Sidelink is a 3GPP-standardized technology that provides direct UE-to-UE communication links, originally intended to operate in licensed 5G spectrum. However, due to recent availability of large swaths of unlicensed bands (notably 6 GHz), 3GPP has initiated studies to support for sidelink operation in such unlicensed spectrum. This paper discusses potential new use cases for sidelink, identifies challenges for adapting sidelink to these new unlicensed bands and user scenarios, and provides an outline of potential approaches that are currently being considered in the design of sidelink to operate in the unlicensed bands.

1. Sidelink Mode 2 Operations in Unlicensed Bands:
Design Challenges and Potential Approaches
Vijitha Weerackody1
, Sumit Roy2
, Kent Benson1
, and Eric Yang1
1
Johns Hopkins University Applied Physics Laboratory, Laurel, MD
2
University of Washington, Seattle, WA
Abstract—Sidelink is a 3GPP-standardized technology that
provides direct UE-to-UE communication links, originally in-
tended to operate in licensed 5G spectrum. However, due to recent
availability of large swaths of unlicensed bands (notably 6 GHz),
3GPP has initiated studies to support for sidelink operation in
such unlicensed spectrum. This paper discusses potential new use
cases for sidelink, identifies challenges for adapting sidelink to
these new unlicensed bands and user scenarios, and provides
an outline of potential approaches that are currently being
considered in the design of sidelink to operate in the unlicensed
bands.
Keywords-5G NR, Sidelink, Unlicensed, Cellular V2X,
Mode 2.
I. INTRODUCTION
Sidelink was developed about a decade ago to support direct
device-to-device connectivity for Proximity Services (ProSe)
with applications intended mainly in public safety, i.e., as in
FirstNet [1]. Recently, sidelink has seen a rapid growth of
applications in commercial and governmental sectors resulting
in significant enhancements resulting in the Release 16 sidelink
that supports higher data rates at very low latencies. Some key
applications of the current sidelink include its use in supporting
the rapidly expanding V2X applications (see 3GPP TS 22.186
for V2X requirements) and in providing the node-to-node link
connectivity in mesh networks in disaster relief areas shown
in Fig. 1. In this figure, sidelink mode 2 links are used to
connect the widely separated and isolated nodes to a drone
network, with one of the drones connected to a base station.
Such mesh connectivity of air-, sea- and land-based nodes in
infrastructureless environments is of extreme importance in
military applications as well.
The ongoing Release 18 efforts look to enhance 5G Sidelink
(SL) technology [2] to multi-Gbps (peak) data rates while
requiring very low end-to-end latencies, cognizant of emerg-
ing requirements such as Augmented Reality/Virtual Realty
(AR/VR) applications. To achieve these goals, 3rd Gener-
ation Partnership Project (3GPP) is developing approaches
to support operations at FR2 (mmWave) frequency bands
(24.25 − 71 GHz) and in unlicensed bands. SL operations
at unlicensed bands is the focus of this paper and it is
of significant interest due to recent allocations (e.g., nearly
{vijitha.weerackody, kent.benson, eric.yang}@jhuapl.edu, {sroy}@uw.edu.
Fig. 1: Sidelink-based mesh network in disaster recovery.
1.2 GHz in 6 GHz band in US by the FCC [3] and similar
allocations in many other regions globally) within the FR1
frequency range (410−7125 MHz) for unlicensed operations.
Other emerging uses for Sidelink includes provisioning the
extremely high data rates required in some AR/VR applica-
tions such as training for various operational scenarios, video
gaming and live-streaming of remote conferences/meetings
[4]. Fig. 2 shows the Use Case 9 (Critical Mission with
AR) described in [4] where a security squad is deployed
to accomplish a difficult task. The deployed personnel wear
helmet-mounted AR/VR devices that are inter-connected via
local communication links, and traffic backhauled live to a
command center via a drone. This application requires AR/VR
object tracking of up to six Degrees of Freedom (DoF), highly
accurate positioning and high-fidelity, realtime video and audio
streaming. Release 18 sidelink is well-suited to support the
communication needs of such applications.
Fig. 2: Sidelink used in Police Critical Mission use case [4].
