Reconfigurable Radio Using Software Technologies Santosh Shah, S L Maskara & V Sinha The LNM Institute of Information Technology Jaipur-303012 [email protected], [email protected], [email protected] ABSTRACT The role of radio in the telecommunication systems have extended from simple radio telephone to mobile communications and beyond. Minimization/replacement of hardware in communication technologies through software has resulted in the birth of new technology, widely known as Software-Defined Radio (SDR). This has come with the challenges to replace the functioning of hardware components by the software on a single chip like DSPs, ASICs, or FPGAs with high speed of operations. The depth of digitization in the radio communication engineering up to the RF front-end from the baseband is stated as an Ideal software radio. In this paper we cover the basic radio communication architecture, SDR architecture, technological challenges and applications. INTRODUCTION Radio has always fascinated human being since its discovery and invention. It provides entertainment and information to people anywhere at low cost. As the digital technologies and the computer systems advanced, emphasis started shifting from digital hardware to software implementation of systems. Digital signal processing (DSP) started playing a very revolutionary role in the design and implementation of many practical systems because of the software. DSP can carry out variety of functions using the same hardware.

Progress

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micro-electronics

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VLSI

technologies

enable

the

miniaturization of the systems and gadgets, however a major limitation of the digital signal processors are the speed of operation. Therefore they were initially used only for the baseband low speed processing. Today the DSPs are available which operate at very high speed thus it is possible to use the DSP for intermediate and radio frequencies.

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Introduction of software into the radio systems has brought the concept of software radio. Software radios have brought a revolution in the radio engineering. It is now possible to define various radio functions using suitable software on the same hardware. Such radios have been referred to as Software-Defined Radio (SDR). It is obvious that the SDRs are programmable and reconfigurable. Programmability / reconfigurability have become necessity of the day, because of the multiple standard and platforms, multiple frequency bands, and variety of applications. Crux of the DSP and, therefore, the SDR is high speed A/D and D/A conversion. A radio system can be partitioned into three stages namely the baseband, the IF, and the RF. With the availability of wideband A/D and D/A converters, it has now become feasible to digitize the RF / IF signals. The baseband signals can anyway be digitized without much difficulty. Thus, at every stage since a radio digital signal is available they can be easily processed using DSPs. Thus the entire radio systems can be now programmed in software resulting in the desired flexibility. The SDRs can provide multifunctionality, global mobility, compactness and power efficiency, ease of manufacture, and ease of upgrades. Key enabling technologies for software radio particularly for handset terminal implementation are signal digitization, silicon capability, signal processing, SIM cards, downloadable software, and personal Java / Java card. Design of SDRs required definition of suitable architecture and proper partitioning of the function in a radio system. Suitable algorithms need to be developed for implementation of various functions. The development of digital techniques in communication systems has resulted in additional performance improvement, because of use of source coding, channel coding, encryption, multiplexing, and multiaccessing techniques. All these techniques can easily be defined and implemented in SDR. Thus the SDR is going to play a key role in the next generation of complex telecommunication systems. This paper describes complexity of radio architecture of SDR, key technologies and some applications. The paper consists of four segments; (i) basic radio communication and complex radio architectures, (ii) Software-Defined Radio (SDR) architecture, (iii) technological challenges, (iv) applications, besides conclusions.

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I. BASIC RADIO COMMUNICATION & COMPLEX RADIO ARCHITETURES A communication system transfers information from one place to other over a channel. Information may include audio, video, data etc. A simplistic way of looking at the system can be represented by blocks shown in fig.1 and is self explanation. At the receiver end processing has to be done to undo the distortion of waveforms caused by the channel and also to eliminate the effect of noise / interference on the received signal. For example, in the figure, the RF downconversion converts the received signal to compatible baseband signal for processing, and then it is decoded to obtain the original transmitted information.

Beginning from the era of 70’s and 80’s the radio systems started migrating from analog domain to digital domain. Source coding was employed to map input information to appropriate optimal digital bitstream. Similarly channel coding was used to provide error protection, etc. Constant evolution of digital domain has enabled early digital microwave [T-carrier] with the data rate of few megabits per second (Mbps) to migrate to modern digital microwave [622 Mbps] with the hundreds of Mbps data rates in the pointto-point communication. Peer-to-peer communication has a more complex architecture than point-to-point communication, and has the variety of evolutions from early analog [Citizen’s band] to military direct sequence Joint Tactical Information Dissemination System [JTIDS]. Similarly multiple hierarchy rather than single hierarchy have more complex architecture than previous two network organizations [1]. Today’s trend is in multiple hierarchy network organization with its evolution towards software based implementation. In a multiple hierarchy application a single radio unit, typically a mobile

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terminal participates in more than one network hierarchy. A software radio terminal, for example, could operate in an AMPS network, a GSM network, an IS-95 network, and a future satellite mobile network. II. SOFTWARE-DEFINED RADIO (SDR) ARCHITETURES The architecture of a SDR for mobile communication system as shown in fig. 2 [2], basically comprises of higher end processing including channel codec and modem, and baseband processing. Higher end processing would include subsystems to perform operations on the signal after the baseband processing while transmitting or before the baseband processing while receiving. The basic objective of SDR in a mobile application will be to replace hardware processing of signals as close to antenna as possible by the appropriate software modules. This end of processing includes programmable RF channel access, intermediate frequency (IF) processing, and modem. RF channel access selects the desired RF band of signals. IF processing may includes filtering, frequency (Up/Down) -conversion (i.e. frequency translation), space / time diversity processing, beamforming, and related functions. Modem has to be principally defined for multiple air interface waveforms to cater to different network configuration, which is generated by multimode radios. And then RF modulation and demodulation takes place. Ciphering and authentication of the information come in to the information security section. They maintain security and reduce the fraud. The role of

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service and network support has to be for creating a connection between the remote source to the radio by Synchronous Digital Hierarchy (SDH), and a Local Area Network (LAN) or other networks. Other sources like data, facsimile, video, and multimedia are coded / decoded in the source coding and decoding section. System stability, error recovery, isochronous streaming of voice and video, and data flow timing is assured by joint control. Complexity of joint control becomes more with the advancement of the radio systems. The layered virtual machine gave a new direction to designing SDR architectures According to the SDR forum engaged in evolving GSM standards [3], the computational efficiency is the trade-off of flexibility versus power dissipation. Various signal processors of a typical system would deliver hundreds of MIPS. On an average a general signal processor of a CDMA chip requires 100 MIPS. Now a new DSP chip can deliver 400 to 1000 MFLOPS. Naturally interfaces among software components have to be streamlined. Baseband standards, controller standards, real-time kernel drivers, standard selectors, and other functional components may be implemented in power-efficient ASICs, FPGAs, DSPs or general purpose processors. III. TECHNOLOGICAL CHALLENGES Software radio has emerged because of the evolution and convergence of several different technologies, tabled in table 1 [4]. They are signal digitization, role of silicon, DSP power, PersonalJava and JavaCard, smart card technology, and software download. Evolution and convergence of signal digitization technology are mainly due to extensive breakthroughs in A/D and D/A capability, improved with dynamic range, accuracy, linearity, sampling and resolution. A/D and D/A capabilities, however, still pose a serious bottleneck in widespread use of SDR. Semiconductor industries are widely going to higher frequency and high speed of operations, but still there exist a trade-off between performances and sampling rate. At high sampling rate the power consumption may become a limitation. Role of silicon in the DSPs is an interesting subject. Today silicon geometries go down 0.35µm to 0.18µm by size, 5v to 3V to 1.8V to 1V by power.

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Moor’s law is playing an amazing role in memory size of DSPs, it is seen that the size of memory becomes double every 18-24 months. Till the year 2000 we have achieved low power consumption DSPs with 200 MIPS, with power consumption at 0.5W. Today 1 GIPS devices at reasonable power consumption and cost are still not feasible. Various DSPs of LMS320xxx series are there for example. The concept of “write ones run anywhere” came only with the development of layered virtual machine and enabling Java programs on only processing platform. New software components and programs can be installed by downloading over the air interface, if Java program is running in a particular platform. As stated above that the silicon geometries continue to shrink, so the

processing and memory capability within the DSP chip increases. Smart card application like, e-cash and pay per-view conditional access to broadcast services as well as cell phone SIM cards are widely growing in the market. The sales, pricing, purchase and distribution of software have been revolutionized by the advent of internet. The internet provides us through the wire or wireless media, easy access try before buy, online upgrades, and upgradeable modems. But downloading through the wireline media (like

