Antenna topology impacts on measured MIMO capacity

The achievable spectral efficiency of multiple antenna radios is greatly improved by Multiple Input Multiple Output (MIMO) signaling. However, the benefit of MIMO is lessened in practice due to mutual coupling in the antenna arrays. This problem is exacerbated by the size restrictions on antenna arrays in portable devices. We present a compact antenna array that is designed to limit mutual coupling and has good radiation efficiency. The array was fabricated and integrated into a MIMO radio testbed. Using this array, we evaluate the testbed's achievable capacity and compare it with the performance achieved using arrays of dipoles with varied spacing. The results provide valuable insight on the design requirements for compact antenna arrays in portable MIMO sytems. We find that, for rich multipath environments, MIMO transceivers with weak coupling-induced correlation at one end of the link and a moderate coupling-induced correlation at the other end have very little loss of system capacity compared to the uncorrelated MIMO case. However, although mutual coupling may not cause appreciable loss in system capacity, mutual coupling should be minimized in compact MIMO array designs if the radiating efficiency is to be maximized.