1

2. In addition to the above enhancements, Release 18 sidelink
will also support one-hop UE-to-UE relaying feature which is a
first-step toward a multi-hop relaying feature. Although mesh
networking is not directly supported in Release 18 sidelink,
there are proposals for Release 19 to include this feature in
future releases of sidelink [5], [6]. With multi-hop relaying
support, next-generation sidelink will be inherently suited to
provide mesh or full direct connectivity in many scenarios
including subterranean (e.g. mining) facilities, autonomous
vehicles, and broader set of sensor network applications.
A. 5G NR in Unlicensed Bands
New Radio in Unlicensed Bands (NR-U) is the 5G NR based
channel access mechanism developed for use in unlicensed
spectrum, as a successor to 3GPP Release 13/14 LTE License
Assisted Access (LAA) [7]. The guiding principle used in
this development was fair coexistence of NR-U with other
users of the unlicensed spectrum1
, e.g., IEEE 802.11 WiFi
networks. 3GPP is currently considering enhancing sidelink
(D-2-D mode) operations in these unlicensed bands, also using
the key features of NR-U channel access mechanism.
Fig. 3: Spectrum coexistence of IEEE 802.11 WiFi, NR-U and the
proposed 3GPP SL-U systems in the unlicensed bands.
Fig. 3 shows a generic spectrum sharing scenario with two
IEEE 802.11 WiFi networks with respective access points AP-
1 and AP-22
, co-located with a 3GPP NR-U system with the
New Radio (NR) base station, and two SL links operating
in mode 1 and mode 2, respectively. As specified in [2], the
5G NR-U access in mode 1 is anchored with licensed gNB
communications via the Uu interface, whereas all direct UE-
to-UE communications (SL-U) use the PC5 interface.
In Release 16, 3GPP developed the specification for NR-U
channel access mechanism [8]. To promote fair coexistence
with WiFi DCF/EDCF, [2] states that the channel access
mechanisms for the Sidelink in Unlicensed Bands (SL-U)
should reuse those specified for NR-U. Hence, the sidelink
UEs use the NR-U channel access mechanism to contend for
1Note that besides unlicensed-unlicensed sharing, unlicensed systems must
also coexist with incumbent licensees in portions of the 6 GHz band, an issue
that is not the subject of this work.
2The channel access mechanisms for the IEEE 802.11 WiFi (Enhanced
Distributed Coordination Function (ECDF)) and NR-U are shown as DCF-1
(DCF-2) and NR-U.
the channel and then employ a potentially modified version
of the autonomous resource allocation scheme used in current
sidelink to reserve resources among themselves3
.
The WiFi 6 and NR-U systems contend for channel access
based on Carrier Sense Multiple Access/Collision Avoidance
(CSMA/CA) with an exponential backoff window size algo-
rithm in the event of collisions. The channel sensing time (slot)
interval in both cases is 9 µs; once the channel is accessed, the
transmitting node may transmit a burst of data whose duration
is limited by the Transmission Opportunity (TXOP) for IEEE
802.11 WiFi, or Maximum Channel Occupancy Time (MCOT)
for NR-U system, respectively. While WiFi transmissions are
synchronized to the sensing slot boundaries, the baseline NR-U
transmits only at slot boundaries which occur at intervals of
125 µs, 250 µs, 500 µs or 1 ms, depending on the numerology
used. However, the NR-U system supports features such as
Cyclic Prefix Extension (CPE) and mini-slot transmissions
where the mini-slot size can be as short as two symbol4
lengths
and the transmissions can start at any symbol boundary [10].
Hence, it is reasonable to assume that the NR-U system is
capable of accessing the channel using these features at any
sensing slot time but such features are not supported in the
Release 16 sidelink. Part of the Release 18 work on SL-U is
to introduce these NR-U features to sidelink to improve its
channel access properties.
In addition to the time-synchronous transmission nature
of SL, its resource reservations are such that transmission
bursts from a single UE on consecutive time slots are in
general not supported. The significant difference between
the channel sensing slot (Tsl = 9 µs) and the SL slot
(TSL = 125, 500 or 1000 µs) durations, coupled with
the inability to transmit data in consecutive SL time slots,
introduce significant disadvantages for SL-U channel access
relative to the WiFi or NR-U systems. To support very high
data rates for SL in the unlicensed bands, some of these
technical challenges are currently being addressed at the 3GPP
Technical Specification Group Radio Access Network Working
Group 1 (RAN1) [9]. In this paper, we examine in detail some
of these challenges and the proposed mitigation techniques.
We emphasize that SL-U at RAN1 meetings is currently at an
advanced development stage with significant detailed proposals
made for enhancing it; the contents presented here address
only some of the key features under consideration that will
most likely be included in the final specification.
II. 5G NR SIDELINK OVERVIEW
The 5G NR sidelink5
operates in two modes - modes 1 & 2
- where the former relies on assistance from the base station
but the latter does not. In both modes, a direct UE-to-UE com-
munication link is established; however, in mode 1 the base
3This is still under active consideration at 3GPP [9] and will not be
addressed further.
4In general, an NR-U slot consists of 14 symbols.
5This discussion is based on Release 16; Release 17 introduces additional
enhancements such as resource allocations for power saving mode and inter-
UE coordination [11].
2

3. station sends a resource grant (via selecting and configuring
the resource blocks/sub-channels) to the UEs for subsequent
use, whereas in mode 2 the UEs use Semi-persistent Sensing
(SPS) - a distributed, sensing-based resource allocation scheme
that primarily serves use cases where the traffic consists
of predictable (periodic) messages with fixed packet sizes
(Transport Block (TB)). In this work, we exclusively focus
on mode 2 sidelink operation, whereupon a UE broadcasts
reservation of selected sub-channels in subsequent subframes
for transmission of data (TB), defined by two variables: the
Resource Reservation Period (Prsvp) and Sidelink Resource
Reselection Counter (SLRRC). The first represents the (peri-
odic) interval for transmission of the messages, and the second
the random length of a reservation duration in multiples of
Prsvp. The values of Prsvp and SLRRC (random variable,
uniform within an interval determined by the Prsvp duration)
are selected to satisfy a target mean reservation duration as
function of the traffic load.
We next provide a succinct overview of key aspects of mode
2 channel sensing based and associated resource allocation
aspects relevant to the developments in the sequel. More
detailed description of sidelink for the interested reader can
be found in [12], [13]. Channel sensing is conducted by all
UEs during respective Sensing Window duration (in terms of
time slots) to monitor channel status for usage by other UEs;
the resultant channel state is used for its own subsequent reser-
vation decisions for future transmissions within the Selection
Window duration. The intent of the SPS-based reservation
resource allocation is to prevent the UE from selecting the
same resources for its own transmission that are already in
use by other nodes (and hence, predictably, may continue to
be used in the future) and avoid potential collisions. However,
since the selection of future resources for reservation by a
UE is based on past channel state information determined by
channel sensing that is inherently imperfect, such SPS-based
allocation does not fully eliminate collisions.
A. Sidelink Data Channel Slot Structure
The resources reserved by the SPS scheme are used to
transmit the TBs in the Physical Sidelink Shared Channel
(PSSCH) with each such transmission contained in a single SL
slot. Fig. 4 shows an example of the slot structure used for the
PSSCH [14] that consists of 14 Orthogonal Frequency Division
Multiplexing (OFDM) symbols. In addition to the PSSCH
data, this slot contains the following: Physical Sidelink Con-
trol Channel (PSCCH), Physical Sidelink Feedback Channel
(PSFCH), Automatic Gain Control (AGC) and guard symbols.
The number of frequency resources or Physical Resource
Block (PRB)s in the PSSCH/PSCCH symbols are determined
by the size of the TB. The last three symbols in this slot are
reserved for packet feedback information from other UEs; the
Demodulation Reference Signal (DMRS) symbol is used for
demodulation of the signals in the slot. This figure shows two
guard symbols: Guard 1 and Guard 2. The former provides
the time necessary for the UE to switch off its transmission
so that it can receive feedback transmission from other UEs;
the latter provides the time needed to switch the UE from
transmission to sensing or receive mode so that it can monitor
other UE transmissions that occur at the start of the next
sidelink slot (which is at the end of Guard 2 period.) Note that
guard symbol period6
can be significantly longer than the 9 µs
sensing slot period of the NR-U. Because of this NR-U and
WiFi nodes could gain access to the channel during this guard
symbol period of the SL-U transmission. This issue and ways
of mitigating it are discussed in detail in the next sections.
AGC
PSSCH
PSSCH
PSSCH
PSSCH
PSSCH
PSSCH
PSSCH
AGC
PSFCH
DMRS
DMRS
Guard
2
PSCCH
UE
Guard
1
Feedback UEs
Slot (𝑇𝑇𝑠𝑠𝑠𝑠)
Symbol
PRBs
Fig. 4: Example of a PSSCH slot transmission. The last three
symbols are reserved to receive ACK/NACK from other UEs.
B. Synchronization in Sidelink
Synchronization of the sidelink UEs is a critical function
that has to be accomplished prior to exchanging messages
among the UEs in the network. Fig. 5 shows the Sidelink Syn-
chronization Signal Block (S-SSB), which is sent by a specific
UE known as the SyncRef UE, used for synchronization of the
sidelink.
Fig. 5: S-SSB slot [14] used for synchronization of sidelink UEs.
The S-SSB consists of the Sidelink Primary Synchroniza-
tion Signal (S-PSS) and Sidelink Secondary Synchronization
Signal (S-SSS), which are together known as the Sidelink
Synchronization Signal (SLSS), and the Physical Sidelink
Broadcast Channel (PSBCH) signal. The S-PSS is used for
initial clock and frequency synchronization and the S-SSS is
used to establish subsequent frame timing. In addition, these
two synchronization signals are used to determine the priority
of the synchronization signals when a UE receives signals from
multiple SyncRef UEs. The S-PSS consists of two possible
sequences that indicate whether the SyncRef UE is either in
coverage or out of coverage of GNSS/gNB. The S-SSS consists
of 336 possible sequences resulting in 2×336 distinct IDs for
the SLSS [12], [15]. The S-SSBs sent from SyncRef UEs that
are in coverage of GNSS/gNB have the highest priority. These
6Guard symbol durations are 17.84, 35.68 and 71.35 µs when the SL slot
sizes are 250 µs, 500 µs and 1 ms, respectively.
3

4. are followed by the S-SSBs sent by SyncRef UEs that are one-
hop away from GNSS/gNB coverage. All other S-SSBs are at
the next lower priority level, which is the lowest.
The S-SSB transmissions are sent on known resources that
are outside the sidelink resource pool so the sidelink UEs
can accomplish the initial synchronization process without
a complex search process of the available resources. In the
unlicensed bands, the S-SSB transmissions have to contend
for channel access with other Radio Access Technology (RAT)
networks resulting in loss or delay of S-SSB transmissions.
Section IV presents some approaches considered for designing
the S-SSB transmissions for SL-U.
III. SIDELINK-BASED UNLICENSED CHANNEL ACCESS
3GPP RAN1 currently has an active Work Item Description
(WID) to study support for sidelink operations in the FR1
unlicensed bands (SL-U) [2]. This seeks to bootstrap existing
3GPP established channel access mechanisms for UE commu-
nications involving a base station (NR-U) for sidelink. This
WID has determined that the channel access mechanisms used
in SL-U should reuse that specified in NR-U, hence we briefly
review key features of the NR-U channel access mechanisms
specified in 3GPP TS 37.213 [8]. The two main types of
downlink channel access - Type 1 and Type 2 - require sensing
of channel status to ensure that the channel is idle, prior to
transmission. In Type 1 the channel sensing time duration is
random, while in Type 2 this duration is deterministic and it
is relatively short.
Fig. 6: Downlink channel access types in NR-U [8].
The table in Fig. 6 lists the key features of these chan-
nel access mechanisms for the downlink of the NR-U. The
Type 1 channel access procedure is a Carrier Sense Multi-
ple Access/Collision Avoidance (CSMA/CA) algorithm often
referred to as Listen Before Talk (LBT) (Category 4) and is
similar in concept to the 802.11 Enhanced Distributed Channel
Access (EDCA) procedure. In Type 1, a random backoff
counter value is generated and channel state is observed at
the sensing slot duration (9 µs) boundaries; if the channel
is idle, the counter is decremented; otherwise, the counter is
held at its value. The sensing process is repeated until the
channel is determined to be idle for a period of defer duration
prior to decrementing the backoff counter value. Once the
counter reaches zero, the node (UE or gNB) may transmit.
On the other hand, in Type 2 access (as shown in this table)
the channel sensing time before transmission is fixed at least
to either 25 µs, 16 µs or 0 µs (no sensing), respectively.
The Type 2A transmissions in general are used to send short
discovery bursts for synchronization purposes, and Types 2B
and 2C transmissions are used to exchange messages following
transmissions from a UE, e.g., in a Channel Occupancy Time
(COT) share.
The stated use of NR-U channel access mechanisms with
sidelink as specified in the WID [2] presents some challenges
that have to be addressed by appropriately adapting these
channel access mechanisms for use in mode 2 access. Some of
these challenges are discussed in detail next and approaches
for mitigating these challenges presented in Section IV.
A. Challenge 1: Impact of Sidelink Slot Size and Slotted
Transmissions
The slot size for channel sensing in NR-U is Tsl = 9 µs;
further, NR-U makes use of mini-slots which allows transmis-
sions to start at any OFDM symbol position [16]. In contrast,
the minimum time slot size in sidelink is (TSL) is 250 µs;
i.e. sidelink transmissions may only occur synchronously with
sidelink slot boundaries7
. As a result of this discrepancy,
sidelink UEs are at a significant disadvantage when contending
for channel access with NR-U or other radio networks. Fig. 7
shows an SL-U UE using the Type 1 procedure to access the
channel after an NR-U transmission. Because of the very short
time slots used for channel sensing in the backoff process, the
end of back-off contention window countdown will typically
not coincide with the sidelink slot boundary8
. This results in
a gap duration [17], [18] during which the SL-U node does
not transmit. This potentially leads to the sidelink UE loosing
access opportunity to the channel because an NR-U/WiFi node
- that is not constrained to transmission only at sidelink slot
boundaries - may win contention and start transmitting on the
channel during this gap duration.
Fig. 7: Gap duration during SL-U channel access.
B. Challenge 2: Effect of Absence of SL Transmissions in
Contiguous Slots
To support low-latency applications and fair access to UEs
in a SL network, the SPS scheme in general does not support
7Sidelink UEs are all assumed time-synchronized via common time slot
boundaries shared by all UEs.
8Alternatively the NR-U backoff procedure [8], can be used to extend the
contention window size so that the end of the contention window period
coincides with the sidelink slot boundary.
4

5. Fig. 8: Example of a transmission resource reservation for UEs A,
B and C in the SL.
resource reservations in contiguous slots for a single UE.9
Fig. 8 shows an example of transmission resources reserved
for a 3-UE network using the SPS scheme. These transmission
resource reservations cannot be used in a straightforward man-
ner when accessing the channel using the NR-U channel access
procedure. To illustrate this, suppose UE A uses the Type 1
access procedure to gain access to the channel. Then, because
the sidelink is in general unable to allocate transmission
resources to a single UE in contiguous slots, the transmission
is limited to only the first slot shown in this figure; all other
UE transmissions shown in this figure have to contend for
channel access using the Type 1 access procedure after this
transmission from UE A. This will significantly degrade the
SL-U network throughput performance when co-existing with
NR-U or WiFi systems because transmission duration for an
SL node is limited to TSL at each access instance of the
channel. Additionally, because each UE has to contend for
channel access, transmissions from multiple UEs in adjacent
time slots as shown in this figure cannot be supported in a
straightforward manner in SL-U.
C. Challenge 3: Effect of Guard Periods
As discussed in Section II-A and shown in Fig. 4, the
guard period included prior to the PSFCH symbol could result
in another radio network accessing the channel before the
arrival of the PSFCH symbol from the feedback UE. Here,
the transmitting UE is expecting feedback (ACK/NACK) after
the last PSSCH in the 10th
−symbol position. However, when
this guard symbol (Guard 1) period is significantly longer than
the sensing slot duration, another radio network may access the
channel during this interval by determining the channel to be
idle.
In addition, the Guard 2 symbol will also degrade the
performance of the SL-U system in case of a UE transmission
immediately following another UE’s transmission. Suppose
Type 2B/2C channel access procedures in Fig. 6 are used for
transmissions from a UE immediately following the transmis-
sion from another UE10
. Then, the absence of a transmission
in the last guard symbol (Guard 2) period may cause loss of
channel access to the UE that follows.
D. Challenge 4: Absence of UE-to-UE COT Sharing in NR-U
Fig. 9 shows an example scenario where the channel is first
occupied by the WiFi and NR-U systems and then followed by
9The minimum value of Prsvp = 1 ms [15].
10This is UE-to-UE COT sharing and has been accepted as a preliminary
feature that will be supported in the new SL-U design [9].
Fig. 9: Channel occupancy for an example coexistence scenario for
WiFi, NR-U and sidelink UE systems.
the sidelink UEs for a maximum allowed duration of MCOT.
In this scenario, the sidelink UE, UE0, gains access to the
channel at time T2 and shares its COT with other sidelink UEs,
after using the channel for a sidelink slot time. Although such a
COT sharing scheme could effectively mitigate the Challenge 2
described in Section III-B, the NR-U channel access procedure
does not specify UE-to-UE COT sharing because it is not
intended for direct UE-to-UE communication links.
E. Challenge 5: Contention-Based Channel Access for S-SSB
Because the S-SSB transmissions are used synchronize the
timing of sidelink UEs, they have to be sent at specific times
known to the UEs. As discussed in Section II-B the S-SSB
transmissions in the unlicensed bands, unlike in the licensed
bands, have to contend for channel access resulting in delay or
loss of some of the S-SSB transmissions. Hence, to maintain
synchronization of the sidelink network, additional S-SSB
transmission opportunities have to be devised.
F. Flexibility in NR-U Type 1 Backoff Procedure
The occurrence of a gap duration shown in Fig. 7 has been
recognized in License Assisted Access (LAA) and NR-U sys-
tems previously, and its effect on coexistence of NR-U/NR-U
systems investigated in [18] and for NR-U/WiFi systems in
[19]. However, any gap duration of NR-U system is negligible
with respect to the corresponding gap duration in the SL-U
system. This is because 3GPP Release 16 supports mini-slots,
enabling transmissions that start at symbol positions within
an NR-U slot [10]; hence any ensuing gaps in NR-U systems
are significantly reduced. Additionally, as discussed later in
Section IV-A, the NR-U system supports the Cyclic Prefix
(CP) extension feature [20] that helps to reduce any residual
gap durations further. For example, when the NR-U slot size
is 250 µs, the symbol duration is as small as 18 µs. Although
this is still longer than the sensing slot duration (9 µs), the
CPE feature allows the flexibility of closing the gap up to a
symbol length. Therefore, by using the CP transmission up to
one symbol length before the symbol boundary, it is possible
to eliminate any gap durations in NR-U system.
Another key fact that is not addressed in the literature
is the flexibility allowed in the backoff process used in the
Type 1 NR-U channel access procedure. The backoff process
established in the 3GPP specification [8] allows the flexibility
of holding the backoff counter at the same value during the
gap duration period, even when the channel is sensed idle
5

6. after a busy period. The specific statement that allows this
flexibility in the backoff process (Clause 4.1.1, Step 2 of 3GPP
TS 37.213) is quoted below:
2) if N > 0 and the eNB/gNB chooses to decrement
the counter, set N = N − 1;
Fig. 10: Gap duration when backoff counter is one. Top figure:
channel idle. Bottom figure: channel busy during part of the gap
duration.
The consequent advantage from using this backoff process
in the presence of a gap duration is explained next. Fig. 10
shows an SL-U node accessing the channel using the Type 1
procedure, with a gap duration when the backoff counter value
equals to one. In the top, the channel is idle during the backoff
countdown, and hence a SL transmission could occur at the
SL slot boundary when the backoff counter is decremented to
zero. Using the quoted clause specified in 3GPP TS 37.213, the
backoff counter value may now be frozen at one. However, as
shown in bottom, a transmission from another RAT node (e.g.
WiFi6/NR-U) may occur during the gap duration. Now, the
backoff counter will be held at one until the channel is sensed
idle for Td duration, and then decremented to zero only at the
SL slot boundary. Note that per [8], the gap duration could
occur at any position in the backoff countdown process; an
example is presented next to illustrate that the gap placed at
the end of the backoff countdown process (as shown in this
figure) is more advantageous for SL-U channel access. This is
discussed next.
Fig. 11: Gap duration when backoff counter is N. Top figure:
channel idle. Bottom figure: channel busy during the gap duration.
