HF Communications: A Systems Approach

201 Pages·1987·11.74 MB·English

by  Nicholas M Maslin

HF Communications:
A Systems Approach
Nicholas M Maslin
MA, PhD, CEng, MIERE
Principal Consultant, Software Sciences Ltd.
Pitman
PITMAN PUBLISHING
128 Long Acre, London WC2E 9AN
© N M Maslin 1987
First published in Great Britain 1987
This edition published in the Taylor & Francis e-Library, 2005.
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British Library Cataloguing in Publication Data
Maslin, N.M.
HF communications: a systems approach.
1. Telecommunication systems
I. Title
621.38 TK5101
ISBN 0-273-02675-5
All rights reserved. No part of this publication may be reproduced,
stored in a retrieval system, or transmitted, in any form or by any
means, electronic, mechanical, photocopying, recording and/or
otherwise without the prior written permission of the publishers.
This book may not be lent, resold, hired out or otherwise disposed of
by way of trade in any form of binding, or cover other than that in
which it is published, without the prior consent of the publishers.
ISBN 0-203-16889-5 Master e-book ISBN
ISBN 0-203-26418-5 (Adobe eReader Format)
ISBN 0 273 02675 5 (Print Edition)
Contents
Preface
1 HF Radio: Past and Present 1
1.1 Early Developments 1
1.1.1 Origins of long-range communications 1
1.1.2 Experiments with the short waveband 2
1.1.3 HF gains the upper hand 3
1.2 Changes in Fortune 4
1.2.1 The reliance upon the operator 4
1.2.2 The fall from favour 4
1.2.3 A resurgence of interest 5
1.3 Present Usage 5
1.3.1 HF spectrum allocation 5
1.3.2 Non-military users 6
1.3.3 Military users 7
1.3.4 Evolving technology 8
2 System Considerations 9
2.1 Concepts 9
2.1.1 The systems approach 9
2.1.2 System characteristics 9
2.1.3 Design considerations 10
2.2 Radio Communications Systems 11
2.2.1 System definition 11
2.2.2 System decomposition 12
2.2.3 Open systems interconnection 13
2.2.4 Transmitting sub-system 13
2.2.5 Channel sub-system 14
2.2.6 Receiving sub-system 15
2.2.7 Design considerations 16
2.3 HF Communications Systems 17
2.3.1 Propagation characteristics 17
2.3.2 A unique role for HF 18
iv
2.3.3 System design overview 20
2.3.4 The power level diagram 21
3 Ground Wave Propagation 23
3.1 Propagation over a Plane Earth 23
3.1.1 Free space propagation 23
3.1.2 Electrical characteristics of the ground 23
3.1.3 Ground wave components 24
3.2 The Surface Wave 24
3.2.1 General principles 24
3.2.2 Zonal relationships 25
3.2.3 Effect of antenna height 26
3.3 The Space Wave 26
3.3.1 Zonal relationships 26
3.3.2 Effect of antenna height 29
3.4 Deviations from Simplified Model 29
3.4.1 General considerations 29
3.4.2 Ground conductivity 30
3.4.3 Terrain irregularities 31
3.4.4 Shadowing 31
3.4.5 Mountainous terrain 32
3.4.6 Vegetation 32
3.5 Field Strength Computation 33
3.5.1 The prediction problem 33
3.5.2 Field strength values 33
3.5.3 Variability of ground conditions 33
3.5.4 Mixed propagation paths 35
3.5.5 Sea state 35
3.5.6 Obstacles 36
3.5.7 Vegetation 37
4 Sky Wave Propagation 41
4.1 The Ionosphere 41
4.1.1 Structure 41
4.1.2 Ionisation 42
4.1.3 D-region 42
4.1.4 E-region 42
4.1.5 F-region 43
4.1.6 Ionospheric disturbances 43
v
4.2 Wave Propagation in the Ionosphere 44
4.2.1 Physical processes 44
4.2.2 Reflection at oblique incidence 45
4.2.3 Ray paths 46
4.2.4 Virtual height 46
4.2.5 Maximum usable frequency 47
4.2.6 Lowest usable frequency 48
4.2.7 Effect of the Earth’s magnetic field 49
4.3 Variation of the Critical Frequency 49
4.3.1 Typical values 49
4.3.2 Solar cycle dependence 50
4.3.3 Annual, seasonal and diurnal variations 50
4.4 Characteristics of the Received Signal 52
4.4.1 Components 52
4.4.2 Multipath propagation and time dispersion 55
4.4.3 Fading 57
4.4.4 Frequency dispersion 58
4.4.5 Delay distortion 59
4.5 Nearly Vertically Incident Sky Waves 60
4.5.1 The need for short-range sky wave links 60
4.5.2 Selecting the frequency 60
4.5.3 Signal strength 61
4.5.4 Dependence upon range 61
4.5.5 Mode structure and multipath 61
4.5.6 Wave polarisation 62
4.5.7 Summary of attributes 63
5 Noise and Interference 65
5.1 Noise 65
5.1.1 Sources 65
5.1.2 Noise power 65
5.1.3 Atmospheric noise 66
5.1.4 Man-made noise 67
5.1.5 Galactic noise 67
5.1.6 Statistical variations 67
5.1.7 Condition for external noise limitation 68
5.2 Interference 68
5.2.1 Sources 68
vi
5.2.2 Magnitude of effects 69
5.2.3 Variation with time and frequency 69
5.2.4 Variation with bandwidth 70
5.3 Local Noise and Interference Effects 71
5.3.1 Sources 71
5.3.2 Precipitation static 73
5.3.3 Local electromagnetic interference 73
5.3.4 Electromagnetic compatibility 74
5.3.5 Magnitude of effects 74
5.4 Audio Noise 75
5.4.1 Sources 75
5.4.2 Effects upon speech transmission 75
5.4.3 Effects upon speech reception 75
6 System Performance Assessment 76
6.1 Antenna Considerations 76
6.1.1 Matching 76
6.1.2 Gain and directivity 76
6.1.3 Polarisation 77
6.1.4 Arrival angles 77
6.1.5 Transmitting antennas 78
6.1.6 Receiving antennas 79
6.1.7 Diversity operation 80
6.2 Signal-to-Noise Ratios 81
6.2.1 The received signal-to-noise ratio 81
6.2.2 The required signal-to-noise ratio 81
6.2.3 The median signal-to-noise ratio 81
6.2.4 Variability of signal-to-noise ratio 82
6.3 Circuit Performance Criteria 83
6.3.1 Ground wave variability 83
6.3.2 Sky wave variability 84
6.3.3 Sky wave availability 84
6.3.4 Circuit reliability factor 85
6.3.5 Effect of signal-to-noise on reliability 85
6.4 Ground Wave Performance 86
6.4.1 Field strength and received power 86
6.4.2 Baseline assessment 87
6.4.3 The range factor 88
vii
6.4.4 Effect of short antennas 89
6.5 Sky Wave Performance 91
6.5.1 Representative example 91
6.5.2 Monthly signal-to-noise ratios 92
6.5.3 Frequency dependence 92
6.5.4 Assessment of reliability improvement 93
7 Air-Ground Communications 96
7.1 The Mobile User 96
7.1.1 Comparison with point-to-point circuits 96
7.1.2 Parameters critical to the mobile user 96
7.2 Characteristics of the Airborne Terminal 97
7.2.1 Antenna radiation efficiency 97
7.2.2 Antenna radiation patterns 98
7.2.3 Aircraft generated noise 99
7.2.4 Flight paths and frequency selection 99
7.3 Impact upon System Performance 100
7.3.1 Effect of antenna efficiency 100
7.3.2 Effect of aircraft noise 100
