Underwater Acoustic MIMO OFDM: An experimental .Underwater Acoustic MIMO OFDM: An experimental analysis

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  • Underwater Acoustic MIMO OFDM:An experimental analysis

    Guillem PalouAdvisor: Milica Stojanovic

    Massachusetts Institute of Technology

    September 2009

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  • Contents

    List of Figures 4

    Acknowledgements i

    Resum iii

    Abstract v

    I Introduction 1

    1 The Underwater Acoustic Channel 3

    1.1 Attenuation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    1.2 Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    1.3 Multipath . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    1.4 Doppler Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    2 Orthogonal Frequency Division Multiplexing 7

    2.1 OFDM Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    Mathematical description . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    Coding and Interleaving . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    Advantages, Drawbacks and System Design . . . . . . . . . . . . . . . . 11

    2.2 Intercarrier Interference . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    Signal model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

    3 MIMO Systems overview 15

    3.1 Forms of MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    Single-Input Single-Output . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    Single-Input Multiple-Output . . . . . . . . . . . . . . . . . . . . . . . . 16

    Multiple-Input Single Output . . . . . . . . . . . . . . . . . . . . . . . . 16

    Multiple-Input Multiple-Output . . . . . . . . . . . . . . . . . . . . . . . 16

    3.2 The MIMO channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

    3.3 Space Time Coding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

    3.4 MIMO OFDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

    2

  • CONTENTS 3

    II Data detection Algorithms 21

    4 State of the Art of OFDM UWA Systems 23

    Low-complexity OFDM detector . . . . . . . . . . . . . . . . . . . . . . . 23

    Phase tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

    Channel Estimation . . . . . . . . . . . . . . . . . . . . . . . . . . 25

    5 Adaptive Algorithm for MIMO systems 27

    Channel estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

    Channel sparsing . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

    Channel estimated length . . . . . . . . . . . . . . . . . . . . . . 29

    6 ICI Algorithms 31

    6.1 Estimating the channel matrix . . . . . . . . . . . . . . . . . . . . . . . . 32

    Pilot aided estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

    Adaptive Frequency Channel Estimator . . . . . . . . . . . . . . . . . . . 33

    Frequency Domain - Decision Feedback Equalizer . . . . . . . . . . . . . 34

    Taylor approximation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

    6.2 Inverting the channel matrix . . . . . . . . . . . . . . . . . . . . . . . . . 36

    LDLH Factorization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

    LSQR Iterative method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

    Matrix Decoupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

    Jacobi Stationary Iterative Method . . . . . . . . . . . . . . . . . . . . . 38

    Matrix Simplification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

    III Results and Conclusions 41

    7 Results on experimental data 43

    7.1 MIMO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    System description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    Channel sparsing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

    Phase Tracking & Doppler factor . . . . . . . . . . . . . . . . . . . . . . 47

    MSE & BER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

    Environmental correlation . . . . . . . . . . . . . . . . . . . . . . . . . . 52

    7.2 ICI Compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

    Taylor approximation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

    Compensation on SIMO systems . . . . . . . . . . . . . . . . . . . . . . . 54

    8 Conclusions 59

    Bibliography 61

  • List of Figures

    1.1 Absorption coefficient in [dB/km] . . . . . . . . . . . . . . . . . . . . . . . . 4

    1.2 Sources of ambient noise and analytical approximation . . . . . . . . . . . . 4

    1.3 SNR depending on the frequency and transmission distance for a fixed trans-mitted power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

    2.1 Typical block diagram of an OFDM system . . . . . . . . . . . . . . . . . . 7

    2.2 OFDM Signal Spectrum with K = 128 subcarriers . . . . . . . . . . . . . . . 9

    2.3 Example of time interleaving with the original and the interleaved data (topand bottom respectively). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

