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

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... tween the users' encoders/decoders is sufficient to allow for multiple access and channel error correction, while providing rates close to channel capacity.