Soft multiuser demodulation and iterative decoding for FH/SSMA with a block turbo code

It has been shown that the number of users that can be supported by frequency-hopped, spread-spectrum multiple-access systems can be increased greatly by using multiuser demodulation and iterative decoding as compared to a receiver, which employs noncollaborative demodulation and decoding at the same probability of error under similar conditions of noise levels, fading, and bandwidth. The above receiver employed hard-decision multiuser demodulation followed by an iterative decoder With users exchanging their decoded information with each other. Additional information from the multiuser demodulation in the first decoding iteration is limited by hard-decision output of multiuser demodulator. Error-correcting code used was an errors-and-erasures Reed-Solomon (RS) decoder. In this paper, we revisited hard-decision demodulation and conventional RS decoding. Hard-decision multiuser demodulation was modified to provide a soft output, which is then given to a nonbinary block turbo code with shortened RS codes as the constituent codes. An iterative multiuser decoding algorithm is developed to do soft multiuser interference cancellation. This soft receiver with soft demodulation and decoding is shown to be more resistant to multiuser interference and channel noise, especially at lower values of signal-to-noise ratio. The results show a great improvement in the ability of the system to support more users (more than three times in some cases), as compared with systems that erase all hits or employ hard-decision multiuser demodulation followed by RS code. We examine the proposed method for synchronous as well as asynchronous frequency-hopped systems in both additive white Gaussian noise and fading channels.