# Secrecy Performance by Power Splitting in Cooperative Dual-Hop Relay Wireless Energy Harvesting

Authors: Nabila Sehito; Shouyi Yang; Abdullh G. Al Harbi; Muhammad Inam Abbasi; Muhammad Abbas Khan; Muhammad Amir Khan; Mian Muhammad Kamal
Journal: Wireless Communications and Mobile Computing
 (2022)

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## Abstract

In wireless communication systems, for secure communication between a transmitter and receiver over the communication channel, the physical layer security is widely utilized. The paper presents a dual-hop wireless full-duplex relay (FDR) network with a source relay and destination relay between two nodes and listening devices. The relay and source use energy harvesting to gain energy from power beacon. Two cooperative techniques are utilized to investigate the amplify-forward (AF) and decode-forward (DF) secrecy capacity in the energy harvesting power splitting system. It is shown that the secrecy performance of an AF relay is better than the secrecy performance of a DF relay in the given form. At 40-meter distance between the relay and the eavesdropper in an energy harvesting system, the AF relay outperforms the DF relay. The simulation is performed using the Monte-Carlo method in MATLAB.


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## Body

## 1. Introduction

Cooperative techniques (AF and DF) are significant techniques in wireless communication which play a vital role in wireless networks utilized widely in different branches and applications, including green monitoring systems, social networks, Internet of Things (IoT), and banking systems [1–4]. Thus, the physical layer security of cooperative relaying systems has recently become a major research scope of direction. Significant amount of work has been done in physical layer security to improve secure communication and reduce limitations in cooperative relay networks at both indoor and outdoor propagating environments [5–9].In [5], a radio signal transmission was secured using an optimum design model from source node to destination through the DF network. In [6], a new two-phase protocol was presented for efficient energy transfer and information relaying, in which the relay operates in full-duplex mode with simultaneous energy harvesting. Ref [7] presents cooperative schemes in dual-hop relaying systems with EH over indoor communication channels categorized by log-normal fading. Similarly, in [8], the cooperative systems are considered with joint time allocation and power splitting schemes.In addition to enhancing spectrum efficiency, wireless nodes also have major issues in cooperative wireless networks, particularly in energy considerations. In [10], the authors investigated EH to improve the system performance and secrecy rate in a jamming system but did not show the full power of the EH system. In [11], the performance and the secrecy rate for both TS and PS protocols were investigated, while [12] investigated the outage probability (OP) over the Rayleigh channel for DF and AF schemes in multiple antennas, respectively. Nevertheless, the EH from the interference node is not considered, and no interference is used, such that only EH from the source is analyzed [13, 14].In [15], a secrecy performance is investigated comprising a single-hop relay system with improvement of EH of 8.89% for the AF relay and of 9.83% for the DF relay between the eavesdropper and the relay. Ref [16] investigated the secrecy performance of a single-hop relay network and observed a performance improvement of 30.47% for the DF cooperative scheme and of 23.63% for the AF cooperative scheme between the eavesdropper and the relay. A geometric programmed (GP) method in [17] was implemented on a full duplex relay network for physical secrecy in a dual-hop relay system. The GP program was applied to power allocation problem on the transmitter side. In [18], the secrecy rate performance of nonorthogonal multiple access and backscatter communication is considered. In [19], they investigated physical layer security ambient backscatter nonorthogonal multiple access in channel estimation errors and imperfect successive interference cancellation with emphasis on reliability and security. The authors in [20] investigated three hop relay cooperative communication networks to perform the two schemes AF and DF, improved by 50.55% (AF) and by 44.2% (DF).In the current study, we have investigated a dual-hop relay cooperative network with a source, relay, eavesdropper, and EH scheme. The goal of this research is to increase the security performance of such a system, using cooperative scheme AF and DF techniques. We have examined the secrecy performance of our proposed model. The contribution of the present work is summarized as follows.(i)
First, we have investigated a dual-hop wireless relay system containing a single source and a single destination relay, along with two nodes and one eavesdropper(ii)
To propose AF and DF schemes and increase the system secrecy performance in the cooperative network(iii)
Various parameters, like the time switching protocol, energy harvesting, power splitting ratio, interference relay, and eavesdropper distance, are investigated. The secrecy performance of designed schemes is discussed and compared to the secrecy performance in previous publications


### 1.1. Paper Organization

The layout of the paper is shown in Table1.Table 1
Paper layout.
Section ISection II1. Introduction1.1. Paper organization2. System model2.1 Energy harvesting technique2.1.1. Decode-forward relay scheme2.1.2 Amplify-forward relay schemeSection IIISection IV3. Secrecy capacity performance3.1. DF scheme3.2. AF scheme4. Simulation results and performanceSection V5. Conclusions


## 1.1. Paper Organization

The layout of the paper is shown in Table1.Table 1
Paper layout.
Section ISection II1. Introduction1.1. Paper organization2. System model2.1 Energy harvesting technique2.1.1. Decode-forward relay scheme2.1.2 Amplify-forward relay schemeSection IIISection IV3. Secrecy capacity performance3.1. DF scheme3.2. AF scheme4. Simulation results and performanceSection V5. Conclusions


## 2. System Model

In this section, we consider a network that relies on a single hop and consists of a relay node (R), source (S), eavesdropper (E), and destination node (D) as presented in Figure 1. It contains power from a beacon (PB). In Figure 1, h∗SR, h∗RD, h∗RE, and h∗SE define the complicated channel gain from PB to S, while h∗BS and h∗BR define the complex channel gain from PB to R. Additionally, we note that noise is complex. Additive white Gaussian noise (AWGN) at each node has zero mean and variance σ2 such that no interference is available. Additionally, the relay function is performed with mutual full-duplex relay and half-duplex relay mode techniques.Figure 1
Dual-hop relay network.


### 2.1. Energy Harvesting Technique

In the designed technique, the nodes relay (R) and source (S) harvest energy from the power beacon (PB). That energy is used to transmit the signals from source S to R and R to D. For high quantities, the time switching- (TS-) based protocol is used for energy harvesting as shown in Figure 2.Figure 2
Time switching-based protocol.The energy harvesting byS and R is given by [17]
(1)ES=ηαPBhBS∗2T2,ER=ηαPBhBR∗2T2.The competence coefficient of the proposed method is denoted byη0<η<1, and the power transmitted by B is denoted by PB and 0<α<1. T represents the time taken to transmit the specific block by S and R [17]. Thus, the power transferred by R and S is given by
(2)PS=ηαPBhBS∗21−α,PR=ηαPBhBR∗21−α.


#### 2.1.1. Decode-Forward Relay Scheme

This scheme consists of two phases, the first of which is shown in Figure3. During the initial transmission, the source sends a signal Xn to the relay and the relay sends the jamming signal q2n to the eavesdropper at the same time. In time slot 2n, the established signal at the R and E is given by [17]
(3)YR2n=ρPShSR∗xn+nR2n,(4)YE2n=ρPShSE∗xn+ρPRJhRE∗xn+nE2n,where the strength of the interference signal from R is given by nR2n. In the next time slot, as presented in Figure 4, the R simply sends the already decoded signal to a valid junction and no longer receives the signal. The source at this point sends the jamming signal to the eavesdropper E, and the signal established in time slots 2n+1 at R and D is expressed as [17]
(5)YR2n+1=ρPRhRE∗xn+ρPSJhSE∗qxn+1+nE2n,(6)YD2n+1=ρPRhRD∗xn+nD2n+1.Figure 3
Illustration of the signal transmission for the2n time slot.Figure 4
Illustration of the signal transmission in the2n+1 time slot.


