A Comparative Study of Downlink MIMO Cellular Networks With Co-Located and Distributed Base-Station Antennas

Despite the common belief that substantial capacity gains can be achieved by using more antennas at the base-station (BS) side in cellular networks, the effect of BS antenna topology on the capacity scaling behavior is little understood. In this paper, we present a comparative study on the ergodic capacity of a downlink single-user multiple-input-multiple-output (MIMO) system where BS antennas are either co-located at the center or grouped into uniformly distributed antenna clusters in a circular cell. By assuming that the number of BS antennas and the number of user antennas go to infinity with a fixed ratio L ≫ 1, the asymptotic analysis reveals that 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 BS antenna layouts, respectively, where α > 2 denotes the path-loss factor. The analysis is further extended to the multiuser case where a 1-tier (7-cell) MIMO cellular network with K ≫1 uniformly distributed users in each cell is considered. By assuming that the number of BS antennas and the number of user antennas go to infinity with a fixed ratio L ≫ K, an asymptotic analysis is presented on the downlink rate performance with block diagonalization (BD) adopted at each BS. It is shown that the average per-antenna rates with the co-located and distributed BS antenna layouts scale in the orders of log₂ L/K and log₂ (L-K+1)^α/2/K, respectively. The rate performance of MIMO K cellular networks with small cells is also discussed, which highlights the importance of employing a large number of distributed BS antennas for the next-generation cellular networks.