1. Introduction
Given the efficiency of superior spectral sharing and the possibility for a large number of connections at the same time slot/frequency [
1,
2], Non-Orthogonal Multi-Access technology (NOMA) in future wireless networks (5G) could serve a large user base. NOMA’s main technology is a superimposed signal sent to all users in a network by multiplexing the channel in the same power domain, but is different in terms of power factors [
3]. At each receiving terminal, the end device, which has stronger conditional channel, is allocated a lower power coefficient than other devices and performs successive interference cancellation (SIC) by treating other users’ information as interference before detecting its own information [
4]. The user, which has the weakest channel condition, only has to decode its own information by applying SIC.
Some initial studies have contributed significantly to the implementation of NOMA in the future. A complete survey in the field of NOMA includes early introduction, recent technologies and future research trends, especially discussions about NOMA’s outstanding advantages over previous technologies [
5]. The authors analyzed the system performance based on resource allocation [
6,
7]. In [
6], the algorithm distributes power to users in different clusters to balance the system throughput and QoS fairness. The authors ensure fairness for users based on a reasonable allocation of power. The allocation coefficients can be allocated by the user’s channel status information (CSI) [
8,
9]. In another study, the authors investigated the system performance with the assumption of imperfect channel state information (CSI) over the amplify-and-forward (AF) protocol [
10]. However, the authors also assumed that only a single antenna has been installed at each node. The impact of the loop interference (LI) channel generated when a relay is equipped with a twin antenna and operated in the FD protocol has therefore not been analyzed. It is a motivation for us to investigate the impact of the LI channel in the AF protocol.
Recently, relaying technology has raised much research interest as an effective solution to the fading resistance. In the cooperative NOMA model, a user with the strongest channel condition is selected as a receiving device and forwards the superimposed signals to users with weaker channel conditions. Therefore, the scope/distance of the network is expanded and the reliability of the network is enhanced by improving QoS for users [
11,
12,
13,
14,
15]. In [
16], the authors investigated the outage performance of the AF and decode-and-forward (DF) relaying schemes. The authors also proposed using a full-duplex (FD) protocol instead of the half-duplex (HD) protocol to avoid wasting time slots [
17]. Although a cooperative NOMA network improves QoS for remote users, it also increases bandwidth costs. This problem can be solved by applying the FD relay technique. The FD relay receives and forwards a signal simultaneously in the same frequency band [
18]. A disadvantage of FD relaying is the impact of the loop interference channel from its own transmitter antenna modeled as a fading channel. Loop interference channels are the main challenge in implementing FD relays [
19]. The authors proposed interference cancellation techniques, including passive cancellation, active analog cancellation, and active digital cancellation [
20]. Studies [
21] and [
22] discussed two main types of FD relay techniques, namely FD AF relaying and FD DF relaying. The authors also investigated a cognitive radio NOMA in FD/HD relay [
23]. The mechanism of random switching between HD/FD relays is on transmit power adaptation [
24]. Another full study on HD/FD relay DF protocol is evaluated in [
25]. Taking up on previous research results, the question whether HD or FD protocol is more suitable arises. A disadvantage of FD protocol is that it is affected by the LI channel, while HD protocol does not have any LI channel. The FD protocol, however, has a better frequency efficiency than the HD protocol. This study proposes a protocol switching mechanism to effectively use the advantages of each protocol. This mechanism can be deployed as a sensor for relaying in future wireless networks. The authors also investigated the HD/FD relay and AF protocol with a fixed gain (FG) [
26]. Through the results, the authors demonstrated that the NOMA system outperforms compared to orthogonal multiple access (OMA) system over the Nakagami-
m fading channels. AF with a variable gain (VG) is less interesting in research because of its complexity. Last, this paper also investigated HD/FD relays not only using the AF protocol with FG but also AF with VG.