#### 2.1.2. Amplify-Forward Relay Scheme

This scenario part has two phases; further, in the first phase, it is the DF scheme. During the initial stages, the signal acquired atR and the eavesdropper E is applied; relay transmits enhanced performance of signals received at the destination in the second phase. The source sends jamming signals to D and E, in the time slot 2n+1, which are received at the same time, given
(7)YD2n+1=GρPShRD∗YR2n+nD2n+1,(8)YE2n+1=GρPSJhSE∗qx2n+1+nE2n+1.Here, we defined the scaling factorG. The fluctuation of the channel coefficient is
(9)G=1PR1hSR12+N0,where N0 is the AWGN noise variance.


## 2.1. Energy Harvesting Technique

In the designed technique, the nodes relay (R) and source (S) harvest energy from the power beacon (PB). That energy is used to transmit the signals from source S to R and R to D. For high quantities, the time switching- (TS-) based protocol is used for energy harvesting as shown in Figure 2.Figure 2
Time switching-based protocol.The energy harvesting byS and R is given by [17]
(1)ES=ηαPBhBS∗2T2,ER=ηαPBhBR∗2T2.The competence coefficient of the proposed method is denoted byη0<η<1, and the power transmitted by B is denoted by PB and 0<α<1. T represents the time taken to transmit the specific block by S and R [17]. Thus, the power transferred by R and S is given by
(2)PS=ηαPBhBS∗21−α,PR=ηαPBhBR∗21−α.


### 2.1.1. Decode-Forward Relay Scheme

This scheme consists of two phases, the first of which is shown in Figure3. During the initial transmission, the source sends a signal Xn to the relay and the relay sends the jamming signal q2n to the eavesdropper at the same time. In time slot 2n, the established signal at the R and E is given by [17]
(3)YR2n=ρPShSR∗xn+nR2n,(4)YE2n=ρPShSE∗xn+ρPRJhRE∗xn+nE2n,where the strength of the interference signal from R is given by nR2n. In the next time slot, as presented in Figure 4, the R simply sends the already decoded signal to a valid junction and no longer receives the signal. The source at this point sends the jamming signal to the eavesdropper E, and the signal established in time slots 2n+1 at R and D is expressed as [17]
(5)YR2n+1=ρPRhRE∗xn+ρPSJhSE∗qxn+1+nE2n,(6)YD2n+1=ρPRhRD∗xn+nD2n+1.Figure 3
Illustration of the signal transmission for the2n time slot.Figure 4
Illustration of the signal transmission in the2n+1 time slot.


### 2.1.2. Amplify-Forward Relay Scheme

This scenario part has two phases; further, in the first phase, it is the DF scheme. During the initial stages, the signal acquired atR and the eavesdropper E is applied; relay transmits enhanced performance of signals received at the destination in the second phase. The source sends jamming signals to D and E, in the time slot 2n+1, which are received at the same time, given
(7)YD2n+1=GρPShRD∗YR2n+nD2n+1,(8)YE2n+1=GρPSJhSE∗qx2n+1+nE2n+1.Here, we defined the scaling factorG. The fluctuation of the channel coefficient is
(9)G=1PR1hSR12+N0,where N0 is the AWGN noise variance.


## 2.1.1. Decode-Forward Relay Scheme

This scheme consists of two phases, the first of which is shown in Figure3. During the initial transmission, the source sends a signal Xn to the relay and the relay sends the jamming signal q2n to the eavesdropper at the same time. In time slot 2n, the established signal at the R and E is given by [17]
(3)YR2n=ρPShSR∗xn+nR2n,(4)YE2n=ρPShSE∗xn+ρPRJhRE∗xn+nE2n,where the strength of the interference signal from R is given by nR2n. In the next time slot, as presented in Figure 4, the R simply sends the already decoded signal to a valid junction and no longer receives the signal. The source at this point sends the jamming signal to the eavesdropper E, and the signal established in time slots 2n+1 at R and D is expressed as [17]
(5)YR2n+1=ρPRhRE∗xn+ρPSJhSE∗qxn+1+nE2n,(6)YD2n+1=ρPRhRD∗xn+nD2n+1.Figure 3
Illustration of the signal transmission for the2n time slot.Figure 4
Illustration of the signal transmission in the2n+1 time slot.


