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JOURNAL OF COMMUNICATIONS AND NETWORKS, VOL. 26, NO. 2, APRIL 2024 207
Towards 5G-Advanced NR-Unlicensed Systems:
Physical Layer Design and Performance
Julius Ssimbwa, Seok-Hyun Yoon, Yeongrok Lee, and Young-Chai Ko
Abstract—In the pursuit of a highly reliable and low-latency-
enabled 5G-advanced new radio unlicensed (NR-U) system,
addressing the challenge of high error rates and high signal-
ing overhead transmissions remains key to improving network
performance. In this context, to reduce error rates, mechanisms
such as retransmissions can be employed. However, performing
multiple retransmissions comes at the cost of utilizing extra trans-
mission resources, which in turn affects the spectral efficiency of
the network. This would further necessitate proper scheduling
to alleviate resource wastage and undesirable collisions during
data transmission. In this article, we provide an overview of the
design specifications of the long-term evolution-license assisted
access (LTE-LAA) technology and the prospective enhancements
to enable NR-U operation in bands beyond 7 GHz. Additionally,
we examine the configurations of selected design features to
enable NR-U scheduling. Specifically, we illustrate the benefits
and the limitations of the choice of the switching pattern
under the frame structure, the feedback value type under the
hybrid automatic repeat request (HARQ) procedure, and the
timing parameters under the radio link control (RLC) layer.
Besides, we present simulation results to depict the impact of the
configurations mentioned above on the performance of NR-U.
Index Terms—Coexistence, high reliability and low-latency,
new radio unlicensed, NR-U scheduling, unlicensed band.
I. INTRODUCTION
WITH the expeditious development of powerful data
processing-capable handheld mobile devices, network
operators have been compelled to scramble for the unli-
censed spectrum to support the resulting data traffic demands.
Fortunately, the federal communications commission (FCC)
has availed more bandwidth in the unlicensed spectrum to
accommodate more radio access technologies (RATs) such
as 5G new radio unlicensed (NR-U), thereby presenting an
opportunistic platform for offloading traffic to extend capacity
and improve coverage [1]–[4]. In this article, attention is
directed towards 5G NR-U, whose features were developed
with inspiration from the 5G technology [5]. The benefit of
operating in the unlicensed band lies in the increased data rates
and capacity in the presence of wider bandwidth. However, the
Manuscript received January 6, 2024; approved for publication January 9,
2024. This paper is specially handled by EIC and Division Editor with the
help of three anonymous reviewers in a fast manner.
This work was supported by the Institute of Information and Commu-
nications Technology Planning and Evaluation (IITP) grant funded by the
Korean government(MSIT) (2022-0-00704, Development of 3D-NET Core
Technology for High-Mobility Vehicular Service).
Y.-C. Ko is the corresponding author.
The authors are with the School of Electrical Engineering, Korea Uni-
versity, Seoul 02841, South Korea, email: {kayjulio, daniel419, spiritica,
koyc}@korea.ac.kr.
Digital Object Identifier: 10.23919/JCN.2024.000002
coexistence among RATs presents a greater concern due to the
disparity in their design specifications. Moreover, unregulated
operational procedures could result in the wastage of resources
and undesirable interference leading to the degradation of
system performance. Furthermore, operating in the unlicensed
spectrum makes the network user vulnerable to experiencing
services of unreliable quality with high latency and low relia-
bility [6]. It is therefore beneficial to investigate the spectrum
usage and network product design to ensure an amicable co-
existence environment among unlicensed spectrum-occupying
RATs and improve the quality of service (QoS).
A. Related Works
Related works by both industry and academia indicate that
various investigations, in the form of analytical modeling
and experiments, are in progress to attain efficient spectrum-
sharing techniques, spearheaded by several bodies such as
3GPP and IEEE 802.11 [7], [8]. In [9], the authors discuss
challenges and potential solutions and perform simulation
evaluations for the channel access procedure based on beam
transmissions to ensure the coexistence of NR-U systems
among different RATs in millimeter wave (mmWave) bands.
A survey discussing opportunities and challenges in designing
NR-U features for the coexistence of Wi-Fi and 5G NR-U in
the 6 GHz bands is presented in [10]. The authors in [11]
study the coexistence between Wi-Fi and 5G NR-U systems
by conducting simulations and evaluating results in terms of
delay and throughput. In [12], the authors propose a scheme
to cope with continuous channel access failures for uplink
control transmissions and improve the latency and reliability of
cellular networks in unlicensed spectrum. The authors in [13]
build an NR-U system-level simulator to evaluate several
aspects such as numerology and channel access procedure,
for the coexistence of NR-U and IEEE 802.11 technologies
in mmWave bands. References [14] and [15] suggest resource
management schemes related to time, bandwidth, and power
allocation to ensure the harmonious coexistence of NR-U
systems with the incumbent Wi-Fi systems.
