Security Enhancement for dual-hop RF Protocols with Nth-best Partial Relay and EH-based Jammer Chu Tien Dung∗ , Nguyen Toan Van† , Tran Trung Duy† , Vo Nguyen Quoc Bao† , and Nguyen Luong Nhat† ∗
Telecommunications University, Vietnam Email:
[email protected] † Posts and Telecommunications Institute of Technology, Vietnam Email:
[email protected] and {trantrungduy,baovnq,nhatnl}@ptithcm.edu.vn
Abstract—In this paper, we propose an energy harvestingbased jamming scheme to enhance secrecy outage probability (SOP) for cooperative relaying protocols. In the proposed scheme, one of available relays is selected to forward data from a source to a destination, while an energy-constrained relay harvests energy from the source’s radio frequency signals to generate artificial noise to an eavesdropper. Moreover, we consider a generalized relay selection scheme, in which the N th-best relay is resorted to help the source-destination communication. We derive exact and asymptotic expressions of the SOP over Rayleigh fading channels. Monte-Carlo simulations are performed to verify the analysis results and confirm the advantage of the proposed scheme. Index Terms—Physical-layer security, energy harvesting, partial relay selection, secrecy outage probability.
I. I NTRODUCTION Recently, energy harvesting (EH) technique has gained much attention as a promising technique to ensure energy for power-constrained devices [1]. In particular, wireless devices can harvest energy from their surrounding natural resources (solar, electromagnetic, radiation, vibration, etc.) to power communicating operation. Very recently, the concept of simultaneous wireless information and power transfer (SWIPT) has emerged to enable radio frequency (RF) signals to carry both the information and energy [2]. To enhance performance of energy-constrained systems, dual-hop cooperative protocols can be used efficiently. In [3], the authors proposed power-splitting relay protocols in which a part of RF received is transferred to operating energy and another one is used to process signals. In [4], new exact closed-form expressions of achievable throughput and ergodic capacity of decode-and-forward (DF) relaying networks based on EH were derived. A wireless cooperative scenario with multiple source-destination pairs communicating with each other via an EH relay was proposed in [5]. Furthermore, the authors in [5] proposed various power allocation strategies and evaluated their performance via both theory and simulations. Published works [6]–[8] studied outage probability in cognitive radio (CR) with secondary EH relays. In particular, [6] evaluated outage performance of both primary and secondary networks in overlay mode, while [7], [8] studied the performance of secondary networks operating on underlay mode. Recently, physical layer security (PLS) [9], which exploits physical characteristics of wireless channels to guarantee sec 2015 IEEE 978-1-4673-6547-5/15/$31.00
cured communication, has become a promising solution to ensure system security. In PLS, the most important metric to evaluate the security level is the system secrecy capacity, defined as the maximal achievable rate at which message can be reliably sent from a source to its intended receiver without being decoded by eavesdropper(s). The secrecy performance can be enhanced by using cooperative relaying protocols with relay and/or jammer selection methods. In [10], [11], the authors considered relay selection methods to increase the secrecy rate at the cooperative phase, where the communication at the broadcast phase is assumed to be perfectly secure due to the short distance between the source and relays. In [12], [13], the best relay selection strategies for dual-hop secured networks were proposed and analyzed. In [14], [15], the authors proposed joint relay and jammer selection methods. Results in [14], [15] presented that these methods outperform the conventional relay selection ones without using jamming nodes. To the best of our knowledge, there have been several research works concerning with PLS systems and energyconstrained jammers. In particular, the authors in [16] introduced a harvest-and-jam protocol with multi-antenna helpers and a multi-antenna amplify-and-forward (AF) relay to improve the secrecy rate. However, similar to [10], [11], this scheme just focuses the secrecy performance in the second phase with a single relay used to forward the source data to the destination. In this paper, we propose a multi-relay cooperative protocol and the main contributions can be listed as follows: •
•
•
First, we propose a generalized relay selection scheme, where the N th-best relay among available ones is selected to help the source-destination communication [17], [18] while an energy-constrained relay using the harvested energy from the source in the first time slot interferes the eavesdropper in the second time slot. Second, we consider the end-to-end secrecy rate for both the broadcast and cooperative phases, which is different with the scheme proposed in [16]. Furthermore, the selected relay in our scheme uses randomize-and-forward (RF) strategy [12], [19] to avoid the eavesdropper from combining the received data at two time slots. Finally, we derive exact and asymptotic closed-form expressions for secrecy outage probability (SOP) over
Rayleigh fading channels. We then perform computer simulations to verify the analytical derivations. Numerical results show the advantages of the proposed scheme as compared with the corresponding protocol without using a jammer. The rest of the paper is organized as follows. The system model is described in Section II. In Section III, the performance evaluation is analyzed. The simulation results are presented in Section IV and Section V concludes the paper.
transmission of the proposed protocol, named PP, is split into two orthogonal time slots. At the first time slot, the source transmits its data to the best relay, which is selected by the partial relay selection strategy as in [19, eq. (1)]: RB : γSRB = N th
max
m=1,2,...,M
γSRm .
(1)
From (1), we can formulate the achievable rate of the S−RB and S − E links, respectively as CSRB =
II. S YSTEM M ODEL
1 log (1 + ΨγSRB ) , 2 2
(2)
and 1 log (1 + ΨγSE ) , (3) 2 2 where Ψ = P/N0 is the average transmit signal-to-noise ratio (SNR) with P being the transmit power of the source and the relays and N0 denoting Gaussian noise variance. Considering jammer RJ , it harvests energy from the RF signals transmitted from the source. Denoting η with 0 < η < 1 as the energy conversion efficiency, the total power harvested by RJ can be formulated by [4], [8] CSE =
Data link Jamming link Eavesdropping link
R1 R2 RB
S
γSRB γSRJ
RM
γSE
γ RBD
D
γ RBE
PJ = ηP γSRJ .
E Jammer
RJ
γRJE
Fig. 1. Secured communication with N th best relay selection and energy harvesting-based jammer.
We consider a secured communication system as shown in Fig. 1, which consists of one source (S), one destination (D), one eavesdropper (E), one jammer (J), and one set of relays (R). We assume that the relays are close together and form a cluster. In this cluster, assume that there are M relays, i.e., R1 , R2 , ..., RM , which have enough energy to help the source forward the data to the destination, and there exists an energyconstrained relay, denoted by RJ , which plays a role as a jamming node. We denote hXY and dXY as channel coefficient and distance between two nodes X and Y, respectively, where X, Y ∈ {S, R, D, E}. Over Rayleigh fading channels, the channel gain γXY γXY = |hXY |2 follows an exponential distributions whose parameter is λXY = 1/E {γXY } = dβXY , where β is the path-loss exponent and E {.} is an expectation operator. Through this paper, independent and identically distributed (i.i.d.) channels are considered, i.e., λSRm = λSRJ = λSR , λRm D = λRJ D = λRD and λRm E = λRJ E = λRE for all m ∈ {1, 2, ..., M }. Assume that there does not exist the links from the source to the destination and to the eavesdropper. It is also assumed that all of the nodes are equipped with a single antenna and operates on half-duplex mode. The operation of the data
(4)
In the second time slot, the best relay RB forwards the source data to the destination by using randomize-and-forward (RF) relaying, while jammer RJ generates an artificial noise to interfere eavesdropper E. Here, we assume that the interference noise created by RJ can be canceled by the destination as in [15]. Hence, the achievable capacity of the data and eavesdropping links can be given, respectively, by 1 log (1 + ΨγRB D ) , 2 2
(5)
1 ΨγRB E log2 1 + . 2 1 + ηΨγSRJ γRJ E
(6)
CRB D = and CRB E =
Finally, the end-to-end secrecy capacity of the PP protocol can be expressed as + Sec CPP = min C1Sec , C2Sec , (7) +
where [x] = max{x, 0}, C1Sec = CSRB − CSE and C2Sec = CRB D − CRB E . III. P ERFORMANCE E VALUATION In this section, we derive new closed-form expressions of the SOP of the PP protocol. The SOP is defined by the probability that the end-to-end secrecy capacity is lower than a positive secrecy rate, i.e., Rth (Rth > 0) and is given as Sec SOP PPP = Pr CPP < Rth . (8) Proposition 1: The SOP of the PP protocol can be expressed by an exact closed-form expression as presented at the top of next page, where ρ = 22Rth and ω = λSR /η. Proof: See Appendix A.
