On the Scaling Behavior of the Average Rate Performance of Large-Scale Distributed MIMO Systems

To support the massive data traffic in the near future, the distributed antenna system has become a promising candidate for the next-generation cellular system. Due to the lack of a closedform expression, how the average rate performance scales with a large number of distributed base-station (BS) antennas is not well understood. This paper focuses on the average rate performance of the downlink channel of a large-scale distributed antenna system. By assuming that the number of BS antennas at each cluster Nc and the number of user antennas Nc go to infinity with N/Nc → η, asymptotic lower-bounds of the average per-antenna capacities with and without channel state information at the transmitter side (CSIT) in the single-user case are characterized as an explicit function of the ratio η and the number of BS antenna clusters L. Simulation results verify that the average per-antenna capacities with and without CSIT logarithmically increase with L in the orders of Θ(α/2 log
₂
L) andΘ((α/2 - 1) log
₂
L), respectively, where α > 2 is the path-loss factor. The analysis is further extended to the multiuser case with K uniformly distributed users. By assuming that N, N
_c
→ ∞ with N/N
_c
→ η, an asymptotic lower-bound of the average per-antenna rate with block diagonalization (BD) is derived. Simulation results verify that the average per-antenna rate scales in the order of Θ(log
₂
K )⌊L-η(K -1)⌋
^α/2
/K) if the ratio η is fixed. The effect of the cluster size on the average rate performance is further analyzed. Simulation results verify that for a given number of BS antennas, the average per-antenna capacities with and without CSIT in the single-user case and the average per-antenna rate with BD in the multiuser case increase monotonically as the number of BS antennas at each cluster decreases, which indicates that a fully distributed BS antenna layout can achieve the highest average rate performance.