  In this paper, we study the information theoretic maximal degrees of freedom
(DoF) for the symmetric multi-input-multi-output (MIMO) interfering broadcast
channel (IBC) with arbitrary antenna configurations. For the G-cell K-user MXN
MIMO-IBC network, we find that the information theoretic maximal DoF per user
are related to three DoF bounds: 1) the decomposition DoF bound
d^{Decom}=MN/(M+KN), a lower-bound of linear interference alignment (IA) with
infinite time/frequency extensions (called asymptotic IA); 2) the proper DoF
bound d^{Proper}=(M+N)/(GK+1), an upper-bound of linear IA without
time/frequency extensions (called linear IA); and 3) the quantity DoF bound
d^{Quan}, a zigzag piecewise linear function of M and N. The whole region of
M/N can be divided into two regions, Region I and Region II. Specifically, for
most configurations in Region I, the information theoretic maximal DoF are the
decomposition DoF bound and can be achieved by the asymptotic IA. For all
configurations in Region II, the information theoretic maximal DoF are the
quantity DoF bound and can be achieved by the linear IA. To obtain the tight
upper-bound, we propose a unified way to construct genies, where the genies
help each base station or user resolve the maximal number of interference.
According to the feature that the designed genies with the same dimension can
derive identical DoF upper-bound, we convert the information theoretic DoF
upper-bound problem into a linear algebra problem and obtain the closed-form
DoF upper-bound expression. Moreover, we develop a non-iterative linear IA
transceiver to achieve the DoF upper-bound for the networks with antenna
configurations in Region II, which means that the derived DoF upper-bound is
tightest. The basic principles to derive the DoF upper-bound and design the
linear IA transceiver to achieve the DoF upper-bound can be extended into
general asymmetric networks.
