Power saving has been an important part of the evolution of 5G NR. Some techniques, such as Discontinuous Reception (DRX), allow a device to switch parts of its receiver off for periods of time. Other mechanisms try to reduce the amount of work that the UE needs to perform while it remains connected to the network.
One such mechanism is Search Space Set Group Switching, or SSSG, which allows the network to dynamically adapt how frequently a UE needs to monitor the Physical Downlink Control Channel (PDCCH).
The feature recently received some attention after Rohde & Schwarz announced the initial verification of a 3GPP Release 17 NR conformance test case for SSSG (TC 7.1.1.12.5). While SSSG is not necessarily one of the better-known 5G power-saving features, it is a useful example of how relatively small changes to control-channel operation can help reduce UE power consumption.
To understand SSSG, it helps to first understand why PDCCH monitoring consumes power. The PDCCH carries Downlink Control Information, or DCI. Among other things, DCI tells a UE when and where downlink data is scheduled or provides information relating to uplink transmission. A connected UE therefore needs to wake up at configured times, monitor the appropriate PDCCH search spaces and determine whether any of the control information is intended for it.
Even when there is no user data waiting, the UE may still need to carry out this monitoring. That means parts of the RF and baseband processing chain have to be active simply to discover that there is nothing for the UE. A Search Space Set essentially defines where and when the UE should look for possible PDCCH transmissions. With SSSG, Search Space Sets can be organised into different Search Space Set Groups, allowing the network to change the monitoring behaviour of the UE.
For example, one group could result in relatively frequent PDCCH monitoring, while another group could result in much sparser monitoring. When traffic is arriving frequently, dense monitoring makes sense because the network may need to schedule the UE quickly. When traffic becomes sparse, repeatedly waking the UE to check the PDCCH provides little benefit. Switching the UE to a group with fewer monitoring occasions allows it to spend more time in a lower-power state during periods when it would otherwise be monitoring the PDCCH, including during C-DRX Active Time.
This is illustrated nicely in the Rohde & Schwarz presentation, where the network switches a UE from dense PDCCH monitoring to sparse monitoring when data is arriving less frequently. The same slide also shows a related Release 17 mechanism, PDCCH skipping, where the UE can stop monitoring the PDCCH altogether for a period when there is no data to be transmitted.
The terminology around SSSG can be slightly confusing because the basic concept predates Release 17. Search Space Set Group switching was originally introduced in Release 16 as part of NR-U, allowing a UE to switch between two groups of Search Space Sets with different PDCCH monitoring behaviour. Release 17 extended the concept under the dedicated UE Power Saving Enhancements work item, applying it more directly to connected-mode power saving and introducing enhanced SSSG switching together with PDCCH skipping.
In Release 17, scheduling DCI can carry a PDCCH monitoring adaptation indication as specified in 3GPP TS 38.212 and TS 38.213. Depending on how the UE has been configured via RRC (3GPP TS 38.331), this field can be used to switch between Search Space Set Groups or to instruct the UE to skip PDCCH monitoring.
The Release 17 mechanism can support up to three SSSGs. The Rohde & Schwarz presentation shows scheduling DCI formats 1-1, 0-1, 1-2 and 0-2 carrying a monitoring adaptation indication of up to two bits, which can be used for switching between the configured groups as well as controlling PDCCH skipping.
The network does not simply switch monitoring behaviour indefinitely. Additional configuration defined in 3GPP TS 38.331 controls how the mechanism operates, including how long the UE stays within a Search Space Set Group, the minimum delay before switching takes effect and the duration for which PDCCH monitoring may be skipped. Parameters highlighted in the R&S presentation include searchSpaceGroupIdList, searchSpaceSwitchTimer, searchSpaceSwitchDelay and PDCCHSkippingDurationList. Up to three skipping-duration values can be configured, with the appropriate duration selected dynamically using DCI.
It is worth distinguishing SSSG from DRX because both mechanisms ultimately try to reduce unnecessary PDCCH monitoring. Connected-mode DRX (C-DRX) creates periods during which a UE can sleep rather than continuously monitor the PDCCH. During its Active Time, however, the UE may still have a number of PDCCH monitoring occasions. SSSG adds another level of adaptation during periods when the UE would otherwise be monitoring the PDCCH, including C-DRX Active Time. Instead of treating all active periods in the same way, the network can change the density of PDCCH monitoring according to the expected traffic.
PDCCH skipping takes this one step further by allowing the UE to temporarily skip PDCCH monitoring for a configured duration, subject to conditions defined by 3GPP.
| Mechanism | Operating Scope | Primary Power-Saving Action |
|---|---|---|
| Connected-Mode DRX (C-DRX) | Active and sleep periods | Reduces WHEN the UE needs to monitor PDCCH |
| SSSG Switching (Rel-16/17) | PDCCH monitoring on the active DL BWP | Reduces HOW OFTEN selected PDCCH search spaces are monitored |
| PDCCH Skipping (Rel-17) | Temporary PDCCH monitoring suspension | Temporarily SKIPS PDCCH monitoring for a configured duration |
These mechanisms are therefore complementary rather than alternatives. This becomes particularly useful because mobile traffic is inherently bursty. A UE may remain in RRC_CONNECTED even though there are periods when very little data is being exchanged. Maintaining aggressive PDCCH monitoring throughout these quiet periods wastes energy.
With SSSG, the network can adapt the monitoring behaviour to the actual traffic situation. When activity increases, the UE can use denser monitoring to maintain responsiveness. When traffic falls, the network can move the UE to sparser monitoring. If nothing needs to be scheduled for a period, PDCCH skipping can provide an additional opportunity to save power.
The Rohde & Schwarz presentation places SSSG and PDCCH skipping alongside a wider set of Release 17 UE power-saving enhancements, including paging improvements, TRS occasions for idle and inactive UEs, and relaxation of Radio Link Monitoring and Beam Failure Detection measurements.
The recent conformance-test work is therefore significant not because SSSG has suddenly appeared as a new 5G feature, but because it shows the Release 17 enhancements moving further into practical implementation and verification. There is often a considerable gap between a feature appearing in a 3GPP specification and it becoming something that can be reliably implemented, tested and eventually used in commercial devices and networks. Conformance testing is an important part of closing that gap.
SSSG may sound like a fairly obscure control-channel optimisation, but the underlying idea is simple: if the UE does not need to look for scheduling information so frequently, do not make it do so. Combined with DRX and PDCCH skipping, it is another example of 5G becoming smarter about when a device really needs to be awake and processing control information, rather than simply trying to make individual radio operations more energy efficient.
Related Posts:
- The 3G4G Blog - Energy Efficiency Improvements in 3GPP Technologies: From LTE to 5G-Advanced
- The 3G4G Blog: Low Latency Power Saving with Low Power-Wake Up Signal/Receiver (LP-WUS/LP-WUR)
- The 3G4G Blog: Four Ways 5G Can Improve the Battery Life of User Equipment (UE)
- The 3G4G Blog: What is RF Front-End (RFFE) and why is it so Important?
- The 3G4G Blog: Dummy Loads in RF Testing for Dummies
- The 3G4G Blog: Energy Consumption in Mobile Networks and RAN Power Saving Schemes
- The 3G4G Blog: Reducing 5G Device Power Consumption Using Connected-mode Discontinuous Reception (C-DRX)
