Field Guide · concept

Also known as: FDD, Frequency-division duplex, paired spectrum

Frequency-division duplex (FDD) is a two-way radio scheme in which the uplink and downlink run simultaneously on two separate, paired frequency bands.1 The two bands are held apart by a duplex gap — a slice of unused spectrum — so a duplexer can let a device transmit and receive at the same instant without its own transmitter deafening its receiver. FDD is the classic arrangement for GSM, UMTS, and most macro-cell LTE bands, and it stands in contrast to time-division duplex.

frequency → uplink band device → tower downlink band tower → device duplex gap both bands carry traffic at the same time
FDD splits uplink and downlink onto two paired bands separated by a duplex gap; both are active simultaneously, so the link is continuous in each direction.

How it works

An FDD deployment allocates two blocks of spectrum a fixed distance apart. The lower block usually carries the uplink (device to tower) and the upper the downlink (tower to device), with the duplex spacing between their centre frequencies fixed by the band plan. Because transmit and receive sit on different frequencies, the device’s duplexer — a pair of sharp filters — passes the receive band to the receiver while blocking the device’s own transmit energy from swamping it. The duplex gap gives those filters room to roll off; a guard band at the edge of each block keeps adjacent operators from interfering.

The defining property is continuity: since each direction owns its own frequency full time, data flows both ways without interruption and with low, constant latency. There is no switching overhead and no need to reserve time for turning the link around. The cost is paired spectrum — a regulator must hand out two matched blocks, and the duplex gap between them cannot be used for traffic, so some spectrum is spent purely on separation. FDD is naturally symmetric: uplink and downlink get equal bandwidth whether or not the traffic is balanced.

Band plans and paired spectrum

Cellular frequency bands are numbered, and each FDD band specifies both halves of the pair plus the spacing between them — for example, LTE Band 1 pairs a 1920–1980 MHz uplink with a 2110–2170 MHz downlink, 190 MHz apart. Multiple users share each band through FDMA and other multiplexing on top of the duplex split. Because download traffic now dominates, the rigid symmetry of FDD is sometimes a poor fit for data-heavy use, which is one reason newer mid-band allocations lean on time-division duplex instead. Established FDD bands remain the backbone of wide-area macro coverage, where their simplicity, range, and predictable latency are assets.

Relevance to SDR

FDD is why a cellular signal you find on an SDR spectrum display appears as two mirror blocks of activity separated by a quiet gap: the busy tower downlink is the one an uncoordinated receiver hears strongly, while the uplink sits in its own band far below the noise unless a handset is nearby. Recognising the paired structure — and the fixed duplex spacing of each band — helps identify which cellular technology and band a signal belongs to. GopherTrunk targets land-mobile trunking rather than cellular, but the same duplexing concepts describe how trunked repeaters separate their inbound and outbound paths, so FDD is useful background for reading any two-way RF spectrum.

Sources

  1. Frequency-division duplexing — Wikipedia, for the definition of FDD, paired bands, and the duplex gap. ↩

See also