## 2.1.2. Amplify-Forward Relay Scheme

This scenario part has two phases; further, in the first phase, it is the DF scheme. During the initial stages, the signal acquired atR and the eavesdropper E is applied; relay transmits enhanced performance of signals received at the destination in the second phase. The source sends jamming signals to D and E, in the time slot 2n+1, which are received at the same time, given
(7)YD2n+1=GρPShRD∗YR2n+nD2n+1,(8)YE2n+1=GρPSJhSE∗qx2n+1+nE2n+1.Here, we defined the scaling factorG. The fluctuation of the channel coefficient is
(9)G=1PR1hSR12+N0,where N0 is the AWGN noise variance.


## 3. Secrecy Capacity Performance

Further, we present a performance secrecy model in terms of secrecy rate. The signal information that may be transferred over a wireless channel to an eavesdropper is defined as the secrecy rate. The next two sections investigate the secrecy rate in AF and DF cooperative networks.


### 3.1. DF Scheme

The secrecy capacity atD and E is defined by equations (3)–(6) for a full-duplex relay in [17]. Thus, we obtain
(10)Rd=12log21+ρPRαRD,(11)Re=12log21+PSαSE1+PRJαRE+ρPRαRE1+ρPRJαSE.Here, we define(12)αRD=hRDσ22,αSE=HSEσ22,αRE=hREσ22.Using equations (6) and (7), the achievable secrecy rate is given by RS=maxRd−Re,0, where it is given in
(13)Rd−Re=12log21+ρPRαRD1+ρPSαSE/1+ρPRαRE+PRαRE/1+PSJαSE.


### 3.2. AF Scheme

Using further equations (7) and (8) at D and E, the secrecy capacity rate can be obtained from
(14)Rd=12log21+G2ρPSαRD,(15)Re=12log21+ρPSαSE1+ρPRJαRE+G2ρPSαRE1+PSJαSE.The secrecy rate is here obtained as given asRS=maxRd−Re,0, whereas
(16)Rd−Re=12log21+ρPRαRD1+PSαSE/1+PJαRE+G2ρPSαRE/1+PSJαSE.


## 3.1. DF Scheme

The secrecy capacity atD and E is defined by equations (3)–(6) for a full-duplex relay in [17]. Thus, we obtain
(10)Rd=12log21+ρPRαRD,(11)Re=12log21+PSαSE1+PRJαRE+ρPRαRE1+ρPRJαSE.Here, we define(12)αRD=hRDσ22,αSE=HSEσ22,αRE=hREσ22.Using equations (6) and (7), the achievable secrecy rate is given by RS=maxRd−Re,0, where it is given in
(13)Rd−Re=12log21+ρPRαRD1+ρPSαSE/1+ρPRαRE+PRαRE/1+PSJαSE.


## 3.2. AF Scheme

Using further equations (7) and (8) at D and E, the secrecy capacity rate can be obtained from
(14)Rd=12log21+G2ρPSαRD,(15)Re=12log21+ρPSαSE1+ρPRJαRE+G2ρPSαRE1+PSJαSE.The secrecy rate is here obtained as given asRS=maxRd−Re,0, whereas
(16)Rd−Re=12log21+ρPRαRD1+PSαSE/1+PJαRE+G2ρPSαRE/1+PSJαSE.


