1
Introduction
The
UE needs measurement gaps to perform measurements when it cannot measure the
target carrier frequency while simultaneously transmitting/receiving on the
serving cell.
In
the case of LTE, the UE needs measurement gaps in order to perform
inter-frequency and inter-RAT measurements. Typical LTE gap length is 6 ms
which accommodates 5 ms measurement time (PSS and SSS are transmitted once
every 5 ms) and RF re-tuning time of 0.5 ms before and after the measurement
gap. The measurement gap repeats with a periodicity of either 40 ms or 80 ms.
Similarly,
in NR, the measurements that the UE performs can be gap-assisted (network configures
measurement gap) or non-gap-assisted.
1.1 Measurements in NR
The
need for measurement gap in NR depends on the capability of the UE, the active BWP of the UE and the
current operating frequency.
In
NR, measurements gaps might be required for intra-frequency, inter-frequency
and inter-RAT measurements.
Unlike
LTE intra-frequency case, intra-frequency measurements in NR might require a measurement
gap in cases for example, if the intra-frequency measurements are to be done
outside of the active BWP.
Measurement
gap lengths of 1.5, 3, 3.5, 4, 5.5, and 6 ms with measurement gap repetition
periodicities of 20, 40, 80, and 160 ms are defined in NR.
In
NR, the RF re-tuning time is 0.5 ms for carrier frequency measurements
in FR1 range and 0.25 ms for FR2 range. For example, a gap length of 4 ms for
FR1 measurements would allow 3 ms for actual measurements and a gap length of 3.5
ms for FR2 measurements would allow 3 ms for actual measurements.
During the measurement gaps, the measurements are to
be performed on SSBs of the neighbour cells. The network provides the timing of
neighbour cell SSBs using SS/PBCH Block Measurement Timing Configuration (SMTC).
– The measurement gap and SMTC duration are configured
such that the UE can identify and measure the SSBs within the SMTC window i.e.,
the SMTC duration should be sufficient enough to accommodate all SSBs that are
being transmitted.
For
SSB based intra-frequency measurements, the network always configures
measurement gap in the following case:
-
If
any of the UE configured BWPs do not contain the frequency domain
resources of the SSB associated to the initial DL BWP.
For
SSB based inter-frequency measurements, the network always configures
measurement gap in the following cases:
-
If
the UE supports per-FR measurement gaps (section 2.1) and if the carrier
frequency to be measured is in same frequency range (FR) as any of the serving
cells.
-
If
the UE only supports per-UE measurement gaps. In this case, the measurement
object can be configured on any frequency range (FR1 or FR2) but the gap will anyway
be configured by the network.
Inter-RAT measurements in NR are limited to E-UTRA. For a UE configured with
E-UTRA Inter-RAT measurements, a measurement gap configuration is always
provided when:
-
The
UE only supports per-UE measurement gaps; or
-
The
UE supports per-FR measurement gaps and at least one of the NR serving cells is
in FR1.
1.2 Measurements in MR-DC configurations
For
MR-DC configurations, a measurement gap configuration is always provided to the
UE in the following cases;
-
In
EN-DC, NGEN-DC and NE-DC, for UEs configured with E-UTRA inter-frequency
measurements.
-
In
EN-DC and NGEN-DC, for UEs configured with UTRAN and GERAN measurements.
-
In
NR-DC, for UEs configured with E-UTRAN measurements.
Additionally,
in EN-DC, 3GPP release 15 didn’t specify a way (in UE Capability Information) for
the UE to inform the E-UTRAN about the measurement gap requirements for individual
EN-DC band combinations. So, the E-UTRAN always configures measurement gap for
NR measurements.
2
Measurement Gap Configuration
Depending
on the UE capability to support independent FR (frequency range) measurement
and network preference, there are two types of measurement gaps defined in NR;
per-UE and per-FR.
In
per-FR gap, two independent gap patterns (i.e. FR1 gap and FR2 gap) are defined
for FR1 and FR2 respectively.
Per-UE
gap applies to both FR1 (E-UTRA and NR) and FR2 (NR) frequencies.
In
the case of MR-DC:
-
For
per-UE and per-FR1 gap patterns, the MN decides the gap pattern. The MN is also
responsible for providing the gap pattern to the UE via MN RRC. This is applicable
for EN-DC, NGEN-DC, NE-DC and NR-DC.
-
For
per-FR2 gap, in EN-DC and NGEN-DC, the SN (NR) decides the FR2 gap pattern. In
NE-DC and NR-DC, the MN (NR) decides FR2 gap pattern and the related gap sharing
configurations. The node (MN/SN) which decides the gap pattern is also responsible
for providing the configuration to the UE.
Node responsible for decision making and providing measurement gap configuration to the UE
|
|||
MR-DC
|
Per-UE gap
|
Per-FR1 gap
|
Per-FR2 gap
|
EN-DC
NGEN-DC
|
E-UTRAN (MN)
|
E-UTRAN (MN)
|
NR (SN)
|
NE-DC
|
NR (MN)
|
NR (MN)
|
NR (MN)
|
NR-DC
|
NR (MN)
|
NR (MN)
|
NR (MN)
|
2.1 Configuration provided by NR RRC
As
discussed already, in the following cases, NR RRC is responsible for providing measurement
gap pattern configuration to the UE. This is done using MeasGapConfig IE
within the MeasConfig IE and is carried by RRC Reconfiguration message. NR RRC is responsible for the
following:
- Configure
the UE with either gapUE or gapFR1 in NR standalone operation
(with single carrier, NR CA and NR-DC) or in NE-DC configuration.
- Configure
the UE with gapFR2 in any configuration i.e., NR standalone operation
(with single carrier, NR CA and NR-DC) or EN-DC or NE-DC.
MeasGapConfig
IE specifies the measurement gap configuration and controls
setup/release of measurement gaps. The details of this IE are given below;
| MeasGapConfig | |
| gapFR2 | SetupRelease { GapConfig } |
| gapFR1 | SetupRelease { GapConfig } |
| gapUE | SetupRelease { GapConfig } |
| GapConfig | |
| gapOffset | INTEGER (0..159) |
| mgl | ENUMERATED { 1.5 msec, 3 msec, 3.5 msec, 4 msec, 5.5 msec, 6 msec } |
| mgrp | ENUMERATED { 20 msec, 40 msec, 80 msec, 160 msec } |
| mgta | ENUMERATED { 0 msec, 0.25 msec, 0.5 msec } |
| refServCellIndicator | ENUMERATED { pCell, pSCell, mcg-FR2 } |
2.1.1 gapFR1
This
field indicates measurement gap configuration that applies to FR1 frequency
range only (both E-UTRAN and NR serving cells). gapFR1 cannot be configured together with gapUE.
In
MR-DC, gapFR1 can only be setup by measConfig associated with MCG
(see below):
-
In
(NG) EN-DC, gapFR1 cannot be set up by NR RRC i.e. only LTE RRC can
configure FR1 measurement gap using LTE RRC Connection Reconfiguration
(discussed in section 2.2).
-
In
NE-DC, gapFR1 can only be set up by NR RRC using NR RRC Reconfiguration (i.e.
LTE RRC cannot configure FR1 gap).
-
In
NR-DC, gapFR1 can only be set up in the measConfig associated
with MCG.
2.1.2 gapFR2
This
field indicates measurement gap configuration that applies to FR2 only. gapFR2 cannot be configured together with gapUE.
MR-DC:
-
In
(NG) EN-DC or NE-DC, gapFR2 can only be set up by NR RRC using NR RRC Reconfiguration
(i.e. LTE RRC cannot configure FR2 gap).
-
In
NR-DC, gapFR2 can only be set up in the measConfig associated
with MCG.
2.1.3 gapUE
This
field indicates measurement gap configuration that applies to all frequency ranges
(FR1 and FR2) which are applicable for E-UTRAN and NR serving cells. If gapUE is configured, then neither gapFR1
nor gapFR2 can be configured.
In
MR-DC, gapUE can only be setup by measConfig associated with MCG
(see below):
-
In
(NG) EN-DC, gapUE cannot be set up by NR RRC i.e. only LTE RRC can
configure FR1 measurement gap using LTE RRC Connection Reconfiguration (discussed
in section 2.2).
-
In
NE-DC, gapUE can only be set up by NR RRC using NR RRC Reconfiguration (i.e.
LTE RRC cannot configure per UE gap).
-
In
NR-DC, gapUE can only be set up in the measConfig associated with
MCG.
