OFDM/FM FOR MOBILE RADIO DATA COMMUNICATION By Eduardo F. Casas B. A. Sc., The University of Ottawa, 1981 M. Eng., McMaster University, 1983 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE FACULTY OF GRADUATE STUDIES DEPARTMENT OF ELECTRICAL ENGINEERING We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1989 © Eduardo F. Casas, 1989 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of c TVC3-/ £<\a\^e* y The University of British Columbia Vancouver, Canada DE-6 (2/88) Abstract The use of Orthogonal Frequency Division Multiplexing (OFDM) for digital communica• tions over Rayleigh-fading mobile radio channels was proposed by Cimini [1]. OFDM transmits blocks of bits in parallel and reduces the bit error rate (BER) by averaging the effects of fading over the bits in the block. This thesis studies the performance of OFDM/FM, a new modulation technique in which the OFDM baseband signal is used to modulate an FM transmitter. OFDM/FM can be implemented simply and inexpensively by retrofitting existing FM communication systems. Expressions are derived for the BER and word error rate (WER) within a block when each subchannel is QAM-modulated. Several numerical methods are developed to eval• uate the overall BER and WER. An experimental OFDM/FM system was implemented using unmodified VHF FM radio equipment and a fading channel simulator. The BER and WER results obtained from the hardware measurements agree closely with the nu• merical results. The effects of forward error correction (FEC), switching diversity, automatic gain con• trol (AGO, and squelch were tested. A new technique, decision feedback correction (DFC), was developed to reduce the crosstalk interference between the OFDM subchannels. This method significantly improves the BER performance of OFDM/FM. At BERs below 10~2 the experimental OFDM/FM system has better power efficiency than the serial modulation techniques conventionally used for mobile radio (NCFSK, 3 GTFM, or GMSK). At a BER of IO" and a normalized block duration (Tfd) of 2.6, the experimental OFDM/FM system is 5 dB more power efficient than serial techniques. The use of DFC can significantly increase this advantage. However, current GTFM and GMSK systems have better spectral efficiency than the OFDM/FM prototype. ii Table of Contents Abstract ii List of Tables xi List of Figures xii Acknowledgement xv 1 Introduction 1 1.1 Mobile Data Communication 1 1.2 OFDM/FM for Mobile Radio Data Communication 2 1.3 Scope of the Thesis 3 1.4 Organization of the Thesis 4 2 Review of Previous Work 6 2.1 The Mobile Radio Channel 6 2.1.1 Frequency-Selective Fading 8 2.1.2 Log-Normal Fading 9 2.1.3 Noise 9 2.2 Digital Modulation for Mobile Radio 9 2.2.1 Performance Criteria 9 2.2.2 Modulation Suitable for Non-Coherent Receivers 10 2.2.3 Modulation Suitable for Coherent Receivers 10 2.2.4 Spread Spectrum Modulation 10 2.3 Orthogonal Frequency Division Multiplexing 11 iii 2.3.1 Description 11 2.3.2 Implementation of OFDM/FM 12 2.3.3 Advantages of OFDM 13 2.3.4 Disadvantages of OFDM 14 2.3.5 Previous Applications of OFDM 14 2.4 The FM Channel 15 2.4.1 The FM Transmitter 15 2.4.2 The FM Receiver 16 2.4.3 Discriminator Output Noise Spectrum and Probability Distribution . 17 2.4.4 Receiver SN Curves 18 2.4.5 Random FM 20 2.4.6 Capture Effect . 20 2.4.7 Preemphasis and Deemphasis 20 3 Modelling and Error Rate Analysis 22 3.1 Introduction 22 3.2 The Equivalent Baseband Channel Model 22 3.3 BER Analysis 24 3.3.1 Mean and Variance of the Received Value 24 3.3.2 Bit Error Rate Within a Block 24 3.3.3 Evaluating the BER 25 3.4 Modelling the OFDM/FM Channel 25 3.4.1 Baseband Noise Distribution 25 3.4.2 Baseband Noise Spectrum 25 3.4.3 Random FM Noise 26 3.4.4 Clipping Noise 26 3.4.5 Squelch 26 3.4.6 FM Receiver SN Curves 27 iv 3.5 Modelling the OFDM/SSB Channel 28 3.5.1 Frequency and Phase Synchronization 28 3.5.2 AGC 28 3.5.3 Example of SSB Receiver SN Curves 29 3.6 Word Error Rate Analysis 29 4 Numerical Evaluation of Bit and Word Error Rates 31 4.1 Introduction 31 4.2 BER Bounds for Large and Small Blocks 31 4.3 Monte-Carlo Integration of the Block BER Equations 32 4.4 Simulation of the Equivalent Baseband Channel . 