Feasibility of closed loop operation for MIMO links with MIMO interference

In a network with several transmitting nodes, array antennas at both ends of MIMO links can be exploited to have multiple links operate in the same channel. Depending on the network topology, each interfering link can transmit fewer modes than it would in an isolated environment, and use the remaining degrees of freedom to avoid or suppress interference. With interference, channel state information (CSI) must be fed back to the transmitter to reach the highest level of performance. We show that the difference between closed- and open-loop capacities when links interfere is usually much larger than with isolated links. This implies that less frequent CSI updates are sufficient to keep closed-loop MIMO performance above open-loop MIMO. We analyze the effect of CSI aging for MIMO links. To understand the effect of fading and study the feasibility of closed-loop operation in MIMO links, we utilize two models for the MIMO channels. First, we use a Rayleigh fading model, and a filtered Gaussian noise method to simulate a fading channel. Then, we consider a more realistic scenario, where we simulate propagation in a typical indoor environment using the finite-difference time-domain (FDTD) method. The results, especially with the realistic channels, indicate that, even with the overhead of CSI updates multiplied by the number of iterations, spatial multiplexing may be attractive for static and low-mobility networks, such as in indoor environments.