Packet Duplication for URLLC in 5G Dual
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Packet Duplication for URLLC in 5G Dual
Connectivity Architecture
JAbstract – This paper addresses the problem of satisfying the
extreme requirements related to Ultra-Reliable Low Latency
Communications (URLLC) in 5G Radio Access Network (RAN).
Complementary to the existing Physical (PHY) layer techniques,
this paper focuses primarily on higher layer solutions,
particularly, on Packet Duplication (PD) as a practical and low
complexity technique for URLLC. The theoretic framework
behind PD is investigated and the recent enhancements made in
the 5G Dual Connectivity (DC) architecture for supporting PD are
discussed. For improving the radio resource utilization and to
dynamically control the activation of PD, an optimization problem
subject to URLLC constraints is formulated and solved
heuristically to give the resource configuration in terms of MCS
and PRB allocation over multiple links. Following this, it is shown
numerically that performing PD in various deployment scenarios
results in better utilization of radio resources compared to using a
single highly reliable link while effectively satisfying the URLLC
requirements.
Keywords- URLLC, Dual Connectivity Architecture, 5G NR
I. INTRODUCTION
Ultra-reliable low-latency communications (URLLC) are
characterized by extreme requirements targeted for supporting use
cases requiring high criticality, resilience and robustness. For
URLLC, the reliability requirement is intertwined with latency and
both performance metrics have to be jointly considered in system
design. This is because transmitting packets with high reliability is
consequential only if the packets are received within the sub-
milisecond latency constraint. The corresponding use cases
evaluated by 3GPP for URLLC include industry automation,
mobile eHealth, interactive augmented reality, drone
communications and connected vehicles [1]. The requirements for
these use cases as defined by 3GPP are a maximum round trip time
of 1ms on the user-plane (UP) and transmission reliability of 1-10-5
for a packet size of 32 bytes [2]. For more advanced URLLC use
cases the latency and reliability requirements can range between
0.5ms to 10ms and 1-10-5 to 1-10-9, respectively for packet sizes of
up to 300 bytes.
The existing PHY layer techniques in LTE, designed primarily
for providing high spectral efficiency, cannot be straightforwardly
extended to support the typical URLLC requirements in 5G New
Radio (NR). As an illustration, to achieve a block error rate (BLER)
target of 1-10-5 on a single link, it is necessary to have highly
favorable channel conditions at all times, use a low and robust
modulation and coding scheme (MCS) index (e.g. QPSK with 1/3
coding rate) and a bandwidth allocation of at least 20MHz [3]. For
higher reliability requirements, it is necessary to use either high
diversity orders (up to 16) or significantly increase the signal-to-
noise (SNR) and bandwidth allocation [4]. The implementation of
other PHY layer solutions based on link adaptation [5] and link
combining [6] reveal certain improvement in terms of reliability but
not without trading off major increase in functional complexity.
Also, such schemes can be applied only in certain highly restrictive
scenarios (e.g. cell center, low load, very low mobility), thus
limiting their usage to support only a moderate number URLLC-
capable devices in most deployment scenarios.
In legacy LTE RAN, the reliability requirements are
conventionally satisfied at the Radio Link Control (RLC) and
Medium Access Control (MAC) layers using the Automatic Repeat
Request (ARQ) and Hybrid ARQ (HARQ) retransmission
techniques. Although these techniques enable certain reliability
level to be achieved, the resulting latency due to packet
retransmissions exceed the sub-milisecond latency requirements for
most of URLLC use cases. Moreover, incorporating enhancements
to these techniques come at the expense of architectural
modification, increase in resource usage and in standardization
effort. As such, it is necessary to consider a fundamentally new
technique that is not only effective for URLLC but can also be
practically implemented with low complexity.
Considering the different architectural options available in 5G,
a promising solution to address the extreme requirements is packet
duplication (PD). Particularly, the PD technique can be directly
applied in the DC architecture [7] without excessively increasing
the complexity in the RAN. The fundamental principle underlying
PD involves generating multiple instances of a packet at higher
layers and transmitting the packets simultaneously over different
uncorrelated channels or transmission links [8]. At the receiver, the
redundancy and diversity in the channel conditions is exploited
such that higher transmission reliability is achieved.
The PD technique has been recently adopted by 3GPP for
satisfying the reliability and latency requirements in 5G. The
corresponding standardization effort to incorporate PD in the NR
RAN protocol stack is currently underway [9]. In light of the recent
progress, this paper provides an overview of the URLLC related
standardization activities in NR RAN. Particularly, more focus is
given towards the solutions and the enhancements made at the
higher layers (i.e. above PHY) and in the RAN architecture for
supporting the PD technique. In this regard, the system model and
the DC architecture enhancements for PD are discussed in Sections
II and III. This is followed by techniques and system design
considering fast triggering of PD via dynamic control in Section IV.
Finally, the PD related performance evaluations as well as the
conclusions are provided in Sections V and VI, respectively.
II. SYSTEM MODEL FOR PACKET DUPLICATON
At the fundamental level, the reliability of a wireless system can be
increased by transmitting the same packet over multiple redundant
links, each experiencing a different channel condition [10]. To
2018 IEEE Wireless Communications and Networking Conference (WCNC)
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realize this in practical network implementations, consider a RAN
architecture consisting of ܰ transmission links, each utilizing a
non-overlapping frequency carrier for transmitting simultaneously
to the UE (User Equipment). In this case, the overall system
reliability R can be determined as:
ܴ = 1 −ෑ(1 − ܴ)
ே
ୀଵ
(1)
where ܴ is the success probability over link ݅ ∈ ܰ. Assuming ܮ is
the overall latency, ܴܵܰ is the SNR achievable on link ݅ and Φ is
the bandwidth allocation over link ݅, then ܴ is defined as:
ܴ = ܲݎ(ܮ ≤ ߣ்)ܲݎ൫ܴܵܰ > ்ܴܵܰ |݉൯ܲݎ(Φ > ݂(ܶܤܵ)|݉) (2)
where i) ߣ் is the latency requirement for URLLC, ii) ்ܴܵܰ =
݂(ܤܮܧ்ܴ ) is the SNR threshold for achieving a BLER target
ܤܮܧ்ܴ on link ݅ when using MCS index ݉ ∈ ࡹ (from the
available MCS set M) and iii) ܶܤܵ is the transport block size of
the URLLC packet when using MCS index ݉ . Note that ܮ
includes the latency due to processing, propagation and
transmission between the transmitting and receiving Packet Data
Convergence Protocol (PDCP) entities in the RAN (i.e. UE and
access node).
From Eq. (1) and (2), clearly to increase the overall reliability
ܴ it is necessary to increase either the reliability of each link ܴ or
the number of links ܰ carrying the same packet. The
straightforward approach to improve the robustness of each link
against channel effects, and consequently ܴ , is to increase the
transmit power and allocate more radio resources over each link.
Both of these techniques, however, may adversely affect the
spectral efficiency, power efficiency and interference, hence are
not applicable for NR system design. On the other hand, to satisfy
the latency requirements in URLLC, the packets transmitted in
each link have to be received within the latency deadline of less
than 1ms. To this end, the use of PD where the packets are
proactively transmitted simultaneously, addresses both the latency
and reliability requirements without having to rely on feedback
and retransmissions as done in ARQ and HARQ in LTE.
At first glance, duplication may imply potential loss in
throughput and spectral efficiency. However, at closer inspection it
becomes clear that exploiting the diversity from using multiple
links provides the means to achieve high reliability on a statistical
basis without actually expending more resources. More precisely,
targeting a lower BLER value on a particular link and,
correspondingly, using a higher MCS index on that link makes it
possible to minimize the radio resources usage on that link.
Consequently, when multiple links are configured to support PD, it
is possible to minimize the total amount of resources required over
all links to be less than that of using a single highly reliable link.
Such techniques can be applied in practical systems in NR RAN to
meet the URLLC requirements and to balance the reliability-
resource usage trade off.
III. ARCHITECTURE ENHAMCEMENTS IN 5G NR RAN
A. NR RAN Protocol Stack Enhancements for PD
The NR RAN protocol stack in the user plane (UP) is collectively
responsible for ensuring reliable over-the-air transmission of
protocol data units (PDUs) in both uplink (UL) and downlink (DL)
directions [9]. The RRC entity, which is the primary control plane
(CP) function in RAN, is responsible for configuring the protocol
layers in both the network and the UE. The RRC is also responsible
for establishing, maintaining and releasing of the radio bearers
between the network and the UE. The radio bearers in NR are
categorized into two types namely, the data radio bearers (DRBs)
and signaling radio bearers (SRBs), both of which are used for
transmission of UP and CP packets, respectively. The DRBs are
generally configured to satisfy a set of Quality of Service (QoS)
requirements which include a guaranteed bit rate and priority level.
In comparison to DRBs, SRBs are characterized by less frequent
transmissions, smaller PDU sizes and higher scheduling priority.
In NR, a new Service Data Adaptation (SDAP) layer is
introduced for performing the mapping between the QoS flows and
DRBs. This is to ensure that the QoS flows, which originate and
terminate in core network (CN) are handled appropriately in the
RAN with the right priority treatment and resource provisioning.
Next, sequence numbering, header compression and ciphering
operations are performed in the NR PDCP to ensure in-order and
secure delivery of both the UP and CP packets. To enhance
transmission reliability, a new duplication function is incorporated
in PDCP whose role is to make duplicates of the PDUs associated
with a set of DRBs and SRBs configured by RRC. Also, each
instance of the duplicate PDU carries the same PDCP sequence
number in order to facilitate the receiving PDCP entity to detect and
remove the duplicates.
This is followed by the RLC layer which is responsible for PDU
segmentation and handling of different transmission modes which
include the Acknowledged Mode (AM) and Un-acknowledged
Mode (UM). When PD is configured the original and duplicated
PDUs are handled by two RLC entities, each correspondingly
assigned to a unique logical channel. While both RLC transmission
modes are supported with PD in NR, for URLLC the RLC operates
in the UM mode where the reception status of the RLC PDUs does
not require to be acknowledged to the transmitter to further
minimize the latency.