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While multiple-input multiple-output (MIMO) wireless systems can improve communication performance over single-antenna systems in multipath propagation environments, there has been concern about integrating the technology into small mobile devices. Specifically, the necessary close antenna spacing and integration of multiple radio front-ends on a single chip lead to coupling effects at the antenna and circuit levels that impact the operation. It is therefore important to carefully characterize this coupling. In this paper, we provide a detailed framework based on network theory for analyzing the performance of MIMO systems where coupling is present. We apply the approach to multi-antenna handset topologies characterized using full-wave finite-difference time-domain (FDTD) simulations of the antennas and a simple model for front-end amplifier coupling. The results show the relative performance for different matching conditions between the receive antennas and amplifiers as well as different amplifier coupling for the handsets operating in a realistic propagation environment.