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Study on User Equipment (UE) power saving in NR (Release 16)V16.0.0 (2019-06)

Note:
  • this page is a summary of 3GPP TS 38.840, for details please check the TS directly.
  • this page shows various power saving options studied for release 16. Please refer to Release 16 technical specifiations for the final solution chosen by 3GPP.

Table of content:

  1. Introduction
  2. UE power saving schemes
    1. UE adaptation to the traffic and power consumption characteristic
      1. Adaptation to the variation in frequency
        1. BWP
        2. CA/DC
      2. Adaptation to the variation in time
        1. cross slot scheduling
        2. single slot scheduling with a gap between PDCCH and PDSCH reception
        3. multi-slot scheduling
      3. Adaptation to number of antenna
      4. Adaptation to DRX operation
      5. Adaptation to achieve reducing PDCCH monitoring/decoding
        1. Triggering of PDCCH monitoring
        2. PDCCH skipping
        3. Multiple CORESET/search space configurations
        4. L1 signaling triggering to assist UE in reducing the number of PDCCH blind decoding
        5. Reduced PDCCH monitoring on SCell
        6. Network assistance
      6. UE ssistance Information
    2. Power saving signal/channel/procedure for triggering adaptation to UE power consumption
      1. Power saving signal/channel
      2. Power saving procedure
      3. Additional RS
    3. Power consumption reduction in RRM measurements
      1. Components to be considered for RRM measurement power saving evaluation
      2. Adaption mechanism of RRM measurement activities for UE power saving
      3. Power consumption reduction schemes
  3. Higher layer procedure for UE power saving

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.

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: a) Efficient data transmission in a loaded case; b) Low energy consumption when there is no data. Efficient data transmission in a loaded case is demonstrated by the average spectral efficiency. Low energy consumption when there is no data can be estimated by the sleep ratio".

Because NR system supports high speed data transport, the bursty user data would be served by network in very short durations. One efficient UE power saving mechanism is to trigger UE for network access from power efficient mode. UE would stay in the power efficient mode, such as micro sleep or OFF period in the long DRX cycle, unless it is informed of network access through UE power saving framework. Alternatively, network can assist the UE to switch from the "network access" mode to the "power efficient" mode when there is no traffic to deliver, e.g. dynamic UE transition to sleep with network assistance signal.

In addition to minimizing the power consumption with the new wake up/go-to-sleep mechanism, it is equally importance to reduce the power consumption during the network access in RRC_CONNECTED mode. More than half of the power consumption in LTE is UE in the access mode. 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. From the field data collected in the LTE network, most of subframes contain no data or small data. The power saving scheme for the dynamic adaptation to the different data arrival should be studied in RRC_CONNECTED mode. Dynamic adaptation to traffic in different dimensions, such as carrier, antenna, beamforming, and bandwidth, would be studied in the studied. Furthermore, methods to enhance the transitions between "network access" mode and power efficient mode should be considered. Both network-assisted and UE-assisted approaches should be considered for UE power saving mechanism.

UE also consumes a lot of power for RRM measurements. In particular, UE would need to power up before the DRX ON period to track the channel in preparation for the RRM measurement. Some of the RRM measurements are not necessary but consumes a lot of UE power, for example, the low mobility UEs does not have to measure as frequent as high mobility UEs. Network would provide the signalling to assist UE to reduce the power consumption on unnecessary RRM measurements. Additional UE assistance, for example the UE status information, etc, is also useful for the network to enable the UE power consumption reduction on RRM measurements.

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.

