Seminar on Topics in Communications Engineering Master of Science in Communication Engineering Munich University of Technology

Seminar Topic :

The IEEE 802.16 WiMAX Broadband Wireless Access; Physical Layer (PHY), Medium Access Control Layer (MAC),Radio Resource Management (RRM)

Author Advisors Date

Jakub Wolnicki Maran Kumar Pereirasamy Dr.Ing.Christian Hartmann 2005-01-14

Institute for Communcation Engineering (LNT) Institue for Communication Networks (LKN)

Introduction [1] The term WiMAX (Worldwide Interoperability for Microwave Access) has become synonymous with the IEEE 802.16 Wireless Metropolitan Area Network (MAN) air interface standard. In its original release the 802.16 standard addressed applications in licensed bands in the 10 to 66 GHz frequency range. Subsequent amendments have extended the 802.16 air interface standard to cover non-line of sight (NLOS) applications in licensed and unlicensed bands from 2 to 11 GHz bands. Filling the gap between Wireless LANs and wide area networks. WiMAX-compliant systems will provide a cost-effective fixed wireless alternative to conventional wire-line DSL and cable in areas where those technologies are readily available. The ongoing evolution of IEEE 802.16 will expand the standard to address mobile applications thus enabling broadband access directly to WiMAX-enabled portable devices ranging from smartphones and PDAs to notebook and laptop computers. WiMAX technology in fixed wireless applications in the sub 11 GHz frequency range. The latest 802.16e amendment is supporting for mobility in WiMAX system. WiMAX Architecture and Applications[1] A wireless MAN based on the WiMAX air interface standard is configured in much the same way as a traditional cellular network with strategically located base stations using a point-to-multipoint architecture to deliver services over a radius up to several kilometers depending on frequency, transmit power and receiver sensitivity. In areas with high population densities the range will generally be capacity limited rather than range limited due to limitation in the amount of available spectrum. The base stations are typically backhauled to the core network by means of fiber or point-to-point microwave links to available fiber nodes or via leased lines from an incumbent wire-line operator. The range and NLOS capability make the system very attractive for users and providers.The three frequency bands that are of primary interest with today’s prevailing regulations.The license-exempt 5.8 GHz (known as Universal National Information Infrastructure (UNII)Band in the U.S. The licensed 2.5 GHz - known as Multipoint Distribution Service - MDS Band, also known as Broadband Radio Service (BRS) in U.S. The licensed 3.5 GHz Band OFDM vs. OFDMA [2] IEEE 802.16 will specify two flavors of OFDM systems: one simply identified as OFDM, the other OFDMA. Orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique that has been recently recognized as an excellent method for high speed bi-directional wireless data communication. Its history dates back to the 1960s, but it has recently become popular because economical integrated circuits that can perform the high speed digital operations necessary have become available. OFDM effectively squeezes multiple modulated carriers tightly together, reducing the required bandwidth but keeping the modulated signals orthogonal so they do not interfere with each other. Today, the technology is used in such systems as asymmetric digital subscriber line (ADSL) as well as wireless systems Figure 1: FDM with Nine Sub-carriers Using Filters[2] such as IEEE 802.11a/g (64 subcarriers) and IEEE 802.16 (WiMAX).

It is also used for wireless digital audio and video broadcasting. It is based on frequency division multiplexing (FDM), which is a technology that uses multiple frequencies to simultaneously transmit multiple signals in parallel. Each signal has its own frequency range (subcarrier) which is then modulated by data. Each sub-carrier is separated by a guard band to ensure that they do not overlap. These subcarriers are then demodulated at the receiver by using filters to separate the bands. OFDM is similar to FDM but much more spectrally efficient by spacing the sub-channels much closer together (until they are actually overlapping). This is done by finding frequencies that are orthogonal, which means that they are perpendicular in a mathematical sense, allowing the spectrum of each subchannel to overlap another without interfering with it. In Figure 2, the effect of this is seen as the required bandwidth is greatly reduced by removing guard bands and allowing signals to overlap. In order to demodulate the signal, a discrete Fourier transform (DFT) is needed. Fast Fourier transform (FFT) chips are commercially available, making Figure 2: OFDM with Nine Sub-carriers[2] this a relatively easy operation.

In OFDM we have 256 sub-carriers with 192 data sub-carriers, 8 pilot sub-carriers and 56 nulls. In its most basic form, each data sub-carrier could be on or off to indicate a one or zero bit of information. However, either phase shift keying (PSK) or quadrature amplitude modulation (QAM) is typically employed to increase the data throughput. So in this case, a data stream would be split into n (192) parallel data streams, each at 1/n (1/192) of the original rate. Each stream is then mapped to the individual data sub-carrier and modulated using either PSK or QAM. Pilot subcarriers provide a reference to minimize frequency and phase shifts during the transmission while null carriers allow for guard bands and the DC carrier (center frequency).All subcarriers are sent at the same time

Orthogonal frequency division multiple access (OFDMA) allows some sub-carriers to be assigned to different users. For example, sub-carriers 1, 3 and 7 can be assigned to user 1 and sub-carriers 2, 5 and 9 to user 2. These groups of sub-carriers are known as sub-channels. Scalable OFDMA allows smaller FFT sizes to improve performance (efficiency) for lower bandwidth channels. This applies to IEEE 802.162004 which can now reduce the FFT size from 4096 to 128 to handle channel bandwidths ranging from 1.25–20 MHz. This allows sub-carrier spacing to remain constant independent of bandwidth which reduces complexity while also allowing larger FFT for increased performance with wide channels. As shown in [1], in order to mitigate the frequency selective fading, the carriers of one subchannel are spread along the channel spectrum. Figure 4 depicts the principles of division into subchannels. The usable carrier space is divided into a number of NG successive groups. Each group contains a number of NE successive carriers, after excluding the initially assigned pilots. A subchannel has one element from each group allocated through a pseudorandom processbased on permutations, so NG is the number of subchannel elements. For N = 2048, downstream NG = 48 and NE = 32, while upstream NG = 53 and NE = 32.

Figure 4 Subchannels in OFDMA[1].

In essence the principle of OFDMA consists of different users sharing the upstream FFT space, while each transmits one or more sub-channels. The division in sub-channels is a form of frequency-division multiple access (FDMA), where the subscriber transmits 1/NE = 1/32 of the available channel bandwidth for the 2048-carrier OFDMA. A low upstream data rate is consistent with the traffic asymmetry where the streams from each subscriber add up in a multi-point-to-point regime, while downstream all the subchannels are transmitted together. One of interesting aspects taken by Mr. Koffman in [1] is that upstream subchannels related to coverage. A BWA system involves a high-power transmitter in the head-end and a multitude of low-cost lowtransmission- power BWSUs. For the OFDMA option of N = 2048, the BWSU concentrates its power into a subchannel that has 1/32 of the channel bandwidth. For equivalent modulation and coding, this results in 15 dB premium for the upstream link budget against the downstream. For a 6 MHz channel, one subchannel has an equivalent bandwidth of 187 kHz. But this low-bandwidth signal does not undergo flat fading since its 53 carriers are spread across the entire channel bandwidth. Regarding interference, the subchannels constitute a form of frequency hopping spread spectrum (FHSS). In every group a BWSU transmits one pseudo-randomly selected carrier out of NE possible ones. A BWSU in an interfering cell does the same type of selection, but statistically independent. The probability of collision is 1/NE. This is a classic scenario of FHSS with partial band jammer [9]. The hopping scenario repeats for every group in an FFT symbol. For N = 2048 there are NG = 53 such groups. The data from carriers with low SNR is corrected through interleaving and coding. The parameter that characterizes the degree of spreading in a spread-spectrum system is the processing gain. Next great advantage of OFDM and OFDMA modulation is tolerance to multipath propagation and selective fading. It can overcome its negative influence utilizing parallel, slower bandwidth nature. This has made it not only ideal for such new technologies like WiMAX,but also currently one of the prime technologies being considered for use in future fourth generation (4G) networks. PHY Layer Physical layer was defined for a wide range of frequency from 2 up to 66 GHz. In sub-range 10-66 GHz system the is an asumption of Line-Of-Sight propagation . In this scheme single carrier modulation was chosen, because of low complexity of system. Downlink channel is shared among users with TDM signals. Subscriber unit are being allocated individual time slots. Access in uplink is being realized with TDMA. Channel bandwidths are 20 or 25 MHz in USA and 28MHz (Europe). Duplex can be realized with either TDD or FDD scheme. In the 2-11 GHz bands communication can be achieved for licensed and non-licensed bands. The communication is also available in NLOS conditions. The 802.16a Draft3 air interface specification describes three formats :

-

Single Carrier modulation (SC) OFDM with 256 point transform OFDMA with 2048 point transform

