Turbo Codes for HSDPA
The release 99 version of the 3GPP standard uses a rate 1/3 turbo encoder. A new rate ¼ turbo code based on the repetition of bits from the 1/3 code has been proposed for use in the HSDPA (High Speed Down-Link Packet Access) enhancement to be published early in 2002. In this project MERL-MHL has been investigating the performance within an HSDPA system of rate ¼ turbo codes based on rate 1/3 codes and bit repetition. In addition, we have investigated techniques to improve BER performance of sub-optimal decoding algorithms like the SOVA as well as VLSI implementation issues.
Background & Objective: There are two reasons behind the use of bit repetition for the HSDPA enhancements. First is it reliance on an already standardized 1/3 rate turbo code. Secondly, is the ability to use a less complex rate 1/3 turbo decoder in the receiver/handset. However, the bit repetition method will offer weaker BER performance when compared to more traditional rate ¼ turbo codes based on puncturing from lower rate codes.
Technical Discussion: It is well known that the MAP (Maximum A Posterior) decoding algorithm is the optimal decoding algorithm for turbo codes. However, it has a drawback in its complexity and cost of implementation. Sub-optimal algorithms such as the SOVA (Soft Output Viterbi Algorithm) have been developed to simplify decoder implementation. Also it has been shown that simple modifications to these sub-optimal decoding algorithms can improve BER performance. Specifically, scaling the extrinsic information that is feedback to the decoder can reduce the BER of the decoding algorithm.
We have found that the use of a simple scaling of the extrinsic information in the decoding process can improve the BER performance of SOVA based turbo decoders for the HSDPA ¼ rate turbo code. In an HSPDA system were the preference is to use the repetition method, the scaling technique is shown to help minimize the penalty incurred by using this sub-optimal encoding scheme. Additional investigations on optimal methods to combine the repeated bits were carried out.
Contact: Philip Orlik
Technology Area: Digital Communications
Modification Date: July 23, 2003
