  Wireless random access protocols are attracting a revived research interest
as a simple yet effective solution for machine-type communications. In the
quest to improve reliability and spectral efficiency of such schemes, the use
of multiple receivers has recently emerged as a promising option. We study the
potential of this approach considering a population of users that transmit data
packets following a simple slotted ALOHA policy to a set of non-cooperative
receivers or relays (uplink phase). These, in turn, independently forward -
part of - what decoded towards a collecting sink (downlink phase). For an
on-off fading channel model, we provide exact expressions for uplink throughput
and packet loss rate for an arbitrary number of relays, characterising the
benefits of multi-receiver schemes. Moreover, a lower bound on the minimum
amount of downlink resources needed to deliver all information collected on the
uplink is provided. The bound is proven to be achievable via random linear
coding when no constraints in terms of latency are set. We complement our study
discussing a family of simple forwarding policies that require no packet-level
coding, and optimising their performance based on the amount of available
downlink resources. The behaviour of both random linear coding and simplified
policies is also characterised when receivers are equipped with finite buffers,
revealing non-trivial tradeoffs.
