7.1 INTRODUCTION

In the preceding chapters we examined analog and digital modulation techniques in terms of their ability to convey information from a single source over a communication channel. Modulation determines how information is represented on a carrier and how efficiently power and bandwidth are used.

In practical systems, however, communication channels are rarely dedicated to a single source. Most networks must support many simultaneous information streams—multiple users, services, or data flows—sharing the same transmission medium. The problem therefore shifts from representing one signal efficiently to accommodating many signals within the available spectral and temporal resources.

Multiplexing techniques address this requirement by combining multiple information sources into a single composite signal for transmission over a common channel. At the receiving end, the reverse process—demultiplexing—separates the composite signal back into its individual message streams.

Because only one (albeit more complex) transmitter–receiver chain is required instead of many parallel systems, multiplexing significantly reduces equipment, spectrum usage, and infrastructure cost. Each individual information stream within the composite is referred to as a channel.

Two fundamental forms of multiplexing are widely used in communication systems:

Frequency-division multiplexing (FDM). FDM is directly applicable to continuous-wave or analog sources. It forms a composite signal by allocating each channel a distinct frequency band within the overall spectrum. The channels are arranged side by side in frequency, separated by guard bands to prevent overlap. The resulting frequency-multiplexed signal may then be transmitted directly (in baseband systems) or used to modulate a higher-frequency carrier. After reception and initial demodulation, individual channels are recovered through bandpass filtering and frequency selection.
Time-division multiplexing (TDM). TDM is primarily associated with digital or pulse-based systems. It combines multiple discrete data streams by interleaving their symbols in time to form a single composite digital signal. At the receiver, demultiplexing is performed by time synchronization and gating, directing the appropriate time slots to their respective output channels.

Historically, multiplexing evolved in parallel with transmission media. Early long-distance telephony relied on analog FDM over coaxial cable and radio links. With the advent of digital transmission, TDM became dominant, particularly in PCM systems and synchronous digital hierarchies such as SONET and SDH. As transmission capacity has increased—especially in optical fiber systems—additional dimensions of multiplexing have been exploited, including wavelength-division multiplexing and spatial multiplexing, which are examined later in this chapter.

Since multiplexing first emerged in analog telephony systems, its earliest and most intuitive form was frequency-division multiplexing. By exploiting the frequency domain—already central to our discussion of modulation—engineers were able to transmit many voice channels simultaneously over a single transmission path. We therefore begin with frequency-division multiplexing, which provides both the historical foundation and the conceptual basis for later multiplexing techniques.