The Forward Error Correction (FEC) is used with Reed-Salomon Codes GF(256). It is also paried inner block convolutional code to robustly transmit critical data, like Frame Control or Initial Access. Grant-requesy mechanism[3] One aspect of WiMAX QoS provisioning is a grant-request mechanism for letting users into the network. This mechanism’s operation and value become apparent from a comparison of WiMAX with the CSMA/CD or CSMA/CA mechanisms used in LAN technologies such as 802.11. When a CSMA/CAbased wireless LAN has fewer than 10 users per access point, the network experiences little contention for use of airtime. Occasional packet collisions occur, and they require back-off and retransmissions, but the resulting overhead does not waste a significant amount of bandwidth. If the number of CSMA/CA accesspoint users goes up to dozens or hundreds of users, many more users tend to collide, back-off and retransmit data. In such an environment, average network loading factors can easily rise past 20 to 30 percent, and users notice delays—especially in streaming-media services. WiMAX avoids such issues by using a grant-request mechanism that allocates a small portion of each transmitted frame as a contention slot. With this contention slot, a subscriber station can enter the network by asking the base station to allocate an uplink (UL) slot. The base station evaluates the subscriber station’s request in the context of the subscriber’s service-level agreement and allocates a slot in which the subscriber station can transmit (send UL packets). MAC Layer The 802.16 MAC protocol was designed for point-multi-point broadband wireless access. Access and bandwidth allocation algorithms accomodate hundreds of user terminals per single channel. User terminals may also be shared among many end-user equipment like phones or PC’s. To support variety of services, 802.16 needs to accomodate bursty and continous traffic, with reqiured QoS of every service. On downlink data is multiplexed with TDM. Uplink is shared with TDMA. 802.16 is connection-oriented. All services, even those inherently connectionless, are mapped to co a connection. It provides mechanism forrequired bandwidth, associating Grade of Service(GoS) and traffic parameters, transporting and routing data to the appropriate sublayer. Connections are refered with 16-bit Connection Identifiiers (CID) and may require continously bandwidth or band-on-demand. Upon entering the network, three management connections are established, in both directions. Every connection is used for different QoS connection type: - basic connection – used to transfer of short, time critical MAC and RLC messages - secondary management Connection – used for transfer of standard-based protocols such as DHCP,TFTP,SNMP - Other types of connection, like connection reserved for braodcasting Adaptive modulation Adaptive modulation allows WiMAX system to adjust channel modulation scheme, according to SNR ratio in radio link. If good SNR is achieved, system can switch to the highest throughtput modulation (64QAM). If fadings accur system can shift to otherlow-throughput modulation, but still not dropping connection. Figure 5. Adaptive modulation radius

Power Control Power control mechanisms are used to improve overall system work. PC is implemented at Base Station. BS sends the steering signal to subscriber station(SS) to achieve, pre-determined signal level at BS. It is required also to reduce interferences with neighbouring cells. Mobility support [4] Mobility support is the feature which is defined in 802.16e amendment for MAC layer. The basic handover type is an inter-sector handover for multisector AP. Amendment describes flags , that represent level of handover information context, that is shared. After [4], „Optimization flags consequently enable modeling of all possible handover scenarios from the most basic nomadic access scenario (where no network entry context is shared between APs across a handover) to scenarios involving inter-subnet, inter-frequency assignment, Idle mode, and interphysical AP handovers”. The goal is to achieve more demanding features like, Soft handover (with PHY layer macro diversity) and Fast Base Station Switchin, Figure 6. Mobility scenarois for 802.16e currently being defined to support zero packet loss, low latency inter-sector handover. Under term of portability, we understand fast intra-RAN switching with potential data loss during handovers and even more latency and data loss during inter-subnet handovers. Full mobility requires zero-low packet loss and low latency handovers that are acceptable to real-time applications such as VoIP. Mobility management refers to micro-mobility and macro-mobility scenario. Within the RAN, Intel recommends [5] the use of Multiprotocol Label Switching (MPLS). For macromobility they recommend the use of SIP mobility for real-time low-latency interactive applications such as VoIP, and Mobile IP. Harmonization and Interworking with Public Wi-Fi and 3G Networks The 802.16 networks are designed either as a data overlay network or as a standalone broadband access network. In terms of mobility, system is designed to co-operate with other public networks. The document [4]describes Intel’s proposed interworking framework for public Wi-Fi hotspots. They recommend adopting and extending the same principles for inter-operator 802.16 interworking, supporting the following goals: - Support reuse of credentials and cryptographically strong bilateral authentication and session key management across these networks - A provisioning and access framework for advanced IP services that is compatible with the architecture for Wi-Fi hotspots - Enable offering of multiple IP services with attributes such as provisioned bandwidths, SLAs, QoS, and variable tariff profile

The all-IP architecture framework for Wi-Fi hotspots and 802.16 permit both loosely and tightly coupled harmonization scenarios. WiMAX Forum is excpecting the system to interoperate with other systems in 2006.

Figure 7. Interworking scenarios for 802.16e systems [4]

Future Spectrum for WiMAX – More Room and Service Options Additional bands are being considered today by different regions around the world for the deployment of WiMAX and other similar broadband wireless access services. In Japan the 4.9GHz – 5.0GHz band will be used after 2007 while the 5.47GHz – 5.725GHz band is also being considered for future use. The first one will require a license for BS deployment and will support 5MHz, 10MHz and 20MHz bandwidths, while the second one will possibly not require a license and would support 20MHz bandwidths. The North American market is indicating some interest in deploying WiMAX in the 4.9GHz broad-spectrum publicsafety band. There is even some interest in using lower frequency bands such as the licensed 800MHz and he unlicensed 915MHz ISM bands for WiMAX and similar types of services and deployments. The WiMAX standard is set to bring the long-awaited spectral efficiency and throughput to meet users’ needs for combined mobility, voice services and high data rates. It will enable access for more users due to its non-line-of-sight capability, lower deployment costs, wide range capability and penetration into the mass consumer market with lower CPE costs as a result of standardization and interoperability. Conclusions The IEEE 802.16 WiMAX is a reliable broadband metropolitian wireless technology. Designed to provide DSL, cable or T1 connectivity, allowing many types of services with the respect to their QoS, which will revolutionize our thinking in connection end-user to broadband access system. WiMAX System is deployed to provide service for hundreds of users. Emerging countries, with low developed wireline infrastructure, are well suited for a quick and cost-effective deployment of broadband access. The system can work in effective range up to 50 kilometers (802.16), without laying cables or fibers. The 802.16e standards mobility feature, will allow users hardware (notebooks, PDA’s), to access highspeed internet, while roaming outside of the WiFi hotspots. The IEEE 802.16 WiMAX system will be deployed in three stages. First stage,by the first half of 2005, will bring fixed wireless access with outdoor antennas to end-user, to avoid laying high-cost cables. Second part, by the end of 2005, involves fixed wireless access using indoor antennas for notebooks, PDA’s. In phase 3, new portable products will be introduced, which will allow to access to broadband networks while roaming between wireline and wireless network. This system has a geat chance for a marketing success, because of quick deployment time and among of services, that can be applied. References : [1] – „Broadband wireless access solutions based on OFDM Access in 802.16”, Isreali Koffman, Runcom Technologies [2] - www.intel.com/netcomms/technologies/wimax/index.htm, Web Seminar Papers [3] - http://www.fujitsu.com/us/news/pr/fma_20050105.html [4] - http://developer.intel.com/technology/itj/2004/volume08issue03/art01_globalwirelessnet/ /p08_interop_interface.htm [5] - http://www.intel.com/technology/IWS/WLAN_study.pdf [6] - RF Spectrum utilization in WiMAX, White Papers,Fujitsu Microelectronics America, Inc.November 2004

IEEE 802.16: WiMAX Broadband Wireless Access: Physical Layer, MAC, and RRM Advisors

Author

Maran Kumar Pereirasamy Dr Christian Hartmann

Jakub Wolnicki

Agenda • • • • • •

Introduction to 802.16 systems OFDM and OFDMA MAC Layer PHY Layer RRM Mobility support and Interworking with WiFi/3G Systems • Conclusions The Seminar on Topics in Communications Engineering

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Global Wireless Standards IEEE 802.16e -

WAN

IEEE 802.20 WirelessMAN - still working...

3GPP,EDGE,GSM

MAN

IEEE 802.16 WirelessMAN

ETSI HiperMAN HIPERACCESS

LAN

IEEE 802.11 WierelessLAN IEEE 802.15 - Bluetooth

ETSI HiperLAN

PAN

ETSI HiperPAN

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802.16 Introduction FRACTIONAL T1 for SMALL BUSINESS BACKHAUL for HOTSPOTS

T1+ LEVEL SERVICE ENTERPRISE

RESIDENTIAL & SoHo DSL Sevice

BACKHAUL

ALWAYS BEST CONNECTED

802.16 Multi-Point BACKHAUL

802.11 802.11 802.11

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Wireless MAN Evolution

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What is WiMAX ? • WiMAX - Worldwide Interoperability for Microwave Access • WiMAX is Worldwide Forum , involving world-wide leading IT manufacturers • Formed to promote and certify compatibility and interoperability of broadband wireless products based on IEEE 802.16 standard The Seminar on Topics in Communications Engineering

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IEEE 802.16 Bands

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Additional UHF Licensed Band

802.11vs.802.16 Spectrum

UNII

ISM

1

International Licensed

2

ISM

US Licensed

3

International Japan Licensed Licensed

4

ISM

5

GHz

802.16a has both licensed and license-exempt options ISM: Industrial, Scientific & Medical Band – Unlicensed band UNII: Unlicensed National Information Infrastructure band – Unlicensed band

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SC vs. OFDM

LOS

NLOS

10-66GHz

2-11GHz

Single carrier

Multiple subcarriers Involves InversedFFT to split the information in subcarrier

SC vs. OFDM Channel attenuance

OFDM vs. OFDMA • 802.16 specifies two types of OFDM – One of them is simply called OFDM • Uses 256 point FFT • All subcarriers transmitted at once • Downlink (TDM), Uplink (TDMA)

– Second one is called OFDMA • Uses 2048 or 4096 point FFT, devided to subchannels • In every channel different modulation types can be used (QPSK ,16 or 64 QAM) • Assignment to subchannels in Downlink by Media Access Protocol (MAP) The Seminar on Topics in Communications Engineering

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OFDMA subchannels

For N = 2048 :

Downstream : Ng = 48 , Ne = 32 Upstream : Ng = 53 , Ne = 32 The Seminar on Topics in Communications Engineering

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Duplex in UL/DL for 802.16a • Time Division Duplex(TDD) – SS doesn’t receive/transmit simultaneously (lower cost)

– DL&UL use the same RF

• Freq Division Duplex (FDD) – simultaneous Tx/Rx work – DL&UL separae RF channels – SSs support half duplex

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TDD Frame Structure

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802.16a MAC • Access to the shared 802.16a medium, preventing simultaneous transmissions from separate SS • Consists of three sub-layers: – The MAC Common Part Sublayer (MAC-CPS), provide basic functions : Scheduling, Bandwidth Request & Allocation, Ranging, Coonection Control – Service Specific Protocol Layer (MAC-SSPL): interface with higher layer protocols (IP v4, IP v6, ATM) – Privacy Sublayer (MAC-PS): authentification,data encryption

MAC 802.16a

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QoS support for 802.16a • UNSOLICITED GRAT SERVICES (UGS) : designed to support constant bit rate (CBR), such as T1/E1 link,or delay-jitter dependend services like VoIP, • REAL-TIME POLLING SERVICES (rtPS) :to support variable data packets on periodic basis, like the MPEG video • NON-REAL-TIME PS (nrtPS) : to support variable grant burst profiles : FTP • BEST EFFORT (BE) : access to web surfing

Delay Spread • One of the parameters that characterizes the multipath signals is delay spread (expressed in rms) • While tranismittiong in NLOS environment signal scattering plays a big role in PHY Layer • The more complexed is the terrain, the higher the rms value is (measured in usec) - Stanford University Interim Modelling • To avoid Inter Signal Interference (ISI) physical layer needs to accomodate values of rms delay spread

Guard Interval • 802.16a PHY, designed to accomodate rms spread delay, depends on deploy scenarios, including : • Cell radius • Scattering Environment

• Tg can be configured to a fraction of Tb – (Tg/Tb : 1/32 , 1/16, 1/8 , 1/4)

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OFDMA Link Adaptation

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802.16e Combined Fixed and Mobile Operation in Lincesed Bands (under development)

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Mobility evolution - 802.16e

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Handoff • The 802.16e defines primitive MAC messages to support network or MSS handovers • The HO scenarios involve simple inter-sector AP HO,two different AP HO • HO can be deployed with information drop/ connection loss (Nomadic connectivity), • More demanding scenarios are considered (Soft HO, Fast BS Switching) to support no packet losses → e.g connection VoIP while HO The Seminar on Topics in Communications Engineering

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Mobility 802.16e • Interoperability, functionality, handoff possibilities with other wireline and wireless systems 3G/802.xx

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Conclusions • The IEEE 802.16a standard provides wireless alternative for cable and DSL systems • OFDM and OFDMA modulation schemes allow 802.16a/d/e systems to work in NLOS environment • Scalability and amonut of provided services, with respect to their QoS demands, guarantees its economical success • Mobility features amended in 802.16e are still not specified and not fully standardized

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Any questions ?? :)

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