Multi-user MIMO-OFDM cell throughput under real-world propagation conditions

Abstract

An innumerable number of papers has been published on MIMO (multiple input multiple output) systems. The original and most of the current work deals with a single link between a multi-element antenna (MEA) transmitter and a MEA receiver. However, link-level capacity gain does not necessarily translate into a similar system-level gain. Therefore, several researchers have investigated the performance of MIMO systems on cell or even system level, including intracell and/or intercell interference. While these multi-user MIMO investigations are very helpful, presented (simulation) results have been mostly limited to simple propagation scenarios (e.g., flat fading and/or i.i.d. Rayleigh fading). Here we present results for the uplink sum capacity of multiple MIMO users in a single cell (i.e., intercell interference is not explicitly accounted for) based on a channel model which accounts for partial MIMO correlations, large-scale fading effects, variable delay spread, non-vanishing Rician factor, and random user orientation. We concentrate on the cell capacity. However, estimation of the maximum uplink cell throughput for real-world AMC (adaptive modulation and coding) is similarly possible via the SNR gap approximation and power/bit loading. According to the simulation results, beamforming is the capacity-achieving strategy for a large number of users within the cell. This is consistent with theoretical results derived by other researchers. However, the simulated results here cover the entire range from optimum single-user performance (i.e., transmission on spatial eigenmodes plus water filling) to the capacity when the number of users greatly exceeds the number of base station antennas (i.e., with beamforming being the best strategy).