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Relay selection is proposed in this paper as an efficient solution to secure information transmission of secondary users against eavesdroppers in energy harvesting cognitive networks. The proposed relay selection method selects a secondary relay among available secondary relays, which are capable of harvesting radio frequency energy in signals of the secondary transmitter and correctly restore secondary message, to curtail signal-to-noise ratio at the wire-tapper. In order to evaluate the security performance of the suggested relay selection method, an exact intercept outage probability formula accounting for peak transmit power confinement, Rayleigh fading, and interference power confinement is firstly derived. Monte-Carlo simulations are then generated to corroborate the proposed formula. Numerous results expose that positions of relays, the number of relays, and parameters of the energy harvesting method significantly influence the security performance while the power confinements on secondary transmitters cause the performance saturation.

The explosion of emerging wireless applications, significantly increasing spectrum utilization demand, and green-and-sustainable communication induce energy efficiency and spectral efficiency to become critical design metrics for modern wireless communication networks (e.g., Fifth Generation (5G)) [

The cognitive radio technology is an appropriate and feasible solution to improve the spectral efficiency [

Several energy efficiency improving solutions for wireless communications networks have been proposed such as network planning [

Energy harvesting cognitive networks (EHCNs) combine two emerging technologies (cognitive radio and RF energy harvesting). Therefore, EHCNs are expected to achieve multiple design criteria of modern wireless communication networks (e.g., 5G), such as high spectral and energy efficiencies [

This subsection solely surveys works pertaining to the relay selection in EHCNs for secure information transmission against eavesdroppers. More specifically, this review relied on notable characteristics (Existing works (e.g., [

Although the relay selection has several advantages, rare attention has been paid on the relay selection in EHCNs for PLS. This motivates us to further study it in order to have a complete evaluation on many aspects (information security, spectral efficiency, energy efficiency, secondary transmitter-destination connection probability) of EHCNs before practical deployment. This paper reconsiders the system model in [

Our paper suggests a different relay selection method in which the chosen relay from the successfully decoding set is the one which minimizes the SNR at the eavesdropper. This prevents the eavesdropper from decoding legitimate information as much as possible

All relays in this paper harvest the energy with the power splitting (PS) method which differs the TS method in [

This paper analyzes the intercept outage probability (IOP) in an exact form while [

Our contributions are briefly listed as:

Suggest a relay selection method in EHCNs to hinder the eavesdropper from overhearing as much as possible

Derive an exact IOP formula for quickly assessing the security measure of the suggested relay selection method in EHCNs under Rayleigh fading channels and the (peak transmit and interference) power confinements

Prove the existence of optimum key system parameters for the best security performance

Provide insightful results on the security performance:

This paper continues with channel and system models in Part II. Then, Part III derives the IOP in detail. Illustrative results and conclusions are delivered in Part IV and Part V, correspondingly.

Figure _{s}_{1}, I_{2}, ..., I_{N}_{r}_{r}

System model.

In Figure _{r}_{r}_{s}_{r}_{r}_{r}_{r}_{r}_{r} in ∆ sets the value of its timer which is proportional to the SNR of the I_{r}-

Message processing at I_{r}

In Figure _{1}_{2}_{N}_{1}_{2}_{N}, E, D, R

The relay I_{r}_{T}_{r}

The T’s transmit power, _{T}_{p}_{p}

According to Figure _{r}_{r}

Figure _{r}

Plugging (

It is inferred from (_{r}

The channel capacity that the relay I_{r} can obtain is _{r}_{r}

The Phase 1 ends by grouping relays which exactly restore the secondary message into a set ∆ as

Then, the relay in ∆ which minimizes the SNR at E is chosen (It is noted that [_{r}

Such a relay selection in (

The Phase 2 is for I_{s} to broadcast the decoded message _{s}

E obtains the following SNR in the Phase 2, which is computed from (

Generally, the SNR at E through the I_{r}_{r} is

I_{r}

The channel capacity at E in the Phase 2 is given by

The IOP is the possibility which the wire-tapper E fails to decode the secondary message. As such, it is a critical performance indicator to assess the security capability of the relay selection in EHCNs. This section proposes an exact IOP formula for quickly measuring the secrecy performance without invoking exhaustive simulations.

The IOP is defined as

Inserting (

It is recalled that I_{s}

Additionally, the formation of the set ∆ implicitly means that the relays in ∆ (i.e.,

Because

Since

Without loss of generality, the current paper assumes that relays are closely located (i.e.,

Now, two terms of (

Please see Appendix

_{x, y}(

Please see Appendix

Inserting (

It is well-known that the single integral in (

The IOP of the proposed relay selection in EHCNs is evaluated through critical system parameters. For illustration purposes, some specifications are selected as follows: T at (0.0, 0.0), I_{r}

Figure _{p}/N_{0} for _{p}/N_{0} =15 dB. The results illustrate that the simulation coincides with the theory, verifying the preciseness of (_{p}/N_{0}. This comes from the fact that increasing _{p}/N_{0} allows the relays to exactly restore the secondary message and to scavenge more radio frequency energy in signals of T, hence increasing the SNR at E in the Phase 2 and reducing the IOP. Nevertheless, the IOP bears the error floor at large _{p}/N_{0}. This error floor is because of the power allocation for secondary transmitters (please recall (_{p}/N_{0} make the transmit powers of T and I_{r}_{p}/N_{0} (i.e., large _{p}/N_{0} neglects the peak transmit power confinement), inducing the constant IOP. Moreover, the IOP is proportional to the number of relays, confirming the effectiveness of the relay selection in improving the secrecy performance.

IOP w.r.t _{p}/N_{0}.

Figure _{p}/N_{0}, with parameters of Figure _{p}/N_{0} =10 dB. The results expose that the theory coincides the simulation, again proving the validity of (_{p}/N_{0}. This result is comprehended from the power distribution of T and I_{r}

IOP w.r.t _{p}/N_{0}.

Figure _{p}/N_{0} =16 dB and _{p}/N_{0} =12 dB. The results prove that the theory agrees with the simulation, again asserting the accuracy of (_{s}_{worst}_{worst}

IOP w.r.t

Figure _{worst}

IOP w.r.t

Figure

IOP w.r.t

This paper proposes the relay selection method to improve the information security in energy harvesting cognitive networks against eavesdroppers. The relays are able to harvest radio frequency energy in the signals of the power-unconstrained secondary transmitter and the relay which creates the smallest SNR at the eavesdropper is adopted to decode and forward the secondary message to the secondary destination. The security performance of the proposed relay selection method considering both (peak transmit and interference) power confinements and Rayleigh distribution is quickly measured by the suggested precise IOP formula that is asserted by Monte-Carlo simulations. Multiple results indicate that the positions of the relays and the parameters (power and time splitting ratios) of the energy harvesting method can be properly adjusted to increase the IOP, eventually improving the security performance. Moreover, the IOP experiences the error floor as the transmit power is high.

Conditioned on _{r}

Using (

It is recalled that

Inserting (

The above integrals are straightforwardly computed; hence,

Conditioned on

The term

The term

In order to compute

Plugging (

Inserting (

Given

By the variable change

By defining

By the variable change

With the help of [

Plugging (

Because

The authors declare that all data used to support the findings of this study are included within the article.

The authors declare that they have no conflicts of interest.

This research is funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 102.04-2019.318. We would like to thank Ho Chi Minh City University of Technology (HCMUT), VNU-HCM, for the support of time and facilities for this study.