Description
Channel Occupancy Time (COT) is a fundamental parameter in shared and unlicensed spectrum operation, defined within 3GPP specifications for technologies like Licensed-Assisted Access (LAA), MulteFire, and New Radio in Unlicensed spectrum (NR-U). It specifies the maximum time a gNB (base station) or UE (user equipment) can continuously occupy a channel after successfully gaining access through a Listen-Before-Talk (LBT) procedure. The COT duration is not fixed by 3GPP but is constrained by regional regulatory requirements, which dictate maximum values (e.g., 4 ms, 6 ms, 10 ms) and associated rules, such as the mandatory idle period (or Channel Occupancy Time) that must follow the transmission burst.
Architecturally, COT management is integrated into the Medium Access Control (MAC) and Physical (PHY) layers of the radio protocol stack. The process begins with a Category 4 LBT procedure, which is a carrier sensing mechanism with random backoff, similar to Wi-Fi's CSMA/CA. Upon winning the channel, the initiating node (which can be the gNB or a UE in uplink) starts a timer corresponding to the maximum allowed COT. During this COT, the node can transmit downlink data, uplink grants, and associated control information. A key feature is COT sharing, where the initiating node can grant portions of its remaining COT to other devices (e.g., a gNB sharing with its scheduled UEs for uplink transmission) without requiring those devices to perform a full LBT, thereby improving efficiency and reducing latency.
The COT structure is divided into transmission bursts and idle gaps. Within a single COT, transmissions can be discontinuous, with short gaps (e.g., up to 16 μs) not counting against the occupancy time, allowing for scheduling flexibility. After the COT expires, the node must vacate the channel for a minimum idle period, allowing other systems to contend for access. This cyclical process of LBT, COT, and idle period forms the basis for fair coexistence. In 5G NR-U, COT management has become more dynamic, supporting flexible numerology and adaptive COT durations based on traffic priority and channel conditions, which are critical for meeting stringent QoS requirements for ultra-reliable low-latency communications (URLLC) and enhanced mobile broadband (eMBB) services in unlicensed bands.
Purpose & Motivation
COT was introduced to enable cellular systems to operate fairly and legally in unlicensed spectrum bands, such as the 5 GHz U-NII bands and the 6 GHz band, which are globally available but subject to strict regulatory requirements to ensure coexistence with incumbent technologies like Wi-Fi and radar systems. Prior to LAA and NR-U, cellular networks operated exclusively in licensed spectrum, guaranteeing interference-free operation but with limited bandwidth availability. The explosion of mobile data traffic drove the need for additional capacity, leading 3GPP to standardize access to unlicensed spectrum. However, without a mechanism like COT, cellular base stations could transmit continuously, causing harmful interference to and monopolizing shared channels, which would violate regulations and degrade performance for all users.
The primary problem COT solves is regulatory compliance for fair spectrum sharing. Regulations in regions like Europe (ETSI EN 301 893) and the United States (FCC Part 15) mandate LBT and impose maximum transmission durations to prevent channel hogging. COT provides the technical framework for 3GPP systems to adhere to these rules. Furthermore, COT addresses the technical challenge of coexistence with non-3GPP systems, primarily Wi-Fi. By limiting continuous transmission time and enforcing idle periods, COT ensures Wi-Fi devices have periodic opportunities to access the channel, creating a level playing field. This was a critical design goal to gain regulatory approval and industry acceptance for cellular technologies in unlicensed bands.
Historically, the concept was first detailed in 3GPP Release 13 for LTE-LAA, drawing from existing Wi-Fi and radar system regulations. Its creation was motivated by the need to boost LTE capacity through carrier aggregation with unlicensed carriers while being a 'good neighbor' to other technologies. The evolution through Releases 14, 15, and beyond for NR-U expanded COT's role to support more complex 5G use cases, requiring more sophisticated sharing mechanisms and adaptive timing to meet diverse service requirements, all while maintaining the core principle of enforced transmission limits to ensure shared spectrum remains a viable resource for multiple access technologies.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (32 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the COT function was clarified and corrected, specifically for Autonomous Uplink (AUL) transmissions, including the COT sharing indication and the COT length. Corrections were also made to the general channel access procedures. The technical details involve the transfer of the COT message between network entities like the oMSC, iMSC, tMSC, and GMSC servers.
- Clarification on CRC attachment for DL-SCH and PCH transport channels in NB-IoT TS 36.212CR0285
- CR on Correction of a reference to 'COT sharing indication for AUL' TS 36.212CR0323
- Correction on COT length for AUL transmission TS 37.213CR0003
- Corrections on channel access procedures in 37.213 TS 37.213CR0004
In Release 16, the COT function was significantly enhanced for operation in unlicensed spectrum, introducing new channel access procedures for NR and correcting specific aspects for semi-static channel occupancy, multi-channel transmission, and SRS. Key technical refinements included corrections to the LBT type and CP extension indication for semi-static access, as well as adjustments to the conditions for the downlink channel access procedure. These updates, detailed in changes to TS 37.213 and TS 38.212, provided a more robust and regulated framework for managing Channel Occupancy Time.
- Introduction of channel access procedures to unlicensed spectrum for NR-based access TS 37.213CR0005
- CR to 37.213 to correct channel access for SRS TS 37.213CR0013
- Correction on LBT Type and CP Extension Indication for Semi-Static Channel Occupancy TS 37.213CR0015
- Correction on Channel Occupancy Time for Semi-Static Channel Access TS 37.213CR0016
- Correction on Channel Access for Multi-Channel transmission TS 37.213CR0017
- Correction on the conditions for DL channel access procedure TS 37.213CR0018
+ 4 more changes
In Release 17, the key new development for the Channel Occupancy Time (COT) function was the introduction of the UE initiating a channel occupancy in semi-static channel access mode, primarily to enhance IIoT and URLLC operation on shared spectrum. This release also included numerous corrections and refinements to this new semi-static mode, covering procedures for PUSCH scheduled via RAR, conditions for channel sensing in FR2.2, and indications in both fallback and non-fallback DCI. Additionally, corrections addressed intra-UE multiplexing, consecutive UL transmissions, and multi-channel access procedures to solidify the feature's operation.
- Introduction of UE initiating a channel occupancy in semi-static channel access mode for enhanced IIoT and URLLC operation on shared spectrum for NR TS 37.213CR0023
- Corrections of the semi-static channel access mode with UE initiating channel occupancy TS 37.213CR0028
- Correction to semi-static channel access procedures for PUSCH scheduled via RAR TS 37.213CR0032
- Corrections to the conditions for channel sensing in FR2-2 in TS37.213 TS 37.213CR0035
- CR on ChannelAccess-Cpext in Fallback DCI TS 38.212CR0118
- CR on channel access type indication in non-fallback DCI TS 38.212CR0125
+ 4 more changes
In Release 18, the COT function was enhanced with specific corrections and new procedures for multi-channel access and shared spectrum operation. Key updates included corrections to the multi-channel access procedure, such as CAPC determination and PSFCH prioritization, and the introduction of transmissions within a shared COT across multiple channels. Furthermore, the release defined an energy detection threshold formula for shared spectrum channel access and provided corrections for NR-U channel access procedures mirrored for Release 18.
- Correction on restrictions of performing multi-channel access TS 37.213CR0055
- Correction on CAPC determination in multi-channel access procedure TS 37.213CR0056
- Correction on reference section numbers for PSFCH prioritization in multi-channel access procedures TS 37.213CR0059
- CR on transmission within a shared COT with multiple channels TS 37.213CR0064
- Correction on SL-U COT sharing flag indication TS 38.212CR0205
- CR on energy detection threshold formula for shared spectrum channel access for Rel-18 TS 37.213CR0049
+ 1 more changes
In Release 19, the new work for the COT (Continuity) function involved corrections to its adaptation for common channels and signals, specifically for the R19 NES (Network-Enabled Services) framework. These corrections refined the procedure where the COT message is sent from the oMSC server to either an iMSC or GMSC server upon access assignment completion. The updates ensured proper alignment and interoperability for this continuity signaling across the network elements involved.
- Corrections on R19 NES adaptation of common channel/signals TS 38.212CR0243
Explore further
Broader topics and technologies where COT plays a role.
Defining Specifications
3GPP specifications that define or reference COT, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.284 vj00 | Local Call Local Switch Stage 2 | Rel-19 |
| TS 29.235 vj00 | SIP-I CS Core Network Interworking | Rel-19 |
| TS 36.212 vj10 | LTE Multiplexing and Channel Coding | Rel-19 |
| TS 36.789 vd00 | LAA Multi-Node Coexistence Test Methodology | Rel-13 |
| TS 37.213 vj00 | Shared Spectrum Physical Layer Procedures | Rel-19 |
| TS 38.212 vj10 | NR Multiplexing and Channel Coding | Rel-19 |