Power control
There are five classes of mobile stations defined, according to their peak transmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel interference and to conserve power, both the mobiles and the Base Transceiver Stations operate at the lowest power level that will maintain an acceptable signal quality. Power levels can be stepped up or down in steps of 2 dB from the peak power for the class down to a minimum of 13 dBm (20 milliwatts).
We need only enough power to make a connection. Any more is superfluous. If you can't make a connection using one watt then two watts won't help at these near microwave frequencies. Using less power means less interference or congestion among all the mobiles in a cell.
The mobile station measures the signal strength or signal quality (based on the Bit Error Ratio), and passes the information to the Base Station Controller, which ultimately decides if and when the power level should be changed. Power control should be handled carefully, since there is the possibility of instability. This arises from having mobiles in co-channel cells alternatingly increase their power in response to increased co-channel interference caused by the other mobile increasing its power. This in unlikely to occur in practice but it is (or was as of 1991) under study.
Two points. The first is that the base station can reach out to the mobile and turn down the transmitting power the handset is using. Very cool. The second point is that a digital signal will drop a call much more quickly than an analog signal. With an analog radio you can hear through static and fading. But with a digital radio the connection will be dropped, just like your landline modem, when too many 0s and 1s go missing. You need more base stations, consequently, to provide the same coverage as analog