B. Motivation and Contributions
While several authors provide different perceptions on im-
proving spectrum usage as shown by the extensive studies on
NR and long-term evolution (LTE) operation in the unlicensed
bands, more explorations on advanced NR-U systems for
operation in bands beyond 7 GHz will be required. In this
article, motivated by the inter-RAT coexistence concerns in
conjunction with the scarcity of spectrum resources resulting
1229-2370/24/$10.00 © 2024 KICS
208 JOURNAL OF COMMUNICATIONS AND NETWORKS, VOL. 26, NO. 2, APRIL 2024
from the demand for massive connectivity and high data
traffic, we study the performance of NR-U under varying
configurations of the design features. Moreover, the disparity
in the design of NR-U and other RATs, say Wi-Fi, renders
it more aggressive when contending for the channel with Wi-
Fi, resulting in deterioration of Wi-Fi performance. Besides,
to guarantee fairness in channel access, it is recommended
that the impact of NR-U on an existing Wi-Fi system should
not be more than the impact of another Wi-Fi system on the
existing Wi-Fi system [16]. We also derive motivation from
the need to achieve high reliability and low latency in the
next generation of NR-U systems. It is worth mentioning that
contained herein is the current and ongoing NR-U work. We
summarize our contributions in this article as follows.
•We briefly discuss various transmission protocols of
long-term evolution-license-assisted access (LTE-LAA)
and NR-U. Particularly, we compare the physical layer
design of LTE-LAA and NR-U, including channel access,
signal detection, and others. Furthermore, we review the
operating features of NR-U for frequency bands up to 7
GHz and above, respectively.
•We examine the configurations of selected design features
to enable NR-U scheduling. We share design rationales
comprising the frame structure, the hybrid automatic
repeat request (HARQ) procedure, and the radio link
control (RLC) layer.
•We present simulation results to provide a comprehensive
understanding of the benefits and limitations of different
configurations associated with the switching patterns,
HARQ feedback value type, and the RLC layer timing
parameters, on the performance of NR-U. The results
indicate that an appropriate configuration can improve
performance in terms of high reliability and low latency.
The remainder of this article is organized as follows. In
Section II, we provide a brief background about LTE-LAA
and NR-U and their corresponding features. Next, we present
investigations on the configurations for selected NR-U design
features in Section III, backed up by a couple of simulations in
Section IV. We then conclude this article in the final section.
II. LTE-LAA AND NR-U OVERVIE W
A. LTE-LAA Key Functional Features
The legacy LTE-LAA was designed to operate in four
main deployment scenarios based on carrier aggregation under
licensed and unlicensed bands. Carrier aggregation involves
communication between the user equipment (UE) and a single
cell, say primary cell (Pcell), through two or more contiguous
or non-contiguous component carriers. The defined deploy-
ment scenarios may include scenario 1, which consists of
carrier aggregation between a licensed macro cell and an
unlicensed small cell, scenario 2, characterized by carrier
aggregation between a licensed small cell and an unlicensed
small cell without macro cell coverage, among others [3].
The operation of LTE-LAA was originally supported as a
supplemental downlink (DL) secondary cell (Scell) only, aided
by a licensed Pcell through carrier aggregation. Nonetheless,
the enhancements in LTE-LAA, say enhanced-LAA (eLAA)
and further enhanced-LAA (feLAA) in Rel. 14 and 15, paved
the way to supporting uplink (UL) transmissions as well. We
highlight some of the peculiar features of a generic LTE-LAA
system as follows.
•Channel access scheme: Before accessing the chan-
nel, LTE-LAA-enabled systems are required to perform
Listen-before-talk (LBT), a process that involves a clear
channel assessment (CCA) to determine whether it is
busy or idle. Transmission can only be guaranteed if
the channel is idle, otherwise, several CCAs are per-
formed depending on the specified type of LBT cate-
gory (CATx-LBT) [3], ranging from CAT1-LBT, in which
no LBT is performed, to CAT4-LBT where LBT is done
alongside random back-off with a variable contention
window (CW). In other categories, LBT is done with
random back-off with a fixed CW size or without random
back-off.
•Detection of LTE-LAA and other RATs’ signals:
During spectrum sensing, the device uses a mechanism
called energy detection (ED), which involves measuring
the energy in the detected wireless signal and comparing
it with a predetermined ED threshold, say -72dBm. If
the measured energy is less than the ED threshold, the
channel is considered idle, otherwise, transmission cannot
take place since the channel is busy. This prompts the
device to execute CCA for an extended period of time
while comparing the detected energy level with the ED
threshold until channel access is obtained.
