  In two-way channels, each user transmits and receives at the same time. This
allows each encoder to interactively adapt the current input to its own message
and all previously received signals. Such coding approach can introduce
correlation between inputs of different users, since all the users' outputs are
correlated by the nature of the channel. However, for some channels, such
adaptation in the coding scheme and its induced correlation among users are
useless in the sense that they do not help enlarge the capacity region with
respect to the standard coding method (where each user encodes only based on
its own message). In this paper, it is shown that adaptation is not helpful for
enlarging the capacity region of two classes of two-way discrete channels: the
modulo additive-noise channel with memory and the multiple access/degraded
broadcast channel.
