Description
The Uplink Sounding Reference Signal (UL-SRS) is a known sequence transmitted by the UE that allows the gNodeB (gNB) to perform uplink channel estimation. Unlike demodulation reference signals (DM-RS) which are tied to a specific PUSCH/PUCCH transmission, SRS is transmitted independently, often on a configured, periodic, semi-persistent, or aperiodic basis. The gNB configures the SRS transmission parameters via RRC signaling and can trigger aperiodic SRS via DCI. Key parameters include bandwidth, time/frequency position, transmission comb, cyclic shift, and sequence ID, allowing for flexible and UE-specific sounding.
The primary function is channel sounding. By analyzing the received SRS, the gNB can estimate the uplink channel state information (CSI), including the channel frequency response across the sounded bandwidth. This estimation is crucial for several network functions. First, it enables frequency-selective scheduling for the uplink UL-SCH. The scheduler can allocate PUSCH resources in frequency regions where the UE has a strong channel, thereby maximizing throughput and spectral efficiency. Second, it facilitates uplink link adaptation by providing the information needed to select the appropriate modulation and coding scheme (MCS) for a given UE's PUSCH transmission, balancing data rate and error probability.
In Time Division Duplex (TDD) systems, where uplink and downlink share the same frequency band, the channel reciprocity principle can be applied. The uplink channel estimate derived from SRS can be used to infer the downlink channel conditions. This is a cornerstone for massive MIMO and beamforming. The gNB can use the SRS-based channel knowledge to calculate precoding weights for downlink transmissions, effectively steering energy towards the UE and creating narrow, focused beams. This improves downlink signal strength, reduces interference to other users, and increases overall network capacity. Furthermore, SRS can be configured with antenna switching, allowing a UE with multiple transmit antennas to sound the channel from different antennas sequentially, enabling the gNB to acquire channel knowledge for all UE transmit antennas, which is vital for uplink MIMO and downlink beamforming based on reciprocity.
Purpose & Motivation
UL-SRS was introduced to provide the network with a dedicated and flexible mechanism for obtaining uplink channel state information, a capability that became increasingly critical with the evolution of LTE and the advent of 5G NR. Early LTE releases focused on downlink CSI feedback (CQI, PMI, RI) reported by the UE. However, for optimal uplink performance and to fully exploit the potential of advanced antenna systems, the network needed direct, wideband knowledge of the uplink radio channel.
The motivation stems from several limitations of relying solely on downlink measurements or demodulation references. Demodulation reference signals (DM-RS) are only present during a PUSCH transmission and cover only the scheduled bandwidth, providing no information about channel quality on other frequencies. SRS solves this by allowing the network to proactively sound the channel over a configurable, potentially system-wide, bandwidth even when the UE has no data to send. This enables proactive scheduling decisions.
With the push towards massive MIMO and beamforming, especially in TDD deployments, channel reciprocity became a highly efficient alternative to explicit CSI feedback. UL-SRS is the enabling signal for this approach. By sounding the uplink, the gNB, equipped with a large antenna array, can estimate the spatial characteristics of the channel and derive accurate beamforming vectors for downlink transmission. This is far more scalable and provides more timely information than having each UE measure and report downlink CSI for dozens of antenna ports. Thus, UL-SRS is a key enabler for the high spectral efficiency and capacity targets of 5G networks.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (22 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the UL-SRS function was enhanced to support new positioning methods, including SL-RTT (Sidelink Round-Trip Time) and SL-TOA (Sidelink Time of Arrival) performance test cases, as defined in the associated test specifications. This expansion facilitated more comprehensive uplink sounding capabilities for device-to-device positioning scenarios alongside traditional network-based measurements.
- Signalling between an LMF and NG-RAN node/UE TS 38.305CR0001
- Minor restructuring of sensor references and addition of sensor methods (IMU) TS 38.305CR0009
- Adding missing reference for autonomous and measuremnts gaps for Inter-RAT RSTD measurements TS 38.305CR0010
- gNB-DU UE Aggregate Maximum Bit Rate Uplink correction TS 38.473CR0273
In Release 16, a key update for the UL-SRS function was a signalling sequence correction for the Uplink SRS Configuration. This change specifically addressed procedural errors in the configuration signalling sequence to ensure correct operation. No other new UL-SRS capabilities or procedures are indicated by the provided grounding context or change request titles for this release.
- Introduction of B1C signal in BDS system in A-GNSS TS 38.305CR0013
- Update B1I signal ICD file to v3.0 in BDS system in A-GNSS TS 38.305CR0024
- Signalling sequence correction for UL SRS Configuration TS 38.305CR0033
- Correction on reference to RACH-Report TS 38.473CR0744
- gNB-DU UE Aggregate Maximum Bit Rate Uplink correction TS 38.473CR0762
In Release 17, there were no specific new features or enhancements introduced for the UL-SRS (Uplink Sounding Reference Signal) function, as none of the provided Change Requests or the grounding specification text mention any modifications to it. The listed corrections and introductions for this release focus on other areas such as NRPPA, SDT, uplink GapFR2, MBS multicast, and various RRC reference fixes. Therefore, for UL-SRS, Release 17 maintained the functionality defined in the previous release without changes.
- NRPPA corrections of references to RRC TS 38.455CR0100
- Correction to SDT for supporting delta signaling TS 38.473CR0890
- Introduction of uplink GapFR2 [NR_RF_FR2_req_enh2-Core] TS 38.473CR1025
- Provision of MBS Multicast F1-U references to UE Context in gNB-CU enabling retrieval of data forwarding progress information TS 38.473CR1043
- Correction of RRC references for SLrelay TS 38.473CR1120
- Correction of RRC references for DRX TS 38.473CR1121
+ 2 more changes
In Release 18, the UL-SRS function was enhanced to support the new state transition procedure for **Switching from SDT to RRC connected state** during Large SDT Uplink Data transmission. This provides a more efficient mechanism for uplink channel sounding when a device moves from the Small Data Transmission state to a fully connected state. Additionally, corrections were made to the **Reference configuration and RRC Complete configuration_Option 1** to ensure proper UL-SRS signaling.
- Switching from SDT to RRC connected state [Large SDT Uplink Data] TS 38.473CR1213
- Introduction of separate uplink and downlink PDU set QoS parameters TS 38.473CR1278
- Correction on Reference configuration and RRC Complete configuration_Option 1 TS 38.473CR1280
- Correction of Time Reference Information TS 38.473CR1377
In Release 19, the UL-SRS function saw no specific new enhancements detailed within the provided grounding context. The context exclusively covers test parameters and assistance data for positioning technologies like A-GNSS and OTDOA. The listed Change Request title regarding a low-power wake-up signal is unrelated to UL-SRS functionality.
- Introduction of low-power wake-up signal and receiver for NR TS 38.473CR1443
Explore further
Broader topics and technologies where UL-SRS plays a role.
Defining Specifications
3GPP specifications that define or reference UL-SRS, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 37.571 vj00 | UE Conformance for Positioning | Rel-19 |
| TS 38.305 vj00 | NG-RAN UE Positioning Stage 2 | Rel-19 |
| TS 38.455 vj10 | NR Positioning Protocol A (NRPPa) | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |