se is subject to the Terms of Use as given at the website. The User Terminal Key Radio Concepts transceiver to transmit and receive at the same time, whereas the GSM air interface maintains separate time slots for the forward and reverse signals. Since they do not have to transmit and receive at the same time, GSM mobile telephones can share such components as high-frequency oscillators. Not having to duplicate these components a cost advantage for the GSM mobile telephone relative to AMPS and CDMA. The demodulator is similar to the modulator. At the lowest level, the forward modu- lation is the same as the reverse modulation, but some of the other rules in the air in- terface are not symmetric in the two directions, as we will discuss in Secs. 8.8 and 8.9. The speaker turns the demodulated forward link signal into sound for the user to hear. This requires an audio amplifier that consumes power, but it can operate at much lower power levels than the radio transmitter amplifier. After all, the signal from the audio speaker only has to travel about 1 cm to a user's ear, whereas the signal from the radio transmitter travels several kilometers to a base-station receiver. 3.1.2 Antennas and the air interface Lastly, we come to the antenna, where electrical signals become radio waves. Unlike antenna arrays at base stations, the antenna of a mobile telephone is severely limited in size and directional control. In Chap. 4 we will go much further into the technical is- sues of antenna design, but it is essential to include some crucial points here. All antennas have a radiation pattern that describes the antenna's gain in varying directions, essentially the fraction of the radio power going in each direction, as de- scribed in Sec. 1.8. The radiation pattern of an antenna is also its reception pattern when it is used to receive radio signals. In terms of antenna function, radio waves be- have the same way in either direction. 5 Thus we refer to antenna radiation patterns for both transmitting and receiving. This reciproci