  In this paper, transmission over time-selective, flat fading relay channels
is studied. It is assumed that channel fading coefficients are not known a
priori. Transmission takes place in two phases: network training phase and data
transmission phase. In the training phase, pilot symbols are sent and the
receivers employ single-pilot MMSE estimation or noncausal Wiener filter to
learn the channel. Amplify-and-Forward (AF) and Decode-and-Forward (DF)
techniques are considered in the data transmission phase and achievable rate
expressions are obtained. The training period, and data and training power
allocations are jointly optimized by using the achievable rate expressions.
Numerical results are obtained considering Gauss-Markov and lowpass fading
models. Achievable rates are computed and energy-per-bit requirements are
investigated. The optimal power distributions among pilot and data symbols are
provided.
