us taking advantage of frequency-selective multiuser diversity. The QoS is satisfied by allocating enough subcarriers, even if there are not enough "good" ones available. The paper assumes perfect knowledge of CSI, which is a challenge. Multiuser multiple input multiple output MIMO is a radio technique that transmits and/or receives over multiple antennas to exploit spatial diversity for increasing system throughput. Consider the system in Fig. 7-6 as an example. Different MIMO implementations exist, including single-user MIMO which uses transmit and receive diversity to increase throughput to a single user, multiuser MIMO where spatial diversity across users is used to simultaneously transmit and receive to multiple users and Antenna 1 User 1 Base Station User K Antenna M FIGURE 7-6 Multiuser MIMO system. MIMO, multiple input multiple output. 248 Open Radio Access Network (O-RAN) Systems Architecture and Design massive MIMO, which is a scaled-out version of multiuser MIMO to a high antenna count (say, 64 antennas). Find a primary user using opportunistic or QoS-based scheduler algorithms and then pair the primary users with others that cause minimum interference between each other. This pairing is done by maintaining a metric of "orthogonality" between all user pairs that mea- sures the spatial compatibility between MIMO users. Users are spatially compatible if the multiuser MIMO channels of those users can be separated in the spatial domain by means of beamforming and precoding. Consider a broadcast channel between a single transmitter (base station) and many users, 46 like shown in Fig. 7.6: y=Hx+n is the r X t complex matrix where the (i,j)th entry defines the path between the jth transmit antenna and the ith receiver. y is the r X 1 vector representing the received signal for the ith receiver where I = {1, r). n is the complex Gaussian noise vector. Denote hi as the ith row of H corresponding to the channel between the base station and the ith receiver. The channel is assumed to be memoryless and quasistatic