Spatial Diversity for IEEE 802.11p V2V Safety Broadcast in a Highway Environment

The Fully Networked Car Geneva, 3-4 March 2010

1Spatial Diversity for IEEE 802.11p V2V SafetyBroadcast in a Highway Environment

Nor Fadzilah Abdullah, Robert Piechocki and Angela Doufexi

Centre for Communications Research,University of Bristol, UK.


Need for Vehicular Communication…

o In 2007, the EU recorded ~43,000 deaths and >1.8 million injuries (€160 billion loss)

o Steady growth of car usage and ownership (>200 millions cars in Europe) • congestion built-up, unpredictable journey time• impact on the economy: significant vehicle operating

costs overhead, burden for travellers• impact on the environment: harmful emissions, worsen

air qualityo Allocated bandwidth for C2X serviceso Lowering cost of WiFi and GPS

2

European Road Safety Observatory, Annual statistical report 2007. [Online] http://euroris.swov.nl/safetynet/xed/WP1/2007/SN-1-3-ASR-2007.pdf


Research Contribution

o Vehicular communication requires longer communication range (than 802.11a/g/n), in extreme multipath and high speed environment• Spatial diversity: a low complexity and low cost solution

o Accurate and realistic vehicular communication modelling by means of:• BER curves from detailed PHY simulator specific to

modulation types, vehicular speeds and range of SNR values

• Integration of PHY simulator and realistic mobility model into network simulator

3


4Scenario: Post-crash warning in highway environment

o Realistic mobility traces: 3 lanes bidirectional highway.

o 2 types of traffic density models (Low & High).

o 2 types of ad-hoc V2V safety messages• Emergency message• Periodic message

o Rayleigh channel with 103ns rms delay spread (ETSI channel B)

D. W. Matolak, I. Sen, W. Xiong, and N. T. Yaskoff, “5GHZ Wireless Channel Characterization for Vehicle to Vehicle Communications,” Proceedings of IEEE Military Communications Conference (MILCOM ’05), vol. 5, pp. 3022–3016, Atlatnic City, NJ, USA, Oct 2005.


5Midamble symbol spacing as a function of channel coherence time, data rate, and packet size

Space-time correlation,Midamble spacing chosen: 30 symbols

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2x 10- 3

-0.5

0

0.5

1

Time(s)

Corr

elatio

n co

efficie

nt

500 bytes data using IEEE 802.11p QPSK 1/2 (6Mbps)

Jakes PSD (fd=1100Hz)Total pkt duration (84sym=0.672ms)Tc = 0.225ms (28sym)Jakes model PSD (fd=550Hz)Tc = 0.44ms (55sym)


6Frame Structure for Proposed Multi Antenna System with Midamble Channel Estimation

o Channel tracking: reuses long preamble sequence for midamble channel estimation

S. I. Kim, H. S. Oh, and H. K. Choi, "Mid-amble Aided OFDM Performance Analysis in High Mobility Vehicular Channel," IEEE Intelligent Vehicles Symposium, Eindhoven, Netherlands, Jun 2008.


7Need for midamble in fast fading vehicular channel

0 5 10 15 20 25 3010-3

10-2

10-1

100

SNR (dB)

PER

Midamble vs. Preamble at 50 km/h

SISO: preambleSISO: midambleSTBC 2x2: midambleSTBC 4x4: midamble

o Midamble vs. Preamble at 10 symbols midamble spacing

SISO: Single Input Single OutputSTBC: Space Time Block CodesModulation: QPSK 1/2

Antenna configuration

Rank

SISO 1

STBC 2x2 1

STBC 4x4 3/4


Low density

STBC 2x2 vs. SISO

STBC 4x4 vs. SISO

No priority

60% (200m/125m)

132%(290m/125m)

EDCA 80% (225m/125m)

164%(330m/125m)

8Low Density Traffic: Emergency Message Packet Delivery Ratio in Rayleigh channel

Low density traffic:6 vehicles/km/lane

* EDCA (Enhanced Distributed Channel Access)

0 100 200 300 400 5000

10

20

30

40

50

60

70

80

90

100

Distance (m)

Pack

et D

eliv

ery

Ratio

(%)

Emergency broadcast with Interference: Low Density

SISO, LoD, no prioritySISO, LoD, EDCASTBC 2x2, LoD, no prioritySTBC 2x2, LoD, EDCASTBC 4x4, LoD, no prioritySTBC 4x4, LoD, EDCA


High density

STBC 2x2 vs. SISO

STBC 4x4 vs. SISO

No priority

70% (170m/100m)

65%(165m/100m)

EDCA 50% (195m/130m)

138%(310m/130m)

Low density

STBC 2x2 vs. SISO

STBC 4x4 vs. SISO

No priority

60% (200m/125m)

132%(290m/125m)

EDCA 80% (225m/125m)

164%(330m/125m)

9High Density Traffic: Emergency Message Packet Delivery Ratio in Rayleigh channel

High density

STBC 2x2 vs. SISO

STBC 4x4 vs. SISO

EDCA improvement

15% (195m/170m)

88%(310m/165m)

Low density

STBC 2x2 vs. SISO

STBC 4x4 vs. SISO

EDCA improvement

13% (225m/200m)

14%(330m/290m)

High density traffic:11 vehicles/km/lane

0 50 100 150 200 250 300 350 400 450 5000

10

20

30

40

50

60

70

80

90

100Emergency broadcast with Interference: High Density

Distance (m)

Pack

et D

eliv

ery

Ratio

(%)

SISO, HiD, no prioritySISO, HiD, EDCASTBC 2x2, HiD, no prioritySTBC 2x2, HiD, EDCASTBC 4x4, HiD, no prioritySTBC 4x4, HiD, EDCA


10Conclusion

o Performance of safety broadcast messages, for MIMO-STBC vs. SISO in a vehicular environment has been presented.

o Spatial diversity increase the communication range: 50-80% for STBC 2x2 and 65-164% for STBC 4x4 case.

o Traffic prioritization (EDCA) is efficient in high density scenario and extends the communication range by 15% for STBC 2x2 case and 88% for STBC 4x4.


11Spatial Diversity for IEEE 802.11p V2V SafetyBroadcast in a Highway Environment

Appendix


Vehicular Channel Model

o Differing maximum Doppler shifts for low and high density traffic

o RMS delay spread of 103ns [Matolak, 2005]

12

0 10 20 30 40 50 60 70 80-18

-16

-14

-12

-10

-8

-6

-4

-2

0

Number of OFDM symbols per frame

Norm

alize

d po

wer (

dB)

Vehicular time-correlated multipath fading channel at fd=550Hz

path1path2path3path4path5path6path7path8

D. W. Matolak, I. Sen, W. Xiong, and N. T. Yaskoff, “5GHZ Wireless Channel Characterization for Vehicle to Vehicle Communications,” Proceedings of IEEE Military Communications Conference (MILCOM ’05), vol. 5, pp. 3022–3016, Atlatnic City, NJ, USA, Oct 2005.

0 10 20 30 40 50 60 70 80-25

-20

-15

-10

-5

0

Number of OFDM symbols per frame

Norm

alize

d po

wer (

dB)

Vehicular time-correlated multipath fading channel at fd=1100Hz

path1path2path3path4path5path6path7path8


13Physical Layer Simulator Block Diagram


Numerical Analysis Parameters 14

Physical Layer MAC LayerTx Frequency: 5.9 GHz Slot Time: 13 usBandwidth: 10 MHz OFDM symbol: 8 usTx Power: 23 dBm PLCP: 40 usReceiver threshold: -82 dBm SIFS: 32 us, CWmin: 31Antenna gain: 0 dBi DIFS: 58 us, BO = 208 usAntenna height : 1.5 m EDCA: High Priority (EM)Channel Model: Rayleigh ECWmin: 7Modulation scheme: QPSK 1/2 AIFS: 58 us, BO = 52 usApplication Layer EDCA: Low priority (PM)Pkt Generation Rate: 10 pkt/s ECWmin: 31Packet size: 500 bytes AIFS: 123 us, BO = 208 us



0 5 10 15 2010-3

10-2

10-1

100

SNR (dB)

PER

SISO vs MIMO: 100 km/h, with midamble spacing of 30 symbols

SISO: 100 bytesSISO: 500 bytesSISO: 1000 bytesSTBC 2x2: 100 bytesSTBC 2x2: 500 bytesSTBC 2x2: 1000 bytesSTBC 4x4: 100 bytesSTBC 4x4: 500 bytesSTBC 4x4: 1000 bytes

Lower SNR requirement for higher spatial diversity and smaller payload size.• STBC 4x4 reduces maximum data rate



0 10 20 30 40

10-4

10-3

10-2

10-1

100SISO, 100 km/h, with midamble spacing of 30 symbols

SNR (dB)

BER

BPSK 1/2BPSK 3/4QPSK 1/2QPSK 3/416QAM 1/216QAM 3/464QAM 2/364QAM 3/4

Higher SNR requirement for higher modulations.

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Spatial Diversity for IEEE 802.11p V2V Safety Broadcast in a Highway Environment