GermanAerospace Center
Novi Sad, March 23, 2006
Increasing Transmit Diversity at the Cell Border with Smart Antennas
Simon Plass
German Aerospace Center (DLR)
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GermanAerospace Center
Future communications systems should have frequency reuse of onehigh spectral efficiencylarge performance degradation at the cell border
Mobile terminal at the cell border can be broadcasted by two base stationsCan we increase the transmit diversity anddecrease the inter-cellular interference?
Motivation
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Outline
OFDMA Downlink SystemTransmit Diversity Techniques
• Delay Diversity (DD)• Cyclic Delay Diversity (CDD)
Cellular Cyclic Delay Diversity (C-CDD)
Simulation ResultsConclusions
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OFDMA Downlink System
…… π
Source COD π MOD
OFDM+ TG
-TGIOFDM
Mul
tipat
hC
hann
el
CSI
Sink DECOD π−1 DEMOD π−1
Transmitter
Receiver…
Source COD π MOD
MUX
… …DEMUX
Desired User
User Ku
User 1
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Delay Diversity (DD)
Idea: Increase number of propagation pathsDelay of TX-signal in time domain
OFDMModulation
M
CyclicPrefix
LT
1N
( )s k
T 1( )Ns k−
1( )s k
0 ( )s k
T 1Nδ −
1δ
Advantage: very low complexity, no additional complexity at mobile
Drawback: additional delay
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Channel Transfer Function |H(f,t)|2
Delay spread: Δτ ≈ 0,750 μs Delay-Diversity, Δτ ≈ 1,850 μs
Symbol Duration TU = 224 μs, Carrier Spacing ΔfC = 4,464 kHz, fD = 10 Hz
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From Delay Diversity to Cyclic Delay Diversity (CDD)
OFDM SymbolCyclicPrefix
maxτ
1δ
ISIinfluence of multipath channel
)(0 ks
)(1 ks
DD
Time Window forOFDM Demodulation
0 time index k
OFDM SymbolCyclicPrefix
gN
gN−
OFDM SymbolCyclicPrefix
cyc1δ
CDD
Time Window forOFDM Demodulation
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Cyclic Delay Diversity (CDD)
Cyclic delay of OFDM-symbols in time domain
OFDMModulation
M
LT
1NT
1N
CyclicPrefix
CyclicPrefix
CyclicPrefix
( )s k%
T
cyc1Nδ −
cyc1δ
T 1( )Ns k−
1( )s k
0 ( )s k
Advantage: no additional inter-symbol interferenceno additional complexity at mobile
Drawback: higher complexity compared to DD
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OFDMModulation
CyclicPrefix
OFDMModulation
CyclicPrefix
Cellular Cyclic Delay Diversity (C-CDD)
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Cellular Cyclic Delay Diversity (C-CDD)
No additional complexity at the mobile terminalPossibility of exploiting the unused ‘interfering’sub-carriersReduction of inter-cellular interferenceInherent delay diversity
Network has to distribute the same signal to both base stationsAvailable sub-carriers are needed
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System Parameters & Channel Model
14Taps
IEEE 802.11n CChannel model
0 mph or 100 mphVelocities
30 samplesCyclic delay
Conv. Code, R=1/2Coding
4-QAM or 16-QAMModulation
16OFDM symbols / Frame
2048FFT length
100 MHzBandwidth
5 GHzCarrier frequency
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Impact on Error Performance
Robustness to high velocitiesBest performance with C-CDD at the cell borderPerformance gain and decreased interference with halved TX power
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Impact on Throughput
High throughput gains with C-CDD techniqueReliable throughput throughout the cell for 4-QAM
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Conclusions
Introduction of new cellular transmit diversity technique for the severe cell border area:
Cellular Cyclic Delay Diversity (C-CDD)
C-CDD offers:• Increased transmit diversity at the cell border• No additional complexity for the mobile terminal • Robustness to different velocities• Reliable throughputs all-over the cell edge • Reliable handoff procedures