Physical and Link Layer Implications in Vehicle Ad Hoc Networks
Dr Mosa Ali Abu-Rgheff, Reader in Mobile Communications, University of Plymouth
Tuesday 18 May 2010, 1200-1300
Vehicle Ad hoc Networks (VANET) provide safety on the road and deliver road traffic information and route guidance to drivers along with commercial applications.
However the challenges facing VANET are numerous. Nodes move at high speeds, road side units and base-stations are scarce, the topology is constrained by the road geometry and changes rapidly, and the number of nodes peaks suddenly in traffic jams.
In this talk, I will focus on issues related to the physical and link layers of VANET to achieve high data rates and high throughput.
For the physical layer, we examine the use of multiple antennas installed on top of the vehicles structured in Vertical BLAST (VBLAST) systems to provide higher capacities in rich fading environments. To study the capability of the proposed system, a vehicular channel model was developed from a rich scattering environment.
However, the performance of VBLAST system drops as the line of sight strength increases due to the correlation between the antennas. The analysis of VLAST-OFDM systems showed that they experience an error floor due to intercarrier interference (ICI) which increases with speed, number of antennas transmitting and number of subcarriers used.
The IEEE 802.11p MAC layer standard, under development for VANET, uses a variant of Carrier Sense Multiple Access (CSMA). IEEE 802.11 protocols were evaluated to prove the saturation throughput of the basic access method drops as the number of nodes increases thus yielding very low throughput in congested areas.
RTS/CTS access provides higher throughput but it applies only to unicast transmissions.
To overcome the limitations of 802.11 protocols, we designed a protocol known as SOFT MAC which combines Space, Orthogonal Frequency and Time multiple access techniques. In SOFT MAC the road is divided into cells and each cell is allocated a unique group of subcarriers. Within a cell, nodes share the available subcarriers using a combination of TDMA and CSMA. The throughput analysis of SOFT MAC showed it has superior throughput compared to the basic access and similar to the RTS/CTS access of 802.11.