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We study coded multichannel random access schemes for ultra-reliablelow-latency uplink transmissions. We concentrate on non-orthogonal access inthe frequency domain, where users transmit over multiple orthogonal subchannelsand inter-user collisions limit the available diversity. Two different modelsfor contention-based random access over Rayleigh fading resources areinvestigated. First, a collision model is considered, in which the packet isreplicated onto $K$ available resources, $K' \leq K$ of which are receivedwithout collision, and treated as diversity branches by a maximum-ratiocombining (MRC) receiver. The resulting diversity degree $K'$ depends on thearrival process and coding strategy. In the second model, the slots subject tocollisions are also used for MRC, such that the number of diversity branches$K$ is constant, but the resulting combined signal is affected by multipleaccess interference. In both models, the performance of random anddeterministic repetition coding is compared. The results show that thedeterministic coding approach can lead to a significantly superior performancewhen the arrival rate of the intermittent URLLC transmissions is low.

Non-Orthogonal Contention-Based Access for URLLC Devices with Frequency Diversity