Relay-Assisted Scheduling in Wireless Networks with Hybrid-ARQ

This paper studies the problem of relay-assisted user scheduling for downlink wireless transmission. The transmitting base station or access point employs hybrid automatic-repeat-request (ARQ) transmission with the possible assistance of a set of fixed relays to serve a set of mobile users. By minimizing a cost function that depends on the queue lengths at the base station and the number of retransmissions of the head-of-line packet for each user, the base station can determine an appropriate set of users to be serviced in each time slot. In the system model of interest, though, minimizing the cost function at the base station may entail selecting a set of relays to service the scheduled user set. Three cost functions are considered, including a linear function with packets arriving according to a Poisson process, a discounted version of this function with rewards, and an increasing convex function with packets that need to be drained from the queues at the base station. It is shown that a priority-index policy is optimal in all cases. Performance comparisons illustrate the benefits of relay-assisted scheduling with hybrid-ARQ transmission.