Performance of Turbo-coded OFCDM Systems with Soft MCI Cancellation

This paper investigates the performance of turbo-coded orthogonal frequency and code division multiplexing (OFCDM) systems with soft multi-code interference (MCI) cancellation. In order to regenerate the soft interference signal, the conventional turbo decoding algorithm must be modified to provide log likelihood ratio (LLR) values for all coded bits. Based on the LLR outputs of turbo decoder, two soft decision functions are proposed, i.e., LLR-soft-decision and Gaussian-soft-decision functions. By means of computer simulations, it is shown that in a highly frequency selective channel, soft MCI cancellation and MMSE detection can significantly improve the performance of turbo-coded OFCDM systems. Two iterations in turbo decoding are sufficient for both hard and soft decision functions. The proposed soft decision functions outperform the hard decision function with various channel conditions and system parameters.