2.1.4 Other
fields within MeasGapConfig provided by NR RRC
mgrp
(Measurement Gap Repetition Period) is the
periodicity (in ms) at which measurement gap repeats. Periodicities of 20, 40,
80, and 160 ms are defined in NR.
gapOffset
is the gap offset of the gap pattern. Not all 160 offset
values applicable for all periodicities. As the offset values points to the
starting subframe within the period, its value range is from 0 to mgrp-1. For example, if
the periodicity is 40 ms, the offset ranges from 0 to 39.
mgl
(Measurement Gap Length) is the length of measurement
gap in ms. Measurement gap lengths of 1.5, 3, 3.5, 4, 5.5, and 6 ms are defined
in NR.
mgta
(Measurement Gap Timing Advance) – If this is
configured, the UE starts the measurement mgta ms before the gap
subframe occurrence i.e., the measurement gap starts at time mgta ms
advanced to the end of the latest subframe occurring immediately before the measurement
gap. The amount of timing advance can be 0.25 ms (FR2) or 0.5 ms (FR1).
refServCellIndicator
indicates the serving cell whose SFN and subframe are
used for gap calculation for this gap pattern. Value pCell corresponds
to the PCell, pSCell corresponds to the PSCell, and mcg-FR2
corresponds to a serving cell on FR2 frequency in MCG. This field is used only
in NE-DC or NR-DC configuration.
2.2 Configuration
provided by E-UTRAN RRC (EN-DC)
In
the case of EN-DC configuration, E-UTRAN RRC is responsible for configuring the
UE with measurement gap using E-UTRAN RRC MeasGapConfig. This is
applicable for LTE and NR serving cells on FR1 only.
-
In
summary, the UE may either be configured with a single (common) gap or with two
separate gaps i.e. a first one for FR1 (configured by E-UTRA RRC for measuring
E-UTRAN and NR serving sells) and a second one for FR2 (configured by NR RRC).
3GPP
release 15 has introduced several new gap configurations within E-UTRAN RRC MeasGapConfig
in order to accommodate the EN-DC measurement requirements.
The
MeasGapConfig IE configured by E-UTRAN RRC is shown below;
| MeasGapConfig | ||
| gapOffset |
MGL (msec)
|
|
| gp0 | INTEGER (0..39) |
6
|
| gp1 | INTEGER (0..79) |
6
|
| gp2-r14 | INTEGER (0..39) |
3
|
| gp3-r14 | INTEGER (0..79) |
3
|
| gp4-r15 | INTEGER (0..19) |
6
|
| gp5-r15 | INTEGER (0..159) |
6
|
| gp6-r15 | INTEGER (0..19) |
4
|
| gp7-r15 | INTEGER (0..39) |
4
|
| gp8-r15 | INTEGER (0..79) |
4
|
| gp9-r15 | INTEGER (0..159) |
4
|
| gp10-r15 | INTEGER (0..19) |
3
|
| gp11-r15 | INTEGER (0..159) |
3
|
In
general, the MeasGapConfig is applicable for all LTE and NR serving
cells (including FR2). Hence, this is same as gapUE (per-UE gap) configuration.
The
network can limit the use of MeasGapConfig to LTE serving cells and NR serving
cells on (only) FR1 by setting the field fr1-Gap (within E-UTRAN RRC MeasConfig) to
TRUE. In this case the gap configuration
is same as gapFR1 (per-FR1 gap).
Now,
the question is how the measurement gap timing advance (mgta) discussed
in section 2.1.4 (for NR measurements) is handled when the gap is configured by
E-UTRAN RRC. Release 15 version of 36.331 has introduced a field (mgta-r15) for this
purpose.
- mgta indicates whether a timing advance value of 0.5 ms is applicable to the
measurement gap configuration provided by E-UTRAN. E-UTRAN sets mgta to
TRUE only when the UE is configured to perform NR measurements. As E-UTRAN RRC
is only responsible for measurement gaps within FR1, only 0.5 ms for mgta
is applicable.
The
fields mgta and fr1-Gap within MeasConfig (configured by E-UTRAN
RRC) are shown below;
| MeasConfig | |
| . . . | |
| fr1-Gap-r15 | BOOLEAN |
| mgta-r15 | BOOLEAN |
| . . . | |
3
Measurement Gap Calculation
After
receiving the measurement gap configuration, the first subframe of each gap
occurs at an SFN and subframe meeting the following condition:
SFN mod (MGRP/10) = FLOOR (gapOffset/10)
subframe = gapOffset mod 10
The
values of MGRP (mgrp) and gapOffset are configured by the network
within the corresponding measurement gap configuration of gapFR1/gapFR2/gapUE.
3.1 Determination of SFN and subframe for measGAP calculation
For
gapFR2 configuration:
-
For
the UE in NE-DC or NR-DC, the SFN and subframe of the serving cell indicated by
the refServCellIndicator in gapFR2 is used in the gap
calculation.
-
Otherwise,
for UE in NR standalone operation (with single carrier or NR CA) or (NG) EN-DC,
the SFN and subframe of a serving cell on FR2 frequency is used in the gap
calculation.
For
gapFR1 or gapUE configuration:
-
For
the UE in NE-DC or NR-DC, the SFN and subframe of the serving cell indicated by
the refServCellIndicator in corresponding gapFR1 or gapUE is
used in the gap calculation.
