Description
Quasi Co-Location (QCL) is a fundamental concept in 3GPP New Radio (NR) that defines an assumed relationship between different reference signal antenna ports or between a reference signal port and a data channel port. When two antenna ports are configured as QCL, the UE is allowed to assume that certain large-scale properties of the radio channel experienced on the first port can be inferred and applied to assist in the reception of signals on the second port. These large-scale properties include parameters like Doppler shift, Doppler spread, average delay, delay spread, and spatial Rx parameters (which relate to the receiving beam). This assumption significantly reduces the complexity and time required for channel estimation, particularly for channels like the Physical Downlink Shared Channel (PDSCH).
The specification defines several QCL types (Type A, B, C, D) in 38.214, each allowing the inference of a different subset of these large-scale parameters. For example, Type A includes Doppler shift, Doppler spread, average delay, and delay spread. Type D is particularly critical for beam management as it includes spatial Rx parameters, meaning the UE can assume the same receive beam can be used for ports with a Type D QCL relationship. In practice, the gNB configures the UE with Transmission Configuration Indicator (TCI) states via RRC signaling and/or MAC CE activation. Each TCI state contains information that links a target reference signal (like a CSI-RS or SS/PBCH block) to a QCL type and a source reference signal. The UE then uses measurements from the source RS to derive channel estimates for the target RS or the PDSCH.
Architecturally, QCL is essential for enabling efficient beamformed transmission, especially in Frequency Range 2 (FR2 - mmWave). Due to high path loss at these frequencies, communication relies on narrow, high-gain beams. QCL Type D allows the gNB to indicate that the PDSCH is transmitted using the same beam (and thus similar spatial characteristics) as a previously measured CSI-RS or SSB. The UE can then apply the same receive beamforming weights, avoiding an exhaustive beam search for every transmission. This is managed through beam management procedures (P-1, P-2, P-3) and is tightly integrated with the control signaling for scheduling grants (where the TCI state is indicated in the DCI).
Purpose & Motivation
QCL was introduced in NR (Rel-15) to address the significant challenges of channel estimation and beam management in advanced MIMO and millimeter-wave systems, which were not sufficiently handled by LTE's antenna port quasi co-location framework. In LTE, QCL assumptions were simpler and implicit for many ports, but NR's use of massive beamforming, wider bandwidths, and higher frequencies created a scenario where the channel characteristics for different reference signals could be vastly different, especially if they were transmitted from different analog beams or different TRPs (Transmission Reception Points). Without explicit QCL relationships, the UE would need to perform independent, complex channel estimation for every signal, increasing latency, power consumption, and reducing reliability.
The primary problem QCL solves is enabling efficient UE receiver processing in a highly dynamic beamformed environment. It allows the network to explicitly inform the UE about which reference signals are 'alike' in terms of their channel statistics, so the UE can reuse prior measurements. This is critical for achieving low latency in beam switching and tracking, which is vital for maintaining connectivity for mobile users in mmWave bands where beams are narrow. It also facilitates advanced multi-TRP and coordinated multipoint (CoMP) operations by allowing the network to define relationships between signals from different geographical points, providing a flexible framework for managing spatial diversity and multiplexing gains in 5G networks.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (62 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the QCL function was enhanced with specific clarifications for initial access and connected mode operation. This included defining QCL assumptions for receiving the PDCCH for the Random Access Response (RAR) and for the PDSCH carrying the RAR, as well as for CORESETs other than CORESET 0. Furthermore, corrections were made to the CSI-RS configuration and its triggering mechanisms.
- QCL properties of Msg4 in CONNECTED Mode TS 38.213CR0023
- CR on QCL assumption for receiving PDCCH for RAR TS 38.213CR0026
- CR on QCL assumption for a CORESET other than 0 TS 38.213CR0030
- Correction to aperiodic CSI-RS triggering with different numerology between PDCCH and CSI-RS TS 38.214CR0007
- Correction on CSI-RS configuration in 38.214 TS 38.214CR0009
- QCL properties of Msg4 in CONNECTED Mode TS 38.214CR0020
+ 2 more changes
In Release 16, enhancements to QCL (Quasi Co-Location) included specifying an additional timing delay for applying QCL relations on a PDSCH scheduled via cross-carrier scheduling with different subcarrier spacings. The release also introduced corrections for the default TCI state of aperiodic CSI-RS in multi-TRP scenarios and provided clarifications on PDSCH QCL procedures. Furthermore, it refined the triggering mechanisms and timing for aperiodic CSI-RS, particularly for beam switching timing values of 224 and 336.