Certain studies have made significant contributions in the field of cooperative NOMA. Research results have shown that system performance can be improved by selecting the appropriate relay. Ding et al. [
27] proposed a two-stage relay selection strategy that outperforms max-min relay selection. Another potential technology in the future 5G network is radio frequency energy harvesting (EH) [
28]. However, the initial studies on high-power wireless power transmission show that high-power devices are potentially dangerous to health, thus inhibiting further development of wireless EH. A complete survey of the advantages of simultaneous wireless information and power transfer (SWIPT) over other wireless power transfer (WPT) techniques is in [
29]. In [
30], the authors surveyed most SWIPT technologies, including SWIPT enabled multi-carrier systems, full-duplex SWIPT systems, etc. Given the explosion in the number of networked devices, e.g., Internet of things (IoTs) devices, the energy issue is particularly important when it comes to research and implementation in G-WNs . A solution for simultaneous data and energy transmission is proposed in [
31]. Although the wireless EH solution has not achieved practical effectiveness yet, this study suggests a solution for energy savings that can be easily deployed in applications rather than wireless EH, based on capacity surveys for the best EE of the system.
The main contributions of this study are:
An investigation into the system performance of cooperative NOMA under six scenarios: (1) HD and DF relay; (2) FD and DF relay; (3) HD and AF with FG relay; (4) FD and AF with FG relay; (5) HD and AF with VG relay; (6) FD and AF with VG relay. The outage probability of each scenario is presented in a closed form.
A proposal of a mechanism for switching protocols and optimizing system performance by selecting the best protocol to forward a signal to the next user.
An investigation into the system performance on different signal-to-noise-ratios (SNRs) to find a suitable means of transmitting power to avoid wasting energy. Energy saving is required in G-WNs.
The results coming from the analysis and simulation of outage probability, system throughput and EE are performed by Matlab (This paper used Matlab software version R2017b, made by The MathWorks, Inc., 3 Apple Hill Drive Natick, MA 01760 USA 508-647-7000) software. In addition, an algorithm used for Monte Carlo simulation is also proposed for investigating the outage probability of individual scenarios. The simulation results are used for verifying the analysis results. The figures are presented clearly and accurately in order to demonstrate our propositions.
The article is structured as follows: first, an experimental model is proposed. Next, six different scenarios are analyzed. The third section analyzes system performance on outage probability, system throughput and energy efficiency in all six proposed scenarios. In the fourth section, numerical results are presented and the figures are clearly and accurately discussed. A summary of the study is presented in the conclusions.
2. Experimental Models
In the system model (
Figure 1), two users are waiting to receive the signals with the assumption that the user
’s channel is in a better condition than that of user
. Both users are over Rayleigh fading channels. As
has poor channel conditions, it, instead of receiving the down-link signals directly from the base station, requires a support of a relay for the relaying signals. This paper assumes that
can be used as a cooperative relay. Another assumption is that
can work in all six protocols: HD and DF, FD and DF, HD and AF with FG, FD and AF with FG, HD and AF with VG, FD and AF with VG. This paper analyzes all six protocols to find out the best protocol. Based on these facilities, the study proposes using a wireless sensor to switch between and select protocols to optimize the system performance.
In
Figure 1, although the FD protocol can send and receive data simultaneously, an initial mixed signal is sent from the BS to the relay in the first time slot. The relay decodes the
symbol and removes
from the mixed signal before decoding its own
symbol. The information in the
symbol is then restored and forwarded to
in the second time slot. However, the forwarded signal from the transmitter antenna generates a loop channel to the receiver antenna while
receives another signal from the BS. Thus, the cooperative NOMA system requires two time slots to transmit a superposed signal from the BS to the second user
.
2.1. First Time Slot (FTS)
According to the NOMA theory, the BS sends a superimposed signal to
in the FTS expressed by
where
and
are in accordance with condition
, and
the transmission power of the BS and
for
is the information symbol of each user, sequentially.
Therefore, the received signal at
can be expressed as:
where
is denoted as the transmission channel from BS to
,
is the LI channel from transmitter antenna to receiver one at
and
is the additive white Gaussian noise (AWGN) at
for
with zero mean and variance
.
is the HD/FD switching mode with
state factor. If
, the relay operates in the HD mode. If
, the relay operates in the FD mode.
needs two phases to decode its own information symbol. In the first phase,
decodes the
symbol by dealing with the
symbol, the LI channel
and AWGN
. The signal-to-interference-plus-noise ratio (SINR) can then be expressed as:
where
.
