A Differential OFDM Approach to Coherence Time Mitigation in DSRC

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 Overview Measurement Differential OFDM Simulation Summary Dedicated Short Range Communications • at vehicular speeds and • in urban, rural, highway, or other scenarios Aim: Enhance roadway safety via wireless communication while: This implies that: Physical layer properties are rapidly changing (high Doppler and delay spreads)

4. PHY Modified from 802.11a Overview Measurement Differential OFDM Simulation Summary transmission time doubled for same packet length

5. Expected Design Properties Overview Measurement Differential OFDM Simulation Summary Doppler spread related to coherence time:

6. Channel Sounding System Overview Measurement Differential OFDM Simulation Summary TX Vehicle RX Vehicle

7. Measured Delay Spreads Tolerable Overview Measurement Differential OFDM Simulation Summary 1.6 us GI should be sufficient for channel delay spreads Mean excess + RMS < 1.6 s

8. Measured Coherence Times Small Overview Measurement Differential OFDM Simulation Summary Causes problems for channel estimation • Example: For a 200 bytes packet at 3 Mbps, • Packet duration = 200*8 bits /3Mbps = 0.53 ms

9. Potential Solutions Overview Measurement Differential OFDM Simulation Summary • 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

10. Coherent OFDM Operation Overview Measurement Differential OFDM Simulation Summary Time-frequency view of OFDM symbols: Xi[n]: the ith subcarrier’s contents at time n Frequency Time n-2 n-1 n Received Signal (subcarrier i, time n): Yi[n] = Hi[n]Xi[n] + Wi[n] Channel Response (frequency) Gaussian noise

11. Differential OFDM Operation - TX -90o 10 0o 00 0o 00 90o 01 0o 00 180o 11 0o 00 Reference Constellation n n-1 Data to send at time n Corresponding Phases Sent at time n-1 Send this at n Passes through fading channel Channel rotates both symbols by same angle Overview Measurement Differential OFDM Simulation Summary Information encoded in relative phases between symbols and system has a one-symbol memory: Translates to Top subcarrier symbols:

12. Differential OFDM Operation - RX -90o 10 0o 00 0o 00 90o 01 0o 00 180o 11 0o 00 Reference Constellation n n-1 Data received at time n Recovered Phases n -90o n-1 Receiver takes phase difference between symbols Overview Measurement Differential OFDM Simulation Summary Receiver recovers data by measuring phase difference between sucessive symbols: Translates to Top subcarrier symbols: Receiver sees current symbol and remembers previous symbol

13. COFDM vs. DOFDM Overview Measurement Differential OFDM Simulation Summary • 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)

14. Simulation Platform DBPSK Modulation BPSK Modulation DBPSK Demodulation BPSK Demodulation Overview Measurement Differential OFDM Simulation Summary Tx Packet Convolution Encoder Interleaver S/P Conversion IFFT Append CP P/S Conversion Rayleigh Fading Error Rate Calculation BER AWGN Rx Packet Viterbi Decoder Deinterleaver P/S Conversion FFT Remove CP S/P Conversion

15. Simulation Parameters Overview Measurement Differential OFDM Simulation Summary

16. Simulation Results - 1000 byte Packets Overview Measurement Differential OFDM Simulation Summary 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

17. Simulation Results – 100 Byte Packets Overview Measurement Differential OFDM Simulation Summary packet duration = 0.27 ms, coherence time =0.2 ms packet duration = 0.27 ms, coherence time =2.5 ms noise penalty

18. Summary Overview Measurement Differential OFDM Simulation Summary • Measured DSRC channel • Identified shortened coherence times as a problem • Proposed TDOFDM as a solution • Performed simulations to verify improvement

19. Postscript (opinions only, not included in paper) • With current IEEE 802.11p, to avoid high Packet Error Rates: • Shorten packet lengths • Reduce vehicle speeds • What can we do? Three options: • Accept above constraints. • Change standard to include DOFDM. • Advanced equalization (higher hardware costs) Solution for current 5.9 GHz need not be same as 700 MHz or other future VANET technologies.