UE power saving schemes

UE adaptation to the traffic and power consumption characteristic

  1. Adaptation to the variation in frequency
    1. BWP - UE adaptation to different BWP
      • RS to assist UE channel tracking and measurements to assist BWP switching
        • The assistance may also include CSI measurements (UE supports and processes one BWP at a time)
      • Enhancement of L1 signaling, e.g., power saving signal or DCI for power saving, in triggering the BWP switching
      • Association of BWP and DRX
      • UE assistance information could be considered
    2. CA/DC
      • Quick activation/de-activation (e.g.,L1 signaling, MAC CE enhancement)
      • Adaptation of PDCCH monitoring/search space on activated SCell
        • Including cross carrier scheduling
      • Adaptation based on the operation in a group of cells in power efficient way
        • Including group of cells in bundle operation
      • CSI/RRM measurements and beam management at non-active SCell
      • UE assistance information could be considered
      • Including the combined operation with the BWP switching
  2. Adaptation to the variation in time
    1. cross slot scheduling
      Minimum K0 > 0 and aperiodic CSI-RS triggering offset is not within the duration - UE could switch to micro sleep after PDCCH reception – no addition PDSCH and CSI-RS signals reception within the given duration (e.g. the same slot)
      • It is known to the UE at PDCCH decoding
      • Extended micro sleep time and reduce the PDCCH processing in reducing UE power consumption
      • Minimum K2 > 0 is essential to avoid the requirements of fast PDCCH processing
      • UE assistance information can be considered
    2. single slot scheduling with a gap between PDCCH and PDSCH reception
      • Adaptation of TDRA configurations to achieve UE power saving – ensure the gap between PDCCH reception and PDSCH transmission known to the UE
      • Adjustment of TDRA configuration
        • Selection of TDRA entry in the TDRA table.
        • Note: cross-slot scheduling could be incorporated in the TDRA configuration
      • Power model for TDRA power saving scheme is the slot-averaged power for a slot with N-symbol PDSCH aligned to the end of the slot and PDCCH at the beginning of the slot reported by the proposed scheme.
      • UE assistance information can be considered
      • Note: For both cross-slot scheduling and single slot scheduling, the UE may achieve power consumption by switching to micro sleep after PDCCH reception
    3. multi-slot scheduling
      PDCCH decoding in one slot (e.g., one DCI, multiple DCI) supports scheduled PDSCH/PUSCH transmission over multiple slots.
      • Achieving UE power consumption reduction by potentially skipping PDCCH monitoring at subsequent slots of PDSCH/PUSCH transmission.
  3. Adaptation to number of antenna
    The UE power saving schemes with UE adaptation to the traffic is to reduce the maximum number of antenna/panels or MIMO layers indicated by the network semi-statically or dynamically to achieve the UE power saving. This may be done with potential UE assistance information. The dynamic approach may also be associated with a transition time, a timer, etc.
  4. Adaptation to DRX operation
    The power saving scheme of power saving signal/channel triggering UE adaptation to DRX operation is to configure the power saving signal/channel before or at the beginning of the DRX ON duration to trigger UE waking up only when there is DL data arrival. UE is not required to wake up at the DRX ON at least for PDCCH monitoring, if the power saving signal is not detected. The go-to-sleep signaling is used as the indication allowing UE going back to sleep state after completion of PDSCH reception during the DRX ON period to further reduce the UE power consumption.
    UE adaptation of its behavior to the DRX operation for UE power consumption reduction:
    • When is configured with power saving signal/channel, power saving signal/channel as the indication whether to wakeup or not before or at the beginning of DRX ON duration
      • At least for the indication of PDCCH monitoring
    • Preparation period is used for (e.g., to perform channel tracking, CSI measurements, beam tracking).
      • Preparation period can be used in preparation for the PDCCH decoding
      • Preparation period could be before or during the DRX ON duration
      • Network can indicate UE to report CSI before or after the power saving signal/channel (if configured) during the preparation period
      • Network can indicate additional RS transmission (e.g., CSI-RS, TRS, SSB and power saving signal) at the preparation period
    • Go-to-sleep signaling as the indication allowing UE going back to sleep state
      • MAC-CE
      • DCI
      • Power saving signal/channel
    • Constraints on scheduling DCI during DRX_ON
  5. Adaptation to achieve reducing PDCCH monitoring/decoding
    The UE power consumption can be reduced when the number of UE PDCCH monitoring occasions and/or the number of PDCCH blind decoding is reduced. The power saving schemes to reduce PDCCH monitoring and blind decoding for further studies are as follows
    1. Triggering of PDCCH monitoring – dynamic trigger through L1 signal/signaling
      • Power saving signal triggering PDCCH monitoring
      • Go-to-sleep signaling to skip PDCCH monitoring
    2. PDCCH skipping
      • DCI based indication for PDCCH skipping (e.g., indication in DCI content, new SFI state).
      • L1 signal/signaling (other than DCI) based triggering
    3. Multiple CORESET/search space configurations
      • Configuration of different PDCCH periodicities with dynamic signaling
      • Adaptation of CORESET/search space configuration – DCI/timer/HARQ-ACK based indication
      • Dynamic/semi-persistent CORESET/search space ON/OFF
      • Adaptation between DRX ON duration timer and inactivitytimer
      • Separated PDCCH monitoring of DL and UL
    4. L1 signaling triggering to assist UE in reducing the number of PDCCH blind decoding
    5. Reduced PDCCH monitoring on SCell (including cross carrier scheduling)
    6. Network assistance – RS is dynamically transmitted based on the need to assist UE performing synchronization, channel tracking, measurements and channel estimations before PDCCH decoding
  6. UE assistance Information
    The UE assistance information for any UE power saving scheme is for UE to provide the assistance information to the network in configuration for UE adaptation to achieve power saving gain. UE assistance information reported by UE for power saving is a UE recommendation and the network shall make the final decision on whether and how to use that information. The UE assistance information for the power saving schemes for further studies are as follows
    • UE preferred processing timeline parameters, e.g., K0, K1, K2 values
    • UE preferred BWP information/configuration
    • UE preferred antenna configuration, including MIMO layers, antenna panel awareness information
    • UE assistance/feedback on the DRX configurations/parameters
    • UE preferred BWP provided to assist network in BWP switching
    • UE request on SCell/SCG activation/de-activation/configuration
    • UE preferred PDCCH monitoring parameters/search space configuration/maximum number of blind decoding

