Sounding Reference Signals in 5G NR

Sounding Reference Signal (SRS) is an uplink reference signal transmitted by the UE that enables the gNB to estimate uplink channel conditions across frequency, time, and spatial domains independently of data transmission. Unlike Demodulation Reference Signals (DMRS) which are tied to specific physical channels, SRS provides broader channel sounding capabilities across the entire bandwidth or specific frequency regions.

SRS operates as an uplink orthogonal frequency division multiplexing (OFDM) signal filled with Zadoff-Chu sequences on different subcarriers. The key characteristic that distinguishes SRS from DMRS is its independence from PUSCH scheduling - while PUSCH DMRS is only transmitted when PUSCH is scheduled and only covers the PUSCH bandwidth, SRS can be transmitted independently with configurable bandwidth coverage. Please see my article on “5G NR DMRS https://www.nxgconnect.com/post/5g-nr-dm-rs-demodulation-reference-signals

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Real-time Example: Consider a UE operating in a 100 MHz carrier with varying channel conditions across frequency. While PUSCH might only be scheduled on 20 MHz at any given time, SRS can sound the entire 100 MHz bandwidth, allowing the gNB to identify optimal frequency regions for future scheduling decisions.

What are Sounding Reference Signal Functions?

SRS serves multiple critical functions in 5G NR networks, each enabling different network optimization capabilities:

1. Uplink Channel-Aware Scheduling and Link Adaptation

The gNB uses SRS measurements to estimate Signal-to-Noise Ratio (SNR), path loss, and frequency-selective fading across the bandwidth. This enables:

• Frequency-selective scheduling: Assigning PRBs with better channel quality

• MCS selection: Adapting modulation and coding schemes based on measured SNR

• Power control optimization: Adjusting UE transmission power based on path loss estimates

Real-time Example: A UE transmits periodic SRS every 20ms. The gNB observes that frequencies around 3.7 GHz show 3dB better SINR than 3.5 GHz. Future PUSCH allocations prioritize the 3.7 GHz region, improving throughput by 15-20%.

2. Downlink Channel Estimation via TDD Reciprocity

In TDD systems, SRS enables reciprocity-based downlink precoding where uplink channel measurements inform downlink beamforming decisions. This is crucial for:

• Multi-user MIMO (MU-MIMO) precoding weight calculation

• Interference mitigation between spatially multiplexed users

• Beam alignment for massive MIMO systems

3. Codebook-based and Non-codebook Transmission Support

• Codebook transmission: gNB uses SRS to recommend PMI (Precoding Matrix Indicator) from standardized 3GPP codebooks

• Non-codebook transmission: UE measures downlink CSI-RS and generates its own precoding weights, with SRS providing uplink verification

4. Uplink Beam Management

SRS enables the gNB to select optimal uplink transmission beams by measuring different spatial directions. This includes:

• Beam sweeping: Sequential transmission on different antenna ports

• Beam refinement: Fine-tuning based on measured beam quality

• Beam failure recovery: Switching to backup beams when primary beams degrade

Time Domain and Frequency Domain Resource allocation:

The basic time/frequency structure of an SRS is exemplified in below fig. In the general case, an SRS spans one, two, or four consecutive OFDM symbols and is located somewhere within the last six symbols of a slot. In the frequency domain, an SRS has a so-called “comb” structure, implying that an SRS is transmitted on every Nth subcarrier where N can take the values two or four (“comb- 2” and “comb-4,” respectively).

Time-Domain Structure of SRS

An SRS can be configured for periodic, semi-persistent, or aperiodic transmission:

• A periodic SRS is transmitted with a certain configured periodicity and a certain configured slot offset within that periodicity;

• A semi-persistent SRS has a configured periodicity and slot offset in the same way as a periodic SRS. However, actual SRS transmission according to the configured periodicity and slot offset is activated and deactivated by means of MAC CE signaling;

• An aperiodic SRS is only transmitted when explicitly triggered by means of DCI.

It should be pointed out that, similar to CSI-RSI, activation/deactivation and triggering for semipersistent  and aperiodic SRS, respectively, is actually not done for a specific SRS but rather done for a so-called SRS resource set which, in the general case, included multiple SRS.

Example: NumSRSSymbols=4, SymbolStart=8 (thus occupying symbols 10–13 in normal CP), Repetition=1, SRSPeriod=[20 4] meaning every 20 slots with offset 4.

Frequency-domain allocation

• Bandwidth triplet C_SRS, B_SRS, B_Hop: These three parameters map to instantaneous RBs per transmission and, if enabled, the total hopping span via 38.211 Table 6.4.1.4.3‑1; increasing C_SRS expands the overall sounded bandwidth, while B_SRS controls the instantaneous width, and B_Hop determines hopping step/enablement.

• Hopping enable rule: Setting B_Hop ≥ B_SRS disables frequency hopping; choosing B_Hop < B_SRS enables hopping per the standardized patterns—engineering tools document and visualize this rule explicitly.

• Frequency position NRRC: The freqDomainPosition parameter, NRRC ∈ {0..67}, places the SRS allocation in blocks of 4 PRBs within the BWP and also selects among distinct hopping patterns when hopping is enabled.

• Transmission comb and combOffset: SRS uses comb sizes {2,4,8} to interleave occupied subcarriers; combOffset selects the phase within the comb and enables orthogonal multiplexing across UEs/ports without overlapping REs.

• Additional shifts: freqDomainShift can fine-align the allocation to the common RB grid when supported by analysis tools, complementing NRRC-based placement.

• Sequences/cyclic shift: The resource carries a sequenceId and may use cyclic shifts to orthogonalize UEs/ports sharing time–frequency resources, consistent with the ZC-based construction in 38.211.

Example: C_SRS=19, B_SRS=2, B_Hop=0, NRRC=14, comb=4 with combOffset=1 yields a medium instantaneous bandwidth with no hopping and interleaved subcarriers suited for multiplexing; changing B_Hop from 0 to a value less than B_SRS enables hopping across subbands per the table.

High-level call flow

SRS call flow consists of configuring SRS at RRC, optionally activating or triggering it at MAC/PHY, transmitting SRS from the UE, and using the measurements at the gNB for uplink channel estimation and subsequent scheduling. The key messages are RRCReconfiguration (carrying SRS-Config), optional MAC CE for SP-SRS activation, and DCI with SRS request/trigger for AP‑SRS, followed by SRS on UL and PDCCH/PUSCH scheduling using the SRS-derived channel quality and spatial signatures.

1. RRC connection established → gNB sends RRCReconfiguration including SRS-Config with SRS-ResourceSets and SRS-Resources; UE applies and responds with RRCReconfigurationComplete.

2. Optional activation: for semi‑persistent SRS (SP‑SRS), gNB activates/deactivates via MAC CE; for aperiodic SRS (AP‑SRS), gNB later triggers via DCI with an SRS request/trigger field and a slot offset k/t.

3. On configured/triggered occasions, UE transmits SRS based on mapping rules (comb, bandwidth, ports, repetitions, hopping); gNB measures per-port and per-frequency channel response.

4. gNB derives UL channel estimates, wideband/narrowband SINR, path loss, timing, and spatial signatures; then issues PDCCH with PUSCH grant using PRBs/MCS/beam optimized by SRS; UE sends PUSCH with DMRS accordingly.

RRC message and parameters

• RRCReconfiguration → SRS-Config: defines SRS resources and sets; each set has a usage and common timing rules; each resource defines frequency/time/sequence/spatial mapping; aperiodic sets carry slot offset semantics; parameters are specified in TS 38.331 ASN.1 and field descriptions.

• MAC CE: for SP‑SRS on/off, complementary to RRC so periodicity is RRC-defined but runtime activation follows MAC; behavior aligns with 38.331/38.321 procedures and resource release notes during reconfigurationWithSync.

• DCI: SRS request/trigger bits map to configured aperiodic resource set(s) and govern which set fires and in which slot relative to the trigger; legacy two‑bit and one‑bit forms exist depending on DCI format, with the actual slot computed from n+k or newer t/slotOffset fields.

SRS-Resource {

    resourceId 10,

    nrofSRS-Ports port2,

    transmissionComb {

        comb4,

        combOffset-n4 1

    },

    resourceMapping {

        startPosition 8,

        nrofSymbols ms1,

        repetitionFactor n1

    },

    freqHopping {

        c-SRS 19,

        b-SRS 2,

        b-hop 0,

        nRRC 12

    },

    resourceType {

        periodic {

            periodicityAndOffset {sl20, 4}

        }

    }

}

Parameter Interpretation:

• 2-port SRS on comb-4 with offset 1

• Periodic transmission every 20 slots with offset 4

• Frequency hopping enabled (BHop < BSRS) across bandwidth determined by CSRS=19, BSRS=2

• Starting frequency position: NRRC=12 → starts at RB 48 (12×4)

This configuration enables wideband channel sounding with frequency diversity while maintaining orthogonal multiplexing capabilities for multiple UEs.

The interaction between these parameters determines SRS performance characteristics:

• Channel estimation accuracy vs interference resilience (comb size trade-off)

• Frequency diversity vs estimation complexity (hopping trade-off)

• Spatial resolution vs overhead (port count trade-off)

• Measurement reliability vs resource efficiency (repetition trade-off)

RRC parameters: SRS-Config structure

SRS-Config contains four lists and per-set/per-resource fields; below are the principal parameters and their roles with practical decoding tips:

• srs-ResourceSetToAddModList: List of SRS-ResourceSet; each set groups one or more resources and shares a resourceType and usage.

• SRS-ResourceSet fields:

  • resourceType: periodic, semiPersistent, or aperiodic; defines how occasions arise (fixed pattern, MAC CE controlled, or DCI triggered).

  • usage: beamManagement, codebook, nonCodebook, antennaSwitching; constrains mapping rules and scheduling assumptions for that set.

  • aperiodic timing: slotOffset (k) and, in newer releases, t/slotOffset‑r17 semantics indicating the offset from the trigger slot to the TX slot, counting only available UL/flexible slots in some proposals.

  • spatialRelationInfo: associates SRS with SSB/CSI-RS or another SRS for QCL/spatial alignment, ensuring beam correspondence.

  • p0/powerControl fields: optional set‑level power control normalization and relations used by gNB for consistent measurement scaling.

• srs-ResourceToAddModList: List of SRS-Resource; per-resource granularity for time/frequency/sequence.

• SRS-Resource fields:

  • nrofSRS-Ports: {port1, port2, port4}; Rel‑18 adds 8‑port variants and optional TDM mapping across symbols; check decode naming alignment across releases.

  • transmissionComb: comb {2,4,8} with combOffset‑n2/‑n4/‑n8; defines interleaved RE positions for collision avoidance and UE multiplexing.

  • freqDomain parameters: c‑SRS (bandwidth configuration), b‑SRS (sub‑band width), bHop (hopping enable), start/position (NRRC) to place the resource within BWP; the combination determines instantaneous bandwidth and hopping steps used each occasion.

  • resourceMapping / resourceMapping‑r16: symbol start (startPosition), number of symbols (ms1..ms4), and repetition factor R (n1..n4); r16 variant allows any symbol as start; R>1 repeats within the slot for robustness.

  • resourceType details: for periodic/semi‑persistent, periodicityAndOffset {sl1..sl2560, offset}; for aperiodic, the set’s DCI trigger and slotOffset apply.

  • sequenceId and cyclicShift: initialize ZC-like base sequence and orthogonalize UEs/ports; may be combined with cyclicShiftHopping in newer releases.

  • hoppingId and combOffsetHopping: pseudo‑randomize comb offsets across occasions; reduces collision and decorrelates interference patterns.

• Release and reconfiguration behavior: on reconfigurationWithSync, implementations reset MAC, which can implicitly cause PUCCH/CSI/SR/SRS resources to be released and require full reconfiguration rather than delta updates; logs often show removal of previous IDs followed by new addMod lists.

Aperiodic SRS trigger details (DCI)

• DCI carries an SRS request that selects which configured aperiodic SRS resource set is triggered; some DCI formats use a two‑bit field selecting among three sets and “no trigger,” others use one bit to trigger a single configured set; mapping depends on the DCI format family.

• Transmission timing: the UE sends SRS in the first available slot satisfying the configured offset rule. Legacy rules use “slot n+k”; newer proposals introduce t that counts only available UL/flexible slots for robustness in TDD patterns.

Example call flow with parameters

1. RRCReconfiguration(SRS-Config)

2. srs-ResourceSetToAddModList:

   • SetId=1, usage=codebook, resourceType=aperiodic, slotOffset=k=2, spatialRelationInfo=CSI-RS#3

3. srs-ResourceToAddModList:

   • ResId=10: nrofSRS-Ports=port2, transmissionComb=comb4/offset 1, c‑SRS=19, b‑SRS=2, bHop=0, resourceMapping‑r16: startPosition=10, nrofSymbols=ms2, repetition=n1, sequenceId=7

UE applies and returns RRCReconfigurationComplete.

2. DCI trigger

3. DCI format with SRS request = “01” selects SetId=1; UE derives transmit slot as n+k where k=2 and transmits on the next eligible UL/flexible slot if TDD

4. UE transmits SRS

5. Port mapping over 2 symbols with comb‑4; gNB measures per‑port channel estimates, noise, and computes wideband/narrowband UL CQI.

6. Scheduling based on SRS

7. gNB sends PDCCH with PUSCH grant targeting PRBs around frequency where SRS SINR is best, sets MCS accordingly, and may reuse UL channel for DL precoding in TDD reciprocity scenarios.

How to Check SRS UE Capability from UE Logs

UE SRS capabilities are reported in the UECapabilityInformation message within the featureSetsUplink information element, as defined in 3GPP TS 38.306. Here's how to decode these capabilities from logs:

Key Capability Parameters to Look For:

1. supportedSRS-Resources Structure

supportedSRS-Resources {

    maxNumberAperiodicSRS-PerBWP n16,

    maxNumberAperiodicSRS-PerBWP-PerSlot 6,

    maxNumberPeriodicSRS-PerBWP n16,

    maxNumberPeriodicSRS-PerBWP-PerSlot 6,

    maxNumberSemiPersistentSRS-PerBWP n2,

    maxNumberSemiPersistentSRS-PerBWP-PerSlot 2,

    maxNumberSRS-Ports-PerResource n4

}

Real-time Log Interpretation Example:

• maxNumberAperiodicSRS-PerBWP n16: UE supports up to 16 aperiodic SRS resources per BWP

• maxNumberSRS-Ports-PerResource n4: UE can transmit SRS on up to 4 antenna ports simultaneously

• maxNumberPeriodicSRS-PerBWP-PerSlot 6: UE can handle 6 periodic SRS transmissions per slot

2. SRS Antenna Switching Capabilities

srs-TxSwitch {

    supportedSRS-TxPortSwitch t1r2,

    txSwitchImpactToRx true

}

Capability Interpretation:

• t1r2: UE has 1 transmit chain, 2 receive antennas - can switch between RX antennas for SRS transmission

• t1r4: 1 transmit chain, 4 receive antennas

• t2r4: 2 transmit chains, 4 receive antennas

• t4r4: 4 transmit chains, 4 receive antennas (no switching needed)

• t1r1-t2r2-t4r4: Supports multiple configurations

Practical Example: If logs show supportedSRS-TxPortSwitch t1r2, the UE will be configured with antenna switching where one transmit path alternates between two receive antennas for SRS transmission.

LTE SRS vs 5G SRS

Below is a concise comparison of LTE SRS and 5G NR SRS.

Aspect	LTE SRS	5G NR SRS
Core purpose	Uplink channel sounding for UL scheduling and link adaptation.	Uplink sounding plus beam/port management, TDD reciprocity for DL precoding, wideband probing over large BWPs/FR2.
Time-domain structure	Typically 1 symbol near slot end; limited periodicities; simpler aperiodic support.	1/2/4 symbols with optional in-slot repetition; periodic, semi-persistent (MAC-activated), and aperiodic (DCI-triggered) with configurable slot offsets.
Frequency allocation	Coarser bandwidth configuration; effectively single comb behavior with modest hopping.	Bandwidth triplet (C_SRS, B_SRS, B_Hop) for instantaneous RBs vs total hop span; comb sizes {2,4,8} with combOffset for UE/port multiplexing; richer hopping patterns.
Spatial ports	Mostly single-port, limited multiplexing via cyclic shift/orthogonality.	Multiple ports per UE (1/2/4, later extended to 8 with TDM mapping); resource-set usage modes (codebook, non-codebook, antennaSwitching, beamManagement).
Beamforming role	Auxiliary for UL; limited role in DL reciprocity.	Central to TDD massive MIMO; SRS-based UL estimates guide DL precoding and MU-MIMO pairing.
Triggering/activation	Primarily periodic; simpler aperiodic.	Rich aperiodic triggering via DCI; semi-persistent activation via MAC; flexible periodicities and offsets.
UE multiplexing	Lower density; orthogonality mainly via cyclic shifts.	High-density multiplexing using comb size, combOffset, cyclic shifts, sequence IDs, and hopping.
Antenna switching	Not a major standardized focus.	Formalized SRS antenna switching (e.g., t1r2, t2r4, beyond 4 RX) to sound more spatial paths than TX chains.
Integration with DL beams	Limited, deployment-dependent.	Tight spatialRelationInfo linkage to SSB/CSI-RS for beam correspondence (especially FR2).
Typical bandwidths	LTE carriers up to 20 MHz (wider only via CA).	Very wide FR1/FR2 carriers; SRS scalable over BWPs up to full carrier bandwidth.

References:

1.      3GPP TS 38.213

2.      3GPP TS 38.211 , https://www.etsi.org/deliver/etsi_ts/138200_138299/138211/16.02.00_60/ts_138211v160200p.pdf

3.      3GPP TS 38.300

4.      https://www.sharetechnote.com/html/5G/5G_SRS.html

5.      5G NR: The Next Generation Wireless Access Technology Erik Dahlman Stefan Parkvall Johan Sköld