  Since Maddah-Ali and Tse showed that the completely stale transmitter-side
channel state information (CSIT) still benefits the Degrees of Freedom (DoF) of
the Multiple-Input-Multiple-Output (MISO) Broadcast Channel (BC), there has
been much interest in the academic literature to investigate the impact of
imperfect CSIT on \emph{DoF} region of time correlated broadcast channel. Even
though the research focus has been on time correlated channels so far, a
similar but different problem concerns the frequency correlated channels.
Indeed, the imperfect CSIT also impacts the DoF region of frequency correlated
channels, as exemplified by current multi-carrier wireless systems.
  This contribution, for the first time in the literature, investigates a
general frequency correlated setting where a two-antenna transmitter has
imperfect knowledge of CSI of two single-antenna users on two adjacent
subbands. A new scheme is derived as an integration of Zero-Forcing Beamforming
(ZFBF) and the scheme proposed by Maddah-Ali and Tse. The achievable DoF region
resulted by this scheme is expressed as a function of the qualities of CSIT.
