In the setting of the time-selective two-user multipleinput 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 is an element of{gamma, alpha} for any two positive current-CSIT quality exponents gamma, alpha; as a result, the overall current CSIT - for both users' channels - alternates between any four states I1I2 is an element of {gamma gamma, gamma alpha, alpha gamma, alpha alpha}. 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 lambda(I1I2) of this total duration (naturally forcing lambda(alpha gamma)+lambda(gamma alpha)+lambda(alpha alpha)+lambda(gamma gamma) - 1), we focus on the symmetric case of lambda(alpha)gamma = lambda(gamma alpha), and derive the optimal degrees-of-freedom (DoF) region to be the polygon with corner points {(0, 0), (0, 1), ((lambda) over bar, 1), (2+(lambda) over bar /3, 2+(lambda) over bar /3), (1, (lambda) over bar), (1, 0)}, for some (lambda) over bar= (lambda(gamma alpha) + lambda(gamma gamma)) gamma + (lambda(alpha gamma) +lambda(alpha alpha)) alpha, 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 I-1, I-2 is an element of {0, 1}, as well as the Yang et al. and Gou and Jafar setting by forcing alpha = gamma. Finally, motivated by recent interest in frequency correlated channels with unmatched CSIT, we also analyze the setting where there is no delayed CSIT.
