Antenna Matching for Capacity Maximization in Compact MIMO Systems

As MIMO technology slowly matures, it is finding its way into more wireless applications. However, some important applications, including mobile communications, require compact implementations. One important challenge in miniaturizing MIMO systems for compact terminals is to overcome capacity performance degradation resulting from mutual coupling among closely separated antennas. In this contribution, we begin with a review of the state-of-the-art, with particular emphasis on impedance matching and its impact on capacity. Whereas it has been shown that a multiport extension of the conjugate match is optimum in a reference environment with uniform 3D angular power spectrum, its bandwidth is severely reduced by decreasing antenna separation. On the other hand, noncoupled, individual port matching is inherently simpler to implement and broader in bandwidth, but offers a smaller capacity. Here, we demonstrate that mean capacity can be easily maximized with respect to individual port matching in a given random field. The extent of capacity gains provided by the optimized matching network over existing individual port matching networks strongly depends on the propagation environment.