- Aperiodic CSI-RS Triggering for UE reporting beamSwitchTiming values of 224 and 336 TS 38.214CR0060
- Sensor Provide Location Information Elements Correction TS 37.355CR0002
- Correction for cancellation due to PDSCH/CSI-RS/SFI TS 38.213CR0186
- Correction on aperiodic CSI-RS triggering with beam switching timing of 224 and 336 and on CSI reporting TS 38.214CR0107
- Correction on aperiodic CSI-RS triggering with beam switching timing of 224 and 336 TS 38.214CR0121
- Corrections for default TCI state of AP CSI-RS in multi-TRP TS 38.214CR0131
+ 9 more changes
In Release 17, the QCL function was enhanced with the introduction of a **default QCL for a unified TCI state for PDSCH and A-CSI-RS**, providing a streamlined beam management framework. This was accompanied by several corrections and clarifications to multi-TRP (mTRP) operations, including aspects of **CSI-RS port restriction**, **slot offsets for CSI-RS resource pairs**, and **aperiodic CSI-RS timing for mixed numerologies**. These changes improved the robustness and specification clarity for QCL assumptions in advanced multi-beam and multi-TRP deployments.
- NMEA GGA sentence info in high accuracy GNSS location estimates [HA-GNSS-NMEA] TS 37.355CR0349
- CR for CSI-RS power for inter-cell mTRP TS 38.214CR0313
- CR on default QCL for unified TCI state for PDSCH and A-CSI-RS TS 38.214CR0314
- Correction on CSI-RS port restriction for mTRP CSI TS 38.214CR0319
- Correction on slot offsets of CSI-RS resource pairs for MTRP TS 38.214CR0320
- Correction on aperiodic CSI-RS for tracking for fast SCell activation TS 38.214CR0321
+ 5 more changes
In Release 18, a key enhancement for the QCL function was the introduction of QCL-TypeD priorities for overlapping CORESETs in Multi-DCI/Multi-TRP (M-DCI/M-TRP) operation. This provides a defined rule for resolving quasi co-location assumptions when a UE is configured with multiple control resource sets from different transmission-reception points that overlap in time and frequency. The release also included corrections to QCL assumptions following a Layer 1/Layer 2 triggered mobility (LTM) cell switch command.
- Introduction of network verification of UE location in TS 37.355 TS 37.355CR0482
- LPP support for sub 1s location information reporting periodicity [Sub_1s_periodicity] TS 37.355CR0501
- Introduction of QCL-TypeD priorities for overlapping CORESETs in M-DCI/M-TRP operation [QCL-TypeD CORESET priority for M-TRP] TS 38.213CR0569
- Correction on network verification of UE location TS 37.355CR0511
- CR to TS 38.106 with correction of co-existence and co-location requirements TS 38.106CR0047
- CR to TS 38.174 with correction of co-existence and co-location requirements TS 38.174CR0079
+ 11 more changes
In Release 19, the QCL function was refined through corrections to the QCL assumption for CLTM and the QCL properties from a default beam. Furthermore, the framework for co-location and co-existence requirements was simplified across multiple technical specifications, including TS 38.174 and TS 38.176-2. These changes also included corrections on the association between NZP CSI-RS and CSI-IM resources.
- TEI19 Counting of CSI-RS resource referred by N CSI reporting settings [SimCSI_count] TS 38.214CR0681
- TEI19 Simultaneous NZP-CSI-RS resource counting with NES [SimCSI_countNES] TS 38.214CR0689
- (TEI19)CR for 38.106, on framework simplification for co-location/co-existence requirement [BDaT_simp_improvement] TS 38.106CR0119
- (TEI19)CR for 38.174, on framework simplification for co-location/co-existence requirement[BDaT_simp_improvement] TS 38.174CR0143
- (TEI19-BDaT_simp_improvement) CR to TS 38.176-2 spec structure simplification for co-location and co-existence requirements [BDaT_simp_improvement] TS 38.176CR0088
- Correction on Semi-persistent CSI/Semi-persistent CSI-RS for LTM TS 38.214CR0733
+ 5 more changes
Explore further
Broader topics and technologies where QCL plays a role.
Defining Specifications
3GPP specifications that define or reference QCL, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 37.355 vj20 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 38.106 vj20 | NR Repeater Radio Transmission and Reception | Rel-19 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| TS 38.174 vj10 | NR Integrated Access and Backhaul Radio Spec | Rel-19 |
| TS 38.176 vj20 | IAB Conformance Testing Specification | Rel-19 |
| TS 38.213 vj10 | NR Physical Layer Control Procedures | Rel-19 |
| TS 38.214 vj10 | NR Physical Layer Procedures for Data | Rel-19 |
| TS 38.831 vg10 | UE RF Requirements for FR2 Enhancements | Rel-16 |
| TR 38.833 vh00 | NR Demodulation Performance Enhancement | Rel-17 |
| TR 38.878 vi40 | Technical Report on Advanced Receiver for MU-MIMO | Rel-18 |