•Channel occupancy structure: Once the channel access
is secured, the device can only transmit for a specified
period known as channel occupancy time (COT), which
depends on the channel access priority class (CAPC). For
instance, the maximum channel occupancy time (MCOT)
of devices categorized under CAPC 3 or 4 is specified as
8 ms. Nonetheless, when the absence of any other RAT
sharing the carrier on a long-term basis is guaranteed, it
is specified as 10 ms [17].
B. NR-U Key Operating Features for Bands up to 7 GHz
Contrary to LTE-LAA whose deployment was done in
the 5 GHz (5150–5925 MHz) unlicensed spectrum, NR-U
considers roll-out in 5 GHz and beyond, including bands up to
7 GHz and 60 GHz [3], [4]. In addition to carrier aggregation,
NR-U supports dual connectivity and standalone modes. Dual
connectivity constitutes a UE communicating with several
Pcells and Scells. The standalone application allows operation
in the unlicensed band only, without support from the licensed
network. The five NR-U deployment scenarios are outlined
below.
•Scenario A: It is characterized by carrier aggregation
between a licensed band NR Pcell and an unlicensed NR-
U Scell.
•Scenario B: It involves dual connectivity between a
licensed band LTE Pcell and an unlicensed NR-U PScell.
•Scenario C: It is standalone.
SSIMBWA et al.: TOWARDS 5G-ADVANCED NR-UNLICENSED SYSTEMS ... 209
•Scenario D: It constitutes an NR Cell with UL in the
licensed band and DL in the unlicensed band.
•Scenario E: It involves dual connectivity between a
licensed band NR Pcell and an NR-U PScell.
We concisely summarize some of the design features of
NR-U as follows.
•Channel access and detection of wireless signals: The
conventional LBT procedure, including the CW adjust-
ment, and the ED-based mechanism in LTE-LAA will
be adopted for carrier sensing in NR-U. Besides, gNB
or UE can utilize CAT4-LBT to initiate COT for usual
data transmissions. On the contrary, gNB can deploy
CAT2-LBT to perform transmissions for the discovery
reference signal (DRS) to enable UE to detect an active
channel. Despite ED being an easy scheme to implement,
several system designers have shown interest in the use
of preamble detection (PD) or hybrid (ED and PD) signal
detection for advanced NR-U systems to enhance relia-
bility, power efficiency, and so forth [4]. PD involves the
transmission and the detection of a well-known preamble
signal, whose energy is compared with a pre-determined
PD threshold. If the energy level is less compared to the
PD threshold, the channel is idle, and the reverse is true.
Before adopting PD or hybrid ED and PD, the issues
concerning reliability, detection/decoding complexity, and
power consumption need attention.
•Random access channel (RACH) procedure: Unlike
the legacy LTE-LAA which supports a contention-free
RACH procedure [3], NR-U targets both the contention-
free and contention-based random-access schemes. For
the contention-free RACH, gNB controls the access by
assigning the dedicated preambles to each UE, while in
the case of contention-based RACH, each UE randomly
chooses a preamble to establish a link with gNB. Owing
to the possibility that several devices can select the same
preamble, contention resolution is required. Therefore,
instead of a 4-step messaging (Msg) RACH, which can
elevate the delay even more alongside accumulated LBT
failures, leading to reduced transmission opportunities, a
2-step RACH has been identified as a potential remedy to
minimize random access delay and overhead [4]. Simply
put, the conventional 4-RACH procedure, constituting the
random access preamble (Msg1), random access response
(Msg2), scheduled transmission (Msg3), and contention
resolution (Msg4), is compressed to the 2-step approach
made up of combined Msg1 and Msg3, and combined
Msg2 and Msg4.
•Scalable subcarrier spacing (SCS) and wideband op-
eration: LTE-LAA operation is specified under 20 MHz
bandwidth whereas NR-U targets a much wider band-
width with flexible SCS options to allow the efficient use
of resources with lower control overhead. The alternatives
of SCS such as 15 kHz and 30 kHz, will enable support
for operation in bands below 6 GHz while greater SCS
options such as 60 kHz will be key for operation in higher
frequency bands. Besides, by supporting several SCS
options and carrier aggregation, a wider bandwidth can
be achieved through leveraging the bandwidth part (BWP)
concept. This BWP plays a big role in determining the
bandwidth over which a given device is assumed to
receive the transmissions of a certain numerology [18].
Having a wider bandwidth, say 80 MHz and 100 MHz,
will be essential in achieving the overly desired higher
data rates. Moreover, the bandwidth will be split into
chunks of 20 MHz (LBT bandwidth) each to support the
channel access especially when the absence of any other
RAT such as Wi-Fi cannot be guaranteed.
