UE Power Saving in NR
UE battery life is an important aspect of the user’s experience. The RAN1 study of the Rel-16 UE power saving had shown substantial power saving gain comparing to considered Rel-15 NR features such as DRX operation, with UE adaptation in frequency domain, time domain, antenna domain, tight control of DRX operations, and reducing PDCCH monitoring with different traffic types.
The Rel-16 UE power saving in NR includes the power saving techniques, such as DRX
adaptation, cross-slot scheduling, and maximum MIMO layer adaptation in
CONNECTED state, fast transition out of CONNECTED state, and reduced RRM
measurements in idle/inactive states.
The UE assistance information is part of the work to enable the UE to
feedback its preferred configuration to achieve desired power saving.
The UE
power saving work in Rel-16 focuses on the power saving techniques in CONNECTED state, which includes DRX
adaptation, cross-slot scheduling, maximum MIMO layer adaptation, and fast
transition out of CONNECTED state. The
RRM measurement reductions are the power saving techniques specified in
idle/inactive states. UE assistance
information is supported for the UE to feedback its preferred configuration of
the specific power saving technique.
Power
Saving Techniques in CONNECTED state
The
power saving techniques are dynamically triggered by L1 signaling indicated
from PDCCH-based power saving signal/channel or semi-statically configured by
RRC signaling. The PDCCH-based power
saving signal/channel reuses the existing PDCCH search space and CORESET
configurations with dynamic TCI states with DCI field indicating the adaptation
to achieve UE power saving, such as UE wakeup in the DRX operation, cross-slot
scheduling, and maximum MIMO layer adaptation through BWP switching.
- DRX adaptation
The DRX
adaptation power saving technique is to configure the PDCCH-based power saving
signal/channel at the active BWP before the beginning of DRX ON for UE
monitoring with the indication of UE wakeup or not depending on whether there
is data for UE to receive. A new DCI
format 2_6 is introduced with CRC scrambled by PS-RNTI (DCP) which contains the
wakeup indication as well as SCell dormancy indication if configured. A PS-offset is semi-statically configured
before DRX ON defining the start of the interval for the DCP monitor occasion
as shown in Figure 1. More than one
monitoring occasions could be configured for DCP on PCell for CA and SpCell for
DC based on the search space and CORESET configurations. Minimum time gap is specified as the UE
processing time as shown in Figure 1. UE
is not required to monitor DCP at the interval of minimum time gap and within
Active Time.
When DCP
monitoring occasion collides with other procedures with higher priority in
PDCCH monitoring, the monitoring occasion is considered invalid. UE follows legacy behavior when all the
configured monitoring occasions are invalid.
UE is configured by RRC to wake up or not when no DCP is detected with
valid monitoring occasions. One DCP can
be configured to control PDCCH monitoring during on-duration for one or more
UEs independently. UE is also configured
by RRC whether to report periodic L1-RSRP or periodic CSI/L1-SINR when UE is
not indicated to wake up at the DRX ON.
- Cross slot scheduling
Power
saving technique with cross-slot scheduling facilitates UE to achieve power saving
with the assumption that it won’t be scheduled to receive PDSCH, triggered to
receive A-CSI or transmit a PUSCH at the scheduling slot within Active
Time. A 1-bit minimum scheduling offset
in DCI format 1_1 and 0_1 enables dynamic switching of DL and UL minimum
scheduling offset values.
- Maximum MIMO Layer Adaptation
UE power
saving techniques with the adaptation to the DL maximum number of MIMO layers
could be achieved by dynamic switching of BWPs, which the DL maximum number of
MIMO layers are configured to be different.
-
Fast transition out of CONNECTED state
UE can
feed back the assistance information of its preference to be released/suspended
for gNB to get UE transitioning out of CONNECTED state quickly when there is no
further data arrival.
Power
Saving Techniques in idle/inactive state
- Reduced RRM measurements in idle/inactive
state
Power
saving in RRC_IDLE and RRC_INACTIVE can also be achieved by UE relaxing
neighbour cells RRM measurements when it meets the criteria determining it is
in low mobility and/or not at cell edge.
UE
assistance information
UE assistance information allows the UE to feedback its preferred configuration, such as c-DRX configuration, aggregated bandwidth, SCell configuration, MIMO configuration, RRC state, minimum scheduling offset values in order for network to assist UE achieving power saving gain.