Evaluating the performance of turbo codes and turbo-codedmodulation in a DS-CDMA environment

摘要

Turbo codes have received great attention due to their outstandingnperformance. Unfortunately, a high performance is associated with largentransmission delays, prohibiting an application for speech transmission.nHence, the aim of this paper is the comparison of turbo codes employingnshort interleavers with convolutional codes in terms of bit error ratenperformance and computational effort. Additionally, a pragmatic approachnof bandwidth-efficient turbo-coded modulation is considered. Analyzingnthe structure of the transmitter and receiver, interesting results arenpresented concerning the design of the mapper. Furthermore, a new verynsimple soft-output demodulation algorithm is derived. In order toncompare turbo codes with convolutional codes under realistic conditions,nboth are embedded in a direct sequence (DS) code division multiplenaccess system. Besides this comparison, a compromise between a highncoding gain (low code rate) and high direct-sequence spreading is workednout, including the consideration of the turbo-coded modulation scheme.nSimulation results indicate that turbo codes with small blockninterleavers do not outperform conventional convolutional codes.nFurthermore, it is shown that for coherent demodulation, low code ratesnand low DS spreading is superior to high code rates and high DSnspreading