C. NR-U Key Operating Features for Bands Beyond 7 GHz
To achieve superior performance in the prevailing NR-U
standard, having abundant spectrum resources remains a criti-
cal prerequisite. Therefore, 3GPP has considered expanding
NR-U operation to higher frequency bands. According to
NR-Rel. 15, two operating frequency regimes (FR), namely,
FR1 (410 MHz to 7.125 GHz) and FR2 (24.25 GHz to
52.6 GHz) had been addressed. In Rel. 17 [19], additional
frequency bands in the 52.6 GHz to 71 GHz range (60 GHz
band) were found to be available to support the operation of
5G in both licensed and unlicensed bands. However, these
frequencies form the mmWave band. Hence, more advanced
mmWave-based operating procedures will be required.
•Enhancements to the initial access: 5G-advanced NR-U
will operate in deployment scenarios similar to the
sub-7 GHz band-based NR-U standard with support for
both LBT and no-LBT operation as baseline channel ac-
cess modes. However, due to the nature of the mmWaves,
characterized by severe attenuation owing to oxygen
absorption and poor diffraction in the presence of ob-
stacles, techniques such as combining LBT with beam-
based transmissions will be essential for improved system
performance. During initial access, UE relies on the
quality of the reference signal associated with each of the
multiple synchronization signal block (SSB) beams trans-
mitted by gNB to establish the best-receiving direction.
On that note, 5G-advanced NR-U is expected to support
up to 64 SSB beams, which is more than the sub-7 GHz
band-based NR-U standard. The proposed beamforming
approaches include omnidirectional LBT (Omni-LBT), a
method of attempting channel access by forming beams
in all directions, and directional LBT (Dir-LBT), in which
a beam is formed in a specific direction rather than all
directions. Omni-LBT is associated with exposed node
problems and deters spatial reuse. Contrarily, Dir-LBT
can allow spatial reuse at the cost of hidden node prob-
lems. Therefore, more research to ensure fairness, accu-
rate beam prediction, and lower overhead for LBT-beam-
based transmissions is required.
•Enhancements to the numerology: Additional values
of the numerology µ, which is significant in determining
the SCS, slot length, and bandwidth, to mention but
a few, were defined to support next-generation NR-U.
These values which are greater than 4, result in wider
SCS sizes say 480 kHz and 960 kHz, computed by
15 ×2µ, with shorter slot lengths calculated using 1/2µ.
210 JOURNAL OF COMMUNICATIONS AND NETWORKS, VOL. 26, NO. 2, APRIL 2024
Accordingly, the defined maximum bandwidths in Rel. 17
may lie between 400 MHz and 2160 MHz, which are
larger than those defined in the current NR-U system.
Flexibility in the choice of µis important since different
services may require specific values. For example, to
achieve high throughput, a short SCS is required while
a large SCS is necessary for reduced latency. Unfortu-
nately, enhancements to the numerology can incur more
implementation complexity. It would be beneficial to
conduct more studies for the appropriate application of
SCS values with minimal complexity to improve latency
without jeopardizing spectrum efficiency.
•Enhancements to the energy detection threshold:
During channel access, any RAT is required to set the
ED threshold to be less than or equal to the maximum
ED threshold, Eth. For the scenario where the absence
of any other technology sharing the channel cannot be
guaranteed on a long-term basis, Eth for sub-7 GHz-
based NR-U [20] can be defined as
Eth = max
−72 + 10 ·log10 BWMHz
20MHz dBm,
min (Tmax (dBm) ,
Tmax (dBm) −TA+PA)
,
(1)
where Tmax = 10 ·log10 3.16228 ·10−8mW/MHz·
BWMHz and PA=PH+ 10 ·log10 (BWMHz )
20MHz −PTX.
For transmissions including discovery burst(s), TAis set
to 5 dB otherwise, TA= 10 dB. PH=23 dBm is
the reference power, PTX is the maximum eNB/gNB
outpower in dBm for the channel, and BWMHz is the
channel bandwidth in MHz. Put differently, Eth for
5G-advanced NR-U [21] can be expressed as
Eth =−80 dBm + Pmax −Pout + 10 ·log10 (BWMHz),
(2)
where Pout ≤Pmax. Note that Pmax is the radio
frequency (RF) output power limit in dBm and Pout is the
maximum equivalent isotropically radiated power (EIRP)
of the intended transmission(s) by gNB/UE to acquire
a channel occupancy in dBm. Supplemental research
to achieve higher power efficiency will be required to
compensate the challenges associated with the large prop-
agation losses in frequencies above 52.6 GHz.
•Enhancements to the physical uplink control channel
(PUCCH): PUCCH is used to convey the uplink control
information (UCI) from UE to gNB. Several PUCCH
formats (PF) distinguished by the UCI payload size and
the duration of PUCCH were defined in a range of 0 to 4.