In the second phase, after
has decoded the
symbol,
would be removed from the superimposed signal as noise.
decodes its own symbol
after removing
by dealing with AWGN
and the LI channel
. SINR can then be expressed as:
The instantaneous achievable bit rate of
when
decodes the
symbol can therefore be expressed by:
where
.
2.2. Second Time Slot (STS)
In STS, will forward a mixed signal to using either the DF protocol or the AF protocol with FG or VG by the PSS.
2.2.1. DF Protocols at the Relay
Once the
symbol has been decoded and removed from the superimposed signal,
is restored and sent to
. Therefore,
will receive a signal expressed as:
where
is the transmission channel from
to
,
is the transmission power of
and
is the AWGN of
.
decodes its own
symbol by removing AWGN
from the received signal. Meanwhile, SINR can be expressed by:
The instantaneous achievable bit rate of
in the DF protocol is expressed as:
2.2.2. AF with FG/VG Protocols at the Relay
Where the relay uses the AF protocol, will amplify the received signal by the amplification factor , for , before forwarding the superimposed signal to .
The
amplification coefficient for FG and VG, respectively, are given as follows:
where
or
.
Therefore, the received signal at
is expressed as:
where
is given by (9a) or (9b) and
is given by (
2).
By submitting (
2) and (9a) or (9b) into (
10),
decodes its own
symbol by removing the
symbol, removing the LI channel if
works in
mode, removing AWGN
of
and removing its own AWGN
. Therefore, SINR can be expressed as follows:
where
is given by:
for
or
.
As with (
8), the instantaneous bit rate threshold of
in AF protocols with FG/VG can be rewritten as:
3. System Performance Analysis
Previous research results showed the feasibility of deploying a cooperative relay with HD/FD and DF protocols to resist fading. A mixed signal was transmitted through the network with the support of the
HD/FD relay before reaching the
N-th user [
31]. However, the AF protocol is less studied than the DF protocol because of its complexity in SIC. By contrast, the authors had a full study of the AF protocol with FG [
26]. Even so, it lacks a comparison to the AF protocol with VG. These research results were our motivation to seek a complete analysis and evaluation of the advantages of each protocol.
In this section, we analyze outage probability, system throughput and EE to evaluate the system performance of a cooperative NOMA system in six proposed scenarios: (1) HD and DF protocols at the relay; (2) FD and DF protocols at the relay; (3) HD and AF with FG protocols at the relay; (4) FD and AF with FG protocols at the relay; (5) HD and AF with VG protocols at the relay; (6) FD and AF with VG protocols at the relay, respectively. The article then proposes a mechanism for switching protocols to optimize the system performance.
3.1. Outage Probability
The probability density function (PDF) and cumulative distribution function (CDF) of the Rayleigh fading channel can be expressed as follows, respectively:
and
where random independent variable
Theorem 1. The outage of signal transmission of will occur when cannot successfully decode either or symbol. Specifically, this outage will occur when there is one of the following cases:
Case 1: The instantaneous bit rate cannot reach to the bit rate threshold , in other words .
Case 2: The instantaneous bit rate can reach to the bit rate threshold but the instantaneous bit rate cannot reach to the bit rate threshold , in other words , and .
Ultimately, the outage probability of can be expressed as:where is the minimum bit rate threshold of that needs to be achieved. The expression (
16) can be solved and represented in a closed form as:
where
for
. Where
or
for
or
, then it is paired.
Theorem 2. The outage of signal transmission of will occur when either or cannot successfully decode symbol. Specifically, this outage will occur when there is one of the following cases:
Case 1: The instantaneous bit rate cannot reach the bit rate threshold , in other words .
Case 2: The instantaneous bit rate can reach to the bit rate threshold but the instantaneous bit rate cannot reach to the bit rate threshold , in other words, , and .
