In MIMO communications, information about the transmitted signal is conveyed to the receiver through fields and currents on the surface of the receive antenna array. However, this information can only be observed indirectly through the ports of the array. This raises fundamental information-theoretic questions: How much useful information is contained in the fields and currents on the surface of the receiver array? How much of this information is captured by the array ports? Do conventional arrays efficiently extract the information contained in their conducting surfaces? In this paper, we consider these questions in the context of a multiuser MIMO (MU-MIMO) uplink where users are separated by spatial beamforming at the receiver. Our main results can be summarized as follows: To quantify the information contained in the EM fields and currents on the surface of the receiving antenna array, we first introduce a new model, the surface receiver model. We then use this model to derive upper bounds on the spectral efficiency that can be approached with any location of M ports on the surface of the receiving antenna array. We denote this the surface modal bound. Furthermore, we also derive upper bound on the spectral efficiency approached with any number or location of ports on the receiver array. We call it the surface spectral efficiency. Finally, we apply the analytical results to a MU-MIMO uplink with an array of patch antennas at the receiver. The results suggests that the conventional arrays of single and dual-polarized patches fails to capture most of the information contained in the surface currents. The results further suggest ways to modify the number of antenna ports, together with the receiver front-end, to extract the information in the surface currents more efficiently.
