  We consider a multiple-input-single-output (MISO) broadcast channel with
mixed channel state information at the transmitter (CSIT) that consists of
imperfect current CSIT and perfect outdated CSIT. Recent work by Kobayashi et
al. presented a scheme which exploits both imperfect current CSIT and perfect
outdated CSIT and achieves higher degrees of freedom (DoF) than possible with
only imperfect current CSIT or only outdated CSIT individually. In this work,
we further improve the achievable DoF in this setting by incorporating
additional private messages, and provide a tight information theoretic DoF
outer bound, thereby identifying the DoF optimal use of mixed CSIT. The new
result is stronger even in the original setting of only delayed CSIT, because
it allows us to remove the restricting assumption of statistically equivalent
fading for all users.
