Page 116 - Kaleidoscope Academic Conference Proceedings 2020

2020 ITU Kaleidoscope Academic Conference

2.2.2 URLLC reliability research in Rel-16 eliminate jitter in the TSN. DS-TT and NW-TT support a
hold and forward mechanism to schedule traffic as defined
Support for multi-TRP transmission mode: Rel-16 proposes in IEEE 802.1Qbv [25], if 5GS is to participate transparently
that transmission blocks can be transmitted repeatedly based as a bridge in a TSN network. The hold and forward
on space division, frequency division, intra-slot division and buffering mechanism allows PDB (packet delay budget)
inter-slot division on the basis of Rel-15. In order to improve based 5GS QoS to be used for TSC traffic since packets need
the diversity gain, it also supports the combination of the only arrive at NW-TT or DS-TT egress prior to their
above modes and the dynamic handover between different scheduled transmission time. How hold and forward buffer
modes (including combined modes). is supported by the TSN translator is up to implementation.

Support for PDCP duplication enhancement mechanism: In addition, time synchronization precision is defined
Rel-15 supports two-branch PDCP duplication, in order to between a sync master and a sync device. The detailed
achieve higher reliability. Rel-16 supports up to four-branch objectives for NR TSC-related enhancements include:
PDCP duplication. This mechanism can be implemented Specify accurate reference timing delivery from gNB to the
through CA (carrier aggregation) duplication, DC (dual UE using broadcast and unicast RRC signaling for
connectivity) duplication and the combination of CA synchronization requirements defined in [26]. To meet the
duplication and DC duplication. high-precision time synchronization requirements of the
TSN, a high-precision reference time transmission
Support for redundant transmission scheme: NG-RAN mechanism is introduced to NR. Broadcast messages (SIB9)
duplicates uplink packets and sends them to the UPF via two or dedicated RRC messages (DLInformationTransfer
redundant link (N3 interface) channels, where each N3 messages) with the high-precision time can be sent. The time
channel is associated with a PDU session, and two granularity is enhanced from 10 ms to 10 ns. According to
independent N3 channels are established to transmit data. the simulation result of RAN1, RAN2 assumes that delay
The gNB, SMF and UPF will provide different routes for the compensation is required in the scenario where the service
two links [24]. range is greater than 200m for the user with the subcarrier
interval of 15 kHz. However, in Rel-16, RAN1 only provides
2.3 Little jitter transmission delay compensation for the base station and UE
in the TDD and FDD scenarios according to half of the
Time requirements are typically specified with two values: timing advance, that is, NTA×Tc/2. In addition, although
characteristic time and jitter. Characteristic time is the target RAN1 discussed a lot about when and how to implement the
value of the time parameter in question, e.g. end-to-end transmission delay supplement, it did not make a conclusion
latency. The jitter is the variation of a (characteristic) time in the Rel-16 phase.
parameter, and the maximum deviation of a time parameter
relative to a reference or target value. 2.4 High transmission efficiency

As depicted in [19], power distribution poses the jitter In the field of IIoT, the small packets of TSN are transmitted
requirements and the traffic pattern is deterministic as well. frequently in the ordinary communication network. TSN is
In such a case, the maximum value of the characteristic time also introduced into the 5G system, which has the
parameter needs to be known. Sometimes, a minimum value characteristics of small packets with frequent transmission,
may also be given, and should not be undershot. A minimum low latency and high reliability. In which case, reducing the
value is only used in particular use cases; for instance, when packet overhead can effectively improve the effective
putting labels at a specific location on moving objects. In utilization of system bandwidth. Therefore, for TSN packets,
R15, a common understanding in RAN is that a delay- header compression can be used to further reduce the size of
sensitive URLLC service with periodic traffic can be data packets, thus saving the wireless resources used by a
accommodated by the semi-persistent CG. That means the single packet and improve the utilization of wireless
periodicity of the traffic should be a prerequisite in RAN to resources. The data stream transmitted by TSN is mainly an
meet the data size and jitter requirements. An example is the Ethernet data packet, so the Ethernet header compression
variation of the end-to-end latency. If not stated otherwise, (EHC) is introduced to reduce the overhead caused by
jitter specifies the symmetric value range around the target Ethernet header transmission.
value (target value ± jitter/2). If the actual time value is
outside this interval, the transmission was not successful. [19] EHC may be particularly beneficial when the payload size of
shows an example of transmissions with jitter. Note that the an Ethernet frame is small relative to the overall size of the
end-to-end latency may scatter even for successful frame, which is typical in Ethernet-based IIoT network. The
transmissions. EHC protocol compresses Ethernet header as shown in
Figure 2. The fields that are compressed by the EHC protocol
As an important feature of TSN, jitter requirement is to are: DESTINATION ADDRESS, SOURCE ADDRESS,
provide a deterministic service with bounded delay. Typical 802.1Q TAG, and LENGTH/TYPE. The fields PREAMBLE,
characteristic parameters to which jitter values are ascribed SFD, and FCS are not transmitted in a 3GPP system, and thus
are transfer interval, end-to-end latency, and update time. not considered in an EHC protocol. There may be more than
Further, the buffering mechanism is held and forwarded to one 802.1Q TAG fields in the Ethernet header, and all are

– 58 –