# Physical Layer Security for NOMA Systems: Requirements, Issues, and
Recommendations

IEEE Publication Technology,  Saeid Pakravan, Jean-Yves Chouinard, Xingwang
Li, Ming Zeng, Wanming Hao, Quoc-Viet Pham
and Octavia A. Dobre, _Fellow, IEEE_ The work of M. Zeng and O. A. Dobre has
been supported in part by the Natural Sciences and Engineering Research
Council of Canada, through its Discovery program.S. Pakravan, J-Y. Chouinard,
and M. Zeng are with the Department of Electric and Computer Engineering,
Laval University, Quebec City, QC, G1V 0A6, CA. email:
<EMAIL_ADDRESS><EMAIL_ADDRESS><EMAIL_ADDRESS>Li is with the School of Physics and Electronic
Information Engineering, Henan Polytechnic University, Jiaozuo, China. email:
<EMAIL_ADDRESS>Hao is with the School of Information Engineering,
Zhengzhou University, Zhengzhou 450001, China. email<EMAIL_ADDRESS>Pham is with the School of Computer Science and Statistics, Trinity College
Dublin, Dublin, Ireland. email<EMAIL_ADDRESS>A. Dobre is with the
Faculty of Engineering and Applied Science, Memorial University, St. John’s,
Canada. email<EMAIL_ADDRESS>(c) 20xx IEEE. Personal use of this
material is permitted. However, permission to use this material for any other
purposes must be obtained from the IEEE by sending a request to pubs-
<EMAIL_ADDRESS>
###### Abstract

Non-orthogonal multiple access (NOMA) has been viewed as a potential candidate
for the upcoming generation of wireless communication systems. Comparing to
traditional orthogonal multiple access (OMA), multiplexing users in the same
time-frequency resource block can increase the number of served users and
improve the efficiency of the systems in terms of spectral efficiency.
Nevertheless, from a security viewpoint, when multiple users are utilizing the
same time-frequency resource, there may be concerns regarding keeping
information confidential. In this context, physical layer security (PLS) has
been introduced as a supplement of protection to conventional encryption
techniques by making use of the random nature of wireless transmission media
for ensuring communication secrecy. The recent years have seen significant
interests in PLS being applied to NOMA networks. Numerous scenarios have been
investigated to assess the security of NOMA systems, including when active and
passive eavesdroppers are present, as well as when these systems are combined
with relay and reconfigurable intelligent surfaces (RIS). Additionally, the
security of the ambient backscatter (AmB)-NOMA systems are other issues that
have lately drawn a lot of attention. In this paper, a thorough analysis of
the PLS-assisted NOMA systems research state-of-the-art is presented. In this
regard, we begin by outlining the foundations of NOMA and PLS, respectively.
Following that, we discuss the PLS performances for NOMA systems in four
categories depending on the type of the eavesdropper, the existence of relay,
RIS, and AmB systems in different conditions. Finally, a thorough explanation
of the most recent PLS-assisted NOMA systems is given.

###### Index Terms:

Ambient backscatter systems, Non-orthogonal multiple access, Physical layer
security, Reconfigurable intelligent surfaces, Relay, Security properties.

##  I Introduction

Non-orthogonal multiple access (NOMA) has been widely regarded as a promising
candidate for the upcoming generation of wireless communication networks
[1]-[3]. The primary purpose of NOMA is to support simultaneous information
transfer of multiple users over the same radio resources at the same time [4].
However, this introduces severe inter-user interference, which is reduced with
the aid of successive interference cancellation (SIC) technique at the
receiver [5]. Owing to the channel disparity among different users, it has
been theoretically demonstrated that NOMA for single-input single-output
(SISO) systems achieves a larger capacity region than traditional orthogonal
multiple access (OMA) [6]. In [7-9], the effectiveness of these systems in the
multiple-input multiple-output (MIMO) scenario has been studied. Results from
both theoretical and numerical analyses have shown that MIMO-NOMA can achieve
a higher sum rate than MIMO-OMA. Besides spectral efficiency, it has been
illustrated that NOMA can lead to increased energy-efficiency, less delay,
improved coverage region, and massive connectivity compared to OMA under
different system settings [10-12]. However, from a security point of view,
when multiple users are utilizing the same time-frequency resource, there may
be concerns regarding keeping information confidential.

In order to address the security challenges, physical layer security (PLS)
strategies have been identified as a reliable and effective approach that can
supplement cryptographic-based approaches [13]. Through utilizing the dynamic
aspects of wireless communication, like random channel, fading, noise, and
interference, PLS can protect the information from being decoded by
eavesdropper while guaranteeing that the legitimate user can decode the data
without an issue. The future generation of wireless communication systems can
also benefit from flexible and scenario-specific security due to PLS’s ability
to design channel-dependent resource allocation and link adaptability [13].
Considering the potential application of PLS in future networks, designing PLS
techniques for NOMA and investigating the related security issues represent an
interesting subject of study. While the primary objective of NOMA is to enable
simultaneous information transfer of multiple users over the same radio
resources, its unique characteristics can also be harnessed to mitigate
security threats and bolster the confidentiality and integrity of wireless
communications. By leveraging NOMA’s capabilities, such as user grouping,
power allocation (PA), and SIC, we can devise novel security mechanisms to
counteract eavesdropping and strengthen the overall security performance of
wireless networks [14]. The inherent inter-user interference in NOMA
transmissions can serve as a deterrent to eavesdroppers, as the non-orthogonal
signals make it challenging for them to distinguish the intended user’s signal
from others. The deliberate introduction of this interference can confuse
potential eavesdroppers, thereby enhancing the physical layer security
[14-18]. Additionally, the unequal PA in NOMA can deliberately weaken the
signals received by unintended users, reducing the likelihood of successful
eavesdropping. By allocating higher power levels to users with sensitive
information and lower power levels to other users, the security of
confidential data can be enhanced. Furthermore, the SIC technique employed at
NOMA receivers not only enhances system capacity but also contributes to PLS
[14-18]. By successively decoding and subtracting the signals of stronger
users, the NOMA receiver can extract the information intended for each user,
while simultaneously rejecting interference. This inherent interference
management technique strengthens the security of individual user
transmissions, as it becomes more challenging for eavesdroppers to decode the
desired information.

The most recent research studies mainly focus on analyzing the impacts of
relay and reconfigurable intelligent surfaces (RIS) on the performances of the
PLS metrics for NOMA systems [19-40]. The secrecy performance of these systems
is often analyzed under passive eavesdropper and has been focused less on
active eavesdropper. In recent years, cases with active eavesdropping have
attracted increased attention, and related studies can be found in [41-43].
Reliability and security analysis of ambient backscatter (AmB)-NOMA systems is
another hot topic [44-50]. Additionally, PLS has been considered for systems
in which NOMA is integrated with other cutting-edge transmission techniques,
like visible light communications [51-53] and mmWave networks [54-56].

The security design specifications for NOMA in the downlink domain and the PLS
solutions satisfying these specifications have been described in [57]. Several
advantages of using NOMA in comparison to OMA in specific situations with
regard to PLS have been addressed in this study. In [58], the authors
categorized the present PLS-aided NOMA frameworks into three distinct groups
depending on the number of antennas at the base station (BS): SISO-, MIMO- and
massive MIMO systems. Then, for each category, an overview of the research
developments were provided. Most of the studies in this field focus mostly on
data privacy and confidentiality and not on other security features such as
message integrity, key generation, and device and source authentication.
Various approaches to deal with the remaining security features have been
suggested in [59], which also explored the data confidentiality of PLS for
NOMA systems and their limits, difficulties, and solutions for it. In Table 1,
a summary of the recent reviews on PLS for NOMA systems is provided.

Figure 1: Organization of this paper (RIS is a technology that can control the transmission of electromagnetic waves by modifying its electrical and magnetic attributes.). TABLE I: A Summary of Existing Surveys on PLS for NOMA Systems. Ref |  Theme |  Limitations |  Main Contribution
---|---|---|---
[57] |  Security design objective |  A specific focus on the classification of users as trusted or untrusted in analysing of security design of various scenarios. |  \- Surveys the objectives for security design and the solutions offered by PLS. \- Presents the advantages of using NOMA in comparison OMA.
[58] |  The application of PLS to NOMA systems |  It does not discuss the trade-off between security and other important system parameters, such as throughput, latency, and energy efficiency. |  \- Classified the PLS-aided NOMA framework into three distinct groups depending on the number of antennas at the BS.
[59] |  Robustness of NOMA systems / PLS authentication and availability solutions |  Mainly focused on data confidentiality and privacy and not on other security properties such as device and source authentication. |  \- Reviews various PLS-based security approaches that aim to protect the confidentiality of NOMA systems. \- Utilizes the dynamic nature of the physical layer to safeguard information using a single iteration and with the least amount of operations necessary. \- Presents the potential PLS-based key deployment methods for users of NOMA that are secure and robust.