-
Otherwise,
for (with single carrier or NR CA) or (NG) EN-DC, the SFN and subframe of the
PCell is used in the gap calculation.
4
Measurement Gap Pattern Configurations
As
discussed in section 2, NR supports MGRP of 20, 40, 80 and 160 ms and MGL of 1.5,
3, 3.5, 4, 5.5, or 6 ms.
A
measurement gap pattern is characterized by MGRP and MGL. There are 24 gap pattern
configurations defined in 38.133 to accommodate all the needs for NR and E-UTRAN
measurements.
The
measurement gap patterns are presented in the table below.
GAP
Pattern Id
|
MGL
(msec)
|
MGRP
(msec)
|
GAP
Pattern Id
|
MGL
(msec)
|
MGRP
(msec)
|
|
0
|
6
|
40
|
12
|
5.5
|
20
|
|
1
|
6
|
80
|
13
|
5.5
|
40
|
|
2
|
3
|
40
|
14
|
5.5
|
80
|
|
3
|
3
|
80
|
15
|
5.5
|
160
|
|
4
|
6
|
20
|
16
|
3.5
|
20
|
|
5
|
6
|
160
|
17
|
3.5
|
40
|
|
6
|
4
|
20
|
18
|
3.5
|
80
|
|
7
|
4
|
40
|
19
|
3.5
|
160
|
|
8
|
4
|
80
|
20
|
1.5
|
20
|
|
9
|
4
|
160
|
21
|
1.5
|
40
|
|
10
|
3
|
20
|
22
|
1.5
|
80
|
|
11
|
3
|
160
|
23
|
1.5
|
160
|
When
the measurement gap is configured by NR RRC (section 2.1), the configuration provides
all the required fields (mgl, mgrp, mgta and gapOffset)
in order for the UE to calculate the measurement gap.
Similarly,
when measurement gap is configured by E-UTRAN RRC (section 2.2), the configured
gapOffset value provides information such as mgl and mgrp in
addition to gapOffset value itself. mgta needs to be explicitly
informed to the UE using the filed mgta-r15 (section 2.2).
5
Measurement Gap Timing Advance (MGTA)
As
discussed in section 1, RF re-tuning time is required at the beginning and at
the end of the measurement gap and the UE uses SMTC information together with
measurement gap configuration to perform neighbour cell NR measurements.
In
some cases, the SMTC window and the measurement gap start at the same time
which leads to overlapping of RF re-tuning time and SMTC window. As the UE cannot
perform measurements during RF re-tuning time, it would miss to measure SSBs
during the start of SMTC window.
To
overcome this problem, 3GPP has introduced Measurement Gap Timing Advance (MGTA).
With this mechanism, the start of measurement gap is advanced by an amount of
RF re-tuning time i.e., 0.5 ms for measurements in FR1 and 0.25 ms for FR2.
For
measurement gap configured by NR RRC (section 2.1.4), the parameter mgta
configured within MeasGapConfig informs the UE about the amount of
timing advance (0.25 ms or 0.5 ms).
For
measurement gap configured by E-UTRAN RRC (EN-DC), the field mgta-r15
is used for this purpose. The presence of this field indicates
to the UE whether a timing advance value of 0.5 ms is applicable to the
measurement gap configuration provided by E-UTRAN. E-UTRAN sets mgta to
TRUE only when the UE is configured to perform NR measurements.
6
Handling of measurement gaps (from MAC perspective)
During
a measurement gap, on the Serving Cell(s) in the corresponding frequency range (FR)
of the measurement gap, the MAC entity shall;
-
not
perform the transmission of HARQ feedback, SR, and CSI
-
not
report SRS
-
not
transmit on UL-SCH except for Msg3
-
not
receive on DL-SCH
-
not
monitor the PDCCH except for the case if the UE is waiting for Msg2 or Msg4
during RA procedure.
7
UE capability
UE
conveys its measurement capabilities in UE Capability Information message for
standalone NR and MR-DC and NR-DC.
In
this case of LTE, the UE indicates its ‘need for gaps’ for every band combination
e.g., via interFreqNeedForGaps or interRAT-NeedForGaps.
To
configure inter-RAT NR measurements to an LTE UE that is not yet configured
with EN-DC, the LTE network needs to know whether the UE needs gaps for such
measurements. RAN2 concluded that adding NeedForGap capability to each
NR bands into LTE RRC signaling is not supported in Rel-15. The
E-UTRAN RRC always configures measurement gap for NR measurements.
