Piggyback Retransmissions over Wireless MIMO Channels: Shared Hybrid-ARQ (SHARQ) for Bandwidth Efficiency

Hybrid ARQ transmissions are effective means for on-demand error correction in wireless networks against unpredictable channel distortions and interferences. By requesting retransmission of failed data packets upon detecting packet errors, receivers can integrate original and retransmitted packet signals for more effective packet detection and error correction. Often, only a small fraction of the packet bits is in error. For channels with sufficiently high capacity such as in MIMO wireless systems, it is typically unnecessary to reuse the entire channel for hybrid-ARQ retransmissions in order to correct the few remaining errors. To improve the wireless channel utility, we propose and investigate a shared hybrid ARQ (SHARQ) mechanism in MIMO systems where failed data can piggyback on new data frames over the diversity MIMO channel. To guarantee certain quality of HARQ retransmission, we design a linear transmit precoder based on channel state information at the transmitter and receiver to achieve a predetermined {minimum} signal-to-noise ratio (SNR) gain for the SHARQ signals by minimizing the transmission power. Our proposed SHARQ system takes advantage of a simple successive interference cancellation receiver to facilitate a simpler and better transmitter.