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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 I1 and I2 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 I1, I2 &#x03F5; {&#x03B3;,&#x03B1;} for any two positive current-CSIT quality exponents &#x03B3;, &#x03B1; as a result, the overall current CSIT - for both users' channels - alternates between any four states I1I2 &#x03F5; {&#x03B3;&#x03B3;,&#x03B3;&#x03B1;, &#x03B1;&#x03B3;, &#x03B1;&#x03B1;}. In a fast-fading setting where we consider communication over any number of coherence periods, and where each CSIT state I1I2 is present for a fraction &#x03BB;I1I2 of this total duration (naturally forcing &#x03BB;&#x03B1;&#x03B3; ; &#x03BB;&#x03B3;&#x03B1; ; &#x03BB;&#x03B1;&#x03B1; ; &#x03BB;&#x03B3;&#x03B3; = 1), we focus on the symmetric case of &#x03BB;&#x03B1;&#x03B3; = &#x03BB;&#x03B3;&#x03B1;, and derive the optimal degrees-of-freedom (DoF) region to be the polygon with corner points {(0,0), (0,1), (&#x03BB;&#x0305;, 1), (2+&#x03BB;/3, 2+&#x03BB;/3), (1, &#x03BB;&#x0305;), (1,0)}, for some &#x03BB;&#x0305; &#x0394; (&#x03BB;&#x03B3;&#x03B1; ; &#x03BB;&#x03B3;&#x03B3;)&#x03B3; ; (&#x03BB;&#x03B1;&#x03B3; ; &#x03BB;&#x03B1;&#x03B1;)&#x03B1;, representing a measure of the average CSIT quality. The result, which is supported by novel communication protocols, naturally incorporates the aforementioned `Perfect current' vs. `No current' setting by limiting I1, I2 &#x03F5; {0,1}, as well as the Yang et al. and Gou and Jafar setting by forcing &#x03B1; = &#x03B3;. Finally, motivated by recent interest in frequency correlated channels with unmatched CSIT, we also analyze the setting where there is no delayed CSIT.

Optimal DoF region of the two-user MISO-BC with general alternating CSIT