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arXiv:1705.08709v1 [cs.IT] 24 May 2017
1
V2X Meets NOMA: Non-Orthogonal Multiple
Access for 5G Enabled Vehicular Networks
Boya Di∗, Lingyang Song∗, Yonghui Li†, and Zhu Han‡
∗School of Electrical Engineering and Computer Science, Peking University, Beijing, China,
Email: {diboya, lingyang.song}@pku.edu.cn.
†School of Electrical and Information Engineering, University of Sydney, Sydney, Australia,
Email: yonghui.li@sydney.edu.au.
‡Electrical and Computer Engineering Department, University of Houston, Houston, TX, USA,
Email: hanzhu22@gmail.com.
Abstract
Benefited from the widely deployed infrastructure, the LTE network has recently been considered
as a promising candidate to support the vehicle-to-everything (V2X) services. However, with a massive
number of devices accessing the V2X network in the future, the conventional OFDM-based LTE network
faces the congestion issues due to its low efficiency of orthogonal access, resulting in significant
access delay and posing a great challenge especially to safety-critical applications. The non-orthogonal
multiple access (NOMA) technique has been well recognized as an effective solution for the future 5G
cellular networks to provide broadband communications and massive connectivity. In this article, we
investigate the applicability of NOMA in supporting cellular V2X services to achieve low latency and
high reliability. Starting with a basic V2X unicast system, a novel NOMA-based scheme is proposed
to tackle the technical hurdles in designing high spectral efficient scheduling and resource allocation
schemes in the ultra dense topology. We then extend it to a more general V2X broadcasting system.
Other NOMA-based extended V2X applications and some open issues are also discussed.
I. INT RODUCTIO N
In the past few years, there have been greatly increasing applications of vehicular networks
developed for future intelligent transportation, such as advanced driver assistance on active
safety and traffic efficiency. To support various applications, an integrated system of the ve-
hicular networking, namely vehicle-to-everything (V2X), has been proposed to enable vehicles
to communicate with each other and beyond [1]. It provides three types of communications,
2
i.e., vehicle-to-vehicle (V2V), vehicle-to-pedestrian (V2P), and vehicle-to-infrastructure/network
(V2I/N) referring to the communication between a vehicle and a roadside unit/network. Although
IEEE 802.11p has established security and upper layer specifications for V2X services, its
unpredictable latency and limited transport capacity have confined its ability to achieve low
latency and high reliability (LLHR). Recently, the widely deployed Long Term Evolution (LTE)
networks [2] are being considered as a very promising solution for supporting V2X services [3],
because large cell coverage, controllable latency, and high data rates can be achieved even in a
high-mobility scenario.
Various techniques have been proposed in LTE to provide reliable communications, which
can potentially be used to achieve LLHR in V2X applications. For V2I services naturally
supported by LTE via the downlink/uplink transmission, the multimedia broadcast/multicast
service (MBMS) can be utilized for the base station (BS) broadcasting [4] to achieve resource-
efficient transmission, thereby reducing the latency to a relatively low level. For V2V services
supported by the LTE device-to-device (D2D) communications [5], end-to-end latency can be
largely reduced via the direct link between users in proximity bypassing the BS, enabling the
key support especially for the safety-critical applications.
Several works have discussed the proposal design for LTE-based V2X services to approach
the LLHR requirement [1], [6], [7]. In [1], enhanced design aspects to support LTE-based V2X
services have been presented and a new demodulation reference signal sequence design has been
performed. In [6], a distributed media access control (MAC) scheme has been proposed for the
D2D OFDMA-based cellular networks. Fair sharing of spectrum among transmitter nodes has
been performed to ensure the reliability of delivery. In [7], a unified radio frame structure and
a MAC protocol have been proposed to enable reliable heterogeneous V2X services in different
use cases.
