MIMO for Millimeter Wave Wireless Communications: Beamforming, Spatial Multiplexing, or Both?
Shu Sun, Theodore Rappaport, Robert W. Heath Jr, Andrew Nix, and Sundeep Rangan
Submitted to the IEEE Communications Magazine, Sep. 2014.
The use of millimeter wave (mmWave) 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, multiple-input multiple-output (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 Long Term Evolution (LTE) 4G cellphone standards. Given the tiny wavelengths associated with mmWave, coupled with differences in the propagation and antennas used, it is unclear how well spatial multiplexing with multiple streams will be suited to future mmWave mobile communications. This tutorial explores the fundamental issues involved in selecting the best communications approach for mmWave frequencies, and provides insights, challenges, and appropriate uses of each MIMO technique based on early knowledge of the propagation environment.