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PSTN) is still slow and frustrating. But this limitation has been almost removed by the wireless technology. The evolution of DSP-based architecture for mobile communication systems started from 1996. In 1983 the Texas Instrument’s launched first DSP i.e. TMS32010, 16-bit ALU with 5MIPS, with operating voltage was 5V. This was used in GSM systems. Uses of DSPs in wireless communication systems have given new directions in software radio communication. Even otherwise 3G communication employs higher end DSOs at high MIPS. Here we are summarizing the DSPs manufactured by the Texas Instrument. Series TMS320Cxx are using in wireless communication systems. Since the power is directly proportional to the supply voltage, lowering the power supply level would reduce power dissipation. However lowering the supply voltage decreases performance. Therefore, technology scaling and power supply scaling are combined to improve performance while decreasing the total power consumption of the DSP. Various TI’s DSP of TMS320Cxx series with operating voltage, year of development, and power dissipation in mW/MMACS are shown in table 2 demonstrating the statement [5].

There are some other DSPs in the market that have been designed for wireless application, they are Lucent 1600 series, and ADI 21xx series.

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Some other challenges for the SDR handset implementation are for power management, clock generation & distribution, receiver architecture, handset production, and software computational efficiency. Mobile node challenges include areas like protocols, multimode personalities, wideband modes, co-site interference, and radio interference with itself. IV. APPLICATIONS

Key software radio components and functions can be divided in three major canonical architectures the real-time channel processing stream, the near real-time environment management streams, and associated software tools and are shown in fig. 3. The real-time channel processing stream includes channel coding and decoding. Translation of baseband signal to an intermediate frequency (IF) or vice versa may be performed by multiplying a discrete time-domain baseband or IF waveform by a discrete reference carrier to yield a sampled IF or baseband. Data interface and other network interfaces also come under this processing stream. The radio environment may be characterized by using frequency, time, and space. This comes under the near-real time environment management. Environment management also specifies the channel identification, channel interference level, and subscriber location. Here one would 8

employ the operations like Fast Fourier Transform (FFT), wavelet transform, and matrix multiplication. Some on-line and off-liner system analysis, signal processing, and rehosting tools, are mapped with the VHDL [1]. A basic functional block diagram of a GSM phone is shown in fig. 4. The assumption was made in the early days of GSM phone development that the low power requirement could be fulfilled by only with implementation in ASIC. At that time DSPs were used only for vocoding. As the technologies evolved, ASIC area increased due to additional implementation of some other functions especially at application layer. Therefore the power requirement and cost increased. After 1994 many industries like Motorola and Texas Instrument developed a DSP, which was powerful enough to do all the DSP function on a single chip. Finally DSP took the empire over ASIC and hence application and layer 1 functions started being implemented in DSP. Now DSP can take care of part of layer 2 & 3 and man made machine interface, interfaces the terminal adapters (i.e keyboard, display, SIM card etc.) and also other hardware like RAM, ROM, and EPROM etc.

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V. CONCLUSSIONS Reconfigurability [6] as promised by software radio is in its infancy. The impacts of development come from the desire of people wanting to roam under different air interface standards with a single handheld terminal. This can only be provided by implementing communication functions in software. Software radio can provide flexibility and intensive personalized subscriber services by on-the-air download or fly. Operators could customize the handset interface, and also maximize handset loyalty and sell added-value services to its subscriber base. The third generation systems like Universal Mobile Telecommunication Services (UMTS) / International Mobile Telecommunications (IMT-2000) technologies will permit software downloadable reconfiguration on the terminal radio interface.

References: [1] Joe Mitola, “The Software Radio Architecture” IEEE Communication Magazine, pp. 26-38, May 1995. [2]. Joseph Mitola, III, “Software Radio Architecture: A Mathematical Perspective” IEEE Journals on Selected Area in Communications, Vol. 17, No. 4, pp. 514-538, April 1999. [3] Joseph Mitola III, “Technical Challenges in the Globalization of Software Radio” IEEE Communication Magazine, pp. 84-89, February 1999. [4] Walter H. W. Tuttlebee, “Software-Defined Radio: Facets of a Developing Technology” IEEE Communication Magazine, pp. 38-44, April 1999. [5]. Alan Gatherer, “DSP-Based Architectures for Mobile communications: Past, Present and Future”, IEEE Communication Magazine, pp. 84-90, January 2000. [6] V Sinha, “Software Radio for Personal Communication Services”, Proceedings of NCC-2000, pp 8-11, January 2000.

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