{"id":829,"date":"2026-06-07T10:06:59","date_gmt":"2026-06-07T08:06:59","guid":{"rendered":"https:\/\/www.i8zse.it\/en\/?page_id=829"},"modified":"2026-06-07T10:08:38","modified_gmt":"2026-06-07T08:08:38","slug":"fdm","status":"publish","type":"page","link":"https:\/\/www.i8zse.it\/en\/modulation\/multiplexing\/fdm\/","title":{"rendered":"FDM"},"content":{"rendered":"

Why there is a need to divide the spectrum<\/h3>\n

The radio spectrum is a finite and regulated resource. Each band has a limited width, and every transmission occupies a portion of it. The problem arises when multiple signals must coexist in the same radio space at the same time: if they all transmit on the same frequency, they overlap, generating mutual interference and making reception difficult or impossible.<\/p>\n

The most intuitive solution is also the oldest: assign each signal a distinct portion of the spectrum, separated enough to limit interference. In practice, in addition to frequency spacing, real-world filter imperfections must be taken into account by introducing guard bands<\/strong> between signals-portions of spectrum deliberately left unused to absorb the spectral \u201ctails\u201d of each channel.<\/p>\n

This principle is called frequency division multiplexing<\/strong>, or FDM<\/strong>: multiple signals transmitted simultaneously, each in its own sub-channel within the same band.<\/p>\n

FDM: the basic structure<\/h3>\n

Think of the available band as a wide sidewalk. FDM consists of drawing parallel lanes, each reserved for a distinct stream. Each lane is a sub-channel with its own center frequency and bandwidth; what ensures coexistence is the separation between lanes and the quality of the filters that define their edges.<\/p>\n

A sub-channel can be only a few hertz wide or tens of kilohertz wide, and it can carry analog voice, digital voice, low- or high-speed data, control signals, or telemetry. The key characteristic is that all sub-channels are active simultaneously: they do not alternate in time and do not share codes. It is pure separation in the frequency domain.<\/p>\n

FDM is used whenever different data streams-even within the same system or emission-must be carried over distinct portions of spectrum. It does not necessarily imply multiple users.<\/p>\n

From FDM to FDMA<\/h3>\n

When FDM sub-channels are assigned not to internal streams of a single system but to independent users<\/strong>, the principle is called FDMA<\/strong> (Frequency Division Multiple Access). The distinction is functional rather than technical: the spectral structure is identical; what changes is who occupies each sub-channel.<\/p>\n

In an FDMA system, each user is allocated a dedicated portion of bandwidth, independent of the others. There is no time coordination or code sharing: each user transmits whenever they want within their assigned lane. This makes FDMA simple to implement and suitable for scenarios where latency must be minimal and the number of users is relatively limited and predictable.<\/p>\n

The main limitation is efficiency: an allocated but unused sub-channel remains occupied, unlike in TDMA or CDMA systems where idle capacity can be dynamically redistributed.<\/p>\n

FM stereo: FDM applied to a single transmission<\/h3>\n

The FM stereo broadcast system is a real-world, everyday example of FDM applied not to different users but to different components of a single signal-where frequency separation is used to carry multiple types of information within one emission.<\/p>\n

The monophonic signal (L+R) occupies the lower part of the baseband. Stereo-capable receivers detect the 19 kHz pilot tone and use it as a reference to process additional subcarriers. From 23 to 53 kHz lies the L\u2212R component (DSB-SC modulated around 38 kHz, twice the pilot tone), which combined with the mono signal allows reconstruction of the left and right channels. At 57 kHz (3 \u00d7 19 kHz) there is the RDS digital subcarrier. Additional optional SCA channels (typically at 67, 76, and 92 kHz) may carry extra services.<\/p>\n

\"\"<\/p>\n

It is a mosaic of independent sub-bands, each with a specific role. The FM receiver filters these components and reconstructs the original signals. None of this information is transmitted in sequence or with separate codes: they coexist simultaneously in the same spectrum, each in its own portion.<\/p>\n

For a radio amateur, this is a valuable example: it shows how multiple different information streams can be carried within a single emission with a structure clearly visible on a spectrum analyzer or waterfall, without requiring complex digital techniques.<\/p>\n

FDMA in amateur radio practice<\/h3>\n

FDMA-and more generally the FDM principle-is the basis of many techniques radio amateurs use every day, often without explicitly naming them.<\/p>\n

The SSB channel as a shared band<\/strong><\/p>\n

A frequently overlooked aspect is that a single SSB channel-typically 2.4\u20133 kHz of passband-can host many simultaneous digital signals, each in its own sub-channel. This is not accidental: it is exactly the application of FDM within an already allocated band.<\/p>\n

PSK31, with a bandwidth of only a few tens of hertz, is designed precisely for this: dozens of stations can transmit simultaneously within the same SSB band, each on a slightly different audio frequency, visible on the waterfall as parallel traces. The operator chooses a free slot and their signal coexists with others thanks to frequency separation and receiver selectivity. This is spontaneous FDMA: no central system assigns sub-channels, yet the resulting structure is exactly that of a frequency-division system.<\/p>\n

FT8 follows a similar but more structured approach: each signal occupies about 50 Hz of bandwidth, with internal tones spaced around 6.25 Hz, and stations distribute themselves across the available spectrum to avoid overlap. The waterfall clearly shows this comb-like structure, with signals neatly separated. The difference from PSK31 is that FT8\u2019s frequency structure is defined by the protocol rather than left to operator choice.<\/p>\n

Digital voice systems<\/strong><\/p>\n

In systems such as C4FM System Fusion, frequency division is explicitly applied at the design level: a single RF channel is divided into sub-channels carrying different content-voice, data, digital identification-each in its own portion of bandwidth. It is FDM becoming FDMA when sub-channels are assigned to independent users or streams.<\/p>\n

Experimental links and SDR<\/strong><\/p>\n

Those working with SDR and DSP can build multi-subchannel systems directly in software: one sub-channel for voice, one for node telemetry, one for a digital beacon, one for remote control. This is one of the most effective experimental environments for understanding modern radio systems-and for observing how bandwidth allocation, guard bands, and filtering affect signal coexistence.<\/p>\n

Spectrum analysis with SDR<\/strong><\/p>\n

On a waterfall display, any frequency-division system is immediately recognizable: each sub-channel appears as a distinct stripe, spacing reflects spectrum planning, and signal shapes reveal filter quality. This is one of the most effective exercises for developing a concrete intuition about radio spectrum behavior.<\/p>\n

In summary<\/h3>\n

Frequency division exists to solve a practical need: allowing multiple signals to coexist in the same band without interference, by assigning each one a distinct portion of spectrum.<\/p>\n

The general principle is called FDM, and it applies whenever multiple streams-even within the same emission-are separated in frequency. When those streams belong to independent users, the system is called FDMA.<\/p>\n

For radio amateurs, this principle is present at every level: in the coexistence of many digital stations within a single SSB channel, in digital voice systems such as C4FM, in SDR and DSP experimentation, and even in everyday broadcast systems like FM stereo. Recognizing it means better understanding how the radio spectrum works-and how to use it more consciously and efficiently.<\/p>\n","protected":false},"excerpt":{"rendered":"

Why there is a need to divide the spectrum The radio spectrum is a finite and regulated resource. Each band has a limited width, and every transmission occupies a portion of it. 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