However, as an orthogonal multiple access based (OMA-based) system, LTE mainly supports
mobile devices sharing resources in an orthogonal manner, leading to serious congestion problems
due to the limited bandwidth [3]. To be more specific, resource collision may occur between
OMA-based vehicle users, and the user access rate is difficult to be guaranteed via the orthogonal
spectrum management in a dense moving environment. In a general V2X broadcasting scenario,
this may also result in even more severe collision problems with significant packet loss [6],
especially for a dense topology. Therefore, a more spectrally efficient radio access technology
3
is still required for V2X services.
To tackle the challenges of access collision reduction and massive connectivity, non-orthogonal
multiple access (NOMA) schemes have been introduced as a potential solution, which allow
users to access the channel non-orthogonally by either power-domain [8] or code-domain [9]
multiplexing. Multiple users with different types of traffic requests can transmit concurrently on
the same channel to improve spectrum efficiency and alleviate the congestion of data traffic,
thereby reducing the latency. To make the NOMA scheme more practical, various multi-user
detection (MUD) techniques, such as successive interference cancellation (SIC), are applied at
the end-user receivers for decoding to cope with the co-channel interference caused by spectrum
sharing among various users. In addition, uplink contention based NOMA [10] has been proposed
to reduce the control signaling overhead, especially for the small packet transmission, in order
to overcome the shortage of large latency produced in LTE uplink.
Capable of achieving high overloading transmission given limited resources, NOMA provides
a new dimension for V2X services to alleviate the resource collision, thereby improving the
spectrum efficiency and reducing the latency. Due to the non-orthogonal nature of NOMA as
well as the mobility and dense topology of vehicular network, the design of scheduling and
resource allocation schemes for NOMA-based cellular V2X services becomes different from the
traditional OMA-based system in many aspects, including:
•Scheduling scheme: In the OMA-based V2X network, semi-persistent scheduling (SPS) is
applied in which the resources are booked by the vehicles every few transmission periods [4].
However, since the NOMA-based Rx decoding requires real-time channel state information
(CSI) of multiple Tx users for enhancing the quality of SIC decoding, a new scheduling
scheme combing the dynamic power control with SPS needs to be considered.
•Spectrum management: Compared to the OMA-based case, the NOMA-based V2X net-
work introduces co-channel interference by allowing multiple vehicles to share the same
subband, which provides an extra dimension influencing the spectrum efficiency as well as
user fairness.
•Power control: Due to the dense topology of vehicular network, when the Tx users
broadcast to their neighborhood, cross interference is brought to those Rx users in the
overlapping region of multiple Tx users’ communication range, which requires the design
of new power control strategy of each Tx user in the NOMA-based V2X network.
4
•Signaling control: Due to the requirement of prior knowledge for joint decoding such as
the CSI of Tx users, signaling between the Rx users and Tx users for information exchange
is an important issue which couples with the power control of Tx users. In the traditional
OMA-based case, such prior knowledge is usually provided by the BS, which may introduce
great latency to V2X applications.
In this article, we discuss the applicability of the NOMA technique for supporting V2X services
starting with the basic V2X unicast systems and then extending to other typical scenarios as
listed below.
•NOMA-based V2X unicast systems: The basic V2X unicast model consists of multiple
V2V pairs sharing the same channel simultaneously for direct communication. One receiver
(Rx user) may suffer from co-channel interference from neighboring transmitters (Tx users),
and one Tx user may cause interference to multiple Rx users nearby. SPS-based spectrum
management of the BS and dynamic distributed power control of the users need to be
considered given the requirement of LLHR.
•NOMA-based V2X broadcast systems: V2X broadcasting is essential for safety-critical
applications, in which each vehicle is required to broadcast a small data packet containing
safety-critical information to its neighborhood within every short transmission period. Beside
of the issues brought up in the unicast case, time domain resource allocation and Tx-Rx
selection of the BS need to be further considered for interference management.
•NOMA-based uplink V2I networks: The uplink V2I networks consist of multiple devices
transmitting to the network/infrastructure by utilizing the contention-based code-domain
NOMA technique [10]. Main issues include resource allocation based codebook selection
and contention-based backoff mechanism design.
