Achievable throughput and service delay for imperfect cooperative retransmission MAC protocols

Abstract

Cooperative retransmission medium access control protocols may be used in a cellular uplink setting to achieve a higher throughput than, say, slotted Aloha, while retaining the advantages of random channel access. When multiple nodes contend for channel access simultaneously, the base station initiates a collision resolution epoch where a set of cooperating nodes retransmit either their own packets, if they were involved in the collision, or the signal that they heard during the collision slot, until the base station has enough linearly independent collided signals to recover the packets. The throughput and service delay of such protocols depends critically on the rate that independent signals may be generated, which in turn depends critically on the underlying channel diversity. We consider two natural models for "imperfect" retransmission schemes where insufficient diversity extends the duration of the retransmission epochs, thus lowering the effective throughput. The throughput optimal medium contention probabilities for both models are obtained as a function of the measure of imperfection. The analysis permits a meaningful and quantitative framework for assessing the strengths and weaknesses of cooperative retransmission MAC protocols.