  This paper investigates how multiuser dimensions can effectively be exploited
for target degrees of freedom (DoF) in interfering broadcast channels (IBC)
consisting of K-transmitters and their user groups. First, each transmitter is
assumed to have a single antenna and serve a singe user in its user group where
each user has receive antennas less than K. In this case, a K-transmitter
single-input multiple-output (SIMO) interference channel (IC) is constituted
after user selection. Without help of multiuser diversity, K-1 interfering
signals cannot be perfectly removed at each user since the number of receive
antennas is smaller than or equal to the number of interferers. Only with
proper user selection, non-zero DoF per transmitter is achievable as the number
of users increases. Through geometric interpretation of interfering channels,
we show that the multiuser dimensions have to be used first for reducing the
DoF loss caused by the interfering signals, and then have to be used for
increasing the DoF gain from its own signal. The sufficient number of users for
the target DoF is derived. We also discuss how the optimal strategy of
exploiting multiuser diversity can be realized by practical user selection
schemes. Finally, the single transmit antenna case is extended to the
multiple-input multiple-output (MIMO) IBC where each transmitter with multiple
antennas serves multiple users.
