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Reconfigurable Structures for Direct Equalisation in Mobile Receivers

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Reconfigurable Structures for Direct Equalisation in Mobile Receivers Reconfigurable Structures for Direct Equalisation in Mobile Receivers Variable Length Linear and Decision Feedback Equalisers for Power- Constrained Receivers Operating in Time-Varying Environments Felip RIERA PALOU Submitted for the degree of Doctor of Philosophy 2002 Department of Electronics and Telecommunications University of Bradford Reconfigurable Structures for Direct Equalisation in Mobile Receivers. Variable Length Linear and Decision Feedback Equalisers for Power- Constrained Receivers Operating in Time-Varying Environments. Any communication channel will usually distort the transmitted signal. This is especially true in the case of mobile systems, where multipath propagation causes the received signal to be seriously degraded. Over the years, many techniques have been proposed to combat channel effects. Two of the most popular are linear equalisation (LE) and decision feedback equalisation (DFE). These methods offer a good compromise between performance and computational complexity. LE and DFE are implemented using finite impulse response (FIR) filters whose frequency spectrum approximates the inverse of the channel spectrum plus noise. In mobile systems, the equaliser is made adaptable in order to be able to respond to the channel variations. Adaptability is achieved using adaptive FIR filters whose coefficients are iteratively updated. In principle, an infinite number of filter coefficients would be needed to achieve perfect channel inversion. In practice, the number of taps must be finite. Simulations show that, in realistic scenarios, making the equaliser longer than a certain (undetermined) number of taps will not yield any benefit. Moreover, computation and power will be wasted. In battery powered devices, like mobile terminals, it would be desirable to have the equaliser properly dimensioned. The equaliser’s optimum length strongly depends on the particular scenario, and as channel conditions vary, this optimum is likely to vary. This thesis presents novel techniques to perform equaliser length adjustment. Methods for the LE and the DFE have been developed. Simulations in many different scenarios show that the proposed schemes optimise the number of taps to be used. Moreover, these techniques are able to detect changes in the channel and re-adjust the equaliser length appropriately. KEYWORDS: Equalisation, Mobile, Reconfigurable, Adaptive Filter, Variable Length. Acknowledgments I want to most gratefully thank my supervisor Dr James M. Noras for his guidance, help, support and encouragement throughout this research project. I also want to thank my lab colleague Kostas Chaikalis for useful discussions and for being such an easy-going lab mate. I hope, and I am sure, that the friendship with both of them will last beyond this project. I also extend my sincere gratitude to all academic and secretarial staff of the Department of Electronics and Telecommunications at the University of Bradford for their help during my time at Bradford. The financial support from MobileVCE during the first two years of this project is gratefully acknowledged. It is a pleasure to acknowledge the support, during the whole project, of various members from the Department of Electronics and Electrical Engineering at the University of Edinburgh. Very specially, I want to thank Dr. David G. M. Cruickshank for the many fruitful discussions we have had over the last three years. Additionally, I am also grateful to Dr. Ian W. Band who provided crucial advice on different aspects of the simulation environment implementation. Although not directly related to the PhD work, I want to express my appreciation to my undergraduate project supervisor Dr. Guillem Bernat, currently at the University of York, previously at the University of the Balearic Islands (UIB), for his support and encouragement during my last year at UIB to come to UK for further study. I thank the many good friends I have made in Bradford for making my time here most enjoyable and to my great friends in Mallorca, who constantly make me wish go home and also, for their visits to Bradford. I consider myself extremely fortunate to have a wonderful family who have always encouraged me to study, especially during my unmotivated years (age 3 to 18), and it is to them that this thesis is dedicated. Finally, I want to thank Isabel for her full-time love, support and understanding. Felip Riera Palou, Bradford, April 2002. A la meva família, i molt especialment, als meus pares List of Acronyms 2G 2nd Generation (of mobile systems) 3G 3rd Generation (of mobile systems) 3GPP 3rd Generation Partnership Project ADC Analog to Digital Converter ADPCM Adaptive Differential Pulse Coded Modulation AM Amplitude Modulation ASIC Application Specific Integrated Circuit ASK Amplitude Shift Keying AWGN Additive White Gaussian Noise BER Bit Error Rate BPSK Binary Phase Shift Keying CDMA Code Division Multiple Access CIR Channel Impulse Response DFE Decision Feedback Equaliser DQPSK Differential Quaternary Phase Shift Keying DSP Digital Signal Processor/Processing EMSE Excess Mean Square Error FAEST Fast A-priori Error Sequential Technique FBF FeedBack Filter FDD Frequency Division Duplex FDMA Frequency Division Multiple Access FFF FeedForward Filter FM Frequency Modulation FPGA Floating Programmable Gate Array FSE Fractionally Spaces Equaliser FSK Frequency Shift Keying GMSK Gaussian Minimum Shift Keying GSM Global System for Mobile communications LE Linear Equaliser LMS Least Mean Square LNA Low Noise Amplifier LPC Linear Predictive Coding MASK M-ary Amplitude Shift Keying MFSK M-ary Frequency Shift Keying MMSE Minimum Mean Square Error MPSK M-ary Phase Shift Keying MSE Mean Square Error NLMS Normalised Least Mean Square OQPSK Offset Quaternary Phase Shift Keying PCM Pulse Coded Modulation PLL Phase Locked Loop PM Phase Modulation PROM Programmable Read Only Memory PSK Phase Shift Keying QAM Quadrature Amplitude Modulation QPSK Quaternary Phase Keying RF Radio Frequency RLS Recursive Least Squares SDMA Spatial Division Multiple Access SFAEST Stabilised Fast A-priori Error Sequential Technique SNR Signal to Noise Ratio SRAM Static Random Access Memory SRRC Square Root Raised Cosine SS-MSE Steady State Mean Square Error TCM Trellis Coded Modulation TDD Time Division Duplex TDMA Time Division Multiple Access UMTS Universal Mobile Telecommunications System VHDL VLSI Hardware Description Language VL LE Variable Length Linear Equaliser VL FBF DFE Variable Length FeedBack Filter Decision Feedback Equaliser VSLMS Variable Step size Least Mean Square TABLE OF CONTENTS 1 Introduction ....................................................................................................... 1 1.1 Introduction to mobile digital systems ........................................................................ 1 1.1.1 Block diagram of a generic mobile transceiver ...........................................................................2 1.1.2 Mobile channels ........................................................................................................................12 1.1.3 Review of mobile Standards......................................................................................................16 1.2 Reconfigurable communication systems................................................................... 18 1.3 Objectives of the thesis.............................................................................................. 19 1.4 Organisation of the thesis .......................................................................................... 21 1.5 Thesis notation........................................................................................................... 22 1.6 Original contributions................................................................................................ 22 1.7 Author's publications................................................................................................. 23 2 Adaptive equalisers: Structures, algorithms and applications................... 25 2.1 Wiener filter theory ................................................................................................... 25 2.2 Equaliser structures ................................................................................................... 28 2.2.1 Linear equaliser.........................................................................................................................29 2.2.2 Decision feedback equaliser......................................................................................................32 2.2.3 Fractionally spaced equaliser ....................................................................................................34 2.2.4 Other multipath mitigation structures........................................................................................36 2.3 Adaptive algorithms .................................................................................................. 39 2.3.1 Least mean squares algorithm and variants...............................................................................40 2.3.2 RLS algorithm...........................................................................................................................44 2.3.3 Fast Kalman algorithms.............................................................................................................47 2.3.4 Comparison of gradient and least squares techniques ...............................................................48 2.4 Applications of equalisers to mobile
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