Passive intermodulation distortion (PIM), or the nonlinear intermodulation distortion (IMD) products from the mixing of two or more signals in passive media, is a concern for high power multi-band systems. As technology progresses, these systems grow in antenna complexity with the introduction of more channels and antennas with MIMO techniques. The need to maintain low noise floors by mitigating PIM in the antenna and connector heads is important particularly for distributed antenna systems (DAS) or cellular installations.
Understanding the fundamentals of MIMO wireless communications can better illuminate the current and future methods leveraged to overcome fade and multipath in various environments. In the diversity-multiplexing tradeoff, designers have to decide where it is best to put resources depending on the bit error rate (BER), data rate, and signal-to-noise ratio (SNR) of the channel. A system with a high SNR and low BER could then take better advantage of diversity gain by spreading resources over multiple channels. While a system with a high BER would be able to take advantage of spatial diversity to increase the data rate of the system. The antennas used in various MIMO applications can vary in polarity and radiation pattern depending upon system requirements.
 Kshetrimayum, Rakhesh Singh. Fundamentals of MIMO Wireless Communications. Cambridge University Press, 2017.
 Jensen, M.a., and J.w. Wallace. “A Review of Antennas and Propagation for MIMO Wireless Communications.” IEEE Transactions on Antennas and Propagation, vol. 52, no. 11, 2004, pp. 2810–2824., doi:10.1109/tap.2004.835272.
 J. W. Wallace, M. A. Jensen, A. L. Swindlehurst and B. D. Jeffs, "Experimental characterization of the MIMO wireless channel: data acquisition and analysis," in IEEE Transactions on Wireless Communications, vol. 2, no. 2, pp. 335-343, March 2003.