Efficient energy utilization with a time constraint and time varying channels

In this paper the efficiency in energy utilization under time constraints is compared between two data transmission strategies: transmission through sending a fixed number of copies, and the ARQ technique. The analysis was held assuming that each packet repetition is decoded separately, and also for the case where the combined decision is used under time varying channel conditions. The first example is under constant channel conditions where decisions are held without packet copy combining, for various values of probability of failure (P) and a limited maximum number of transmissions (n). Results indicate that there is a large saving of energy by the ARQ technique compared to transmitting a fixed number of copies for rather small values of n, and the energy saving ratio increases even more with a larger n and also in the case for smaller P values. In the second experiment, packet combining is carried out on a per symbol basis. The overall packet is presumed to have near perfect error detection and possibly also error correction. For numerical comparison purposes a Rayleigh distribution was selected to describe the received power variation of the time varying channel. The results indicate that the saving in energy is substantial, though not as great as in the non-combining case. Unlike the previous experiment, the energy saving ratio increases only for decreasing P values and not for increasing n.