Multiplexing

The basic idea

A multiplexer combines several input streams onto a shared medium. A demultiplexer separates them at the far end. The medium might be a copper pair, optical fibre, radio channel or waveguide.

Nothing is created for free. Each stream consumes some distinguishable resource, and the combined traffic cannot exceed the useful capacity of the link. Guard bands, time gaps, synchronisation, coding and error correction consume some capacity too.

Multiplexing usually describes several signals sharing infrastructure. Multiple access describes several users sharing it. The physical technique can be the same; the wording changes with the system boundary.

FDM — Frequency Division Multiplexing

FDM divides the available spectrum into non-overlapping frequency bands. Every channel transmits continuously, but on a different carrier frequency. Filters at the receiver select one band and reject the others.

The unused guard bands between channels allow imperfect filters and drifting oscillators without overlap. Traditional broadcast radio is the clearest example: every station occupies a different slot, and tuning moves the receiver’s filter. Traditional cable television and early analogue telephone carrier systems also use FDM.

Strength: simple, continuous access. Cost: fixed bands sit unused when their owner has nothing to send, and guard bands waste spectrum.

TDM — Time Division Multiplexing

TDM gives signals turns on the same channel. Time is divided into repeating frames and each source receives a slot. The receiver needs timing alignment so it knows which source owns each arriving slot.

Synchronous TDM reserves a regular slot for every input, even if it is silent. It gives predictable delay and was central to digital telephony. Statistical TDM labels and assigns slots only to active inputs, using capacity more efficiently but making delay variable. Packet switching is a flexible form of statistical sharing.

Strength: clean separation and predictable allocation. Cost: precise synchronisation; fixed slots can be wasted.

WDM — Wavelength Division Multiplexing

WDM is frequency division for optical fibre. Several lasers use different wavelengths—often described as colours—so one fibre carries many independent optical channels. A passive optical multiplexer combines them and a demultiplexer splits them again.

• CWDM (Coarse WDM) uses fewer, widely spaced wavelengths. The optics are cheaper and suit shorter or moderate distances.
• DWDM (Dense WDM) packs many tightly controlled wavelengths together. It needs precise lasers and filtering but can carry enormous capacity across long-haul networks.

The light is often in infrared bands and therefore invisible; “colour” is still a useful shorthand. Each wavelength can itself carry a very fast modulated data stream and can be routed or amplified alongside its neighbours.

CDM / CDMA — Code Division

Code division allows signals to occupy the same time and frequency region. Each stream is multiplied by a distinctive high-rate code. A receiver correlates the mixture with the desired code: that signal adds coherently, while sufficiently different codes mostly appear as noise.

This spreads a signal over a wider band. Accurate power control is important: a nearby strong transmitter can swamp a distant weak one, called the near–far problem. Capacity degrades gradually as more users add interference rather than ending at a neat fixed slot count.

CDMA was prominent in 3G cellular systems. GPS satellites use distinct ranging codes so a receiver can separate very weak signals sharing a band and measure their arrival times.

SDM — Space Division Multiplexing

SDM separates channels by physical path or location. The simplest examples are different copper pairs in one cable, separate fibres in a bundle, or separate cores in a multi-core fibre. Directional antennas can reuse the same frequency in different places or directions.

Cellular networks are fundamentally spatial: the same channel can be reused by cells far enough apart. Sector antennas divide one site into different coverage areas. The challenge is crosstalk—unwanted coupling between paths—and interference where spatial regions overlap.

Strength: capacity can scale by adding paths. Cost: more physical hardware, space, alignment or signal processing.

OFDM — Orthogonal Frequency Division Multiplexing

OFDM divides a fast data stream among many closely spaced, slower subcarriers. Unlike ordinary FDM channels separated by generous guard bands, the subcarriers mathematically overlap but are orthogonal: sampled correctly, each sits at a zero point of all the others and can be separated.

An inverse fast Fourier transform efficiently creates the combined waveform; a fast Fourier transform separates it. A cyclic prefix copies the end of each symbol onto its beginning, giving delayed reflections time to settle and turning difficult multipath into simpler per-subcarrier corrections.

Subcarriers can use different modulation and coding according to channel quality. OFDM resists frequency-selective fading and multipath, but its waveform can have high peak-to-average power and needs accurate frequency/timing control. It appears in Wi-Fi, DSL, DVB-T/T2, 4G and 5G.

OFDMA — Orthogonal Frequency Division Multiple Access

OFDM describes how one waveform uses subcarriers. OFDMA schedules subsets of those subcarriers—and time symbols—to different users. A base station or access point can give a small allocation to a sensor and a large one to a laptop during the same transmission opportunity.

The schedulable units are often called resource units or resource blocks, depending on the system. A scheduler considers demand, radio quality, fairness and latency. Wi-Fi 6, LTE and 5G use OFDMA.

OFDMA is especially efficient when many users have small bursts: they need not each occupy a whole wide channel in turn.

PDM — Polarisation Division Multiplexing

An electromagnetic wave has an electric-field orientation. Two orthogonal polarisations—such as horizontal and vertical, or two circular senses—can carry independent signals on the same frequency.

Satellite systems reuse frequencies with opposite polarisations, provided antennas reject the unwanted orientation well. Coherent optical systems commonly send two data streams on two orthogonal light polarisations, doubling capacity. Real links mix the polarisations, so the receiver uses signal processing to untangle them.

Do not confuse this with pulse-density modulation, which also uses the abbreviation PDM in digital audio.

Mode Division Multiplexing (MDM)

A mode is a stable field pattern that can propagate through a waveguide or fibre. MDM launches separate data streams into different modes and separates them at the receiver.

Few-mode and multi-core optical fibres are researched and deployed for space-division capacity, while waveguides and some wireless systems use analogous modes. Imperfections cause modes to couple, so multiple-input signal processing may be needed to recover the original streams. That complexity is the main barrier: modes are mathematically distinct but not perfectly isolated in real material.

MIMO and spatial multiplexing

Multiple-input, multiple-output uses several transmit and receive antennas. In spatial multiplexing mode, antennas send independent streams at the same time and frequency. The streams take slightly different paths; the receiver estimates the channel and solves the resulting set of mixed signals.

MIMO needs enough independent spatial paths and adequate signal quality. In a rich indoor environment, reflections can help make streams distinguishable. In clear line-of-sight with antennas too close together, adding antennas may not add independent capacity.

MIMO can also be used for diversity (the same data over several paths for reliability) or beamforming (coordinating antennas to concentrate energy). Those are valuable MIMO techniques, but they are not spatial multiplexing. Modern Wi-Fi, LTE and 5G use combinations of all three.

How the methods fit together

Real systems stack methods. A coherent optical link may use WDM, two polarisations and complex modulation at once. 5G uses frequency bands, OFDMA, time scheduling and MIMO. The receiver reverses each layer in the appropriate order.