In many texts on communication, including this one, it is often assumed that transmitters and receivers are ideal. This assumption is never true. In practice, to get a communication system to operate, more time and effort are often invested in compensating for imperfections than in designing the ideal communication system. Furthermore, many system and algorithm design choices can increase or decrease the sensitivity of the communications system to the imperfections of the constituent components.
In this chapter, a few practical issues are addressed, including noise models, noise figure, power consumption, antennas, local oscillators, and dynamic range. These are only a small fraction of all practical issues faced by radio designers, but these are presented to sensitize the system designer to these potential issues. Many of these issues are addressed with greater depth in texts such as References [261, 252].
In order for signals to be radiated or received, the electromagnetic energy is coupled through an antenna . As discussed in Section 5.1, antennas can significantly affect the signal being transmitted or received. Each antenna has some directional and polarimetric response. In addition, the antenna has a frequency response. In the context of this text, the effects of the antennas are typically folded into the channel. Finally, if the impedance of the antenna is not matched well to the transmitter or receiver, then inefficiencies are introduced. There is constant drive to reduce the size of radio systems.