    2.4 Frequency synchronization in OFDM systems. . . . . . . . . . . . . . . . . . 12

    2.5 Effect of the Doppler spread in the ICI phenomenon . . . . . . . . . . . . . . 13

    3.1 Different forms of MIMO and their configuration . . . . . . . . . . . . . . . 16

    3.2 Simplyfied scheme of the MIMO channel . . . . . . . . . . . . . . . . . . . . 17

    4.1 Example of non-uniform Doppler shift . . . . . . . . . . . . . . . . . . . . . 24

    4.2 Diagram of the algorithm described in [1] . . . . . . . . . . . . . . . . . . . . 24

    6.1 Typical channel matrix for an ICI problem. Dark points mean highest coefficients 32

    6.2 Scheme of a Frequency Domain DFE . . . . . . . . . . . . . . . . . . . . . . 34

    6.3 Example of decoupling the diagonal of the channel matrix . . . . . . . . . . 38

    7.1 Geometry of the experiment. . . . . . . . . . . . . . . . . . . . . . . . . . . 44

    7.2 Wind speed with the wind direction indicated, wave height and wave periodduring the experiment. Stars mark the exact points in time when OFDMsignals were recorded. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

    7.3 Scatter plot for received QPSK and 8-PSK signals . . . . . . . . . . . . . . . 46

    7.4 A typical channel impulse response. . . . . . . . . . . . . . . . . . . . . . . . 47

    7.5 Channel Impulse Response estimated for a different number of threshold.From left to right and top to bottom: no sparsing, 10, 30, 60. . . . . . . . . 48

    7.6 MSE and Coefficients kept (left to right) for a different number of sparsingthresholds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

    7.7 Phases of three experiments with 128 and 1024 subcarriers and 1 transmitter. 49

    7.8 Doppler of three experiments with 1024 subcarriers and 1 transmitter . . . . 49

    7.9 Phases of three experiments with 1024 subcarriers and 2 transmitters . . . . 50

    7.10 MSE for QPSK (top) and 8-PSK (bottom) for varying number of transmitters,MT=1, 2, 3 and 4 from left to right. . . . . . . . . . . . . . . . . . . . . . . 50

    7.11 BER without coding for QPSK (top) and 8-PSK (bottom) for varying numberof transmitters, MT=1, 2, 3 and 4 from left to right. . . . . . . . . . . . . . 51

    4

  • List of Figures 5

    7.12 Wave height for the days of the experiment (top) and MSE (single transmitter,QPSK and 8-PSK, K=128, 256, 512, 1024). . . . . . . . . . . . . . . . . . . 53

    7.13 Autocorrlation of a received signal (QPSK, K = 1024) after FFT demodulation. 547.14 Performance of ICI suppression on a QPSK signal set: linear equalization and

    time-domain channel estimation based on Taylor series model are used. Thebackground light-grey curve corresponds to the ICI equalizer MSE. . . . . . 55

    7.15 Scheme of ICI equalization prior to receiver combination . . . . . . . . . . . 557.16 Scheme of receiver combination before ICI suppression . . . . . . . . . . . . 567.17 Performance of ICI suppression on a SIMO system with a variable number of

    receivers with EGC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577.18 Performance of ICI suppression on a SIMO system with a variable number of

    receivers with MRC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

  • Acknowledgements

    I first would like to thank people who helped me to be able to do this work. Milica,as my advisor, helped me in all the problems and sugggestions I had. Her open mindshowed me how to look at the problems in many different ways, and try to solve them byseeking another perspective. Im also thankful to MIT, specially the Sea Grant College,who offered me a lab and treated me like all the others in the team. My labmates in MIT,Jordi, Willy, Thang and Roman, were wonderful persons to share work with. Neverthe-less, regular meetings at Northeastern University, made the work more interesting, beingable to learn from other people, namely Ashish, Parastoo, Yashar, Rameez, Francesco,Baosheng and Joao.The work would have not be the same if life outside the laboratory didnt exist. SpeciallyI would like to mention my roommate in the best house ever, 357 Columbia St., Jordi.Funny, intelligent and very... lets say critic. I want to mention also