From practical and theoretical viewpoints, performance analysis of communication systems by using information-theoretic results is important. In this paper, first, we obtain a general achievable rate region for three-user discrete memoryless (DM) Multiple Access Relay Channel (MARC) via partial decode and forward (DF) strategy and regular block Markov encoding/ backward decoding. Also by extending the discrete alphabet results to the continuous alphabet channel, an inner bound for three-user Gaussian MARC (GMARC) is achieved. Next, based on the inner bound of three-user GMARC, we analyze the impact of a relay on the coverage region of MARC, as a basic model for uplink communications between a base station and users with the help of a relay. This analysis includes the coverage region for the MAC and two-user GMARC as special cases. Considering rate regions for the Gaussian MARC and fading Gaussian MARC, it is proved that in a fixed transmission rate, the relay extends coverage region of the MARC. Numerical illustrations confirm our theoretical results.

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