Power saving signal/channel/procedure for triggering adaptation to UE power consumption

  1. Power saving signal/channel
    • Existing signal/channel based power saving signal/channel
      • PDCCH channel
      • TRS, CSI-RS type RS, SSS-like and DMRS
      • PDSCH channel carried MAC CE and/or RRC signaling
    • New power saving signal/channel – sequence based
  2. Power saving procedure
    • triggering UE adaptation to DRX operation
      • The power saving signal/channel can be configured along the DRX configuration as the indication for UE to wake up from the sleep state.
      • RS resources can be considered to assist UE in performing RRM/CSI measurement and channel time/frequency and/or beam tracking.
    • triggering UE to achieve reducing PDCCH monitoring
      • The power saving signal/channel can be used to trigger UE to skip the PDCCH monitoring and/or to go to sleep for a period of time.
    • triggering UE frequency domain processing adaptation
      • The power saving signal/channel can be used to trigger the indication of RS configuration for channel tracking, CSI measurements, and beam management for the additional assistance of dynamic switching of BWP or activation of SCell in achieving the power saving gain.
      • The power saving signal/channel can be used for BWP switching, activation/deactivation of SCell or adaptation of PDCCH monitoring and/or CORESET/search space of PCell/SCell.
    • triggering adaption to the UE processing, such as MIMO configuration/layers, antenna configuration, UE processing time, and background processing
      • The power saving signal/channel in triggering UE adaptation to the processing is to allow UE in reducing the power consumption by indication of the processing time, such as PDCCH/PDSCH/PUSCH/PUCCH processing or the essential background processing, such as periodic CSI and RRM measurements.
  3. Additional RS
    the RS provided by gNB to assist UE in performing e.g., fine synchronization, channel/beam tracking, and/or CSI/RRM measurements in addition to the existing RS in Rel-15. The additional RS was proposed for study for power saving schemes for e.g,, UE adaptation to the DRX operation, BWP switching, fast SCell activation, reducing PDCCH monitoring, and/or RRM measurements. RS design is assumed to reuse Rel-15 waveform. Power saving signal could be used to meet the purpose of additional RS.

Power consumption reduction in RRM measurements

The RRM power saving study should consider RRC IDLE, INACTIVE and CONNECTED states. The deployment scenarios for further studies of RRM power saving focus on the stationary (e.g., 0km/h), pedestrian (e.g., 3km/h) and vehicular (e.g., 30km/h) scenarios when considering UE power saving techniques for RRM measurements.

Components to be considered for RRM measurement power saving evaluation:

  1. For IDLE/INACTIVE state, at least the following power components are recommended to be considered for RRM measurement power saving evaluation
    • Loop convergence (AGC, TTL and FTL) / time-frequency tracking
      • How many SSB bursts are used for Loop convergence, with consideration of being potentially confined in the same SSB burst or different SSB bursts for serving cells and neighboring cells measurement/ time-frequency tracing
      • FFS: The power value for loop convergence /time-frequency tracking is as the same as SSB processing.
    • Paging
    • SIB1 decoding (PDCCH+PDSCH)
    • Neighboring cell search (within SMTC), if any
    • SSB measurement (serving cell only / severing cell and neighboring cells, if any)
    • Sleep
  2. For CONNECTED state, at least the following power components are recommended to be considered for RRM measurement power saving evaluation,
    • Loop convergence (AGC, TTL and FTL) / time-frequency tracking
      • How many SSB bursts are used for Loop convergence, with consideration of being potentially confined in the same SSB burst or different SSB bursts for serving cells and neighboring cells measurement/ time-frequency tracing
      • Based on SSB and/or TRS(if configured)
    • PDCCH-only monitoring during active time
    • SSB measurement (serving cell only / severing cell and neighboring cells, if any)
    • Neighboring cell search (within SMTC), if any
    • Sleep

The following adaption mechanism of RRM measurement activities for UE power saving:

  1. gNB controlled RRM measurement operation with UE assistance information reported to gNB, e.g.,
    • mobility related information (e.g., mobility state, history of mobility state, UE's visited cells and cells not reselected due to the ping-pong effect, the number of handovers for certain period, etc.)
    • channel condition (e.g. change in serving RS/signal)
  2. gNB controlled RRM measurement operation without UE assistance information reported to gNB based on certain conditions, e.g.,
    • Doppler estimation for RRC CONNECTED states
    • cell type (e.g., small cell/macro cell)
  3. gNB controlled threshold to support UE autonomous RRM measurement adaptation based on e.g.,
    • signal measurements (e.g., RSRP)
    • UE mobility state (e.g., low/medium/high mobility)
    • UE location in the cell (e.g., cell-center/cell-edge)
    • S-measure enhancement (e.g. S-measure for SCell, CSI-RS)
  4. Other mechanisms/approaches are not precluded

Power consumption reduction schemes:

  1. Adapting/Relaxing RRM measurement in time domain
    • For certain conditions (e.g., low mobility deployment/UE speed/favorable RSRP conditions), the number of RSRP measurement samples for a given duration (e.g., measurement period / evaluation period) can be relaxed with negligible impact on accuracy achieved by existing Rel-15 measurement.
    • For certain conditions (e.g., favorable RSRP conditions, etc.), reducing RRM measurement activities (e.g., measurement, reporting) for a given time period is beneficial from UE power saving perspective for RRC IDLE/INACTIVE/CONNECTED states.
  2. Adapting/Relaxing intra-frequency measurements
    • Reducing the number of cells for intra-frequency measurement can be beneficial for UE power saving,
      • Assuming that UE can limit the processing for measurement within a constrained time period and/or with reduced complexity.
      • Assuming number of neighbouring cells to be measured is reduced.
    • For UE power saving perspective, reducing the need in neighbour cell intra-frequency measurement can be beneficial.
  3. Power saving schemes for RRM measurements with additional resource
    under certain conditions and deployment scenarios, additional resource for RRM measurement can be beneficial for UE power saving, including at least the following aspects:
    • Minimizing/reducing the timing gap between measurement (e.g., SSB) and DRX ON duration (e.g., paging monitoring occasion/reception, data reception, etc.)
    • Additional resource around the measurement occasion, e.g., for AGC assistance
    • Reducing measurement activities by providing additional resource may provide sufficient measurement/T-F accuracy.
    • The followings can be considered as the usage of additional resource for RRM measurement:
      CSI-RS (including TRS); SSS only; SSB, PSS, SSS and wake-up signaling/paging; DMRS for RMSI PDCCH/PDSCH for standalone; Additional new RS/signal (e.g, configuring additional RS next to SSB) in addition to existing RS/signal in Rel-15
    • Adapting/Relaxing inter-frequency measurements