The three previous reviews tackle important problems in NOMA systems with PLS,
but they also carry certain limitations. Indeed, none of them has
systematically looked into the works when NOMA is combined with other state-
of-the-art transmission technologies, e.g., RIS, full-duplex, and AmB
communication. Given that the research on PLS-NOMA with these novel
technologies has gained popularity in recent years, there is a need for a
comprehensive survey to discuss the recent development of PLS-NOMA in the
context of other advanced transmission technologies. In this regard, we carry
out this study and attempt to shed insight into this topic. Hence, we
thoroughly discuss and describe the PLS for NOMA systems in four scenarios:
active and passive eavesdropper, the presence of a relay, RIS, and AmB
communication systems. The corresponding security levels of these scenarios
are evaluated and analysed. A summary of the different aspects of each scheme
is provided as well. Furthermore, a thorough overview of the most current
research problems and unsolved PLS concerns in NOMA are presented. Suggestions
for future research in this area are made as well.

The organization of this survey is illustrated in Fig. 1. In the beginning, we
provide a detailed explanation of the concepts and principles of NOMA in
Section II, and PLS in Section III. Following that, in Section IV, we
thoroughly describe the current PLS-aided NOMA systems that have been proposed
in the literature. The challenges of the PLS in NOMA, potential solutions and
directions for future works are discusses in Section V. Finally, in Section
VI, the conclusions of this survey are presented.

Notation: $\left[x\right]^{+}=\max(x,0)$. $f(X,Y)$ represents the joint
probability density function (PDF) of two random variables $X$ and $Y$.
Additionally, the abbreviations are listed in Table II. The summary Tables V
and VII employs a standardized notation, whereby a ’+’ symbol signifies that a
particular factor was incorporated in the analysis, whereas a ’-’ symbol
signifies its exclusion from the analytical framework.

TABLE II: A List of Abbreviations in Alphabetical Order. Item |  Description
---|---
AF |  Amplify-and-Forward.
AmB |  Ambient Backscatter.
AN |  Artificial Noise.
AWGN |  Additive White Gaussian Noise.
BS |  Base Station.
CD-NOMA |  Code-Domain Non Orthogonal Multiple Access.
CSI |  Channel State Information.
DF |  Decode-and-Forward.
ESC |  Ergodic Secrecy Capacity.
FDR |  Full Duplex Relay.
FU |  Farthest User.
HDR |  Half Duplex Relay.
IoV |  Internet-of-Vehicle.
IQI |  In-phase and Quadrature-phase Imbalance.
MIMO |  Multiple-Input Multiple-Output.
MISO |  Multiple-Input Single-Output.
MTS |  Maritime Transportation Systems.
NOMA |  Non Orthogonal Multiple Access.
NU |  Nearest User.
OMA |  Orthogonal Multiple Access.
PA |  Power Allocation.
PLS |  Physical Layer Security.
PD-NOMA |  Power-Domain Non Orthogonal Multiple Access.
QoS |  Quality-of-Service.
RIS |  Reconfigurable Intelligent Surface.
SCA |  Successive Convex Approximation.
SC |  Secrecy Capacity.
SCR |  Secrecy Coverage Region.
SIC |  Successive Interference Cancellation.
SIS0 |  Single-Input Single-Output.
SNR |  Signal-to-Noise Ratio.
SOP |  Secrecy Outage Probability.
STAR |  Simultaneous Transmitting and Reflecting.

##  II Fundamentals of NOMA

The concept of NOMA and its various types are described in this section,
representing the foundation for understanding the security strategies
presented in the paper.

The existing NOMA structures can be classified in two main categories: power-
domain NOMA (PD-NOMA) and code-domain NOMA (CD-NOMA). By employing different
power levels, PD-NOMA provides services to multiple users simultaneously in
the same time-frequency resource [1-4]. In order to reduce the interference
between users, the receiver employs SIC technique. CD-NOMA assigns non-
orthogonal resources to the users, like codebooks, scrambled patterns,
broadcast sequences, and scrambled sequences [5]. Besides to the two main
types of NOMA, exists several NOMA approaches which are not as widely known,
like bit division multiplexing and pattern division multiple access [60].
Generally speaking, PD-NOMA has received more attention than CD-NOMA because
of its convenience, effectiveness, and compatibility to existing systems.
Indeed, most PLS solutions largely concentrate on PD-NOMA instead of CD-NOMA.
As a result, the emphasis of this study is on PD-domain NOMA. For the
simplicity of the notation, it will be referred to as NOMA in the rest of the
paper. Interested readers can refer to [61], as well as any references therein
for more information on CD-domain NOMA.

###  II-A Downlink NOMA

The BS superimposes the user signals into a signal waveform on the transmitter
side using different power coefficients. The amount of power assigned to each
user is based on the quality of its relative channel. Generally, users with
worse channel conditions are allocated with a higher power and vice versa. In
other words, the user equipment that is farthest away from the BS receives the
highest PA, while the user equipment that is closest to the BS receives the
lowest allocation. We will refer to them as the farthest user (FU) and the
nearest user (NU), respectively. The downlink signal to be transmitted can be
represented as follows:

$x_{D}(t)=\sum_{k=1}^{K}\sqrt{\alpha_{k}P}{x_{D,k}(t)},$ (1)

where $K$ is the number of users in the network; $x_{D,k}(t)$ is the
individual information of user equipment $k$; $t$ represents the time here;
$P$ denotes the total power transmitted by the BS, and $\alpha_{k}$ for
$k=\left\\{1,...,K\right\\}$ represents the power coefficient for the signal
of user $k$ where $\sum_{k=1}^{K}\alpha_{k}=1$. Then, the signal that user $k$
has received is specified as follows:

$y_{k}(t)={h_{k}}x_{D}(t)+n_{k}(t),$ (2)

where $h_{k}$ represents the channel gain between user $k$ and the BS, and
$n_{k}(t)$ is the additive white Gaussian noise (AWGN) at user equipment $k$,
with zero-mean and variance $\sigma^{2}$. It is supposed that users are
hypothetically indexed based on the order of their channel gains for decoding,
i.e., $\left|h_{1}\right|^{2}\geq...\geq\left|h_{K}\right|^{2}$.

Each user equipment executes SIC to subtract the interference signals with
higher power levels. This procedure is carried out sequentially until the user
equipment locates its signal. Note that the user with the worst channel
condition, namely, the maximum power coefficient can recover the desired
signal without performing SIC, wherein additional signals are regarded as
noise [5]. As previously stated, SIC is carried out at user $k$,
$k=\left\\{1,...,K-1\right\\}$ to eliminate the interference from the users
with lesser channel gains at the receiver. Therefore, the downlink rate that
can be achieved by user $k$ is determined as follows:

$R_{k}^{\mathrm{DL}}=\log_{2}\bigg{(}1+\frac{\alpha_{k}P\left|{h_{k}}\right|^{2}}{\sum_{i=1}^{k-1}\alpha_{i}P\left|h_{k}\right|^{2}+\sigma^{2}}\bigg{)},$
(3)

where $\sum_{i=1}^{k-1}\alpha_{i}$ is assumed to be zero for $k=1$. Fig. 2 (a)
illustrates the detailed downlink network for a scenario with two users.

(a)  (b)

Figure 2: Two scenarios for NOMA system models: a) Downlink NOMA system models
with two users, b) Uplink NOMA system models with two users.

Theoretically, there is no limit to the number of users that can be served via
downlink NOMA. Nevertheless, in practice, downlink NOMA is normally only used
for a few users, usually $K=2$ or $3$. The reason is that as $K$ increases,
the bit error rate performance suffers greatly as a result of error
propagation from an imperfect SIC. In the meantime, decoding the signal of
other users requires more computing power and energy, making it less appealing
for user devices with limited resources. User scheduling is required when the
system consists of many users. First, users are separated into various
clusters. Afterwards, NOMA is used to provide services to users who are inside
the same cluster, whereas OMA is used to serve users within different clusters
as time division multiple access or frequency division multiple access.

###  II-B Uplink NOMA

In the NOMA network for uplink transmissions, each user sends its signal to
the BS. Then, these signals are combined into one signal using multiplexing.
To distinguish and characterize each user’s signal at the BS, SIC is employed
[5]. The signal that is received at the BS, which consists of the signals from
each user, can be written as:

$y(t)=\sum_{k=1}^{K}\ {P_{k}x}_{U,k}(t)h_{k}+n(t),$ (4)

where $x_{U,k}(t)$ represents the individual uplink information of user $k$,
$P_{k}$ indicates the transmitted power of user $k$; $h_{k}$ indicates the
channel gain between user $k$ and the BS, and $n(t)$ is the AWGN; zero-mean
and variance $\sigma^{2}$. It is supposed that the signals received are
indexed in decreasing order, i.e., $P_{1}\left|h_{1}\right|^{2}\geq...\geq
P_{K}\left|h_{K}\right|^{2}$. Consequently, the uplink rate that can be
achieved by user $k$ is determined as follows:

$R_{k}^{\mathrm{UL}}=\log_{2}\bigg{(}1+\frac{P_{k}\left|{h_{k}}\right|^{2}}{\sum_{i=k+1}^{K}P_{i}\left|h_{i}\right|^{2}+\sigma^{2}}\bigg{)}.$
(5)

It is assumed that $\sum_{i=k+1}^{K}P_{i}\left|h_{i}\right|^{2}$ is zero for
$i=K$. Fig. 2 (b) illustrates the uplink network for a scenario with two
users.

The above-mentioned system models are studied for SISO systems where scalars
have been used to represent the channels. Similarly, MIMO-NOMA systems can be
found in [7-9].

