In this chapter, we consider the basic functions performed by the physical layer. In particular, we consider the following:
Message signals are classified as either analog (or continuous-time) or digital (or discrete-time). Analog (or continuous-time) signals, which include speech, audio, and video, have an infinite number of values. Digital signals are predominantly binary in nature and thus are represented by two values: 0 and 1. These two values are called binary digits, or bits.
Signals found in communication systems are complex waveforms. However, in many instances, these waveforms can be analyzed as one or more sine waves of the form:
where A is the amplitude of the sinusoid, f the frequency in Hertz (Hz), and ϕ the phase in radians.
As mentioned in Chapter 1, a transmitter is required to convert signals into a form suitable for transmission over a channel. A key operation is to change the signal's frequency range to match the frequency range, or bandwidth (BW), offered by the channel. In some systems, the signal's frequency content (or frequency spectrum ...