Incremental redundancy and bit-mapping techniques for high speed downlink packet access

A major evolution of the UMTS standard is high speed downlink packet access (HSDPA), which provides higher cell throughput and peak data rates of 10.8 Mbps. Key enabling technologies include fast scheduling, adaptive modulation and coding (AMC), as well as hybrid automatic repeat request (HARQ). To ease the implementation of HSDPA all coding elements are identical to standard WCDMA. In particular, the rate 1/3 turbo code is reused and no trellis coded modulation scheme has been introduced. The resulting drawbacks are analyzed and it is shown how they are efficiently counteracted by bit-mapping techniques accounting for the different bit reliabilities within a 16-QAM symbol. This paper focuses on the implementation and performance of HARQ and bit-to-symbol mapping functions. The performance of different HARQ schemes is compared for QPSK and 16-QAM and varying code rates. It is shown that depending on the code rate and the user terminal capabilities different HARQ types are favorable.