Recently, turbo coding schemes involving block codes have been studied, significantly augmenting its overall performance. Usually, a turbo code uses different sets of parity information for the same set of information bits. In this paper we investigate the usage of a turbo scheme using two different sets of information bits, each encoded by separate constituent block coders. Their encoded bits are then linked by a simple module-2 sum, originating an extra amount of parity bits. The overall code can be seen as a turbo code, yielding enhanced performance at an expense of a small increase in complexity, since few decoding iterations are required to obtain good results. Using these turbo schemes it is possible to obtain codes with low coding rate, which makes them especially suitable for spread spectrum systems where they can be used for achieving simultaneously error correction and bandwidth expansion. Both TCH (Tomlinson, Cercas, Hughes) codes and biorthogonal codes were considered for the constituent encoders of the turbo schemes, due to their properties which allows the decoder to have low complexity and at the same time to achieve good performance. Bound expressions were developed for quantifying the performance of the turbo schemes and they were compared with the original constituent codes. Simulations were also performed in order to effectively determine the performance of the sub-optimal iterative decoding algorithm versus the Maximum Likelihood (ML) decoding algorithm.

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Block Turbo Codes (BTCs),Parity concatenation,Performance bounds,TCH codes,Biorthogonal codes,Code spread CDMA

• Matemáticas - Ciências Naturais
• Engenharia Eletrotécnica, Eletrónica e Informática - Engenharia e Tecnologia