Cooperative broadcast channels with hybrid ARQ

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.