The Effect of Spatial Multiplexing of the Interference on MIMO Communication Performance

This work sheds light on the effects of spatially multiplexed interference on multiple-input-multiple-output (MIMO) networks. In particular, any increase in the number of interfering data streams (while keeping the total interference power constant) is shown to degrade the quality of the interfered link. Although this statement appears very intuitive, it has yet to be proven. In this work, we first give a mathematical definition of the intuitive notion of `increasing the number of streams' leads to proof that the achievable rate of a link decreases when any of its interferers increases its number of data streams. The achievable rate is measured by the mutual information of the link or by the spectral efficiency of the optimal linear Minimum-Mean-Square (MMSE) receiver. Correlatively, we show that the worst power allocation for an interferer is the equal power allocation for all the streams. This result highlights the importance of the optimization of the number of data streams at each transmitter in MIMO networks.