he term modem includes the individual voice channel equipment that supports the radio transmit function as well. These radio modems that modulate and demodulate the sig- nal from the telephone link to the air interface should not be confused with the com- puter modems we use at home to modulate digital signals onto the analog landlines of the local loop of the PSTN. FDMA and TDMA receivers have filters to find the specific frequency of their chan- nels, and TDMA systems have demodulators and processing equipment to find their time slots. As we will discuss in Sec. 8.2, CDMA receivers are more complicated because each receiver demodulates the entire signal to find its own specific code. In most cellular systems, the receiver equipment is connected to two antennas, and switched diversity is used to combat Rayleigh fading. Even in wideband CDMA sys- items, where fading is a minor problem, having two receivers combats multipath delay spread by using a rake filter to combine the two signals. We have more to say about rake filters in Sec. 8.4. 4.5 The Power Supply and Environmental Controls Each piece of equipment in a base station uses electrical power. It is important to add up all the power consumption figures and build an ample power supply. If the current demand of the base-station equipment exceeds the power-supply capability, the voltage can drop, and the radio equipment can perform poorly or can fail altogether. The base station must be protected from the weather by a small building or large inet, and it also may need climate control to operate in cold or warm conditions. Air con- ditioning in a hot climate is important because the base station will cease to provide service if it overheats. The wireless subscribers expect their telephones to work, even when the power mains do not work. Even in the United States there are areas with frequent power out- ages. The wireless service provider has to decide how much backup power to build into a base station. In the early AMPS days, we had racks of huge batteries that 