MIMO in particular, takes advantage of spatial diversity with multiple antennas at both the transmitter and receiver. The benefits yielded from spatial diversity actually increase with the number of antennas employed. However, there is the tradeoff between transmission rate and spatial diversity. The multitude of antennas elements can either send different data over different paths (multiplexing gain), or the same signal over different paths (diversity gain). Commonly known as the diversity-multiplexing tradeoff, there is a balancing act between these two methods of transmission (Figure 1). Some standard MIMO configurations are 2x2, 3x3, 4x4 (802.11n/ah), and 8x8 (802.11ax/ac).
Also known as multi-user MIMO (MU-MIMO), the concept of spatial multiplexing can also be applied towards multiple receivers with or without antenna diversity (Figure 2). This increases system efficiency while maintaining link reliability. Wi-FI has support for MU-MIMO in the 802.11ac/ax standards (downlink only) while 4G LTE (rel 8) and LTE-A (Rel. 10) include MU-MIMO. As opposed to SU-MIMO systems, MU-MIMO systems are more susceptible to co-channel interference, often requiring nearly perfect CSI and a low relatively SNR.