Page 115 - Kaleidoscope Academic Conference Proceedings 2020

Industry-driven digital transformation

such as mini-slot. 2) Uplink access uses the no-scheduling 2.1.2 URLLC delay research in Rel-16
permission mechanism, and the terminal can directly access
the channel. 3) Supporting an asynchronous process to save Support for grant free configuration: In the scheduling based
uplink time synchronization overhead. 4) Adopting fast on grant configuration, the UE needs to obtain resources
hybrid automatic repeat request (HARQ) and fast dynamic through the scheduling request. In order to reduce the delay,
scheduling, etc. the resource [23] can be pre-allocated to some UEs according
to the service characteristics.
Based on the above discussion, this section will introduce the
progress of URLLC delay research in Rel-15 in Section 2.1.1. Support for intra-UE priority and multiplex mechanism: In
In order to fully support URLLC services, 3GPP further Rel-15, eMBB DG (dynamic grant) takes precedence over
enhanced [22] on URLLC in the Rel-16 phase, and proposed URLLC CG (configured grant). In order to ensure the delay
the delay reduction scheme described in Section 2.1.2. of a URLLC service, Rel-16 proposes a selection scheme
based on LCP (logical channel prioritization), so as to
2.1.1 URLLC delay research in Rel-15 transmit URLLC services with a higher priority.
Support for a more flexible frame structure: The 5G NR (new Support for TSN and 5G convergence: The following
radio) supports the carrier spacing of 15 KHz in the LTE mechanisms are adopted to realize TSN: 1) Support SPS
system. It also supports more spacing schemes, including 30 (semi-persistent scheduling) with a shorter period. 2)
KHz, 60 KHz, 120 KHz, and 240 KHz. The higher the carrier Support configuring multiple SPS and CG for a bandwidth
spacing, the lower the delay performance. In addition, 5G part (BWP) of the UE. 3) Support TSN services that do not
NR supports frame structure adjustment. Compared with the match the CG/SPS period.
LTE, a fixed subframe includes 2 slots. The NR can flexibly
switch between 1, 2 and 4 slots and flexibly configure the 2.2 High reliability
uplink/downlink ratio, thus reducing the air interface
transmission time of each slot. At present, the reliability index of 5G URLLC is 99.999%
for a 32-byte packet in the user plane delay 1ms. In addition,
Support for more flexible scheduling units: The slot is the if the delay allows, 5G URLLC can also use the
unit of the minimum scheduling period. The LTE includes a retransmission mechanism to further improve the success
slot consisting of 14 symbols. However, the NR supports rate. In terms of improving system reliability, 5G URLLC
mini-slot. Mini-slots support the length of 2 symbol, 3 adopts the following technologies: 1) Adopting a more
symbol, and 4 symbol, and a shorter slot can reduce the robust multi-antenna transmit diversity mechanism. 2)
feedback delay. Adopting robust coding and modulation order to reduce the
bit error rate. 3) Adopting super robust channel state
Support for flexible PDCCH configuration: The search space estimation. Sections 2.2.1 and 2.2.2 show that the reliability
consists of a group of candidate PDCCH (physical downlink of URLLC has been enhanced in Rel-15 and Rel-16 of 3GPP,
control channel), and the search space can be configured with respectively.
parameters such as search type, period, slot offset, number
of slot, CORESET, downlink control information (DCI) 2.2.1 URLLC reliability research in Rel-15
format and so on. By configuring a reasonable monitoring
period and offset of the PDCCH and the monitoring of the Support for PDCP duplication mechanism: The sender
PDCCH in a slot, the PDCCH monitoring opportunity can be replicates the data at the PDCP layer, and then sends the two
achieved densely. The slot has multiple PDCCH monitoring duplications to two independent logical channels for
moments, which can meet the requirements for the burst transmission, so as to achieve the frequency diversity gain
service scenarios of the URLLC and meet the requirements and improve reliability.
for low latency.
Support for optimizing MCS\CQI tables: The MCS
Support for URLLC high-priority transmission: To meet the (modulation and coding scheme)\CQI (channel quality
URLLC service requirement of high priority, 5G/NR indication) of the LTE cannot meet the requirements of NR
proposes that a URLLC service can preempt enhanced for system reliability and transmission rate. Therefore, NR
mobile broadband (eMBB) service resources to reduce the adds two lower bit rates in the CQI table, and the
delay. corresponding gNB adds two MCS low frequency options.
A lower bit rate can be chosen between the UE and the gNB
Introduction of mobile edge computing (MEC): In a 5G to ensure reliability.
network, the user-plane function (UPF) can be deployed on
the user side. The edge computing server and the UPF are Support for less load DCI design: By reducing the DCI
co-located. The UPF recognizes that the destination address overhead and improving the aggregation level (AL), the
of the service flow is local, so it distributes the service to the PDCCH encoding rate is reduced. The decoding error rate is
local edge computing server for service processing, which reduced, and the reliability is improved.
reduces the redundant transmission path of the service and
reduces the delay.

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