A covert communication system under block fading channels is considered where
users experience uncertainty about their channel knowledge. The transmitter
seeks to hide the covert communication to a private user by exploiting a
legitimate public communication link while the warden tries to detect this
covert communication by using a radiometer. We derive the exact expression for
the radiometers optimal threshold which determines the performance limit of the
wardens detector. Furthermore for given transmission outage constraints the
achievable rates for legitimate and covert users are analyzed while maintaining
a specific level of covertness. Our numerical results illustrate how the
achievable performance is affected by the channel uncertainty and required
level of covertness.Comment: to appear in IEEE VTC2017-Sprin
