Soft decision multi-user iterative decoding using a block turbo code for SFH/SSMA

It has been shown that the capacity of a slow frequency hopped (SFH) spread spectrum multiple access (SSMA) system can be increased by using the joint demodulation and iterative decoding by a hard decision demodulator with Reed Solomon code as a forward error correcting code. In this paper, we revisited the FSK demodulation and the conventional RS decoding. The FSK demodulation was modified to do joint demodulation and provide the soft decision of the type in the presence of multi-user interference. The conventional RS decoding was replaced by a block turbo code consisting of two shortened RS codes with interleaving between them. An iterative multi-user decoding algorithm is developed to do a soft collaborative decoding. This 'soft core' of the decoder is shown to be more resistant to the multi-user interference and the channel noise. The results show a great improvement in the capacity of the system as compared to systems which erase the hits or which employ hard decision in the joint demodulation.