NR UE Power Saving Mechanisms

Introduction:

• UE battery life is an important aspect of the user's experience, which will influence the adoption of 5G NR handsets and/or services. It is critical to study UE power consumption to ensure that UE power efficiency for 5G NR UEs can be better than that of LTE, and techniques and designs for improvements are identified and adopted.
• In General as you may know packet data traffic is often very unpredictable, with short bursts of activity followed by longer periods of silence. To minimize delays, it’s important to monitor downlink control signals in every slot to catch uplink grants or downlink data transmissions and quickly respond to traffic changes. However, this approach increases power usage on the device, which can impact battery life—one of the key metrics for end-user satisfaction.

• ITU-R defines energy efficiency as one of the minimum technical performance requirements for IMT-2020. According to ITU-R report – Minimum requirements related to technical performance for IMT-2020 radio interface(s), "energy efficiency of the device can relate to the support for the following two aspects:
  • Efficient data transmission in a loaded case; is demonstrated by the average spectral efficiency.
  • Low energy consumption when there is no data an be estimated by the sleep ratio".

• The study of UE power saving is to identify the feasibility and benefit of techniques to allow UE implementations which can operate with reduced power consumption. The study of the UE power saving framework would also take into consideration of latency and performance in NR as well as network impact.
• The power saving scheme would focus on minimizing the dominate factor of power consumption during the network access, which includes the processing of aggregated bandwidth, number of active RF chains and active reception/transmission time, and dynamic transition to power efficient mode.  The following figure summarizes the key NR UE power-saving schemes.

NR UE power saving Mechanisms breakdown

Now, let’s explore each of the mentioned power-saving mechanisms to understand their impact on reducing UE battery consumption, as well as their potential negative effect on latency.

(1) Bandwidth Adaptation (BWP Switching)

• NR supports very wide transmission bandwidths, up to several 100 MHz on a single carrier. While this is beneficial for quickly delivering large payloads, it is unnecessary for smaller payloads or monitoring downlink control channels when no data is scheduled. To address this, 3GPP introduced the bandwidth part (BWP) concept to reduce UE power consumption while ensuring adequate data transmission rates.
• The bandwidth part (BWP) concept in NR allows receiver bandwidth adaptation, enabling the device to use a narrow bandwidth for monitoring control channels and activate the full bandwidth only when a large amount of data is scheduled. This approach reduces device power consumption and can be seen as discontinuous reception in the frequency domain.

Observation: Source 3GPP TR 38.840

• Rel-15 DCI and timer based BWP adaptation between full bandwidth and a narrow bandwidth can achieve power saving gain in the range 8.5% - 31%. (Mediatek)
• The power saving schemes with UE adaptation to BWP bandwidth show 16% - 45% power saving gain if suitable BWP bandwidth is applied.  (vivo)

(2) S-Cell Operation (S-Cell Dormancy)

SCell Dormancy feature is introduced to improve the power consumption in the scenarios with the carrier aggregation.

• For a dormant cell, the device stops PDCCH monitoring but continues to perform CSI measurement and bream management. Although a dormant cell is still considered as active and is not activated, there is considerably less activity from a device perspective which saves power.
• Deactivating a cell is another possibility to save power but in this case no CSI reports are provided and the activation of an SCell takes longer time than returning from the dormancy.

Observation: Source 3GPP TR 38.840

(3) Cross Slot Scheduling

• NR allows the data to start immediately after the PDCCH. From a latency perspective this is clearly beneficial, but it also requires the device to keep the receiver open and buffer the received signal at least until the PDCCH decoding is ready.
• In many cases, the device is not scheduled and the buffering the received signal is done in vain. From this perspective, cross-slot scheduling, where the PDCH is transmitted in a later slot than the PDCCH, is beneficial as no buffering of the received signal is required.
• The Cross-Slot Scheduling is a UE power saving scheme with UE adaptation to the traffic arrival in the time domain includes cross slot scheduling, single slot scheduling with a gap between PDCCH and PDSCH reception, and multi-slot scheduling. 
  • For both cross-slot scheduling and single slot scheduling, the UE may achieve power consumption by switching to micro sleep after PDCCH reception. 
  • For multi-slot scheduling, the UE may achieve power consumption by skipping the PDCCH monitoring at subsequent slots of PDSCH/PUSCH transmission.

Observation:

(4) Discontinuous reception (DRX)

• The basis for DRX is a configurable DRX cycle in the device. With a DRX cycle configured, the device monitors the downlink control signaling only when active, sleeping with the receiver circuitry switched off the remaining time.
• This allows for a significant reduction in power consumption- the longer for the cycle, the lower the power consumption.
• Naturally, this implies restrictions to the scheduler as the device can be addressed only when active according to the DRX cycle.

(5) DRX with Wake-up signals

• The DRX mechanism gives significant improvements in device power consumption compared to being continuously active. Nevertheless, further improvements are possible if the network could inform the device to sleep for another long DRX cycle if no downlink data is expected instead of waking up regularly and monitor PDCCHs for a certain time before going back to sleep.
• Therefore, release 16 introduces up a configurable time before the start of the long DRX cycle, checks for the wake-up signal and if told not to wake up, return to sleep for the next long DRX cycle.

Observation:

(6) CONTROL CHANNEL DESIGN: PDCCH monitoring control

• Blind decoding of PDCCHs is carrier out at regular time instants. The periodicities of the blind decoding are configurable but a typical configuration is to monitor PDCCHs ate the beginning of each slot.
• However, in most cases the device is not scheduled and the blind decoding are done in vain, resulting in a waste of the device energy.
• To mitigate this and reduce the device energy consumption in connected mode, release 17 introduces PDCCH monitoring adaptation, based on two components: search-space-set-group(SSSG) switching and PDCCH skipping.

(7) Early indication of paging

• In the idle state, the device sleeps most of the time and wakes up at the paging occasions to see whether it is paged or not. Each of these paging occasions is associated with, relative to the sleeping, significant processing in the device and a corresponding energy consumption.
• Prior to each paging occasion, the device needs to wake up to obtain time/frequency synchronization, followed by PDCCH and PDSCH reception. This is a large of energy consumption in the device, compared to the PDCCH reception, energy that is wasted in the cases when the device is not paged.
• To address this, release 17 introduces a mechanism to indicate to the device in advance whether it is likely to be paged in an upcoming paging occasion. The indicator, know as the paging early indicator (PEI) is conveyed on a PDCCH using DCI format 2_7.

Source:

• The New generation wireless access technology Book page 352 to 363.
• 3GPP TR 38.840