On the degrees of freedom of the K-user time correlated broadcast channel with delayed CSIT

The degrees of freedom (DoF) of a K-User MISO broadcast channel (BC) is studied when the transmitter (TX) has access to a delayed channel estimate in addition to an imperfect estimate of the current channel. The current estimate could be for example obtained from prediction applied on past estimates, in the case where feedback delay is within the coherence time. Prior results in this setting are promising, yet remain limited to the two-user case. In contrast, we consider here an arbitrary number of users. We develop a new transmission scheme, called the K_α-MAT scheme, which builds upon both the principle of the MAT alignment from Maddah-Ali and Tse and zero-forcing (ZF) to achieve a larger DoF in the channel state information (CSI) configuration previously described. We also develop a new upper bound for the DoF to compare with the DoF achieved by K_α-MAT. Although not optimal, the K_α-MAT scheme performs well when the CSIT quality is not too delayed or K is small. The K_α-MAT scheme can be seen as a robust version of ZF with respect to the delay in the CSI feedback.