# A New Relay and Jammer Selection Schemes for Secure One-Way Cooperative Networks

## Abstract

This paper presents different relay and jammer selection schemes for one-way cooperative networks to increase the security against malicious eavesdroppers. We consider a single source-destination cooperative network with multiple intermediate nodes and one or more eavesdroppers. The selection in the proposed schemes is made with the presence of direct links and the assumption that the broadcast phase is unsecured. The proposed schemes select three intermediate nodes. The first selected node operates in the conventional relay mode and assists the source to deliver its data to the corresponding destination via a Decode-and-Forward strategy. The second and third selected nodes are used in different communication phases as jammers to create intentional interference at the eavesdroppers’ nodes. Moreover, a hybrid scheme which switches between jamming and non-jamming modes is introduced in this paper. The proposed schemes are analyzed in terms of ergodic secrecy capacity and secrecy outage probability. Extensive analysis and a set of simulation results are presented to demonstrate the effectiveness of the different schemes presented in this work. The obtained results show that the proposed schemes with jamming outperform the conventional non-jamming schemes and the hybrid switching scheme further improves the secrecy capacity. The impact of changing both the eavesdroppers and the relays location on ergodic secrecy capacity and secrecy outage probability is also discussed. Finally, the impact of the presence of multiple eavesdroppers is studied in this paper.

### Keywords

Relay and jammer selection Cooperative networks Security## 1 Introduction

Information privacy in wireless networks has been taken a considerable attention for several years due to the broadcast nature of the wireless medium which allows all users in the coverage area of a transmission to overhear the source message. Traditionally, security in wireless networks has been mainly focused on higher layers using cryptographic methods [1]. However, as the implementation of secrecy at higher layers becomes the subject of increasing potential attacks, physical (PHY) layer based security has drawn increasing attention recently [2, 3, 4, 5, 6]. The main objective of PHY-based approaches is to enable source and destination exchanging secure messages at a non-zero rate when the eavesdropper channel is a degraded version of the main channel. In [7] joint optimal power control and optimal scheduling schemes were proposed to enhance the secrecy rate of the intended receiver against cooperative and non-cooperative eavesdropping models. In [8] decode and forward (DF) cooperative protocol was considered to improve the performance of secure wireless communications in the presence of one or more eavesdroppers. The interaction of the cooperative diversity concept with secret communications has also recently been reported as an interesting solution [9, 10, 11]. In [12] the authors proposed a variety of cooperative schemes to achieve secure data transmission with the help of multiple relays in the presence of one or multiple eavesdroppers. In [13] a four-node model was proposed and it was shown that if the relay is closer to the destination than the eavesdropper, a positive secrecy rate can be achieved even if the source-destination rate is zero. In [14] some relay selection metrics have been proposed with different levels of feedback overhead. The authors in [15] extended the work presented in [14] for cooperative networks with jamming protection without taking direct links into account. In [16, 17] different relay selection strategies were introduced for improving the secrecy rate in [14]. In [18, 19, 20] jamming schemes which produce an artificial interference at the eavesdropper node in order to reduce the capacity of the related link seem to be an interesting approach for practical applications. In [21] the interaction between relay and jammer is introduced as a non-cooperative game where both nodes have conflicting objectives and the Nash equilibrium (NE) of the system was derived.

The main contribution of this paper is to investigate relay and jammer selection schemes to increase one-way cooperative networks security in the presence of one or more eavesdroppers. In contrast to [15], the selection in the proposed schemes is made with the presence of direct links, the assumption that broadcast phase is unsecured, and when one or more eavesdroppers are present in the system. We consider a single source-destination cooperative network with one or more eavesdroppers and multiple intermediate nodes to increase security against malicious eavesdroppers. In the proposed schemes an intermediate node is selected to operate in the conventional DF relay mode and assists the source to deliver data to the corresponding destination. Meanwhile, another two intermediate nodes that perform as jamming nodes are selected and transmit artificial interference in order to degrade the eavesdroppers’ links in the first and second phase of data transmission, respectively. The proposed schemes are analyzed for different complexity requirements based on global instantaneous knowledge of all links and average knowledge of the eavesdroppers’ links. The obtained results reveal that the proposed schemes with cooperative jamming can improve the secrecy capacity and the secrecy outage probability of the cooperative network. In addition to the investigation of these jamming-based selection schemes, we show that jamming is not always beneficial for security. According to this observation, a switching scheme between jamming and non-jamming relay selection is proposed. This hybrid scheme overcome jamming limitations and seems to be efficient solutions for practical application with critical secrecy constraints. Moreover, the impact of changing both the eavesdroppers and the relays location on the system performance is also discussed in this paper. Finally, the impact of the presence of multiple eavesdroppers is studied.

The rest of this paper is organized as follows. Section 2 introduces the system model and formulates the problem. Section 3 presents the proposed selection schemes. Numerical results are shown and discussed in Sect. 4, followed by concluding remarks in Sect. 5.

