Open Loop Power Control, Solution to the Direct Sequence Spread Spectrum Near-Far Problem
Campbell, Kelly Scott
Today's highly mobile, information based society is demanding personal wireless communications. Present solutions, including pagers, cordless telephones, and cellular systems have deficiencies that do not permit roaming telephone communication throughout an office building. A truly mobile, universal telephone service in an office building environment requires a different technology. Many of the systems proposed for the task are based on wireless radio with a modulation format known as Direct Sequence Spread Spectrum (DS-SS). Such systems have the advantages of being spectrum efficient and operating at very low power levels. Unfortunately, the spectral efficiency is dependent on accurate control of the portable telephone's transmission power. This power control is necessary to combat the inherent near-far problem. Numerous studies have been made on power control issues relevant to Frequency Division Duplex (FDD) systems. The portables and the base station transmit simultaneously on independent ·frequency bands when they use this protocol. In contrast to FDD, Time Division Duplex (TDD) systems operate using the ping-pong principle where the base station transmits, then the portables transmit -- on the same frequency band. This work concentrates on portable transmission power control for a TDD system. This thesis examines a control method based on fitting Minimum Square Error (MSE) curves to the forward link power received at the portable. These MSE curves are extrapolated to provide portable transmission power estimates for the next burst. The effect of low order antenna diversity and burst length on this form of power. control are also examined. It is shown that MSE control increases the DS-SS system capacity, especially in the presence of diversity and moderate burst lengths.