Cooperative broadcast channels with hybrid ARQ

Abstract

The throughput performance is studied for a two-user wireless broadcast channel with hybrid automatic repeat request (HARQ) and user cooperation. Two transmission schemes are considered: superposition coding (SC) and frequency-division multiplexing (FDM). During the initial HARQ rounds, the access point (AP) transmits messages to the users using either SC or FDM. If a user is successful, the AP uses all resources to transmit to the other user in subsequent HARQ rounds. Meanwhile, the successful user attempts to decode the other user's message and cooperate simultaneously with the AP, as long as the maximum number of HARQ rounds L has not been reached. The cooperative transmissions use distributed space-time coding to avoid the half-duplex penalty. HARQ transmissions continue until success for both users or until L HARQ rounds have elapsed. Cooperation provides throughput gains subject to quality-of-service (QoS) constraints, even when the path loss between users is greater than that between each user and the AP.