A massive MIMO system, represented by a base station with hundreds ofantennas, is capable of spatially multiplexing many devices and thus naturallysuited to serve dense crowds of wireless devices in emerging applications, suchas machine-type communications. Crowd scenarios pose new challenges in thepilot-based acquisition of channel state information and call for pilot accessprotocols that match the intermittent pattern of device activity. A joint pilotassignment and data transmission protocol based on random access is proposed inthis paper for the uplink of a massive MIMO system. The protocol relies on theaveraging across multiple transmission slots of the pilot collision events thatresult from the random access process. We derive new uplink sum rateexpressions that take pilot collisions, intermittent device activity, andinterference into account. Simplified bounds are obtained and used to optimizethe device activation probability and pilot length. A performance analysisindicates how performance scales as a function of the number of antennas andthe transmission slot duration.
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