In summary, PF0 and PF2 represent short PUCCH of 1
or 2 symbols with small and large UCI payloads, respec-
tively while PF1, PF3, and PF4 represent long PUCCH
of 4–14 symbols with small, large, and moderate UCI
payloads, respectively. In addition, PF0, PF1, and PF4
support multi-UE multiplexing. The sub-7 GHz-based
NR-U standard supports PF2 and PF3 whereas PF0, PF1,
and PF4 are considered for NR-U operating in frequen-
cies above 52.6 GHz. PF0, PF1, and PF4 had not been
considered for the sub-7 GHz-based NR-U standard since
they support only a single physical resource block (PRB)
under the scenario of contiguous allocations. PF2 and
PF3 were identified to be configurable with a bandwidth
that satisfies the minimum temporal allowance of 2 MHz,
hence supporting more PRBs say 12, 6, and 3 PRBS for
15 kHz, 30 kHz, and 60 kHz SCS [4], [5].
It should be emphasized that the aforementioned features
are not exhaustive. Therefore, interested readers can refer to
[3], [4], [17], [19] for extensive details.
III. CON FIG UR ATIO NS F OR NR-U SCHEDULING
In the sequel, we investigate the design of selected NR-
U features to facilitate scheduling and unravel the problem
of decreased reliability and increased latency in the next
generation of NR-U systems.
A. Frame Structure
In the NR-U system, gNB can transmit signals when the
channel access procedure is successfully performed in the time
domain [17]. Therefore, NR-U can be operated using time
division duplexing (TDD), a scheme based on NR TDD. The
DL/UL configuration of NR TDD can be categorized into three
types, namely, static, semi-static, and dynamic TDD. However,
by applying the static or semi-static TDD to the NR-U opera-
tion, the system performance can be severely disrupted since
the channel access procedure should be executed according
to a predetermined DL/UL pattern (switching). To deal with
the above problem, it would be beneficial to operate NR-
U using the dynamic TDD approach. Moreover, once gNB
obtains channel access, dynamic TDD can allow flexibility
in the configuration of the DL/UL pattern. Furthermore, the
following specifications were defined to compensate the prob-
lems that arise when NR TDD is applied to NR-U. Firstly,
gNB has a slot format indicator (SFI) and COT duration
information with OFDM symbol units in downlink control
information (DCI) format 2 0, which is included in the group
common physical downlink control channel (GC-PDCCH).
The switching of the physical downlink control channel (PD-
CCH) monitoring window configuration for a group of UEs
can be done dynamically using GC-PDCCH. Moreover, NR-
U supports skipping the PDCCH monitoring, a feature that
enables power saving. Secondly, NR-U supports the physical
downlink shared channel (PDSCH) mapping type B (mini-slot)
with a symbol length of 2–13. Additionally, PDSCH grouping
was specified to overcome the missed PUCCH due to LBT
failure.
In the Rel. 16 NR-U standard, after a successful channel
access procedure, the node can occupy the channel during the
MCOT whose value is dependent on channel access priority.
This COT is shared by gNB and UE to perform DL and
UL transmission. COT can be categorized into gNB/UE-
initiated COT depending on the type of node that successfully
accesses the channel [17]. For gNB-initiated COT, there are
two options to configure the DL/UL pattern, namely, the
single DL/UL switch and multiple DL/UL switches. In the
former case, it has the advantages of low complexity and
SSIMBWA et al.: TOWARDS 5G-ADVANCED NR-UNLICENSED SYSTEMS ... 211
relatively high throughput since switching is performed once
and the channel is unlikely to be taken over by another node.
However, gNB can only transmit the PDSCH for a negative
acknowledgment (NACK) received during the current COT
at the next COT, which causes high latency. In the latter
case, on the contrary, it can achieve low latency and a low
retransmission rate, at the expense of large complexity and a
lower throughput compared to the former case.
B. Hybrid Automatic Repeat Request
The HARQ procedure of Rel. 16 NR-U is based on
Rel. 15 [22]. However, when applying the existing syn-
chronous HARQ to NR-U, several issues can be encountered
since DL/UL scheduling in NR-U is asynchronous and de-
pendent on channel access success time. Specifically, in the
existing NR TDD, gNB sends a PDSCH-to-HARQ feedback
timing indicator with a numerical value of DCI Format 11,
and UE sends HARQ acknowledgment (ACK)/ NACK at a
specified timing. Nevertheless, if UE fails the channel access
procedure and cannot transmit the UL feedback at the timing to
be transmitted, gNB considers PDSCH as NACK and retrans-
mits it even though the PDSCH arrived well. Therefore, both
the retransmission rate and the latency increase accordingly. To
compensate this problem, a non-numerical PDSCH-to-HARQ
feedback timing indicator (inapplicable) value can be used
such that UE stores the ACK/NACK of PDSCH and transmits
at the next COT.
