On the use of mutual information at bit level for accurate link abstraction in LTE with incremental redundancy H-ARQ

Link abstraction methods are able to predict the BLER in multicarrier wideband systems using AMC techniques under multipath fading. The basic idea is to map the vector of the subcarrier SNR's to a single scalar, the Effective SNR (ESNR), which is then used to predict the BLER. This procedure is important during real system operation, since the mobile terminal must report the Channel Quality Indicator (CQI) to the base station, and also for the system level simulators. In order to take into account H-ARQ, it is also interesting to predict the BLER after one or more IR retransmissions have happened. With IR H-ARQ there is an additional complexity, since the BLER performance of a given redundancy version (rv) depends on the SNR experienced by the receiver at current rv but also on the SNR of previously received rv's. We propose a novel link abstraction method that can predict the BLER with good accuracy in multipath fading and including the effects of H-ARQ retransmissions. The proposed method is based on estimating the mutual information between the transmitted bits and the received LLRs. We show that, by working at bit level, the effect of unequal error protection in 16 and 64QAM modulations is properly captured without resorting to any set of calibration constants. We also show how to reduce the set of reference BLER curves when working with multiple modulation and code schemes (MCS). Simulation results for LTE downlink with SISO transmission mode are included in the paper.