A turbo coded DS/CDMA system with embedded Walsh-Hadamard codewords: coder design and performance evaluation

This paper presents the design of a turbo encoding scheme with embedded Walsh-Hadamard (WH) codewords, and a suitable decoder structure based on an iterative decoding technique, akin to the work on "superorthogonal" turbo codes presented by Pehkonen and Komulainen (see ISSSTA'96: International Symposium on Spread Spectrum Techniques & Applications, Mainz, Germany, p.580-4, 1995). The main contribution of this paper is a detailed design of the turbo decoder structure required to perform the required decoding steps, omitted by Pehkonen et al. The performance of the proposed iterative turbo coded DS/CDMA receiver is then studied in an mobile radio environment. Attention is focused on an 8-state rate 2/64 coded QPSK DS/CDMA system, in agreement with the rate offered by the IS-95 coding scheme. The bit error rate performance of the proposed coding scheme is evaluated by means of simulation and compared to that of a DS/CDMA system employing the IS-95 front-end coder for the reverse link channel.