ARQ with Channel Gain Monitoring

Automatic repeat request (ARQ) schemes with cyclic redundancy check (CRC) codes are designed specifically for additive white Gaussian noise channels based on average performance measures. Average performance measures are known to be inadequate for high data rate wireless communication where the channel gain varies randomly in time, because they do not reflect the poor QoS in deep fades. The instantaneous accepted packet error outage (IAPEO) probability is a more meaningful performance measure. Therefore, we examine the IAPEO of conventional ARQ and observe that, to achieve a certain IAPEO performance, the system must operate above a minimum signal-to-noise ratio (SNR), which may be too high to achieve. This motivates us to propose ARQ with channel gain monitoring (CGM), in which a packet is only demodulated and checked for errors when the channel gain exceeds a threshold; otherwise retransmission is requested. We show that it does not have a minimum SNR requirement. The average performance measures of ARQ-CGM with selective-repeat, go-back-n and stop-and-wait retransmission protocols, i.e. the average accepted packet error probability, throughput and goodput, are also obtained as functions of the pilot length and the CRC code length. The allocation of bandwidth and energy between channel estimation and CRC coding is studied.