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Effects of capture, power loss factor, and variable transmit power level in multiple-access systems Wong, Victor J. K. (Jack Keung)


The problem of data transmission in a slotted ALOHA system with one central base station and a number of mobiles is addressed. Of particular interest is the probability qi of successful packet reception ("capture") when i mobile users transmit in a given time slot. This probability depends on the propagation law, as well as fading, transmit power variations, spatial distribution and capture models. The capture probability in a noisy system is studied and the values of qi for large i appear to be independent of the noise level as long as the noise is reasonable (i.e. the normalized noise power is less than one). Conditions under which qi decreases monotonically with i are derived. Examples are also given to illustrate cases in which qi may not decrease monotonically with i. The effect of the power loss factor, β, on different spatial distribution and capture models is studied. It is shown that in a noiseless system, qi increases with β for the two capture models considered. A new spatial distribution, referred to as the inverse-distance spatial distribution, is proposed which allows an exact expression for qi to be obtained. A transmit power selection scheme, which decreases with distance over a selected range of distance, is proposed. Simulation results are used to show that under certain conditions, the throughput performance can be greatly improved with such a transmit power law.

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