I. Introduction
Receivers that are designed to work in unknown time-selective correlated fading channels perform channel estimation for reliable communications. The quality of channel estimation is an important factor that affects the receiver performance. Most current mobile communication systems use constant envelope phase modulations. For this class of modulation schemes, estimation of the phase component of the channel response is essential and poses a great challenge for the receiver design, especially at high fading rates. Usually, to achieve reasonable precision in phase estimation, explicit periodic training bits are inserted among channel symbols. These bits are predetermined and convey no detectable information. The main advantage of this method is that during the training period good precision of channel estimation is attainable. Explicit training drawbacks are summarized as:
The receiver does not get any supervision between the training periods and if the channel state changes during this interval, performance is degraded.
Better channel estimation precision in the training period does not necessarily result in better channel estimation in the tracking period. In fact we have recently shown that the precision of channel state information degrades exponentially for two-state Markov channels as we move further from the last training bit [10].