## 2 System Model and Problem Formulation

In this paper two different scenarios of eavesdropper are considered. The first scenario discusses the effect of presence of one eavesdropper while the second scenario studies the effect of presence more eavesdroppers on cooperative network as follows.

### 2.1 The Presence of One Eavesdropper

#### 2.1.1 System Model

As in most existing cooperative network topology [22], the direct links (\(\mathrm{{S}}\!\!\rightarrow \!\! \mathrm{{D}}\) and \(\mathrm{{S}} \rightarrow \mathrm{{E}})\) are available.

The broadcasting phase is unsecured. Therefore, the eavesdropper can overhear the transmitted information.

In both two phases, a slow, flat, and block Rayleigh fading environment is assumed, i.e., the channel remains static for one coherence interval and changes independently in different coherence intervals with a variance \(\sigma _{i,j}^2 =d_{i,j}^{-\beta } \), where \(d_{i,j}\) is the Euclidean distance between node \(i\) and node \(j\), and \(\beta \) is the path-loss exponent.

Furthermore, additive white Gaussian noise (AWGN) is assumed with zero mean and unit variance.

#### 2.1.2 Problem Formulation

### 2.2 The Presence of Multiple Eavesdroppers

#### 2.2.1 System Model

#### 2.2.2 Problem Formulation

## 3 Relay and Jammers Selection Schemes

In order to investigate the effect of jamming we will distinguish between the following three cases; no jammer selection, conventional jamming (where the jamming signal is unknown at the destination) and controlled jamming (where the jamming signal is known at the destination) as will be explained in the next subsections.

### 3.1 The Presence of One Eavesdropper

#### 3.1.1 Selection Schemes Without Jamming

*Conventional Selection (CS):*This solution does not take the eavesdropper channels into account and the relay node is selected according to the instantaneous quality of the S \(\rightarrow \) D link [14]. Although it is an effective solution for non-eavesdropper environments, it cannot support systems with secrecy constraints. The conventional selection is written as$$\begin{aligned} R^{*}=\arg {\mathop {\max }\limits _{R\in C_d}} \left\{ {1+\gamma _{S,D} +\gamma _{R,D}} \right\} \end{aligned}$$(5)*Optimal selection (OS):*This solution takes into account the relay-eavesdropper links and decides the relay node according to the knowledge set \(\Psi _{0}\).The optimal selection is given as [14]$$\begin{aligned} R^{*}=\arg {\mathop {\max }\limits _{R\in C_d}} \left\{ {\frac{1+\gamma _{S,D} +\gamma _{R,D}}{1+\gamma _{S,E} +\gamma _{R,E} }} \right\} \end{aligned}$$(6)*Suboptimal Selection (SS):*It avoids the instantaneous estimate of the relay eavesdropper links by deciding the appropriate relay based on the knowledge set \(\Psi _{1}\). It is a solution which efficiently fills the gap between optimal and conventional selection with a low implementation/complexity overhead. The suboptimal selection is expressed as [14]$$\begin{aligned} R^{*}=\arg {\mathop {\max }\limits _{R\in C_d}} \left\{ {\frac{1+\gamma _{S,D} +\gamma _{R,D} }{1+\mathrm{E}\left[ {\gamma _{S,E} } \right] +\mathrm{E}\left[ {\gamma _{R,E} } \right] }} \right\} \end{aligned}$$(7)

#### 3.1.2 Selection Schemes with Conventional Jamming

*Optimal Selection with Jamming (OSJ)*

*Optimal Switching (OW)*

*Suboptimal Selection with Jamming (SSJ)*

*Suboptimal Switching (SW)*

#### 3.1.3 Selection Schemes with Controlled Jamming

### 3.2 The Presence of Multiple Eavesdroppers

#### 3.2.1 Selection Schemes Without Jamming

#### 3.2.2 Selection Schemes with Conventional Jamming

#### 3.2.3 Selection Schemes with Controlled Jamming

## 4 Numerical Results and Discussion

In this section, we investigate the effectiveness of the proposed selection schemes via computer simulations. The simulation environment follows the model explained in Fig. 1 and consists of a 2-D square topology where the nodes S, D and E are located as {X\(_{\mathrm{S}}\), Y\(_{\mathrm{S}}\)} \(=\) {0, 0}, {X\(_{\mathrm{D}}\), Y\(_{\mathrm{D}}\)} \(=\) {1, 0}, {X\(_{E}\), Y\(_{E}\)} \(=\) {0, 1}, respectively and the direct paths S \(\rightarrow \) D, S \(\rightarrow E\) are available. For simplicity, the source and relay nodes transmit with the same power, i.e. P\(^{(\mathrm{S})} =\) P\(^{(\mathrm{R})}\). The relay and jammer nodes transmit with a relay-jammer power ratio \(L\)\(=\) 100. The number of the relays N \(=\) 4 and the relays are located randomly in the 2-D space considered; their exact location is given for each example considered. The path-loss exponent is set to \(\beta \)=\( 3\), the area of the network is a 1 \(\times \) 1 unit square, the transmission spectral efficiency is equal to R\(_{0}\)\(=\) 2 bits per channel use (BPCU) and the target secrecy rate is equal to R\(_{\mathrm{s}}\)\(=\) 0.1 BPCU. In this paper, the adopted performance metrics are the ergodic secrecy capacity and secrecy outage probability.

