  In the setting of the time-selective two-user multiple-input single-output
(MISO) broadcast channel (BC), recent work by Tandon et al. considered the case
where - in the presence of error-free delayed channel state information at the
transmitter (delayed CSIT) - the current CSIT for the channel of user 1 and of
user 2, alternate between the two extreme states of perfect current CSIT and of
no current CSIT.
  Motivated by the problem of having limited-capacity feedback links which may
not allow for perfect CSIT, as well as by the need to utilize any available
partial CSIT, we here deviate from this `all-or-nothing' approach and proceed -
again in the presence of error-free delayed CSIT - to consider the general
setting where current CSIT now alternates between any two qualities.
Specifically for $I_1$ and $I_2$ denoting the high-SNR asymptotic
rates-of-decay of the mean-square error of the CSIT estimates for the channel
of user~1 and of user~2 respectively, we consider the case where $I_1,I_2
\in\{\gamma,\alpha\}$ for any two positive current-CSIT quality exponents
$\gamma,\alpha$. In a fast-fading setting where we consider communication over
any number of coherence periods, and where each CSIT state $I_1I_2$ is present
for a fraction $\lambda_{I_1I_2}$ of this total duration, we focus on the
symmetric case of $\lambda_{\alpha\gamma}=\lambda_{\gamma\alpha}$, and derive
the optimal degrees-of-freedom (DoF) region. The result, which is supported by
novel communication protocols, naturally incorporates the aforementioned
`Perfect current' vs. `No current' setting by limiting $I_1,I_2\in\{0,1\}$.
  Finally, motivated by recent interest in frequency correlated channels with
unmatched CSIT, we also analyze the setting where there is no delayed CSIT.