In the HARQ mechanism of the existing NR TDD, the
dynamic HARQ codebook size is determined through a 2-bit
downlink assignment indicator (DAI) field. However, with the
2-bit DAI, more than 4 consecutive missing DCIs cannot be
detected by UE, which results in the improper calculation of
the dynamic HARQ codebook size. To tackle the problem
of the limited DAI size, PDSCH groups are introduced. Up
to two PDSCH groups can be configured such that gNB can
request retransmission of HARQ-ACK for a group while the
other group is being scheduled. In that case, the DAI works
independently between the two groups. Besides, Type 3 code-
book has been introduced in Rel.16 to support one-shot HARQ
request in NR-U such that the status, the ACK/NACK of all
HARQ processes for all the carriers and PDSCH groups can
be reported. In addition, with the new feedback indicator (NFI)
field, gNB will notify UE whether the previous ACK message
for each group was received correctly.
Fig. 1 shows an example of HARQ-ACK feedback operation
in the Rel. 16 NR-U standard [23]. In Fig. 1, during the 1st
slot, we assume that gNB transmits the corresponding PDSCH
belonging to group 0 and UE receives them successfully.
Meanwhile, the PDSCHs of the 2nd, the 3rd, and the 4th slots
do not arrive at UE due to several reasons such as collision
and the weak channel. Nevertheless, gNB expects feedback for
four PDSCHs in group 0. In the 7th slot, UE sends a HARQ-
ACK feedback for PDSCH group 0. However, this feedback
is for PDSCH of the 1st slot only, hence, the decoding of
PUCCHs fails because their sizes are mismatched. Moreover,
the HARQ-feedback-timing of the PDSCH transmitted in the
6th slot is set to a non-numerical value and the HARQ value
DL DL DL DL DL DL UL DL DL UL
GAP
GAP
Codebook
Codebook
NACK
NACK
TI = 6
DAI = 0
Group = 0
TI = 5
DAI = 1
Group = 0
TI = 4
DAI = 2
Group = 0
TI = 3
DAI = 3
Group = 0
TI = 2
DAI = 0
Group = 1
TI = “later”
DAI = 1
Group = 1
TI = 2
DAI = 2
Group = 1
TI = 1
DAI = 3
Group = 1
Acknowledgement
NACK
Fig. 1. An illustration of HARQ-ACK feedback operation.
will be transmitted at the UL of the next COT after holding.
At the next COT, gNB sends PDSCHs belonging to group 1
during the first two slots and the NFI for group 0 without
toggling, since gNB does not receive proper HARQ-ACK
feedback for group 0. Then, at the end of COT, UE sends
a NACK to the PDSCH belonging to group 0 that gNB did
not receive, and the HARQ value of group 1 PDSCH including
the PDSCH of the 6th slot that was previously withheld.
C. Radio Link Control Layer
In the NR-U system, the channel access procedure affects
not only the media access control (MAC) layer but also the
RLC layer timing parameter [24]. Specifically, in the case of
RLC unacknowledged mode (RLC UM), if the t-Reassembly
value is set to be short, the amount of dropped RLC segmented
PDU increases due to channel access procedure and collision.
On the other hand, when the t-Reassembly value is set to
be long, the amount of RLC service data unit (SDU) to
be dropped increases when the receiving buffer size is not
large. Therefore, gNB and UE must set the t-Reassembly value
appropriately. Furthermore, gNB can adjust the UL slot portion
by considering the statistics of the packet dropped rate. Com-
pared to RLC UM, the RLC acknowledged mode (RLC AM)
supports the ARQ feedback mechanism to ensure reliable
packet delivery. Therefore, the PDU status message is peri-
odically sent by UE to indicate the current status of the RLC
PDUs received at UE. Specifically, gNB can request UE to
send the current PDU status via transmitting a poll.
To regulate the frequent current PDU status transmissions,
gNB and UE utilize the t-PollRetransmit and t-StatusProhibit
timers as follows. Firstly, the t-PollRetransmit timer is used
by gNB AM RLC entity to retransmit a poll. If the
t-PollRetransmit timer expires, gNB sends a poll when the
transmission and retransmission buffers are empty or when the
window is stalled. In this case, gNB includes either the highest
segment number that has been submitted for retransmission or
any RLC SDU that has not been positively acknowledged in
the polling. Secondly, the t-StatusProhibit timer is used by
UE AM RLC entity to prohibit the transmission of a STATUS
PDU. Consequently, these timers can have a significant impact
on the system throughput and latency. For instance, due to the
channel access procedure and MCOT, it would be beneficial
to control the timers and allocate the UL portion to send the
RLC PDU status.
212 JOURNAL OF COMMUNICATIONS AND NETWORKS, VOL. 26, NO. 2, APRIL 2024
Table I
NR-U AND WI-FI SIMULATION ASSUMPTIONS.
