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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Simulation Results

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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