\t. s-qb ON TTIE DESIGN OF HF RADIO MODEMS TIMOTHY C. GILES Thesis submitted for the degree of Doctor of Philosophy in The University of Adelaide (Department of Electrical and Electronic Engineering.) December, 1995 CONTENTS Abstract 1 Statement of Originality 2 Glossary 3 Acknowledgment 5 I. INTRODUCTION 6 1.1 HIGH FREQUENCY RADIO COMMUNICATONS 6 1.2 HF MODEM TECHNOLOGY 8 1.3 THESIS SUMMARY 11 1.4 CONCLUSION 13 1.5 APPENDX I4 1.5.1 Details pertaining to Figures 1.3 & 1'4 t4 2. A REVIEW OF THE MODELLING AND SIMULATION OF DISTORTION ON THE HIGH FREQUENCY VOICE BAND CHANNEL 15 2.1 INTRODUCTON 15 2.2 HF SIMULATON - A REVIEW r6 2.2.1Watterson Model 16 2.2.2 Wide-Band Model 22 2.2.3 A^ Pseudo-Deterministic Model 27 2.4 DISCUSSION 29 2.5 CONCLUSION 30 3. A REVIEW OF THE MODELLING AND SIMULATION OF NOISE AND INTERFERENCE FOR HF MODEM DESIGN 31 3.1 INTRODUCTION 3l 3.2 NARRO\MBAND INTERFERENCE 32 3.2.1 Conclusion - Na¡rowband Interference 36 3.3 IMPULSIVE NOISE 36 3.3.1 Mathematical Models 36 3.3.2 A Physically Based Model 37 3.3.3 CCIR Atmospheric noise model 39 3.3.4 Man-Made Noise Pdf s 39 3.3.5 Statistics in time 39 3.3.6 Impulsive Noise Simulators 39 3.3.7 Conclusion - Impulsive Noise 40 3.4 CONCLUSION 40 4. THE ADVANCED HF MODEM USING TRELLIS CODED MODULATION 42 4.1 INTRODUCTION 42 4.2 MODULATION STRUCTURE 43 4.3 TRELLIS CODED MODULATION 45 4.4 THE SOFT DECISION DECODER METRIC 46 4.4.1 Metric Design for a Differential Phase Shift Keyed (DPSK) System in Gaussian Noise 46 4.5 INTERLEAVING 47 4.6 CONCLUSION 50 5. REPLAY SIMULATION 5l 5.1 INTRODUCTON 51 5.2 PHASOR BASED REPLAY SIMULATOR 52 5.3 NOISE AND INTERFERENCE REPLAY SIMULATOR 53 5.4 FULL HF CHANNEL REPLAY SIMULATOR 56 5.5 CONCLUSION 60 5.6 APPENDD( 61 5.6.1 Phasor Based Simulator Recording Trial 61 5.6.2 Impulsive Noise Sample Recordings 62 5.6.3 DSTO HF Radio Replay Simulator Recordings 62 6. DESIGN OF NARROW-BAND INTERFERENCE TOLERANT MODEMS 63 6.I INTRODUCTION 63 6.2 REVIEW OF INTERFERENCE TOLERANT MODEM DESIGNS æ 6.2.I A Modem for More Reliable Communications at HF u 6.2.2 The Chtrp Modem 66 6.2.3 Frequency Shift Keyed Modems 68 6.2.4Inærference Rejection in Direct Sequence Spread Spectrum '70 6.2.5 Optimum Detection Techniques 7r 6.2.6lmproving High Speed HF Parallel-Tone Modems 71 6.3 THE SIMULATION SYSTEM 72 6.4 THE SOFT DECISION DECODER METzuC 72 6.5 FURTHER IMPROVEMENTS 74 6.5.1 V/indowing 74 6.5.2 Inærleaving 74 6.6 NARROW BAND EXCISION FILTERING 76 6.7 CONCLUSION 76 6.7.1 Further Resea¡ch 79 7. THE DESIGN OF AN IMPULSNE NOISE TOLERANT MODEM 80 7.1 INTRODUCTION 80 7.2 A REVIEW OF DETECTION THEORY FOR DIGITAL COMMUNICATIONS 81 7.2.I The Optimum Detector 81 7.2.2 T\e Matched Filær 82 7.2.3 A Blanking or Limiting Non-Linearity 84 7.2.4 Locally Optimum Bayes Detectors 84 7.2.5 Some Optimum Detectors for specific types of Non- Gaussian Noise 84 7.2.5.1 Optimum detector for Independent and Identically Distribuæd Laplacian Noise 84 7 .2.5.2 The Log-Correlator 85 7.2.6 Designing for Robustness 85 7.2.7 Discussion 85 7.3 THE SIMULATION SYSTEM 87 7.4 THE SOFT DECISION DECODER METRIC 87 7.5 A BLANKING NON-LINEARITY 87 7.6 AN OPTIMAL DETECTOR FOR INDEPENDENT AND IDENTICALLY DISTRIBUTED LAPLACIAN NOISE 90 7.7 OPTIMAL DETECTOR FOR INDEPENDENT AND IDENTICALLY DISTRIBUTED NON-GAUSSIAN NOISE 90 7.8 THE ADAPTIVE GAUSSIAN METRIC 93 7.9 ADAPTIVE LAPLACIAN METRIC 93 7.10 DISCUSSION 94 7.11 CONCLUSION 94 7.12 APPENDIX 95 7.12.1The Median Filær as a Detector in Laplacian Noise. 95 7.I2.2 Derivation of the Non-Linearity for the tncally Optimum Bayes Detector 96 7.I2.3 Probability Density Function Estimation using a Histogram Matching Technique 97 8. A COMPLETE HF RADIO MODEM 99 8.1 INTRODUCTION 99 8.2 NOISE VARIANCE ESTIMATION 101 8.2.1 Noise Variance Filær Type 105 8.2.2 Noise Variance Filær Shape 108 8.2.3 Noise Variance Filær Size 110 8.3 CONCLUSION 110 9. CONCLUSION TT2 9.1 REVIEW OF ACHIEVEMENTS rt2 9.2 CONTINUING RESEARCH 113 9.3 THE FUTURE 115 9.4 APPENDIX 116 Bibliography 118 APPENDICES List of Publications r27 Selected Papers r29 On the Design of HF Radio Modems ABSTRACT The subject of this thesis is the design of HF radio modems. The investigation begins with a review of the distortion, noise and interference characteristics specific to modems designed for the HF channel. Based on the findings from this review a number of techniques have been invented which substantially improve HF modem performance. An HF modem is confronted with distortion and additive perturbations, i.e. noise and interference. In the past statistical simulators were commonly used to guide the design of HF modems but these risked ignoring pertinent aspects of the channel. Replav simulators, constructed by the author overcame this