Probabilistic retransmission strategy for single-relay cooperative ARQ

In wireless networks with cooperative automatic repeat request (C-ARQ) protocols, a relay node, placed within the range of the sender node and the destination node, assists the sender in the process of frame retransmission. In a collision-free scenario, the sender and the relay use different physical channels for retransmissions. This paper highlights the tradeoff between throughput increment and efficiency in the use of radio resources. By using a probabilistic retransmission strategy, the sender only retransmits in some time-slots after a frame error notification. At the other time-slots, the source can assign the radio resources to other communication processes, resulting in more efficient use of the bandwidth. The retransmission probability must be carefully adjusted according to the network parameters. In this paper we propose a Markov model to compute the throughput performance and a complementary reward model to compute the retransmission rate of the Source. In order to keep the throughput at a high value and to reduce the retransmission rate at the same time, we present a multi-objective optimization method that is capable of balancing both objectives in any scenario. It is shown that the increment in bandwidth efficiency can be very high, especially for degraded links, compensating the small throughput reduction associated with a probabilistic retransmission at the source.