•NOMA-based V2V networks with multiple operators: In such a network, at lease a
set of carriers for direct communication is shared by the vehicles subscribed to different
operators, and joint data transmission and reception is performed by the operators to improve
the spectrum efficiency of cell-edge vehicle pairs. Coordination between the operators to
jointly allocating antennas, spectrum, and power is required.
The rest of this article is organized as follows. Section II provides an overview of existing
cellular V2X techniques and NOMA-based communications. A NOMA-based mixed central-
5
ized/distributed (NOMA-MCD) scheme is proposed in Section III for resource allocation and
signaling control in the aforementioned basic V2X unicast system. Main challenges, possible
solutions, and performance evaluations are discussed in detail. Other NOMA-based V2X commu-
nication extensions are presented in Section IV focusing on both benefits and research problems
brought by NOMA. In the final section, we draw the main conclusions, and also discuss some
open problems and potential research directions.
II. OVE RVI EW OF CELLULAR V2X AND NOMA-BA SED COMM UNI CATION S
A. LTE-supported V2X Services
Traditionally, V2X services can be classified as two types, safety-critical and traffic efficient
applications, with different requirements of packet size and latency. Safety-critical messages are
usually short broadcast messages with strict latency constraints, putting high requirement on
scheduling and packet loss. Different from the safety-critical messages, traffic efficient messages
refer to a large amount of sensed data of vehicles sent to the infrastructure or other vehicles.
Instead of achieving stringently low latency, the key point of traffic efficient message transmission
is how to achieve continuous communication in a moving environment [11] while guaranteeing
the delivery of other human-to-human (H2H) traffic.
To support both safety-critical and traffic efficient applications, LTE has provided two com-
munication modes for V2V and V2I, i.e., LTE-direct (LTE-D) and cellular UL/DL, respectively,
as illustrated in Fig. 1.
•LTE-D for V2V: LTE-D refers to direct communication between two devices bypassing
the BS in a proximal way. The design of LTE-D has made up for the native feature of LTE
that message passing between vehicles via the infrastructure may produce large end-to-end
latency. Resource allocation problems involving multiple D2D pairs and cellular users have
been discussed in the literature [12]. However, unlike traditional H2H LTE-D networks, the
V2V network has brought new concerns such as intractable cross interference due to the
moving environment and a dense topology.
•Cellular UL/DL for V2I: Cellular UL/DL refers to the common communication mode
between devices and the BS/road side units. For the case in which the density of vehicles
is particularly sparse, cellular UL/DL can be considered for assistance. Note that multiple
transmissions via unicast in the DL may lead to great resource waste given the expensive
6
DL
Uu
UL
Base
Station
LTE-D
Fig. 1. A basic framework illustrating two communication modes in LTE-based vehicular network.
licensed spectrum. To achieve resource-efficient transmission, BS broadcasting/multicasting
can be adopted via MBMS. Based on coordination between multiple cells, MBMS helps
improve the cell-edge performance and reduce the latency.
Note that the mobility of vehicles leads to the rapid variation of fading, which poses a great
challenge to the scheduling schemes for V2X services. Various existing scheduling schemes in
LTE are listed as below suitable for different latency requirements:
•In Dynamic Scheduling, users are allocated resources for every transmitted packet in each
time slot based on real-time CSI, which requires accurate channel estimation. It is suitable
for sudden and frequently size-varying data transmissions that potentially consume wide
bandwidth.
•In Semi-Persistent Scheduling, the BS allocates the predefined sets of resources to those
users requesting for transmission every SPS period. The length of each SPS period is set
as the same order of the required latency. By removing unnecessary signaling exchange in
each slot, SPS reduces a great deal of latency, which is particularly suitable for transmitting
periodically short messages with a fixed packet size, such as the basic safety messages.