Ultimately, the outage probability of can be expressed as: 3.1.1. HD and DF Protocols at the Relay ()
Remark 1. In this scenario, is operated by HD and DF protocols. Therefore, Theorem 2 shown as (18) can be rewritten as well as solved in closed form as:where is given by (17) for . 3.1.2. FD and DF Protocols at the Relay ()
Remark 2. In this scenario, is operated by FD and DF protocols. Therefore, the Theorem 2 shown as (18) can be rewritten as well as solved and in closed form as:where and are given by (17) and (19) for , respectively. If ε in (20) equals zero, (20) becomes (19). 3.1.3. HD and AF with FG Protocols at the Relay ()
Remark 3. In this scenario, is operated by HD and AF with FG protocols. Before forwarding a signal to , amplifies the received signal shown as (2), where , by the amplification coefficient given by (9a). The outage probability of is then expressed in closed form as:where is given by (17) for and is denoted as a modified BesselK function. 3.1.4. FD and AF with FG Protocols at the Relay ()
Remark 4. In this scenario, is operated by FD and AF with FG protocols. Before forwarding a signal to , amplifies the received signal shown as (2), where , by the amplification coefficient given by (9a). The outage probability of is then expressed in closed form as:where ε in (22) equals zero, (22) becomes (21). 3.1.5. HD and AF with VG Protocols at the Relay ()
Remark 5. In this scenario, is operated by HD and AF with VG protocols. Before forwarding a signal to , amplifies the received signal shown as (2), where , by the amplification coefficient given by (9b). The outage probability of is then expressed in closed form as: 3.1.6. FD and AF with VG Protocols at the Relay ()
Remark 6. In this scenario, is operated by FD and AF with VG protocols. Before forwarding a signal to , amplifies the received signal shown as (2), where , by the amplification coefficient given by (9b). The outage probability of is then expressed in closed form as:where ε in (24) equals zero, (24) becomes (23). 3.2. System Throughput
The sum of achievable received data at
, which is also referred to as the system throughput
, is the sum of the throughput results of all
in the system, expressed as:
where
and
.
3.3. Energy Efficiency
Technological development has significantly increased the amount of electricity consumed and seriously affected the living environment. Thus, the minimal energy consumption on every bit of data transmitted through the network is an essential requirement of next generation mobile networks. In this section, the paper evaluates the EE of each scenario:
3.4. Protocol Switching Mechanism
In this section, the study proposes implementing a protocol switching mechanism. Of the six protocols analyzed above, none surpasses any of the other protocols significantly. Each protocol has its own advantages in different situations. The results of the analysis presented in the next section will demonstrate the advantages of each protocol more clearly. Therefore, it is necessary that the relay needs to be equipped with a sensor that can switch between the six protocols in order to optimize the system performance.
Figure 2 shows the mechanism of switching between protocols HD/FD and DF/AF with FG/VG. Each protocol is submitted into its corresponding analysis function. The results of the analysis are used to decide which protocol is optimal and should be applied to forward a signal to the next user at the moment of evaluation.
Proposition 1. Investigation of the outage probability in all six scenarios by the relevant functions to select the best protocol with the lowest outage probability result in order to optimize QoS for the network users.
The minimum outage probability results in HD and DF/AF with FG/VG protocols is selected by:
and the minimum outage probability results in FD and DF/AF with FG/VG protocols is selected by:
where
is given by (
17) and
is given by (
19)–(
24) for
and
, in pairs and respectively.
Since HD protocol is not impacted by the LI channel,
is therefore obvious. The study proposes an outage threshold denoted by
. The mechanism switches between HD and FD protocols as follows:
or
where (
29) is for
, (
30) is for
and
is the outage threshold.
Proposition 2. The outstanding feature of NOMA is that all users are served in the same time slot by sharing the same power domain to improve the user throughput. In this research, the mechanism switches the protocols to optimize the throughput of the cooperative NOMA system. The system performance is directly proportional to the system throughput. With a higher throughput, users can reach a higher data bit rate. The throughput results of all six scenarios of and were evaluated, and the choice of the best protocol to reach the optimal system throughput was determined as: Proposition 3. Given the battery capacity limitations, G-WN technology requires as little as possible energy to be spent. In this study, the EE results of all six scenarios were investigated and presented. In these results, the mechanism selects the best EE protocol for bits of data per joule (b/J) transmitted through the network considered by the as: