Multi-Cellular Wireless With Base Stations Employing Unlimited Numbers of Antennas
Thomas L. Marzetta
Bell Labs, Alcatel-Lucent

A wireless system comprises autonomous base stations, each serving a multiplicity of autonomous single-antenna terminals in its respective cell, through multi-user MIMO on both the forward and the reverse links. Reverse-link pilot sequences, which are orthogonal among the users within each cell, directly provide the reverse-link channel state information (CSI) to the base station, and indirectly the forward-link CSI through time-division duplex operation combined with reciprocity; the base station has no prior knowledge of the channel. The channel is assumed constant only for the duration of a coherence interval over which occur three distinct transmission activities: reverse-link data, reverse-link pilots, and forward-link data. The assumption of an infinite number of antennas at each base station permits a rigorous but simple multi-cellular analysis/simulation which accounts for all relevant phenomena: channel noise, Rayleigh fading, log-normal shadow fading, geometric attenuation, channel estimation inaccuracy, and inter-cell interference. A number of exact, far-reaching conclusions follow: the effects of uncorrelated receiver noise and Rayleigh fading vanish, there is no residual intra-cell interference, the throughput per cell and the number of users per cell which can be simultaneously served are independent of the cell-size, the spectral efficiency is independent of channel bandwidth, and the required transmission energy per bit is equal to zero. The maximum number of users which can be simultaneously served in each cell is approximately half of the ratio by the coherence interval duration to the channel delay-spread. The only remaining impairment is inter-cell interference that arises from the re-use of pilot sequences in other cells (pilot contamination).