1 A Differential OFDM Approach to Coherence Time Mitigation in DSRC Youwei Zhang, Ian Tan, Carl Chun Ken Laberteaux*, Ahmad Bahai UC 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: OverviewMeasurementDifferential OFDMSimulationSummary

4 PHY Modified from a PHY ParameterDSRCIEEE a Bandwidth10 MHz20 MHz Date Rate3, 4.5, 6, 9, 12, 18, 24 and 27Mbits/s 6, 9, 12, 18, 24, 36, 48, and 54Mbit/s. ModulationBPSK,QPSK, 16-QAM,64-QAM BPSK,QPSK, 16-QAM,64-QAM Number of Subcarriers52 Subcarrier Spacing KHz KHz Frequency Range GHz GHz Symbol Duration8 us4 us Guard Interval1.6 us0.8 us transmission time doubled for same packet length OverviewMeasurementDifferential OFDMSimulationSummary

5 Expected Design Properties MetricDesired RelationshipReasoning Delay Spread (T S ) T S < OFDM GI = 1.6 s Prevent intersymbol interference in time Doppler Spread (D S ) D S < f = KHz Prevent intercarrier interference in frequency Coherence Time (T C ) T C > Packet Duration Allow one equalization setting per packet Doppler spread related to coherence time: OverviewMeasurementDifferential OFDMSimulationSummary

6 Channel Sounding System TX Vehicle RX Vehicle OverviewMeasurementDifferential OFDMSimulationSummary

7 Measured Delay Spreads Tolerable LocaleDistance (m) Delay Parameters (ns) Mean ExcessRMSMax Excess (30 dB) Urban LOS Urban NLOS Highway LOS Highway NLOS Rural Mean excess + RMS < 1.6 s 1.6 us GI should be sufficient for channel delay spreads OverviewMeasurementDifferential OFDMSimulationSummary

8 Measured Coherence Times Small LocaleDistance (m) Frequency Parameters (Hz) Estimated Coherence Time (ms) Frequency Shift Avg. Doppler Spread Urban LOS Urban NLOS Highway LOS Highway NLOS Rural Example: For a 200 bytes packet at 3 Mbps, Causes problems for channel estimation OverviewMeasurementDifferential OFDMSimulationSummary 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 OverviewMeasurementDifferential OFDMSimulationSummary

10 Coherent OFDM Operation Time-frequency view of OFDM symbols: Time Frequency X i [n]: the i th subcarriers contents at time n n-2n-1n Received Signal (subcarrier i, time n): Y i [n] = H i [n]X i [n] + W i [n] Gaussian noise Channel Response (frequency) OverviewMeasurementDifferential OFDMSimulationSummary

11 Differential OFDM Operation - TX Information encoded in relative phases between symbols and system has a one-symbol memory: n-1 n Reference Constellation Passes through fading channel Send this at nSent at time n-1 Info bitsPhase diff. 000o0o 0190 o o o Channel rotates both symbols by same angle OverviewMeasurementDifferential OFDMSimulationSummary Data to send at time n Translates to -90 o 0o0o 0o0o 90 o 0o0o 180 o 0o0o Corresponding Phases Top subcarrier symbols:

12 Differential OFDM Operation - RX Info bitsPhase diff. 000o0o 0190 o o o Reference Constellation Receiver recovers data by measuring phase difference between sucessive symbols: n-1 n Data received at time n Translates to -90 o 0o0o 0o0o 90 o 0o0o 180 o 0o0o Recovered Phases OverviewMeasurementDifferential OFDMSimulationSummary Receiver sees current symbol and remembers previous symbol n n-1 Receiver takes phase difference between symbols -90 o 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) OverviewMeasurementDifferential OFDMSimulationSummary

14 Simulation Platform Tx Packet Convolution Encoder Interleaver S/P Conversion IFFTAppend CP P/S Conversion Rayleigh Fading AWGN Rx Packet Viterbi Decoder Deinterleaver P/S Conversion BPSK Modulation BPSK Demodulation FFTRemove CP S/P Conversion Error Rate Calculation BER DBPSK Modulation DBPSK Demodulation OverviewMeasurementDifferential OFDMSimulationSummary

15 Simulation Parameters ParameterValue Packet Size100 bytes, 1000 bytes Data Rate3 Mbps Transmission SchemeOFDM ModulationBPSK, DBPSK Channel Coding½ Convolution Coding Carrier Frequency5860 MHz Channel Bandwidth10 MHz Subcarrier Spacing KHz OFDM Symbol Length8 us Channel EstimationLong preambles used, BPSK only Channel ModelOne tap Rayleigh flat fading Doppler Spread0 Hz, 100 Hz (6 mph), 1300 Hz (75 mph) OverviewMeasurementDifferential OFDMSimulationSummary

16 Simulation Results byte Packets 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 OverviewMeasurementDifferential OFDMSimulationSummary

17 Simulation Results – 100 Byte Packets packet duration = 0.27 ms, coherence time =0.2 ms packet duration = 0.27 ms, coherence time =2.5 ms noise penalty OverviewMeasurementDifferential OFDMSimulationSummary

18 Summary Measured DSRC channel Identified shortened coherence times as a problem Proposed TDOFDM as a solution Performed simulations to verify improvement OverviewMeasurementDifferential OFDMSimulationSummary

19 Postscript With current IEEE p, 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)