Uri Erez Stephan ten Brink
Previous chapters have focused on the Multiple-Access (MAC) channel, or the uplink in a typical communication scenario. In this chapter, we consider communication on the downlink. In multiuser broadcast (BC) channels, a transmitter communicates with a multiplicity of users simultaneously. While it is possible to restrict transmission to schemes such that signals arriving at different users are orthogonal, this is in general suboptimal. To approach the capacity of the Gaussian broadcast channel, we need to use transmission schemes that result in interference between users. Thus, for a given user, we are now faced with a detection problem of recovering the desired signal in the presence of interfering (undesired) signals. In other words, we are faced with a multiuser detection problem.
In this chapter, we shall see that this multiuser detection problem may be circumvented using precoding techniques. In effect, we will see how (capacity-achieving) detection at each receiver may be performed without joint decoding of the messages of the interfering users. This insight hinges on the “dirty paper” coding result  which will be described.
We will outline recent advances in the field and discuss multiuser detection versus multiuser precoding for the Gaussian MIMO broadcast scenario. In fact, the latter approach allows us to achieve the capacity ...