Does Low Mutual Coupling Imply Low Antenna Correlation?

Multiple-input multiple-output (MIMO) systems promise high data rate and have been employed ubiquitously in modern telecommunications. However, due to limited space for MIMO antennas and finite angular spread of the propagation channel, antenna mutual coupling and correlations are inevitable, which can degrade the MIMO performance considerably. Since the antenna correlation is a function of antenna patterns and angular distribution of the spatial channel, it is less intuitive than the mutual coupling for antenna engineers. As a result, MIMO antenna designers usually focus on mutual coupling reduction, hoping that the correlation will be improved as well. But, does mutual coupling reduction guarantee low antenna correlation? Unfortunately, this important question has not been well answered in the literature. We try to address this issue in this work.