Power Control
Definition: The process of controlling the transmitted power so as to achieve the required communications performance, but without transmitting more power than is needed to achieve that performance.
NOTE: In IS-95, power control is used to describe processes for controlling the transmitted power with the time interval between power changes on the order of every second down to about every millisecond. Other powers in the system may be adjusted from time to time, but not nearly as frequently as those links which use power control.
Power Control
Application: Only the traffic and Access Channels use power control. The power levels transmitted from mobiles to their base stations are very closely controlled. Three different control loops are used to ensure that just enough, but not too much, power is transmitted. One loop is called the open loop control. It is based on the level of power received in the total 1.25 MHz physical channel bandwidth. A second loop is called the closed loop. It uses measurements of power on the reverse traffic channels to determine if the reverse-link power is just at the level required. If it is not, a one-bit control message is sent at a rate of up to 800 bits/sec on the forward traffic channel which adjusts the mobile transmit power in steps of approximately 1 dB. A third loop, called the outer loop, appraises the performance of the closed loop using reverse-link frame quality statistics. If the frame-error rate is too high, the closed loop power control is used to request a higher power be transmitted.
The power in the forward link is controlled by the forward-link power control algorithm. The mobile reports back to the base station forward-link frame quality statistics. The base station uses these statistics to determine if the power transmitted on each forward-link traffic is appropriate.
Power Control
Example: The open loop power control law is based on the equation that the power transmitted in dBm equals the negative of the total power received in the receiver bandwidth (in dBm) minus 73 plus any adjustments which might be communicated to the mobile. As an example, suppose a rather weak signal of - 96 dBm is received. The negative of this power equals 96. Then 96 minus 73 equals 23 dBm. In the absence of any adjustments, the transmitted power would be 200 mW, the maximum level for many mobile handsets. The weak received signal suggests a very strong transmitted signal is required to reach the base station. Strong received signals cause low transmit power.
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