Higher layer procedure for UE power saving

  1. UE paging procedure based on power saving signal/channel/procedure: down-prioritized in the study.
  2. UE power saving procedure in transition from RRC_CONNECTED to RRC_IDLE/RRC_INACTIVE state.
    A mechanism for a UE to indicate its preference of transitioning out of RRC_CONNECTED state to the network is beneficial to reduce UE's power consumption.
  3. Higher layer procedures for the UE power saving schemes in RRC_CONNECTED
    1. Higher Layer procedure related to PDCCH-based power saving signal/channel scheme for wake-up purpose
      The PDCCH-based power saving signal/channel scheme for wake-up purpose is considered jointly with DRX i.e. it is only configured when DRX is configured. If the PDCCH-based power saving signal/channel for wake-up purpose is not configured, the legacy DRX operation applies.
      • When configured, the PDCCH-based power saving signal/channel scheme for wake-up purpose is monitored at occasions located at a known offset before the start of the drx-onDurationTimer. The offset is part of physical layer design.
      • The PDCCH-based power saving signal/channel for wake-up purpose can indicate the UE to monitor or skip the PDCCH during the next occurrence of the drx-onDurationTimer. In the latter case, the UE does not start the drx-onDurationTimer at its next occasion. From higher layer perspective, in order to minimize the rate of such PDCCH-based power saving signal/channel transmission from the network, the preference is that the PDCCH-based power saving signal/channel for wake-up purpose is used to indicate to the UE to wake up to monitor the PDCCH during the next occurrence of the drx-onDurationTimer.
      • Except for the drx-onDurationTimer, the PDCCH-based power saving signal/channel for wake-up purpose has no impact on other DRX timers and does not impact the Active Time due to other triggers than the drx-onDurationTimer. The UE behaviour when the occasion of PDCCH-based power saving signal/channel for wake-up purpose collides with Active Time due to other triggers than the drx-onDurationTimer (e.g. drx-InactivityTimer) should be addressed in the WI phase.
    2. Higher Layer Procedure for DCI-based PDCCH monitoring reduction
      If enabled, it is assumed that DCI-based PDCCH monitoring skipping could be configured with or without DRX.
      DCI-based PDCCH monitoring skipping aims to operate on a short time scale (i.e. shorter time scale then the L2 DRX). Under this condition, it has not been identified that DCI-based PDCCH monitoring skipping duplicates the DRX functionality.
      The DCI-based PDCCH monitoring skipping is a physical layer procedure with minimal impact, if any, on the MAC layer. The MAC timers are not affected by the DCI-based PDCCH monitoring skipping command, except for timers related to UL triggered activities (e.g. RA, SR and BFR), namely how/if the DCI-based PDCCH monitoring skipping command impacts L2 operation while a Random Access or scheduling request procedure is on-going.
    3. Power saving in CA/DC: reducing PDCCH monitoring on activated SCells
    4. UE assistance information
      UE's power consumption may be reduced if the UE could provide following assistance information to the network: mobility history information (e.g. similarly as in LTE via mobilityState, and MobilityHistoryReport in TS 36.331), power preferred information (baseline LTE PPI in a well-defined manner) related to C-DRX, BWP and SCell configurations.
    5. Adaptation to the number of antennas: It is recommended to support the possibility to configure a different MIMO layer configuration for the initial/default BWP compared with other BWPs of a Serving Cell. It should be further considered if this can be extended to a per-BWP MIMO layer configuration.
  4. Higher layer procedures for power consumption reduction in RRM measurements
    RRM measurement relaxation for serving cell is down-prioritized for UE in any RRC state. RRM measurements for neighbour cells in both intra and inter-frequencies can be relaxed for UEs in RRC_CONNECTED and RRC_IDLE/INACTIVE. Measurement relaxation for UEs in RRC_CONNECTED is under network control.
    The relaxed monitoring criteria may include the following aspects, but are not limited to:
    • UE mobility status (e.g. serving cell variation, speed, movement, direction, cell re-selection, UE type …)
    • Link quality (e.g. serving cell threshold/quality, position in cell …)
    • Serving cell beam status (e.g. beam change, direction, beam specific link condition…)
    The exact relaxation criteria are to be defined, and the following two should be treated with higher priority:
    1. UE is not at cell edge,
    2. UE is stationary or with low mobility.

    It is beneficial to perform RRM measurement relaxation by allowing measurements with longer intervals, and/or by reducing the number of cells/carriers/SSB to be measured.