Multiple-input, multiple-output (MIMO) antenna systems offer capacity benefits over their single-input single-output (SISO) counterparts, thus attracting considerable current research. MIMO performance depends on a wide range of parameters including radiation efficiency, correlation, mutual coupling, matching efficiency and polarization misalignment. No single simulation method has been described that includes each of these effects so critical to handset array designs. This work synthesizes a comprehensive model to incorporate each of these effects. In order to manage the complexity of such a model, the MIMO Transmission Equation is introduced - similar to the well-known Friis transmission equation.