7
B. Non-Orthogonal Multiple Access Technique
NOMA has been proposed as a new access technique for next generation mobile communi-
cations, supporting massive connectivity and sufficient spectrum usage. Two types of NOMA
schemes have drawn great attention as comprehensively introduced below:
•Power domain NOMA (PD-NOMA) allows multiple users to share the same channel
simultaneously by power domain multiplexing at the Tx, and SIC can be applied at the
end-user Rx users to decode the received signals which suffer from co-channel interference.
It smartly exploits the differences of received power levels to obtain higher spectrum
efficiency than the OMA scheme. Industry standards have been widely discussed for future
deployment [13], and several efficient algorithms for the resource allocation have been
proposed [14].
•Code domain NOMA (CD-NOMA) is also known as SCMA, short for sparse code multiple
access. It uses sparse (or low-correlation) spreading sequences to integrate data streams of
various users and then spread over multiple subchannels to realize overloading. Each user is
identified by a codebook containing multiple codewords, and one codeword is represented
by the spreading sequence of which length equals to the size of subcarrier set. At the
transmitter, bit streams of each user are directly mapped to different sparse codewords of
the corresponding codebook. All mapped codewords are then multiplexed over the dedicated
subchannels, followed by a near-optimal detection of over-laid receiving sequences benefited
from the sparsity of codewords. Protocols for the SCMA schemes have been released in [9].
III. NOMA APPLICA BIL I TY TO CELLULAR V2X
In this section, we elaborate how to apply the PD-NOMA technique in the cellular vehicular
network for resource collision reduction, thereby achieving low latency. To better explain this,
we first present a basic V2X unicast model in which the cross interference is considered. The
NOMA-MCD scheme is then proposed and evaluated as below. Extension of this scheme to a
more general V2X broadcast case will be presented in detail in Section IV.A.
A. NOMA-based V2X Unicast System Model
Consider a NOMA-based V2X unicast network as shown in Fig. 2. Multiple V2X pairs
communicate in a NOMA-based mode in which one sub-channel can be assigned to multiple
8
Interference
Rx 2
Tx 2
Intended V2V link
R
x
2
Base
Station
Rx 1
Control signaling
Tx 1
Power
Freqency
P1of Tx 1
P2of Tx 2
R
x
1
T
x
2
Superposed
received signals
Fig. 2. System model of the NOMA-based V2X unicast systems.
pairs, and one V2X pair can occupy multiple sub-channels. The BS is capable of frequency
resource allocation requiring global position information of the network. For those conflicting
Rx users who suffer co-channel interference from neighboring Tx users, SIC is performed based
on the decreasing channel gains of Tx users for decoding the target signals. To improve the
system performance, we aim at maximizing the total number of successfully decoded packets,
for which the data rate of a target signal exceeds a given threshold. Due to the severe cross
interference caused by overlapping interference ranges of each Tx user as well as the prior
knowledge requirement of SIC decoding, new challenges have emerged in the resource allocation
and signaling control as presented in sequel.
B. Key Problems and Solutions of Resource Allocation and Signaling Control
Before presenting the key problems, we first illustrate the reason why we adopt a mixed
centralized/distributed scheduling scheme as follows. Considering the mobility of vehicles and
complicated cross interference caused by the dense topology, selection of scheduling scheme
9
is a non-ignorable factor affecting both latency and data rates. Traditional dynamic centralized
resource allocation may cause significantly large delay since the users need to send resource
request messages to the BS for every data packet. In addition, accurate CSI is hard for the BS
to obtain in a mobile environment. For a dynamic distributed scheduling scheme, CSI can be
updated in time via direct links. It may be suitable for the case where the BS is out of function;
however, it is very costly and unwise for the users to access the channel in a contention-based
method within the coverage of the BS. Full centralized SPS has a good performance in reducing
the delay; nevertheless, it fails to capture the rapidly changing CSI caused by the mobility,
leading to potentially large resource collision during each SPS period.
