1.3.7 The Communications Channel, Noise, And Interference

Once transmitted, the modulated signal propagates through the communications channel, where it is inevitably impaired. The channel may attenuate the signal, distort its waveform, delay different frequency components by different amounts, or introduce time-varying effects such as fading and dispersion. In addition, the signal is corrupted by noise arising from natural physical processes and by interference generated by other communications systems and electrical equipment.

The nature of these impairments depends strongly on the transmission medium. Copper cables may suffer attenuation and electrical interference, optical fibers experience dispersion and optical losses, while radio signals may be affected by reflections, atmospheric conditions, terrain, buildings, and movement of the transmitter or receiver. No practical communications channel is perfect, and every system must be designed to accommodate these unavoidable imperfections.

Although real channels are often highly complex, engineers frequently model their behavior using simplified mathematical representations that capture the dominant impairments while avoiding unnecessary detail. Such models make it possible to predict system performance, compare alternative designs, and optimize equipment before it is constructed.

Understanding the characteristics of the communications channel is one of the central themes of communications engineering. Many of the techniques discussed throughout this book—including modulation, coding, diversity, equalization, multiplexing, and adaptive signal processing—have been developed specifically to overcome the limitations imposed by real communication channels.