Description
Layer 1, the Physical Layer, is the foundational layer in the 3GPP protocol architecture, interfacing directly with the transmission medium. It is responsible for the conversion of data frames from the Data Link Layer (Layer 2) into a signal suitable for transmission over the physical medium, which can be radio waves, optical fiber, or copper cable. This involves critical functions such as modulation, where digital data is impressed onto an analog carrier signal; channel coding, which adds redundancy for error detection and correction; and multiplexing, which allows multiple data streams to share the same physical channel. In the radio access context, it handles the precise timing and synchronization required for over-the-air transmission, including the generation of radio frames, slots, and symbols according to the specific numerology of the technology (e.g., UMTS, LTE, NR).
The architecture of L1 is tightly coupled with the Radio Access Network (RAN) and is implemented in both User Equipment (UE) and base stations (NodeB, eNB, gNB). Key components include the transmitter and receiver chains, which encompass power amplifiers, filters, analog-to-digital and digital-to-analog converters, and oscillators. The layer defines the physical channels (e.g., PDSCH, PUSCH in LTE/NR) and physical signals (e.g., synchronization signals, reference signals) that carry user data and control information. It also manages procedures like cell search and initial synchronization, where a UE detects a cell's presence and aligns its timing and frequency.
L1 operates based on detailed specifications for spectrum allocation, duplexing methods (FDD/TDD), and multiple access schemes (e.g., CDMA for UMTS, OFDMA for LTE and NR downlink, SC-FDMA for LTE uplink). Its performance parameters, such as throughput, latency, and block error rate (BLER), are fundamental to the overall system performance. Advanced techniques like MIMO (Multiple-Input Multiple-Output), beamforming, and adaptive modulation and coding are implemented at this layer to enhance spectral efficiency and link robustness. The physical layer provides measurement reports (e.g., for channel quality) to higher layers, which are used for scheduling and mobility decisions.
Purpose & Motivation
The Physical Layer exists to establish the fundamental, raw communication link necessary for any digital network. It solves the problem of reliably transmitting bits over a potentially noisy and shared physical medium. Before standardized L1 specifications, proprietary and incompatible physical interfaces would hinder interoperability between equipment from different vendors. The creation of a unified L1 within 3GPP ensures that UEs can communicate with network infrastructure regardless of manufacturer, enabling global roaming and economies of scale.
Historically, each generation of mobile technology (2G GSM, 3G UMTS, 4G LTE, 5G NR) introduced a new physical layer to address the limitations of its predecessor. For example, GSM's GMSK modulation and TDMA provided digital voice but limited data rates. UMTS introduced W-CDMA to support higher-speed data services. The motivation for evolving L1 has consistently been to achieve higher data throughput, lower latency, improved spectral efficiency, and support for new use cases. The physical layer must efficiently utilize scarce and expensive radio spectrum, a primary driver for innovations like OFDMA and advanced antenna systems in 4G and 5G.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (63 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the L1 (Physical Layer) saw the introduction of new UE capabilities and configurations for advanced FD-MIMO, including specific processing capabilities for EN-DC and parameters for per-serving cell MIMO layer configuration. This release also involved updates to L1/RF capabilities and the invalidation of the specific L1 parameter `nrofCQIsPerReport`. Furthermore, support for handling multiple numerologies in FeMBMS and for SRS carrier switching was standardized through updated UE capability signalling.
- Advanced CSI CBSR CBSR parameter and related capability for FD-MIMO TS 36.331CR3397
- Indications of RRC connection resumption and establishment to upper layers during EDT TS 36.331CR3709
- PDU session release indication to upper layers during Full Configuration in eLTE TS 36.331CR4044
- Corrections to Application layer measurement reporting and UE capability signalling TS 36.331CR4142
- RIL I118 on release case to upper layers for CN paging for a UE in RRC_INACTIVE TS 38.331CR0244
- Update of L1/RF capabilities TS 38.331CR0295
+ 11 more changes
In Release 16, the Layer 1 (Physical Layer) introduced enhancements focused on improving MIMO efficiency for both downlink and uplink. These included new UE capabilities specifically for DL MIMO efficiency enhancement and for UL MIMO coherence in support of uplink transmission switching. Additionally, corrections and refinements were made to existing Layer 1 procedures, such as those concerning repetition for L1-SINR.
- Introduction of DL MIMO efficiency enhancement TS 36.331CR4219
- Introduction of UE capabilities for DL MIMO efficiency enhancement TS 36.331CR4334
- Introduction of MIMO enhancements TS 38.331CR1500
- Correction on repetition for L1-SINR TS 38.331CR2586
- Adding UE capability of UL MIMO coherence for UL Tx switching TS 38.331CR2786
In Release 17, key Layer 1 enhancements focused on improving uplink performance, specifically by removing the maximum number of MIMO layers restriction for Supplementary Uplink (SUL) and adding corresponding test applicability for SUL with UL MIMO. This required corrections and clarifications to test applicability for UL MIMO test cases and L1-RSRP measurements. Furthermore, the release introduced new test applicability for enhanced MIMO (e-MIMO) and L1-SINR measurement cases to validate these physical layer improvements.
