How LTE Stuff Works?
5G NR: Measurement GAPs
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 interfrequency 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-DC
NR (MN)
NR (MN)
NR (MN)
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
gapFR1
gapUE
GapConfig
gapOffset
mgl
mgrp
mgta
refServCellIndicator
2.1.1
SetupRelease { GapConfig }
SetupRelease { GapConfig }
SetupRelease { GapConfig }
INTEGER (0..159)
ENUMERATED { 1.5 msec, 3 msec, 3.5 msec, 4 msec, 5.5 msec, 6 msec }
ENUMERATED { 20 msec, 40 msec, 80 msec, 160 msec }
ENUMERATED { 0 msec, 0.25 msec, 0.5 msec }
ENUMERATED { pCell, pSCell, mcg-FR2 }
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 EUTRAN 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
gp0
INTEGER
gp1
INTEGER
gp2-r14
INTEGER
gp3-r14
INTEGER
gp4-r15
INTEGER
gp5-r15
INTEGER
gp6-r15
INTEGER
gp7-r15
INTEGER
gp8-r15
INTEGER
gp9-r15
INTEGER
gp10-r15
INTEGER
gp11-r15
INTEGER
(0..39)
(0..79)
(0..39)
(0..79)
(0..19)
(0..159)
(0..19)
(0..39)
(0..79)
(0..159)
(0..19)
(0..159)
MGL (msec)
6
6
3
3
6
6
4
4
4
4
3
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 EUTRAN 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
mgta-r15
BOOLEAN
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) ENDC, 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
2
3
4
5
6
7
8
9
10
11
6
3
3
6
6
4
4
4
4
3
3
80
40
80
20
160
20
40
80
160
20
160
13
14
15
16
17
18
19
20
21
22
23
5.5
5.5
5.5
3.5
3.5
3.5
3.5
1.5
1.5
1.5
1.5
40
80
160
20
40
80
160
20
40
80
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, ENDC, 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}
...
Reference: 3GPP TS 38.133, 38.331, 38.321, 38.912, 38.306, 36.306 and 36.331