Whereas in the previous chapter we considered a single-user MIMO link
where data streams intended for a single user are multiplexed spatially, in this
chapter we consider multiuser MIMO channels where data streams to/from
multiple users are multiplexed in this way. In particular, we consider two
multiuser Gaussian MIMO channels that are relevant for cellular networks.
The uplink can be modeled by the Gaussian MIMO multiple-access channel
(MAC), a many-to-one network where independent signals from K >1 users
are received by a single base receiver. The downlink can be modeled by the
Gaussian MIMO broadcast channel (BC), a one-to-many network where a
single base transmitter serves K > 1 users. To avoid needless repetition, we
will henceforth drop the “Gaussian MIMO” qualifier when referring to these
channels.