1
FHSS
For modulation, the FHSS scheme uses 2-level Gaussian FSK for the 1-Mbps system. The bits zero and one are encoded as deviations from the current carrier frequency. For 2 Mbps, a 4 level GFSK scheme is used, in which 4 different deviations from the center frequency define the four 2 bit combinations.
3.8. The PHY Layer: A Glance
49
The PLCP additional bits consist of a preamble and the header, the preamble is a sequence of alternating 0 and 1 symbol for 80 bits that is used to extract the received clock for carrier and bit synchronization. The start of the frame delimiter (SFD) is a specific pattern of 16 bits indicating start of the frame. The next part of the PLCP is the header that has three fields. The 12 bit packet length width (PLW) field identifies the length of the packets that could be up to 4 Kbytes. The 4 bits of the packet signaling field (PSF) identifies the data rates in 0.5 Mbps steps starting with 1 Mbps.
Figure 3.5: PLCP Frame for the FHSS of the IEEE 802.11 The FHSS PMD hops over 78 channels of 1 mega hertz each in the center of the 2.44 gigahertz ISM bands. The modulation technique is the GFSK for 1 Mbps two levels and for two Mbps four levels.
3.8.2
DSSS
The PMD of the DSSS version of the IEEE.802.11 uses a barker code of length 11. It should be remembered that FHSS receiver is a narrow band receiver (1 MHz bandwidth) whose center frequency hops over 76 MHz, but DSSS communicates using non overlapping pulses at the chip rate of 11 Mcps which occupies around
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Chapter 3. An Introduction To IEEE Standard: 802.11g
26 MHz of band width. The modulation techniques used for the 1 and 2 Mbps are DBPSK and DQPSK, respectively, which sends one or two bits per transmitted symbol. The ISM band at 2.4 GHz is divided into 11 overlapping channels spaced by 5 MHz.
Figure 3.6: Overlapping frequency bands for the DSSS in the IEEE802.11
The overall format is similar to the FHSS, but the length of the field is different because transmission techniques are different and different manufacturers design the model product for development of the FHSS and DSSS standards. The MPDU from the MAC layer is transmitted either at 1 or 2 Mbps, however, analogous to the FHSS version of the standard, the PLCP of the DSSS version also uses the simpler BPSK modulation at 1 Mbps all the time.
3.8.3
DFIR
The infrared scheme is omni directional rather than point to point. A range of up to 20 m is possible. For 1 Mbps data the modulation scheme is 16-PPM (pulse position modulation). Where each group of 4 data bits is mapped into one of the
3.8. The PHY Layer: A Glance
51
Figure 3.7: PLCP Frame for the DSSS of the IEEE 802.11 16-PPM symbols; each symbol is a string of 16 bits. For the 2 Mbps data rate, each group of 2 data bits is mapped into one of four 4-bit sequences. To understand the structure of the diffused infrared we need to study the PLCP frame of it. The PMD of DFIR operates based on transmission of 250 ns pulses that are generated by switching the transmitter LEDs on and off for the duration of the pulse.
Figure 3.8: PLCP Frame for the DFIR of the IEEE 802.11 The PLCP packet format for the DFIR is shown as the figure. The PLCP signals are shown in the unit of slots of 250 ns for one basic pulse. The sync and SFD fields are shorter than FHSS and DSSS because non coherent detection using
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Chapter 3. An Introduction To IEEE Standard: 802.11g
photosensitive diode detectors do not need carrier recovery or elaborate random code synchronizations. The 3 slot data rate indication field starts by 000 for 1 Mbps and 001 for 2 Mbps. The length and FCS are identical to the DSSS. The only new field is the dc level adjustments (DCLA) that sense a sequence of 32 slots, allowing the receiver to set its level of the received signal to set threshold for the deciding between zeros and ones. The MPDU length is restricted to 2500 bytes.
3.9
802.11g: A Natural Demand in Wireless Communication
3.9.1
Introduction
The next mainstream wireless LAN standard is 802.11g. This technology satisfies the bandwidth needs of the market while remaining compatible with the installed base of mainstream products like 802.11a, b.
In July 1999 IEEE subcommittee was tasked to extend the 2.4 GHz unlicensed spectrum to data rates faster than 20 Mbps. the result was a standard with OFDM and CCK as the mandatory modulation schemes with 24 Mbps as the maximum mandatory data rate, but it also provides for optional higher data rates of 36, 48, and 54 Mbps.
3.9. 802.11g: A Natural Demand in Wireless Communication
3.9.2
53
Data rates
This standard supports multiple data rates to allow clients to communicate at the best possible speed. Data rate selection is a trade-off between obtaining the highest possible data rate while trying to minimize the number of communication errors. Whenever there is an error in the data, the systems must spend time to retransmit the data until it is error free. Each 802.11 client performs a procedure to select the best data rate. The 802.11g clients can select from the widest possible range of both OFDM data rates of 54, 48,36,24,18,12,9 and 6 Mbps, and the CCK rates of 11, 5.5,2 and 1 Mbps.
3.9.3
The PHY Layer
The PHY layer uses OFDM to combat frequency selective fading and to randomize the burst errors caused by a wideband fading channel. The physical layer modes are as follows • For the DSSS modulation: DBPSK, DQPSK, CCK • For the OFDM modulation: BPSK, QPSK, 16 QAM, 64 QAM These PHY modes with different coding and modulation schemes are selected by a link adaptation scheme. The exact mechanism of this process is not specified in the standard.
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Chapter 3. An Introduction To IEEE Standard: 802.11g
The transmitted data is supplied to the physical layer in the form of an input protocol data unit from data link control. Then it is scrambled and put to half rate convolutional code, after that puncturing, interleaving, mapping is done to prepare for OFDM operation. And hence it is turned to PHY bursts.
3.9.4
The MAC Layer
The medium access is based on CSMA/CA. A station, Mobile terminal or access point, must listen to the channel and sense it idle for a certain time before they can start to transmit. If a collision should occur the participants must wait a random backoff time before trying to re-transmit. The MAC layer also takes care of error control, authentication, key management, association, and disassociation and encryption seed distribution. The EC mechanism supports both acknowledged and unacknowledged modes. The acknowledged mode is based on a stop- and- wait automatic repeat request protocol (SW-ARQ). A cyclic redundancy code (CRC) is used for error detection. There are 3 different types of frames used for transmission; management, control and data frames. The management frames are used for association and authentication of an MT to an AP. The control frames are used for handshaking and acknowledgement, and data frames are used for transmitting data. The control frames are Request To Send (RTS), Clear To Send (CTS), and Acknowledge (ACK). Each frame has following basic components: • A MAC header that consists of frame control, duration, address, and se-
3.9. 802.11g: A Natural Demand in Wireless Communication
55
quence control information. The length of it depends on the frame type. • A data field of variable length. • A 32-bits CRC used for error detection.
3.9.5
Characteristics of 802.11g
Range & Data rates As distance from the access point increases, 802.11 based products provide reduced data rates to maintain connectivity. The IEEE802.11g standard has the same propagation characteristic as 802.11b, because it transmits in the identical 2.4 GHz frequency band. Because 802.11b and 802.11g products share the same propagation characteristics, implementations provide roughly the same maximum range at the same data rate. Because 5-GHz radio signals do not propagate as well as 2.4-GHz radio signals, the 802.11a product range is limited compared to the 802.11b or 802.11g product range.
THROUGHPUT Throughput is not the same as data rate for networking systems, because of overhead, environment and network composition. The throughput of it can depend on whether there are 802.11b products nearby. Performance is best in environments where an 802.11g access point is only “talking” with 802.11g clients in a homogeneous WLAN. In these environments, the data rate within 75 feet is 54 Mbps and the throughput is 22-24 Mbps when using transmission control protocol
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Chapter 3. An Introduction To IEEE Standard: 802.11g
(TCP). In the interest of maximizing performance in the presence of 802.11b products, the 802.11g APs coordinate the use of the transmission medium with protection mechanisms. Because the protection mechanisms require overhead communication, compatibility is provided at the expense of throughput. The CTS-to self protection mechanism lowers the maximum TCP throughput to approximately 15 Mbps.
Protection Mechanism
The 802.11g standard allows its clients to use one of several protection mechanisms in a mixed 802.11b/g environment. The Wi-Fi alliance will test for one of two signaling methods: RTS/CTS and CTS-to self. Request to send (RTS) is analogous to a pilot’s takeoff request to an air traffic control tower- the pilot waits to use the airspace until verifying with the control tower that the airspace is clear. The clear to send (CTS) message is like the clearance from the tower. The CTS-to-self protection mechanism method sends a CTS message using an 802.11b rate to clear the air, and then immediately follows with data using an 802.11g data rate. The CTS-to-self protection mechanism provides a maximum TCP throughput of 14.7 Mbps. With any of the protection mechanisms, 802.11g throughput is still grater than that of 802.11b at the same distance.
3.10. Comments
57
Compatibility As 802.11g uses the same radio signaling (CCK) as 802.11b at the lower four data rates, it is fully backward compatible with 802.11b. This enables networks to continue supporting 802.11b enabled devices when migrating to the higher performance standard. Modifying protocol parameters allow 802.11g to operate in a ”g only’ mode or a ’mixed g & b’ mode. allowing backwards compatibility to 802.11b with different performance results. As it will be difficult for many system managers to know how to configure these parameters to achieve the best performance for their network, Proxim provides three easy-to-understand set-up configurations for ORiNOCO 802.11b/g Access Points: Choosing the mode automatically configures the appropriate parameters for the best possible performance. The next section discusses how ORiNOCO provides two solutions for maximizing performance in a mixed 802.11b and 802.11g environment.
3.10
Comments
802.11g lays out the ground rules for Wireless LAN equipment that is capable of at least 24Mbps (megabits per second) and up to 54Mbps, while remaining backward compatible with existing 802.11b equipment that runs at a maximum 11Mbps. Both use radio spectrum in the 2.4GHz radio band. Another standard, 802.11a, defines 54Mbps gear in the 5GHz range. Certification means that products are compatible with one another.
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Chapter 3. An Introduction To IEEE Standard: 802.11g
Mode Type
Clients Supported
Performance
802.11b only
802.11b
Maximum throughput of 6Mbps for 802.11 clients in optimum environments
802.11g only
802.11g
No backwards compatibility for 802.11b clients.Maximum throughput of 27Mbps for 802.11g clients in optimum environments
Mixed 802.11b
802.11b
Support for legacy 802.11b clients
&
&
on a single 802.11g radio. Maximum
802.11g
802.11g
throughput with .11g clients on radio - 18 Mbps. Maximum throughput with both .11g and .11b clients on radio– 9Mbps
Table 3.2: Compatibility in 802.11g
3.10. Comments
59
The growing number of amendments to IEEE’s family of 802.11 wireless Internet standards and vendors’ use of different chip sets within the same product line have created a need for interpretability testing and certification. The key issue with 802.11 standards is that they are mainly focussed on the functionality of WiFi equipment. There is little specific detail regarding the securing of WiFi equipment in a networked environment. 802.11g is frequently referred to in relation to network security. It is relevant in terms of interpretability but that is the only part it can play in a security discussion. It is only when a system (group of devices) is fully functional that the security problem can be tackled. In short 802.11g enables security rather than ensuring it.
Chapter 4 Diversity Techniques
4.1
Introduction
In this chapter we discuss some concepts and techniques of diversity in wireless communication. Basic diversity techniques in space, time and frequency are discussed in the beginning. Space diversity is investigated in details. Finally we focus on single input multiple output(SIMO) systems.
4.2
Fundamental Concepts of Diversity
Diversity is a technique of transmitting multiple instances of the same signal. Then the redundancy of signal is used to achieve better performance. Diversity can be achieved in many ways and using different means. A signal can be diversified in frequency, time and space. A combination of any of them is also possible. Spacetime coding is an example of exploiting both space and time for diversity. 60
4.2. Fundamental Concepts of Diversity
61
Diversity can be used both ways in two way communication. Although receive diversity is more popular and older concept, in recent years transmit diversity is gaining more considerations. Transmit diversity can be open loop or close loop. Open loop diversity does not assume any prior knowledge about channel while close loop diversity uses channel estimation using feedback loop from receiver. Cyclic Delay Diversity (CDD) is an example of open loop transmit diversity. Beamforming at transmission end is another technique which uses close loop system. The different diversity techniques are • Space Diversity • Frequency Diversity • Time Diversity • Multipath Diversity • Polarization Diversity • Pattern Diversity • Angle Diversity Here these techniques are discussed in brief
4.2.1
Space Diversity
To exploit space diversity we use different antennas at different spatial locations. We use M antennas to send signal in transmit diversity and M different antennas to receive signals in receive diversity. The important advantage in space diversity is that it does not require any additional time or frequency to achieve diversity.
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Chapter 4. Diversity Techniques
Figure 4.1: Space Diversity (MIMO)
The main shortcomings of space diversity lies in the fact that it needs different signal instances which goes under independent fading. Which means that the antennas should be placed at modest distances so that the signal they receive or transmit goes through channels which are uncorrelated. If the antennas are placed without proper spacing then all the antenna will experience almost same path. So the received copies of signals will be highly correlated. As a result no diversity gain can be achieved.Space diversity can be employed to combat both frequency selective fading and time selective fading.
Figure 4.2: Frequency Diversity
4.2. Fundamental Concepts of Diversity
4.2.2
63
Frequency Diversity
One approach to achieve diversity is to modulate the information signal through M different carriers. Each carrier should be separated from the others by the coherence bandwidth (∆fc ) so that different copies of the signal undergo independent fading. At the receiver, the M independently faded copies are “optimally” combined to give a statistic for decision. Frequency diversity can be used to combat frequency selective fading. As available and useful frequency range is not wide, in most cases it is not optimum approach to use frequency diversity only. For frequency diversity uses more frequency which is costly.
Figure 4.3: Time Diversity
4.2.3
Time Diversity
Another way of achieving diversity is to use time instead of space. In time diversity we send same signal in M different time slots. So that M copies of same signal is
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Chapter 4. Diversity Techniques
sent. Here important consideration should be taken to use proper time difference among M time slots. The time between two consecutive instances of same signal should be at least equal to the coherence time (∆tc ) so that the multiple copies of same symbol undergo independent fading. Another shortcoming of time diversity is that it uses more time which is unsatisfactory in case of live communications. This problem can be overcome by using another dimension such as time or space to provide for extra time.
4.2.4
Multipath Diversity
OFDM and MIMO technology hold much promise in realizing broadband wireless systems which are both power efficient and bandwidth efficient. One of the key issue to be confronted in such systems is the frequency selective fading of the wireless channel due to multipath propagation. OFDM helps in transforming this frequency selective channel into multiple narrowband flat sub-channels which facilitate a computationally efficient equalization. It is possible to use this multipath diversity to achieve high performance. By using space sampling it can be shown that very closely placed antennas such as 0.44λ can achieve performance comparable to widely spaced antenna in frequency flat channels[22]. This is possible because OFDM system converts multipath diversity to space diversity in frequency domain. Thus multipath diversity can be a good advantage in OFDM systems.
4.2. Fundamental Concepts of Diversity
65
Figure 4.4: Multipath Diversity
4.2.5
Polarization Diversity
Polarization diversity is the transmission and reception where the same information signal is transmitted and received simultaneously on orthogonally polarized waves with fade-independent propagation characteristics.Like space diversity, polarization diversity relies on the decorrelation of the two receive ports to achieve diversity gain. The diversity gain from polarization diversity is maximized if the dual-polarized antenna receive radiation in a cross-polarized fashion with equal field strengths.
4.2.6
Pattern Diversity
Pattern diversity or Field Component Diversity is the use of two antennas with minimally overlapping patterns to provide greater overall pattern coverage. Pattern diversity uses co-located antennas that are of different size, shape, orientation and material. These antennas have dissimilar radiation patterns and their signals are combined in phase due to their collocation.
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Chapter 4. Diversity Techniques
4.2.7
Angle Diversity
Angle Diversity technique is based on the slight difference in the angle of arrival between the indirect delayed waves, occurring during multipath propagation, and the direct wave. If at the receiving site there are two antennas or two antenna feeds pointing partiallv above and below the line of sisht. then the wave components add up in different ways resulting in diiterent trequency-selective dispersions in the two diversity branches. the availability of a single antenna with dual angle diversity beam makes angle diversity cost effective. Recently different coding schemes, like Space-Time Block Codes, are used to exploit more than one diversity. Prominent schemes exploit the following diversities. • Space-Time (ST) Diversity • Space-Frequency (SF) Diversity • Space-Time-Frequency (STF) Diversity
4.3
Different Techniques for transmit diversity
In recent days many interesting techniques are used to achieve transmit diversities. In 1998 S. M. Alamouti presented a simple and attractive scheme for transmit diversity and showed that in transmit diversity it is possible to achieve same performance as receive ,diversity using this scheme[3]. This concept is further investigated and developed into a complete structure of transmit diversity using space and time diversity known as Space Time Block Coding[29]. This coding can
4.3. Different Techniques for transmit diversity
67
be extended to utilize space and frequency diversity known as Space Frequency Block Coding[6]. Another approach was proposed in 2001 where frequency selectivity is fully exploited in an OFDM system named Cyclic Delay Diversity (CDD)[31]. The system effectively reduces the deep fades of a frequency selective channel into distributed small fades throughout the whole channel. In this section we will briefly discuss Alamouti scheme and CDD systems and try to convey the concepts behind them.
4.3.1
Alamouti Scheme
In this scheme the transmitting signals of two branches are coded in such a way so that they could be combined to form the actual signal in the receiving end. For two branch diversity the transmitted signals are of the following form The channel time
antenna0 antenna1
time t
s0
s1
time t + T
−s∗1
s∗0
Table 4.1: The encoding and transmission sequence for the two-branch transmit diversity scheme can be modeled as complex values h0 and h1 where it is assumed that the channel will remain same for two consecutive symbols. So we can write h0 (t) = h0 (t + T ) = α0 ejθ0 jθ1
h1 (t) = h1 (t + T ) = α1 e
(4.1)
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Chapter 4. Diversity Techniques
where T is the symbol duration. Then the received signals r0 and r1 in the receiver end in two consecutive time slot can be expressed as r0 = r(t) = h0 s0 + h1 s1 + n0
(4.2)
r1 = r(t + T ) = −h0 s∗1 + h1 s∗0 + n1 where n0 and n1 are the complex random variable modeling received noise and interferences.The scheme is shown in 4.5.
Figure 4.5: Alamouti Scheme
The combining scheme is simple and can be achieved by simple linear combination of the received signals. The two combined signals that are sent to the maximum
4.3. Different Techniques for transmit diversity
69
likelihood detector are as follows s0 = h∗0 r0 + h1 r1∗
(4.3)
s1 = h∗1 r0 + h0 r1∗ By substituting 4.1 and 4.2 into 4.3 we get s0 = (α02 + α12 )s0 + h∗0 n0 + h1 n∗1 s1 =
(α02
+
α12 )s1
+
h0 n∗1
+
(4.4)
h∗1 n0
The resulting combined signals s0 and s1 are equivalent to the two branch maximal ratio receiver combining (MRRC) with two antennas in the receiver. In Alamouti Scheme a phase rotation of the noise is occurred does not has any effect in the performance of the scheme. This scheme can be implemented with higher order of diversity. It can provide diversity of the order 2M where M received antennas are used with 2 transmit antennas [3]. This system can be readily adopted by OFDM systems [13] and it can achieve high performance in OFDM based 802.11g and 802.16 [24].
4.3.2
Cyclic Delay Diversity
Cyclic Delay Diversity (CDD) is another approach to achieve transmit diversity by inducing a delay in the system. This approach is first introduced as delay diversity where the OFDM symbols are delayed to a certain extent and then transmitted from different transmit antennas. These delayed versions of the original OFDM symbols introduce a delay diversity in the channel. As a result delay spread of the channel increases which consequently decreases coherence bandwidth. This is because delay spread is inversely related to coherence bandwidth [15]. Which means
70
Chapter 4. Diversity Techniques
error per unit frequency increases. This is helpful for coded and interleaved transmission scheme, because errors occur more independently. Where channel has no built in frequency diversity, this scheme helps to gain diversity in frequency domain. Delay diversity is bounded by the length of guard interval. Because the highest delay that can be introduced must be less than guard interval. Otherwise one symbol will overlap next symbol which will make the received signals unintelligible [31].
Figure 4.6: Cyclic Delay Diversity Scheme
To overcome the shortcomings of delay diversity another concept of cyclic delay diversity is used. In CDD the the OFDM symbol is not simply shifted rather cyclically shifted. A cyclic shift means that the last n samples of the symbol are shifted and added before the first sample of the OFDM symbol. This way the highest delay which can be introduced is not dependent on guard period. This process randomize the channel and thus make it possible to use interleaving and coding schemes more efficiently. If the channel transfer function is H( f ) then the
4.4. Receive Diversity
71
Figure 4.7: CDD Symbols transfer function becomes after applying CDD M 1 X H (f ) = √ Hi (f )e−j2πτi f M i=1 P
(4.5)
where M is the number of antennas and τi is the cyclic delay. This randomization of noise is dependant on many parameters. One of the main consideration is how to choose delays. There several approach developed for this purpose [12]
4.4
Receive Diversity
In receive diversity different techniques can be used like polarization diversity, angle diversity or space diversity. Space diversity is commonly used. SIMO OFDM system has three basic blocks in receiving end. They are • Receive signals with antenna array • Channel estimation
72
Chapter 4. Diversity Techniques • Convert signals to frequency domain and decision making
these steps are discussed in further detail.
4.4.1
Receive Antenna Array
Antennas can be arranged in various directions and in different ways. They are located in the three dimensional space in some fixed points with respect to some predefined point. The popular geometrical configurations are Linear, Circular and Planar. In linear Array the antennas are aligned in a straight line while in Circular Array they are placed on the perimeter of a circle. In planar array the antenna elements lie on a fixed plane. The radiation pattern of an array is determined by the radiation pattern of the individual elements, their orientation and relative positions in space, and the amplitude and the phase of the feeding current [23]. If each element of the array is an isotropic point source, the radiation pattern of the array will depend solely on the geometry and feeding current of the array. In that case the radiation pattern is commonly known as the array factor. If each of the elements of the array is similar but nonisotropic, by the principle of pattern multiplication [18], the radiation pattern can be computed as a product of the array factor and the individual element pattern [26].
Uniform Linear Array If the spacing between the elements of a linear array is equal, it is known as Uniform Linear Array (ULA). 4.8 shows an N element ULS. The spacing between the
4.4. Receive Diversity
73
array elements is d and a plane wave arrives at the array from a direction θ off the array broadside. The array broadside is perpendicular to the line containing the center of the elements. The angle θ measured clockwise from the array broadside is called Direction of Arrival (DOA) or the Angle of Arrival (AOA) of the received signal.
Figure 4.8: Uniform Linear Array System
The received signal at the first element can be written as, x(k)=A1 (k) cos {2πfc k + γ(k) + β} Where, A1 (k) is the amplitude of the signal fc is the carrier frequency γ(k) is the information β is the random phase. If this data is denoted as d(n) then after IFFT operation in the transmitter, the signal becomes a time domain signal and denoted as d(k). It is told that the receiver is equipped with an antenna array and the radiation pattern of an array is determined by the radiation of individual element. Complex
74
Chapter 4. Diversity Techniques
envelope of received signal at ith array element is, xi (k) = di (k)e−j {2π λ (i−1) sin θ} d
(4.6)
Now if we denote the total signal received by receiver which is combined effect of all antenna elements by x (k) whose each element contains the received signal at the corresponding array element. So, the received signal vector can be defined as, h iT x (k) = x1 (t) x1 (t) · · · xN (t) . And the array response vector could be defined as, h iT d d a(θ) = 1 e−{j2π λ sin θ} ........e−{j2π λ (N −1) sin θ}
(4.7)
Now, if we define the interferer signal as i(k) and array response vector for the interferer is a(φ). Then, the signal received by the receiver is, x(k) = d(k)a(θ) + i(k)a(φ) Considering the additive white Gaussian noise (AWGN), x(k) = d(k)a(θ) + i(k)a(φ) + n(k) Here, n(k) is the additive white Gaussian noise (AWGN), a(θ) is known as the array response vector or the steering vector of an ULA. The array response vector is a function of the AOA, individual element response, the array geometry and the signal frequency. The received signal vector can now be written in a compact vector form as x(t) = a(θ)x(t) [2].
Circular Array A circular array consisting of N identical isotropic antenna elements evenly placed in a circle of radius R is shown in the 4.9. Each element is weighted with the weight
4.4. Receive Diversity
75
Wn where n = 0, 1, 2, . . . , N − 1. Since the N elements are equally placed around the circle the azimuth angle of the nth element is given as φn = 2nπ/K. If a plane wave comes in the direction of the received signal is (θ, φ) then the relative phase of the k t h element with respect to the center is given by
Figure 4.9: Circular Array System
βn = −nRcos(φ − φn )sinθ
(4.8)
It follows that the array response vector for a circular array is given by
a(φ, θ) =
N −1 X
An ej[αn −nR cos(φ−φk ) sin θ]
n=0
Where An ejαn is the complex weight of the nth element.
(4.9)
76
Chapter 4. Diversity Techniques
Planar Array Linear array can be of many form and geometric shape. Linear array and circular array are two variations of planar array. Other common configurations are rectangular array, as shown in 4.10, and hexagon array, as shown in 4.11. Rectangular array can be considered as a linear array consisting of L elements each of which is a linear array of N elements with array response vector aN (u). The array response vector for L-element linear array is given by aL (v) where u and v are function of θ and φ. According to the principle of pattern multiplication, the overall array response vector for the rectangular array is given by a = aN (u)aL (v)
(4.10)
Figure 4.10: Rectangular Planar Array System A hexagon array can be evaluated as a single element at the center and the summation of concentric circular arrays of different radii, as shown in ??. Thus the
4.4. Receive Diversity
77
Figure 4.11: Hexagonal Planar Array System
Figure 4.12: Hexagonal Array Geometry overall array response vector can be given by a = a0 +
XXX n
m
An,m,l
(4.11)
l
where k denotes the number of hexagon.
4.4.2
Channel Estimation
To achieve diversity gain it is needed to estimate the channel. Different techniques can be adopted to achieve this goal. To estimate usually a predefined sequence
78
Chapter 4. Diversity Techniques
of known signals are used. This is called training sequence. Then after reception they are used to find the characteristics of the channel by comparing the statistical properties of the known symbols with received symbols. Diversity gain of a system is dependent on the accurate knowledge of the channel.
The channel estimation can be blind or semi-blind. And they can be adaptive or one-shot. Adaptive filtering techniques are suitable for iterative channel estimation, such as RLS, LMS, Kalman filter (KF), Direct Sample Covariance Matrix Inversion (DSCMI), Neural Network etc, because of their computational efficiency compared to one-shot approaches [19], such as least-squares (LS) and linear minimum mean-square error (MMSE) methods which need matrix inversions. At a cost of complexity, both recursive least-squares (RLS) [1] [27], least-mean-square (LMS) and Kalman filter (KF) [14],[25] algorithms are extensively used for channel estimation in wireless communications. These estimation is also used to compensate for ISI and other distortion in the received signals. When it is used for ISI combatting it is called equalization.
Adoptive techniques for channel estimation are very popular due to its dynamic nature and low complexity. Usually known training symbols are used for estimation. Most popular techniques are RLS, LMS, Kalman filter (KF), Direct Sample Covariance Matrix Inversion (DSCMI), Neural Network etc. Here LMS and RLS techniques are discussed.Two adaptive equalization techniques RLS and LMS are discussed in the appendix.
4.4. Receive Diversity
4.4.3
79
Decision Making
After finding the channel information it is necessary to make the decision. In practical field an optimum weight is calculated for each receive antenna from training symbols by MMSE or any other suitable method. Then the output is formed by the linear combination of the weighted output from each antenna. Then the modified signal is sent to the decision making block. If we do the channel estimation and weight calculation in the frequency domain then simple weighted sum of the multiple antenna signals can easily combat the channel effect. For two receive antenna if the channel vectors H1 and H2 are modeled by H1 = α1 ejθ1 H2 = α2 ejθ2 and the transmit signal is X then the received signal is Y then Y1 = XH1 Y2 = XH2 by multiplying the received signals with suitable weight and adding them we get Y or, Y or, Y or, Y
= (XH1 ) W1 + (XH2) W2 H2∗ H1∗ + (XH2) = (XH1 ) H1 H1∗ + H2 H2∗ H1 H1∗ + H2 H2∗ α12 α22 = X + α12 + α22 α12 + α22 = X
where the weights are H1∗ W1 = H1 H ∗ +H2 H ∗ 1 2 H2∗ W2 = H1 H ∗ +H2 H ∗ 1
2
80
Chapter 4. Diversity Techniques
So it is possible get back the actual signal if we have perfect knowledge of the channel.
Chapter 5 Simulation and Result
5.1
Transmitter Model
IEEE 802.11g specified standard is used for data transmission. Let Dk , 0 ≤ k ≤ N − 1, be a complex data in frequency domain which we get after mapping the original digital data with different modulation schemes like BPSK, QPSK or 16/64 QAM. The time domain signal Dk is transmitted. The first 4 time domain slots are identical and consist of known training symbols. They are used for channel estimation in receiving end. After transmission of training symbols next 20 time slots consist of data. Each time slot comprises 64 sub carriers. 48 of them are used for data and 4 more are used as pilots. Numbering the sub carriers from -26 to +26, the pilot symbols are located at -21, -7, +7, +21 positions. Position 0 is associated with central frequency and filled with zero. Next 11 sub carriers are padded with zeroes to complete 64 sub carriers needed for 64 point IFFT according to 802.11g standard. After IFFT a cyclic prefix is added with the OFDM symbol 81
82
Chapter 5. Simulation and Result
as guard period to compromise the Inter Symbol Interference (ISI). According to 802.11g standard cyclic prefix is equal to one fourth of length of total OFDM symbol. Information signal used in this simulation consists of random symbols of uniform distribution.
Some parameters which are followed in this simulation, according to the IEEE802.11g standard are shown in Table 5.1.
Table 5.1: Different Parameters of the transmitter, according to the IEEE802.11g Standard Parameter
Value
No. of Data Subcarriers
48
No. of FFT Points
64
Pilot Subcarriers
4 (+1)
Modulation Schemes
BPSK, QPSK, 16QAM and 64QAM
Data Rate
6, 9, 12, 18, 24, 36, 48 and 54 Mbps
Coding Rate
1/2, 2/3 and 3/4
Channel Spacing
20 MHz
Occupied Bandwidth
16.6 MHz
Subcarrier Spacing
312.5 kHz
Sample Rate
20 MHz
Sample Spacing
0.5 µs
Symbol Duration
3.2 µs
Guard Interval
0.8 µs
Symbol Duration with Cyclic Prefix 4 µs
5.2. Channel Model
83
Table 5.2: Coding Rate and Different Modulation Schemes for 802.11g PHY Layer with Allowed Constellation error Modulation Coding Data Rate
5.2
Relative Constellation
Schemes
Rate
Nominal
Error (dB)
BPSK
1/2
6 Mbps
-5
BPSK
3/4
9 Mbps
-8
QPSK
1/2
12 Mbps
-10
QPSK
3/4
18 Mbps
-13
16QAM
1/2
24 Mbps
-16
16QAM
3/4
36 Mbps
-19
64QAM
2/3
48 Mbps
-22
64QAM
1/2
54 Mbps
-25
Channel Model
We used vector channel model which uses modified Jakes model [? ]. The parameters of the channel are listed in 5.3. Parameter Name
Parameter Values
Maximum Doppler Spread
10 Hz
Angle Spread of Sub paths
50
Normalized Antenna Separation
0.5
Random Phase of each Sub Path
0 ∼ 3600
Angle of Arrival of each Sub Path
0 ∼ 3600
Table 5.3: Vector Channel Model Parameters
84
5.3
Chapter 5. Simulation and Result
Receiver Model
Receiver Model consists of one or multiple receiving antenna situated in an antenna array. The array used in this simulation was Uniform Linear Array (ULA). The number of antenna was varied from 20 to 25 in the simulation. Each antenna has its own set of receiver blocks, for removing cyclic prefix, extracting pilot and training symbols, performing 64 Point FFT operation, undertaking serial to parallel conversion and demapping the received equalized signal into information signal. In ?? the receiver model is shown.
Figure 5.1: Transmitter and Receiver Model
5.4
Weight Calculation
There are many adaptive weight estimation techniques.
In this paper Mini-
mum Mean Squared Error (MMSE) is used. The Minimum Mean squared Error (MMSE) norm expects to find weight vector which will minimize the Mean Squared Error (MSE) between the reference signal, in this case known training
5.5. Linear Combination of Weighted Signal
85
symbol defined by the standard IEEE 802.11g and the signal received by the antenna array. Different adaptive algorithms may be used for estimation of weight vector according to MMSE criterion. Here Least Mean Square (LMS) technique is used. According to LMS method the updated value of the weight vector at time n + 1 is computed by using the simple recursive relation w(k + 1) = w(k) + µX k e∗k Where ek is error signal and is step size. µ is calculated according to the following equation ek = Dk − wH X k Where Dk is known training symbol. H denotes to Hermitian transpose. We can see that the LMS has a computational complexity of O(2N ). This low computational complexity is the most attractive feature of LMS algorithm. In this paper 0.001 is used as step size .
5.5
Linear Combination of Weighted Signal
If the received signal vector is X k and weight vector is wmmse then the combined signal Rk can be expressed as Rk = w H X k Then received combined signal Rk is demodulated to get the binary data.
86
5.6
5.6.1
Chapter 5. Simulation and Result
Simulation Results
Performance Comparison by varying Receiving Antenna
By changing the number of receiving antenna we can get diversity gains. From 5.3 and 5.2 it is clearly visible that there is almost 3 dB gain in performance which was achieved by making the number of antenna twice. In 5.2 simple AWGN channel is used while in 5.3 results from Jakes channel is shown. So With increase of receiving antenna the performance of 802.11g based system will improve quite clearly.
Figure 5.2: Performance comparison for different receiving antenna in AWGN channel
5.6. Simulation Results
87
Figure 5.3: Performance comparison for different receiving antenna in vector channel
5.6.2
Performance Comparison by varying the number of receiver antenna with 64 and 16 QAM
As the number of element in receiver antenna array is increased, better performance was obtained considering the Bit Error Rate. Here we have simulated both 16 QAM and 64 QAM systems. From 5.4 it can be seen that for both 16 QAM and 64 QAM the gain is achieved in performance while using multiple antenna.
5.6.3
Some Test Cases
The transmitted signal is shown here. Then received signals were shown and then the weighted outputs from different antenna’s are shown.
88
Chapter 5. Simulation and Result
Figure 5.4: Performance for different receive antennas In 16 QAM and 64 QAM
Figure 5.5: Transmitting signal for 64 QAM
5.6. Simulation Results
Figure 5.6: Signals in different receive antennas for 2 antenna
Figure 5.7: Signals in different receive antennas for 4 antenna
89
90
Chapter 5. Simulation and Result
Figure 5.8: Signals in different receive antennas for 8 antenna
Figure 5.9: Weighted output signal for single receive antenna
5.6. Simulation Results
Figure 5.10: Weighted output signal for 2 receive antenna
Figure 5.11: Weighted output signal for 4 receive antenna
91
92
Chapter 5. Simulation and Result
Figure 5.12: Weighted output signal for 8 receive antenna
Figure 5.13: Weighted output signal for 16 receive antenna
5.6. Simulation Results
Figure 5.14: Weighted output signal for 32 receive antenna
93
Chapter 6 Conclusion
6.1
Summary
The objective of this thesis was to develop a system to mitigate effect of noisy environment and channel distortion in OFDM based WLAN systems. IEEE standard for WLAN system IEEE802.11g was simulated in this thesis and SIMO system is observed in this environment. Monte-Carlo simulation in MATLAB shows that the performance improves significantly in SIMO system than SISO system. In chapter 2 basic OFDM systems are discussed. IEEE standard 802.11g was observed and its necessary properties are discussed in chapter 3. It gave a overview on the MAC and PHY layer of 802.11g. In chapter 4 we have discussed about different diversity techniques currently available for wireless system. Both receive and transmit diversities are considered. In chapter 5 we have shown the simulation results. Simulation was done in MATLAB. During simulation all the necessary conditions for 802.11g PHY layer was maintained. Transmitter and Receiver mod94
6.2. Contribution
95
els were discussed. Different Monte-Carlo simulations were shown varying different parameters of system. Overall it was evident that for more antenna better performance can be achieved. This conclusively shows that the receive diversity system is suitable for WLAN 802.11g standard.
6.2
Contribution
In recent years there are an astonishing number of products that uses ISM band it became a necessity to improve performance of WLAN. Our research is a little contribution towards that goal. As 802.11g standard is accepted by the leading industrialists the receive diversity in small devices may be a very good solution to achieve improved performance. With this technology it will be possible to satisfy the consumer market with high data rate demand. In our research it was simulated and shown that receive space diversity performs nicely in 802.11g environment.
6.3
Future Works
The research in this thesis can be extended to the following topics • Designing transmit diversity using Alamouti scheme, CDD and Space Time Block Code. • Multiple receive antennas in receiving end may be used for diversity with combination of smart antenna system or beamforming to mitigate the effect of co channel interference
96
Chapter 6. Conclusion • The research should be done for different propagation scenario or different path models like Rayleigh channel model or Rician channel model. • The SIMO model can be extended to the other OFDM based systems or other OFDMA systems like 802.16e (WinMax). • Other adoptive algorithm like RLS or neural network should be tried to get the best performance. • Impact of channel coding should be observed. • Research can be done to find the effect of MIMO systems. • MAC layer can be simulated for SIMO system. • SIMO system may be implemented in MC-CDMA and OFDM-CDMA systems.
Appendix A Least Mean Square (LMS) Method According to LMS method the updated value of the weight vector at time n + 1 is computed by using the simple recursive relation w(k + 1) = w(k) + µX k e∗k Where Xk is received signal vector,ek is error signal and µ is step size. ek is calculated according to the following equation ek = Dk − wH X k Where Dk is known training symbol. H denotes to hermitian transpose. We can see that the LMS has a computational complexity of 0 (2N ). This low computational complexity is the most attractive feature of LMS algorithm while RLS has a much higher computational complexity than LMS. The response of the LMS algorithm is determined by the step-size, the size of the weight vector and Eigen-value distribution of the received signal covariance matrix. 97
Appendix B Recursive Least-Squares (RLS) Method In RLS method the updated value of the weight vector at time n + 1 is computed by using the recursive relation w(k) = w(k − 1) + qk Dk∗ − wH (n − 1)Xk Where Xk is received signal vector,Dk is known training symbol and qk is the gain vector. H denotes to hermitian transpose. qk is given by −1 γ −1 Rk−1 Xk qk = −1 1 + γ −1 XkH Rk−1 Xk
where weighted factor γ has the value between 0 and 1. The vector R can be calculated by the following equation Rk =
K X
γ k−1 Xi XiH
i=1
An important feature of the RLS algorithm is that the inversion of the covariance matrix is replaced at each step by a simple scalar division. The convergence rate 98
99 of the RLS algorithm is typically an order of magnitude faster than that of the LMS algorithm, provided that signal to noise ratio is high.
Appendix C IQ Diagrams
Figure C.1: IQ Diagram of BPSK (left) and QPSK (right)
100
101
Figure C.2: IQ Diagram of 8-QAM (left) and 16-QAM (right)
Figure C.3: IQ Diagram of 32-QAM (left) and 64-QAM (right)
102
Chapter C. IQ Diagrams
Figure C.4: IQ Diagram of 128-QAM (left) and 256-QAM (right)
Figure C.5: IQ Diagram of 512-QAM (left) and 1024-QAM (right)
Glossary 2G
Second Generation mobile phone system
3G
Third Generation mobile phone system
4G
Fourth Generation mobile phone system
ADSL
Asymmetric Digital Subscriber Line
AoA
Angle of Arrival
AMPS
Advanced Mobile Phone System
AP
Access Point
AWGN
Additive White Gaussian Noise
BER
Bit Error Rate
bps
Bits per second
BPSK
Binary Phase Shift Keying
CCK
Complementary Code Keying
CDF
Cumulative Distribution Function
CDMA
Code Division Multiple Access
CSMA/CA
Carrier Sense Multiple Access with Collision Avoidance
dB
Decibel (ratio in log scale)
dBc
Decibel relative to main signal power
dBm
Decibel relative to 1 milliwatt 103
104
Chapter C. Glossary
DFIR
Diffused Infra-Red
DFT
Discrete Fourier Transform
DoA
Direction of Arrival
DSSS
Direct Sequence Spread Spectrum
EBNR
Energy per Bit to Noise Ratio
FCC
Federal Communications Commission
FDD
Frequency Division Duplexing
FDM
Frequency Division Multiplexing
FEC
Forward Error Correction
FFT
Fast Fourier Transform
FHSS
Frequency Hopping Speard Spectrum
Fs
Sample Frequency
GHz
Gigahertz - 109 Hz
GSM
Global System for Mobile communications
HDTV
High Definition Television
HiperLAN2
High Performance Radio Local Area Network
Hz
Hertz (cycles per second)
ICI
Inter-Carrier Interference
IEEE
Institute of Electrical and Electronics Engineers
IEEE802.11
WLAN standard set by U.S.
IF
Intermediate Frequency
IFFT
Inverse Fast Fourier Transform
IMD
Inter-Modulation Distortion
IP
Internet Protocol
IQ
Inphase Quadrature
105 IR
Infra-RED
IS-95
Mobile phone standard using CDMA transmission method.
ISI
Inter-Symbol Interference
ISI
Inter Symbol Interference
ISM
Industrial Scientific Medical
kbps
Kilo bits per second (103 bps)
kHz
Kilohertz - 103 Hz
LOS
Line Of Sight
LAN
Local Area Network
MAC
Media Access Control
Mbps
Mega bits per second (106 bps)
MHz
Megahertz - 106 Hz
MIMO
Multiple Input Multiple Output
ML
Maximum Likelihood
MMSE
Minimum Mean Square Error
MPDU
MAC Protocol Data Unit
MSE
Mean Square Error
MSINR
Maximum Signal to Interference and Noise Ratio
MSNR
Maximum Signal to Noise Ratio
OFDM
Orthogonal Frequency Division Multiplexing
NMT
Nordic Mobile Telephony
PAN
Personal Area Network
PC
Personal Computer
PCF
Point Coordination Function
PCS
Personal Communication System
106
Chapter C. Glossary
PDA
Personal Digital Assistant
PHY
Physical Layer
π
3.14159265....
