Efficient energy utilization with time constraints in mobile time varying channels

In this paper, the efficiency in energy utilization under time constraints is compared between two data transmission strategies: (1) transmission through sending a fixed number of copies, and (2) the ARQ technique. The analysis includes a case where each packet repetition is decoded separately, and cases where a combined decision is used. (1) For constant channel conditions and no combining, results are compared for various specifications of probability of failure (P) and maximum number (n) of transmissions. There is a large saving of energy for the ARQ technique, which increases greatly for smaller P values and significantly for larger n. (2) Variable channel conditions and decision combining techniques are assumed for two sub-cases: (a) fast fading channel with a Rayleigh distribution, where it is assumed that successive transmissions or copies relating to a given packet experience independent channel conditions. (b) lognormally shadowed Rayleigh slow fading channel, where successive transmissions or copies relating to a given packet experience identical channel conditions. The (2a) case yields a substantial energy saving compared to sending fixed multiple copies, though not as great as the noncombining case (1). The (2b) case shows a very large saving in energy utilization with some examples of about a 20:1 or more factor reduction in energy utilization when the strategy includes increasing the energy increments with successive transmissions