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In many 5G NR deployments, a UE is anchored on a low-band FDD carrier for reliable uplink coverage, while a mid/high-band TDD carrier is added for huge downlink capacity with massive MIMO. The problem is that the gNB cannot fully exploit DL beamforming on the high band unless it can learn the uplink channel there, because TDD beamforming typically depends on reciprocity and an uplink reference such as SRS. However, the UE may not have enough simultaneous Tx RF chains to keep transmitting on the high band, and the uplink coverage on that band may be too weak for sustained PUSCH. SRS Carrier Switching solves this by letting the UE time-share the same transmitter: it pauses uplink on the source carrier, retunes the RF to the target carrier, transmits an SRS burst only to “sound” the channel, and then retunes back to resume normal operation. This switching creates an unavoidable interruption gap, so the scheduler must avoid UL/DL timing conflicts during the retune window, and it must also apply collision rules where high-priority signals like SR or ACK/NACK can override the planned SRS. This is different from SRS antenna switching, which rotates the Tx chain across antenna ports on the same carrier, while carrier switching moves the same chain across different frequencies, and it also explains why UL CA is not required, because the transmission is sequential rather than simultaneous.
Clearing Up Confusion with other UL switching featuresIn NR uplink discussions, terms like UL Tx switching, SUL, SRS antenna switching, and SRS carrier switching often sound like the same idea because they all involve “moving something” in the uplink side, either across frequency or across antenna resources. But the key is to separate the object being switched and the purpose of the transmission. SUL and UL Tx switching are uplink capacity/coverage features, so they exist to carry real uplink traffic, meaning PUSCH and sometimes PUCCH are part of the intention. SRS antenna switching is also a reference-signal driven mechanism, but it stays on the same carrier and mainly rotates limited Tx resources across antenna ports so the gNB can build a better MIMO picture on that band. SRS carrier switching is the special case that looks like inter-carrier uplink activity but is not trying to move any user or control data to the target carrier at all. The UE temporarily retunes only to “probe” the target band by sending SRS, so the gNB can estimate that band’s channel for downlink beamforming, and then the UE immediately returns to its normal uplink carrier. The defining point is that SRS carrier switching is an SRS-only operation on the target carrier, while the other uplink switching features are designed to deliver uplink data or uplink coverage improvement
SRS Carrier Switching vs UL CASRS Carrier Switching is used mainly for DL only CA setup only. If it is in DL/UL CA, this switching would not be needed because in UL CA SRS for both carrier is being transmitted without switching.
SRS Carrier Switching is mainly used in a DL-only CA setup. In this case, the UE is configured with multiple CCs for downlink reception, but it still uses only one main uplink carrier for real UL transmission. The gNB still wants to run strong DL MIMO and beamforming on the secondary high-band carrier, so it needs uplink sounding on that carrier to estimate the channel. However, since the UE is not transmitting UL on that secondary carrier, there is no “natural” uplink signal there, and the UE may not have enough simultaneous Tx RF chains to transmit uplink on both carriers at the same time. This is why the UE temporarily retunes to the target carrier and transmits SRS only, and then it returns back to the source uplink carrier. If the deployment is DL/UL CA, the situation changes. In UL CA, the UE can transmit uplink on multiple carriers simultaneously, which implies that the uplink RF chain resources are available to support those carriers at the same time. In that condition, the UE can transmit SRS on each uplink carrier directly as part of the normal UL operation, so the gNB can obtain channel measurements for each carrier without any special retuning procedure. As a result, the “borrow-and-return” mechanism of SRS Carrier Switching is typically not needed, because the UE does not need to time-share a single transmitter to create uplink sounding on the other carrier. The fundamental logic behind this feature is.
Here is the comparison to make it concrete. Scenario 1: Full Uplink CA (No Switching Needed)
Scenario 2: SRS Carrier Switching
Why not just always use UL CA?If UL CA avoids the "switching gap" and the complexity, why don't we use it everywhere?
Summary Visual
UE Capability InformationIn the UE capability report, the UE provides the network with the required switching interruption times and, in newer releases, which other bands in the same band combination are affected during the switch. The network uses this information to schedule appropriate gaps, avoid collisions with uplink data, and apply the correct drop/overlap rules when SRS switching coincides with other transmissions. BandParameters-v1540 ::= SEQUENCE { srs-CarrierSwitch CHOICE { nr SEQUENCE { srs-SwitchingTimesListNR SEQUENCE (SIZE (1..maxSimultaneousBands)) OF SRS-SwitchingTimeNR }, eutra SEQUENCE { srs-SwitchingTimesListEUTRA SEQUENCE (SIZE (1..maxSimultaneousBands)) OF SRS-SwitchingTimeEUTRA } } OPTIONAL, srs-TxSwitch SEQUENCE { supportedSRS-TxPortSwitch ENUMERATED { t1r2, t1r4, t2r4, t1r4-t2r4, t1r1, t2r2, t4r4, notSupported }, txSwitchImpactToRx INTEGER (1..32) OPTIONAL, txSwitchWithAnotherBand INTEGER (1..32) OPTIONAL } OPTIONAL }
SRS-SwitchingTimeNR ::= SEQUENCE { switchingTimeDL ENUMERATED {n0us, n30us, n100us, n140us, n200us, n300us, n500us, n900us} OPTIONAL, switchingTimeUL ENUMERATED {n0us, n30us, n100us, n140us, n200us, n300us, n500us, n900us} OPTIONAL }
BandParameters-v1730 ::= SEQUENCE { -- R1 39-3-2 Affected bands for inter-band CA during SRS carrier switching srs-SwitchingAffectedBandsListNR-r17 SEQUENCE (SIZE (1..maxSimultaneousBands)) OF SRS-SwitchingAffectedBandsNR-r17 }
BandCombination-UplinkTxSwitch-v1900 ::= SEQUENCE { bandCombination-v1900 BandCombination-v1900 OPTIONAL, -- R1 67-5: Enhanced handling of simultaneous SRS carrier switching and uplink Tx switching simultaneousSRS-UplinkTxSwitch-r19 ENUMERATED {max, sum} OPTIONAL, supportedBandPairListNR-v1900 SEQUENCE (SIZE (1..maxULTxSwitchingBandPairs)) OF ULTxSwitchingBandPair-v1900 OPTIONAL, uplinkTxSwitchingBandParametersList-v1900 SEQUENCE (SIZE (1..maxSimultaneousBands)) OF UplinkTxSwitchingBandParameters-v1900 OPTIONAL }
It is designed to work together with the switching-time reporting, so the UE includes the same number of entries and in the same order as the corresponding srs-SwitchingTimesListNR. For each inter-band “source–target” switching case represented in the switching-times list, the UE provides a bit string that marks which other bands in the combination are affected by that SRS switch. The network uses this information to apply the correct dropping rules / timelines when SRS switching overlaps with other uplink activity in the same symbol. For intra-band cases (no true inter-band retune), the UE sets the bit string to all zeros, meaning there are no “other-band” impacts to account for beyond the band itself. The content is typically split into switchingTimeDL and switchingTimeUL, describing the blocking time seen on downlink reception and uplink transmission respectively. The values are represented in discrete steps such as n0us for 0 µs, n30us for 30 µs, and so on, so the scheduler can treat them as well-defined timing penalties rather than free-form delays. This IE is signaled per pair of bands per band combination, and the switching-time fields are mandatory when that NR band-pair switching capability is supported; otherwise the fields are absent. The network uses these values to avoid scheduling DL/UL activity during the retune window and to prevent collisions when SRS switching would otherwise overlap with data transmissions.
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