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Part III Baseband Technology ‘Software runs on Silicon’ – and in the case of SDRs today, the competition between approaches and technologies for exponentially increasing baseband processing requirements is proving a fertile ground for innovation, in both conceptual approaches and implementation architectures. Software Defined Radio Edited by Walter Tuttlebee Copyright q 2002 John Wiley & Sons, Ltd ISBNs: 0-470-84318-7 (Hardback); 0-470-84600-3 (Electronic) 7 Baseband Processing for SDR David Lund a and Bahram Honary b a HW Communications Ltd. b Lancaster University Many technologies require substantial research and development to facilitate the emergence of mature and generic software defined radio architectures, as will be evident from a perusal of the contents of this volume or the other literature in the field. Our own chapter focuses upon the broad ranging topic of baseband processing, an important and central element of such architectures. Alongside networking of these enhanced capability and flexible systems, the radio frequency processing aspects of the physical layer are required to accommodate a flexible range of different frequencies, formats, and environments. Baseband processing is perhaps one of the most potentially fruitful areas of development anticipated over the next few years – indeed, significant progress is already evident. 7.1 The Role of Baseband Architectures The baseband of any radio system is responsible for digitally transforming raw data streams into the correct format ready for transmission over a known wireless channel. In a transmitter this simply consists of formatting of the data and introduction of any redundancy required to improve reception. At the receiver, the information from the radio frequency front end has to be carefully analyzed in order to extract correctly the data which was intended for reception. This requires synchronization, demodulation, channel equalization, channel decoding, and multiple access channel extraction. These, and many more functions, are linked together to form a chain of processing functions, providing a pipeline through which the data and its associated overhead are passed. The architecture of this baseband processing chain is structured to reflect the type of wireless channel and other supporting functions over which data is to be transmitted. Differ- ent modulation, channel coding schemes are chosen to maximize throughput over the parti- cular channel, which itself may be influenced by the application (e.g. required data rate). Multiple access methods are chosen to maximize the number of transmitters which can effectively and simultaneously share the spectrum. Support functions such as synchroniza- tion, equalization, and spatial diversity algorithms all enhance the basic transmission format Software Defined Radio Edited by Walter Tuttlebee Copyright q 2002 John Wiley & Sons, Ltd ISBNs: 0-470-84318-7 (Hardback); 0-470-84600-3 (Electronic) at the expense of extra processing and power consumption, the latter being particularly important within the context of portable devices. The majority of today’s air interfaces are subject to a standardized specification which strictly defines the transmission formatting for a particular system. For example, GSM, UTRA, and IS-95 specify just a few of the individual formats for mobile wireless telecom- munication systems. Digital audio broadcasting (DAB) and DVB-T specify formats for terrestrial broadcasting systems and DVB-S for satellite broadcasting. Many others define high bit rate wireless systems for short range networking, fixed wireless access, and other applications. A software defined radio may be designed to tranceive using a variety of such available standards. It may even participate ad hoc, as and when bandwidth may be available. Any of the standard transmission formats may be chosen which may provide the necessary level of service as the users’ application requires. Research in SDR commonly takes a top down approach. Evaluation of the market looks at what the user requires in terms of application. Network developers look at how to provide such applications and services to the user. Equipment developers develop and use compo- nents to build the infrastructure needed to implement the networks and terminals. Equipment developers will frequently take predominantly off-the-shelf component tech- nologies and, maybe after some specific modification, integrate them into infrastructure equipment. This is certainly how second-generation mobile equipment has been developed, with GSM terminals commonly containing hybrid application specific integrated circuit (ASIC) devices based upon a particular microprocessor (mP) or digital signal processor (DSP) core. New component technologies are rapidly emerging which complement the now traditional mP and DSP technologies. Field programmable gate array (FPGA) and new reconfigurable fabric processors give an alternative edge to the use of aging mP and DSP technologies. The remainder of this chapter describes a range of technologies and techniques presently available for implementation of the baseband processing subsystem. The concept of flexible processing is introduced to describe the higher level problems associated with using such technology. The status of currently available component technologies is described, illustrat- ing the wide range of processing resource already available today. These are presented to developers of SDR-based equipment, providing insights into how they may be used for different algorithmic purpose. The introduction of new processing technologies also requires development of new design tools and methods. The status of such tools is also described with discussion of requirements not only for initial design of an SDR system but also its maintenance. A discussion of object- oriented methods illustrates how the high level software developer may be given the neces- sary visibility and understanding of the processing resources available. With such visibility, SDR operators and maintainers can then allocate the necessary time critical baseband proces- sing chains to a multitude of processing resources with maximum efficiency. 