Abstract
In communication over time-varying Rayleigh fading channels, adaptive coded modulation for Pilot Symbol Assisted Modulation (PSAM) without feedback has been shown to yield significant benefits in terms of achievable rates. This technique adapts transmission rate at the sender to the quality of the channel estimate at the receiver but keeps the mean power constant throughout. In this paper, we show that this adaptive PSAM scheme can be further improved if power and rate are jointly adapted to the quality of the measurement at the receiver. We study the optimal power distribution scheme and find it to apply the following principle: more power is allocated to symbols corresponding to better estimates at the receiver, while maintaining the average energy constraint satisfied within a period. We find that this simple scheme is optimal and performs better than adapting to channel quality using schemes akin to 'water filling'. Our model is a Rayleigh fading channel where time-variance is described by a Gauss-Markov model [2]. The transmitter periodically sends pilot tones to measure the channel at the receiver. We interleave different codes, while maintaining the power constant over a codebook, and the average power over codebooks satisfying the constraint. Performance is quantified in terms of achievable rates. Our scheme does not require any real time computation or adaptation at the transmitter, and so comes at no extra cost with respect to. When considering causal and non causal estimation strategies at the receiver, considerable improvement was attained without any added complexity.