Single-RAT operation

Description

Single-RAT operation is a fundamental deployment concept in 3GPP radio access networks, defined as the operation of a base station in an operating band with only one Radio Access Technology (RAT) configured in that operating band. This means that within a given frequency band, the base station supports only one cellular technology, such as LTE or NR, rather than simultaneously supporting multiple RATs like GSM, UMTS, LTE, or NR. The concept is specified in 3GPP TS 37.104, which covers multi-standard radio (MSR) base station radio frequency requirements, and TS 37.141, which details MSR base station conformance testing. Single-RAT operation is the baseline configuration for most initial deployments of a technology, providing a clean spectrum environment where all radio resources in the band are dedicated to a single RAT.

In a single-RAT deployment, the base station's radio equipment, antennas, and baseband processing are optimized for one technology. This simplifies radio resource management, scheduling, and interference coordination because there is no need to handle inter-RAT interference within the same band or to share spectrum dynamically between technologies. For example, an LTE base station operating in band 3 (1800 MHz) with only LTE configured would be a single-RAT operation. The base station can use the full bandwidth of the operating band for LTE carriers, and all mobility and session management procedures are handled within the LTE network. This contrasts with multi-RAT operation, where a base station might support both LTE and NR in the same band, requiring complex coexistence mechanisms such as spectrum sharing, dynamic power allocation, and inter-RAT handover.

The primary advantage of single-RAT operation is simplicity and performance optimization. Since the base station does not need to support multiple RATs, its hardware and software can be tailored to the specific requirements of one technology, potentially reducing cost and power consumption. Interference is easier to manage because all transmissions in the band follow the same radio frame structure, numerology, and power control rules. This leads to more predictable coverage and capacity. Additionally, single-RAT operation simplifies network planning and optimization, as engineers do not need to account for inter-RAT interference or complex coexistence scenarios. For operators, single-RAT operation is often the first step in deploying a new technology, allowing them to refarm spectrum from older technologies gradually.

However, single-RAT operation also has limitations. It does not allow for the efficient use of spectrum when multiple technologies need to coexist, such as during the transition from LTE to NR. In such cases, multi-RAT operation or dynamic spectrum sharing (DSS) is required. Single-RAT operation also means that if an operator wants to support legacy users, they must maintain separate bands or deploy multi-RAT base stations. Despite these limitations, single-RAT operation remains the most common deployment scenario for many frequency bands, especially those dedicated to a single technology. It is also the reference configuration for many 3GPP specifications, providing a baseline for performance requirements and testing.

In the context of 3GPP specifications, single-RAT operation is defined in TS 37.104 and TS 37.141, which are part of the multi-standard radio (MSR) series. These specifications define the radio frequency (RF) requirements and conformance testing for base stations that support one or more RATs. For single-RAT operation, the requirements are typically less complex than for multi-RAT operation, as the base station only needs to meet the RF requirements for one technology. The definition has remained stable across releases from Rel-11 to Rel-19, indicating that the concept is well-established and not subject to frequent changes. This stability reflects the fundamental nature of single-RAT operation as a basic deployment mode.

Purpose & Motivation

Single-RAT operation exists to provide a clear, simple, and optimized deployment scenario for cellular networks where a base station operates in a frequency band with only one radio access technology. The primary motivation is to simplify network design, reduce interference, and maximize performance for a single technology. By dedicating an entire operating band to one RAT, operators can avoid the complexities of multi-RAT coexistence, such as inter-RAT interference, spectrum sharing, and complex scheduling. This is particularly important during the initial deployment of a new technology, when operators want to ensure reliable coverage and capacity without the overhead of supporting legacy technologies in the same band.

Historically, before the introduction of multi-standard radio (MSR) base stations, each technology was deployed in its own dedicated spectrum. For example, GSM operated in 900/1800 MHz bands, UMTS in 2100 MHz, and LTE in various bands. As spectrum became scarce and operators sought to refarm spectrum from older technologies to newer ones, the need for multi-RAT operation arose. However, single-RAT operation remained the baseline for many deployments, especially in bands that were newly allocated for a specific technology. The concept was formally defined in 3GPP Rel-11 as part of the MSR specifications, which aimed to standardize the requirements for base stations that could support multiple RATs. Single-RAT operation was defined as the simplest case, providing a reference for more complex multi-RAT scenarios.

The limitations of previous approaches that single-RAT operation addresses include the lack of a standardized definition for base station operation in a single RAT within an operating band. Before Rel-11, the term was not explicitly defined in 3GPP specifications, leading to potential ambiguities in testing and deployment. By defining single-RAT operation, 3GPP provided a clear baseline for manufacturers and operators to design and test equipment. It also helped regulatory bodies and operators to understand the performance expectations for single-RAT deployments. Today, single-RAT operation remains relevant for new spectrum bands, such as those allocated for 5G NR, where operators may initially deploy only NR before considering multi-RAT operation.