##  III Fundamental of PLS

Wireless networks are subjected to eavesdropping attacks because of the
broadcast nature of wireless channels. How to ensure that the private messages
are not accessed by eavesdroppers is a fundamental security requirement.
Traditionally, cryptographic algorithms have been used to achieve this [13].
PLS is an alternate strategy that makes use of the randomness property of
wireless channels at the physical layer. The idea of using random
characteristics of fading wireless channels to prevent eavesdropping has been
proposed using an information-theoretic perspective [61]. In matter of
confidentiality, if the eavesdroppers entirely disregard the transmitted
information and only randomly guess the original information bit by bit,
perfect secrecy can be obtained. The early PLS research was directly affected
by the entropy and equivocation concepts developed for communication issues
since this definition of security is closely associated to communications when
there is noise. The most efficient way to send a confidential message would be
to use wiretap channel coding to reach the highest possible data transmission
rate, which Wyner referred to as the secrecy capacity (SC) in [62]. Wyner only
demonstrated that secure communications are feasible in degraded broadcast
channels. The ideas of PLS have gained further popularity as a result of the
development of non-degraded channels [63], Gaussian channels [64], fading
channels [65] and [64], multi-antenna channels [67], and relay channels [68].

Researchers in this field have introduced several metrics for evaluating and
assessing PLS systems, and we outline some of them below.

Ergodic Secrecy Capacity (ESC):  The ESC defines a limit on the capacity,
based on the principles of information-theoretic secrecy, for a system where
the coded message is sent over a large enough number of channel realizations
to take advantage of the ergodic properties of the fading channel. The maximum
achievable transmission rate, subject to the limits of reliability and
information-theoretic secrecy, can be assessed through the SC [63]. This value
can be used to measure the secrecy performance of an AWGN wiretap channel
[61]. The SC of a single wiretap fading channel is determined as follows

$C_{s}=\left[C_{m}-C_{e}\right]^{{}^{+}},$ (6)

where $C_{m}$ and $C_{e}$ respectively represent the instantaneous capacity of
the legitimate receiver and eavesdropper, and are given by

$C_{m}=\log_{2}(1+\gamma_{m}),$ (7)

and

$C_{e}=\log_{2}(1+\gamma_{e}),$ (8)

where the instantaneous received signal-to-noise ratio (SNR) at the legitimate
receiver and the eavesdropper, respectively are
$\gamma_{m}=\frac{P\left|h_{m}\right|^{2}}{\sigma_{m}^{2}}$ and
$\gamma_{e}=\frac{P\left|h_{e}\right|^{2}}{\sigma_{e}^{2}}$. Here, $P$ is the
average power of the transmitted signal. The fading coefficients of the
channels between the transmitter and a legitimate receiver and the transmitter
and an eavesdropper are denoted by $h_{m}$ and $h_{e}$, respectively, and
$\sigma_{m}^{2}$ and $\sigma_{e}^{2}$ are the receiver noise variances.

Two possibilities of channel state information (CSI) available at the
transmitter, namely full CSI and legitimate CSI, are taken into consideration
in order to determine the ESC. In the first case, the sender is aware of the
CSI at the legitimate and the eavesdropper channels. As a result, the
transmitter only sends the information when the SNR of the legitimate channel
is higher than the SNR of the eavesdropper channel, i.e.,
$\gamma_{m}>\gamma_{e}$. The average SC over all fading realizations is used
to calculate the ESC, which is represented as follows:

$C_{s-f}^{avg}=\int_{0}^{\infty}\int_{\gamma_{e}}^{\infty}\log_{2}\bigg{(}\frac{1+\gamma_{m}}{1+\gamma_{e}}\bigg{)}f(\gamma_{m},\gamma_{e})d\gamma_{m}d\gamma_{e}.$
(9)

Likewise, the ESC when only legitimate CSI is available at the transmitter is
given as follows:

$C_{s-l}^{avg}=\int_{0}^{\infty}\int_{0}^{\infty}\bigg{(}\log_{2}\bigg{(}\frac{1+\gamma_{m}}{1+\gamma_{e}}\bigg{)}\bigg{)}^{+}f(\gamma_{m},\gamma_{e})d\gamma_{m}d\gamma_{e}.$
(10)

Secrecy Outage Probability (SOP): This metric is defined as the probability
that the SC is less than the already specified secure transmission rate
$R_{s}>0$. Based on this definition, the SOP is characterized as follows

$P_{SOP}={\text{Pr}(C_{s}\leq R_{s})}.$ (11)

Secrecy Coverage Region (SCR): In information theory, achieving the maximum
rate is one of the most important issues that we deal with. However, one of
the key issues in this subject is how to improve the coverage region at a
specified and desired rate. For this purpose, [69] and [70] provide in-depth
analysis of the concepts of coverage region. The concept of coverage region,
which is often thought of as an area outside of which the target transmission
rate cannot be achieved, is closely associated with the definition of outage
capacity [71]. The path-loss impact is added to the SNRs by taking into
consideration the fixed distance between the transmitter and receiver (as well
as the eavesdropper). Thus, the coverage region is obtained by calculating the
ergodic rates, which are dependent on the distances. As a result, the
geographic region, which ensures the secrecy rate to be at least $R_{s}>0$ is
considered as the coverage region for the fading wiretap channel, i.e.,

$\zeta(d_{m},d_{e}):=\left\\{d_{m},d_{e}:C_{s}(d_{m},d_{e})>R_{s}\right\\},$
(12)

where $C_{s}(d_{m},d_{e})$ indicates the average channel capacity when
legitimate receiver and eavesdropper are located at distances $d_{m}$ and
$d_{e}$, respectively. We can characterize SCR as the smallest distance
$d_{e}$ between legitimate transmitter and eavesdropper for which secure
communication is ensured by $d_{m}$.

Effective secrecy throughput: This measure has been recently presented to
evaluate dependability and safety [59], [72]. The channel throughput with
regards to the reliability and security requirements is known as the effective
secrecy throughput. This ensures that the average network latency is within
acceptable limits and that the data being transmitted is safe and secure [59],
[72].

Low probability of detection: A cutting-edge transmission method called low
probability of detection communication concentrates on the confidentiality and
security of wireless networks. Some studies in recent years have investigated
the basic constraints of low-probability detection communication by assessing
the amount of information bits transmitted between two users while placing
restrictions on the probability of a warden making a detection error [73].

##  IV PLS for NOMA Systems

Most recently, the PLS for NOMA systems has received a great deal of interest
from the community. In order to comprehensively address the topic of PLS in
NOMA, we will first discuss the technical difference between the PLS in uplink
NOMA and the PLS in the downlink NOMA. Following that, in the following sub-
sections, we classify and evaluate the current works into four scenarios based
on the type of eavesdropper, the presence of a relay, RIS, and analyzing the
PLS concept for AmB-NOMA systems.

###  IV-A Technical Difference between PLS in Uplink NOMA and Downlink NOMA

The PLS in uplink NOMA and downlink NOMA transmissions exhibits technical
differences that stem from the distinct characteristics of each communication
direction. Understanding these differences is crucial for designing robust and
efficient PLS mechanisms in NOMA systems. In this sub-section, we analyze and
compare the technical disparities between uplink NOMA and downlink NOMA in
terms of PLS.

In uplink NOMA, the primary focus of PLS lies in safeguarding user privacy
against potential eavesdroppers and ensuring the secure transmission of
sensitive information. Various techniques have been proposed to enhance PLS in
uplink NOMA, such as PA schemes that distribute power levels among users based
on their channel conditions [74]. Cooperative jamming techniques have also
been suggested, whereby selected users deliberately generate interference to
confuse eavesdroppers [75]. On the other hand, the focus of PLS in downlink
NOMA revolves around ensuring confidential and reliable transmission to the
intended users while minimizing the impact of potential eavesdroppers.
Researchers have explored different approaches to enhance PLS in downlink
NOMA, including transmit beamforming techniques that direct the signal towards
the intended user while minimizing leakage to unintended recipients [76].
Moreover, AN injection techniques have been proposed, involving the
intentional introduction of additional noise by the BS to confuse
eavesdroppers [77].

A comparative analysis of the technical disparities between PLS in uplink NOMA
and downlink NOMA unveils unique characteristics and design considerations for
each transmission direction. Uplink NOMA places a strong emphasis on
protecting user privacy against eavesdropping attacks, while downlink NOMA
focuses on ensuring secure transmission to intended users while mitigating the
impact of potential eavesdroppers. These dissimilarities arise due to
fundamental variances in signal propagation, receiver architectures, and user
roles within NOMA systems. By comprehending and addressing these technical
distinctions, it becomes feasible to develop tailored solutions for PLS that
effectively tackle the specific challenges and requirements of uplink NOMA and
downlink NOMA transmissions.

Table III provides a comprehensive analysis of the technical differences
between uplink and downlink NOMA.

TABLE III: A comparison of the technical disparities between uplink and downlink NOMA. Technical Disparities |  Uplink NOMA |  Downlink NOMA
---|---|---
Access Direction |  User to Base Station |  Base Station to Users
Channel Allocation |  Power domain |  Power and/or code domain
User Multiplexing |  Multiple users share the same resources simultaneously |  Single user per resource block
Interference Management |  SIC |  Multiuser Detection or SIC
Complexity |  Moderate complexity due to SIC |  High complexity due to Multiuser Detection or SIC
User Grouping |  Based on channel conditions and power levels |  Based on user characteristics and quality requirements
User Fairness |  Challenging to achieve fairness |  Easier to achieve fairness
Power Efficiency |  Higher power efficiency due to simultaneous access |  Lower power efficiency due to single-user access
Application Suitability |  IoT devices, low-latency applications |  High data rate applications, diverse user requirements
Performance Trade-offs |  Spectral efficiency vs. user fairness |  Spectral efficiency vs. complexity

###  IV-B Types of Eavesdroppers in NOMA Systems

Generally, the reliability, throughput, security, and other requirements that
different NOMA users can have should be considered while designing PLS
approaches. The basic system model for NOMA-based networks for PLS is
demonstrated in Fig. 3. Furthermore, there are two types of eavesdroppers:
internal and external. An internal eavesdropper is a legitimate user who also
eavesdrops other users’ information, whereas an external eavesdropper does not
belong to any legitimate user. Another factor to consider when designing PLS
methods is whether or not an eavesdropper is active (disrupting wireless
communication by initiating jamming or channel estimation attacks) or passive
(just observes the communication without interfering) [78-80].

Figure 3: The PLS scenario for NOMA.

This study focuses on internal eavesdropping and passive external
eavesdropping. In the beginning, we classify eavesdroppers into the two
categories described below:

Internal eavesdroppers: Internal eavesdroppers are legitimate users who share
the same time-frequency resources as other network users and try to intercept
other users’ information.

External eavesdroppers: In contrast, external eavesdroppers are illegitimate
users who are using the same bandwidth.

The objectives for security design in NOMA can be classified into three main
distinct groups, as illustrated in Fig 4, depending on its requirements, which
we will discuss below.

Figure 4: A classification of NOMA systems with $K$ legitimate users under
eavesdropping conditions: a) Internal eavesdroppers, b) External
eavesdroppers, c) Both internal and external eavesdroppers.

####  IV-B1 Security Designs against Internal Eavesdroppers

In this scenario, it is presumed that the internal users (NUs and FUs) are
untrusted while creating security protocols to combat internal eavesdroppers.
Here, the design objective is to secure user information from one another
while ensuring that the SIC operates normally. The following explanations will
cover the two types of internal eavesdropping.

Eavesdropping of FU by NU: The primary security concern for FU under the NOMA
principle is caused by the fact that the NU must decode the FU’s signal in
order to use SIC. The signal from the FU is given additional strength, which
is another important factor that facilitates the NU’s ability to detect it.
Here, maintaining SIC functionality while preventing information from FU to be
obtained by NU is the design objective. In order to further clarify this
matter, it is important to note that there exist two different types of SIC
receivers: the first is a symbol-level SIC receiver, which demodulates the FU
signal without decoding it in order to use SIC, and the second is a codeword-
level SIC receiver, which demodulates and decodes the FU signal simultaneously
in order to use SIC. The messages can only be secured by encryption methods in
the codeword-level-SIC case. However, PLS approaches can be used in the
situation of symbol-level-SIC. By converting FU’s messages into a different
domain and applying a special sequence, security can be provided to FU’s
messages in symbol-level based SIC so that NU can use SIC but cannot decode
FU’s messages [78]. The use of channel-dependent characteristics is another
method for doing this transformation.

Eavesdropping of NU by FU: According to the fundamentals of NOMA, the FU can
directly decode its signal while treating NU’s information as noise.
Nevertheless, it may detect the NU signal after acquiring its own signal.
Here, the objective of this design is to secure NU’s messages from FU while
ensuring that SIC functions normally. As compared to the security issue with
FU’s messages, designing security techniques in this case is easier. In order
to meet the security necessities of NU and ensure that the fundamental data
rate requirement of FU is satisfied, the BS can use PLS approaches based on
PA, beamforming, or any other method that relies on adaptability. For
instance, during beamforming, the design must consider a compromise among
providing security at the NU and dependability at the FU [78].

####  IV-B2 Security Designs against External Eavesdroppers

In this case, NU and FU are trusted. Therefore, the objective of the design is
to secure NU and FU messages from an external eavesdropper. It is essential to
consider eavesdropper’s location while designing security algorithms since it
can impact the NOMA system’s security performance. The legitimate users are
given different levels of power, which results in an uneven level of security
for them relative to Eve’s location. Accordingly, Eve has a set of abilities
to eavesdrop on their signals in various dimensions. The following are two
main conditions that must be considered while designing algorithms for this
situation: first, the fundamental SIC process should be performed regularly
with the security algorithms, which implies that the suggested algorithms
should not interfere with how the usual NOMA operates; secondly, it is
anticipated that the methods will function even when there is a significant
spatial similarity among channels belonging to legitimate and illegitimate
parties. A number of PLS techniques have been proposed to combat external
eavesdropping, such as channel-based PA optimization for each user, assigning
subcarriers to users, channel ordering of NOMA users together with the
decoding order, adding interfere signal, optimize beamforming policies,
transmit antenna selection approaches, phase manipulation, key generation, and
exploiting user interference [79].

####  IV-B3 Security Designs against both Internal and External Eavesdroppers

There is an internal as well as an external eavesdropper in this case, and the
network users are not to be trusted. How to ensure the security of signals
intended for NU and FU from external eavesdroppers as well as from one another
is an objective of this design. In terms of security design, this situation is
the most challenging. In order to achieve the aforementioned objectives, the
design methods must guarantee that SIC will perform properly. In this
situation, transforming the signal of nearby and distant users into a
different domain using various randomization sequences is one potential
security measure [80].

The objectives of security designs for the scenarios explained above are
summarize in Table IV.

TABLE IV: A Summary of Security Design Objectives for the Various NOMA.  Ref |  Security scenario |  Design purpose |  Some potential solutions
---|---|---|---
[78] |  Security Designs against Internal Eavesdroppers |  Ensuring normal SIC while protecting users’ information from one another |  Transformation of FU to another domain, Power allocation for beamforming.
[79] |  Security Designs against External Eavesdroppers |  Ensuring normal SIC while protecting NU and FU data from external eavesdroppers |  Beamforming based on PA, interference use, and relay selection.
[80] |  Security Designs against both Internal and External Eavesdroppers |  Ensuring normal SIC while protecting users’ information from each other and external eavesdroppers |  Phase rotation based on a channel, interference mitigation, and transformation of users’ signal into another domain.

###  IV-C PLS for NOMA Systems with Relay

First introduced in [81], the relay can improve transmission rate as well as
facilitate communication between the users and BS. Since then, the relay has
been considered in different communication systems, including studying the PLS
for such systems. Indeed, the relay was first introduced for the wiretap
channel in [62]. The author of this work studied the coding issue with the
relay channel when some of the transmitted messages are private to the relay.
After that, in [82], Lifeng Lai et al. proposed several cooperation strategies
for the wiretap channel with relay and obtained corresponding achievable
performance bounds. Additionally, the some recent studies have investigated
how the relay affects PLS performance for NOMA networks [14-25]. Numerous
studies have been done under cooperative NOMA, namely when NOMA and relaying
are combined. Two models for the relay, trusted and untrusted relay, are taken
into consideration when studying the secrecy performance for such channels. In
the existing works, the relay is referred to be untrusted in the sense that
the messages transmitted to the users should remain a secret from it.
Nevertheless, it is presumed that the relay is unmalicious in which it would
not deviate from its purpose of transmission or try to attack the users; it
could only be inquisitive enough to try to decode the users’ messages. There
are practical applications for such a communication scenario. For instance,
users of a network that provides data could have varying access to different
information contents depending on their subscription plans or could have
varying hierarchical security approvals for various types of data. Users must
cooperate with one another and follow the network protocols since they are
legitimate members of the same network. In contrast, an untrusted relay can be
inquisitive sufficient to decode the signals’ contents before transmitting
them to the users.

In the following, we go into details on how the relay affects NOMA networks’
secrecy performance.

