On Medium Access for Wireless Ad-hoc and Sensor Networks

Licentiatavhandling, 2007

The medium access control (MAC) is a sub-layer of the data link layer, the second layer of the open system interconnection (OSI) model. The MAC sub-layer is responsible for organizing the access to the shared channel and it is due to this important responsibility that the performance of networks is substantially influenced by the MAC protocol. This thesis is based on three contributions, addressing different issues related to the MAC layer of a variety of wireless networks. The first contribution in this thesis, paper A, deals with the problem of designing a MAC protocol for vehicle-to-vehicle (V2V) communication for traffic safety purposes. Traffic safety applications often require to broadcast information or warning packets to all the vehicles in a close vicinity with as little delay as possible. CSMA-based protocols, which is one of the candidates for the V2V communication MAC, suffer from lack of an efficient and reliable broadcast method. To address this problem, a new broadcast scheme called Long Range Busy Tone (LRBT) is proposed. Two important ranges in the MAC layer, namely the transmission and interference ranges, are the topics of the second contribution of this thesis. Weaknesses of the current model for interference range are discussed and an improved model is obtained by removing some of the simplifying assumptions. Additionally, the effects of physical layer parameters and a fading channel on transmission and interference ranges are discussed. The last part of this thesis is devoted to scheduling co-existing time-slotted clusters in a Rayleigh fading channel. In this fading channel, no distance separation guarantees interference free communication. Therefore, the co-channel interference can be only minimized, if a joint scheduling among all the clusters is considered. In paper C, this joint scheduling problem, which is known to be NP-hard, is sub-optimally solved based on Lagrangian relaxation and the auction algorithm.