MIMO for millimeter-wave wireless communications: Beamforming, spatial multiplexing, or both?

The use of mm Wave frequencies for wireless communications offers channel bandwidths far greater than previously available, while enabling dozens or even hundreds of antenna elements to be used at the user equipment, base stations, and access points. To date, MIMO techniques, such as spatial multiplexing, beamforming, and diversity, have been widely deployed in lower-frequency systems such as IEEE 802.11n/ac (wireless local area networks) and 3GPP LTE 4G cellphone standards. Given the tiny wavelengths associated with mm Wave, coupled with differences in the propagation and antennas used, it is unclear how well spatial multiplexing with multiple streams will be suited to future mm Wave mobile communications. This tutorial explores the fundamental issues involved in selecting the best communications approaches for mm Wave frequencies, and provides insights, challenges, and appropriate uses of each MIMO technique based on early knowledge of the mm Wave propagation environment.