Optimised Turbo Codes for Wireless Channels

Error control codes have become a vital part of modern digital wireless systems, enabling reliable transmission to be achieved over noisy and fading channels. Over the past decade, turbo codes have been widely considered to be the most powerful error control code of practical importance. In the same time-scale, mixed voice/data networks have advanced further and the concept of global wireless networks, integrating satellite and terrestrial links, has emerged. Such networks present the challenge of optimising error control codes for different channel types, and for the different qualities of service demanded by voice and data. This thesis begins with an investigation of the optimisation of binary turbo codes through good interleaver design. An analysis is made of what makes a good interleaver, and a novel interleaver design is proposed. The design is a hybrid between random and deterministic designs and will be shown to exhibit a very low error floor; comparable with the best interleavers investigated here. The design, however, has simpler implementation than the other interleavers. A search is then made for bandwidth-efficient code schemes which produce good performance over both AWGN and Rayleigh channels. The search focusses on a technique called Multi-level Coded Modulation (MLCM). The MLCM architecture allows flexibility in the choice of component code, thus offering the possibility of optimisation for different channels. The optimisation of Turbo coded MLCM (T-MLCM) at target bit error rates (BERs) suited to voice and data services is investigated. It is shown that some gain, in terms of a reduction in required , can be achieved by optimising both component code rates and generator polynomial at target BERs. Optimisation of T-MLCM for AWGN and Rayleigh channels is demonstrated. Coding solutions based on T-MLCM and binary turbo codes are presented for wireless applications including ATM-over-satellite and Digital Video Broadcasting. It is found that T-MLCM can be optimised effectively for the AWGN channel, so as to produce performance comparable with the most powerful turbo coded modulation techniques currently available. However, a comparison of T-MLCM results obtained here with results by other authors for Bit-Interleaved Coded Modulation (BICM) schemes suggests that BICM may provide better overall performance when considered jointly over both AWGN and Rayleigh channels.