TOWARD UNDERSTANDING CHARACTERISTICS OF DEDICATED SHORT RANGE COMMUNICATIONS (DSRC) FROM A PERSPECTIVE OF VEHICULAR NETWORK ENGINEERS GM Global R&D Mobicom 2010 Jihun Ha 1

Contents Introduction Background and Motivation IEEE p Standard Channel Impairment Measurement platforms and Experimental methodology Impact of Uncontrollable factors Impact of Controllable radio parameters Correlation of DSRC characteristics Summary 2

Introduction The emergent IEEE p-based Dedicated Short Range Communication standard – one of the IEEE standards customized for highly mobile, severe fading vehicular environments – paves a new direction for creating cooperative Vehicle Safety Communications (VSC) systems Single-hop periodic broadcast of vehicle kinematic information 3

Problems of Single-hop Broadcast Tx It is a well-known fact that the broadcast mechanism of IEEE cannot provide reliable broadcasting services Needs upper-layer compensation schemes. Unfortunately, DSRC characteristics still remain unclear to the VANET community. Observations made toward WLAN or WiFi technology cannot be directly applied to DSRC technology, because these two types of technology are significantly different Spectrum Channel bandwidth Targeted user mobility Deployment environment Radio specification 4

In this paper... To bridge this gap, we conducted a series of empirical experiments and analyzed DSRC characteristics under freeway, rural, suburban and open field environments. 5

Background - IEEE p Standard OFDM modulation as the IEEE a/g Except carrier frequency bandwidth (change 2.4/5.8 GHz to 5.9 GHz) Channel bandwidth (change 20 MHz to 10 MHz) OFDM is an effective wireless communication scheme for non-mobile environments Both the symbols and their sub-channels are orthogonal Zero ISI and zero ICI But, in reality, these properties might be affected by Doppler spread/shift and fading environment 6

Motivation - Channel Impairments 7

Motivation - Questions? How do fading effect and Doppler effect affect DSRC characteristics? How do other contributing factors (e.g., radio operation parameters) affect DSRC characteristics? Do temporal correlation, spatial correlation and symmetric correlation exist in vehicular networks? 8

Measurement Framework To better understand DSRC characteristics in realistic vehicular environments, we take a black box approach to analyze how the key DSRC characteristic (PDR) is affected by various factors separation distance signal propagation environment relative velocity and effective velocity received signal strength transmission power modulation rate 9

Empirical Measurement Platforms DSRC-compatible Radio Atheros AR5000 chipset Tx power and data rate are 20 dBm and 6 Mbps, respectively IEEE p Wave BSS(WBSS) mode RSSI sensitivity level : up to -94 dBm Omni-directional antenna Antenna gain : 0 dB Signal attenuation between cable and connectors : 2 dB GPS Receiver DSRC Protocol Stack Frequency : 10Hz Packet length : 300 bytes excluding PHY and MAC-layer headers Vehicle Safety Communication(VCS) applications 10

Experiment Settings Route Urban Freeway (UF) a large number of walls, tunnels and overhead bridges, as well as heavy vehicle traffic are present Rural Freeway (RF) The number of walls, tunnels and vehicle traffic are slightly less than its UF counterpart Rural Road (RR) The traffic was heavy on these routes because they lead toward a vehicle testing facility. Suburban Road (SR) controlled Open Field (OF) no buildings and other vehicles. 11

Impact of Uncontrollable factors - Impact of Separation Distance and Environment We study how DSRC characteristics are impacted by environment factors. DSRC radios used default values of 20 dBm power and 6 Mbps data rate. 12 Prevalence of Gray-Zone Phenomenon We find that 'intermediate reception(gray- zone)' prevails throughout the whole communication range (i.e., up to 450 m) while the perfect-reception zone does not exist in vehicular networks.(unlike sensor network mentioned in some papers) RR is the harshest.(somewhat to surprise) Due to the remote houses, tress and hills, a high volume of bypassing vehicles)

Impact of Uncontrollable factors - Impact of Relative Velocity and Effective Velocity 13

Impact of Relative Velocity and Effective Velocity 14 PDRs of DSRC radios are insensitive to relative velocity for any given separation distance. Similar conclusions were made for the impact of effective velocity

Impact of Relative Velocity and Effective Velocity - Controlled Experiments on the Impact of Relative Velocity The pre-determined speeds included 10, 20, 30, 40, 50 and 60 mph 15 A C B We measured that the Doppler spread in suburban, highway and rural area is 0.58KHz, 1.53KHz, and 1.11KHz, respectively. At the same time, the OFDM carrier spacing in the IEEE p (DSRC) standard is 156KHz.

Impact of Uncontrollable factors - Impact of Transmission Power We find that the difference of PDRs between different power settings is not as big as we expected. As power increases from 10 dBm to 15 dBm, an increased transmission power leads to an increased SNR, resulting in a significantly improved PDR however, as the power is further increased, the channel distortion becomes the bottleneck rather than SNR. 16 Our experiment results confirm that a low transmission power (e.g., 15 dBm) is sufficient to provide reasonable communication quality for VSC systems while improving spatial reuse in heavy traffic.

Impact of Uncontrollable factors - Impact of Data Modulation Rate The data modulation rate was set to 3 Mbps (BPSK with 1/2 data rate, the lowest data rate) 6 Mbps (QPSK with 1/2 data rate, the default value used in the current industry practice) 18 Mbps (16-QAM with 3/4 data rate, one of the highest data rates.) The transmission power was 20dBm. 17

Impact of Uncontrollable factors - Impact of Data Modulation Rate We find that the option of 6 Mbps generates higher PDR than the other two options (3 Mbps or 18 Mbps.) It seems to be counter-intuitive that the option of 6 Mbps is superior to the option of 3 Mbps in certain scenarios. 18

Impact of Uncontrollable factors - Impact of Data Modulation Rate A conjecture for explaining this surprising phenomenon is as follows: Packet transmission duration with BPSK and with QPSK are 0.8 msec and 0.4 msec, respectively.(300-byte packet) We found that the channel coherence time of vehicle-to-vehicle channel in rural and freeway environment is msec If BPSK is used, DSRC radio is unable to keep track of the channel variation after the channel coherence time expires, resulting in a poor performance 19

Correlation of DSRC characteristics - Temporal correlation 20

Correlation of DSRC characteristics - Temporal correlation Except OF, bursty effects are quite common, indicating a weak temporal correlation in vehicular networks. Realistic vehicular environments are surely harsher, because the LOS component of signal might not exist Because of mobility, the surrounding environment of a vehicle changes quickly, resulting in a high degree of signal fluctuation and thus a weak temporal correlation. 21 UF RF OF RR SR

Correlation of DSRC characteristics - Temporal correlation the temporal correlation decreases with increasing separation distance. Impact of Separation Distance 22

Correlation of DSRC characteristics - Spatial correlation 23

Correlation of DSRC characteristics - Spatial correlation We observe that the highest values of spatial correlation for the UF, RF and RR environments are 0.482, and respectively suggesting that there is weak spatial correlation in the majority of environments that we investigate. The spatial correlation in the UF and RF environments seems to be much stronger than that in the RR environment. 24

Correlation of DSRC characteristics - Symmetric correlation 25

Correlation of DSRC characteristics - Symmetric correlation It is observed that incoming and outgoing links exhibit a strong degree of symmetry across all the environments. symmetric links and slightly asymmetric links are characterizing the majority of wireless links in vehicular environments. 26

Summary VSC systems pave a new direction for providing actively managed safety alerts to drivers, protecting drivers and passengers from dangerous driving situations. Taking an empirical measurement approach and collecting about three million data samples, we not only analyze how DSRC communication characteristics are impacted by a number of environmental and radio factors, but also reveal various types of correlation functions in vehicular environments. 27