Sub-block gap

Description

In modern cellular networks, a base station's RF bandwidth can be composed of multiple sub-blocks, each potentially serving different operators, frequency layers, or radio access technologies. A sub-block gap is the unused frequency region between two consecutive sub-blocks within the overall base station RF bandwidth. These gaps are not arbitrary; they are defined to ensure that the RF requirements in the gap—such as unwanted emissions, blocking, and intermodulation—are based on co-existence for un-coordinated operation. This means that the network can deploy sub-blocks without requiring precise synchronization or coordination with other systems, simplifying deployment and reducing operational complexity.

The concept was introduced in 3GPP Release 10 as part of the work on multi-standard radio (MSR) base stations, which aimed to support multiple technologies (e.g., GSM, UMTS, LTE) within a single base station. Sub-block gaps enable flexible spectrum usage by allowing operators to deploy non-contiguous spectrum chunks while maintaining acceptable interference levels. The size and placement of sub-block gaps are specified in 3GPP TS 37.104 and 37.141 for MSR, and in 38.104 for NR. They are typically expressed as a minimum frequency separation that must be maintained to meet regulatory and co-existence requirements.

From a technical perspective, the sub-block gap is characterized by its width in Hz (or MHz). Within the gap, the base station must meet certain RF requirements, such as spurious emissions limits, to avoid causing harmful interference to other systems. These requirements are less stringent than those for in-band transmissions but more stringent than for out-of-band emissions. The gap also affects the base station's ability to perform measurements and may influence scheduling if the gap is used for other purposes (e.g., guard bands). The gap is defined at the base station antenna connector and is part of the overall RF bandwidth configuration.

Key parameters associated with sub-block gaps include the maximum number of sub-blocks, the minimum gap size, and the applicable RF requirements. For example, in LTE, a sub-block gap of at least 5 MHz might be required between two sub-blocks of different operators to ensure co-existence. The gap size depends on factors such as the operating band, the sub-block bandwidths, and the coexistence scenario (e.g., co-located base stations). 3GPP specifications provide tables mapping these parameters to specific requirements.

Sub-block gaps are essential for enabling spectrum sharing and flexible deployment. They allow operators to use non-contiguous spectrum, which is common in refarmed bands. They also facilitate the coexistence of different radio access technologies within the same base station, supporting the smooth evolution from 2G/3G to 4G and 5G. Without sub-block gaps, deploying multiple sub-blocks would require tight synchronization and coordination, increasing cost and complexity. As networks move towards 5G NR and beyond, the concept remains relevant for multi-band and multi-operator deployments, including spectrum sharing scenarios like CBRS and shared spectrum in 6 GHz.

Purpose & Motivation

The sub-block gap was introduced to address the challenge of deploying multiple sub-blocks within a single base station's RF bandwidth without requiring tight coordination between them. Before its definition, operators faced difficulties in sharing spectrum or deploying multi-operator base stations because uncoordinated transmissions could cause severe interference. The sub-block gap provides a buffer zone where RF requirements are relaxed but still controlled, enabling coexistence in un-coordinated scenarios. This is particularly important for spectrum refarming, where legacy technologies and new ones must coexist, and for shared spectrum access, where multiple operators use the same band. The gap concept allows flexible spectrum usage and simplifies deployment, reducing the need for complex coordination and synchronization mechanisms.