APPLICATION OF SPACE-TIME CODING TECHNIQUES IN THIRD GENERATION SYSTEMS - A. G. BURR ADAPTIVE SPACE-TIME SIGNAL PROCESSING AND CODING – A. G. BURR.
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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
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
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
SYSTEM MODEL n T transmit antennas, n R receive antennas S total power transmitted on all elements Receive Signal Model r = Hs + n r - (n R x 1), s - (n T x 1), n – (n R x 1) ~ iid (0,N) H, (n R x n T ) memoryless, complex channel matrix.
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. 1. Outage capacity gain due to diversity Use variety of combining methods. 2. n fold capacity for n antennas where n = min (n T,n R )
Channel Code Design Criterion Euclidean distance d 2 (D) determines BER of ML receiver. TX codeword s ~ (n T x m), m = symbol length RX codeword r ~ (n R x m) Codeword Difference Matrix (D) D = s i – s j r i – r j = H.D d 2 (D) = |D.H| 2 = trace (A. ) A = D.D H, = H H.H d(D) 2 maximized if maximize diagonal elements of A and minimize its off diagonal elements. Diversity order = rank (A) x n R
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
SAME DATA, DIFFERENT ORTHOGONAL CODE ON EACH OF N T ANTENNAS (scenario 1) E.g. A is full rank, diagonal with an optimum spread of eigenvalues Maximum diversity of order n T 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
DIFFERENT DATA, DIFFERENT CODE ON EACH ANTENNA (scenario 2) E.g. A has unity rank, no diversity improvement Capacity increase by factor n T Receiver identical to multicode receiver No advantage over multicode system with one TX antenna
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 d min, also possible coding gain Net capacity increase from rate of FEC code (R) ~ n T R. Can use conventional receiver Different fade states on antennas could cause phase reference problems – Carrier recovery or differential demodulation
DIFFERENT DATA, DIFFERENT ORTHOGONAL CODES, MULTIPLEXED OVER ANTENNAS (scenario 4) A has full rank and optimum eigenvalue spread Diversity order n T, 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
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
RESULTS 4 Tx/4 Rx MIMO, length 16 spreading codes (scheme 4) single antenna multicode system with 4 spreading codes.
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
OPTIMUM ADAPTIVE SYSTEM Adapt Power in channel by “water-filling” principle Capacity becomes
Assumes knowledge of Channel Power directed at user(s) of interest Beam pattern shows smart antenna property of antenna array being used.
COMMENTS Adapting power but by beamforming Significant Gains over Non-Adaptive system even at low SNR