n explaining the most relevant issues on handover as the most important feature of cellular networks. However, before that the main characteristics of power control are highlighted, as it is a feature that influences the handover algorithms and decisions. Power Control Power control is a feature of GSM that enables control of output power of either the MS or BTS in order to reduce interferences. In fact, the idea is that when the distance between the two stations is small, there is no need to transmit 'speak' at full power. An analogy to assimilate this idea is to state that when someone is close to you, you reduce your voice to the level that the other person can hear (there is no need to shout), and vice versa, when the person is far from you, you try to shout at your maximum power to reach that person. The same algorithm exists in power control. There are two level values that delimit the range where power can vary: upper and lower thresholds, as illustrated in Figure 2.66. Whenever the signal level goes beyond the upper value, the mechanism of PC brings back the power to the upper value by means of a power decrease step size. Whenever the signal is below the lower value the mechanism Radio Network Planning and Optimisation Signal Level Figure 2.67 Variation of real signal level values adjusts the power to reach the minimum lower value through progressive increment of a defined step size. Generally the upper and lower values are not the same in order to prevent the ping-pong phenomenon. As power control can work in both directions - uplink and downlink - there is a separate threshold range for each direction. Uplink power control means that the BTS controls the emission of the MS. This is the one most commonly used in almost all GSM networks. Downlink power control is the one where the MS controls the power of the BTS, but is less commonly used because of its complexity. Especially when the DTX is activated, DL power control does not work properly. In fact, one BTS controls one MS but many MSs may control one 