Cooperative NOMA: In cooperative NOMA, nearby users to the BS who have better
channel conditions decode information for others and serve as relays for those
further away users who have bad channels to the BS. This will improve
reception reliability for them. A cooperative NOMA system with two users, a
BS, an eavesdropper, and a relay was explored in [14]. It was assumed that the
BS had no direct link with either the users or the eavesdropper. For relaying,
both decode-and-forward (DF) and amplify-and-forward (AF) protocols were taken
into consideration. The results of the SOP analyses showed that DF and AF
reach almost identical secrecy performance levels. [15] considered a similar
system model in the presence of a direct link and a relay link, while the
secrecy performance was studied under different PAs and target rates for both
trusted and untrusted relays. To assess the dependability and security
performance of the underlying NOMA system with arbitrary system parameters,
the SOP and strictly positive SC were obtained. The resulting findings showed
that the secrecy performance could be remarkably improved by judicious
selection of such parameters. The authors of [16] studied a NOMA system with
two-way relay and multiple preassigned user pairs. Then, a cooperative scheme
of the PA and subcarrier assignment was proposed for the secrecy energy
efficiency minimization challenge. Many-to-many matching was used to manage
the subcarrier assignment, and on this basis, the PA problem was resolved by
using geometric programming. The authors of [19] considered a cooperative NOMA
system based on DF in which an eavesdropper could wiretap information being
transmitted from the relay. It was shown that the use of relay could reduce
the SOP of the opportunistic user. In contrast of this study, a cooperative
NOMA system based on AF that comprises of a multi-antenna source, a single
antenna relay, and a destination was investigated in [20]. The relay has been
considered untrusted and it acted as an eavesdropper. The antenna that
increases the secrecy rate at the BS was selected when channel state
information was available. Here, a straightforward strategy based on choosing
an antenna that maximizes the connection quality between the source-
destination links has been presented to decrease the signaling overhead. In
[21], the authors suggested a two-stage secure relay selection strategy with
NOMA to improve PLS in a cooperative network with multiple source-destination
pairs and multiple relays. In the suggested approach, multiple eavesdroppers
were present while two sources simultaneously communicated with their
respective destinations over a single selected relay. The selected relay
ensures successful, secure transmission of another source-destination pair at
a predefined rate while maximizing the channel capacity of one source-
destination pair. The suggested technique greatly outperforms this approach
for OMA systems where the transmit power of the sources and chosen relay is in
the middle and low regimes, according to an analysis of the SOP performance
for this scheme.

Full Duplex Relay: The secrecy performance of a NOMA system with half duplex
and full duplex relay (HDR and FDR) has been studied in [22]. The considered
system consists of two legal NOMA users and one eavesdropper where a devoted
FDR (or HDR) assists a far user. For NOMA users in these networks, exact
expressions for SOP have been obtained. It has been demonstrated that FDR
system always has lower SOP than HDR system. Additionally, due to the
existence of a relay that decodes and forwards messages from the far user in
both FDR and HDR systems, the NU has much less SOP than the FU. A cooperative
NOMA system with two source-destination pairs sharing a single FD-DF relay was
investigated in [23]. It is presumed that there are no direct links between
the sources, destinations, and eavesdroppers. Therefore, the sources use
uplink NOMA to transmit information to the relay, while the relay employ
downlink NOMA to forward information to the destinations. In this scheme, the
relay generates AN during information transmission to prevent eavesdropping.
To maximize the system capacity, the optimal PA between the AN signal and the
information signal is established. Furthermore, analytical and numerical
outcomes over SOP show that the suggested method greatly outperforms the joint
NOMA and AN in HDR scheme. The authors of [24] created a NOMA-based
cooperative network that is secure against eavesdropping, in which the source
transmits confidential NOMA symbols to the destination using rate-splitting
technique under the assistance of a FDR. Moreover, the source divides its
symbol into two sub-symbols for superposition coding through PA and uses the
channel gain differences created by the relay to make more efficient use of
spectral resources. In fact, the relay uses FD operation to simultaneously
receive NOMA symbols and send jamming signals in the same frequency band. This
is done in order to increase the capacity of the legitimate channel and create
confusion for any possible eavesdroppers.

Another secure two-way relay network based on NOMA with regard for different
eavesdropping scenarios has been introduced in [25]. It has been demonstrated
that the capability of the relay to prevent eavesdropping without affecting
the legitimate users’ ability can be improved through the employment of FD and
AN techniques. Additionally, it is demonstrated that the data transmission
efficiency has been increased with the use of FD mode to the user nodes in the
first phase without the need for additional bandwidth resources. Finally, by
obtaining closed-form expressions for the ESC, it has been determined that the
relay performs two crucial functions in this networks: not only transmits the
confidential information to two sub-users, but it clogs any probable
eavesdropper.

The existing works show that in high SNR regimes, the SOP of NOMA systems for
both the DF and AF protocols tends to be a constant. Moreover, to provide
dependable communication in such a NOMA system, suitable quantities for the
objective rate, PA coefficients, and the power level of the eavesdropper link
under the influence of jamming signals should be selected. Despite the fact
that secure objective rates in the scenarios under study have no impact on
SOP, the best SOP performance is shown by the optimal PA parameters for NOMA.
Additionally, by carefully choosing the PA for users in NOMA and raising the
level of power provided to the untrusted relay, the strictly positive SC can
be increased.

Table V presents a description of the relaying strategy, structure of links,
type of duplex, obtained metrics for PLS, and main contributions for secure
NOMA networks in the presence of the relay.

TABLE V: A Summary of Secure NOMA Networks in the Presence of the Relay. Ref |  Type of Duplex |  Relaying Strategy |  Structure of the Links |  Obtained PLS Metrics |  Main Contribution
---|---|---|---|---|---
[14] |  HDR |  AF/DF |  No direct links between BS and the users/ eavesdropper |  SOP, strictly positive secrecy rate |  PLS investigation for cooperative NOMA systems by considering AF and DF protocols
[15] |  HDR |  DF |  Relay NOMA network with a direct/indirect link |  SOP, strictly positive secrecy rate |  DF relay scheme to give secure operation between BS and NOMA users in three different schemes considering relay and direct links
[16] |  FDR |  AF |  Direct links between the users and relay |  Secrecy energy efficiency |  Exploring NOMA two-way relay wireless networks’ secure communications in the presence of eavesdroppers in situations in which cooperative jamming is used and when it is not used at the relay station
[19] |  HDR |  DF |  No direct links between the source and the destinations |  SOP |  Proposing an optimal antenna selection for cooperative NOMA networks
[20] |  HDR |  AF |  Direct links between the source and the destinations |  Ergodic secrecy rate |  Suggested two transmission antenna selection schemes to further improve security
[21] |  HDR |  DF |  No direct links between the source and the destinations |  SOP |  A concept for a secure relay with two stages that aims to increase one source-destination pair’s capacity while assuring the reliable transmission of the other source-destination pair
[22] |  FDR /HDR |  DF |  No direct links between BS and FUs |  SOP |  Investigation PLS of a NOMA system with FDR and HDR
[23] |  FDR |  DF |  No direct links from source to destination/ eavesdropper |  SOP |  Investigation an AN aided secure transmission for NOMA FDR network
[24] |  FDR |  DF |  All the links are available |  Ergodic secrecy rate |  Development of a NOMA-based cooperative network against multiple eavesdropping where under FDR assistance and protection, the source uses a rate-splitting strategy to transmit secret information to the destination.
[25] |  FDR |  DF |  All the links are available |  Ergodic secrecy rate |  Proposed a secure two-way relay network based on NOMA, where the relay not only prevents the network from eavesdropping but also improves spectral efficiency by giving NOMA users access to heterogeneous channels.

###  IV-D PLS for NOMA Systems with RIS

RIS is a novel technology that has been proposed recently as a means of
addressing the randomness and uncontrollability of wireless signal propagation
[83]. RIS can reduce the damaging effects on radio waves that occur due to
natural wireless transmission by scattering, managing the reflection, and
refraction properties. Furthermore, RIS offers a new approach to the planning
and optimization of wireless communication networks [84]. RIS can also improve
the received signals [85] or reduce undesirable signals like co-channel
interference [86] by modifying the reflection amplitude and phase
coefficients. As was already noted, power domain NOMA has the ability of
providing services to numerous users simultaneously within the same physical
resource block, thereby enhancing secrecy efficiency and connection density
[87]. Since NOMA is more effective when the differences between the user
channel gains are greater and RIS can proactively adjust user channels to
accomplish this aim [87], it is anticipated that combining RIS with NOMA can
further improve the network performance. The private information is
susceptible to eavesdropping due to the broadcast nature of wireless channels.
Therefore, in order to accomplish secure communications, PLS, which makes use
of wireless channels’ properties, is proposed. Since RIS has the possibility
to adjust the wireless propagation environment, it can be used to improve PLS
by intelligently altering the reflection coefficients for signal enhancement
at the receiver and signal mitigation/cancellation at the eavesdropper [88].
Indeed, the emergence of RIS technology has provided a new solution for PLS
problems. In the following, we will go into details on how RIS affects NOMA
networks’ secrecy performance.

