Relay selection scheme for amplify-and-forward ...

SECURITY AND COMMUNICATION NETWORKS Security Comm. Networks 2015; 00:1–7 DOI: 10.1002/sec

RESEARCH ARTICLE

Relay selection scheme for amplify-and-forward cooperative communication system with artificial noise Nan Run Zhoua,b∗ , Xiao Rong Lianga , Zhi Hong Zhoub , Ahmed Faroukc a

Department of Electronic Information Engineering, Nanchang University, Nanchang 330031, China Shanghai Key Laboratory of Integrate Administration Technologies for Information Security, Shanghai Jiao Tong University, Shanghai 200240, China. c Information Technology Department, Al-Zahra College for Women, Oman b

ABSTRACT Cooperative communication can improve the performance of communication system under multi-path fading conditions by relaying. If there is an eavesdropper, the secrecy capacity of the system will decrease. Sending artificial noise can enhance the secrecy capacity of communication system. A novel scheme combining relay selection with artificial noise for amplify-and-forward cooperative communication system in the presence of an eavesdropper is designed, which seeks the relay with the highest signal-to-noise ratio via relay selection based on distance. The location of optimal relay node is found out in this scheme by the relay selection scheme based on distance. However, there is not always a relay node at the optimal location. If there is no relay at the optimal location, then the suboptimal relay node can be found out by drawing circles centered on the optimal location. After that, the optimal relay forwards the signals and sends the artificial noise in the null space of the legitimate channel to confuse the eavesdropper. A close-form expression for maximizing the secrecy capacity is derived, and it is used as the objective function to select the optimal or suboptimal relay node. The algorithm complexity of the relay selection scheme based on distance is lower than that of the relay selection scheme based on instantaneous channel states. Moreover, it can achieve higher secrecy capacity compared with the scheme without artificial noise. Simulations are conducted to validate the theoretical analyses, and the results demonstrate the validity and reliable c 2015 John Wiley & Sons, Ltd. security of the scheme. Copyright ⃝ KEYWORDS cooperative communication, secrecy capacity, relay selection, artificial noise ∗

Correspondence Department of Electronic Information Engineering, Nanchang University, Nanchang 330031, China E-mail: [email protected]

1. INTRODUCTION In wireless communication system, cooperation communication can expand the range of wireless communication and achieve greater coverage area with a smaller transmission power. However, the broadcast property of the wireless communication system renders the information vulnerable to the eavesdroppers. Physical layer security, which ensures secure communication by taking advantage of the physical characteristics of wireless channels, has attracted increasing attention. The information theoretic characterization of security in broadcast channel was

c 2015 John Wiley & Sons, Ltd. Copyright ⃝ Prepared using secauth.cls [Version: 2015/07/19 v2.00]

reviewed [1], where various scenarios of two-user broadcast channels were considered. To achieve secure communication, the physical security of cooperative communication system has been studied widely. Wyner revealed that secure communication is theoretically possible if the channels of the legitimate users have better transmission conditions than those of the eavesdroppers [2]. This conclusion was extended to Gaussian wire-tap channels subsequently [3]. The secrecy capacity of relay transmission channel was studied [4], where the classic three points (source node, destination node and relay node) transmission model was proposed. Laneman provided three basic transport protocols based on cooperative communication including fixed relaying, incremental relaying and selection relaying,

