SIMULATION OF PHYSICAL LAYER IMPAIRMENTS ON COMMUNICATION SYSTEM _______________ 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 Anupama Prasad Fall 2012 iii Copyright © 2012 by Anupama Prasad All Rights Reserved iv ABSTRACT OF THE THESIS Simulation of Physical Layer Impairments on Communication System by Anupama Prasad Master of Science in Electrical Engineering San Diego State University, 2012 A practical software radio system (SDR) typically suffers from a number of impairments in its migration from a baseband signal at the transmitter to its replication at the receiver. This gives rise to error generation and degradation in overall system performance. Analog impairments at the intermediate frequency (IF) can have many of the same issues as radio frequency (RF) signals and they also include modulator and demodulator mismatches between the I and Q-channels. These RF and analog impairments are not confined to SDR systems but may be found in any digital radio system. The fact that the SDR uses such a large portion of digital logic and direct conversion receiver (DSP) introduces additional impairment concerns that may not be found in a traditional radio design. The digital radio receiver’s main function is to extract the variable RF signals in the presence of interferences and transform them into a close replica of the original signal that is the baseband signal. Therefore the main objective of this thesis is to help one understand the impact of measure and visualize implementation impairments related to both the transmitter and the receiver at all stages of a communication system and how they affect the overall accuracy of the system and its effect on the performance. Towards this end a basic communication system was set up with ability to insert and simulate user selectable levels of multiple implementation impairments and software was created to measure and identify the most common impairments in the design and results are presented through constellation plots, eye diagrams and power spectrum. v TABLE OF CONTENTS PAGE ABSTRACT ............................................................................................................................. iv LIST OF TABLES .................................................................................................................. vii LIST OF FIGURES ............................................................................................................... viii LIST OF ABBREVIATIONS ....................................................................................................x ACKNOWLEDGEMENTS .................................................................................................... xii CHAPTER 1 INTRODUCTION .........................................................................................................1 2 THE COMMUNICATION CHANNEL ........................................................................3 3 COMMUNICATION SYSTEM ARCHITECTURE ....................................................5 3.1 Tx Architecture ..................................................................................................5 3.1.1 Direct (Homodyne) Up Conversion ......................................................... 6 3.1.2 IF (Heterodyne) Up Conversion .............................................................. 8 3.2 Rx Architecture ..................................................................................................9 3.2.1 Super-Heterodyne Architecture ............................................................. 10 3.2.2 Direct Conversion Architecture or Zero-IF Architecture ...................... 11 3.2.2.1 Merits of Zero-IF Architecture ..................................................... 12 3.2.2.2 De-Merits of Zero-IF Architecture ............................................... 12 3.2.3 Wideband-IF Architecture ..................................................................... 12 3.2.4 Low-IF Architecture .............................................................................. 13 4 MODULATION TECHNIQUES AND APPLICATIONS .........................................15 4.1 QPSK ...............................................................................................................15 4.2 QAM ................................................................................................................18 4.2.1 Analog QAM ......................................................................................... 18 4.2.2 Digital/Quantised QAM ......................................................................... 21 4.2.3 Drawbacks of QAM ............................................................................... 21 4.2.4 Comparison of QAM with Other Modes ............................................... 22 4.2.5 Constellation Diagrams for QAM .......................................................... 23 vi 5 SYSTEM MODEL OF MODULATOR AND DEMODULATOR AND THEIR IMPERFECTIONS .........................................................................................24 6 IMPAIRMENTS IN THE COMMUNICATION CHANNEL ....................................26 6.1 Hardware Independent Impairments ................................................................27 6.1.1 Additive White Gaussian Noise (AWGN) ............................................. 27 6.1.2 Multi Path............................................................................................... 27 6.1.3 Phase/Frequency Offset ......................................................................... 29 6.1.3.1 Frequency Offset ........................................................................... 29 6.1.3.2 Phase Offset (Phase Error) ............................................................ 30 6.1.4 Symbol Rate ........................................................................................... 30 6.2 Hardware Inherent Impairments ......................................................................31 6.2.1 Non-Linearity ......................................................................................... 32 6.2.2 Power Amplifier Non-Linearities .......................................................... 32 6.2.3 Phase Noise ............................................................................................ 32 6.2.4 Quadrature Skew .................................................................................... 34 6.3 Architecture Specific Impairment ....................................................................34 6.3.1 DC Offset ............................................................................................... 34 6.3.2 IQ Imbalance .......................................................................................... 36 6.3.3 I Q-Gain Imbalance................................................................................ 38 6.3.4 IF Filter Ripple or Tilt............................................................................ 41 6.3.5 Group Delay ........................................................................................... 41 6.3.6 Baseband Filtering Problem ................................................................... 44 7 SIMULATION OF THE PHYSICAL LAYER IMPAIRMENT .................................48 7.1 Model Set Up ...................................................................................................48 7.2 Working of Graphical User Interface ..............................................................52 7.3 Significance of Constellation and Eye Diagram ..............................................52 7.4 Simulation Results ...........................................................................................53 8 CONCLUSION AND FUTURE WORK ....................................................................58 BIBLIOGRAPHY ....................................................................................................................59 vii LIST OF TABLES PAGE Table 4.1. Summary of Types of Modulation with Data Capacities .......................................22 viii LIST OF FIGURES PAGE Figure 2.1. Block diagram of a digital radio communication system ........................................4 Figure 3.1. Block diagram of communications system..............................................................5 Figure 3.2. Components of an RF Tx. .......................................................................................5 Figure 3.3. Block diagram of direct up conversion RF Tx. .......................................................6 Figure 3.4. Hardware diagram of direct up conversion. ............................................................7 Figure 3.5. Frequency domain of direct up conversion to RF ...................................................7 Figure 3.6. Hardware diagram of direct down conversion. .......................................................8 Figure 3.7. Block diagram of homodyne up converter. .............................................................8 Figure 3.8. Frequency domain of heterodyne up conversion to RF ...........................................9 Figure 3.9. Block diagram of a typical heterodyne Rx. ...........................................................10 Figure 3.10. Block diagram of DCR architecture. ...................................................................11 Figure 3.11. Block diagram of wideband-IF Rx architecture. .................................................13