To achieve the optimal scheduling given the latency requirement and mobility features, we
adopt a mixed centralized/distributed scheme in which the BS performs the SPS and the Tx
users perform distributed autonomous power control in each time slot. At the beginning of each
SPS period, the BS determines how to allocate frequency resources to the Tx users, which takes
full advantage of the global position information obtained by the BS to perform the interference
management. Dynamic distributed power control by the Tx users is then performed to resolve
the issues that the BS cannot obtain real-time CSI as well as to improve the rate performance
of PD-NOMA.
Below we discuss the main problems and possible solutions in detail from the perspectives
of centralized spectrum management of the BS and distributed power control.
1) Centralized Spectrum Management of the BS: To reduce the resource collision, the BS
performs frequency resource allocation based on the position information of each vehicle updated
at the beginning of each SPS period. Different from the traditional OMA-based case, co-channel
interference needs to be considered here.
Due to mobility of vehicles, not only the full CSI is very costly for the BS to acquire, but
also the CSI can get easily outdated due to the rapid variation of small scale fading. Therefore,
we adopt partial CSI containing the path loss and shadowing during each SPS period. Define
an indicator variable xj,k to denote whether sub-channel kis allocated to Tx j. Considering
the reliability of the network, we aim at improving the number of successfully decoded signals,
10
which can be approximated by the sum of continuous logistic functions1as shown below:
{xj,k}= arg
xj,k
max X
j,k
Y
j′∈Sj∪{j}
xj′,k=1
1
1 + e−η(Ratej′,k −Rateth),(1)
in which Rateth denotes the minimum data rate required for successful decoding, Ratej′,k
denotes the data rate of the link Tx user j′– Rx user k,Sjrepresents the set of Tx users
with higher channel gains 2than Tx user jover subchannel k, and ηis the slope parameter of
the logistic function.
Note that this is a non-convex problem due to the binary variables, which can be converted
into a many-to-many matching problem with externalities. Considering the Tx users and sub-
channels as two sets of objects to be matched, the BS performs a swap-matching algorithm
briefly described as below. Initially each Tx user randomly selects a set of sub-channels based
on its priority. In the following swap-matching phase, the BS keeps searching for two pairs of
Tx and sub-channel to check whether they can swap their matches such that the total utility in
(1) can be improved. If these two pairs exist, the BS swaps the original matches of them. The
largest number of one player’s matches is fixed during the matching phase, and the iteration
stops until there exists no blocking pair in current matching.
2) Distributed Power Control of the Users: After the frequency resource allocation at the
beginning of each SPS period, dynamic power control is then performed by the Tx users in each
time slot. The transmit power of each Tx user has cross influence over the set of neighboring Rx
users, posing a great challenge to perform the interference management in a distributed manner,
especially for the PD-NOMA scheme. Specifically, though a Tx user intends to communicate
with only one target Rx, there may exist multiple Rx users within this Tx user’s interference
disk, each of which affected by the transmit power of this Tx user. Similarly, the target Rx user
may also receive from multiple Tx users, rendering the decoding effect unstable. In addition, we
note that necessary prior knowledge is required by the Rx users to perform the SIC decoding,
such as the number of Tx users in the interference range and the corresponding CSI. Therefore,
1Here we take the logistic function as an example to depict the successful decoding probability. Other approximation methods
such as the experience-based SINR-packet reception ratio curve can also be used.
2According to the SIC principle, the signal of Tx user jcan be successfully decoded by Rx user kif the signal of Tx user
j′with a higher channel gain can be decoded by the Rx user first.
11
the distributed power control requires information exchange between the Tx users and the Rx
users, i.e., the control signaling, as will be explained below.