33 4.5 Description of Software 34 4.5.1 Fading Waveform Generator 35 4.5.2 Data Generator 35 4.5.3 Noise Generators 36 4.5.4 Bit and Word Error Rate Measurement 36 4.5.5 FEC 37 4.5.6 QAM Encoding/Decoding 37 4.5.7 OFDM Modulation/Demodulation 38 4.5.8 IF SNR to Baseband Signal and Noise Power Conversion 38 4.5.9 Simulated Fading Channel 38 5 Experimental Measurements 39 5.1 Introduction 39 5.2 Experimental Hardware 39 5.2.1 Transmitter 40 5.2.2 Fading Channel Simulator 41 5.2.3 Receiver 43 v 5.2.4 DSP Equipment 44 5.3 Experimental Software 46 5.4 Measuring the Baseband SN Characteristics 46 5.4.1 The Modulating Signal Level 48 5.4.2 The Baseband Frequency Range 48 5.4.3 Software Preemphasis 49 5.4.4 Bit Rate 50 5.4.5 Measuring the IF SNR 50 5.4.6 Converting from IF SNR to E6/N0 51 5.4.7 SN Curve Measurement Method 51 5.4.8 SN Curve Results 53 5.5 BER and WER Measurements 54 5.5.1 BER Results 54 5.5.2 Bound Results 55 5.5.3 Effect of Block Duration and Doppler Rate (T/d) 56 5.5.4 WER Results 57 5.6 Random FM 60 5.7 Probability Distribution of the Baseband Noise 61 5.8 Example of Received Signal Values 62 5.9 Conclusions 62 6 Improving Performance 65 6.1 Introduction 65 6.2 Switching Diversity . 66 6.3 Forward Error Correction Coding 68 6.3.1 Testing the Independent Error Assumption 69 6.3.2 BER Performance of Block Codes 70 6.4 Automatic Gain Control 73 vi 6.5 Squelch 74 6.6 Decision Feedback Correction 76 6.6.1 Description of the Method 76 6.6.2 Simulation Results 80 7 System Design 83 7.1 Introduction 83 7.2 Block Size 83 7.3 Reducing Dependence on Vehicle Speed 84 7.4 Timing, Synchronization and Equalization 84 7.4.1 Sampling Timing 85 7.4.2 Phase Synchronization 86 7.4.3 Equalization 87 7.4.4 Differential Coding 88 7.5 Hardware Cost and Complexity 89 7.6 A/D and D/A Quantization 90 7.7 Sampling Frequency Error and Jitter 91 8 Comparisons With Other Modulation Methods 92 8.1 Introduction 92 8.1.1 Description of Other Modulation Techniques 92 8.2 Power Efficiency 93 8.2.1 GTFM 93 8.2.2 GMSK 93 8.2.3 OFDM/SSB 95 8.2.4 OFDM/FM 95 8.3 Bandwidth Efficiency 95 8.3.1 GTFM/GMSK 95 vii 8.3.2 OFDM/SSB 96 8.3.3 OFDM/FM 96 8.4 Delay 97 8.5 Implementation Considerations 97 8.5.1 OFDM/SSB 98 8.5.2 OFDM/FM 99 8.5.3 GMSK/GTFM 99 8.6 Conclusions 99 9 Conclusions 101 9.1 Conclusions 101 9.2 Topics for Further Study 103 Bibliography 105 Appendices A Conditional Mean and Variance of the Error 115 A.l Notation 115 A.2 Assumptions 115 A.2.1 Data 116 A. 2.2 Noise 116 A.3 Conditional Mean 117 A. 4 The Conditional Variance 118 B The Fading Simulator 121 B. l Introduction 121 B.2 Control Section 122 B. 2.1 Software Description 122 B.2.2 Hardware Description 125 viii B.3 The RF Quadrature Modulator 130 B.3.1 Mixers 130 B.3.2 IF Port Network 132 B.3.3 Splitters 132 B.3.4 Delay Line 132 B.3.5 Construction 132 B. 4 Performance Measurements I33 B.4.1 CPDF 134 B. 4.2 Level Crossing Rate 135 C The Analog/Digital Interface 136 Cl Introduction 136 C. 2 Circuit Description i3^ C. 2.1 Timer Circuit 136 C.2.2 A/D I37 C.2.3 A/D Status I37 C.2.4 Address Decoding and D/A 138 C.2.5 A/D and D/A Analog Interface 139 C.3 Software 140 C.4 Testing 141 C.4.1 Noise 141 C.4.2 Distortion I42 C.4.3 Linearity and Accuracy . I42 C.4.4 Jitter 143 C.4.5 Clock Accuracy I43 C.4.6 Overrun Indicator I43 D RF SNR Measurement I44 ix D.l RF SNR Measurement 144 D.2 Accuracy 145 D.3 Noise Power Measurement 146 D.4 Noise Bandwidth Measurements 147 D.4.1 Spectrum Analyzer IF Filter 148 D.4.2 Receiver IF Filter 149 E Program Listings 151 List of Tables 5.1 Summary of parameter values used in experimental measurements 54 8.1 Performance summary for various modulation techniques 100 E.l Types of listings and compilers used 151 E.2 Routines used for BER evaluations 152 xi List of Figures 1.1 A mobile data terminal (MDT) system 1 1.2 A sample plot of the received signal level 2 2.1 Nomograph showing the relationship between fd,v, and fc 7 2.2 An implementation of an OFDM system 13 2.3 Block diagram of an FM transmitter 16 2.4 Block diagram of an FM receiver 16 2.5 Baseband noise spectra for different IF SNRs 18 2.6 The SN curves of a narrowband FM receiver 19 2.7 Preemphasis filter specification 21 3.1 The equivalent baseband channel model 23 3.2 Example of an FM SN curve 27 3.3 Example of SSB receiver SN curves 29 4.1 The numerical integration method 33 4.2 Simulation using the equivalent baseband channel 34 4.3 QAM Encoding 37 5.1 Block diagram of experimental setup 39 5.2 Experimental transmitter 40 5.3 Fading channel simulator 41 5.4 Experimental receiver 43 5.5 Schematic of audio attenuator for transmitter 45 5.6 Signal processing for experimental measurements 47 xii 5.7 Measured IF filter response 52 5.8 Theoretical and measured SN curves 53 5.9 BER results for fd = 20Hz .