Fig. 11 shows a gap at the start of the countdown process
where for comparison, the start of the backoff counter and
the occurrence of the transmission from the other RAT are at
identical positions as in Fig. 10. In the top of Fig. 11, the
gap duration is selected such that when the backoff counter
reaches zero it coincides with the next SL slot boundary. The
bottom shows the other RAT transmission starting when the
backoff counter value is three and it is decremented to two at
the end of the defer duration. As shown in Fig. 11 bottom, the
backoff counter is at one at the SL slot boundary, resulting
in a missed transmission opportunity at this time. Similarly,
it could be argued that placing the gap duration at locations
other than when the backoff counter is equal to one, could lead
to missed transmission opportunities for the SL. Observe that
this missed transmission opportunity may not occur when the
gap is placed at the end of the countdown period as shown in
Fig. 10.
IV. FEATURES UNDER CONSIDERATION FOR ENHANCING
SL-U PERFORMANCE
The preceding section described challenges that are en-
countered when sidelink must use the NR-U channel access
procedure. In this section we discuss some techniques that
3GPP RAN1 is currently developing to support SL-U [9].
A. Cyclic Prefix (CP) Extension
The extension of the OFDM symbol duration using a CP
prefix up to one OFDM symbol duration is supported in the NR
waveform [20]. This CPE feature allows some flexibility in the
transmit timing positions of the waveform so could be useful in
some applications, including in systems with uncertain delays
or when the uplink signal is received and processed by multiple
gNBs. This feature is currently used in the NR-U system
in an advantageous manner to maintain the COT share as
shown in Fig. 12, where the CPE is used to close the gap
between the end of the downlink (DL) transmission and the
start of the OFDM symbol boundary to 16 µs when the UE is
using the Type 2B channel access procedure. The CPE feature
allows the UE to transmit at the end of the required minimum
sensing duration, without waiting for its regular OFDM symbol
boundary, so as to maintain access to the channel.
Fig. 12: CPE for maintaining gNB COT share in NR-U system.
The CPE feature could be used to reduce or eliminate
the guard symbol periods and, consequently, enhance the
SL-U system performance. The Challenge 3 identified in Sec-
tion III-C is due to the SL slot structure containing Guard 1 and
Guard 2 symbols, at the 11th
− and 14th
−symbol positions, in
the SL slot. To reduce the Guard 1 symbol duration in Fig. 4,
the feedback UE can advance its transmission starting time
6

7. so that it starts transmitting using an extended CP prior to
the scheduled 12th
−symbol position. In fact, since the NR-U
system supports extending the CP by as much as 1 symbol
duration, the feedback UE can start transmitting at the start
of the 11th
−symbol time resulting in eliminating the Guard 1
symbol period.
Fig. 13: Higher priority UE1 gaining access to the channel using a
longer CPE.
According to current agreements, the SL-U UE can support
multiple CPE lengths with up to a maximum of two symbols
lengths. To prevent higher priority traffic being blocked or
collided by lower priority traffic, UEs with higher priority
traffic are allowed to select longer CPE lengths when accessing
the channel. Fig. 13 shows a scenario where UE1 with higher
priority traffic using a CPE of length T1 to gain access to the
channel at the SL slot boundary by blocking the UE2’s lower
priority transmission which is specified to use a CPE of length
T2 (< T1).
B. Multiple Starting Positions within an SL Slot
The gap duration shown in Fig. 7 can be significantly
reduced by redesigning the SL waveform so that it is allowed
to transmit at multiple starting positions. The top part of
Fig. 14 shows an illustrative scenario where the backoff
counter countdown is such that the gap duration is longer
than the length of 7 symbols in a 14-symbol SL slot. The
bottom part of this figure shows the case when transmissions
starting at the position of the 7th
−symbol position is allowed.
In addition to this feature, in this figure, the CPE extension
feature described in the preceding subsection is used to com-
pletely close the resulting gap duration. The NR-U system
allows transmissions at multiple starting positions and the CPE
feature but incorporating these features to the SL-U requires
modifying the SL waveform and these are currently under
consideration at RAN1 meetings [9].
C. UE-to-UE COT Sharing
Although UE-to-UE COT sharing is not part of the NR-U
channel access procedure, agreements have been reached at
3GPP RAN1 meetings to support this feature in SL-U but the
specific mechanism is yet to be finalized [9]. A key justification
for supporting this feature in SL-U is because the standard
ETSI EN 301 893, which specifies the requirements to comply
with European regulations for operations in these unlicensed
bands, specifies a framework for UE-to-UE COT sharing. With
respect to Fig. 9, UE0 is the COT initiator and other UEs
that share its COT are the responding UEs. An effective COT
Fig. 14: Multiple starting positions in a SL slot combined with
CPE. Top figure: absence of multiple starting positions.
sharing mechanism should support transmissions from one of
these responding UEs to any other UE that shares UE0’s COT
shown in this figure. Although such a COT sharing mechanism
has not yet been agreed to at 3GPP RAN1 meetings, the
following concepts have been proposed, and are currently
under consideration, to accomplish UE-to-UE COT sharing
[9]: (a) a transmission from a responding UE is allowed to
share the initiator’s COT when it is intended for the COT
initiator; (b) a responding UE is broadly defined as a UE
whose ID is included as an additional ID in the COT initiator’s
transmission; (c) a transmission from a responding UE is
considered to be intended for the COT initiator when the target
UE’s ID is included in the set of additional IDs.
Fig. 15: Proposed approach to support UE-to-UE COT sharing.
Above proposed concepts are explained next with respect
to Fig. 15. UE0 gains access to the channel using the Type
1 procedure so it becomes the COT initiator and sends its
transmission to UE1. In the next slot, because UE1 received
a transmission directly from the COT initiator, UE1 as the
responding UE is allowed to send its data to UE0. The
key challenge is to support the unicast transmission from
UE2 to UE1 in the third slot. Suppose UE0, in addition to
its transmission to UE1, sends COT share information that
includes the IDs of UE1 and UE2. Then, from Proposal (b),
UE2 is a responding UE. Because UE1’s ID is in the set of
additional IDs, from Proposal (c) the transmission from UE2
to UE1 is considered as a transmission intended for the COT
initiator. Finally, from Proposal (a) this unicast transmission
from UE2 to UE1 is allowed to share UE0’s COT. Note that
agreements have not been reached on these specific UE-to-UE
COT sharing proposals.
7

8. D. SL Transmissions in Contiguous Slots
The very low SL-U network throughput discussed in Sec-
tion III-C can be overcome by modifying the SL waveform so
that a single UE can transmit in a block of contiguous slots. In
the NR-U system the maximum transmission time is limited
to MCOT duration so such a feature could be incorporated to
the SL-U design. The SL-U will support this feature and the
details of the design are currently under consideration at 3GPP
RAN1.
E. Synchronization
To overcome the blockage of regularly scheduled S-SSB
transmissions in SL-U, 3GPP RAN1 is considering ways of
supporting more S-SSB transmissions than that specified in
Release 16/17 versions. Fig. 16 shows an example where
the regularly scheduled S-SSB transmission at T1 is blocked
by an NR-U transmission. One of the options considered
is to attempt a specified number of S-SSB transmissions at
pre-specified time intervals until the S-SSB transmission is
successful. In this figure channel access is not available to the
S-SSB transmissions at T1 and T2 but it is available to the
transmission at T3, after Type 2A access. Note that, because
S-SSB transmissions are used for time synchronization, they
have to be sent at pre-speficified times know to the UEs. The
number of the S-SSB transmissions that are attempted and
the time interval between such transmissions are yet to be
determined.
Fig. 16: S-SSB transmissions at multiple opportunities when
channel access is blocked by an NR-U transmission.
For S-SSB transmissions it has been agreed to use Type 2A
channel access mechanism shown in Fig. 6 and to use Type
1 channel access mechanism with the highest priority class
to send additional S-SSB transmissions that exceed the duty
cycle limit of Type 2A [9].
V. CONCLUDING REMARKS
Although this paper discussed many topics that are ad-
dressed at 3GPP RAN1 meetings, some topics such as multi-
channel access, resource allocation within a COT share, and
UEs blocking each other when accessing the channel, were not
discussed here because of the broad scope of the SL-U work.
The SL-U part of sidelink is still in the development phase but
it is expected to be completed by mid-2024. The features that
are expected to be supported in Release 18 sidelink will serve
as a catalyst for the growth of direct peer-to-peer applications.
With such growth of peer-to-peer applications, sidelink has
the potential to disrupt the current base-station-centric com-
munications paradigm so planners in both commercial and
military sectors should pay close attention to the developments
in sidelink.
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