We divide each transmission slot into one control signaling portion and one data transmitting
portion, in which the control signaling portion consists of multiple Tx-Rx iterations for control
message exchange between the Tx users and the Rx users. Power control strategy of each
Tx user is determined in every iteration. To limit the potential signaling costs to a tolerable
level, we assume that there are TcTx-Rx iterations in the control portion, followed by the data
transmission. Each iteration consists of one Tx block and one Rx block, and works as below. In
the Tx block, every Tx user adjusts its transmit power so that the transmitted reference signals
can be successfully decoded by its target Rx user while causing minimum interference to other
Rx users. Each Rx user obtains its neighboring Tx users’ CSI and transmit power via the received
reference signals. In the following Rx block, the Rx users then calculate the potential co-channel
interference brought by Tx users in the neighborhood, and send back to corresponding Tx users
for further processing in the next Tx block.
In each Tx block, every Tx user adjusts its transmit power based on the feedback sent by the
Rx users. To avoid the situation where each Tx user transmits with the maximum power, the
power control strategy for each Tx user is set as below: if the co-channel interference caused
by a Tx user is larger than a threshold, its transmit power is set to zero; otherwise, the power is
set as the minimum value such that the rate of the direct link between this Tx user and its target
Rx user is larger than the decoding rate threshold. Small scale fading is considered during the
power control.
C. Performance Evaluation
In summary, our proposed NOMA-MCD scheme is described as below. At the beginning of
each SPS period, each vehicle user updates its position and velocity information to the BS. The
BS then allocates the frequency resources to the Tx users to maximize the number of successfully
transmitted messages in which large scale fading is considered based on predictable distance
information. After the centralized spectrum assignment, distributed power control coupled with
the Tx-Rx selection is then performed. In each iteration of the control signaling portion, the Tx
users adjust their transmit power based on the feedback from neighboring Rx users. The whole
distributed power control process ends within the control portion of a transmission slot, followed
12
15 20 25 30 35 40 45 50 55 60
0.55
0.6
0.65
0.7
0.75
0.8
Speed (km/h)
Packet reception probability
NOMA−MCD
GA−based NOMA
OMA−based V2X
(a)
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
0.8
0.82
0.84
0.86
0.88
0.9
0.92
0.94
0.96
0.98
1
Decoding rate threshold
Latency satisfication ratio
NOMA−MCD
GA−based NOMA
OMA−based V2X
(b)
Fig. 3. System performance of the NOMA-MCD scheme.
by the data transmission in which Rx users can decode received signals given the CSI obtained
from the control portion.
To evaluate the performance of NOMA-MCD scheme, we compare it with the traditional
OMA-based LTE-D scheme with respect to the packet reception probability as well as the latency
performance3as shown in Fig. 3. We assume that 20%and 80%vehicles on the road serve as
Tx users and Rx users, respectively, and at most 2 Tx users can share the same subchannel. It
is observed that the NOMA-MCD scheme performs better than the OMA-based scheme.
IV. NOMA-BASED V2X COMM UNI CATION EXTENSION
In this section, we extend the basic V2X model introduced in Section III to other applications
in the cellular vehicular network. Some key research problems as well as possible solutions are
also discussed.
A. NOMA-based V2X Broadcast Systems
V2X broadcasting via the direct links is viewed as a key support for utilizing the LTE
technology in vehicular networks, which enables safety-critical applications. Take a dense V2X
3Latency satisfaction ratio refers to the ratio between the number of successfully transmitted signals (given the latency
constraint) and that of successfully decoded signals.
13
PD-NOMA
Base
Station
Transmitter
Receiver
Tx 1
Tx 2
Power
Frequency
P1of Tx 1
P2of Tx 2
Broadcasting range
Fig. 4. System model of the NOMA-based V2X broadcast transmission.
broadcast transmission scenario for example, as shown in Fig. 4. In every transmission period
consisting of multiple time slots, each vehicle is required to broadcast a small data packet
containing safety-critical information at least once to its neighborhood. In the traditional OMA-
based LTE-D mode, hidden terminal problems exist in which a conflicting Rx user may lie in
the overlapping section of multiple Tx users’ communication ranges, and these Tx users are not
close enough to communicate with each other. Existing solutions for this problem usually focus
on the collision avoidance mechanism design in which only one Tx user can transmit in current
slot while other Tx users in the neighborhood yield to this Tx user and keep silent. However,
this may result in insufficient use of the spectrum resources and loss of time-validity for the
sate-critical messages of those silent Tx users.
The proposed NOMA-MCD scheme is then naturally extended to this case such that one
Rx user can receive from multiple Tx users simultaneously, reducing the resource collision and
improving massive connectivity via power-domain multiplexing. Different from the V2X unicast
version, time-domain resource management and user scheduling need to be considered, and the
power control strategy of each Tx user is re-considered, as illustrated below.
1) Tx-Rx selection and time-frequency resource allocation of the BS: During each time slot,
a user can only be either the Tx user or the Rx user due to the half-duplex nature, and thus, any
14
two users in each other’s communication range cannot be assigned to transmit simultaneously
in one SPS period. At the beginning of each SPS period, the BS decides which two subsets of
vehicles act as Tx users and Rx users in each time slot, respectively, and how time-frequency
resources are allocated to the Tx users. Therefore, the centralized resource allocation problem
for the BS is then formulated as a three-dimensional integer programming problem.
2) Power control strategy of each Tx user: Unlike the Tx users in the unicast case each of
which has only one target Rx user while other neighboring Rx users are interfered, Tx users in the
broadcast case aim at adjusting the transmit power so as to maximize the number of successfully
decoding Rx users in the neighborhood. However, it is almost impossible for every Rx user to
successfully decode all the superposed received data in a dense environment. Therefore, each
Tx user minimizes its transmit power during the process of control signaling such that a certain
percent of neighboring Rx users in the communication range are guaranteed to successfully
decode the received signals.
B. NOMA-based Uplink V2I Networks
Note that the LTE uplink transmission can be a bottleneck of achieving LLHR due to the
excessive signaling exchange between each vehicle and the BS. Specifically, this issue can
degrade the performance of safety-critical applications in which short packets containing basic
safety information are frequently updated by the vehicles. Aiming at finding a low-latency access
technique, we observe that the contention based SCMA scheme may be a suitable candidate.
A SCMA-based uplink V2I network works as in Fig. 5, where each vehicle transmits to
the BS by competitively occupying one or more contention transmission units (CTU) from the
dedicated contention region. Given the contention region which is part of the UL bandwidth,
one CTU refers to a combination of time, frequency, SCMA codebook, and pilot sequence. At
the beginning of each SPS period, the CTUs are determined for the users either by the BS
assigning or implicitly deriving from the user ID. When multiple users are assigned the same
CTU for data transmission, user collision occurs, which can be resolved through the random
back-off mechanism similar to that in 802.11p. No transmission grant of the BS is required before
the users send packets, which is different from the traditional centralized UL transmission. For
receiver decoding, the BS attempts to detect received packets by utilizing all possible assigned
access code sequences, in which the message passing algorithm (MPA) for joint decoding can
15
Codebooks
01
01
01
00
00
00
11
11
11
10
10
10
Tx 1
Tx 2
Tx 3
+
+
Time
Frequency
Tx 4
Tx 5
CTU
Tx 1
Tx 2
Tx 3
...
Fig. 5. NOMA-based uplink V2I networks: codebook-based resource occupation, encoding, and multiplexing
be utilized.
Enabling the grant-free UL transmission while allowing system overload via multiplexing,
the SCMA scheme has provided a new solution to achieve massive connectivity and meet very
stringent latency requirements for some V2X services. Note that the number of vehicles set
in one subband influences the collision probability of CTUs, thereby having an impact on the
decoding performance of MPA. Therefore, a trade-off between the massive connectivity and
latency needs to be discussed especially in an ultra dense network. In addition, resource allocation
based codebook selection and back off mechanism design for SCMA are also crucial factors
influencing the reliability of data services as well as latency performance.
C. NOMA-based V2V Networks with Multiple Operators
Consider a V2V network with multiple operators as shown in Fig. 6. A set of carriers for
direct communication is shared by the vehicles subscribed to different operators, i.e., vehicles
belonging to different operators may transmit on the same carrier [4]. For the cell-edge vehicles,
cooperation between multiple operators is necessary for joint spectrum management and power
control to achieve reliable direct communication. More than one cell-edge V2V pairs may contest
for the shared frequency resources, leading to resource collision.
To improve the spectrum efficiency of cell-edge users, NOMA-based joint data transmission
and reception of the vehicles can be performed assisted by the operators. Due to the mobility of
16
Power
Shared
subband
Operator 1
(Subband 1)
Operator 2
(Subband 2)
Shared V2V
frequency
Operator 3
(Subband 3)
O
p
e
(
S
u
b
r
2
d
2
)
User 1
User 2 User 3
Power allocated
to user 3
Power allocated
to user 1
Power allocated
to user 2
Subband 1 Subband 2 Subband 3
Fig. 6. NOMA-based V2X networks with multiple operators.
vehicles, dynamic cell hand-off needs to be considered in the proposed scheme. The association
between users and operators should be carefully designed to obtain the prior knowledge of joint
decoding while maintaining low latency of the system. In addition, precoding of the BS may be
different from the traditional single-cell case, since it is not easy for a precoder associated with
multiple separate antennas to form the physical beam that perfectly fits the distribution area of
those co-channel NOMA users.
V. CONCLUSIONS AND FUT URE OU TLO OK
In this article, we introduce the NOMA technique into the LTE-based vehicular network to
support massive connectivity and reduce resource collision for multiple V2X applications via
either the power-domain or code-domain multiplexing. The new scheduling scheme, resource
allocation algorithm, and control structure are designed in which the expected added value of
network control is exploited to meet the requirement of LLHR. The proposed NOMA-MCD
scheme is illustrated in detail given a basic V2X unicast model, and simulation results indicate
that it can efficiently reduce the resource collision compared to the traditional OMA-based
scheme. An explicit extension of this scheme to a more general safety-critical V2X broadcast
scenario is then elaborated. Other NOMA-based extended V2X applications are also presented
such as networking NOMA and contention based SCMA, which can be applied to cope with
the multiple-operator case and LTE-uplink latency, respectively.
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As an effective approach supporting massive connectivity and high spectrum efficiency, the
NOMA technique shows its potential to enhance the quality of cellular V2X services. Several
open issues still need to be carefully addressed before practical implementation of the NOMA-
based cellular vehicular network, which may drive the future inventions and research. These
issues include synchronization in a high-mobility case, co-existence of LTE and WiFi, and
coordination between various types of services/devices with different requirements. Two future
research problems to be discussed in this field are listed below as examples.
•NOMA for cooperative V2X: Consider a broadcast scenario in which the originating node
(BS or vehicles) broadcasts superposed signals to multiple Rx users. Lack of feedback
from the Rx users to the originating node makes the retransmission not user-specific. Note
that in the NOMA scheme, users with good channel gains are capable of decoding the
information intended to other users with poor channel gains. Such prior information can be
utilized for cooperation-based retransmission, in which a Rx user with good channel gain
forwards the messages to other Rx users with poor channel gains via a direct link [15].
Considering the mobility of vehicles, research topics such as dynamic spectrum management
and communication protocol design need to be redeveloped.
•NOMA for cognitive V2X: For traffic efficient applications, a large amount of data gen-
erated by the vehicles poses great pressure on traditional H2H traffics. To guarantee the
demand of H2H traffics while improving the quality of data transmission of vehicles, a new
scheme named cognitive NOMA can be utilized in which the vehicles opportunistically
access the channels which are originally occupied by the cellular users. Given a dedicated
spectrum band, cellular users and vehicles are regarded as primary users and secondary
users, respectively. Vehicle users can only access the channel when the services of cellular
users are not affected. Different from the traditional cognitive radio scheme, power control
needs to be performed carefully and a scheme coordinating both primary and secondary
users should be designed.
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