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Dr Ghassan Abdalla PhD

Research Student

Brief biographical information



Access thesis on-line

Physical and Link Layer Implications in Vehicle Ad Hoc Networks

Vehicle Ad hoc Networks (VANET) have been proposed to 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 basestations 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 thesis we investigate the physical and link layers of VANET and propose methods to achieve high data rates and high throughput.

For the physical layer, we examine the use of Vertical BLAST (VBLAST) systems as they provide higher capacities than single antenna systems in rich fading environments. To study the applicability of VBLAST to VANET, a channel model was developed and verified using measurement data available in the literature. For no to medium line of sight, VBLAST systems provide high data rates. However the performance drops as the line of sight strength increases due to the correlation between the antennas. Moreover, the performance of VBLAST with training based channel estimation drops as the speed increases since the channel response changes rapidly. To update the channel state information matrix at the receiver, a channel tracking algorithm for flat fading channels was developed. The algorithm updates the channel matrix thus reducing the mean square error of the estimation and improving the bit error rate (BER). The analysis of VBLAST-OFDM systems showed they experience an error floor due to inter-carrier interference (ICI) which increases with speed, number of antennas transmitting and number of subcarriers used. The update algorithm was extended to VBLAST-OFDM systems and it showed improvements in BER performance but still experienced an error floor. An algorithm to equalise the ICI contribution of adjacent subcarriers was then developed and evaluated. The ICI equalisation algorithm reduces the error floor in BER as more subcarriers are equalised at the expense of more hardware complexity.

The connectivity of VANET was investigated and it was found that for single lane roads, car densities of 7 cars per communication range are sufficient to achieve high connectivity within the city whereas 12 cars per communication range are required for highways. Multilane roads require higher densities since cars tend to cluster in groups. Junctions and turns have lower connectivity than straight roads due to disconnections at the turns. Although higher densities improve the connectivity and, hence, the performance of the network layer, it leads to poor performance at the link layer. The IEEE 802.11p MAC layer standard under development for VANET uses a variant of Carrier Sense Multiple Access (CSMA). 802.11 protocols were analysed mathematically and via simulations and the results 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.

Dr Ghassan Abdalla

Director of studies: Dr Mosa Ali Abu-Rgheff
Other supervisors: Dr Zaki Ahmed

Journal papers

A channel update algorithm for VBLAST architecture in VANET
Abdalla GMT, Abu-Rgheff MA, Senouci S-M
IEEE Vehicular Technology Magazine, Vol. 4, Iss. 1, March, 2009
More details | External link available

Joint Channel Tracking and ICI Equalisation for VBLAST-OFDM in VANET
Abdalla GMT, Abu-Rgheff MA, Senouci S-M
Intelligent Transport Systems, IET, vol.3, iss.4, pp409-418, 2009
More details | External link available

An Adaptive Channel Model for VBLAST in Vehicular Networks
Abdalla GMT, Abu-Rgheff MA, Senouci S-M
EURASIP Journal on Wireless Communications and Networking, vol. 2009, Article ID 328706, 8 pages, 2009
More details | External link available

Current Trends in Vehicular Ad Hoc Networks
Abdalla GMT, Abu-Rgheff MA, Senouci S-M
UbiCC Journal-Special Issue of UbiRoads 2007, pp1-9, 2008
More details | External link available

4 Journal papers

Conference papers

Space-Orthogonal Frequency-Time medium access control (SOFT MAC) for VANET
Abdalla GMT, Abu-Rgheff MA, Senouci S-M
Information Infrastructure Symposium, 2009. GIIS 2009. Global, 23-26 June, pp1-8, 2009
More details | External link available

A Channel Update Algorithm for VBLAST Architecture in Vehicular Ad-hoc Networks
Abdalla GMT, Abu-Rgheff MA, Senouci S-M
Vehicular Technology Conference, 2008. VTC 2008-Fall. IEEE 68th, pp1-5, 2008
More details | External link available

Current Trends in Vehicular Ad Hoc Networks
Abdalla GMT, Abu-Rgheff MA, Senouci S-M
UBIROADS Workshop, IEEE GIIS Conference, 2007
More details | External link available

3 Conference papers

Patents filed

Space-Orthogonal Frequency-Time Medium Access Control (SOFT MAC)
Abu-Rgheff MA, Abdalla GMT, Senouci S-M
Institut national de la propriété industrielle - INPI (French patent office), French patent INPI FR #0951620, 2009
Filed: 13/03/2009; Published: 09/2010
More details

1 Patents filed

8 publication(s) - all categories.