### 4.1 The Impact of Changing the N-Relays Set Location with Respect to the Destination and the Eavesdropper

To study the effect of relays location in system performance we have three different scenarios as follows;

*1st scenario: When the N-Relays are located in the middle of the space between D and E*

Regarding to the hybrid schemes, it can be seen that OW outperforms all the selection schemes and provides the best performance where its secrecy capacity converges to 2.6413 BPCU. This result validates that an appropriate mechanism for switching between OS and OSJ overcomes the cases where the interference decreases the secrecy. For the suboptimal case, SW outperforms SS and SSJ selection schemes (its secrecy capacity converges to 2.3876 BPCU). An observation of OSCJ scheme performance shows that it outperforms all the other selection schemes, providing the highest ergodic secrecy capacity when the transmitted power increases due to the ability of the destination to decode the artificial interference in this scheme.

*2nd scenario: When the N-Relays are close to the eavesdropper*

*3rd scenario: When the N-Relays are close to the destination*

### 4.2 The Impact of Changing the Eavesdropper Location with Respect to the Source and the Destination

Assuming the locations of the source, relays and the destination are fixed as in Fig. 3 and the eavesdropper location has two different scenarios.

*1st scenario:*\(\{X_{E}, Y_{E}\} = \{0.2, 0.2\}\)*i.e. the eavesdropper is close to the source*

*2nd scenario:*\(\{X_{E}, Y_{E}\} = \{0.8, 0.2\}\)*i.e. the eavesdropper is close to the destination*

The worst results of the secrecy ergodic capacity for all selection schemes are obtained when the eavesdropper is close to the source.

When eavesdropper is close to the source or close to the destination the non-jamming schemes are inefficient.

There is no need to apply the hybrid switching schemes as they follow the jamming schemes behavior.

OSCJ scheme provides the highest secrecy ergodic capacity than the other selection schemes for both cases.

### 4.3 The Impact of the Presence of Multiple Eavesdroppers

1st scenario: Number of eavesdroppers M \(=\) 2 and their locations are fixed at \(\{x_{E_i } ,y_{E_i } \}_{i=1}^2 =\{(0,1),(1,1)\}\)

2nd scenario: Number of eavesdroppers M \(=\) 3 and their locations are fixed at \(\{x_{E_i } ,y_{E_i } \}_{i=1}^3 =\{(0,1),(0.5,0.5),(1,1)\}\)

The performance of different selection schemes is degraded.

Optimal selection scheme without jamming (OS) becomes inefficient and should not be used in these systems.

Optimal selection scheme with jamming (OSJ) is preferred in these systems due to the ability of jamming nodes to confuse eavesdroppers and increase significantly the secrecy ergodic capacity.

Optimal selection scheme with controlled jamming (OSCJ) achieves the best performance due to the ability of the destination node to decode the jamming signals.

For M \(=\) 1: C_os \(=\) 1.3702, C_osj \(=\) 2.6087, and C_oscj \(=\) 5.7494.

For M \(=\) 2: C_os \(=\) 0.6695, C_osj \(=\) 1.6950, and C_oscj \(=\) 4.6367.

For M \(=\) 3: C_os \(=\) 0.0292, C_osj \(=\) 1.0233, and C_oscj \(=\) 3.7812.

## 5 Conclusion

This paper has studied different relay and jammer selection schemes for one-way cooperative networks with physical layer secrecy consideration. The proposed schemes achieve an opportunistic selection of one conventional relay node and two jamming nodes to increase security against eavesdroppers based on both instantaneous and average knowledge of the eavesdroppers’ channels. Selection in the proposed schemes was made with the presence of direct links and the assumption that the broadcast phase was unsecured. The obtained results showed that the jamming schemes such as OSJ and SSJ are effective for scenarios with strong eavesdropper links. In order to overcome jamming limitations for scenarios with weak eavesdropper links, a hybrid scheme for switching between jamming and non-jamming schemes was introduced which further improves the system performance in terms of ergodic secrecy capacity and secrecy outage probability. The obtained results showed also that as long as the eavesdropper has comparable links with the source and the destination, the ergodic secrecy capacity and the secrecy outage probability are improved. Finally, increasing the eavesdropper nodes in the system degrade the system performance.

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