7.1 Measurement
Gap patterns supported by the UE
Measurement
gap patterns are discussed in section 4. The UE need to first inform the
network (via UE capability signalling) which gap patterns it supports. The
network configures the measurement gap pattern to the UE accordingly.
The
gap patterns capabilities are conveyed by the UE as discussed in the below
sections.
7.1.1 Gap Patterns 0 and 1
It
is mandatory to support gap patterns 0 and 1 for all cases (LTE standalone,
EN-DC, NR standalone etc…). So, the UE need not inform the network.
7.1.2 Gap Patterns 2 and 3
In
LTE standalone as well as in EN-DC, LTE RRC signalling shortMeasurementGap
is used to signal whether UE supports gap patterns 2 and 3.
shortMeasurementGap is part of MeasParameters IE and is introduced
in release-14 version of 36.331. The UE includes this field to indicate whether
the UE supports shorter measurement gap length (i.e. gp2 and gp3)
in LTE standalone and for independent measurement gap configuration on FR1 and
per-UE gap in (NG) EN-DC.
| MeasParameters-v1430 (36.331) | |
| . . . | |
| shortMeasurementGap-r14 | ENUMERATED {supported} |
| . . . | |
In
NR (NR SA, NR-DC, and NE-DC), NR RRC signalling supportedGapPattern (22
bits) is used to signal whether UE supports gap patterns 2 to 23.
supportedGapPattern
is part of MeasAndMobParameters IE and it indicates measurement gap
pattern(s) supported by the UE for NR SA, for NR-DC, for NE-DC and for
independent measurement gap configuration on FR2 in (NG) EN-DC.
-
The
leftmost bit (bit 0) corresponds to the gap pattern 2, the next bit corresponds
to the gap pattern 3 and so on.
| MeasAndMobParametersCommon | |
| supportedGapPattern | BIT STRING ( SIZE (22) ) |
| . . . | |
7.1.3 Gap Patterns 4 to 11
In
LTE standalone as well as in EN-DC, LTE RRC signalling measGapPatterns
(8 bits) is used to signal
whether UE supports gap patterns 4 to 11.
measGapPatterns is part of MeasParameters IE and is introduced
in release-15 version of 36.331. The UE includes this field to indicate, whether
the UE supports NR gap patterns 4 to 11 in LTE standalone as well as for (NG) EN-DC
(per-UE gap and FR1 gap of per-FR gap).
-
The
leftmost bit (bit 0) corresponds to the gap pattern 4, the next bit corresponds
to the gap pattern 5 and so on. Value 1 indicates that the UE supports the
concerned gap pattern.
| MeasParameters-v1520 (36.331) | |
| measGapPatterns-r15 | BIT STRING ( SIZE (8) ) |
In
NR (NR SA, NR-DC, and NE-DC), NR RRC signalling supportedGapPattern (22
bits) is used to signal whether UE supports gap patterns 2 to 23. supportedGapPattern
field is explained in the previous section (7.1.2).
-
The
leftmost bit (bit 0) corresponds to the gap pattern 2, the next bit corresponds
to the gap pattern 3 and so on.
7.1.4 Gap Patterns 12 to 23
The
gap patterns 12 to 23 are applicable for FR2 only and can only be configured by
NR RRC signalling. So, the UE indicates its capability to support these
patterns to NR RRC via supportedGapPattern (this field is explained in section
7.1.2).
-
The
leftmost bit (bit 0) corresponds to the gap pattern 2, the next bit corresponds
to the gap pattern 3 and so on.
Note:
The UE shall set the bits corresponding to the measurement gap pattern 13 and
14 to 1 if one of the following conditions are satisfied the UE is an NR
standalone capable UE that supports at least a band in FR2 or if the UE is an
(NG) EN-DC capable UE that supports independentGapConfig and supports at
least band in FR2. This implies that if a UE supports at least a band in FR2,
it must (mandatorily) support gap patterns 13 and 14.
7.2 UE’s
support for Independent Gap Configuration
The
field independentGapConfig indicates whether the UE supports two
independent measurement gap configurations for FR1 and FR2. This helps the
network providing per-FR measurement gap patterns for each frequency range (FR)
where UE requires per-FR measurement gap for concurrent monitoring of all
frequency layers of each frequency range independently.
-
Moreover,
this field also indicates whether the UE supports the FR2 inter-RAT measurements
without gaps when EN-DC is not (yet) configured i.e., if this LTE UE (not yet
configured with EN-DC) has per-FR measurement capability and it is configured
to measure FR2 inter-RAT MO only, gapless measurement is possible.
| MeasAndMobParametersCommon or MeasAndMobParametersMRDC-Common | |
| . . . | |
| independentGapConfig | ENUMERATED {supported} |
| . . . | |
