Asymptotic capacity analysis of downlink MIMO systems with co-located and distributed antennas

Despite the common belief that substantial capacity gains can be achieved by employing a large number of antennas at the base-station (BS) side in cellular networks, the effect of BS antenna topology on the capacity scaling behavior is little understood. This paper presents a comparative study on the asymptotic capacity of a downlink single-user multiple-input-multiple-output (MIMO) system with multiple BS antennas which are either co-located or grouped into uniformly distributed clusters. The analysis reveals that when the ratio L of the number of BS antennas to the number of user antennas is large, the average per-antenna capacities in both cases logarithmically increase with L, but in the orders of log2 L and α/2 log2 L for the co-located and distributed antenna layouts, respectively, where α>2 denotes the path-loss factor. A higher capacity is achieved by distributing the BS antennas thanks to the reduction of the minimum access distance.