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where detailed analyses of the above three kinds of agreements were presented [5]. The best relay selection (BRS) and the partial relay selection (PRS) are the two main schemes for relay selection [6-8]. BRS selected the relay node of maximum signal-to-noise ratio (SNR) to transmit the signals, which can achieve full diversity and maintain high throughput. An energy-efficient optimal relay selection strategy which jointed the energy-efficient optimal power allocation scheme for amplify-and-forward (AF) cooperative transmission was introduced [9]. Three opportunistic relay selection schemes in cooperative networks with secrecy constraints were studied, where a number of eavesdropper nodes may overhear the signals. The first one chose the relay with the lowest instantaneous SNR to reduce the overheard information at the eavesdroppers. The second scheme sought the relay with the highest SNR to the destination, and the third scheme selected the optimal relay with the ratio between the SNR of a relay and the maximum SNR among the corresponding SNRs to eavesdroppers [10]. Relay selection (RS) for dual-hop decode-and-forward (DF) relay networks over Rayleigh fading channels was studied, which selected the best relay that consumes the minimum power during the cooperation of transmission [11]. RS with the analog network coding (ANC) and time division broadcast (TDBC) was given, where the accurate instantaneous channel state information (CSI) between terminals and relays was required in order to select the best relay [12]. The opportunistic relay selection strategy was studied, which aims to optimize the outage performance of the analogue network coding protocol with multiple mobile relays [13]. Unfortunately, it is not enough to improve the SNR of the legitimate channel by considering the relay selection merely, since the secrecy rate will be very low or even decline to zero if the channel conditions in wireless networks are not favorable for the legitimate users. An efficient way to solve this problem is reducing the SNR of eavesdropper by adding artificial noise (AN) [14]. The existing artificial noise schemes have two cases, one is sending AN by source node, the other is sending AN by relay node. Two scenarios were studied [15], where the Gaussian distributed AN is added in the null space of legitimate receiver to ensure the security of communication and achieve a better secrecy capacity while only degraded the wiretap channel. Secure relay and jammer selection for physical layer security are studied in a wireless network with multiple intermediate nodes and eavesdroppers, where each intermediate node either helps to forward messages as a relay, or broadcasts noise as a jammer [16]. Doaa et al. studied the relay and jammers selection scheme in two-way cooperative networks to improve physical layer security, where three different categories of selection scheme were considered [17]. The jamming noise signals for transmit-beamforming systems were exploited, which is generated from the null space of the channel matrix [18]. Lin Meilu et al. jointed cooperative beamforming and artificial noise for

two-way amplify-and-forward relay networks to improve the security of the date exchange [19].After that, a jointed scheme combining relay selection with cooperative beamforming in two-hop multi-relay decode-and-forward networks was proposed, which selects the best two among several relays and then the selected relays forward their detected symbols in a common channel to the destination [20]. In this paper, a relay selection scheme with artificial noise is proposed, which selects the optimal relay node based on distance and sends the artificial noise by the selected relay nodes to confuse the eavesdropper. Relay selection scheme based on distance can achieve the highest signal-to-noise ratio of the destination, and the optimal location of relay node is found out in this scheme. If there is no relay at the optimal location, then the suboptimal relay node can be found out by drawing circles centered on the optimal location. After that, artificial noise is sent by the optimal or suboptimal relay node, which would not affect the legitimate receiver since the artificial noise is in the null space of the legitimate channel. By deriving a close-form expression for the secrecy capacity maximization, the optimal relay node is selected. This method can not only reduce the burden of the source node, but also enhance the security of AF cooperation system. Besides, the algorithm complexity of the relay selection based on distance is lower than that of the relay selection algorithm based on the instantaneous channel states. Simulations are further provided to verify the effectiveness of the scheme jointed relay selection and artificial noise.

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c 2015 John Wiley & Sons, Ltd. Security Comm. Networks 2015; 00:1–7 ⃝ DOI: 10.1002/sec Prepared using secauth.cls

2. WIRELESS CHANNEL MODEL AND COOPERATIVE COMMUNICATION MODEL 2.1. The wireless channel model

Channel Noise Path loss Input

Output

Modulation Decline

Fig. 1: The wireless channel model.

