Analytical Assessment of the Effect of Backoffs and Retransmissions on the Performance of ALOHA and CSMA in Manets

This paper considers the impact of backoffs and retransmissions on the performance of the ALOHA and CSMA protocols in wireless mobile ad hoc networks. In our network model, packets belonging to specific transmitters arrive randomly in space and time according to a 3-D PPP, and are then transmitted to their intended destinations using a fully-distributed MAC protocol. A packet transmission is considered successful if it is initiated and the received SINR is above a predefined threshold for the duration of the packet. Approximate expressions are derived for the outage probability of ALOHA (slotted and unslotted) and CSMA (with transmitter sensing and with receiver sensing), as a function of the transmission density and the number of backoff and retransmission attempts. Simulations verify our analytical expressions, and it is shown that the outage probability is a decreasing function of the maximum allowable number of backoffs and retransmissions. By allowing for high number of backoffs, CSMA with receiver sensing can even outperform slotted ALOHA. For high number of retransmissions, however, ALOHA may outperform CSMA. Finally, the average transmission delay of packets is also evaluated through simulations.