Evaluating multi-element antennas using equivalent number of antenna elements

Multiple-input, multiple-output (MIMO) systems use multi-element antennas (MEAs) for improving wireless communications links. Over-the-air testing is used to evaluate the communications link of a mobile terminal, but there is no standardized metric for evaluating the design of an MEA. In this paper, the concept of an equivalent number of idealized elements is discussed for the evaluation of different MEA designs. The equivalence refers to the ports of an idealized MEA, viz., lossless, uncoupled, uncorrelated, and having equal mean distributed gains, or SNRs. This equivalent number of idealized elements provides a ready conversion to a statistically expected communications performance, such as a capacity. It also leads to a volumetric efficiency (space efficiency) if considered with a number of antenna spherical modes. Examples are given using the diversity gain and the information-theoretic capacity, in modeled propagation scenarios. Finally, the differences between this equivalent number of elements and the number of eigen-modes of MIMO channels, and the number of spherical wave modes is discussed.