6.8 LOOKING AHEAD
Digital modulation provides the physical-layer foundation upon which synchronization, coding, multiplexing, and multiple-access techniques are built. The selection of an appropriate modulation technique remains a fundamental design decision that determines the achievable balance among bandwidth, power, complexity, and reliability in any communication system.
While modulation determines how a single information stream is represented on a carrier, practical communication systems rarely convey only one stream at a time. Modern networks must support many users, services, and data flows simultaneously within limited spectral resources. The next step in system design is therefore not how to represent information on a carrier, but how to share that carrier efficiently among multiple signals. This is the role of multiplexing.
In the following chapter, we examine the principal multiplexing techniques—frequency-division, time-division, wavelength-division, and spatial multiplexing—and show how they build upon the modulation methods developed here to enable scalable, multi-user communication systems.
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