PLCP
Physical Layer Convergence Protocol
PPDU
Physical Protocol Data Unit
PSK
Phase Shift Keying
QAM
Quadrature Amplitude Modulation
QoS
Quality of Service
QPSK
Quadrature Phase Shift Keying
RC
Raised Cosine (Guard Period)
RF
Radio Frequency
RMS
Root Mean Squared
RTS/CTS
Request to Send/Clear to Send
SINR
Signal to Interference and Noise Ratio
SIR
Signal to Interference Ratio
SMI
Sample Matrix Inversion
SMS
Short Messaging Service
SNR
Signal to Noise Ratio
TDM
Time Division Multiplexing
TDMA
Time Division Multiple Access
U-NII
Unlicensed national information infrastructure
ULA
Uniform Linear Array
UMTS
Universal Mobile Telecommunications System
W
Watt (energy per unit time, one joule per second)
W-CDMA
Wide-band Code Division Multiple Access
107 WLAN
Wireless Local Area Network
WLL
Wireless Local Loop
108
Chapter C. Glossary
Bibliography [1] T. Abe and T. Matsumoto. Space-time turbo equalization in frequencyselective mimo channels. IEEE Transactions on Vehicular Technology, 52:469475, May 2003. [2] F. Alam. Space Time Processing for Third Generation CDMA Systems. PhD thesis, Virginia Polytechnic Institute and State University, November 2002. [3] S. M. Alamouti. A simple transmitter diversity scheme for wireless communications. IEEE J. Select. Areas Commun., 16:14511458, Oct. 1998. [4] Weinstein S. B. and P. M. Ebert. Data transmission by frequency division multiplexing using the discrete fourier transform. IEEE Trans. Comm., 19:628–634, October 1971. [5] Broadcom. The New mainstream Wireless LAN standards, 2003. [6] H. Blcskei and A. J. Paulraj. Spacefrequency coded broadband ofdm systems. Proc. IEEE WCNC00, page 16, Sept. 2000. [7] IEEE computer society. Wireless lan medium access control (mac) and physical layer (phy) specifications. Technical report, IEEE, June 1997. [8] IEEE computer society. Wireless mac and phy specifications: High speed physical layer extension in the 2.4 ghz band. Technical report, IEEE, September 1999. [9] IEEE computer society. Wireless medium access control (mac) and physical layer (phy) specifications: High speed physical layer in the 5 ghz band. Technical report, IEEE, September 1999.
BIBLIOGRAPHY
109
[10] IEEE computer society. Wireless medium access control (mac) and physical layer (phy) specifications: Amendment 4: Further higher data rate extension in the 2.4 ghz band. Technical report, IEEE, June 2003. [11] International Engineering Consortium. Ofdm for mobile data communications. www.iec.org/online/tutorials/ofdm/topic04.html, 2004. [12] A. Huebner, M. Bossert, F. Schuehlein, H. Haas, and E. Costa. On cyclic delay diversity in ofdm based transmission schemes. 7th Internat,SFCional OFDM-Workshop (InOWo), Hamburg, Germany, 2002. [13] Jaekwon Kim, Robert W. Heath Jr., and Edward J. Powers. A decision directed receiver for alamouti coded ofdm systems. IEICE Trans. on Commun., e86-b(10):3141–3143, 2003. [14] C. Komninakis, C. Fragouli, A. H. Sayed, and R. D. Wesel. Multi-input multioutput fading channel tracking and equalization using kalman estimation. IEEE Transactions on Signal Processing, 50:10651076, May 2002. [15] Jr. L. J. Cimini. Analysis and simulation of a digital mobile channel using orthogonal frequency division multiplexing. IEEE Trans. Commun. Technol., COM-33:665675, July 1985. [16] E. Lawrey. Multiuser ofdm paper. Signal Processing Research Centre, Fifth International Symposium on Signal Processing and its Applications, August 1999. [17] Eric Phillip Lawrey. Adaptive Techniques for Multiuser OFDM. PhD thesis, James Cook University, December 2001.
110
Chapter C. Glossary
[18] J. Litva and T. K. Lo. Digital Beamforming in Wireless Communications. Boston, MA: Artech House, 1996. [19] M. Loncar, R. R. Muller, J. Wehinger, C. F. Mecklenbrauker, and T. Abe. Iterative channel estimation and data detection in frequency-selective fading mimo channels. European Transactions on Telecommunications, 15, July 2004. [20] Kaveh pahlavan. Principles of Wireless Networks. Prentice hall of India, 2003. [21] John G. Proakis. Digital Communications. McGraw-Hill International, Inc., fourth edition edition, 2000. [22] G. V. Rangaraj, D. Jalihal, and K. Giridhar. Introducing space sampling for ofdm systems with multipath diversity. Proc. IEEE Symposium on Signal Processing and Information Technology-2003, Darmastadt, Germany, pages 427–430, Dec. 2003. [23] Z. Rong. Simulation of adaptive array algorithms for cdma systems. Master’s thesis, Virginia Tech, Sept. 1996. [24] A. X.
Salvekar, Qian.
S.
Sandhu,
Multiple-antenna
Q.
Li,
technology
M-H. in
Vuong, wimax
and
systems.
http://developer.intel.com/technology/itj/2004/volume08issue03/ art05_multiantenna/p01_abstract.htm (August 2004). [25] D. Schafhuber, G. Matz, and F. Hlawatsch. kalman tracking of timevarying channels in wireless mimo ofdm systems. Proceedings of Asilomar Conf. Signals, Syst. Comput., Pacific Grove, CA, USA, Nov. 2003.
BIBLIOGRAPHY
111
[26] W. L. Stutzman and G.A. Thiele. Antenna Theory and Design. John Wiley & Sons, New York, 1981. [27] Y. Sun, M. Yee, and M. Sandell. Iterative channel estimation with mimo mmse-turbo equalization. Proceedings of IEEE Semiannual Vehicular Technology Conference, Orlando, FL, USA, Oct. 2003. [28] Andrew S. Tanenbaum. Computer Networks. Prentice hall of India, fourth edition edition, 2002. [29] V. Tarokh, H. Jafarkhani, and A. R. Calderbank.
Space-time block
codes from orthogonal designs. IEEE Transactions on Information Theory, 45(5):14561467, July 1999. [30] Richard van Nee and Ramjee Prasad. OFDM for Wireless Multimedia Communications. Artech House, Inc., 2000. [31] K. Witrisal, Y.H. Kim, R. Prasad, and L.P. Ligthart. Antenna diversity for ofdm using cyclic delays. proc. IEEE 8th Symposium on Communications and Vehicular Technology in the Benelux, SCVT- 2001,, October 2001.
