Power Control in GSM for RF Optimization
Power Control adjusts the transmitting power of BTS and MS when needed. It is based on measurement reports of BTS and MS. The purpose of power control is to save the power of BTS and MS, reduce the interference of the network and increase the quality of the network. Power control includes uplink power control and ownlink power control, both of them is performed independently.
Uplink power control:
Adjust TX power of MS to let BTS receive stable signal, reduce the uplink co-channel and adjacent channel interference and reduce power consumption of MS. While downlink power control: Adjust BTS TX power to let MS receive stable signal, reduce the downlink co-channel and adjacent channel interference and reduce power consumption of BTS.
Downlink Power Control
In downlink power control the output power of a Base Transceiver Station (BTS) can be controlled during a connection. The control strategy is to maintain a desired received signal strength and quality in the mobile station (MS). The aim with BTS Power Control is to increase the number of MSs with sufficiently good C/I. BTS Power Control will improve C/I if traffic is maintained, or maintain C/I when traffic is increased or tighter frequency is re-used. The gain is obtained by a reduction of the over all interference level (I) in the network. When BTS Power Control is used in all BTSs in the network, the total amount of radiated power is reduced compared to when it is not used. This implies that the downlink co- and adjacent channel interference in the network is reduced. Since MSs with low signal strength or bad quality use full BTS output power, reduced interference level imply increased C/I for these connections. On the other hand, the C/I is decreased for connections with high signal strength and good quality since they are subjected to a reduced BTS output power. Reduction of C/I will not affect the speech quality of these connections since they have a margin to the lowest tolerable C/I.
Both quality and signal strength is considered by the power control algorithm. Quality is the estimated bit error rate which is represented by RxQual. Signal strength is represented by RxLev. Bad quality as well as low signal strength will increase the output power of the BTS.
In below figure, the BTS output power and the signal strength in the MS versus path loss between a BTS and an MS is shown. A BTS can only transmit at distinct power levels; this is illustrated in the figure.
Base Station Output Power and MS Signal Strength versus Path Loss
When a connection has low path loss (left part of Figure 3-58), the BTS transmits at its lowest possible power level. Although the MS receives a signal that exceeds the desired value, the BTS can not reduce the transmitted power any further.
Conversely, when a connection experiences high path loss (right part of Fig), the BTS transmits at the maximum allowed power level for the cell. The power cannot be increased even if the received signal strength in the MS is low.
When quality is taken into account the output power is regulated up or down depending on the received quality. The base station power then varies with the quality measured by the MS. When an MS has low RxQual (high quality) the base station
sends on low power and when an MS has high RxQual, on high power. The higher the RxQual, the higher will be the power and vice versa.
Example of BTS
Output Power versus RxQual
