Transcript APPLICATION OF SPACE-TIME CODING TECHNIQUES IN …

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
• 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.
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
•
2.
Use variety of combining methods.
n fold capacity for n antennas where n = min (nT,nR)
• Gains proportional to SNR
 S 
S

H
C  W  log2 1  i

W
log
I

H
H

2
n
N
nN


i 1
n
•I = eigenvalues of H
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 – s j
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
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 NT
ANTENNAS (scenario 1)
E.g.
1
1
D
1

1
1
1
1
1
1
1
1
1
1
 1
1

 1
A  16I
• 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
DIFFERENT DATA, DIFFERENT CODE ON EACH ANTENNA
(scenario 2)
E.g.
0
0
D  2
1

0
0
0
1
0
0
0
1
0
0
0
1

0
A  16I
• 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
MULTICODE WITH FEC AND ANTENNA HOPPING (scenario 3)
E.g.
0

0
D  2
1

0
0
0
0 0
0
0
0
1
0
1
0 1
1 0
1
0
1
0
0
0
0 0
0
0
0

1
0

0
0
0
A  4
0

0
0
0
4
0
0
4
0
0
0
0
0

0
• 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
DIFFERENT DATA, DIFFERENT ORTHOGONAL CODES,
MULTIPLEXED OVER ANTENNAS (scenario 4)
1

0
D  2
0

0
1
0
0 0
0
0
0
0
1
0
1 0
0 1
0
1
0
0
0
0
0 0
0
1
0

0
0

 1
A  8I
• 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
DELAY DIVERSITY (scenario 5)
0
D  2
0
0
,

1
0
A
0
0
4
• 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

N 
si  max P 
,0 
i 


N 
S   max P 
,0 
i 
i 1

n
with
•Capacity becomes
Si 

C  W  log2 1  i

N 

i 1
n
• 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
X
C1
X1.C1
C2
X2.C2
C3
X3.C3
C4
X4.C4
SCENARIO 1
X1.C1
X2.C2
X1.C4
X1.C3
X2.C1
X2.C4
…
X3.C3
X4.C4
t1
SCENARIO 2
…
X3.C2
X3.C1
X4.C3
X4.C2
t2
SCENARIO 3
t3
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
SCENARIO 4