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SDSU Template, Version 11.1 AN HF MULTITONE MODEM _______________ A Thesis Presented to the Faculty of San Diego State University _______________ In Partial Fulfillment of the Requirements for the Degree Master of Science in Electrical Engineering _______________ by Louis A. Rey Summer 2017 iii Copyright © 2017 by Louis A. Rey All Rights Reserved iv DEDICATION This thesis is dedicated to my wife, two daughters and all who have loved and supported me throughout this journey. v ABSTRACT OF THE THESIS An HF Multitone Modem by Louis A. Rey Master of Science in Electrical Engineering San Diego State University, 2017 In today’s world, we can find signals being transmitted on every portion of the frequency spectrum from a few Hertz up to several Giga Hertz. Frequencies of upwards of 60GHz are common today. Although it is now common to operate in ever higher frequencies, High Frequency (HF) frequencies (3 to 30 MHz) enjoyed huge popularity during the advent of early long-range communication work. Before satellites were invented, it was the only means of transmitting information at long distances wirelessly. Initial work in the defense industry gave fruit to Multitone modems which were then superseded by single tone modems for both data and voice communications at 2400bps, 9600bps and 19400bps. With the growing popularity of digital signal processing and software radio, these modems are now easily implemented. This thesis revisits Multitone modems, also known as parallel tone modems, which use 16 and 39 tones for data and voice respectively. A comparison with modern communication systems will be explored. A description of the Modem sections will be created using MATLAB. All information pertaining to the Multitone modem can be found in the public domain and fully declassified. Any detailed classified information is excluded from this thesis. vi TABLE OF CONTENTS PAGE ABSTRACT ...............................................................................................................................v LIST OF TABLES ................................................................................................................. viii LIST OF FIGURES ................................................................................................................. ix ACKNOWLEDGEMENTS ..................................................................................................... xi CHAPTER 1 INTRODUCTION .........................................................................................................1 1.1 Problem Statement .............................................................................................1 1.2 Motivation ..........................................................................................................1 1.3 Thesis Outline ....................................................................................................2 2 TECHNICAL OVERVIEW ...........................................................................................3 2.1 Overview and History ........................................................................................3 2.1.1 Modern HF Communications ...................................................................4 2.2 HF Propagation ..................................................................................................4 2.3 Skywave Propagation.........................................................................................5 2.3.1 Channel Effects on HF Signals .................................................................8 2.4 The Watterson HF Channel Model ..................................................................10 2.5 HF Modem Solutions .......................................................................................11 2.6 Modulation Used ..............................................................................................13 2.7 Challenges ........................................................................................................14 3 MODEM DESCRIPTION ...........................................................................................16 3.1 Modem Requirements ......................................................................................17 3.2 OFDM Modulation ..........................................................................................20 3.3 Software Implementation .................................................................................22 4 HF MODEM STANDARD AND IMPLEMENTATION ...........................................24 4.1 The Transmitter ................................................................................................24 vii 4.1.1 The Channel Model.................................................................................27 4.2 Preamble Synchronization ...............................................................................28 4.3 The Data Payload .............................................................................................34 5 MODERN COMMUNICATION SYNCHRONIZATION METHODS .....................37 5.1 Timing Synchronization...................................................................................37 5.2 Frequency Synchronization .............................................................................40 6 RESULTS ....................................................................................................................43 7 FUTURE WORK .........................................................................................................45 REFERENCES ........................................................................................................................46 viii LIST OF TABLES PAGE Table 2.1. Typical HF Fast Fading Periods ...............................................................................8 Table 3.1. Frequency and Initial Phases for the First and Second Part of the Modem Preamble ......................................................................................................................18 Table 3.2. Frequency and Initial Phases of the 16 Tones for the Parallel Tone Modem .........18 Table 3.3. 15 Bit PN Tone Sequence for Each of the 16 Tones ..............................................19 Table 3.4. Dibit Grouping of the 32 Bit Data Frame ...............................................................20 ix LIST OF FIGURES PAGE Figure 2.1. Ionospheric regions throughout the day. .................................................................6 Figure 2.2. Signal path as a function of angle of incident. ........................................................7 Figure 2.3. The Watterson HF Channel model. .......................................................................11 Figure 2.4. Differential Quadrature Phase shift keying block diagram ...................................13 Figure 2.5. DQPSK Constellation Diagram. ............................................................................14 Figure 3.1. Block diagram of Data Modem. ............................................................................16 Figure 3.2. Modem preamble for the 16 tone parallel tone modem. .......................................17 Figure 3.3. OFDM orthogonal subcarriers. ..............................................................................21 Figure 3.4. OFDM transmission structure. ..............................................................................22 Figure 4.1. Time and frequency domain representation of the first part of the preamble. ......................................................................................................................25 Figure 4.2. Time and frequency domain representation of the second part of the preamble. ......................................................................................................................25 Figure 4.3. Time and frequency domain representation of the third part of the preamble. ......................................................................................................................26 Figure 4.4. Time and frequency domain representation of the fourth part of the preamble. ......................................................................................................................27 Figure 4.5. Low pass Chebyshev2 Filter Response. ................................................................28 Figure 4.6. Signal Presence Detector. ......................................................................................29 Figure 4.7. Channel effects on the four tone preamble. ...........................................................29 Figure 4.8. Signal Presence Detector Output. ..........................................................................30 Figure 4.9. Doppler Estimation Block. ....................................................................................31 Figure 4.10. Doppler Estimation of offset signal compared to original signal. .......................31 Figure 4.11. Autocorrelation of the three modulated tones. ....................................................32 Figure 4.12. Autocorrelation output of the three modulated tones. .........................................32 Figure 4.13. Composite 16 tone signal of modem preamble. ..................................................33 x Figure 4.14. Phase offset correction of composite 16 tone signal of modem preamble. .........33 Figure 4.15. Time and frequency spectrum of PN sequence. ..................................................34 Figure 4.16. Correlation of PN sequence with match filter. ....................................................35
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