Whereas superposition coding as above relies on interference cancelation at
the receiver, an alternative technique for achieving the capacity region places
the burden of complexity on the transmitter. This technique, known as dirty
paper coding (DPC) [79], is an encoding process that occurs jointly and in an
ordered fashion among the users so that a given user experiences interference
only from users encoded after it. In other words, a given user receives zero
power from the signals of users that are encoded earlier in the sequence,
as if the signals of these users were “pre-subtracted” at the transmitter. In
our example, user 1 would be encoded in a conventional (capacity-achieving)
manner, then user 2 would be encoded using DPC with non-causal knowledge
of user 1’s data signal.