 Power Control Power control can be used for both the downlink and uplink directions to decrease co-channel interference. Power control increases the spectrum efficiency and at the same time saves the mobile station battery lifetime, because less power needs to be transmitted. The transmission power can be reduced in case the received power level is high and decreased until the received level still remains adequate. The power ranges for the base station and the mobile station as well as the power control steps are specified in the GSM specifications. The mobile station transmission power is defined by the MS classes, according to the specifications. For example, the MS output power with a class 4 mobile (GSM 900) is 2 W. The MS power can be decreased with steps of 2 dB, the range for the MS power control being 15 steps, or in other words 30 dB. Similarly, the base station power control also works in 2 dB steps and the range is 30 dB. Another parameter besides the power ranges and the steps defined in the GSM specifications is time - how fast the power can be decreased or increased. One 2 dB change can be done once during 13 TDMA frames, which is 13 X 4.615 ms = 60 ms. The GSM channel structure is specified in Section 2.6.1 (Capacity Planning in GSM Networks). When the connection is initialised the MS first transmits using a fixed power. The transmission power level is defined for the whole cell, which is always used when initialising a connection. The information is sent to the MS via the BCCH. After a while the power control can be used to find a more suitable MS power. The initial power is selected to be adequate in all situations for that particular cell. The power control is an optional network feature, SO can be activated when desired. The base station subsystem (BSS) controls the power control both in the downlink and uplink directions. The MS measures the received power level and sends the results via the BTS to the BSS. The BTS also measurs the level Radio Network Planning and Optimisation of the received