  Channel estimation at the receiver side is essential to adaptive modulation
schemes, prohibiting low complexity systems from using variable rate and/or
variable power transmissions. Towards providing a solution to this problem, we
introduce a variable-rate (VR) M-PSK modulation scheme, for communications over
fading channels, in the absence of channel gain estimation at the receiver. The
choice of the constellation size is based on the signal-plus-noise (S+N)
sampling value rather than on the signal-to-noise ratio (S/N). It is
analytically shown that S+N can serve as an excellent simpler criterion,
alternative to S/N, for determining the modulation order in VR systems. In this
way, low complexity transceivers can use VR transmissions in order to increase
their spectral efficiency under an error performance constraint. As an
application, we utilize the proposed VR modulation scheme in equal gain
combining (EGC) diversity receivers.
