Monday, October 26, 2009

RF Power Control

RF power control is a process of adjusting the power level of a mobile radio as it moves closer and further away from a transmitter. RF power control is typically accomplished by sensing the received signal strength level to determine the necessary power level adjustments. The base stati then sends power control messages to the mobile device to increase or decrease the mobile device’s output power level. The WiMAX system uses various forms of RF power control including open loop power control and closed loop power control.

Open loop power control is a process of adjusting the transmission power level for the subscriber station using the received power level. Open loop power control in the WiMAX system is used when the subscriber station initially attempts to accessing the system. When accessing the WiMAX system, the subscriber station calculates its’ initial transmit power level using parameters that are broadcasted from the WiMAX system along with the signal strength that it receives. The smaller the signal strength it receives, the larger the initial transmit power level it uses when accessing the WiMAX system.

Closed loop power control is a process of adjusting the transmission power level for the mobile radio using the power level control commands from another transmitter that is receiving its signal (e.g. from a radio base station). The WiMAX system uses closed loop power control to continually adjust the transmitter level of the subscriber station as it moves, or as the signal level varies (such as when signal fading occurs or when obstructions move).

Figure 1 reveals how the radio signal power level output of a subscriber station is first determined by the received signal power level and is then adjusted by commands received from the base station to reduce the average transmitted power from the subscriber station. This lower power reduces interference to nearby cell sites and helps to ensure the signal level received by the base station from all the subscriber stations is approximately the same. As the subscriber station moves closer to the base station, less power is required from the subscriber station and it is commanded to reduce its transmitter output power level. The base station transmitter power level can also be reduced.

Figure 1: WiMax Power Control
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