## 4. Numerical Results and Performance

This section presents the numerical results for the investigation of the secrecy performance of the proposed system model, in dual-hop EH and AF and DF cooperative scenarios in terms of relay distance. The numerical results based on the simulation model are shown in Figure5; the S, R, and D are assumed to be the presence of a line of sight (LOS); moreover, dRD, dRE, dBS, and dBR indicate the distances between R and S nodes, between D and R, between E and R, between PB and S, between R and PB, and between S and R. The distance between S and E nodes can be, respectively, given as
(17)dSE=dSR2+dRE2.Figure 5
Numerical model.Source, relay, and destination are located in the line of sight (LOS). The channel model configuration describes the channel between any two nodes, which is uniformly distributed within0,2π, with path loss exponent c=3.50 [17].We assume that the transmission power of the beacon isPB=30dBM and the noise power is NB=−40dBm.Furthermore, it is assumed thatdBS=idBR=14m. Moreover, α=0.999, η=1, and p=3.5.The parameters used for the simulation of this research are shown in Table2.Table 2
Parameter used for the simulation.
ParameterSingle-relay networkDSR25 mDRE30 mDRD15 mDRB14 mRNTLine of sight (LOS)PLE3.5Number of relay1Communicate energy30 dBmMake noise power-40 dBmα0.999ρ3.5In Figures6 and 7, we give the plot of the security performance AF and DF dual-hop scheme and distance between relay (R) and destination (D), when dSR=25m, dBR=14m, and dRD=30m in the proposed EH system and PS EH system. The plot of secrecy capacity shows that the secrecy capacity decreases with increasing distance between destination and relay, but at the same time due to the power splitting receiver in the EH system, the secrecy rate is quite high which is good. Also, this figure clearly shows that the AF single-relay scheme gives a better secrecy rate than the DF single-relay scheme.Figure 6
Secrecy rate vs.dRD for the EH system.Figure 7
Secrecy rate vs.DRD for the proposed EH and PS system.In Figures8 and 9, we compare the secrecy rate distance between relay and eavesdropper between the AF and DF schemes, when dSR=10m and dRD=30m. The graph shows the secrecy rate in the energy harvesting (EH) system depending on the distance between R and E. In the proposed EH PS system, the secrecy rate is very high for the power splitting receiver. Also, in the AF scheme, the secrecy rate is better than that in the DF Scheme. Therefore, to increase the secrecy rate, it is important to use a share of useful power to relay jamming signals in both AF and DF.Figure 8
Secrecy rate vs.DRE for the energy harvesting system.Figure 9
Secrecy rate vs.DRE for the energy harvesting power splitting system.In Figures10 and 11, we plot the average secrecy rate as a function of the path loss exponent factor in the single-relay AF and DF schemes, when dSR=10m, dRD=15m, and dRE=15m. Path loss plays a vital role in the calculation of the secrecy rate. The graph shows that as we increase the path loss exponent, the secrecy rate decreases gradually in both the existing EH systems and the proposed EH PS system. It is shown that the increment in the path loss exponent degraded the system secrecy capacity in both AF and DF schemes. This implies that self-interference should be minimized.Figure 10
Secrecy capacity rate vs. PLE for the EH system.Figure 11
Secrecy capacity rate vs. PLE for the EH PS system.The comparison of the cooperative proposed techniques with the published literature is shown in Table3.Table 3
Performance comparison of the cooperative scheme concerning their secrecy rate.
ReferenceTechniquesCooperative schemeAmplify-forwardDecode-forward[17]HADF, FDR, EPA29.26%29.26%[15]Energy harvesting8.89%9.83%[16]Energy harvesting and jamming signal23.63%30.47%[21]AF and DF11.9%42.96%[22]HDR, AF, and DF40%41%[20]EH, TS, PS, FDR, SOP50.5%44.2%Present articleEH, PS, AF, DF, single relay, HDR, FDR65.5%52.5%


## 5. Conclusions

In this study, we have proposed the energy harvesting single-relay cooperative system, to enhance the performance of secure wireless communications in the existence of one eavesdropperE and relay R. Under a total transmitted power constraint of 30 dBm and noise power of -40 dBm, the secrecy performance of a single-relay wireless cooperative system is investigated, and an innovative single-relay technique has been applied. In the proposed technique, each R transmits a signal to the E and gets a transmission signal at the same time. Two cooperative schemes have been examined: amplify-forward (AF) and decode-forward (DF) in energy harvesting. The results show that secrecy rate performance is improved by AF FDR for single relay to 65.5% and DF FDR for single relay to 52.5% in the energy harvesting power splitting system. Furthermore, we show that an increase in the path loss exponent degrades the performance of the system.



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