Automatic Neighbour Relationship

Description

Automatic Neighbour Relationship (ANR) is a core Self-Organizing Network (SON) function defined within the 3GPP standards, primarily for the Radio Access Network (RAN). Its primary operational domain is the base station (eNodeB in LTE, gNB in NR), which hosts the ANR function. The process is fundamentally driven by the User Equipment (UE). The UE continuously performs measurements on detected cells, including those not listed in its current Neighbour Relation Table (NRT). When a UE reports a strong, previously unknown cell (identified by its Physical Cell ID (PCI) and, critically, its globally unique E-UTRAN Cell Global Identifier (ECGI) or equivalent), the serving base station's ANR function triggers a procedure to establish a neighbour relation.

This establishment involves several key steps. First, the serving cell instructs the UE to read the System Information Block Type 1 (SIB1) or equivalent broadcast information from the newly detected cell to obtain its global identifier (ECGI/NCGI) and Tracking Area Code (TAC). Once this information is retrieved and reported, the serving cell can then use the X2 (in LTE) or Xn (in NR) interface to establish a direct signalling connection with the neighbour cell's base station. Upon successful setup, a Neighbour Relation (NR) entry is automatically added to the NRT. This entry includes attributes defining the relationship, such as whether it is a handover candidate (No Remove, No HO) and whether it's a neighbour for coverage or capacity purposes.

The ANR function manages the entire lifecycle of neighbour relations. It not only adds new relations but also monitors their usage and performance. Relations that are never used for successful handovers or that consistently lead to handover failures can be automatically removed or de-prioritized, optimizing the NRT over time. This dynamic management is essential in modern networks with features like Carrier Aggregation (CA), Dual Connectivity (DC), and dense small cell deployments, where the radio environment and optimal neighbour sets change frequently. ANR thus ensures the Neighbour Relation Table is always relevant, accurate, and optimized for seamless mobility, forming a foundational automation layer for modern RAN operations.

Purpose & Motivation

ANR was created to solve the significant operational burden and performance limitations associated with manually configuring neighbour cell lists in mobile networks. Prior to SON and ANR, network engineers had to manually define every potential neighbour cell for each base station based on drive tests and propagation predictions. This process was extremely time-consuming, expensive, and prone to human error. Inaccurate or missing neighbour relations directly caused call drops, failed handovers, and poor user experience. Furthermore, as networks evolved with more frequency bands, heterogeneous deployments (macro, micro, pico cells), and dynamic topologies, manual management became utterly unsustainable.

The introduction of ANR in 3GPP Release 8, alongside the LTE system, was a cornerstone of the Self-Organizing Network vision. It addressed the critical need for operational expenditure (OPEX) reduction and network robustness. By automating neighbour discovery and management, ANR eliminates configuration errors, dramatically speeds up network deployment and optimization, and enables the network to self-adapt to changes such as the addition of new cells or changes in radio conditions. This automation is not merely a convenience but a necessity for the scalability and reliability of modern and future networks, including 5G NR, where network density and complexity are orders of magnitude greater than in legacy systems.