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1
A Differential OFDM Approach to Coherence Time Mitigation in
DSRCYouwei Zhang, Ian Tan, Carl Chun
Ken Laberteaux*, Ahmad BahaiUC Berkeley, Toyota Research(*)
VANET 2008
2
Outline
• DSRC overview
• Motivating measurements
• Application of differential OFDM
• Simulation results
• Summary
3
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
4
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
Overview Measurement Differential OFDM Simulation Summary
5
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
Overview Measurement Differential OFDM Simulation Summary
6
Channel Sounding System
TX Vehicle
RX Vehicle
Overview Measurement Differential OFDM Simulation Summary
7
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
Overview Measurement Differential OFDM Simulation Summary
8
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
Overview Measurement Differential OFDM Simulation Summary
Packet duration = 200*8 bits /3Mbps = 0.53 ms
9
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
Overview Measurement Differential OFDM Simulation Summary
10
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)
Overview Measurement Differential OFDM Simulation Summary
11
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
Overview Measurement Differential OFDM Simulation Summary
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:
12
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
Overview Measurement Differential OFDM Simulation Summary
Receiver sees current symbol and remembers previous symbol
n
n-1
Receiver takes phase difference between symbols
-90o
Top subcarrier symbols:
13
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)
Overview Measurement Differential OFDM Simulation Summary
14
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
Overview Measurement Differential OFDM Simulation Summary
15
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)
Overview Measurement Differential OFDM Simulation Summary
16
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
Differential Rayleigh, 6 mph
Coherent Rayleigh, 75 mphCoherent Rayleigh, 6 mph
Differential Rayleigh, 0 mph
Coherent Rayleigh, 0 mph
packet duration = 2.67 ms, coherence time =0.2 ms
packet duration = 2.67 ms, coherence time =2.5 ms
packet duration = 2.67 ms, coherence time =0.2 ms
2 OFDM symbol duration = 16 us
Overview Measurement Differential OFDM Simulation Summary
17
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
Differential Rayleigh, 0 mph
Coherent Rayleigh, 0 mph
packet duration = 0.27 ms, coherence time =0.2 ms
packet duration = 0.27 ms, coherence time =2.5 ms
noise penalty
Overview Measurement Differential OFDM Simulation Summary
18
Summary
• Measured DSRC channel
• Identified shortened coherence times as a problem
• Proposed TDOFDM as a solution
• Performed simulations to verify improvement
Overview Measurement Differential OFDM Simulation Summary
19
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)