o the Terms of Use as given at the website. Radio Engineering Concepts Key Radio Concepts TABLE 1.2 A Link Budget 10-W amplifier 40 dBm Transmit antenna gain +10 dB Radio path 130 dB Receive antenna gain Signal level (total) -79 dBm Receiver noise level 105 dBm Signal-to-noise (S/N) ratio 26 dB Required S/N ratio 20 dB Surplus S/N 1.10 Modulation Analog and digital signals are added to radio transmissions through very different types of modulation. For analog radio, the electrical signal modifies the carrier wave. For digital transmission, encoded bits alter the carrier. The heart of a wireless telephone technology is its radio modulation scheme. We of- ten refer to the full specification of the wireless modulation scheme, complete with all the signaling rules, as the air interface. 1.10.1 Analog modulation We can send analog electrical signals (audio or video) on a radio link in two basic ways, amplitude modulation (AM) or frequency modulation (FM). AM means that the electri- cal voltage is matched by the amplitude of the radio wave. The receiver simply converts varying radio levels back to voltage that goes into an audio amplifier, a television or videocassette recorder (VCR), or some other terminal equipment. As shown on the left side of Fig. 1.8, AM is a continuous sine wave with its amplitude varying as the input signal changes. Should some other, interfering radio wave come along with its fre- quency close to the AM radio wave we are currently receiving (and demodulating), that wave will have its content added as a noise component to our received signal. As its name suggests, FM means the electrical voltage is matched by the frequency of the radio wave, and the receiver converts varying frequency back to signal voltage. As shown on the right side of Fig. 1.8, FM is a continuous sine wave with its frequency varying as the input signal changes. An interfering radio wave is less likely to affect the frequency component at our receiver, SO the FM receiver tends to produce a less noisy reproduction of its signal