DATA
Chapter 12 Conclusion and Recommendations
Chapter 12 Conclusion and Recommendations
Wireless communications, especially in the cellular context, will continue to extend its influence on a global scale. As 1110re users become jnfonned and begin to explore the limits of new technologies, so the demand for improved perfonnance will begin to draw on mathematical and technological innovations. This has already been the case, with new mobile communication standards emerging to meet the needs ofa highly mobile population. However, this has not only placed pressure on engineers to develop powerful communications systems, but also to seamlessly incorporate the existing basis from which the success of ceJiular communications has been launched. This thesis has examined two related phenomenon - the rise of CDMA as a viable wireless communications technique, and the development of the software radio to deliver key flexibility options. CDMA has been shown to be a spread spectrum communications technique, enabling it to coexist with other narrow·band transmissions. Further, the software radio is one means of allowing a single device to be able to access services offered by these various communications systems. This feature is derived from its flexible, re.programmable nature, a characteristic that, as this thesis has shown, enables the device to be far more than just a communications tool.
While GSM presently dominates cellular communications as the wireless protocol of choice, it is envisaged that, with time, the battery of positive characteristics of CDMA will force it further into the limelight. This thesis shows that, while it has had several technological barriers in the past, the protocol offers many advantages over current techniques and is gathering momentum as mobile hardware and technology improves. Further, mathematical progress in the realm of multi-user detection techniques has seen improved performance and extended possibilities for COMA. These techniques make use of, and detect, all the users on a CDMA communications channel simultaneously. This allows for performance gains over traditional correlating detectors, which treat other users as AWGN, thus precluding the use of the infonnation contained in the structured nature of the CD MA signal. The advantages do require copious amounts of processing power, and the software radio is one platform that offers many of the features necessary to implement such a COMA system.
The software radio is a radio where many of the functions can be implemented in a re·
configurable processing block. Many of tbe modules described in this thesis have been implemented on the Flexible Radio Platform, the software radio offered by Alcatel Altech Telecomms. Since FPGA's fonn the backbone of this software radio offering, VHDL was used to programme the devices. This digital description language is ideally suited to the high·speed FPGA environment, and proved to be a capable tool in the development of the core CDMA routines. The design methodology, however, was tedious since, to accurately test a system, it had to be compiled through to completion. This ensured that the system performed adequately.
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Chapter 12 Conclusion and Recommendations
III addition to filting in the FPGA. Newer simulators offer EDlF-Ievel simulation of VHDL systems and thus integrate more tightly with the system design. However, these were unavailable for the implementation or this project. Using VHDL to generate the entities, and schematics to create the top-level interraces, the COMA system was successfully implemented on the software radio.
This thesis has shown that the software radio used in this project is a highly flexible and powerful 1001 for the development of mobile communications systems. It has also shown that the FPGA can incorporate many functions previously relegated to sequential Digilal Signal Processors. Base-band shaping of the COMA signal was undertaken in a set of matched Finite Impulse Response filters. In this implementation phase, it becomes imperative that the developer balance resource usage and speed in the FPGA-based system. As was shown, these two parameters are permanently at loggerheads and the developer needs to carefully balance the design so as to optimise both. With this in mind, many realisation structures were presented as possibilities for the FIR filters. This culminated in a promising new technique known as distributed arithmetic. This solution promises to produce FIR filters capable of running at the system clock speed.
Not only does tbe software radio perform well in a mobile communications environment, but it also extends the developer's tools to incorporate testing and debugging techniques. It was shown that, once the system has been programmed, that it may be subject to channel distortions to test its viability. This requires a few statistical generation entities and higher-level layers to control the distortion parameters. These were incorporated in a simulation environment in which the personal computer set and controlled the necessary parameters, while recording the associated bit error rates. It was seen that, not only could the developer test the system, but that the simulations could also be performed in real time. This offers a speed advantage over traditional personal computer aided simulations.
In spite of the success of the project, there are many recommendations that would allow the flexible radio platform to become an even more useful tool. First, multi-user detectors require the calculation of a matrix inversion that, especially in the presence of a rapidly changing channel, is computationally burdensome. As the thesis examined the software radio, so it became apparent that it is possible to use resource partitioning to overcome this limitation.
Here, a OSP is used to calculate amine information, such as a matrix inverse. However, in this system, the link between the FPGA core and the OSP subsystem is a fairly slow RS-48S communication line. As a result, off-line calculations and updates are possible, but not at the speeds required by normal cellular communications. It is therefore a strong recommendation that, while this route is explored in future research implementation work, that another alternate be examined. This may include the use of the inter-FPGA communication bus for transferring signals to an off-board DSP chip.
Chapter 12 Conclusion and Recommendations
Control FPGA ~
"
Processing FPGA,
\
-V
\7
DSP for Off-line Calculations
Figure 12.1 -Splitting the FPGA bus to transfer signals to an off-board DS ...
Figure 12.1 shows how the off-line processing may be achieved in the DSP. Under these conditions, the processing FPGA can be used to package the data appropriately for serial transmission, while the DSP will have the task of channel estimation and matrix inversion. This resource partitioning would circumvent the speed and resource limitations encountered while implementing this project. Second, a further resource partitioning should be present in the form of a large, high-speed memory bank. When the partial summation techniques were investigated for base-band pulse shaping, it was seen that the FPGA technology used has very limited internal RAM. Further, the external RAM is also very limited. As a result, partial summations cannot be easily stored, especially in the case of high bit accuracy where the number of coefficients escalates exponentially. Nevertheless, the platform did offer most of the functionality expected of a software radio.
This thesis has examined two emerging technologies that serve to complement each other as much as they each stand as individual milestones. CDMA promises to boost the functionality and performance of cellular communications, while the software radio is seen as the tool for easily testing and deploying new communications protocols. Further, due to its re-configurable nature, the software radio also promises to seamlessly bridge the divide between the multitudes of communications protocols presently influencing mobile communications. Together, these technologies will influence tomorrow's world of mobile communications as they set new standards and reach for new limits in global cellular communications.
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Chapter 13 References
Chapter I3 References
[I] R. Prasad, "Overview of CD MA Evolution toward Wideband CDMA", IEEE, 1998.
[2] S. Verdu, "Minimum Probability of Error for Asynchronous Gaussian Multiple·Access Channels", IEEE Transaction on Information Theory, IT·32, No. I, pp. 8S·96, January 1986.
[3] J. Mitola 1Il, "Sof1ware Radio Technology Challenges and Opportunities", First European Workshop on Software Radios Keynote Address.
[4] The Science and Technology Policy Program (SRJ International· Washington, DC), "The Role of NSF's Support of Engineering in Enabling Technological Innovation· Phase 11", Chapte, 4, THE CELLULAR TELEPHONE.
[S] R. L. Pickholtz, L. B. Milstein, and D. L. Schilling, "Theory of Sprcad·Spectrum Communications - a Tutorial", IEEE Transactions on Communications, Volume COM-30, No. 5, pp. 855-882, May 1982.
[6] S. Glisic and B. Vucetic, "Spread Spectrum COMA Systems for Wireless Communications". Artech House Publishers, 1997.
[7] Taub & Schilling, "Principles of Communications Systems". Second Edition, McGraw·
Hilllntemational, 1986.
[8] http://www.xsilogy.com/supportlcodes.html.
[9] R.E. Ziemcr and R.L. Peterson, "Digital Communications and Spread Spectrum Systems".
[10] A. Grant and C. Schlegl, "Iterative Implementations for Linear Multi·User Detectors", Febcuary l, 1999.
[11] Fraleigh & Beuregard, "Linear Algebra, 3'd Edition". Addison Wesley, 1995.
[12] Dr. J. H. Reed & Dr. B. D. Woerner, "Industry Trends Leading to the Development of the Software Radio Concept". http://www.softradios.com/trends.html.
[13] I. Seskar and N. B. Mandayam, "Sofh.vare-Defined Radio Architectures for Interference Cancellation in DS·CDMA Systems", IEEE Personal Communications, Volume 6, Number 4, August 1999.
[14] M. Kruger, "User Manual For The 1/0 Processing Board Of The Dsp·Based Digital Transceiver System", Alcatel Altech Telecomms, 2000,
[IS] Blue Wave Systems, "PCI/C6200 Technical Reference Manual". Version 0.90, June 1998.
[16] J. D. Gibson. "Principles of Digital and Analogue Communications, Second Edition".
Macmillan Publishing Company, \993.
[17] AP. Cl ark, "Principles of Digital Data Transmission". Pentech Press, 1983.
Chapter 13 References
[18] E. C. Ifcachor and B. W. Jervis, "Digital Signal Processing - A Practical Approach".
Addison·Wesley Publishers Limited, 1993.
[\9) ALTERA, "Implementing FIR Filters in FLEX Devices". February 1998, ver. 1.01.
[20] A. Ralston, "Mathematical Methods for Digital Computers". John Wiley & Sons, 1967.
[21J W. H. Press, S. A. Teukolsky, W. T. Venerling, and B. P. Flannery, ''Numerical Recipes in C - The Art of Scientific Computing". Second Edition, Cambridge University Press,
1992.
[22] Nievergelt, Farrar, and Reingo\d, "Computer Approaches to Mathematical Problems".
Prentice·Hall, 1974.
[23] Synopsys, "FPGA Express, VHLD Reference Manual". Synopsys, 1997.
[24] Alcatel Altech Telecomms, "Software Radio Main 110 Board Schematics & Assembly Drawings". AAT,1999.
[25] N.S. Alagha and P. Kabal, "Generalised Raist:U-Cosine Filters", IEEE Transaction on Communications, Volume 47, No. 7, pp. 989·997, July 1999.
[26] James Miller, "Practical Communications Theory - Pan 2". Spread Spectrum Scene, 1998: www.sss-mag.com/G3RUH/indexl.html
(27] K.G. Beauchamp, "Walsh Functions and their Applications". Academic Press, 1975.
[28] H. Saarnisaari, "Robustness of the MUSIC algorithm to Errors in Estimating the Dimensions of the Subspaces: Delay Estimation in DS/SS in the Presence of Interference", University ofOulu, Telecommunication Laboratory, 1999.
[29] E.G. Strom, S. Parkvall, S.L. Miller, B.E. Ottersten, "Propagation Delay Estimation in Asynchronous Direct·Sequence Code·Division Multiple Access Systems", University of Florida/Royal Institute of Technology, 1994.
[30} T. Ostman and B. Ottersten, "Systems with Bandlimited Pulse Shapes'" Royal Institue of Technology.
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Chapter 14 Appendix