Design Strategies for Ensuring PLS of RIS-Aided NOMA Networks: In [24] a new
design of RIS to improve the PLS in the RIS-assisted NOMA network has been
proposed. The issue of how the increase of RIS elements affect the secrecy
performance has been resolved in this study. Additionally, it has been shown
that the network can employ traditional channel coding techniques to achieve
confidentiality. As two common security scenarios in these networks, the joint
precoding and reflecting beamforming approach for the internal untrusted user
and the joint beamforming strategy assisted by artificial jamming for the
external eavesdropping have been displayed in [27]. Furthermore, the
effectiveness and feasibility of these two methods are shown through
simulation results. For the first scenario, it has been shown that; higher SNR
threshold of artificial jamming results in lower sum eavesdropping rate.
Moreover, from the results obtained for the second scenario, it can be seen
that when the RIS is close to the legitimate user, better security performance
can be obtained with the same number of RIS elements. The introduction of RIS
into NOMA networks enables secure communication using artificial jamming,
where the multi-antenna BS transmits the NOMA and jamming superposition to the
legitimated users with the help of RIS, in the presence of a single-antenna
passive eavesdropper [28]. In this investigation, the jamming vector, the
transmit beamforming, and the RIS reflecting vector are jointly optimized to
maximize the sum rate of legitimate users while fulfilling the SIC decoding
condition, the RIS reflecting constraint, and the quality-of-service (QoS)
requirement. Furthermore, to ensure successful cancellation through SIC, the
received jamming power has been adjusted at the highest level to all
legitimate users. The non-convexity of the formulated optimization issue
causes it to be split into two subproblems, i.e., the beamforming optimization
and the RIS reflecting optimization. The successive convex approximation (SCA)
approach was adopted to approximate each subproblem to a convex approximation,
after which an effective solution based on iterative optimization was designed
to solve each subproblem iteratively. The simultaneous transmitting and
reflecting RIS in conjunction with NOMA is a very promising method that can
considerably enhance the performance of coverage. On the other hand,
eavesdroppers could get comparable performance advantages to the legitimate
user. In order to optimize the secrecy rate, a secure communication approach
supported by AN is offered in [29] as a solution to this issue. Then, by
optimizing the passive beamforming, NOMA parameters, and AN signal model, a
secrecy rate maximization issue has been developed under the restrictions of
individual secrecy needs and overall transmit power. The obtained results from
solving this non-convex issue demonstrated that simultaneously transmitting
and reflecting RIS and AN can significantly enhance secrecy performance in
comparison to benchmark schemes and increasing the number of RIS elements can
decrease the required AN power. The ideas presented in this research provides
to useful suggestions for the design of a method to simultaneously aid the
transmission and reflection of signals in a RIS network.

The PLS is studied for a RIS-assisted NOMA 6G network in [30], where a RIS is
placed to help the two NOMA users in the “dead zone” and both internal and
external eavesdropping have been considered. For the scheme with only internal
eavesdropping, the worst-case situation, in which the NU is unreliable and
tries to intercept the information of the FU, has been taken into
consideration. To enhance the PLS, a combined sub-optimal beamforming and PA
strategy has been presented. Then, the scope of this study was expanded to
include a scenario with both internal and external eavesdropping. For this
scenario, there are two sub-scenarios that have been taken into consideration:
one in which eavesdroppers do not have access to CSI, and the other in which
they do. A noise beamforming system has been introduced for both sub-scenarios
to protect from the external eavesdroppers. Moreover, in order to further
increase the PLS for the second sub-scenario, an optimal PA strategy has been
proposed. Finally, it has been demonstrated that increasing the number of
reflecting components improves secrecy performance, and thus, can bring more
gain in secrecy performance than that of the transmit antennas. The secure
beamforming of a two-user uplink NOMA system assisted by a RIS, in which both
users transmit AN to confuse the eavesdropper and simultaneously sending
messages to the BS, has been studied in [31]. This paper formulates the
combined beamforming optimization of the users and the RIS as a quadratic-
fractional problem in order to increase security while ensuring max-min
fairness. The deployment of an uplink transmission framework that
simultaneously transmits and reflects RIS to the relay superimposed signals
from indoor and outdoor users to the BS while preventing malicious
eavesdropping has been discussed in [32]. In this study, two joint beamforming
optimization problems for maximization of the minimal SC and minimization of
the maximum SOP were formulated by considering different eavesdropping CSI
assumptions. It was observed that when using the adaptive rate wiretap code
setting, it was better to deploy simultaneously transmitting and reflecting
RIS close to the users or the BS, whereas when using the constant rate wiretap
code setting, it was better to deploy simultaneously transmitting and
reflecting RIS far from them.

An effective beamforming technique with AN to provide secure NOMA transmission
with the IRS has been presented by considering a practical eavesdropping
scenario with imperfect CSI of the eavesdropper [33]. The transmit power has
been minimized in this study by solving a simultaneous transmit beamforming
and RIS phase shift optimization problem. A secure NOMA network with
distributed RISs helping a BS to transmit private information to NOMA users
while protecting them from passive eavesdroppers has been studied in [34]. In
this study, the objective is to improve the minimum secrecy rate of legitimate
user by jointly designing the reflection coefficients, transmit power, and
beamforming, subject to the transmit power restriction at the BS, the phase
shift restriction of RISs, the SIC decoding restrictions, and the SOP
restrictions. The exact SOP in closed-form terms for the case of a single-
antenna BS has been calculated and a ring-penalty based SCA was used to
simultaneously optimize the power distribution and phase shifts. The general
multi-antenna BS scenario was then considered, and a Bernstein-type inequality
approximation based alternating optimization method was considered to design
the transmit beamforming matrix at the BS and alternatively optimize the
reflection coefficients of RISs. The obtained results in this study clearly
showed that the maximum secrecy rate is attained when distributed RISs shared
the reflecting elements equally. The use of inter-user interference as a
secure transmission method for an RIS-assisted NOMA system without
eavesdroppers CSI has been suggested in [35]. By maximizing the transmit power
of weaker users and changing the SIC order, the authors attempted to minimize
the SNR of the passive eavesdropper to improve eavesdropping. Here, two
transmit power maximization challenges have been established by simultaneously
maximizing the active beamforming vector and passive RIS reflecting matrix,
each of which is based on a distinct set of assumptions on secure users.
Following the decomposition of each non-convex problem into two convex
subproblems using semi-definite relaxation and successive SCA, a different
optimization framework has been adopted to effectively address these
optimization issues. It has been demonstrated that the suggested scheme
outperforms the baseline schemes in terms of security performance under
various QoS conditions.

Finally, to improve the internal secrecy of NOMA users with heterogeneous
secrecy needs, RIS has been implemented in [36]. The goal is to jointly
optimize the beamforming vectors and RIS reflection factors in order to
minimize the overall transmission power under heterogeneous secrecy
restrictions. By utilising the SCA and semi-definite relaxation techniques, an
iterative solution based on alternating optimization has been proposed to
effectively address this issue. It has been observed that under the users’
individual secrecy requirements, the suggested NOMA algorithm with the help of
RIS can greatly reduce the power usage.

PLS of RIS-assisted NOMA Networks over Various Fading Channels: PLS can be
efficiently applied in fading channels for ensuring security measures in the
presence of attacks from illegal eavesdroppers [13]. To provide secure
communications, the randomness of wireless fading channels is specifically
studied, and the wiretap coding method suggested in [62] has been extensively
adopted as a secure channel coding approach in PLS. Recent studies have
examined the critical secrecy performance metrics over a variety of fading
channels, such as Rayleigh [66], [70], Rician [89], [90], Nakagami-_m_ [91],
Fisher-Snedecor _F_ [92], etc. Nevertheless, PLS measures are rarely employed
to measure performance in RIS-assisted NOMA. Some of these studies are
reviewed below. The SOP of a RIS-assisted NOMA network in a multi-user setting
was studied in [37]. It is important to note that the authors of [37] focused
into Rayleigh fading when studying the PLS of RIS-assisted NOMA networks.
Then, the secrecy performance of the RIS-aided NOMA networks with Nakagami-_m_
fading considered for the reflected links has been investigated in [36]. The
closed-form term of the SOP has then been obtained using the channel
statistics. Analytical findings show that the number of RISs and the Nakagami-
_m_ fading parameters affect the secrecy diversity orders and the expectation
of channel gain for the reflected connections, respectively. As an extension
of this study, the effectiveness of the suggested network has been assessed in
terms of the average SC in [39]. [40] analysed the PLS of a NOMA system with
RIS assistance over Fisher-Snedecor _F_ composite fading channels. In more
detail, closed-form terms that are presented in terms of Meijers G-function
are used to determine the outage probability and SOP. It is assumed that the
system is constructed on a RIS access point and that RIS is employed to
enhance the secrecy performance of two legitimate users. The analytical
findings in this study have shown the impact of the quantity of reflecting
components, the degree of fading, and the impact of shadowing on the
performance of PLS metrics of the systems.

Tables VI and VII compare the above realizations for secure RIS-NOMA networks
from two perspectives.

