Description
The Time Resource Indicator Value (TRIV) is a parameter within the Downlink Control Information (DCI) formats used in 3GPP 5G New Radio (NR). The DCI is transmitted via the Physical Downlink Control Channel (PDCCH) and carries scheduling assignments for both downlink and uplink transmissions. The TRIV specifically points to an entry in a configured table of time-domain resource allocations, defining the start symbol and length of the scheduled transmission in terms of OFDM symbols within a slot or across slot boundaries. This mechanism provides the gNB with fine-grained control over when a UE should transmit or receive data, adapting to latency requirements, traffic type, and channel conditions.
Operationally, the gNB's scheduler determines the appropriate time resources for a UE's Physical Downlink Shared Channel (PDSCH) reception or Physical Uplink Shared Channel (PUSCH) transmission. This allocation is mapped to an index in a pre-defined or RRC-configured table (e.g., pdsch-TimeDomainAllocationList or pusch-TimeDomainAllocationList). The index is then encoded into the TRIV field within the DCI. The size of the TRIV field depends on the number of entries in the applicable table. Upon decoding the DCI, the UE extracts the TRIV value, consults the relevant table (which is known via RRC configuration or specification defaults), and identifies the precise start and length indicator (SLIV) or directly the start symbol and duration. This tells the UE exactly which OFDM symbols to use for the scheduled communication.
The introduction of TRIV supports 5G NR's key feature of ultra-lean and flexible design. Unlike LTE's more rigid subframe-based scheduling, NR supports variable slot formats, mini-slots (spanning fewer than 14 symbols), and dynamic TDD. The TRIV, in conjunction with frequency-domain resource assignment, enables this flexibility. It allows for very low-latency transmissions (e.g., starting a PDSCH reception just a few symbols after the PDCCH) and efficient multiplexing of diverse services (e.g., eMBB and URLLC) within the same carrier. The table-based approach also saves DCI overhead, as a few bits can indicate complex time patterns. Proper interpretation of TRIV is thus fundamental for a UE to correctly align its transmission or reception timing, ensuring efficient resource utilization and meeting the stringent performance requirements of 5G.
Purpose & Motivation
The TRIV was created to address the need for highly flexible and efficient time-domain resource allocation in 5G NR, a requirement not fully met by LTE's scheduling mechanisms. LTE used a fixed 1 ms subframe and predefined timing relationships (e.g., k0, k2 values), which were sufficient for broadband data but lacked the granularity and dynamism needed for 5G's diverse use cases like ultra-reliable low-latency communication (URLLC) and massive machine-type communication (mMTC). The rigid structure was inefficient for traffic with variable packet sizes and latency constraints.
Its development was motivated by the desire to decouple scheduling timing from a fixed grid, enabling features like mini-slot scheduling and dynamic adaptation of the TDD uplink/downlink pattern. The TRIV provides a compact, table-driven method to signal arbitrary time allocations, from a single OFDM symbol to a full slot or even multiple slots. This solves the problem of signaling overhead; instead of directly encoding start and length values (which could require many bits), a short index points to a pre-agreed configuration, optimizing control channel efficiency.
Historically, time resource indication in LTE was implicit or semi-static. The TRIV, introduced in NR Rel-15 and further enhanced in later releases, represents a shift towards explicit, dynamic, and highly configurable scheduling. It allows the network to rapidly adapt to changing traffic needs—for instance, immediately scheduling a URLLC packet over a mini-slot that punctures an ongoing eMBB transmission—thereby fulfilling 5G's promise of supporting heterogeneous services on a unified air interface.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (44 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the TRIV function was not explicitly defined or newly introduced within the provided grounding context or the associated Change Request titles. The listed corrections and clarifications for Release 15 focus on other procedures such as PUSCH resource handling for Semi-Persistent CSI reporting, PRACH resource selection for PDCCH order initiated Random Access, and BWP inactivity timer operation.
- Correction on PUSCH resource handling for Semi-Persistent CSI reporting TS 38.321CR0141
- PRACH Resource Selection for RA Initiated by PDCCH Order TS 38.321CR0190
- Addition of NOTE to clarify meaning of available UL-SCH resource TS 38.321CR0206
- Correction to TCI State Indication for UE-specific PDCCH MAC CE TS 38.321CR0243
- PDCCH for BFR termination TS 38.321CR0283
- Correction to RA Resource Selection Procedure TS 38.321CR0304
+ 6 more changes
In Release 16, the TRIV function was refined through corrections and clarifications to sidelink resource (re)selection behavior and associated procedures. Specific updates included corrections for resource overlapping with grants addressed to TC-RNTI, the condition for setting the resource reservation interval for mode 2, and MCS selection during transmission resource checks. These changes also involved clarifications on PUCCH resource handling within Logical Channel (LCH)-based prioritization and the SL HARQ feedback indicator.
- Correction on resource (re)selection TS 38.321CR0858
- Correction on resource overlapping with grants addressed to TC-RNTI TS 38.321CR0927
- Correction on available UL-SCH resource TS 38.321CR1037
- Correction on condition of setting the resource reservation interval for mode 2 TS 38.321CR1127
- Corrections on MCS selection when UE performing TX resource (re-)selection check TS 38.321CR1139
- Clarification of PUCCH resource in LCH-based Prioritization TS 38.321CR1141
+ 5 more changes
In Release 17, the TRIV function was updated through a clarification on Random Access (RA) resource selection during Configured Grant - Small Data Transmission (CG-SDT). This provided specific guidance for how the MAC entity selects the applicable set of Random Access resources when initiating a Random Access procedure under these conditions.
In Release 18, the TRIV function saw refinements primarily through corrections to the underlying resource selection procedures, particularly for sidelink and random access channels. These corrections addressed specific scenarios including sidelink resource selection for random transmissions, resource pool usage for BRID/DAA and A2X communication, and the availability check for RACH resource sets. Furthermore, the release introduced clarifications and corrections for procedures like PDCCH order-triggered early RACH and the handling of the SL-PRS Resource Request MAC CE.
- Correction for SL resource pool usage for BRID/DAA transmission TS 38.321CR1743
- Correction to resource selection for LTE-NR cochannel scenario TS 38.321CR1807
- Correction on resource pool selection for A2X communication TS 38.321CR1813
- Correction on RACH resource set selection---Option 1 TS 38.321CR1884
- MAC correction for resource selection of MCSt TS 38.321CR1973
- Correction on MCSt Restriction for Resource Selection TS 38.321CR2060
+ 10 more changes
In Release 19, the enhancements for the TRIV function included introducing Scheduling Request (SR) resources within the LTM cell switch MAC CE to improve resource indication during cell switching procedures. Additionally, a correction was specified for the processing of sidelink grants when operating on a Dedicated SL-PRS resource pool, which is a sidelink resource pool configured exclusively for SL-PRS transmission. These updates refined the MAC layer's handling of time-resource indications for both access and sidelink operations.
Explore further
Broader topics and technologies where TRIV plays a role.
Defining Specifications
3GPP specifications that define or reference TRIV, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |