7.6 A COMPARISON OF FDM, TDM, WDM AND OFDM

Multiplexing techniques differ not only in their method of resource allocation but also in their historical context, implementation complexity, and typical application domains. Although all four techniques allow multiple information streams to share a common transmission medium, they partition resources in fundamentally different ways:

FDM divides the available bandwidth into distinct frequency bands. Each user occupies a dedicated sub-band continuously.
TDM divides access in time. Each user occupies the full bandwidth, but only during assigned time slots.
WDM divides the optical spectrum into distinct wavelength channels within a single fiber. Each wavelength behaves as an independent transmission path.
OFDM divides a single transmission band into many closely spaced, mathematically orthogonal subcarriers, transmitting parallel low-rate streams within the same spectrum.

In terms of bandwidth efficiency:

FDM requires guard bands between channels, reducing spectral efficiency.
TDM requires synchronization overhead but can approach high efficiency when traffic is continuous.
WDM achieves enormous aggregate capacity by exploiting multiple optical carriers without increasing individual symbol rates.
OFDM improves spectral efficiency by allowing overlapping subcarrier spectra through orthogonality.

In terms of implementation efficiency:

FDM requires stable oscillators and high-quality filters.
TDM requires precise clock synchronization.
WDM requires narrow-linewidth lasers, wavelength stability, and dispersion management.
OFDM requires digital signal processing (DSP), fast Fourier transforms (FFT), and equalization.

In terms of typical applications:

FDM: legacy analog telephony, broadcast radio.
TDM: digital telephony, PCM hierarchies, SONET/SDH.
WDM: long-haul fiber backbones, submarine cables, metro optical transport.
OFDM: wireless broadband (Wi-Fi, LTE, 5G), digital television, DSL.

The progression from FDM to TDM reflected the shift from analog to digital transmission. The emergence of WDM paralleled the widespread adoption of optical fiber, enabling exponential capacity growth. OFDM represents a further evolution driven by the need to combat multipath interference and improve spectral efficiency in wireless and broadband systems.

Thus, while all four techniques share the goal of transmission economy, they represent different engineering responses to the constraints of medium, bandwidth, and system architecture.