A Smart Switched Sector Array for Wideband Interference ...

1726+,%$�5HVHDUFK�(XURSH�/LPLWHG ConfidentialConfidential

3rd IEEE Workshop on WLAN, 27 Sept 2001, Newton MA

A Smart Switched Sector Array for A Smart Switched Sector Array for Wideband Interference Mitigation in an Wideband Interference Mitigation in an

OFDMOFDM--based WLANbased WLAN

Hidehiro Matsuoka, Yong Sun, Steve Parker and Chris Simmonds

Toshiba Research Europe Limited, UK



OutlineOutline

• Introduction• Simulation environment• BER performance with or without interference• Switching success probability• Capacity improvement• Conclusions



IntroductionIntroduction

• 2.4GHz and 5GHz WLAN system will provide high-rate data services

• WLAN service area can cover office, home, conference hall, station, airport, etc.

• OFDM-based wireless LAN provides robustness to multipath

IP Network RT

Service Area

MT

MT

AP



Future problem and solutionFuture problem and solution

In the Near Future• Due to the limited spectrum allocation and predicted uptake of

WLANs, co-channel interference will become an issue

Solution• Antenna technology for interference reduction

– Switched sector array (simple structure solution)

– Beam- and null-forming (good performance)

Competing MT

AP



Antenna pattern and Rx structureAntenna pattern and Rx structure

0.2

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180 0

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180 0

RSSI measurementFor each sector

SectorSwitch

Dem

SWcontrol



Spatial channel modelSpatial channel model

8 SectorSwitch

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-10.0

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Delay ns

Pow

erdB

ETSI-B channel model(each path varies basedon Rayleigh fading)

18 paths are spatially distributed at random over time (frame by frame)

RSSI measurementRSSI measurement






Simulation ParametersSimulation Parameters

The same channel property as required signalsInterference

8 element circular sector array (90o cosine beam)Antenna array

Largest measured RSSI selectionSector switching method

At random on a frame by frame basisSpatial signal distribution

ETSI-B (Excess Delay: 720nsec, Delay Spread: 100nsec)Channel model

312.5kHzSubcarrier spacing

Preamble(4symbols) + payload(180symbols)Frame structure

4usec (0.8usec cyclic prefix)Symbol interval

52 … 48 (data subcarriers) [No DC component]

… 4 (pilot at positions 7, -7, 21, -21)

Number of subcarrier

OFDM (Uncoded QPSK and 16QAM for each subcarrier)

Modulation scheme

ValueParameters



BER performance (ETSIBER performance (ETSI--B channel)B channel)

0 5 10 15 20 25 3010

-6

10-5

10-4

10-3

10-2

10-1

100

Eb/No [dB]

Unc

oded

Bit

Err

or R

ate

(BE

R)

QPSK(omni)16QAM(omni)QPSK(sector)16QAM(sector)10dB SNR

improvement compared to single omni-directional system 10dB



BER performance with multipath BER performance with multipath interference (ETSIinterference (ETSI--B channel)B channel)

0 5 10 15 20 25 3010

-6

10-5

10-4

10-3

10-2

10-1

100

Eb/No (dB)

Unc

oded

Bit

Err

or R

ate

(BE

R)

SIR = 0 dBSIR = 3 dBSIR = 10 dBSIR = 20 dB

Interference will deteriorate the BER performance even using switched sector array



SIR robustness over omniSIR robustness over omni--directional directional systemsystem

0 2 4 6 8 10 12 14 16

10-4

10-3

10-2

10-1

100

SIR [dB]

Irre

duci

ble

BE

R

SW (multipath Interference)Omni (multipath Interference)

10-5

Switched sector array based on RSSI (SW) can improve SIR robustness by 2-3dB compared to single omni-directional system

Irreducible BER:is the value of BERfloor at Eb/No = ∞



Switching success probability in ETSISwitching success probability in ETSI--B B channel with interferencechannel with interference

0 5 10 15 20 25 300

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Eb/No (dB)

Pro

babi

lity

SIR = 0 dBSIR = 3 dBSIR = 10 dBSIR = 20 dBSuccessful

switching probability depends on SIR and SNR



Switching method comparisonSwitching method comparison

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10-5

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10-1

100

Eb/No (dB)

Unc

oded

Bit

Err

or R

ate

(BE

R)

Omni antennaSWOptimal switchingWorst switching

OptimalSwitching:

WorstSwitching:

selects the sectorthat providesthe best BER

selects the sectorthat providesthe worst BER Signal Channel: ETSI-B

Interference Channel: ETSI-BSIR = 10dB



Capacity improvementCapacity improvement

10 12 14 16 18 20 22 24 26 28 3010

-4

10-3

10-2

Eb/No (dB)

Unc

oded

Bit

Err

or R

ate

(BE

R)

BPSK (Omni antenna)BPSK (SW on RSSI)16QAM (optimal SW)64QAM (optimal SW)

Signal Channel: ETSI-BInterference Channel: ETSI-BSIR = 10dB

Approximately five-fold data rate improvement can be expected using optimum switchingCompared to single omni-directional system



Approach to optimum switchingApproach to optimum switching

• Successful switching probability based on RSSI measurement is strongly dependent on SIR rather than SNR

• Interference detection is useful for switching criteria in the presence of strong interference



0 5 10 15 20 25 300

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Eb/No (dB)

Pro

bab

ility

of

succ

ess

swit

chin

g

Interference detection

RSSI-based SW

S: 2path channelI: 2path channel

Successful switching probability is improved at low SIR

Switching success probability using Switching success probability using interference detectioninterference detection--based switchingbased switching



BER performance comparisonBER performance comparison

0 5 10 15 20 25 3010

-5

10-4

10-3

10-2

10-1

100

Eb/No (dB)

Un

cod

ed B

it E

rro

r R

ate

SIR = 15dB

SIR = 3dB


Switching method based on interference detection can approach the optimum sector switching



ConclusionsConclusions

• Performance of switched sector array for OFDM system in the environment with multipath interference was shown:

– 10dB SNR improvement over an omni-directional system– Two-fold SIR improvement over an omni-directional system– Successful RSSI-based switching depends on the prevailing

SIR.– Five-fold increase in data rate using the optimum sector

selection– Switching method based on interference detection can

approach the optimum sector switching at low SIR conditions



Switching success probability using Switching success probability using interference detectioninterference detection--based switchingbased switching

- Successful switching probability is improved at low SINR- Correct switching depends on multipath

Eb/No (dB)

Pro

bab

ility

Eb/No (dB)

Pro

bab

ility


S: ETSI-B channelI: ETSI-B channel

A Smart Switched Sector Array for Wideband Interference ...

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