7.3.3 Effect of frequency choice 101
7.3.4 Effect of flight path 102
7.4 Communications with Small Aircraft 103
7.4.1 Summary of system problems 103
7.4.2 Performance predictions 103
7.4.3 Air-to-ground links 103
7.4.4 Advantage of multi-ground station usage 105
7.4.5 Ground-to-air links 105
7.5 Guidance for the System Designer 107
7.5.1 Criteria 107
7.5.2 Frequency management 109
7.5.3 Antennas 109
7.5.4 Aircraft noise 109
8 Frequency Management 110
8.1 Techniques 110
8.2 Ionospheric Predictions 110
8.2.1 Predicting the frequency window 110
8.2.2 MUF prediction 111
8.2.3 LUF prediction 112
viii
8.2.4 Example format of long-term predictions 112
8.2.5 Limitations of long-term predictions 113
8.2.6 Short-term predictions 114
8.3 Sounding 114
8.3.1 Types of technique 114
8.3.2 Pulse sounding and ionograms 115
8.3.3 Comparison of sounding techniques 116
8.3.4 Linear sweep sounding (chirpsounding) 118
8.3.5 Channel sounding 119
8.4 Real Time Channel Evaluation (RTCE) 119
8.4.1 The need for RTCE 119
8.4.2 The nature of RTCE 121
8.4.3 Automation of system control 121
8.4.4 Matching the channel to the medium 122
8.4.5 An air-ground link example 122
8.4.6 Potential advantages of RTCE 124
9 Data Communications 125
9.1 General Considerations 125
9.2 Digital Modulation Techniques 126
9.2.1 Terminology 126
9.2.2 Phase discontinuous schemes 126
9.2.3 Phase continuous schemes 130
9.2.4 Primary modulation schemes for HF systems 131
9.3 Error Rate Performance 132
9.3.1 Error probability 132
9.3.2 Non-fading signals 132
9.3.3 Fading signals 133
9.3.4 Use of diversity reception 134
9.3.5 Non-ideal receivers 135
9.3.6 Effect of non-Gaussian noise 135
9.3.7 Multipath and interference considerations 136
9.4 High Data Rate Transmissions 137
9.4.1 Types of technique 137
9.4.2 Parallel data modulation methods 138
9.4.3 Equalisation techniques 139
9.4.4 Comparison of techniques 140
9.5 Wide Bandwidth Techniques 141
ix
9.5.1 General limitations 141
9.5.2 Channel dispersion effects 142
9.5.3 Matched filter/wideband correlation 142
9.5.4 Matched filter/signal gating 143
9.5.5 Direct sequence spread spectrum 144
9.5.6 Narrowband interference 144
9.6 Principles of Error Control Coding 145
9.6.1 Causes of data errors 145
9.6.2 Types of coding 145
9.6.3 Block coding, with hard decision 146
9.6.4 Convolutional coding, with hard decision 146
9.6.5 Detection and correction of error bursts 147
9.6.6 Soft decision decoding 148
9.7 Performance of Error Control Coding 149
9.7.1 Effect of error probability 149
9.7.2 Coding gain 149
9.7.3 Analysis of examples 149
9.7.4 Some practical considerations 150
9.7.5 Time delays 151
9.7.6 Interleaving 152
9.7.7 Erasure decoding and soft decision decoding 153
10 Impact of Modern Technology 154
10.1 Overview 154
10.2 Hardware Evolution 155
10.2.1 Receivers 155
10.2.2 Transceivers 155
10.2.3 Power amplifiers 156
10.2.4 Synthesisers 156
10.2.5 Transmitting antennas 156
10.2.6 Receiving antennas 157
10.3 Signal Processing 157
10.3.1 Lincompex 157
10.3.2 Digital receive modules 157
10.3.3 Signal detection 158
10.3.4 Signal identification 158
10.3.5 Transmit processing 159
10.4 Interference Reduction 159