TABLE VI: A Summary of the Various RIS Designs Used to Realise PLS in RIS-NOMA Networks. Ref |  NOMA Scenario |  CSI of Eav |  AN |  Optimization |  Main Contribution
---|---|---|---|---|---
[26] |  Downlink |  Perfect CSI |  - |  Maximize sum rate |  Comparison of the secrecy performance of conventional NOMA and signal-enhance RIS-NOMA scenarios
[27] |  Downlink |  No CSI |  + |  - |  Reduction sum eavesdropping rate due to higher SNR threshold for AN
[28] |  Downlink |  No CSI |  - |  Maximize sum rate |  Maximizing legitimate user sum rate by Jointly optimizing transmission beamforming, jamming vector and RIS reflections vector
[29] |  Downlink |  Perfect CSI |  + |  Maximize secrecy rate |  Maximizing secrecy rate under the restrictions of individual secrecy requirement and total transmit power
[30] |  Downlink |  Perfect CSI |  + |  Maximize secrecy rate |  Investigation of PLS for RIS-assisted NOMA system in two scenarios: a) with only internal eavesdropping, b) with internal and external eavesdropping
[31] |  Uplink |  Perfect CSI |  + |  Maximize minimal secrecy rate |  Active beamforming and phase shifts of RIS are jointly optimized to provide secure beamforming
[32] |  Uplink |  Perfect CSI/ Statistical CSI |  - |  Maximization the minimum secrecy capacity and minimization the maximum SOP |  The joint optimization of the passive and active beamforming for a two-user RIS-assisted uplink NOAM system
[33] |  Downlink |  Imperfect CSI |  + |  Minimize transmit power |  The jamming power has been minimized to mislead eavesdroppers while reducing its interference to legitimate users
[34] |  Downlink |  Statistical CSI |  - |  Maximize the minimum secrecy rate |  It is achievable to enhance the minimal secrecy rate of a legitimate user by jointly designing the transmit power, reflection coefficients, and beamforming
[35] |  Downlink |  No CSI |  - |  Maximize transmit power of weaker user |  Proposing a secure transmission strategy through user interference engineering for a RIS assisted NOMA system
[36] |  Downlink |  Perfect CSI |  - |  Minimize transmit power |  The joint optimization of the RIS reflection parameters and beamforming vectors under heterogeneous secrecy constraints to reduce the total transmission power
TABLE VII: PLS Realisation in RIS-NOMA Networks with Various Fading channels. Ref |  NOMA Scenario |  CSI of Eav |  Fading Distribution |  Obtained PLS Metrics
---|---|---|---|---
[37] |  Downlink |  No CSI |  Rayleigh fading |  SOP, asymptotic SOP
[38] |  Downlink |  No CSI |  Nakagami-_m_ fading |  SOP, asymptotic SOP, secrecy diversity orders
[39] |  Downlink |  No CSI |  Nakagami-_m_ fading |  SOP, ASC, asymptotic approximation of the SOP and the ASC
[40] |  Downlink |  No CSI |  Fisher-Snedecor _F_ Composite Fading |  Novel expression for the SOP

###  IV-E PLS for AmB-NOMA Systems

It should be noted that in NOMA communication systems, a larger number of
access users might cause a significant rise in power consumption. AmB
communication was suggested as a promising solution to deal with the issue of
power consumption since it can accomplish data transmission through the
surrounding radio signal from ambient radio-frequency (RF) sources, without
needing a separate energy source [93]. The backscatter device in AmB
communication uses the incident RF signals such as Wi-Fi, cellular, or TV
transmissions to modulate and reflect its signal to the readers [96]. Due to
its applicability in low-powered Internet-of-Things (IoT) networks, NOMA’s
inclusion in backscatter communication has attracted significant academic
attention. In spite of this, such systems are susceptible to a number of
security risks, including interference and eavesdropping, because of the
simple coding and modulation techniques. In the following, the security aspect
of these networks has been reviewed.

Impact of in-phase and quadrature-phase imbalance (IQI), cognition, and
hardware impaired: Investigations on the dependability and security of NOMA
systems under the assumption that all nodes and the backscattering device
experience IQI can be found in [44]. The results obtained in this study have
demonstrated that while IQI has a negative impact on dependability, it may
improve the target system’s secure performance. Additionally, the suggested
AmB-NOMA system can offer robust secure communication for backscatter devices.
By deriving various security metrics, it has been explored in [45] how the
overlay cognitive AmB-NOMA based on intelligent transportation system performs
in terms of secrecy while there is an eavesdropping vehicle. For applications
such internet-of-vehicle (IoV) enabled maritime transportation systems (MTS),
the security and dependability of the cognitive AmB-NOMA network in the
presence of IQI have been examined in [46]. As expected, these applications
offered large-scale connections, varied QoS, and highly reliable, low-latency
connectivity. Therefore, the significance of this subject has been highlighted
in light of the previously mentioned points. The PLS of the AmB-NOMA systems
with focus on security and reliability have been explored in [47] under the
reasonable hypotheses of remaining hardware issues, imperfect SIC, and
incorrect channel estimate. The RF source in this study serves as a jammer,
sending interference signals to the legitimate receivers and eavesdropper in
order to further increase the security of the system under consideration.
These assumptions have a negative impact on the outage probability, but a
positive impact on the intercept probability, according to the research
outcomes.

Optimization perspective: In [48], it is discussed how NOMA IoT users can
operate simultaneously with a backscatter in the presence of several
eavesdroppers. The basic purpose of this research was to propose an
optimization methods for maximizing the secrecy rate of AmB-NOMA in the
presence of several non-colluding eavesdroppers. The benchmark schemes of
conventional OMA and suboptimal NOMA have been presented in order to evaluate
the effectiveness of the suggested scheme in this study. In [49], a secure
beamforming technique of multiple-input single-output (MISO) NOMA backscatter
symbiotic radio networks has been devised to optimize outage secrecy rate
between backscatter devises and the central user under the circumstances of
feasible secrecy rate restrictions. The results obtained in this study show
that the proposed network achieves a substantially higher secrecy rate region
than the OMA network.

A summary of the important aspects to secure AmB-NOMA networks has been
provided in Table VIII.

TABLE VIII: A Summary of PLS Realisation in AmB-NOMA Networks.  Ref |  AN |  IQI |  Optimization |  Obtained PLS Metrics |  Main Contribution
---|---|---|---|---|---
[44] |  - |  + |  - |  SOP, secrecy intercept probability (SIP), asymptotic SOP |  Investigation the effects of IQI on the PLS of the AmB-NOMA systems
[45] |  - |  - |  - |  SOP |  Describing the secrecy performance of ITS-based AmBC-NOMA cognitive overlay in the presence of an eavesdropping vehicle
[46] |  - |  + |  - |  SOP, SIP, asymptotic SOP |  Analysing the reliability and security performance of the cognitive AmB-NOMA IoV-MTS network with IQI at radio-frequency front-ends and the existence of an eavesdropper
[47] |  + |  - |  - |  SOP, SIP, asymptotic SOP/SIP |  Examining the combined effects of residual hardware impairments, inaccurate channel estimate, and imperfect SIC on the reliability and security of AmB-NOMA systems
[48] |  - |  - |  Secrecy rate maximization |  - |  Providing an optimization framework for maximizing the secrecy rate of AmB-NOMA under multiple non-colluding eavesdroppers
[49] |  - |  - |  Secrecy rate maximization |  - |  Investigating a MISO NOMA backscatter device assisted secrecy rate network with the possibility eavesdropper

##  V Challenges, Possible Solutions, and Future Directions

We point out several open problems and challenges related to PLS in NOMA
networks and discuss the corresponding research directions and opportunities
in this section.

Simultaneous Transmitting and Reflecting RIS (STAR-RIS) Communications in
Secure NOMA Networks: The STAR-RIS has been developed to increase the
effectiveness of RIS communications [95]. This technique enables the RIS to
broadcast and reflect incident signals while simultaneously increasing
coverage and transmission efficiency. However, the fundamental issue of
protecting privacy has not yet well been addressed, despite the fact that much
effort is put into the STAR-RIS enabled NOMA communications. In reality, since
STAR-RIS is able to provide a 360-degree service, it unavoidably results in a
360-degree eavesdropping, which poses more significant security issues to the
transfer of private information than that with a traditional reflecting-only
RIS. Additionally, there are a number of other issues that should be
investigated for the secure NOMA networks with RIS, including an analysis of
the effect of beam split for the transmit beamforming created in RIS,
performance consequences of it, how to optimize both the transmission and
reflection coefficients together, as well as how RIS should be deployed.

Various Transmission Disturbances: Current studies in 6G wireless networks
have not adequately addressed the influence of diverse transmission
disturbances, including partial CSI, discrete phase shift, and random phase
noise, on the performance of RIS-assisted transmissions. It is imperative to
thoroughly investigate the modeling of these impairments and their impact on
SNR. Additionally, it is crucial to assess how these disturbances affect the
performance of RIS-assisted transfers in terms of the ASC and the SOP. By
considering these factors, the evaluation of secure NOMA in Terahertz bands
and non-terrestrial networks can provide valuable insights for optimizing 6G
wireless systems and facilitate better visibility in relevant 6G-related
searches.

PLS Metrics under Correlated Fading Channels: The development of secure
communication techniques for RIS-aided NOMA networks under different fading
channels has received contributions in the literature [37-40]. However, most
of the research studies assume the independency between channel coefficients
in order to obtain PLS metrics such as SOP and ASC, while they are practically
correlated. The strength of this correlation is influenced by the placement of
the antennas, how close the legitimate receiver and eavesdropper are to one
another, and whether there are any scatters in the area [96]. Therefore, it is
essential to investigate the correlated fading wiretap channel in order to
comprehend how fading correlation affect PLS applications in practical
wireless communication networks. For assessing the correlated fading channels
and associated issues, having the appropriate mathematical tools can be highly
beneficial. The idea of statistical correlation between random variables can
be expressed in a variety of ways, among them one of the best adaptable and
effective approaches is the copula theory [97].

PLS for AmB-NOMA systems with delayed QoS consideration: Current research on
AmB-NOMA systems focuses exclusively on the performance analysis with delay
insensitivity, which is insufficient to describe the real-time service. A
crucial suggestion for future work is security analysis with respect to
developing an analytical framework where delayed QoS is considered on AmB-NOMA
systems.