Parameter Value
Wi-Fi System
AP number 1–8
Header length 128 bits
Acknowledgment (ACK ) 240 bits
Request to send (RTS) 288 bits
Clear to send (CTS) 352 bits
Short inter-frame spacing (SIFS) 16 µs
Priority class 1,2,3
NR-U System
gNB/UE number 1/1
DL/UL bandwidth 20 MHz
Subcarrier spacing (SCS) 15 kHz, 30 kHz
gNB/UE antenna number 1
Resource block (RB) number 106 (for 15 kHz), 51 (for 30 kHz)
PDSCH/PUSCH mapping type Type A/Type B
HARQ codebook Type Type 2
COT sharing Enabled (gNB initiated COT)
K0
K1
K2
0 slot
minimum 1 slot
minimum 0 slot
IV. PERFORMANCE EVALUATION
In this section, we present simulation results obtained using
the Matlab 5G toolbox, illustrating the effect of different
configurations for the switching patterns, HARQ feedback
value type, and the RLC layer timing parameters, on the
performance of NR-U in the presence of Wi-Fi. To account
for channel estimation and feedback, we consider the channel
quality indicator (CQI) based on TDD channel reciprocity.
We also assume that each resource block (RB) has one value
randomly chosen from the maximum CQI values based on
the initial configuration of the distance between gNB and
UE. We do not consider the hidden node phenomenon in our
simulations. Moreover, we assume that Wi-Fi and NR-U nodes
confirm channel occupancy based on the minimum remaining
time (defer duration + remaining backoff number ×9) rather
than energy detection. This means that during contention for
channel access, the node with the minimum remaining time
wins the contention, and in case of a tie between nodes,
it is assumed that collision occurs, and the window size is
increased. Regarding data decoding, we assume a block error
rate (BLER) of 10%, and the ACK/ NACK is determined based
on this probability. We also assume an on-off traffic model for
the application layer. Unless stated otherwise, the simulation
assumptions used in this article are summarized in Table I.
A. Effect of the Switching Pattern on the Latency and
Throughput
Fig. 2 compares the DL system performance with a corre-
sponding DL/UL slot configuration pattern (single and multi-
ple switching). We assume that SCS is 30 kHz, the number
of multiple switches is 2 and the access priority of Wi-
Fi is arbitrarily selected from 1 to 3 without hidden nodes
under the carrier sense multiple access with collision avoid-
ance (CSMA/CA) approach. Fig. 2 shows that as the number
of Wi-Fi access points (AP) increases, gNB is subject to
stiff competition for channel access and it is less likely to
occupy the channel. Hence, the DL latency increases while
the DL throughput decreases. We can observe from Fig. 2 that
12345678
Number of Wi-Fi APs
0
5
10
15
20
25
30
DL latency (ms)
Single switching
Multiple switching
(a) DL latency vs. the number of Wi-Fi APs
12345678
Number of Wi-Fi APs
2
3
4
5
6
7
8
9
DL throughput (Mbps)
Single switching
Multiple switching
(b) DL throughput vs. the number of Wi-Fi APs
Fig. 2. DL performance for two DL/UL configuration patterns.
there is a trade-off between the DL latency and throughput.
For example, single switching attains higher DL throughput
at the cost of higher latency and the reverse is true for
multiple switching. Moreover, substituting single switching
with multiple switching leads to a decrease in latency by
35.7%and 40.6%and a decrease in throughput by 12.7%
and 9.6%for 3 APs and 6 APs, respectively. Therefore,
it is necessary to determine DL/UL slot format and sched-
ule DL/UL transmission, while considering the traffic status,
throughput/latency requirement, CQI, PUCCH overhead, RLC
status protocol data unit (PDU), and others.
B. Influence of the HARQ Feedback Value Type on the Latency
and Retransmission Rate
Fig. 3 presents the DL system performance for a corre-
sponding HARQ feedback value type versus the number of
Wi-Fi APs. We set the number of switching points to 1
and consider similar assumptions stated in the evaluation for
Fig. 2. From Fig. 3, it can be seen that as the number of
Wi-Fi APs increases, the DL latency and retransmission rate
SSIMBWA et al.: TOWARDS 5G-ADVANCED NR-UNLICENSED SYSTEMS ... 213
12345678
Number of Wi-Fi APs
0
5
10
15
20
25
30
35
DL latency (ms)
Non-numerical
Numerical
(a) DL Latency vs. the number of Wi-Fi APs
12345678
Number of Wi-Fi APs
10
15
20
25
30
35
40
45
DL ReTx rate (%)
Non-numerical
Numerical
(b) DL retransmission (DL ReTx) rate vs. the number of Wi-Fi APs
Fig. 3. DL performance for HARQ feedback type.
increase. Additionally, the DL latency and retransmission rate
are higher when the numerical value is used compared to
the case when the non-numerical value is used. When the
access priority of Wi-Fi is varied from 1 to 2 and the backoff
value is 0, the CAT2-LBT channel access procedure performed
during DL/UL switching fails which prompts retransmission
and leads to increased latency. Therefore, by employing the
inapplicable HARQ feedback value concept, the DL latency
and retransmission rate can be reduced. For instance, the DL
latency decreases by 16.3%and 12.9%while the retransmis-
sion rate decreases by 4.9%and 4.5%for 3 APs and 6 APs,
respectively, when the non-numerical value is used.