Assume the input of the system is s, the system transmission formula is r = hs + n

(1)

where h is the channel coefficient between transmitter and receiver, n is the white Gaussian noise with mean 0 and variance σ 2 . Considering the path loss, the channel can be

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Relay selection for AF cooperative communication system with AN

denoted as h = αd−m ejβ , where d is the distance between two nodes, m is the channel attenuation coefficient, β ∈ [0, 2π), β is a constant. 2.2. The AF cooperative communication model

D (XD, YD)

R1 (X1, Y1)

i

R3 (X3, Y3)

R2 (X2, Y2)

hS,R

hR ,D

i

R4 (X4, Y4)

S (XS, YS)

R5 (X5, Y5)

RN (XN, YN)

hR ,E i

A cooperative communication system model with one source S, one destination D, one eavesdropper E, and a series of relay nodes R1 , R2 , R3 , · · ·, RN is shown in Fig. 2. This model is used to simulate the user intensive communication scenarios, and the idle users play as a series of relay nodes. Assume that the eavesdropper is passive, which means that it only listens but does not transmit. Assume that both the receiver and the eavesdropper are equipped with a single antenna, and the coordinates of the source, the destination, the relay nodes and the eavesdropper are (XS , YS ), (XD , YD ), (Xi , Yi ) and (XE , YE ), respectively, then the distances between S and Ri , Ri and D, Ri and E are respectively

dRi ,E

g m PS PRi nR nD nE s rR i

rD rE ΓD ΓE CS

i.e.,

dS,Ri =

(2)

Description

S D E Ri C (XS , YS ) (XD , YD ) (XE , YE ) (Xi , Yi ) (XC , YC ) θ hS,Ri

hRi ,E

Fig. 2: The AF cooperative communication model.

dRi ,D

Symbols

hRi ,D

E (XE, YE)

√ (X − Xi )2 + (YS − Yi )2 √ S = (XD − Xi )2 + (YD − Yi )2 √ = (XE − Xi )2 + (YE − Yi )2

Table I. Symbols used in this paper.

√

Source node Destination node Eavesdropper The ith relay node The midpoint between S and D Coordinate of the source node Coordinate of the destination node Coordinate of the eavesdropper Coordinate of the ith relay node Coordinate of the midpoint between S and D The angle between line Ri C and x axis Channel parameter between the source and the ith relay node Channel parameter between the ith relay node and the destination Channel parameter between the ith relay node and the eavesdropper The amplification coefficient The channel attenuation coefficient Sending power of the source Sending power of the ith relay node White Gaussian noise in the ith relay node White Gaussian noise in the destination White Gaussian noise in the eavesdropper Signal transmitted by the source Signal received by the ith relay node Signal received by the destination Signal received by the eavesdropper SNR of the destination SNR of the eavesdropper The secrecy capacity

PRi hRi ,D nZ = 0, the signal at the destination is

√ PRi hRi ,D nRi + nD (4) Then the SNR of the destination is

rD = g

√

PS PRi hS,Ri hRi ,D s + g

ΓD =

3. TRANSMISSION SCHEME The transmission is divided into two phases. In the first phase, the source transmits a signal s. The transmission is assumed to be secure due to the lack of direct link between the source and the eavesdropper. The signal received by the relay is √ rRi = PS hS,Ri s + nR (3) where PS is the sending power of the source, hS,Ri is the channel parameter between S and Ri , nRi is the white 2 Gaussian noise with mean 0 and variance σR in the ith i relay. In the second phase, Ri amplifies the message and forwards it to D, and also attempts to confuse the eavesdropper by injecting independent artificial noise nZ , Assume the artificial noise is produced and lies in the null space of the channel between the relay and the destination, c 2015 John Wiley & Sons, Ltd. Security Comm. Networks 2015; 00:1–7 ⃝ DOI: 10.1002/sec Prepared using secauth.cls

PS PRi |hS,Ri |2 |hRi ,D |2 g 2 PRi |hRi ,D |2 g 2 σRi 2 + σD 2

(5)

where PRi is the sending power of Ri , and hRi ,D is the channel parameter between Ri and D. nZ is the artificial 2 noise with mean 0 and variance σZ sent by the ith relay, nD is the white Gaussian noise in the destination D, and g is the amplification coefficient. √ g=

1 2 PS |hS,Ri |2 + σR

(6)

4. RELAY SELECTION SCHEME BASED ON DISTANCE In this scheme, relay selection depends on maximizing the SNR of the destination. Assume that the power of the source is equal to that of the relay node, i.e., PS = 3

Relay selection for AF cooperative communication system with AN

2 2 PRi = P , and σR = σD = σ 2 . Under the high SNR case, i substituting Eq. (6) into Eq. (5), one obtains

ΓD =

P α2 1 · σ 2 d2m + d2m Ri ,D S,Ri

(7)

Assume that the coordinates of the source S, the destination D and the relays Ri are (XS , YS ), (XD , YD ) and (Xi , Yi ), respectively. [ 2 2 ]m 2m d2m Ri ,D + dS,Ri =[ (XS − Xi ) + (YS − Yi ) ] m + (XD − Xi )2 + (YD − Yi )2 (8) Assume that the coordinate of the source is (0, 0), and the coordinate of the destination is (XD , 0), i.e., the destination is on the X axis, thus [ ] 2 2 m 2m d2m Ri ,D + dS,Ri = [ (XS − Xi ) + Yi ] (9) m + (XD − Xi )2 + Yi2 where m ≥ 2, then

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transmitted in the range space of the channel between the relay and the destination. Therefore, the artificial noise is ignored by the channel between the relay and the destination but not necessarily by the channel between the relay and the eavesdropper, and the signal received by the eavesdropper is √ rE = g√PS PRi hS,Ri hRi ,E √s + nE + g PRi hRi ,D nRi + PRi hRi ,E nZ And the SNR in the eavesdropper is PS PRi |hS,Ri |2 |hRi ,E |2 g 2 2 2 2 + σE + PRi |hRi ,E |2 g 2 σZ PRi |hRi ,E |2 g 2 σR i (13) Assume that the powers of white Gaussian noise received 2 by the relay node and the eavesdropper are σ 2 , i.e., σR = 2 2 σE = σ . The powers of the source and the relay node are equal to each other, i.e., PS = PRi = P >> σ 2 . With the optimal relay adding the artificial noise, the SNR of the eavesdropper is ΓE =

2m 2m d2m + |XD − Xi |2m (10) Ri ,D + dS,Ri ≥ |XS − Xi |

i.e., if the relay node is on the X axis, the value of 2m d2m Ri ,D + dS,Ri reaches the minimum, that’s to say, the optimal relay node is at the midpoint between the source and the destination. Assume C is the midpoint between the source and the destination, then the coordinate of C is (XC , 0), and S XC = XD −X , and the optimal location of the relay node 2 is at C. Assume r is the distance between Ri and C, θ is the angle between line Ri C and x axis, and θ ∈ [0, 2π), then [( ]m )2 XD −XS 2m 2 2 dS,Ri = − r cos θ + r sin θ 2 ]m (11) [( )2 XD −XS 2 2 − r cos θ + r sin θ d2m = Ri ,D 2 If there is a relay node at the midpoint between the source and the destination, then the relay node is an optimal relay. However, if there is not a relay node at the midpoint between the source and the destination, the 2m value of d2m S,Ri + dS,Ri depends on r. Combining the two conclusions, if the relay node is on the midpoint of the source and the destination, the SNR of the destination reaches the maximum. If there is no relay node at the midpoint between the source and the destination, then the suboptimal relay node should be selected by drawing circles centered on the midpoint between the source and the destination.

