Influence of SNR Estimation on HARQ Combining in UMTS-HSDPA

Turbo-codes are employed in the High Speed Downlink Packet Access (HSDPA) mode of UMTS. The Turbo decoder requires Log-Likelihood Ratio (LLR) values on the observed symbols as input variables. In time-variant channels these depend on the instantenous SNR of the symbols. When using the sub-optimum max-log-MAP decoder as well as the max-log 16-QAM demapper the noise variance is a linear factor in the LLR that cancels out during the computation of the decoder output values. Hence, it could be omitted. However, when combining different redundancy versions of symbols, that are transmitted through Hybrid Automatic ReQuest (HARQ), we need to combine LLR values with different SNRs. We illustrate three SNR estimators for the 16-QAM symbol constellation and show the resulting throughput of a H-Set 6 scenario on a Pedestrian-B channel. We assume perfect channel knowledge and apply Linear Minimum Mean Square Error (LMMSE) equalization on the chip level.