Description
The Low Power Synchronization Signal (LP-SS) is a physical layer signal defined in 3GPP Release 18 for New Radio (NR), specifically designed to minimize power consumption during the cell search and initial access procedures. It operates as part of the synchronization signal block (SSB) framework but is optimized for low-power operation. The LP-SS is transmitted by the gNB with specific characteristics, such as reduced bandwidth, lower transmission power, or optimized periodicity, to allow user equipment (UE) to perform synchronization with minimal receiver activation time. This signal is crucial for devices like IoT sensors and wearables that require ultra-long battery life, as it reduces the energy expended during the critical initial network attachment phase.
Architecturally, LP-SS is integrated into the NR physical layer specifications (TS 38.211 series) and is managed by the gNB's radio resource control (RRC) layer. It typically consists of primary and secondary synchronization signals (PSS/SSS) but with modifications to reduce complexity. For instance, it may use simpler sequences or be transmitted less frequently compared to conventional SSBs. The UE's physical layer processing for LP-SS involves low-power receiver circuits that can detect these signals with high efficiency, often leveraging wake-up receiver (WUR) technology to minimize main receiver usage. The signal parameters, such as frequency location and time-domain pattern, are configured via system information or dedicated RRC signaling to align with network energy-saving policies.
In operation, when a UE powered by a battery needs to synchronize with a cell, it activates its low-power receiver to search for LP-SS instead of the full SSB. Upon detection, the UE acquires timing and frequency synchronization, along with basic cell identification, before potentially waking its main receiver for further procedures like reading the physical broadcast channel (PBCH). This two-step process significantly cuts energy use. LP-SS is specified across multiple 3GPP documents, including TS 38.101 for UE radio requirements, TS 38.300 for overall architecture, and TS 38.331 for RRC protocols, ensuring interoperability and performance benchmarks for low-power scenarios.
Purpose & Motivation
LP-SS was created to address the growing demand for energy-efficient communication in 5G and beyond, particularly for Internet of Things (IoT) and massive machine-type communication (mMTC) use cases. Prior to Release 18, NR synchronization relied on standard SSBs, which, while effective for high-performance devices, consumed excessive power for battery-constrained devices during frequent cell searches or in coverage-challenged environments. This limitation hindered the deployment of long-life IoT sensors and wearables in NR networks, as continuous synchronization drained batteries quickly.
The motivation stems from the need to extend device battery life from years to decades, a key requirement for applications like environmental monitoring, smart agriculture, and industrial sensors. Historical approaches in LTE-M and NB-IoT offered low-power features but were not natively integrated into NR's framework. LP-SS fills this gap by introducing a native NR solution that reduces UE power consumption during initial access, aligning with 3GPP's RedCap (Reduced Capability) and IoT enhancements. It solves the problem of energy waste in synchronization by optimizing signal design, allowing devices to stay dormant longer and only activate minimally for network attachment, thus enabling sustainable massive IoT deployments in 5G networks.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (77 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the LP-SS (Low Power Synchronization Signal) function was newly introduced as part of the broader completion of power saving descriptions and mechanisms. This included defining specific UE channel bandwidths for LP-WUS/WUR (Low Power Wake-Up Signal/Wake-Up Receiver) operation. The release also established corresponding synchronization raster entries to support these low-power synchronization procedures.
- CR on signalling introduction of UE overheating support in NR SA scenario TS 38.331CR0729
- Additional capability signalling for 1024QAM support TS 38.331CR1120
- Introduction of UE capability for NR-DC with SFN synchronization between PCell and PSCell TS 38.331CR1265
- Completion of description of power saving TS 38.300CR0050
- Capture signalling flows where the last serving gNB moves the UE to RRC_IDLE TS 38.300CR0087
- Clarification on power ramping counter TS 38.300CR0125
+ 15 more changes
In Release 16, the new LP-SS (Low Power Synchronization Signal) function was introduced as part of broader UE power saving enhancements. Specifically, it involved defining new synchronization raster entries for operating bands and establishing UE channel bandwidth parameters for LP-WUS/WUR (Low Power Wake-Up Signal/Wake-Up Receiver). These additions provided the foundational RF and synchronization requirements needed to enable the low-power monitoring states signaled by the power saving features listed in the CRs.
- Introduction of UE Power Saving in NR TS 38.300CR0193
- CR for 38.331 for Power Savings TS 38.331CR1469
- Introduction of signalling for high-speed train scenarios TS 38.331CR1464
- Configuration for uplink power boosting via suspended IBE requirements TS 38.331CR2008
- CR for 38.331 for Power Savings TS 38.331CR1540
- Duty cycle signalling for power class 1.5 TS 38.331CR2817
+ 6 more changes
In Release 17, the LP-SS (Low Power Synchronization Signal) function introduced new UE power saving enhancements and corrections, as detailed in the core specifications. These enhancements included specific technical refinements for power-saving resource allocation and corrections related to remote UE synchronization procedures. The release also defined new synchronization raster entries for operating bands and detailed transmitter power requirements for various configurations including CA and RedCap.
- Introduction of UE power saving enhancements In 38.300 TS 38.300CR0417
- Introduction of RRC signaling for measurement gap enhancement TS 38.331CR2913
- CR to TS 38.331 on Network assistant signalling for Rel-17 CRS interference mitigation TS 38.331CR3021
- UE Security Capabilities signaling in NG-RAN [UE_Sec_Caps] TS 38.300CR0427
- Corrections to UE power saving enhancements In 38.300 TS 38.300CR0458
- Corrections to UE power saving enhancements in TS 38.300 TS 38.300CR0552
+ 10 more changes
In Release 18, the LP-SS function saw the introduction of new synchronization raster entries for each operating band and a dedicated synchronization raster for Sidelink Unlicensed operation. Furthermore, specific channel bandwidth definitions and arrangements were established for LP-WUS/WUR and for (e)RedCap devices to support their low-power operation. These additions provided the foundational RF and synchronization parameters needed to enable these new UE categories and use cases.
- Signaling support for intra-band non-collocated NR-CA, EN-DC TS 38.331CR4396
- Introduction of network RRC signalling for advanced receiver TS 38.331CR4488
- Correction of network timing synchronization status monitoring TS 38.300CR0817
- Correction on coexistence between CHO and satellite switching with re-synchronization TS 38.300CR0903
- Correction on network RRC signalling for advanced receiver TS 38.331CR4585
- Correction on RRC signalling for advanced receiver TS 38.331CR4673
+ 8 more changes
In Release 19, the key new feature for LP-SS is the formal introduction of the Low-Power Wake-Up Signal and Receiver (LP-WUS/WUR) for NR, which is a new function distinct from previous synchronization signals. This introduction required new RRC signaling support for the feature and included specifications for UE channel bandwidth and operating bands dedicated to LP-WUS/WUR operation. The release also encompassed necessary corrections and refinements, such as defining subgroup IDs and disabling indications, to solidify the LP-WUS framework.
- Introduction of Low-Power Wake-Up Signal and Receiver for NR TS 38.300CR1015
- Introduction of LP-WUS/WUR in RRC TS 38.331CR5416
- Introduction of signaling support for intra-band non-collocated EN-DC/NR-CA deployment Phase 2: new receiver type(s) TS 38.331CR5479
- Miscellaneous corrections for LP-WUS TS 38.300CR1046
- Allocation of subgroup IDs for LP-WUS TS 38.300CR1076
- On introducing LP-WUS disabling indication TS 38.300CR1082
+ 8 more changes
Explore further
Broader topics and technologies where LP-SS plays a role.
Defining Specifications
3GPP specifications that define or reference LP-SS, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.101 vj31 | NR User Equipment Radio Transmissions | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.774 vj00 | Rel-19 LP-WUS/WUR RF Requirements TR | Rel-19 |
| TR 38.869 vi00 | Study on low-power wake up signal and receiver for NR | Rel-18 |