5. ARTIFICIAL NOISE ADDED BY RELAY NODES Due to the artificial noise is in the null space of the channel between the relay and the destination, and the signal is 4

(12)

ΓE =

−2m −2m P α2 dS,R d i Ri ,E

(14)

−2m 2 2 2 −2m d−2m S,Ri σ + dRi ,E σZ + σ dRi ,D

The secrecy capacity is CS = CD − CE = log (1 + ΓD ) − log (1 + ΓE ) (15) So the secrecy capacity of the system is   CS = log2  

1+

−2m −2m P α2 dS,R d i Ri ,D ) ( −2m σ 2 dS,R +d−2m R ,D i

1+

i

P α2 d−2m d−2m S,R R ,E i

i

   (16) 

−2m 2 d−2m σ 2 +d−2m σ 2 +dR σ S,R R ,E Z ,D i

i

i

If the optimal relay node Ri is selected, then ΓD , dS,Ri and dRi ,D are known. The secrecy capacity goes up along with the increasing of the distance between the relay node and the eavesdropper if the power of the artificial noise is determined. If the distance between the relay node and the eavesdropper is given, the secrecy capacity goes up along with the increasing of nZ .

6. SIMULATION RESULTS According to [21], the source is assumed to be at (0, 0), and the destination is at (1, 0), the powers of the source and the relay are equal to each other, i.e., PS = PRi = 1W. And the constant α is assumed to be 0.58, the power of white Gaussian noise is 0.1W, the path loss coefficient is 1. All of the simulation parameters above can be reset according to different situations. Fig. 3 shows the change curve of ΓD while the relay node moves on the area of { (x, y)| x ∈ [0, 1] , y ∈ [−0.5, 0.5]}. The result indicates that the SNR of destination achieves the maximum if relay c 2015 John Wiley & Sons, Ltd. Security Comm. Networks 2015; 00:1–7 ⃝ DOI: 10.1002/sec Prepared using secauth.cls

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Relay selection for AF cooperative communication system with AN

70

on the midpoint between S and D on the circumference of circle C with r = 0.3 on the circumference of circle C with r = 0.5

60 70 50

(dB)

50

40

D

40

Γ

Γ D (dB)

60

30

30 20

20 0.5 1

10

0.8

0

0.6 0.4

YR

−0.5

0.2 0

0 0.1

XR

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

σ2 (W)

Fig. 5. ΓD for different σ 2 .

Fig. 3 ΓD for different (XR , YR ).

0.5

while the relay node is on the circumference of circle C with r = 0.3, i.e., point C is the center of the circle, and the distance between the relay node and the midpoint is 0.3; the green curve expresses the change curve of ΓD with the increasing of σ 2 while the relay node is on the circumference of circle C with r = 0.5, i.e., point C is the center of the circle, and the distance between the relay node and the midpoint is 0.5. As is shown in Fig. 5, the SNR of 2 the destination decreases with the increasing of σR , and the SNR of the red curve is the best one, while the green curve is the worst one.

0.4 0.3 0.2 0.1 YR

0.2

0 −0.1 −0.2 −0.3 −0.4 −0.5

7 0

0.2

0.4

0.6

0.8

on the midpoint between S and D on the circumference of circle C with r = 0.3 on the circumference of circle C with r = 0.5

1

X

6

R

Fig. 4. The contour lines of ΓD .

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node is on the midpoint of source and destination. As discussed in section 3, the SNR of destination achieves the maximum if the relay node is on the midpoint between S and D, i.e., the distance between S and Ri is 0.5, and so is the distance between Ri and D. It means that the coordinate of the optimal relay node is (0.5, 0). Assume C is the midpoint of S and D, then the coordinate of C is (0.5, 0). Fig. 4 shows the contour lines of ΓD with different (Xi , Yi ), and it indicates that ΓD decreases with the increasing of the distance between the relay node Ri and the midpoint C. It indicates that we can find the optimal relay node by drawing circles centered on point C if there is not a relay node at the midpoint between the source and the destination. The change curves of SNR of destination and the power of white Gaussian noise with the relay node of different location to cooperate communication are compared in Fig. 5. Considering three situations, the red curve denotes the change curve of ΓD with the increasing of σ 2 while the relay node is on the midpoint of S and D, so the distance between the source and the relay node is equal to that between the relay node and the destination; the blue curve shows the change curve of ΓD with the increasing of σ 2 c 2015 John Wiley & Sons, Ltd. Security Comm. Networks 2015; 00:1–7 ⃝ DOI: 10.1002/sec Prepared using secauth.cls