7.2 Software Radio – From Silicon to Software It is important here to recognize the context within which, for the purposes of this chapter, we use the term ‘software’ and the consequent breadth of the requirement. The Cambridge International Dictionary of English gives the following simple definition: Software –‘the instructions which control what a computer does’. Software Defined Radio: Enabling Technologies202 So, for an easy method of translation to the context of software radio systems, which is simply a new advanced method of communication, we could replace the term ‘computer’ with the term ‘communication system’ to get: Software –‘the instructions which control what a communication system does’. The computer is a relatively simple system which is controlled by relatively simple soft- ware. It is clear to see that software in the context of SDR is much more elaborate and can in fact be considered to be two-tiered. Software (tier 1) is required to define the computation and software (tier 2) is required to control the mode of operation of this computation within the communication system. 1 Much research is today being carried out in order to determine the best methods to architecturally define software radio with its associated reconfigurable systems and networks. 2 Issues such as quality of service, value added service provision, and the huge task of managing all of this are being defined and will require substantial advances in implementation level technology to automate these aspects. Quality of service (QoS) guarantees are becoming more and more an important function- ality for the future of mobile telephony and data transfer services. QoS applies a policy to a communication service to which the system must adhere. For example, the resource reserva- tion protocol (RSVP) service is such a method for providing a level of QoS in fixed network applications. A data transfer session begins by reserving bandwidth through a network or the Internet. A route of fixed bandwidth is reserved from the data source to its destination by negotiating with routers along the way. Once the bandwidth is reserved, the data transmission can take place with a guaranteed pipeline of fixed bandwidth. The application transferring the data can then guarantee the quality of service supplied to the user. QoS is a topic originally pioneered by researchers in computer-based fixed networks. As third-generation mobile networks are developing, the concept of QoS is emerging as an important requirement for provision of a wide range of high quality data services to the mobile user. Network management is also a hot topic in the quest for improving the mobile and Internet experience. Providing efficiency in the system along with network management allows easy deployment and control of systems and hence efficient service to the user. Second-generation mobile networks consist of mobile terminals, base stations, and switching centers. Third- generation networks provide much more functionality in order to improve user services. Various domains and strata providing different levels of data access and control are defined, allowing the capability to provide advanced, user specific services across a broad range of environments and across multiple networks [1]. Network management of these, already multimode, systems is proving to be a huge task. The concept of software radio and the advent of reconfigurable processing systems makes the organization and management of the network even more complex, as the majority of functionality in the network becomes capable of being modified with only the actual hardware architecture remaining static. The CAST 3 project illustrates the need for advanced management methods by focusing on Baseband Processing for SDR 203 1 A similar distinction, using the terms ‘software signal processing’ and ‘software control’, is made and explained further in Chapter 1. 2 For an overview of such research in Europe see Dillinger and Bourse in Software Defined Radio: Origins, Drivers and International Perspectives’, Tuttlebee, W. (Ed.), John Wiley & Sons, Chichester, 2002, Chapter 7. 3 Configurable radio with Advanced Software Technologies (CAST) project is European Commission Information Society Technologies (IST) funded Project, IST-1999-10287. HW Communications Ltd are primary contractors in this project, within which the authors of this chapter actively participate. the use of organic intelligence to cope with the enormous range of situations and scenarios which have to be managed. Figure 7.1 illustrates how an architecture based on organic intelligence can be used to manage the multitude of reconfigurable elements in a network [6,17,38]. Other approaches to the problem use traditional procedural approaches whereby a fixed set of rules governs the management operation depending upon the status of the system. It becomes quite clear when reviewing the different proposed systems that a combination of intelligence and procedural rules is essential to cope with the immense multitude of operating scenarios which are available. Overall, the high level visionary architectures of reconfigurable mobile networks place a huge demand on the technologies which have the responsibility of processing all of this. Combined with greedy demand for data bandwidth and strict QoS restrictions the resultant demands placed on silicon processing technologies are seen to be growing in an unprece- dented manner. There are two demands made by such systems which quite simply describe the major developments required of the physical processing technology, namely demands for: † increased processing capability † increased dynamic capability These are not, however, just tasks for the silicon engineering industry. Today’s complex semiconductor devices require powerful tools to aid designers in using the technology quickly and efficiently. The days of Karnaugh maps for logic minimization are now far in the distant past. Modern processing technologies exceed tens of millions of logic gates. Manual design, even on circuits which are today considered simple, is impossible. Increasing the dynamic capability of a system not only increases its range of operation, but also increases its lifetime, extending the system maintenance requirements. To summarize, three major areas of advance are required in order to provide the dynamic processing capabilities essential to support future reconfigurable communication systems. A number of key challenges and questions arise in each of these areas, summarized below. It is Software Defined Radio: Enabling Technologies204 Figure 7.1 CAST network management architecture [6] the lack of a single simple criterion which restricts evaluation of the many innovative alter- natives proposed today as possible baseband technology solutions. 1. Baseband component technologies – dynamic capability – how flexible are different processing devices? – processing capability – how powerful are different processing devices? – physical constraints – what are their physical limitations? 2. Design tools and methods – standardized tools and methods – global compatibility and coherence. – specification tools and methods – transferral of design information. – mixed mode capability – mixed component technologies imply the need for mixed tool environments. – tool processing requirements – can a highly complex system be simulated? – compliance to design procedures – design flows for different technologies and combi- nations. – algorithm processing requirements – to provide enhanced automated design decisions. – automated hardware selection for algorithms – also for automated design decisions. – system simulation and emulation – testing methods at different levels. 3. System maintenance – object oriented system control – control of low level processing resource by higher layer distributed control. – configuration and reconfiguration mechanisms – controlling the physical processing resources. – initial set up and configuration – how is a system initialized? – automatic intelligent decisions – higher capability requires more complex decisions. – capability classification – knowledge of the processing system is required for in-system decision making. – resource allocation – efficiently allocating functions to processing resources. – configuration update procedures – methods of securely controlling and updating dyna- mically distributed systems. It is evident that the advances in silicon technology today are outstanding and provide huge capabilities. However, in order to use these technologies efficiently, more development is required in order to support the silicon. From the component technologies viewpoint, the evolution of the personal computer has reflected advancement in microprocessor and RAM technologies. Support and drive has also however been required from providers of operating systems, development tools and system maintenance. As with the PC, provision of generic reconfigurable systems will not just rely on a small number of technologies. Different silicon devices are now essential to provide high capacity dynamic processing. It is shown later how some of these essential new silicon technologies presently lack the required support from development tools and maintenance. Figure 7.2 illustrates the wider context of the real enabling technology required for software radio. Existing methods of using software to define the function of a processing system are well defined when using microprocessor-based resources. Application and service developers are Baseband Processing for SDR 205 able to use this resource, without knowledge of its existence, to provide high quality and efficient desktop based data services such as real time video, radio, and many more. Much development is required in order to support the increased range of processing media avail- able. Software radio systems depend wholeheartedly upon this new multitude of processing resource. Allowing the application and service providers transparent access to this newly defined resource will require substantial development in all three areas described above. The subsequent sections of this chapter are devoted to an examination of the current status of component technologies, design tools and methodologies, and system design and main- tenance. 7.3 Baseband Component Technologies Until relatively recently, the majority of software radio research had focused mainly on the use of software and digital signal processors [12]. Arguably, this is the simplest technology to implement, but performance of a system consisting of processor and software only is not yet powerful enough for the high data rate requirements of systems targetted for third-generation (3G) mobile communications. Several recent papers have described the application of FPGAs in software radio [2,9,10], but few have attempted to tackle the issues relating to the config- uration and reconfiguration during an online service transmission. No single silicon technology is more important than another when it comes to the design of efficient processing systems. Each processing algorithm has a different combination and set of discrete operations. The combination of logic operations, adds, subtracts, multiplies, divides and condition operations is different for each algorithm. The first digital signal processors (DSP) were optimized mainly for the huge demand of pipelined multiply accu- mulate (MAC) operations which form the basis of most discrete signal processing algorithms. Software Defined Radio: Enabling Technologies206 Figure 7.2 The breadth of enabling technology required to support SDR Figure 7.3 illustrates how the FPGA can provide a solution for systems requiring both high performance and a high degree of function capability. At one extreme, hardwired devices, such as an ASIC, can only perform a limited function; they do, however, provide a very high performance. The DSP, being software programmable, can offer an almost unlimited function capability, but, of course, the serial processing nature of traditional DSPs does limit perfor- mance. Figure 7.4 illustrates how the FPGAs processing resource is able to provide this combi- nation of high function with high dynamic capability. Any processing algorithm can be decomposed into subelements which incrementally carry out the computation required by the algorithm. Each of these subelements has dependencies upon the data available as a result of other subelements’ processing. In Figure 7.4, subelement 2 is dependant upon 1 and subelement 4 is dependant upon 3. Subelement 5 is dependant upon the result of 2 and 4. A single mP or DSP software based computation of the algorithm must process each sub- element sequentially, satisfying the dependencies, i.e. 1 ) 2 ) 3 ) 4 ) 5or3) 4 ) 1 ) 2 ) 5. Improved performance may be gained by using multiple processors to compute 1 ) 2 and 3 ) 4 in parallel. Multiple processors do, however, result in higher power consump- tion and the requirement for more silicon area. The FPGA processing resource is fine grained and can carry out this parallelism to a degree as small as individual logic gate operations. An important trade-off here is ease of implementation vs. performance. An FPGA circuit with currently available design tools is more difficult to configure than the well-established programming methods of the DSP. A small price is also paid in time when reconfiguring. Reconfiguring FPGA logic is much slower than a simple function call in the mP or DSP. The DSP and FPGA devices showcased in the following sections are chosen in order to illustrate the types of processing resources available. Although the majority of these devices are currently large and power hungry, it is their function that is important in order to draw conclusions upon which available resources will be required for reconfigurable communica- tion systems. It is the plethora of processing methods used within these current devices which Baseband Processing for SDR 207 Figure 7.3 Current enabling technologies for digital processing is the important consideration today. Once the best methods or processing are known, future silicon, or other fundamental technologies, will be implemented and used efficiently as targeted to the reconfigurable processing or SDR system. 7.3.1. Digital Signal Processors The digital signal processor (DSP) 4 was first introduced in the early 1980s in order to provide a processing machine optimized for interpreting, manipulating, or even generating discrete signals in the time or frequency domain. DSPs provided a method which revolutionized the way in which real physical information is processed. The flexibility, accuracy, and reprodu- cibility of analog components was relatively limited and, hence, superseded by the solidly defined program of the DSP. Dynamic range is a problem associated with analog circuitry; this constraint is still present in the vital analog to digital conversion process (ADC) encoun- tered prior to the DSP. The DSP is in essence simply an optimization of the general purpose microprocessor (mP). On a simple mP only basic functions such as memory load, memory store, add/ subtract, and logic operations were initially available. The DSP’s key innovation which optimized its architecture for analog signal manipulation was the inclusion of the multiply accumulate (MAC) operation. Algorithms for manipulating signals are often based upon the method of convolution. Convolution allows the set of discrete samples to be treated in an equivalent manner to the represented continuous signal. Convolution-based algorithms allow signals to be combined, filtered, and transformed, allowing operations to be imple- mented fully equivalent to the analog case. The MAC operation is optimized for execution in a single DSP clock cycle; indeed, high performance DSPs may even support two or more MACs per clock cycle. Addressing modes are also optimized in DSP architectures, allowing efficient loading and Software Defined Radio: Enabling Technologies208 4 The newsgroup comp.dsp provides a thorough working analysis of DSPs from which some of this historical and tutorial material is sourced. Figure 7.4 Comparison between software and reconfigurable logic storage of discrete data to and from memory circuits. Data access is also improved by using Harvard architectures to allow the DSP to access both data and instructions simultaneously. Functions such as pre/post addressing registers (pointers) store addresses of locations of discrete data. They often also incorporate their own arithmetic function to allow for fast update of the pointer to quickly address the next required data element. Circular addressing is also common, allowing a pointer to rotate around a defined area of memory to provide a cycle-based memory access. Along with the MAC, the DSP may also provide execution Baseband Processing for SDR 209 Table 7.1 A comparison of DSP engines