- Remove the maximum number of MIMO layers restrictions for SUL TS 38.331CR2465
- Clarification on QoE configuration for Layer-2 based UE-to-Network Relay TS 38.331CR3855
- Addition of test applicability e-MIMO test cases TS 38.522CR0119
- Addition of test applicability for UE Enhancements on MIMO TS 38.522CR0146
- Addition of test applicability for L1-SINR measurement cases TS 38.522CR0148
- Removing test case 6.5D.1_1 Occupied bandwidth for UL MIMO (Rel-16 onward) from 38.522 TS 38.522CR0172
+ 9 more changes
In Release 18, the Layer 1 (L1) function introduced MIMO evolution for both downlink and uplink, with corresponding test cases defined for FR1 and FR2. The release also specified enhancements for L1 measurement reporting, including procedures for handling PUCCH resources for L1 LTM reports and clarifications on the maximum number of SSB resources for measurements without gaps. These updates refined the applicability and alignment of test cases for carrier aggregation and MIMO configurations.
- Introduction of MIMO Evolution TS 38.331CR4406
- Correction to MIMO Evolution TS 38.331CR4539
- Correction to MIMO Evolution TS 38.331CR4775
- Correction on the maximum number of SSB rsources for L1 measurement without gaps in LTM TS 38.331CR5302
- Handling of PUCCH resources for L1 LTM reports at TAT expiry TS 38.331CR5332
- Update of RF UL MIMO test case applicability TS 38.522CR0324
+ 9 more changes
In Release 19, the key Layer 1 advancements centered on enhancing MIMO capabilities, specifically introducing MIMO Phase 5 and enabling a new UE capability for 6 Downlink MIMO layers. The release also included corrections and updates to the applicability of various physical layer procedures and tests, such as those for Additional Maximum Power Reduction (A-MPR) and uplink MIMO configurations. Furthermore, specific test case applicabilities were added for RRM L1-RSRP measurements in Non-Terrestrial Networks (NTN) and corrected for uplink transmit switching MIMO coherence.
- Introduction of MIMO Phase 5 TS 38.331CR5441
- Introduction of UE capability on 6 DL MIMO layers TS 38.331CR5476
- Corrections for MIMO Phase 5 TS 38.331CR5548
- Corrections for MIMO Phase 5 TS 38.331CR5686
- Update of applicability for A-MPR, A-SEM and UTRA ACLR for UL MIMO TS 38.522CR0636
- Correction to applicabilities related to 2-layer MIMO capabilities TS 38.522CR0638
+ 5 more changes
Explore further
Broader topics and technologies where L1 plays a role.
Defining Specifications
3GPP specifications that define or reference L1, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 25.201 vj00 | UTRA Physical Layer General Description | Rel-19 |
| TS 25.222 vj00 | UTRA TDD Multiplexing & Channel Coding | Rel-19 |
| TS 25.301 vj00 | UE-UTRAN Radio Interface Protocol Architecture | Rel-19 |
| TS 25.302 vj00 | UTRA Physical Layer Services | Rel-19 |
| TS 25.321 vj00 | MAC Protocol Specification for UTRAN | Rel-19 |
| TS 25.322 vj00 | RLC Protocol Specification | Rel-19 |
| TS 25.324 vj00 | Broadcast/Multicast Control Protocol | Rel-19 |
| TS 25.331 vj00 | UTRAN RRC Protocol Specification | Rel-19 |
| TS 25.402 vj00 | UTRAN Synchronisation Mechanisms | Rel-19 |
| TS 25.433 vj00 | Node B Application Part (NBAP) Protocol | Rel-19 |
| TS 25.709 vf00 | Simplified HS-SCCH for UMTS Study | Rel-15 |
| TS 25.874 vb00 | HSPA Feedback & Signalling Efficiency for LCR TDD | Rel-11 |
| TR 25.931 vj00 | UTRAN Signalling Procedures Examples | Rel-19 |
| TS 29.274 vj50 | GTPv2-C Control Plane Protocol Specification | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 36.938 v900 | E-UTRAN to 3GPP2/Mobile WiMAX Mobility | Rel-9 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |
| TS 38.522 vj11 | UE Conformance Test Applicability Statement | Rel-19 |
| TR 43.901 vj00 | Generic Access to A/Gb Interface Feasibility Study | Rel-19 |