On Repetition Protocols and Power Control for Multiple Access Block-Fading Channels

In this paper we study the long-term throughput performance of repetition protocols coupled with power control for multiple access block-fading channels. We propose to use the feedback bits to inform the transmitter about the decoding status and the instantaneous channel quality. We determine the throughput of simple and practically inspired protocols; we show remarkable throughput improvements, especially at low and moderate SNR, when compared to protocols where the feedback bits are used for acknowledgment only or for power control only; we show that the throughput is very close to the ultimate ergodic multi-user water-filling capacity for small number of feedback bits and/or retransmissions. For symmetric Rayleigh fading channels, numerical results show that the throughput improvement is mainly due to the ability to perform a power control, rather than to retransmit.