MTC enhancements for LTE - Rel 16
Release 16 adds on the LTE features for Machine-Type Communications (MTC) introduced in earlier releases (e.g. low-complexity UE categories M1 and M2, and Coverage Enhancement Modes A and B) by further improving network operation and efficiency in a range of areas.
All New features are optional for the UE and can be supported by Cat-M1 and Cat-M2 and by normal LTE UEs supporting CE mode unless explicitly specified. All features are applicable to both CE modes (A and B) in all duplex modes (HD-FDD, FD-FDD, and TDD) unless otherwise stated.
Improved DL transmission efficiency and UE
power consumption
Reduced UE power consumption is achieved
through reduced downlink monitoring and reduced signalling, building on
features introduced in earlier releases.
• UE-group
wake-up signals (GWUS): Reduced UE power consumption in idle mode was enabled
in Rel-15 by the introduction of the wake-up signal (WUS), a compact signal
transmitted a configurable time before the paging occasion (PO) when a UE is
being paged, allowing the UE to maximize its sleep time during periods when
there is no paging. In Rel-16, an enhancement is introduced that allows a WUS
to wake up a configurable group of UEs rather than all UEs that happen to
monitor the same PO. This helps reduce the power consumption even further. The
mapping of GWUS in the time and frequency domains is highly configurable.
• Mobile-terminated
early data transmission (MT-EDT): For scenarios where the UE only needs to
transmit a small amount of data, the early data transmission (EDT) feature in
Rel-15 enables the UE to transmit up to (slightly more than) 100 bytes of data
already in Msg3 during the random-access procedure, and to receive data already
in Msg4. If needed, eNB can order fallback to legacy random-access procedure
during the EDT procedure. In Rel-16, an enhancement is introduced that allows
not only mobile-originated (MO) EDT access but also mobile-terminated (MT) EDT.
When the MME triggers MT-EDT, an indication is included in the paging message,
after which the UE triggers random access to resume the connection (in case the
UP CIoT EPS optimization is used) or initiate MO-EDT (in case the CP CIoT EPS
optimization is used). MT traffic is received in Msg4. MT-EDT is only supported
when UE is connected to EPC (not 5GC).
• Improved
DL quality reporting: Legacy CE mode A supports both periodic and aperiodic CSI
reporting which can be used to assist PDSCH link adaptation. In Rel-16, a new
type of DL quality reporting is introduced which reflects MPDCCH quality rather
than PDSCH quality. The report represents the required number of MPDCCH
subframe repetitions for reliable MPDCCH reception. It can be sent in connected
mode, but it can also be sent already in Msg3 during the random access
procedure, which means that the report can be used for guiding the UE-specific
MPDCCH configuration, which helps optimize power consumption, latency, and
spectral efficiency.
• MPDCCH performance improvement: In legacy LTE-MTC, MPDCCH demodulation is DMRS-based. With this feature, the UE can use a combination of DMRS and CRS for MPDCCH demodulation to improve the MPDCCH performance. The feature takes the configured DMRS-to-CRS power ratio into account. The feature can be used for transmissions in idle mode and/or connected mode. In idle mode, the DMRS-to-CRS mapping is based on precoder cycling, whereas in connected mode, it can be configured to be precoder cycling based, CSI-based, or (in case of TDD) reciprocity-based.
Preconfigured uplink resources (PUR)
In Rel-15, signalling overhead and power consumption
reductions were introduced by the (mobile-originated) early data transmission
(EDT) feature, where data can be transmitted already in Msg3 during the
random-access procedure.
In Rel-16, the earlier transmission of UL
data payload has been further enhanced by introducing UL transmission using
preconfigured uplink resources (PUR). When the feature is configured, both the
random-access preamble transmission (Msg1) and the random-access response
(Msg2) can be omitted, and the data transmission can be completed in only two
messages (i.e., Msg3 and Msg4).
The UE is configured with PUR via dedicated
RRC signaling while in connected mode. Configuring a UE with PUR can be
triggered by the network or requested by the UE. Before performing a PUR
transmission, the UE must evaluate the validity of the timing advance (TA)
based on either individual or combined usage of any of the following
attributes: a) serving cell change, b) TA timer, c) RSRP change. Additionally,
it is possible to configure the TA as always valid within a given cell.
There are two schemes for transmitting using
PUR, dedicated PUR and shared PUR, the latter allows up to two users to
transmit simultaneously when the number of PUSCH repetitions is greater than or
equal to 64 for full-PRB allocation.
Scheduling of multiple transport blocks
In legacy LTE-MTC operation, each DCI
carried by MPDCCH schedules a single PDSCH or PUSCH transport block (TB). In
Rel-16, a possibility to schedule multiple TBs using a single is introduced.
This can help improve the resource utilization by reducing the number of
physical resource blocks (PRBs) spent on MPDCCH transmission and the number of
subframes spent on guard time for DL-to-UL and UL-to-DL transition (in
half-duplex FDD operation).
• Unicast
multi-TB scheduling: When the feature is configured, a single DCI can schedule
multi TBs for PDSCH or PUSCH (up to 8 TBs in CE mode A, or up to 4 TBs in CE
mode B). The number of TBs is dynamically controlled by the DCI. The TBs can be
configured to be transmitted consecutively or subframe interleaved (in case of
subframe repetition). For PDSCH multi-TB scheduling, HARQ-ACK bundling can
optionally be used to improve the resource utilization further for UEs in good
coverage. For PUSCH multi-TB scheduling, early termination of the PUSCH
transmission is supported through indication of positive HARQ-ACK in the DCI.
• Multicast
multi-TB scheduling: When the feature is configured a single DCI can schedule
up to 8 TBs for PDSCH for a SC-MTCH, with configurable time gaps between the
TBs if desired. The number of TBs is dynamically controlled by the DCI.
