Exploiting Secure Performance of Full-Duplex Decode and Forward in Optimal Relay Selection Networks

In the presence of an illegitimate user, we investigate the secrecy outage probability (SOP) of the optimal relay selection (ORS) networks by applying decode-and-forward (DnF) based full-duplex (FD) relaying mode. The closed-form expressions for the allocations of the end-to-end signal-to-interference-plus-noise ratio (SINR) in each wireless network are derived as well as the closed-form expression for the exact SOP of the proposed ORS system is presented under Rayleigh fading schemes. As an important achievement, SOP is also compared between orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) schemes. Our results reveal that the SOP of the suggested scheme can be considerably influenced by several parameters involved, including the number of relays, the average signal-to-noise ratio (SNR) of eavesdropper links, transmit power and the average residual self-interference (SI) enforced on the FD relays. DOI:  http://dx.doi.org/10.5755/j01.eie.24.4.21483


I. INTRODUCTION
To enable secure communication in wireless networks, it will take advantage of the physical characteristics of wireless channels in term of secrecy rate and amount of securely transmitted information transmitted to the expected receiver.In particular, a wireless relaying system with energy harvesting capability is investigated under the impact of the jamming signal [1].Simultaneously, for improving the spectral efficiency of the wireless networks and widening the radio coverage thanks to relaying schemes [2], [3], a valuable method which has been arising is an implementation of half-duplex (FD) based relaying Manuscript received 20 November, 2017; accepted 23 June, 2018.
technologies in cooperative communication systems.However, the happening at the same time of proper receivers and eavesdroppers in the equivalent network may turn the legitimate responding information of receivers be vulnerable to security attacks, although cooperative communication technology has published several technical advantages [4].In [5], an optimal power allocation design introduced for performing full-duplex relaying (FDR) shows the outperformance between FDR and the typical half-duplex relaying (HDR) in terms of outage performance.The FDR technique investigated in [6] reveals that it could raise the physical layer security rate in multi-hop relaying systems.Furthermore, Shafie et al. considered the physical-layer security cases of an FD multiple-input multiple-output (MIMO) relaying channel with fixed power transmissions as well as suggesting an artificial-noise aided secure system for attaining an improvement in average secrecy rate in [7].
Recently, the current multiple access techniques can be mainly categorized into two classes including orthogonal multiple access and non-orthogonal multiple access (NOMA), by distinctive considering on a specific resource block occupied by more than one user [8].In the NOMA technique, it is further categorized some kinds, so-called as code-domain NOMA and power-domain NOMA through examining the multiplexing gain gathered from the different domains.Ding et al. [9] explored randomly scenario of roaming users to examine the performance of the NOMA downlink.Moreover, Qin et al. considered NOMA systems in large-scale networks with respect to physical layer security in case of NOMA users and eavesdroppers are spatially arranged at random position [10].
Motivated by previous works and to fill the gap related to   relaying scheme in [11], [12], the main aim of this paper is enhancing the secrecy performance of a full duplex relay network with DnF scheme.More importantly, the main contribution is that secrecy performance is compared in both OMA and NOMA scheme.The rest of the paper is organized as follows: in Section II, system model shows secure full-duplex with OSR.In Section III and IV, we investigate the secrecy performance with DnF relay schemes for OMA, NOMA respectively.Section V shows numerical simulations to verify the analysis.Finally, the paper is concluded in Section VI.

II. SYSTEM MODEL
As shown in Fig. 1, consider a system composed of one source S, one destination D, N full-duplex DnF relays Rk with k = {1, ..., N}, and an eavesdropper E. The source, destination, and the eavesdropper are assumed to have a single antenna while the relays have two antennas, one for reception and the other for transmission.The direct links between the source and the destination or the eavesdropper are unavailable due to severe fading and path-loss, and thus communication can be established only via relays.Also, we concentrate on the situation that the eavesdropper may only overhear the confidential message from the source through the relays' transmission.N .We assume that the source and the relays transmit signal with fixed powers S P and R P , respectively.

A. Secrecy Outage Probability for the System with Full-Duplex DnF Relays
Secrecy outage probability is defined as the probability that the instantaneous secrecy rate of the system is less than a predefined target rate Ro (in bits/s/Hz).We denote mn  is SNR of link m n  .Mathematically, the concerned SOP, i.e.Pr( )  can be expressed as [11], [12]   where min( , ), 1 After some analysis we have the following result [13]   where

Secrecy Outage Probability for the System with Half-Duplex DnF Relays
In this section we consider the SOP of the system using HD DnF relays.Similarly, secrecy outage probability for the system with Half-Duplex DnF relays can be expressed as where min( , ) After some analysis we have the following result [11], [12] 1 1 1 .
At relay, the composited signal is decoded to obtain signal X are given respectively by: 2 1 Then, SOP for signal Pr( ) where After some analysis, we have the following result where Proof: See Appendix A.
In special case as and hence, we obtain   Similarly, in OMA case, SOP for signal 2 S X can be computed by where where Remark 1: It difficult of evaluate how the related parameter, i.e. the number of relay, , S R P P , which affect on system performance by analytical expressions.Fortunately, they can be determined through following simulation results for find optimal SOP.

V. SIMULATION RESULTS
In this section, several Monte Carlo simulation results of the proposed relay section network and existing half-duplex relaying (HDR), full-duplex relaying (FDR) schemes are presented.In the simulations, the transmit power of source and relay are changed to find optimal outage performance.In this paper, we assume that the self-interference is the residual self-interference exists small amount after the imperfect self-interference suppression.For outage probability evaluation, we assume that the interference is at noise level and channel gain of each channel as  , which can be achieved through numerical simulation.
Figure 2 plots the SOP of the scenario in which we change the transmit power at the source and the transmit power at the relay.We can observe that as expected, the SOP decreases to an optimal point as approximately with mathematical programs such as Matlab, i.e,   In Fig. 3 and Fig. 4, we compare SOP of OMA and NOMA.The results confirmed that SOP performance of 1 S X and 2 S X lower than SOP performance of OMA due to lower power allocated for each signal.In this case the transmit power at relay R P contribute to change SOP performance at high region S P and the cross point is approximately 25 dB- 30 dB.

VI. CONCLUSIONS
In this paper, we studied the secrecy outage probability of the FDR by comparing performance of OMA and NOMA scenarios with relay selection scheme.We suggested a novel FDR operation, where each FDR receives the information signal from the previous node as well as transmits the jamming signal to the eavesdropper at the same time.The transmit power allocations for FDRs are calculated by a numerical approach to minimize the lower bound of the secrecy outage performance.Numerical results have revealed that the mentioned FDR operation with the high number of the relay significantly enhanced the outage performance compared with the conventional FDR operation with the only relay.

APPENDIX A
It is noted that We consider outage event below ( ) Pr( )

Fig. 1 .
Fig. 1.System Model for OMA and NOMA.The channel coefficients for S → Rk, Rk → D, Rk → E and Rk → Rk are denoted as k SR h , k R D h , k R E h and kk h , respectively.It is considered that a quasi-static block-fading Rayleigh channel between two nodes with k SR h ,

Fig. 2 .
Fig. 2. Secrecy outage probability in OMA versus R P of FD DnF relays

Fig. 3 .
Fig. 3. Secrecy outage probability of the ORS scheme versus S P of FD

:
Mathematical expectation is denoted by  