  This paper investigates the achievable information rate of phase-shift keying
(PSK) over frequency non-selective Rayleigh fading channels without channel
state information (CSI). The fading process exhibits general temporal
correlation characterized by its spectral density function. We consider both
discrete-time and continuous-time channels, and find their asymptotics at low
signal-to-noise ratio (SNR). Compared to known capacity upper bounds under peak
constraints, these asymptotics usually lead to negligible rate loss in the
low-SNR regime for slowly time-varying fading channels. We further specialize
to case studies of Gauss-Markov and Clarke's fading models.