ρ − 1 λ SE t t j−1 SOP PPP = 1 − 1 − exp − (t + j − 1) λSR (−1) CM −j+1 CN λ + (t + j − 1) λ ρ Ψ SE SR j=1 t=0 !# " ρ−1 ρω ρ−1 ρ λRE 0 − − 3,1 . × exp −λRD − λRD exp −λRD G1,3 + λRD ω 0 0 0 Ψ Ψ Ψ Ψ Ψ −j+1 N MX X
Corollary 2: At high Ψ values, the SOP of the PP protocol can be approximated by SOP CPP
Ψ→+∞
≈
1 − 1 −
−j+1 N MX X
(9)
Similarly, we can obtain the exact closed-form expression for the SOP of the CC protocol as follows: Sec SOP PCC = Pr CCC < Rth −j+1 N MX X t t j−1 =1 − (−1) CM −j+1 CN j=1
t
j−1 t (−1) CM −j+1 CN
t=0
λSE λSE + (t + j − 1) λSR ρ λSE × ρ−1 λSE + (t + j − 1) λSR ρ × exp − (t + j − 1) λSR Ψ ρ−1 ρ−1 ρω exp −λRD × exp −λRD − λRD ρ−1 λRE Ψ Ψ Ψ exp −λ × . (14) RD !# λRE + λRD ρ Ψ ρ 0 − − . (10) At high Ψ regimes, (14) reduces to ω ×G3,1 1,3 λRD Ψ 0 0 0 SOP PCC −j+1 N MX X Ψ→+∞ t t j−1 ≈ 1 − 1 − (−1) CM −j+1 CN Proof: At high Ψ values, we have the following approxj=1 t=0 imation: λRE λSE . (15) × λSE + (t + j − 1) λSR ρ λRE + λRD ρ SOP CPP IV. S IMULATION R ESULTS " !!#+ In this section, Monte-Carlo simulations are presented to Ψ→+∞ 1 γSRB 1 + ΨγRB D ≈ . (11) verify our derivations. We consider a two-dimensional netlog2 min , γRB E 2 γSE 1 + ηγ γ SRJ RJ E work, where the source, the relays, the destination and the eavesdropper are placed at (0,0), (xR ,0), (0,1) and (1,0.25), Using the same manner with Appendix A, we can obtain (10). respectively. In all simulations, the path-loss exponent is fixed by 4 (β = 4), the target secrecy rate equals 1 (Rth = 1) and the efficiency is 0.1 (η = 0.1) for illustrative purposes. For baseline comparison, we also evaluate the SOP perforIn Fig. 2, we present the SOP of the PP and CC protocols as mance of the conventional cooperative scheme, named CC a function of the transmit SNR (Ψ) in dB. In this simulation, protocol, which does not use the EH-based jamming node. the number of available relays is 3 (M = 3) and their position Indeed, the end-to-end secrecy capacity of this protocol is is (0.5, 1). We can observe from this figure that the PP formulated by protocol outperforms the CC protocol for all values of N and Ψ. It is due to the fact that the PP protocol uses jamming nodes to reduce the achievable rate obtained at the eavesdropper. + Sec Sec Sec CCC = min C1,CC , C2,CC , (12) In addition, we can also see that the SOP of both protocols significantly decreases when the best relay (N = 1) can be selected to help the transmission between the source and the where destination. In Fig. 3, we present the SOP as a function of the number of relays when the transmit SNR is fixed by 10 dB and the system 1 + ΨγSRB 1 , C1,CC = log2 can choose the best relay for the cooperation. The figure shows 2 1 + ΨγSE that the SOP of the PP and CC protocols decreases with the 1 1 + ΨγRB D C2,CC = log2 . (13) increasing of M . Furthermore, it is shown that the positions 2 1 + ΨγRB E of the relays also impacts on the secrecy outage performance. j=1
×
t=0
10 0
1
0.9
0.8
10
-1
SOP
SOP
0.7
0
5
0.6
0.5
PP-Sim (N=1) CC-Sim (N=1) PP-Sim (N=3) CC-Sim (N=3) Theory-Exact Theory-Asymp 10 -2
PP-Sim (N=1) CC-Sim (N=1) PP-Sim (N=3) CC-Sim (N=3) Theory-Exact
0.4
0.3
10
15
20
25
30
35
40
0.2 0.1
Ψ [dB]
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
XR
Fig. 2. SOP versus Ψ in dB when M = 3 and xR = 0.5.
Fig. 4. SOP versus xR when Ψ = 5 dB and M = 5.
0.8
the secrecy performances of joint relay and jammer selection methods over various fading channels.
0.7
ACKNOWLEDGMENT
0.6
0.5
This research is funded by The Ministry of Education and Training (MOET) under grant number B2014-45-02.
SOP
PP-Sim (x R = 0.25 ) CC-Sim (xR = 0.25 ) PP-Sim (x R = 0.75)
0.4
CC-Sim (x = 0.75 ) R
A PPENDIX A: P ROOF OF P ROPOSITION 1
Theory-Exact
0.3
SOP At first, from (7) and (8), we can rewrite PPP by
0.2
0.1 1
2
3
4
5
6
7
8
9
10
M
Fig. 3. SOP versus M when Ψ = 10 dB and N = 1.
Fig. 4 illustrates the impact of the relays position on the SOP when M = 5 and Ψ = 5 dB. A similar result is presented in Fig. 4 that the secrecy performance of the PP protocol is still better than that of the CC protocol. Moreover, it is seen that when the relays move on x-coordinate, there exists a optimal position at which the SOP of both protocols is lowest. V. C ONCLUSIONS In this paper, an energy harvesting-based jamming strategy is proposed to enhance performance for the cooperative relaying protocols, in terms of secrecy outage probability (SOP). Comparing with the conventional relay selection methods, our scheme significantly enhances the SOP even if the best relay is not available. Our future works are to proposed and evaluate
SOP PPP
1 + Ψγ 1 + Ψγ RB D SRB < ρ , = Pr min ΨγRB E 1 + ΨγSE 1 + 1+ηΨγSRJ γRJ E
1 + ΨγSRB
