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This paper exploits a self-powered secondary relay to not only maintain but also secure communications between a secondary source and a secondary destination in cognitive radio networks when source-destination channel is unavailable. The relay scavenges energy from radio frequency (RF) signals of the primary transmitter and the secondary source and consumes the scavenged energy for its relaying activity. Under the maximum transmit power constraint, Rayleigh fading, the primary outage constraint, and the interference from the primary transmitter, this paper suggests an accurate closed-form expression of the secrecy outage probability to promptly assess the security performance of relaying communications in energy scavenging cognitive networks. The validity of the proposed expression is verified by computer simulations. Numerous results demonstrate the security performance saturation in the range of large maximum transmit power or high required outage probability of primary users. Moreover, the security performance is a function of several system parameters among which the relay’s position, the power splitting factor, and the time splitting factor can be optimized to achieve the minimum secrecy outage probability.

Currently low spectrum utilization efficiency is a great motivation for the application of the cognitive radio technology which enables secondary/unlicensed users to access the allocated spectrum of primary/licensed users in order to better exploit the available spectrum [

This subsection merely surveys published works related to security performance analysis for relaying communications in energy scavenging cognitive networks. Therefore, published works which did not reflect a complete set of specifications such as power constraints for SUs, security performance analysis, relaying communications, and energy scavenging should not be surveyed (e.g., [

The authors in [

In summary, [

This paper extends the system model in [

The decode-and-forward relay is activated merely when it can exactly restore the source information. This limits the error propagation (e.g., [

The relay exploits the interference from the primary transmitter for energy scavenging. This is helpful in turning unwanted signals to useful energy source and differs from [

Periods of two (energy scavenging and information processing) stages are unequal. This facilitates optimizing these periods for minimum secrecy outage probability (SOP). Also, this makes our work distinguished from [

This paper proposes the accurate closed-form SOP analysis, which differs from [

The contributions of the paper are highlighted as follows:

Exploit a secondary relay to guarantee secure communications between the secondary source and the secondary destination in case that their direct communication is in outage. The relay is capable of scavenging the energy from both signals of the secondary source and the primary transmitter. Also, it must be successful in restoring the source information before taking part in the relaying activity

Suggest accurate closed-form expressions for crucial security performance metrics such as the SOP, the probability of strictly positive secrecy capacity (PSPSC), the intercept probability (IP) under both maximum transmit power constraint and primary outage constraint, and interference from the primary transmitters to promptly evaluate the security performance of relaying communications in energy scavenging cognitive networks without time-consuming computer simulations

Employ the suggested expressions to optimize important system parameters

Provide numerous results to obtain helpful insights into security performance such as the security performance saturation in the range of large maximum transmit power or high required outage probability of PUs and the minimum secrecy outage probability achievable with appropriate selection of the relay’s position, the time splitting factor, and the power splitting factor

The paper continues as follows. System model, signal model, secrecy capacity, and secondary power allocation are described in the next section. Section

Figure

System model.

System model

Stage periods

Signal processing at SR

In stage 1, both the secondary source

In stage 2,

In Figure

In Figure

By denoting

Based on the operation principle in Figure

The maximum transmit power which

The signal input to the information decoder in Figure

Plugging (

Then, the channel capacity which

In stage 2,

The SINRs at

Then,

The secrecy capacity of relaying communications in energy scavenging cognitive networks, which is the difference between the channel capacities of the trusted channel (from

The SINR at

Then, the channel capacity that

Similarly, the SINR at

Because the secondary transmitters (

Constraints in (

The transmit powers of

Constraints in (

Transmit power constraints for

Similarly, transmit power constraints for

In (

The derivation of (

The SOP is a crucial performance metric in assessing information security of wireless communications in the information-theoretic aspect. It is defined as the probability that the secrecy capacity

The SOP of relaying communications in energy scavenging cognitive networks is given by

Since

Because the required security degree

The accurate closed-form representation of

Please refer to Appendix.

The accurate closed-form representation of

By imitating the derivation of (

Plugging (

Additionally, the PSPSC refers to the probability that the secrecy capacity is strictly positive, i.e.,

Simulated/numerical results in this section are collected to assess the security performance of relaying communications in energy scavenging cognitive networks in terms of the SOP through typical parameters. Numerical results are produced by (

Simulation parameters.

PARAMETER | VALUE |
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Path-loss exponent | |

| |

Energy conversion efficiency | |

| |

Coordinate of | |

| |

Coordinate of | |

| |

Coordinate of | |

| |

Coordinate of | |

| |

Coordinate of | |

| |

Coordinate of | |

Figure

SOP versus

Figure

SOP versus

Figure

SOP versus the relay’s position.

Figure

SOP versus the time splitting factor.

Figure

SOP versus the power splitting factor.

Figure

SOP versus the required spectral efficiency of SUs.

Figure

SOP versus the required spectral efficiency of PUs.

Figure

SOP versus the required security degree.

This paper evaluated the security performance of relaying communications in energy scavenging cognitive networks in terms of the SOP. For quick performance assessment, the accurate closed-form expression of the SOP was derived under consideration of Rayleigh fading, the primary outage constraint, the interference from PUs, and the maximum transmit power constraint. The validity of the proposed expression was verified by computer simulations. Various results exposed that the self-powered relay considerably enhances the security performance even when the source-destination channel is unavailable owing to deep fading, severe path-loss, and strong shadowing. Moreover, the security performance suffered the error floor in the range of large maximum transmit power or high required outage probability of PUs. Furthermore, the security performance of relaying communications in energy scavenging cognitive networks depends on several system parameters among which the time splitting factor, the relay’s position, and the power splitting factor should be optimally selected to minimize the SOP.

Decompose

Because the required security degree is positive (i.e.,

Because

To numerically evaluate (

The cdf of

The last integral in (

Similarly, the cdf of

Taking the derivative of

Plugging (

By letting

It is obvious that the integrals in the last equality of (

The accurate closed form of (

To obtain the accurate closed form of (

By representing the integrals in the last equality of (

We 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 Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number B2019-20-01.