ON THE RANDOMIZATION OF TRANSMITTER POWER LEVELS TO INCREASE THROUGHPUT IN MULTIPLE-ACCESS RADIO SYSTEMS

To enhance the throughput of a slotted random access protocol in a rad io communication system, we describe the use of a scheme in which mult iple power levels are used at the transmitters. We first consider a si tuation in which n transmitters are simultaneously trying to send a pa cket to a central receiving station using a time-slotted access protoc ol, like slotted ALOHA. Each of these transmitters randomly chooses on e of m discrete power levels during each attempt to send a packet. One of the simultaneously sent packets can often be successfully received due to the power capture effect. We consider two types of capture mod els: (1) one in which the transmitter with the largest received power captures the channel, and (2) one in which the transmitter captures th e channel only if its signal-to-interference ratio is above some thres hold when received at the central station. In this paper, we determine the optimal transmit probabilities for the power levels as well as th e optimal values of the power levels themselves, when their range is c onstrained and for cases both with and without Rayleigh fading. After determining the precise optimal power levels and probabilities for max imizing the capture probabilities (i.e., for a given n), we propose a less complex, but nearly optimal, approximate approach based on using logarithmically equi-spaced levels. After demonstrating the closeness of our suboptimal results to the optimal results, we apply our approac h to the problem of optimizing the throughput of the slotted ALOHA pro tocol for a case in which the input traffic is generated according to a Poisson process. Several numerical examples are presented along with a demonstration of how the optimal choice of power levels and probabi lities can enhance throughput relative to previous ad hoc methodologie s.