Simple upper bound on the information rate of the phase noise channel

A simple upper bound on the information rate (IR) of digital communications over phase noise-impaired channels is proposed. In particular, the transmission of linearly modulated signals, such as QAM, at high-signal-to-noise ratio is considered, thus extending previous literature results for phase modulated signals, e.g. PSK. A simulation-based lower bound on the IR shows that the proposed bound is tight for small-medium size constellations and large phase noise.