CS (bit/s)

4 3 2 1 0 −1

0

0.2

0.4

0.6

0.8

1

σ 2Z (W)

2 Fig. 6. CS for different σZ .

Fig. 6 gives the change curve of security capacity with 2 the change of σR . Similar in Fig. 5, three situations are considered in Fig. 6 where the artificial noise is taken in AF cooperative system. For simplicity, the distance between the source and the relay node is assumed to be equal to the distance between the relay node and the destination, and the distance between the relay node and the eavesdropper is equal to that between the relay node and the destination. It is shown that the security capacity 2 increases with the increasing of σZ , because AN has no effect on the destination while can reduce the SNR of the eavesdropper. The first situation has the highest secrecy capacity, because the SNR of the destination achieves the 5

Relay selection for AF cooperative communication system with AN

maximum if the relay node is on the midpoint of the source node and the destination. 6

on the midpoint of S and S, add AN on the midpoint of S and S, without AN on the circumference of circle C with r = 0.3, add AN on the circumference of circle C with r = 0.3, without AN on the circumference of circle C with r = 0.5, AN on the circumference of circle C with r = 0.5, without AN

5

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communication performance compared with the random relay selection scheme. Simulation results demonstrate that the scheme performs a more reliable security and achieves higher secrecy capacity compared with the scheme without artificial noise.

CS (bit/s)

ACKNOWLEDGEMENT 3

2

1

0

−1 0.5

1

1.5 d

Ri,E

2

(m)

Fig. 7. CS for whether adds AN.

Fig. 7 reveals the change curves of the security capacity with the distance between the relay node and the eavesdropper and compares the performance of whether adds AN. As is analyzed in section 4, sending AN can reduce the SNR of the eavesdropper, while has no influence due to the AN is added in the null space of the legitimate channel. Therefore, the performance of the system with AN is better than that of the system without AN if the location of the relay node is determined. However, when the eavesdropper is far away from the relay node, the secrecy capacity of the system with artificial noise is close to that of the system without artificial.

7. CONCLUSION In this paper, a novel scheme for AF cooperative communication system joint relay selection based on distance and artificial noise added by relay node in the presence of an eavesdropper is devised. The optimal relay with the highest signal-to-noise ratio is selected based on distance.Simulation analyses show that the optimal relay node should be located at the midpoint between the source and the destination. If there is not a relay node at the midpoint between the source and the destination, then the optimal relay node should be selected by drawing circles centered on point C. After that, the optimal relay is used to forward the signals and send the artificial noise to confuse the eavesdropper. The artificial noise is in the null space of the main channel, thus it can reduce the SNR of the eavesdropper and would not affect the SNR of the legitimate receiver. This scheme enhances the secrecy capacity of the wireless communication system and ensures the maximum SNR of destination by adding artificial noise via the selected relay node based on distance. Performance analyses show that the algorithm complexity of the relay selection scheme based on distance is lower than that of the relay selection scheme based on instantaneous channel states and this scheme has reliable 6

This work is supported by the National Natural Science Foundation of China (grant no. 61371115), the Natural Science Foundation of Jiangxi Province, China (grant no. 20132BAB201019), the Foundation for Young Scientists of Jiangxi Province (Jinggang Star) (grant no. 20122BCB23002), the Research Foundation of the Education Department of Jiangxi Province (grant no. GJJ14133) and the Opening Project of Shanghai Key Laboratory of Integrate Administration Technologies for Information Security (grant no.AGK2014004).

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