Modulated Signals and Their Power Spectral Densities

Modulation is a process where the message information is embedded into the radio carrier. Message information can be transmitted in the amplitude, frequency, or phase of the carrier, or a combination of these, in either analog or digital form. For mobile radio applications it is desirable to use bandwidth and power resources most efficiently. The primary objective of bandwidth and power efficient modulation is to maximize the bandwidth efficiency, measured in bits/s/Hz, while achieving a prescribed bit error probability with a minimum expenditure of power resources. Good bit error rate performance must be achieved in the presence of a variety of channel impairments including fading, Doppler spread, intersymbol interference, adjacent and co-channel interference, and thermal noise. Furthermore, portable and mobile radio transmitters normally use power efficient nonlinear amplifiers to conserve battery resources. Because of the amplifier nonlinearities, modulation techniques with a relatively constant envelope are often used. All first generation cellular systems used analog FM. However, the pressing need for greater bandwidth efficiency has lead to the use of digital modulation techniques in second generation digital cellular standards. The North American IS-54 and Japanese PDC systems use π/4-DQPSK, the European GSM system uses Gaussian minimum shift keying (GSMK), and the Motorola Integrated Radio System (MIRS) uses orthogonal frequency division multiplexing (OFDM).