Device Manufacturer Clock (MHz) Performance Precision Optimized for DSP56800 Motorola 80 40 MIPS 16 bit fixed Control applications (peripheral IO) DSP56600 Motorola 60 60 MIPS 16 bit fixed Cell phone and 2-way radio DSP56367 Motorola 150 150 MIPS 24 bit fixed Audio processing MSC8102 (Starcore) Motorola 300 4800 MMAC 16 bit fixed High processing performance ADSP-2191 Analog Devices 160 160 MIPS 16 bit fixed Audio Processing SHARC Analog Devices 100 600 MFLOPS 32/40 float High performance precision TigerSHARC Analog Devices 150 1.2 Billion MACS 40 bit fixed & float Very high processing performance TMS320C24x Texas Instruments 40 20–40 MIPS 16 bit fixed Control applications (peripheral IO) TMS320C54x Texas Instruments (133 30–532 MIPS ( 40 bit fixed Low power consumption 0.32 mW/ MIPS TMS320C55x Texas Instruments 200 400 MIPS 16 bit fixed Low power consumption 0.05 mW/ MIPS TMS320C62x Texas Instruments 150–300 1200–2400 MIPS Fixed Fixed point processing power TMS320C64x Texas Instruments 400–600 3200–4800 MIPS Fixed Fixed point processing power TMS320C67x Texas Instruments 100–167 600–1000 MFLOPS floating Floating point processing power TMS320C8x Texas Instruments 50 100 MFLOPS equiv 32 bit fixed Telecommunications and image parallel processing 2 BOPS (RISC) 32 bit float 4 (C80) or 2 (C82) parallel pProcessors 1 32 bit RISC processor [...]... Mitola, J and Maguire, G., ‘Cognitive radio: making software radios more personal’, IEEE Personal Communications, August, 1999, pp 13–18 [31] Mitola, J., ‘Software radio architecture: a mathematical perspective’, IEEE Journal on Selected Areas of Communications, Vol 17, No 4, 1999, pp 514–538 [32] Pereira, J.M., ‘Re-defining software (defined) radio: re-configurable radio systems and networks’, IEICE Transactions... 1999 [27] Mitola, J., Software Radio Architecture: Object-Oriented Approaches to Wireless Systems Engineering, John Wiley & Sons, 2000 [28] Mitola, J., ‘Software radio architecture evolution: foundations, technology trade-offs, and architecture implications’, IEICE Transactions in Communications, Vol E83-B, No 6, June, 2000, pp 1165–1172 [29] Mitola, J., ‘The software radio architecture’, IEEE Communications... The status of design tools for individual component technologies is known Software defined radio systems, however, require a combination of the different types of processing resource In many cases there are few design tools and methods yet defined for such purposes These systems require careful Software Defined Radio: Enabling Technologies 222 consideration of the interaction between devices and their... have vastly improved As we look ahead to the promise of software radio systems we may perhaps expect a similar advance in the coming decade References and Further Reading [1] 3G TS 23.101, ‘General UMTS architecture’, Ver 3.0.1, 3GPP Partnership Project, April, 1999 [2] Ahlquist, G., Rice, M and Nelson, B, ‘Error control coding in software radios: an FPGA approach’, IEEE Personal Communications, August,... preliminary information’, www.anadigm.com, 2000 [5] Brock, D., Mukhanov, O and Rosa J., ‘Superconductor digital RF development for software radio , IEEE Communications Magazine, Vol 39, No 2, February, 2001, pp 174–179 [6] Madani, K., Lund, D., Patel, P et al., ‘Configurable radio with advanced software techniques (CAST) – initial concepts’ IST Mobile Communications Summit, Galway, Ireland, October, 2000 [7]... time varying channel’, IEE 8th International Conference on HF Radio Systems And Techniques, University of Surrey, 2000 [21] Lund, D., Barekos, V and Honary, B., ‘Convolutional decoding for reconfigurable mobile systems’, IEEE 3G, 2001 [22] Lund, D., Honary, B and Madani, K., ‘Characterising software control of the physical reconfigurable radio subsystem’, presented at the 1st Mobile Summit, Barcelona,... Lund, D and Honary, B., ‘Enhanced trellis extracted synchronisation technique for practical implementation’, IEE Colloquium on Novel DSP Algorithms And Architectures For Radio Systems, London, September, 1999 232 Software Defined Radio: Enabling Technologies [25] Lund, D., Honary B and Darnell, M., ‘Adaptive nested codec’, ISPLC99: 3rd International Symposium on Power Line Communications and Its Applications,... feasible for these to be of use at the GHz frequencies, or even the related IF Software defined radio is, however, not just limited to systems in the higher frequency bands One paper [8] describes the use of a combination of different FPAA devices for the realization of a short wave receiver for the new digital radio mondiale (DRM) digital HF standard This illustrates the applicability of the FPAA in adaptable... at the present time For the medium term development of software defined radio it is expected that devices will appear incorporating a combination of the functionalities available today in the separate technologies described above, optimized according to the required functionality In the long term, when consideration for cognitive radio and ad hoc networks implies very comprehensive functionality, the... layer in software radio applications’, International Symposium 3rd Generation Infrastructure and Services, 3GIS, July, 2001, Athens, Greece [38] Ramos, R and Madani, K., ‘A novel generic disturbed intelligent re-configurable mobile network architecture’, VTC-Spring Conference, Rhodes, Greece, May, (2001), pp 6–9 [39] Sandor, I and Kovacs, J., ‘Resource control and reconfiguration in software radio environment’, . computer does’. Software Defined Radio: Enabling Technologies202 So, for an easy method of translation to the context of software radio systems, which is simply. Defined Radio: Origins, Drivers and International Perspectives’, Tuttlebee, W. (Ed.), John Wiley & Sons, Chichester, 2002, Chapter 7. 3 Configurable radio