problem. The design study began br' examining a recently-developed modem thereby taking advantage of much previous research. The modem, employing a parallel-tone waveform and trellis coded modulation. was uncomplicated yet attained high performance amid HF distortion. Having identified. acquired and, where necessary, constructed atl the experimental tools, the problem of combating intederence was addressed. A combination of inærleaving, windowing and the use of an adaptive decoding metric was found to give excellent performance in na¡rowband interference. Experiments using impulsive noise revealed that interleaving with an adaptive metric substantially improved performance. It was then demonstrated that a combination oi these modifications could be incorporated into one modem giving outstanding performance for a wide range of expected HF channel conditions. The approach adopted here, of giving priority to an undersunding of the channei. has resulted in substantial HF modem performance improvements. These ìmprovemenr would not have even been recognised if it had not been for the advanced simulation techniques developed. 1 This work contains no material which has been accepûed for the award of any other degree or diploma in any university or other tertiary institution and, to the best of knowledge and belief, contains no material previously published or written by another person, except where due reference has been made in the æxt. I give consent to this copy of my thesis, when deposited in the University Library, being available for loan and photocopying. 2 GLOSSARY OF TERMS AND ABBREVIATIONS ADC Analogue to Digital Converter AGC Automatic Gain Control APD Amplitude Probability Distribution ARQ Automatic Repeat Request AV/GN Additive Whiæ Gaussian Noise BER Bit Error Rate BFSK Binary Frequency Shift Keyed bps bits per second BPSK Binary Phase Shift Keyed CCIR International Radio Consultative Committee cdf cumulative density function codec Coder and Decoder CM Coded Modulation CSF Channel Scattering Function CW Carier Wave DAC Digital to Analogue Converter dB decibels DOD Department of Defence (U.S.A.) DPSK Differential Phase Shift Keyed DQPSK Differential Quadrature Phase Shift Keyed DSPSK Differential Octonary Phase Shift Keyed DSP Digital Signal Processing DSSS Direct Sequence Spread Spectrum DSTO Defence Science and Technology Organisation (Ausralia) ECC Error Control Coding FSK Frequency Shift Keyed FIR Finite Impulse Response HF High Frcquency (refening to radio \¡/aves 3-30 MHz) Hz Ilerrz IID Independent and Identically Distributed ISI Inter-Symbol Interference ITU International Telecommunications Union kHz kilo-Hertz 3 km kilo-metre kv/ kilo-Watt LPI Low Probability of Intercept MF Medium Frequency (referring to radio waves 0.3 to 3 Mhz) MFSK Multiple Frequency Shift Keyed MHz Mega-Hertz MIL-STD Military Standa¡d modem modulator and demodulator ms milli-second MUF Maximum Useable Frequency NGN non-Gaussian noise NTIA National (U.S.A) Telecommunications and Information Administration OFDM Orthogonal Frequency Division Multiplexed owF Optimum Working Frequency PDD Pulse Duration Distribution pdf probability density function PSD Pulse Spacing Distribution PSK Phase Shift Keyed SIR Signal to Interference Ratio STP Standing Technical Panel TCM Trellis Coded Modulation TTCP The Technical Cooperation Program UT Universal Time VHF Very High Frequency (referring to radio waves 30 to 300 MHz) 4 ACKNOWLEDGMENT During my period of candidature I have been employed as a senior research engineer with Communication Division in Defence Science and Technology Organisation. Australia. As would be expecæd in this position I have worked in collaboration with many other colleagues. The benefits of this arrangement have been the opportunity to achieve much more than an individual alone and to be able to access a large amount of resources to further research objectives. I wish to acknowledge the following people specifically for their advice and suppoft, Dr Stephen Cook, Dr Bruce Davis, lvfartin Gill, Dr Richard Merchant, Mark Preiss, Dr Jason Scholz and Bruce Vyden. This thesis relied heavily on the data collected during a number of radio communication experiments. I wish lo acknowledge the following people and organisations for assistance in the area of radio trials and data collection, Jeff Ball, Trevor Burford, John Dainty, Andrew Dunning, David Krause, George McKenzie, K.L. Nguyen, Mark Preiss, Dr Jason Scholz, Bruce Vyden, the Austalian Army, DSTO's Communications Integration Group, the Royal Australian Nat)' and Rome Laboratories U.S.A.. The specific contributions of the aforementioned people and organisations will be st¿ted in the main body of this thesis.