Large MIMO systems are systems which use tens to hundreds of antennas in communication terminals. Depending on the application scenario, different MIMO system configurations are possible. These include point-to-point MIMO and multiuser MIMO configurations. In multiuser MIMO, point-to-multipoint (e.g., downlink in cellular systems), and multipoint-to-point (e.g., uplink in cellular systems) configurations are common. In a point-to-point communication scenario (Fig. 1.1), the number of transmit antennas nt at the transmitter and the number of receive antennas nr at the receiver can be large. A typical application scenario for a point-to-point large MIMO configuration is providing high-speed wireless backhaul connectivity between BSs using multiple antennas at each BS. Since space constraint need not be a major concern at the BSs, a large number of antennas can be used at both the transmit and receive BSs in this application scenario.

In multiuser MIMO (Fig. 1.2), the communication is between a BS and multiple user terminals. These user terminals can be small devices like mobile/smart phones or medium sized terminals like laptops, set-top boxes, TVs, etc. In mobile applications where mobile/smart phones and personal digital assistants are the user terminals, only a limited number of antennas can be mounted on them because of space constraints. However, in applications involving user terminals like TVs, set-top boxes, and laptops a larger number of antennas can be used on the user terminal side as well.