Narrowband Physical Uplink Shared Channel

Description

The Narrowband Physical Uplink Shared Channel (NPUSCH) is the key physical channel in NB-IoT responsible for carrying uplink transmissions from the User Equipment (UE) to the base station (eNB/gNB). It is the counterpart to the downlink NPDSCH (Narrowband Physical Downlink Shared Channel). NPUSCH is designed to operate within the constraints of NB-IoT, primarily the 180 kHz bandwidth (or a single-tone transmission of 3.75 kHz or 15 kHz) and the requirement for extreme coverage enhancement and ultra-low power consumption. It serves as the physical layer conduit for the Narrowband Uplink Shared Channel (NUL-SCH), which carries higher-layer data, and for Hybrid Automatic Repeat Request (HARQ) acknowledgments for downlink transmissions.

NPUSCH is defined with two distinct formats, each serving a specific purpose. NPUSCH format 1 is used for carrying uplink data. It can be configured to use either single-tone transmission (with a subcarrier spacing of 3.75 kHz or 15 kHz) or multi-tone transmission (using 3, 6, or 12 subcarriers with 15 kHz spacing). Single-tone transmission, especially with 3.75 kHz spacing, provides a very narrow transmission bandwidth, resulting in a high power spectral density. This is crucial for achieving the maximum coverage extension, as it allows the device to concentrate its limited transmit power into an extremely narrow frequency band to penetrate challenging environments. Multi-tone transmission offers higher data rates for devices in good coverage. The channel coding for format 1 uses Turbo coding for robust error correction.

NPUSCH format 2 is dedicated solely for carrying the Uplink Control Information (UCI), specifically the HARQ acknowledgment (ACK/NACK) in response to a downlink NPDSCH transmission. Format 2 uses a single-tone transmission with a fixed subcarrier spacing of 3.75 kHz. It carries a very small payload (1 or 2 bits for ACK/NACK) and uses repetition coding for reliability. The transmission of NPUSCH is scheduled by the network via Downlink Control Information (DCI) carried on the NPDCCH. The scheduling grants specify parameters like the resource assignment (subcarrier indices), modulation and coding scheme (QPSK for format 1, BPSK for format 2), number of repetitions, and the transport block size. This grant-based access ensures efficient shared medium utilization. The design of NPUSCH, with its tone flexibility and extensive repetition support, is central to meeting NB-IoT's goals of deep coverage, long battery life (enabled by efficient power-amplifier usage in single-tone mode), and support for a massive number of low-throughput devices.

Purpose & Motivation

NPUSCH was created to address the unique uplink requirements of NB-IoT, which were not adequately met by the existing LTE Physical Uplink Shared Channel (PUSCH). The standard LTE PUSCH is designed for wider bandwidths (at least 1.4 MHz) and higher data rates, making it inefficient and power-hungry for IoT devices that need to send only small, infrequent packets over very long distances. The high Peak-to-Average Power Ratio (PAPR) of multi-carrier LTE uplink signals also reduces the power amplifier efficiency in low-cost devices, draining battery life. There was a clear need for a new uplink channel optimized for the IoT paradigm.

The purpose of NPUSCH is to provide an uplink transmission scheme that maximizes coverage, minimizes device power consumption, and supports massive connectivity. It solves the coverage problem through its support for single-tone transmission with low subcarrier spacing (3.75 kHz), which dramatically increases the symbol duration and improves robustness against delay spread and noise. The extensive repetition mechanism (up to 128 or more repetitions) provides the processing gain needed for deep indoor or rural coverage. It solves the power efficiency problem by enabling constant-envelope single-tone signals (with pi/2-BPSK or pi/4-QPSK modulation), which have low PAPR, allowing the device's power amplifier to operate near its saturation point with high efficiency. Furthermore, the two-format structure separates data and small control feedback, optimizing the resource usage for each type of information. NPUSCH was thus a fundamental innovation that made cellular-based, battery-operated IoT feasible on a massive scale.

Key Features

• Supports two formats: Format 1 for uplink data and Format 2 for HARQ ACK/NACK
• Enables single-tone transmission (3.75 kHz or 15 kHz) for maximum coverage and power efficiency
• Supports multi-tone transmission (3, 6, or 12 tones at 15 kHz) for higher data rates
• Utilizes extensive repetition (up to 2048 times) for coverage enhancement up to 164 dB MCL
• Employs low-PAPR modulation (pi/2-BPSK, pi/4-QPSK) for efficient power amplifier operation
• Grant-based transmission scheduled via NPDCCH for controlled uplink access

Evolution Across Releases

Defining Specifications