  We present a novel cross layer approach to random access (RA) that combines
physical-layer network coding (PLNC) with multiuser detection (MUD). PLNC and
MUD are applied jointly at the physical level in order to extract any linear
combination of messages experiencing a collision. The set of combinations
extracted from a whole frame is then processed by the receiver to recover the
original packets. A simple pre-coding stage at the transmitting terminals
allows the receiver to further increase system diversity. We derive an
analytical bound on the system throughput and present simulation results for
the decoding at the physical level as well as several performance measures at
frame level in block fading channels, namely throughput, packet loss rate and
energy efficiency. The results we present are promising and suggest that a
cross layer approach leveraging on the joint use of PLNC and MUD can
significantly improve the performance of RA systems.
