A Comparative Study of Downlink MIMO Cellular Networks with Co-located and Distributed Base-Station Antennas

摘要

Despite the common belief that substantial capacity gains can be achieved byusing more antennas at the base-station (BS) side in cellular networks, theeffect of BS antenna topology on the capacity scaling behavior is littleunderstood. In this paper, we present a comparative study on the ergodiccapacity of a downlink single-user multiple-input-multiple-output (MIMO) systemwhere BS antennas are either co-located at the center or grouped into uniformlydistributed antenna clusters in a circular cell. By assuming that the number ofBS antennas and the number of user antennas go to infinity with a fixed ratio$L\gg 1$, the asymptotic analysis reveals that the average per-antennacapacities in both cases logarithmically increase with $L$, but in the ordersof $\log_2 L$ and $\tfrac{\alpha}{2}\log_2 L$, for the co-located anddistributed BS antenna layouts, respectively, where $\alpha>2$ denotes thepath-loss factor. The analysis is further extended to the multi-user case wherea 1-tier (7-cell) MIMO cellular network with $K\gg 1$ uniformly distributedusers in each cell is considered. By assuming that the number of BS antennasand the number of user antennas go to infinity with a fixed ratio $L\gg K$, anasymptotic analysis is presented on the downlink rate performance with blockdiagonalization (BD) adopted at each BS. It is shown that the averageper-antenna rates with the co-located and distributed BS antenna layouts scalein the orders of $\log_2 \tfrac{L}{K}$ and $\log_2\frac{(L-K+1)^{\alpha/2}}{K}$, respectively. The rate performance of MIMOcellular networks with small cells is also discussed, which highlights theimportance of employing a large number of distributed BS antennas for thenext-generation cellular networks.