  The degrees of freedom (DoF) of the two-user multiple-input single-output
(MISO) broadcast channel (BC) are studied under the assumption that the form,
I_i, i=1,2, of the channel state information at the transmitter (CSIT) for each
user's channel can be either perfect (P), delayed (D) or not available (N),
i.e., I_1 and I_2 can take values of either P, D or N, and therefore the
overall CSIT can alternate between the 9 resulting states, each state denoted
as I_1I_2. The fraction of time associated with CSIT state I_1I_2 is denoted by
the parameter \lambda_{I_1I_2} and it is assumed throughout that
\lambda_{I_1I_2}=\lambda_{I_2I_1}, i.e., \lambda_{PN}=\lambda_{NP},
\lambda_{PD}=\lambda_{DP}, \lambda_{DN}=\lambda_{ND}. Under this assumption of
symmetry, the main contribution of this paper is a complete characterization of
the DoF region of the two user MISO BC with alternating CSIT. Surprisingly, the
DoF region is found to depend only on the marginal probabilities (\lambda_P,
\lambda_D,\lambda_N)=(\sum_{I_2}\lambda_{PI_2},\sum_{I_2}\lambda_{DI_2},
\sum_{I_2}\lambda_{NI_2}), I_2\in {P,D,N}, which represent the fraction of time
that any given user (e.g., user 1) is associated with perfect, delayed, or no
CSIT, respectively. As a consequence, the DoF region with all 9 CSIT states,
\mathcal{D}(\lambda_{I_1I_2}:I_1,I_2\in{P,D,N}), is the same as the DoF region
with only 3 CSIT states \mathcal{D}(\lambda_{PP}, \lambda_{DD}, \lambda_{NN}),
under the same marginal distribution of CSIT states, i.e., (\lambda_{PP},
\lambda_{DD},\lambda_{NN})=(\lambda_P,\lambda_D,\lambda_N). The results
highlight the synergistic benefits of alternating CSIT and the tradeoffs
between various forms of CSIT for any given DoF value.
