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|a We discuss the problem of designing channel access architectures for enabling fast, low-latency, grant-free and uncoordinated uplink for densely packed wireless nodes. Specifically, we extend the concept of random-access code introduced at ISIT'2017 by one of the authors to the practically more relevant case of the AWGN multiple-access channel (MAC) subject to Rayleigh fading, unknown to the decoder. We derive bounds on the fundamental limits of random-access coding and propose an alternating belief-propagation scheme as a candidate practical solution. The latter's performance was found to be surprisingly close to the information-theoretic bounds. It is curious, thus, that while fading significantly increases the minimal required energy-per-bit Eb/N0 (from about 0-2 dB to about 8-11 dB), it appears that it is much easier to attain the optimal performance over the fading channel with a practical scheme by leveraging the inherent randomization introduced by the channel. Finally, we mention that while a number of candidate solutions (MUSA, SCMA, RSMA, etc.) are being discussed for the 5G, the information-theoretic analysis and benchmarking has not been attempted before (in part due to lack of common random-access model). Our work may be seen as a step towards unifying performance comparisons of these methods.
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