C. Impact of the t-Reassembly Value on the Packet Dropped
Rate
Fig. 4 shows the comparison of the DL/UL packet drop rate
with the t-Reassembly value for the varying number of Wi-Fi
APs such as 1, 3, and 7. Three observations can be drawn
from Fig. 4. Firstly, as the number of APs varies, the DL/UL
packet drop rate initially decreases rapidly and gradually
0 20 40 60 80 100 120 140 160 180 200
t-Reassembly (ms)
0
5
10
15
20
25
30
35
40
Packet Dropped Rate (%)
DL (Number of AP=1)
DL (Number of AP=3)
DL (Number of AP=7)
UL (Number of AP=1)
UL (Number of AP=3)
UL (Number of AP=7)
Fig. 4. Packet dropped rate vs. t-Reassembly for RLC UM mode.
increases with higher t-Reassembly values. For example, when
the number of Wi-Fi APs is 3 and the t-Reassembly value
is varied from 10 ms to 100 ms, the packet dropped rate
decreases by 70%and 78%for UL and DL respectively. This
performance justifies the discussions made earlier. Secondly,
as the number of APs increases, so does the packet drop
rate. This is because the probability of occupying the channel
decreases, resulting in the packet being dropped as it is less
likely to come in t-Reassembly. Lastly, since each scenario
(varying number of APs) presents a different optimal DL/UL
t-Reassembly value (marked with a black circle), it is therefore
important for gNB to set this value well based on the statistics
of the channel situation with feedback from UE.
V. CONCLUSION
In this article, we provided an overview of the design
mechanisms for LTE-LAA and NR-U technologies. We further
investigated the impact of the switching pattern, the feedback
value type, and the timing parameters as enablers of NR-U
scheduling. We also presented simulation results to illustrate
the performance of NR-U for varying configurations of se-
lected design features. It has been shown that appropriate
configurations improve NR-U performance in terms of latency
and reliability as well as attaining fair coexistence between
NR-U systems with other RATs operating in unlicensed bands.
To this end, we hope that our findings will trigger further
research interests toward achieving a highly reliable and low-
latency-enabled next generation of NR-U.
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Julius Ssimbwa received his B.S. degree in
Telecommunications Engineering from Makerere
University, Kampala, Uganda, in 2016, and his M.S.
degree in Electrical Engineering from Korea Uni-
versity, Seoul, Korea, in 2021, where he is currently
pursuing a Ph.D. degree with the School of Electrical
Engineering. He was an O&M Engineer at Huawei
Technologies (Uganda) from 2015 to 2016. In 2017,
he joined the I-Engineering group (Uganda) as an
O&M Supervisor. His current research interests are
signal processing, radio resource management, and
multiple antenna technologies.
Seok-Hyun Yoon received his B.S. degree in Com-
puter Science and Communication Engineering from
Korea University, Seoul, South Korea in 2017, where
he is pursuing a Ph.D. degree with the School of
Electrical Engineering. In 2022, he visited Tufts
University, Medford, MA, USA, to conduct col-
laborative research. His research interests include
mmWave/THz beamforming, intelligent reflecting
surfaces (IRS), and full-duplex communication.
Yeongrok Lee received his B.S. and Ph.D. degrees
in Electrical Engineering from Korea University,
Seoul, in 2016 and 2023, respectively. Since March
2023, he has been with Samsung Electronics. His
research interests include wireless LAN standards,
software-defined radio, vehicular communication,
and machine learning in communications and net-
works.
Young-Chai Ko received his B.Sc. degree in Elec-
trical and Telecommunication Engineering from
Hanyang University, Seoul, Korea, and his M.S.E.E.
and Ph.D. degrees in Electrical Engineering from
the University of Minnesota, Minneapolis, MN in
1999 and 2001, respectively. He was with Novatel
Wireless as a research scientist from January 2001
to March 2001. In March 2001, he joined Texas
Instruments, Inc., wireless center, San Diego, CA,
as a Senior Engineer. He is now with the School
of Electrical Engineering at Korea University as a
Professor. His current research interests include the design and evaluation
of multi-user cellular systems, MODEM architecture, mmWave, and THz
wireless systems.
