EXIT chart optimization of Repeat-Accumulate codes for an N-Frequency T-User Multiple Access Channel with noise

In this paper we show how a graph-based system can be designed for an N-FSK T-user multiple access channel with noise. The channel is a frequency selective fast fading channel with additive white Gaussian Noise. The system does not know or attempt to determine the fading incurred at a given moment. The system is composed of a multiuser detector and T different Repeat-Accumulate (RA) decoders. The decoders are optimized as a set and consequently have the same parameters. A structural variation between the users‘ encoders/decoders is sufficient to allow for multiple access and channel error correction, while providing rates close to channel capacity.