  The discrete-time Wiener phase noise channel with an integrate-and-dump
multi-sample receiver is studied.
  A novel outer bound on the capacity with an average input power constraint is
derived as a function of the oversampling factor.
  This outer bound yields the degrees of freedom for the scenario in which the
oversampling factor grows with the transmit power $P$ as $P^{\alpha}$.
  The result shows, perhaps surprisingly, that the largest pre-log that can be
attained with phase modulation at high signal-to-noise ratio is at most $1/4$.
