Outage-based ergodic link adaptation for fading channels with delayed CSIT

To deal with a time-varying nature of the wireless channel, the most modern wireless systems use link adaptation in which the transmission rate is adjusted according to the current fading status to ensure reliable communication. For example, the current long-term evolution (LTE) standards require the user equipment (UE) to report channel quality indicator (CQI) to the base station (BS) to determine the data rate of a transmission (code) block. Such link adaptation effectively eliminates fading-induced outage and provides ergodic throughput in the long run. When there is delay in CQI feedback, however, such knowledge of the channel status is inevitably outdated, and outage becomes problematic again. In this paper, we characterize a future channel in which the reported CQI is actually used as a conditional random variable given current channel observation. The dependence of the future channel to the current observation is captured by channel correlation. By considering an outage event as a failed transmission, we can compute probability of successful transmission for each value of transmission rate to determine the best transmission rate which maximizes the expected throughput. If ergodicity holds for a channel process, then such expected throughput becomes close to an empirical throughput in the long run.