Narrowband Internet of Things In-Band Operation

Description

NB-IoT in-band operation is one of the three defined deployment modes for Narrowband-IoT, alongside stand-alone and guard band operation. In this mode, NB-IoT utilizes resource block(s) within a normal E-UTRA carrier — that is, within the occupied bandwidth of an existing LTE carrier. Unlike guard band operation, which uses unused spectrum outside the LTE carrier, in-band operation shares the same frequency resources as LTE, with the base station dynamically allocating resource blocks between LTE and NB-IoT traffic. This mode is specified in 3GPP TS 36.104 (E-UTRA Base Station radio transmission and reception), TS 36.113 (E-UTRA Base Station EMC), TS 36.141 (E-UTRA Base Station conformance testing), and their NR equivalents in the 37.xxx series.

The technical implementation of in-band operation requires the base station to support multiplexing of NB-IoT and LTE transmissions within the same carrier. The NB-IoT PRB is positioned within the LTE carrier's bandwidth, and the base station scheduler coordinates resource allocation between the two technologies. In the downlink, NB-IoT uses the same OFDMA numerology as LTE, with 15 kHz subcarrier spacing, which simplifies multiplexing. In the uplink, NB-IoT supports both single-tone (3.75 kHz or 15 kHz) and multi-tone (15 kHz) transmissions, with the 15 kHz multi-tone mode being compatible with LTE's SC-FDMA numerology. The base station must ensure that NB-IoT transmissions do not degrade LTE performance, particularly regarding power allocation and interference.

From a device perspective, in-band operation is transparent — the UE tunes to the NB-IoT carrier frequency and operates as it would in any other mode. The network configures the operation mode through system information, and the device uses the appropriate synchronization and reference signals to access the cell. The key difference lies in the base station implementation, where the NB-IoT carrier is positioned within the LTE carrier's bandwidth rather than in a dedicated carrier (stand-alone) or in the guard band. The device must be able to detect the NB-IoT synchronization signals (NPSS and NSSS) and decode the Master Information Block (MIB-NB) and System Information Blocks (SIBs) regardless of the operation mode.

In-band operation offers significant advantages for operators. It allows NB-IoT deployment on existing LTE spectrum without requiring additional spectrum or refarming, which is particularly valuable in markets where spectrum is scarce. It also enables operators to leverage their existing LTE infrastructure, including base stations, antennas, and backhaul, reducing deployment costs. However, it presents challenges including reduced LTE capacity (since some resource blocks are allocated to NB-IoT), potential interference between NB-IoT and LTE transmissions, and the need for careful power allocation. The 3GPP specifications define the necessary requirements and test procedures to ensure that in-band NB-IoT can coexist with LTE without degrading performance.

In practice, in-band operation is widely used by operators who have deployed NB-IoT on their existing LTE networks. It is often combined with guard band operation, with the network dynamically selecting the best mode based on available resources and traffic conditions. The specifications in TS 36.104 and TS 36.141 define the base station RF requirements, while TS 36.113 covers electromagnetic compatibility. The NR equivalents (TS 37.104, 37.105, 37.113, 37.114, 37.141, 37.145) address multi-RAT base station requirements when NB-IoT is deployed alongside NR. The choice between in-band and guard band operation depends on factors such as available spectrum, LTE traffic load, and RF planning constraints.

Purpose & Motivation

NB-IoT in-band operation was created to provide operators with a flexible deployment option that leverages existing LTE spectrum and infrastructure. When NB-IoT was being standardized in Release 13, one of the key requirements was that it should be deployable without requiring new spectrum acquisitions. While stand-alone mode required dedicated spectrum (typically refarmed GSM carriers) and guard band mode required sufficient unused guard band spectrum, in-band mode allowed operators to deploy NB-IoT directly within their existing LTE carriers.

This was particularly important for operators who had limited spectrum holdings or who wanted to deploy NB-IoT quickly without waiting for spectrum refarming. In-band operation allowed them to use their existing LTE base stations and antennas, reducing deployment costs and time-to-market. It also provided a way to monetize existing spectrum assets by supporting IoT services alongside traditional mobile broadband.

The technology also addressed the need for coexistence between NB-IoT and LTE. By defining in-band operation, 3GPP ensured that NB-IoT could share resources with LTE without degrading performance. The specifications define the necessary RF requirements, power allocation rules, and test procedures to ensure that both technologies can operate harmoniously within the same carrier. This coexistence is essential for operators who want to support both mobile broadband and IoT services on the same network.