Deep Reinforcement Learning (DRL) for PLS in NOMA:  DRL has emerged as a
powerful tool for solving complex decision-making problems in wireless
networks [98]. By combining deep neural networks with reinforcement learning,
DRL can learn optimal policies through interactions with the environment. In
the context of PLS in NOMA networks, DRL can be utilized to design intelligent
resource allocation strategies, beamforming techniques, and power control
policies that maximize security while maintaining high-quality communication
[99]. One potential application of DRL is in the optimization of transmit
beamforming in NOMA systems with RIS. DRL algorithms can learn to adaptively
adjust the transmit beamforming weights based on the channel conditions,
interference levels, and security requirements. By considering the
eavesdropping channel characteristics and the presence of RIS, DRL agents can
optimize the beamforming vectors to enhance the security performance of NOMA
transmissions. Moreover, DRL can also be employed to address the challenges
associated with secure transmission under correlated fading channels.
Traditional approaches often assume independent fading channels, which may not
accurately represent real-world scenarios. DRL algorithms can learn to model
and exploit the correlation between fading coefficients, enabling more
accurate assessment of the secrecy performance and the optimization of PA and
beamforming strategies [100].

Federated Learning (FL) for PLS in NOMA:  FL is a distributed learning
paradigm that allows multiple entities, such as BSs or users in a NOMA
network, to collaboratively train a shared model without sharing their local
data [101]. This property makes FL an attractive approach for addressing
security and privacy concerns in PLS. In the context of NOMA, FL can be
employed to jointly optimize PLS-related tasks across multiple BSs or users.
For instance, FL can be used to collaboratively learn interference mitigation
strategies that enhance the security of NOMA transmissions. Each base station
or user can train its local model using its own data while periodically
exchanging model updates with the other entities. By aggregating the knowledge
from diverse sources, FL can enable the extraction of global insights and
facilitate the design of robust and secure NOMA communication systems [102].
Furthermore, FL can be applied to address the challenge of secure
communication in NOMA systems with delayed QoS considerations. By collectively
learning and adapting to the dynamic network conditions and delay
requirements, FL algorithms can optimize resource allocation and power control
policies that simultaneously meet the QoS constraints and ensure secure
communications in AmB-NOMA systems [103-105].

##  VI Conclusion

NOMA provides low latency, high spectral efficiency, and massive connections,
whilst PLS offers simple and efficient security measures. These two
technologies can operate together to meet the needs of 5G and beyond networks
through enabling high spectral-efficiency and security. The effectiveness of
PLS as a solution to these objectives has also been extensively presented,
analysed, and discussed in this study along with the various PLS schemes for
NOMA systems. Systems that are spectrally efficient, adaptable, and secure can
be achieved by applying PLS to NOMA. Additionally, the provided schemes have
also been tabulated, discussed, contrasted, and summarised. Finally, to assist
researchers who are interested in investigating this subject, some key
challenges, possible solutions and future directions have been outlined.

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|  Saeid Pakravan received his Ph.D. degree in Electrical Engineering from
Ferdowsi University of Mashhad, Iran, in 2022, specializing in Network
Information Theory, Channel Coding, Wireless Communication, and Physical Layer
Security. Currently, he is conducting research at Laval University in QC,
Canada, focusing on electrical engineering research with a particular emphasis
on analyzing Reconfigurable Intelligent Surfaces (RIS) and addressing
optimization issues. He has published numerous papers in prestigious
international conferences and reputable journals, highlighting his expertise
in performance analysis of wireless multi-user communication systems, network
information theory, physical layer security, analysis of correlated random
variables, and polar codes.
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|  Jean-Yves Chouinard (S’75–M’83–SM’03) received the Ph.D. degree in
electrical engineering from Laval University, QC, Canada, in 1987. He joined
as a Post-Doctoral Fellow with the Space and Radio Communication Division,
Center National d’études des Télécommunications (CNET), Paris, France. From
1988 to 2002, he was a Professor with the School of Information Technology and
Engineering, the University of Ottawa, ON, Canada. Since 2003, he has been
with the Department of Electrical and Computer Engineering, Laval University.
He is the author/co-author of more than 200 journal, conference papers, and
technical reports. He is an editor of a book: Information Theory and co-author
of book chapters: MIMO Wireless Communication Systems and OFDM-Based Mobile
Broadcasting. His research interests include signal processing for radar
applications, wireless communications, and secure communication networks. Dr.
Chouinard is an Editor for the IEEE Transactions on Vehicular Technology and
Associate Editor for the IEEE Transactions on Broadcasting. He has served on
several conference committees including Technical Program, Co-Chair for the
Vehicular Technology Conference (VTC2012 Fall), 2012, and Publications Chair
for the IEEE International Symposium on Information Theory (ISIT’2008). He was
the co-recipient of Best Propagation Paper Award from IEEE Vehicular Society
and Signal Processing and Best Paper Award from European Journal of Signal
Processing.
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|  Xingwang Li (S’12-M’15) received his M. Sc. and Ph. D. degrees from
University of Electronic Science and Technology of China and Beijing
University of Posts and Telecommunications in 2010 and 2015. From 2010 to
2012, he was working with Comba Telecom Ltd. in Guangzhou China, as an
engineer. He spent one year from 2017 to 2018 as a visiting scholar at Queen’s
University Belfast, Belfast, UK. He is currently an Associated Professor with
the School of Physics and Electronic Information Engineering, Henan
Polytechnic University, Jiaozuo China. He is on the editorial board of IEEE
Transactions on Intelligent Transportation Systems, IEEE Transactions on
Vehicular Technology, IEEE Systems Journal, IEEE Sensors Journal, Physical
Communication, etc. He has serviced as the Guest Editor for the special issue
on Computational Intelligence and Advanced Learning for Next-Generation
Industrial IoT of IEEE Transactions on Network Science and Engineering, “AI
driven Internet of Medical Things for Smart Healthcare Applications:
Challenges, and Future Trends” of the IEEE Journal of Biomedical and Health
Informatics, etc. He has served as many TPC members, such as IEEE ICC,
GLOBECOM, etc. His research interests span wireless communication, intelligent
transport system, artificial intelligence, Internet of things.
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|  Ming Zeng received his B.E. and master’s degrees from Beijing University of
Post and Telecommunications, China in 2013 and 2016, respectively, and his
Ph.D. degree in 2020 in telecommunications engineering. Currently, he is an
assistant professor at the department of Electrical and Computer Engineering,
Université Laval, Canada. He has published more than 80 articles and
conferences in first-tier IEEE journals and proceedings, and his work has been
cited over 3200 times per Google Scholar. His research interests include
resource allocation for beyond 5G systems, and machine learning empowered
optical communications. He serves as an Associate Editor of the IEEE Open
Journal of the Communications Society.
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|  Wanming Hao (Member, IEEE) received the Ph. D. degree from the School of
Electrical and Electronic Engineering, Kyushu University, Japan, in 2018. He
worked as a Research Fellow at the 5G Innovation Center, Institute of
Communication Systems, University of Surrey, U.K. Now, he is an Associate
Professor at the School of Electrical and Information Engineering, Zhengzhou
University, China. His research interests include millimeter wave, RIS/THz
communication, physical layer security and so on. He has served as
2019/2020/2022 IEEE ICC Technical Program Committee (TPC) member.
---|---
|  Quoc-Viet Pham received the Ph.D. degree in telecommunications engineering
from Inje University, Korea, in 2017. He is currently an Assistant Professor
in Networks and Distributed Systems with the School of Computer Science and
Statistics, Trinity College, The University of Dublin, Ireland. He is also an
ADVANCE CRT supervisor and an Associate Investigator at the CONNECT centre,
Ireland. Prior to joining TCD, he was in various academic positions with Kyung
Hee University, Changwon National University, and Pusan National University.
He specialises in applying convex optimisation, game theory, and machine
learning to analyse and optimise cloud edge computing, wireless networks, and
IoT systems. He was granted the Korea NRF funding for outstanding young
researchers for the term 2019-2024. He was a recipient of the Best Ph.D.
Dissertation Award from Inje University in 2017, the Top Reviewer Award from
the IEEE Transactions on Vehicular Technology in 2020, and the Golden Globe
Award from the Ministry of Science and Technology, Vietnam, in 2021, a co-
recipient of an IEEE ATC Best Paper Award in 2022, and Enterprise Ireland
Coordination Support Award in 2023. He is an Editor of IEEE Communications
Letters, Journal of Network and Computer Applications and Scientific Reports,
and a Lead/Guest Editor of the IEEE Internet of Things Journal, IEEE
Transactions on Consumer Electronics, and Computer Communications.
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|  Octavia A Dobre (Fellow, IEEE) is a Professor and Canada Research Chair
Tier 1 at Memorial University, Canada. She was a Royal Society and a Fulbright
Scholar, as well as a Visiting Professor at Massachusetts Institute of
Technology. Her research interests include technologies for beyond 5G, optical
and underwater communications. She published over 450 referred papers in these
areas. Dr. Dobre serves as the Director of Journals of the Communications
Society. She was the inaugural Editor-in-Chief (EiC) of the IEEE Open Journal
of the Communications Society and the EiC of the IEEE Communications Letters,
as well as senior editor and editor for numerous journals and magazines. Dr.
Dobre received various distinctions, such as Best Paper Awards at leading
conferences, was elected as member of the European Academy of Sciences and
Arts, Fellow of the Engineering Institute of Canada, and Fellow of the
Canadian Academy of Engineering.
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