CE mode improvements
for non-Cat-M UEs
The features in this work item can be
supported both by Cat-M UEs and non-Cat-M UEs that support CE mode A or B. In
addition, the following features have been specified specifically for non-Cat-M
UEs that support CE mode A or B.
• Enhancements
to idle mode mobility: A possibility is introduced for a non-Cat-M UE in a
non-standalone LTE-MTC cell to use enhanced coverage functionality to camp in
the cell even if the S-criterion indicates that the UE is in normal coverage.
This functionality is enabled/disabled by a configuration provided in SIB1.
(This is the default behavior for the standalone LTE-MTC case described in the
next section in this document.)
• CSI
feedback based on CSI-RS: In legacy CE mode A, periodic and aperiodic CSI
feedback is based on up to 4 CRS antenna ports. This feature introduces support
for periodic CSI feedback based on 8 CSI-RS antenna ports in TM9 for non-Cat-M
UEs in CE mode A. The feature can help improve the DL link adaptation and hence
the DL performance. As a separate UE capability, the feature can also
optionally be supported in combination with codebook subset restriction.
• ETWS/CMAS
in connected mode: In legacy LTE-MTC, ETWS/CMAS notification indication is
supported using DCI format 6-2 in MPDCCH common search space Type-1 in idle
mode. This feature introduces ETWS/CMAS notification indication using DCI
format 6-1A/B in MPDCCH common search space Type-0 in connected mode for
non-Cat-M UEs in CE mode A/B. This means that a UE can be notified without
releasing the UE to idle mode.
Stand-alone deployment
In legacy LTE-MTC operation, the first few
OFDM symbols in each DL subframe are unused by LTE-MTC since they are assumed
to be occupied by LTE control channels for normal LTE UEs (PCFICH, PDCCH,
PHICH). This feature enables transmission of MPDCCH and/or PDSCH to UEs in CE
mode A/B in the “LTE control channel region” on carriers that are not used for
normal LTE. The feature can be used for transmissions in idle mode and/or
connected mode. The potential DL transmission efficiency gain is about 14%
(corresponding to 2 out of 14 OFDM symbols) for 1.4 MHz carriers and about 7%
(corresponding to 1 out of 14 OFDM symbols) for wider carriers.
Mobility enhancements
In Rel-15, two new LTE-MTC signals were
introduced, the resynchronization signal (RSS) and the wake-up signal (WUS),
and in Rel-16 the following mobility enhancements are introduced which make use
of the Rel-15 signals.
• RSS-based
measurements: In Rel-15, support for a resynchronization signal (RSS) was
introduced and its configuration is provided by the serving cell. In Rel-16,
signaling of RSS configurations for neighbor cells is introduced. Both broadcasted
and dedicated signaling can be used to provide the configurations. The primary
purpose of RSS is to improved synchronization performance, but with the Rel-16
signaling, the UE may also use RSS for improved measurement performance for
intra-frequency RSRP measurements for neighbor cells in both idle and connected
mode.
• RRM
measurement relaxation: The legacy LTE-MTC UE behavior requires the UE to
measure on the serving cell and evaluate the cell selection criterion at least
every DRX cycle. The wake-up signal (WUS) introduced in Rel-15 would allow the
UE to sleep for multiple paging cycles and wake up to receive paging after a
configurable time duration, but the UE power saving gain from WUS cannot be
fully utilized since the UE is still required to wake up for measurements.
Therefore, an RRM measurement relaxation is introduced in Rel-16, which allows
the UE meet the requirements using a longer measurement cycle to save power,
where the cycle is configurable under certain conditions.
Performance improvement for NR coexistence
Spectrum sharing with legacy (Rel-13/14/15)
LTE-MTC is already supported in Rel-15 NR, and the RF coexistence aspects
described in TR 37.823. The following features are introduced in Rel-16 LTE-MTC
in order to further improve the performance of the coexistence with NR.
• DL/UL
resource reservation: Legacy LTE-MTC supports configuration of invalid DL/UL
subframes, which can be used in order to avoid mapping LTE-MTC transmissions to
subframes that are needed for NR transmissions. Rel-16 takes a step further by
introducing finer-granularity LTE-MTC resource reservation in both the time
domain (with subframe, slot, or symbol level granularity) and the frequency
domain (with LTE RBG level granularity) for unicast MPDCCH/PDSCH/PUSCH/PUCCH
transmissions in connected mode in CE mode A/B. The resource reservation
patterns are configurable using parameter combinations based on bitmaps,
periodicities and offsets. For PDSCH/PUSCH, the DCI can indicate that the
resource reservation should be overridden, in which case the PDSCH/PUSCH
transmission becomes continuous.
• DL
subcarrier puncturing: In order to achieve PRB alignment between LTE-MTC and
NR, a possibility to puncture 1 or 2 DL subcarriers at the lower or higher edge
of each 6-PRB narrowband is introduced. The puncturing affects MPDCCH/PDSCH
transmissions in connected mode in CE mode A/B. The performance loss from the
puncturing should typically be insignificant.
Connection to 5GC
In Rel-16, support for connecting LTE-MTC UEs to 5GC is introduced. It resembles the Rel-15 functionality for connecting LTE UEs to 5GC. The RRC_INACTIVE state is supported and additionally the User Plane CIoT 5GS optimisation is supported in RRC_IDLE (similar to the corresponding EPC feature). Some features, such as EDT and PUR are supported only in RRC_IDLE using the UP-optimisation solution and are not supported in RRC_INACTIVE. Long extended DRX in RRC_IDLE is supported, and RAN paging cycles of 5.12 s and 10.24 s are supported in RRC_INACTIVE.