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CONTENTS Introduction System Model Channel Code Design Criterion

APPLICATION OF SPACE-TIME CODING TECHNIQUES IN THIRD GENERATION SYSTEMS - A. G. BURR ADAPTIVE SPACE-TIME SIGNAL PROCESSING AND CODING – A. G. BURR. CONTENTS Introduction System Model Channel Code Design Criterion Application Scenarios for 3-G Networks Optimum Adaptive System.

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CONTENTS Introduction System Model Channel Code Design Criterion

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  1. APPLICATION OF SPACE-TIME CODING TECHNIQUES IN THIRD GENERATION SYSTEMS - A. G. BURRADAPTIVE SPACE-TIME SIGNAL PROCESSING AND CODING – A. G. BURR

  2. CONTENTS • Introduction • System Model • Channel Code Design Criterion • Application Scenarios for 3-G Networks • Optimum Adaptive System

  3. INTRODUCTION • Space-Time Codes - Channel codes designed to approach Shannon capacity for multiple antenna systems without requiring instantaneous channel knowledge at the transmitter. • Advantages • Capacity Improvement / Spectral Efficiency • Provide reliability improvement via diversity

  4. Smart Antennas - form a movable beam pattern that can be steered, using either digital signal processing, or RF hardware, to a desired direction that tracks the mobiles as they move. • Advantages • Multipath/Co-channel Interference Mitigation • Range Extension • Network capacity improvement • Diversity

  5. SYSTEM MODEL • nT transmit antennas, nR receive antennas • S total power transmitted on all elements • Receive Signal Model • r = Hs + n • r - (nR x 1), s - (nT x 1), n – (nR x 1) ~ iid  (0,N) • H, (nR x nT) memoryless, complex channel matrix.

  6. Capacity Gains • Outage capacity is defined as the capacity obtainable in a given • proportion of cases on slowly varying channels. • Capacities calculated as random variables at a certain confidence levels. • Outage capacity gain due to diversity • Use variety of combining methods. • n fold capacity for n antennas where n = min (nT,nR)

  7. Gains proportional to SNR • I = eigenvalues of H

  8. Channel Code Design Criterion • Euclidean distance d2 (D) determines BER of ML receiver. • TX codeword s ~ (nT x m) , m = symbol length • RX codeword r ~ (nR x m) • Codeword Difference Matrix (D) • D = si– sj • ri – rj = H.D • d2(D) = |D.H|2 = trace (A.) • A = D.DH ,  = HH.H • d(D)2 maximized if maximize diagonal elements of A and minimize its off diagonal elements. • Diversity order = rank (A) x nR

  9. APPLICATION SCENARIOS FOR 3-G NETWORKS • Model • WCDMA interface • Short orthogonal spreading codes • Multi-user detection to increase SNR for capacity improvement. • Rake receiver • Multicode transmission • Neglect channel delay spread

  10. SAME DATA, DIFFERENT ORTHOGONAL CODE ON EACH OF NT ANTENNAS (scenario 1) • E.g. • A is full rank, diagonal with an optimum spread of eigenvalues • Maximum diversity of order nT • Can be implemented as orthogonal transmit diversity or different spreading code on each transmit antenna • Increases reliability but does not increase capacity • Receiver implemented as a multicode receiver with diversity combining

  11. DIFFERENT DATA, DIFFERENT CODE ON EACH ANTENNA • (scenario 2) • E.g. • A has unity rank, no diversity improvement • Capacity increase by factor nT • Receiver identical to multicode receiver • No advantage over multicode system with one TX antenna

  12. MULTICODE WITH FEC AND ANTENNA HOPPING (scenario 3) • E.g. • Subsequent code symbols transmitted on different antennas in subsequent code periods • Diversity improvement by factor dmin, also possible coding gain • Net capacity increase from rate of FEC code (R) ~ nTR. • Can use conventional receiver • Different fade states on antennas could cause phase reference problems – Carrier recovery or differential demodulation

  13. DIFFERENT DATA, DIFFERENT ORTHOGONAL CODES, MULTIPLEXED OVER ANTENNAS (scenario 4) • A has full rank and optimum eigenvalue spread • Diversity order nT, capacity increase achieved. • Different antenna paths could cause loss of orthogonality • Need to adapt to distorted code due to channel • Other codes transmitted simultaneously using cyclic shift

  14. DELAY DIVERSITY (scenario 5) • Signal fed to a second antenna with delay of a few chips • Create frequency selective channel from flat channel and use resulting diversity gain with a rake receiver. • No capacity gain • Reduces ability of receiver to cope with real multipath

  15. RESULTS • 4 Tx/4 Rx MIMO, length 16 spreading codes (scheme 4) • single antenna multicode system with 4 spreading codes.

  16. COMMENTS • Could effect a BER reduction, capacity increase or both. • Has no knowledge of channel. • Correlated fading could limit the gains in capacity • Do not provide any directional discrimination. Need smart antennas to do that. • MUD and smart antennas to reduce interference and increase signal power

  17. OPTIMUM ADAPTIVE SYSTEM • Adapt Power in channel by “water-filling” principle • Capacity becomes

  18. Assumes knowledge of Channel • Power directed at user(s) of interest • Beam pattern shows smart antenna property of antenna array being used.

  19. COMMENTS • Adapting power but by beamforming • Significant Gains over Non-Adaptive system even at low SNR

  20. C1 C2 C3 C4 X X1.C1 X2.C2 X3.C3 X4.C4 X1.C1 X2.C2 X3.C3 X4.C4 … … SCENARIO 1 SCENARIO 2 X1.C3 X2.C4 X3.C1 X4.C2 X1.C4 X2.C1 X3.C2 X4.C3 X1.C1/X2.C2/X3.C3/X4.C4 X1.C1/X2.C2/X3.C3/X4.C4 X1.C1/X2.C2/X3.C3/X4.C4 X1.C1/X2.C2/X3.C3/X4.C4 t1 t3 t2 SCENARIO 4 SCENARIO 3

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