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.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (52 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-13, normative work from Rel-15.
In Release 15, corrections and clarifications were introduced for the NPUSCH function, primarily focusing on the HARQ process. These included a correction for the minimum length of the uplink HARQ RTT timer and a clarification on HARQ RTT for specific UE categories. Additionally, the release specified the flushing of the HARQ buffer upon skipping an uplink transmission and made corrections to HARQ retransmission resource selection.
- Clarification on CRC attachment for DL-SCH and PCH transport channels in NB-IoT TS 36.212CR0285
- Correction on the interpretation of HARQ-ACK bitmap for FeLAA in 36.212 TS 36.212CR0297
- 36.300 CR on Correction of Physical Layer Resource to Cell Resource TS 36.300CR1211
- Minor corrections to services provided by physical layer TS 36.302CR1195
- Correction on HARQ RTT Timers TS 36.321CR1328
- Correction on the logical channel selection in sidelink LCP TS 36.321CR1330
+ 21 more changes
In Release 16, the key new development for the NPUSCH was the introduction of performance and test requirements for NPUSCH format 1 during multi-TB interleaved transmission. This involved defining specific conformance testing procedures for this enhanced transmission mode within the relevant technical specifications. The release also included subsequent corrections and cleanup for these new multi-TB interleaved transmission test cases to ensure clarity and consistency.
- CR: Introduce NPUSCH format 1 performance requirements for multi-TB interleaved transmission. TS 36.104CR4909
- CR: Addition of NPUSCH format1 performance requirements for multi-TB interleaved transmission in TS 36.104 TS 36.104CR4915
- CR: Introduce NPUSCH format 1 test requirements for multi-TB interleaved transmission for TS 36.141 TS 36.141CR1271
- Mobility to NR operating with shared spectrum access TS 36.331CR4263
- CR: Cleanup for NPUSCH format 1 conformance testing for multi-TB interleaved transmission in TS 36.141 TS 36.141CR1284
- Corrections on the NPUSCH repetition adjustment field TS 36.212CR0354
+ 8 more changes
In Release 17, key enhancements for NPUSCH included the introduction of uplink RRC segmentation capability and the finalization of demodulation requirements for NPUSCH format 1 using 16QAM. The release also introduced corrections and clarifications for the UL HARQ RTT timer length and the npusch-MCS field description within the dedicated configuration.
- Introduction of uplink RRC Segmentation capability TS 36.331CR4826
- CR on NPUSCH format1 demodulation requirement for TS 36.104 TS 36.104CR4961
- CR 36.141 on Finalization of NPUSCH format 1 16QAM test requirement TS 36.141CR1344
- Correction on figure clarifying HARQ RTT timer TS 36.321CR1561
- Correction on npusch-MCS field description TS 36.331CR4866
- CR to 36.331 on NPUSCH-ConfigDedicated-NB-v1700 TS 36.331CR4903
+ 1 more changes
In Release 18, changes to the NPUSCH primarily consisted of specification corrections to improve reliability, specifically targeting the Hybrid Automatic Repeat Request (HARQ) process and the demodulation requirements for NPUSCH format 1. These corrections, detailed in the updated TS 36.104 and TS 36.213, provided necessary clarifications to ensure consistent implementation and testing of these narrowband uplink procedures.
In Release 19, the NPUSCH function was updated with corrections for Semi-Persistent Scheduling (SPS) and preallocated uplink grants specifically for TDD operation. Furthermore, the release introduced corrections to define the valid uplink subframes for IoT operations over Non-Terrestrial Networks (NTN) in TDD mode.
Explore further
Broader topics and technologies where NPUSCH plays a role.
Defining Specifications
3GPP specifications that define or reference NPUSCH, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 36.104 vj10 | Base Station (BS) radio transmission and reception | Rel-19 |
| TS 36.141 vj00 | E-UTRA BS Conformance Testing | Rel-19 |
| TS 36.201 vj00 | LTE Physical Layer General Description | Rel-19 |
| TS 36.211 vj10 | LTE Physical Layer Specification | Rel-19 |
| TS 36.212 vj10 | LTE Multiplexing and Channel Coding | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.302 vj00 | E-UTRA Physical Layer Services | Rel-19 |
| TS 36.321 vj00 | E-UTRA MAC Protocol Specification | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |