Gap Between Passbands

Description

The 'gap between passbands' is a 3GPP term describing the frequency gap between two consecutive passbands that belong to the same operating band, where the RF requirements applicable in that gap are based on co-existence for un-coordinated operation. In other words, when a single operating band is defined as consisting of two or more separate passbands (sub-bands) rather than one contiguous block of spectrum, the spectrum lying between those passbands is not simply 'unused'; it has defined RF behaviour, and that behaviour is derived from the assumption that systems operating in the two passbands are not coordinated with each other.

Architecturally, this concept belongs to the band-definition and RF requirement framework of NR. An operating band in 3GPP is normally a contiguous range of frequencies with a defined duplex arrangement (FDD, TDD or SUL/SDL). However, regulators and operators sometimes hold spectrum that is split into non-contiguous chunks within what is logically treated as one band. To support such arrangements, 3GPP can define a band with multiple passbands, each with its own lower and upper edges, and with gaps between them. The gap is therefore a first-class part of the band definition, not an implementation detail.

How it works: for each passband, the usual transmitter and receiver requirements apply — output power, unwanted emissions, ACLR, reference sensitivity, blocking, etc. For the gap between passbands, the requirements are set so that equipment operating in one passband does not unacceptably degrade equipment operating in the other, under the assumption that the two systems are uncoordinated. Uncoordinated operation means there is no scheduling, synchronisation or power-control coordination between the transmitters and receivers on either side of the gap; each side must therefore be robust to the other's emissions on its own. This typically leads to requirements on out-of-band emissions and on receiver blocking/adjacent channel selectivity that are referenced to the passband edges rather than to the outer edges of the whole band.

The concept interacts with several other RF constructs. It is distinct from, but related to, 'inter-band' operation (where two separate operating bands are used, e.g., carrier aggregation across bands) and 'intra-band non-contiguous' carrier aggregation (where multiple carriers within one band are separated by a gap). The gap between passbands provides the formal band-definition basis on which intra-band non-contiguous arrangements and their RF requirements can be built. It also interacts with the notions of band edge, operating band unwanted emissions, and coexistence with other services or operators that may occupy the gap spectrum.

In Rel-19 and Rel-20, specifications 38.106 (NR radio transmission and reception) and 38.115 (NR repeater and related RF requirements) address these band definitions and the associated RF requirements. The Rel-19 and Rel-20 definitions of the term are identical, indicating that the concept carried over unchanged between these releases even as the surrounding requirement sets evolved. For engineers, the practical significance is that when designing or testing equipment for a multi-passband band, the gap is not a free-for-all: emissions into the gap and sensitivity in the presence of signals in the gap must meet requirements derived from uncoordinated coexistence, and test configurations must respect the passband edges rather than treating the band as contiguous.

Purpose & Motivation

The gap between passbands exists because real-world spectrum holdings are often non-contiguous, while 3GPP band definitions are traditionally contiguous. Regulators may assign an operator several separate blocks within a frequency range, or a band may be defined to cover spectrum that is partly occupied by other services. To describe and regulate equipment behaviour in such cases, 3GPP needs a formal way to say 'these are the passbands, and this is the gap between them, with requirements based on uncoordinated coexistence'.

Before such a concept was formalised, multi-passband arrangements were awkward to specify: requirements were anchored to a single contiguous band edge, which did not reflect the reality of emissions and blocking in the interior gap. The term solves this by making the gap an explicit part of the band definition and by tying its RF requirements to the assumption that the two sides are not coordinated. This is a conservative and realistic assumption, because in practice the two passbands may be used by different operators, different services or different deployments that cannot rely on each other's cooperation.

The concept also supports the broader 3GPP goal of flexible spectrum use: it enables new band definitions that stitch together fragmented spectrum, facilitates intra-band non-contiguous carrier aggregation, and provides a common vocabulary for coexistence studies. By stating that requirements in the gap are based on uncoordinated coexistence, it gives equipment vendors and test houses a clear, stable basis for design and verification, and it gives regulators a defensible technical framework for assigning fragmented spectrum.