Bringing Robustness into 5.9 GHz ITS WAVE Systems

Átírás

1 Nicolai Czink Laura Bernadó, Roland Tresch, Alexander Paier, Thomas Zemen Bringing Robustness into 5.9 GHz ITS WAVE Systems FTW Forschungszentrum Telekommunikation Wien 2nd ETSI TC ITS Workshop, February 2010 ETSI, Sophia Antipolis, France

2 Outline Motivation IEEE p (draft) road tests - Setup - Performance results - Robustness issues Transceiver performance - Conventional receivers - High-complexity implementation - Standard improvements to reduce complexity Conclusions - 2 -

3 Motivation In vehicle-to-vehicle and vehicle-to-infrastructure scenarios, safety-related messages must be passed on - Robustly - In real-time Currently, these issues are only addressed on the MAC layer - Acknowledgements - Retransmissions However, the main shortcoming lies on the PHY layer - High bit/frame error rates lead to many retransmissions - Strongly depends on environment - Worst case: non-line-of-sight - 3 -

4 WAVE Measurements Vomp, Tyrol, Austria Freeway test track from ASFINAG was available: - Gantries, power supply, LAN connection - 4 -

5 Environments -5-

6 Measurement Equipment Radio equipment: - CVIS testbed (including WAVE and GPS) Antennas: GHz omni-directional monopoles at the RSUs - CVIS antenna at the OBU Test vehicle: - ASFINAG minivan Additionally: - Video and photo equipment GPS WLAN 2-6 GHz - 6 -

7 Performance Measurements Throughput measurements over distance from RSU Measure: - Frame Success Ratio - 7 -

8 Unexpected Throughput Drops Performance jitter Throughput drops Non-line-of-sight (NLOS) environment Obstruction by other cars Radio receiver cannot cope with the channel effects - 8 -

9 Wave Transceiver Architecture Transmitter Encoder Interleaver Mapper Receiver Decoder Deinterleaver Demapper Equalizer Estimator Pilot insert Modulator Pilot remove Demodulator Radio Channel + Noise - 9 -

10 Pilot Distribution vs. Channel Variation Bandwidth 10 MHz OFDM symbol duration 8 µs Number of 52 + subcarriers 4 pilot Channel is changing significantly during one frame Pilot symbols at the beginning are suboptimal!

11 Performance of simple channel estimators PER = 100% PER ~ 70-90% PER < 10 % NLOS channel model (worst case), L = 200 bytes, 3 Mbit/s Least squares channel estimation, hard decision Higher velocities lead to increased BER floors BER = 10-2 only for speed 50 km/h

12 Advanced Receiver Architectures iteration BER iterations E b /N 0 [db] 4 iterations 8 iterations 12 iterations 14 iterations PER < 10 % NLOS channel model (worst case), L = 200 bytes, 3 Mbit/s, v = 150 km/h High-complexity iterative receiver Performance depends on number of iterations

13 Improving Pilot Structure Distributing pilots across time and frequencies - Midamble - Optimized placement (e.g. LTE-like or WiMAX-like)

14 Complexity Reduction by Improved Pilot Pattern iteration, old pattern BER E b /N 0 [db] 1 iteration, midamble 2 iterations, midamble 14 iterations, old pattern More reasonable pilot placement leads to tremendous complexity reduction on chip!

15 Conclusions The PHY layer of the IEEE p (draft) standard is not designed for mobile terminals In non-line-of-sight environments: - Performance problems with conventional receivers (demonstrated by measurements and simulations) - To circumvent the problem: very high receiver complexity is needed - An improved pilot pattern leads to much lower receiver complexity To establish a robust link - A new chip design must be done anyway - Why not do it with low complexity and an improved pilot pattern?