1 A Differential OFDM Approach to Coherence Time Mitigation in DSRC Youwei Zhang, Ian Tan, Carl Chun...

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A Differential OFDM Approach to Coherence Time Mitigation in

DSRCYouwei Zhang, Ian Tan, Carl Chun

Ken Laberteaux*, Ahmad BahaiUC Berkeley, Toyota Research(*)

VANET 2008

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Outline

• DSRC overview

• Motivating measurements

• Application of differential OFDM

• Simulation results

• Summary

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Vehicle Speeds Imply High Doppler

• at vehicular speeds and• in urban, rural, highway, or other scenarios

Dedicated Short Range Communications

Aim: Enhance roadway safety via wireless communication

Physical layer properties are rapidly changing

(high Doppler and delay spreads)

while:

This implies that:

Overview Measurement Differential OFDM Simulation Summary

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PHY Modified from 802.11a

PHY Parameter DSRC IEEE 802.11a

Bandwidth 10 MHz 20 MHzDate Rate 3, 4.5, 6, 9, 12, 18, 24 and

27Mbits/s6, 9, 12, 18, 24, 36, 48, and 54Mbit/s.

Modulation BPSK,QPSK,16-QAM,64-QAM

BPSK,QPSK,16-QAM,64-QAM

Number of Subcarriers 52 52

Subcarrier Spacing 156.25 KHz 312.50 KHz

Frequency Range 5.850 - 5.925 GHz 5.725 - 5.850 GHz

Symbol Duration 8 us 4 us

Guard Interval 1.6 us 0.8 us

transmission time doubled for same packet length


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Expected Design Properties

Metric Desired Relationship ReasoningDelay Spread

(TS)TS < OFDM GI = 1.6 s Prevent intersymbol

interference in time

Doppler Spread (DS)

DS < f = 156.25 KHz Prevent intercarrier interference in frequency

Coherence Time (TC)

TC > Packet Duration Allow one equalization setting per packet

Doppler spread related to coherence time:

SC

DT

4

1


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Channel Sounding System

TX Vehicle

RX Vehicle


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Measured Delay Spreads Tolerable

Locale Distance (m)

Delay Parameters (ns)

Mean Excess RMS Max Excess (30 dB)

Urban LOS

200 303.2 157.5 1681.8

400 370.1 320.6 3781.8

600 515.9 286.6 3625

Urban NLOS 200 521.7 295 2454.5

Highway LOS300 154.1 156.8 2026

400 175.4 141.1 1575.8

Highway NLOS 400 558.5 398 4772.7

Rural 100 85.8 21.6 272.7

Mean excess + RMS < 1.6 s

1.6 us GI should be sufficient for channel delay spreads


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Measured Coherence Times Small

Locale Distance (m)

Frequency Parameters (Hz)Estimated

Coherence Time (ms)

Frequency Shift

Avg. Doppler Spread

Urban LOS

200 -20 341 0.73

400 203 263 0.95

600 -21 294 0.85

Urban NLOS 200 103 298 0.84

Highway LOS300 209 761 0.33

400 261 895 0.28

Highway NLOS 400 -176 978 0.26

Rural 100 201 782 0.32

• Example: For a 200 bytes packet at 3 Mbps,

Causes problems for channel estimation


Packet duration = 200*8 bits /3Mbps = 0.53 ms

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Potential Solutions• Repeated channel estimation

– Pro:• Adaptable to existing systems

– Cons:• Potentially complex (high cost)

• Data rate reduction from overhead

• Differential OFDM (DOFDM)– Pros:

• Simple and targeted - requires small modifications to change from coherent (COFDM) to differential

– Cons:• Requires standards change

• Impact of noise doubled


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Coherent OFDM OperationTime-frequency view of OFDM symbols:

Time

Frequency

Xi[n]: the ith subcarrier’s contents at time n

n-2 n-1 n

Received Signal (subcarrier i, time n):

Yi[n] = Hi[n]Xi[n] + Wi[n]

Gaussian noise

Channel Response (frequency)


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Differential OFDM Operation - TX

Information encoded in relative phases between symbols and system has a one-symbol memory:

n-1 nReference Constellation

Passes through fading channel

Send this at nSent at time n-1

Info bits Phase diff.

00 0o

01 90o

10 -90o

11 180o

Channel rotates both symbols by same angle


10

00

00

01

00

11

00

Data to send at time n

Translates to

-90o

0o

0o

90o

0o

180o

0o

Corresponding PhasesTop subcarrier symbols:

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Differential OFDM Operation - RX

Info bits Phase diff.

00 0o

01 90o

10 -90o

11 180o

Reference Constellation

Receiver recovers data by measuring phase difference between sucessive symbols:

n-1n

10

00

00

01

00

11

00

Data received at

time n

Translates to

-90o

0o

0o

90o

0o

180o

0o

Recovered Phases


Receiver sees current symbol and remembers previous symbol

n

n-1

Receiver takes phase difference between symbols

-90o

Top subcarrier symbols:

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COFDM vs. DOFDM

• For Coherent OFDM– Estimates channel at packet start– Explicitly assumes channel is invariant

over one packet duration on the order of ms

• For Differential OFDM– Channel estimate unnecessary– Implicitly assumes channel is invariant over

two OFDM symbols (16 us)


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Simulation Platform

Tx PacketConvolution

EncoderInterleaver

S/P Conversion

IFFT Append CPP/S

Conversion

RayleighFading

AWGN

Rx PacketViterbi

DecoderDeinterleaver

P/SConversion

BPSKModulation

BPSKDemodulation

FFT Remove CPS/P

Conversion

Error RateCalculation

BER

DBPSKModulation

DBPSKDemodulation


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Simulation Parameters

Parameter ValuePacket Size 100 bytes, 1000 bytes

Data Rate 3 Mbps

Transmission Scheme OFDM

Modulation BPSK, DBPSK

Channel Coding ½ Convolution Coding

Carrier Frequency 5860 MHz

Channel Bandwidth 10 MHz

Subcarrier Spacing 156.25 KHz

OFDM Symbol Length 8 us

Channel Estimation Long preambles used, BPSK only

Channel Model One tap Rayleigh flat fading

Doppler Spread 0 Hz, 100 Hz (6 mph), 1300 Hz (75 mph)


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Simulation Results - 1000 byte Packets

5 10 15 20 25 3010

-4

10-3

10-2

10-1

100

SINR(dB)

BE

R1000 Byte Packet Transmissions

Differential Rayleigh, 75 mph


Coherent Rayleigh, 75 mphCoherent Rayleigh, 6 mph


Coherent Rayleigh, 0 mph

packet duration = 2.67 ms, coherence time =0.2 ms



2 OFDM symbol duration = 16 us


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Simulation Results – 100 Byte Packets

5 10 15 20 25 3010

-4

10-3

10-2

10-1

100

SINR(dB)

BE

R100 Byte Packet Transmissions

Differential Rayleigh, 75mph

Differential Rayleigh, 6mph

Coherent Rayleigh, 75mphCoherent Rayleigh, 6mph


Coherent Rayleigh, 0 mph



noise penalty


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Summary

• Measured DSRC channel

• Identified shortened coherence times as a problem

• Proposed TDOFDM as a solution

• Performed simulations to verify improvement


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Postscript

• With current IEEE 802.11p, to avoid high Packet Error Rates:

– Shorten packet lengths– Reduce vehicle speeds

• What can we do? Three options:1. Accept above constraints.2. Change standard to include DOFDM.3. Advanced equalization (higher hardware costs)

Solution for current 5.9 GHz need not be same as 700 MHz or other future VANET

technologies.

(opinions only, not included in paper)

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