Quantum key distribution (QKD) systems are being developed for possible use in future free space optical communication links to provide high security transmissions. However, these systems restrict the communication to a low bit-rate. To increase the QKD bit rate we suggest implementing a communication system which includes both optical detector and transmitter arrays. The mathematical description of this scheme involves a multi-input multi-output (MIMO) model. The optical detector and transmitter arrays enable the receiver to communicate simultaneously with a number of transmitter elements, which consequently increases the QKD bit rate. However, this method may introduce interference effects when a photon transmitted from one transmitter element reaches the wrong detector element in the receiver, due to scattering and turbulence in the atmospheric channel. This crosstalk and interference reduce the overall system bit rate and increase the quantum bit error rate (QBER). Analyzing the effect of crosstalk and interference on the system performance is accomplished by using the optical transfer function (OTF) theory.