Chapter

Fading and Shadowing in Wireless Systems

pp 109-191

Date:

Modems for Wireless Communications

  • P. Mohana ShankarAffiliated withElectrical and Computer Engineering, Drexel University Email author 

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Abstract

Digital modulation and demodulation techniques together (modem) form the fundamental building block of data transmission in communication systems in general and wireless communication systems in particular (Lucky et al. 1968; Oetting 1979; Amoroso 1980; Feher 1995; Benedetto and Biglieri 1999; Proakis 2001; Simon and Alouini 2005; Schwartz 2005; Couch 2007). The modems can be classified in a multitude of ways (Simon et al. 1995; Sklar 1993, 2001; Haykin 2001). They can be identified in terms of the signal property that is modulated such as amplitude, phase, or frequency. They can also be classified in terms of the number of levels of values (binary, quaternary, or in general M-ary) attributable to the property. Detection methods such as coherent or noncoherent ones can also be used for the classification. We can, in addition, use terms such as “linear” and “nonlinear” modulation to classify the modulation types (Sundberg 1986; Anderson et al. 1986; Gagliardi 1988; Gallager 2008). The output of the frequency modulated system has a constant envelope while the output of the amplitude or phase modulated system has a constant frequency with time varying amplitudes. This makes the amplitude and phase modulation a form of “linear” modulation and the frequency modulation a form of “nonlinear” modulation. Even for a specific modulation type such as phase modulation (phase shift keying for example), it is possible to have a coherent or a noncoherent detector or receiver. Modems cover a wide range of possibilities with some common themes such as the initial building blocks of the “signal space.” The concepts of signal space makes it possible to analyze modems, design them so that they meet certain criteria, provide a uniform framework, and make it possible to compare and contrast the different modems.