# Secrecy Performance of Underlay Cooperative Cognitive Network Using Non-Orthogonal Multiple Access with Opportunistic Relay Selection

^{1}

^{2}

^{*}

## Abstract

**:**

_{1}and D

_{2}, respectively, under wiretapping of an eavesdropper (E). We study the best relay selection strategies by three types of relay selection criteria: the first and second best relay selection is based on the maximum channel gain of the links R${}_{i}$-D${}_{1}$, R${}_{i}$-D

_{2}, respectively; the third one is to ensure a minimum value of the channel gains from the R${}_{i}$-E link. We analyze and evaluate the secrecy performances of the transmissions ${x}_{1}$ and ${x}_{2}$ from the source node to the destination nodes D

_{1}, D

_{2}, respectively, in the proposed UCCN-NOMA system in terms of the secrecy outage probabilities (SOPs) over Rayleigh fading channels. Simulation and analysis results are presented as follows. The results of the (sum) secrecy outage probability show that proposed scheme can realize the maximal diversity gain. The security of the system is very good when eavesdropper node E is far from the source and cooperative relay. Finally, the theoretical analyses are verified by performing Monte Carlo simulations.

## 1. Introduction

_{1}and D

_{2}, respectively, under wiretapping of an eavesdropper. The best relay selection strategy is investigated in three types of relay selection criteria: the first and second best relay selection is based on maximizing the value of the channel gains from the links R${}_{i}$-D

_{1}, R${}_{i}$-D

_{2}, respectively; the third one is to ensure the minimum value of the channel gains from the R${}_{i}$-E link. Then, we analyze and evaluate the secrecy performances of the transmissions ${x}_{1}$ and ${x}_{2}$ in the proposed UCCN-NOMA scheme in terms of the SOPs over Rayleigh fading channels to advance the spectral efficiency and secure communication in which the best intermediate relay supports power to the destination nodes and perform digital network coding (DNC) to compress the received data and then to forward the signals to the destination nodes.

## 2. System Model

## 3. Secrecy Outage Probability Analysis

_{th}.

#### 3.1. The Sum SOP of the Secrecy Transmission in the Proposed UCCN-NOMA System with the Best Relay Selection: Case ST1

**Lemma**

**1.**

**Proof.**

**Lemma**

**2.**

**Proof.**

**Theorem**

**1.**

**Proof.**

#### 3.2. The SOP of the Secrecy Transmission in the Proposed UCCN-NOMA System with the Best Relay Selection: Case ST2

#### 3.3. The SOP of the Secrecy Transmission in the Proposed UCCN-NOMA System with the Best Relay Selection: Case ST3

## 4. Simulation Results

## 5. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## Abbreviations

NOMA | Non-orthogonal multiple access |

UCCN | Underlay cooperative cognitive network |

PLS | Physical layer security |

SIC | Successive interference cancellation |

DNC | Digital network coding |

AF | Amplify-and-forward |

DF | Decode-and-forward |

SOP | Secrecy outage probability |

CR | Cognitive radio |

## Appendix A. Proof of Lemma 1

## Appendix B. Proof of Lemma 2

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**Figure 3.**The sum SOPs of the UCCN-NOMA system versus Q(dB) when M = 3, ${\alpha}_{1}=0.8$, ${\alpha}_{2}=0.2$, $\beta =3$ and SC${}_{th}$ = 1 (bit/s/Hz).

**Figure 4.**The sum SOPs of the UCCN-NOMA system versus Q(dB) when M = 3, ${\alpha}_{1}=0.8$, ${\alpha}_{2}=0.2$, $\beta =3$, SC${}_{th}$ = 0.7 (bit/s/Hz) and SC${}_{th}$ = 1 (bit/s/Hz).

**Figure 5.**The sum SOPs of the UCCN-NOMA system versus ${d}_{{R}_{b}E}$ when M = 3, ${\alpha}_{1}=0.8$, ${\alpha}_{2}=0.2$, $\beta =3$ and SC${}_{th}$ = 1 (bit/s/Hz).

**Figure 6.**The sum SOPs of the UCCN-NOMA system versus Q(dB) when M = 3, ${\alpha}_{1}=0.8$, ${\alpha}_{2}=0.2$, $\beta =3$, SC${}_{th}$ = 1 (bit/s/Hz), ${d}_{{R}_{b}{D}_{2}}=0.6$, and ${d}_{{R}_{b}{D}_{2}}=1$.

**Figure 7.**The sum SOPs of the UCCN-NOMA system versus ${\alpha}_{1}$ when M = 3, $\beta =3$ and SC${}_{th}$ = 1 (bit/s/Hz).

Symbols | Parameter Names | Values |
---|---|---|

$\beta $ | Path-loss | 3 |

M | Number of relays | 3 |

${\alpha}_{1}$,${\alpha}_{2}$ | Power allocation coefficients | 0.8; 0.2 |

SC${}_{th}$ | Threshold | 0.7, 1 (bit/s/Hz) |

${d}_{{R}_{b}Pu}$ | Distance of the R${}_{b}$-Pu link | 1 |

${d}_{{R}_{b}{D}_{1}}$ | Distance of the R${}_{b}$-D${}_{1}$ link | 0.5 |

${d}_{{R}_{b}{D}_{2}}$ | Distance of the R${}_{b}$-D${}_{2}$ link | 0.6, 1 |

${d}_{{R}_{b}E}$ | Distance of the R${}_{b}$-E link | 1–3 |

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**MDPI and ACS Style**

Huynh, T.-P.; Ngoc Son, P.; Voznak, M.
Secrecy Performance of Underlay Cooperative Cognitive Network Using Non-Orthogonal Multiple Access with Opportunistic Relay Selection. *Symmetry* **2019**, *11*, 385.
https://doi.org/10.3390/sym11030385

**AMA Style**

Huynh T-P, Ngoc Son P, Voznak M.
Secrecy Performance of Underlay Cooperative Cognitive Network Using Non-Orthogonal Multiple Access with Opportunistic Relay Selection. *Symmetry*. 2019; 11(3):385.
https://doi.org/10.3390/sym11030385

**Chicago/Turabian Style**

Huynh, Tan-Phuoc, Pham Ngoc Son, and Miroslav Voznak.
2019. "Secrecy Performance of Underlay Cooperative Cognitive Network Using Non-Orthogonal Multiple Access with Opportunistic Relay Selection" *Symmetry* 11, no. 3: 385.
https://doi.org/10.3390/sym11030385