Anchor carrier (NB-IoT)

Description

In Narrowband IoT (NB-IoT), the anchor carrier is a specific carrier frequency on which the User Equipment (UE) can assume that essential synchronization and broadcast signals are always present. These signals include the Narrowband Primary Synchronization Signal (NPSS), Narrowband Secondary Synchronization Signal (NSSS), Narrowband Physical Broadcast Channel (NPBCH), and for Frequency Division Duplex (FDD) operation, the System Information Block for NB-IoT (SIB-NB). For Time Division Duplex (TDD) operation, the anchor carrier is defined as carrying NPSS, NSSS, and NPBCH, but not SIB-NB, reflecting the different duplexing and signal structure in TDD mode.

The anchor carrier serves as the initial access point for NB-IoT UEs. When an NB-IoT device powers on or enters a new cell, it searches for the anchor carrier to acquire time and frequency synchronization via NPSS and NSSS, and to decode the Master Information Block (MIB-NB) carried on NPBCH. The MIB-NB provides critical system parameters, including the system frame number, hyperframe number, and scheduling information for SIB-NB. SIB-NB, transmitted on the anchor carrier in FDD mode, contains additional system information such as cell access restrictions, random access parameters, and configuration for other carriers.

NB-IoT supports operation in three modes: stand-alone, guard-band, and in-band. In stand-alone mode, the anchor carrier occupies a dedicated 200 kHz bandwidth, typically in a GSM spectrum refarming scenario. In guard-band mode, it uses unused resource blocks within an LTE carrier's guard band. In in-band mode, it operates within the resource blocks of a normal LTE carrier. Regardless of the mode, the anchor carrier is the one that carries the synchronization and broadcast signals, allowing UEs to discover and access the cell.

In addition to the anchor carrier, NB-IoT may use non-anchor carriers for data transmission. These non-anchor carriers do not carry NPSS, NSSS, NPBCH, or SIB-NB, and are used to offload traffic or provide additional capacity. A UE first camps on the anchor carrier, reads the necessary system information, and may then be directed to a non-anchor carrier for data transfer. This multi-carrier operation improves spectral efficiency and allows the network to support more devices.

The anchor carrier concept is specified in 3GPP TS 36.331 (Radio Resource Control protocol for E-UTRA), TS 36.355 (LTE Positioning Protocol), and TS 37.355 (LTE Positioning Protocol for NR and LTE). These specifications define the UE procedures for cell selection, synchronization, and system information acquisition on the anchor carrier. The anchor carrier is fundamental to NB-IoT operation, ensuring that low-complexity, low-power devices can reliably find and connect to the network.

Purpose & Motivation

The anchor carrier concept was introduced in Release 13 as part of the initial NB-IoT specification to provide a reliable and predictable access mechanism for low-cost, low-power IoT devices. NB-IoT devices are designed to be extremely simple and power-efficient, often operating for years on a single battery. To achieve this, they cannot perform complex cell searches or continuously monitor multiple carriers. The anchor carrier solves this by defining a single carrier that always carries the necessary synchronization and broadcast signals, allowing UEs to quickly and reliably find the network.

Before NB-IoT, cellular IoT technologies like GPRS and LTE Cat-1 required more complex and power-hungry procedures for initial access. The anchor carrier simplifies this by ensuring that NPSS, NSSS, and NPBCH are always present on a known carrier, reducing the search space and power consumption. It also enables efficient system information acquisition, as SIB-NB is transmitted on the same carrier in FDD mode, eliminating the need to tune to other frequencies during initial access.

The separation of anchor and non-anchor carriers further improves efficiency. Non-anchor carriers can be used for data transmission without the overhead of synchronization and broadcast signals, allowing the network to optimize resource usage. This is particularly important in in-band and guard-band deployments where spectrum is shared with LTE. The anchor carrier concept thus provides a balance between simplicity for the device and flexibility for the network operator.