Cooperative Jammer Selection for Secrecy Improvement in Cognitive Internet of Things

The Internet of Things (IoT) is an emerging wireless application [1] and has many applications [2,3]. Many techniques for IoT have arisen in recent years such as adaptive monitoring techniques [4,5,6]. Moreover, IoT technologies can implement smart homes, which can improve the quality of life. However, the security and privacy problems are very important in smart homes [7] and have received significant interest [8,9,10,11,12]. Furthermore, combining cognitive radio technique and IoT, Cognitive Internet of Things (CIoT) is proposed, which is an enhanced IoT paradigm. However, the available bandwidth for IoT is very limited. Thus, the spectrum efficiency is a key issue for IoT design [13,14]. To improve the utilization efficiency of radio spectrum, Cognitive Radio (CR) [15] is a promising technology [16]. In Cognitive Radio Networks (CRN), unlicensed users opportunistically access to the licensed spectrum band [17]. Furthermore, unlicensed users cannot harm the performance of primary users. However, since the spectrum access is dynamic and the communication mode is broadcast communication in wireless communication, any unlicensed users and eavesdroppers can have access to the shared spectrum. Therefore, the eavesdroppers readily overhear any active transmissions over wireless networks. However, cognitive radio technology also introduces some new security threats, e.g., using the shared spectrum by selfish behavior, reporting false sensing information, etc. Therefore, ensuring security is a key issue in CIoT.

To ensure security, physical-layer security technology is an effective confidentially protection mechanism [18,19,20,21,22]. In Ref. [23], when the wiretap channel condition between a source node and an eavesdropper node is worse than the channel condition between the source node and the destination node, the source node can successfully communicate with the destination node in perfect secrecy. Ref. [22] emphasized that both the primary users (PUs) and secondary users (SUs) must be defended from eavesdropping in cognitive networks. Specifically, it is legitimate that the SUs are allowed to access the primary spectrum by cooperating with the PUs, where the SUs act as a relay or a friendly jammer to elevate the PU’s secrecy [24]. Some studies [25,26] reveal that resource allocation is an efficient approach to ensure the PU’s security requirement while achieving good transmission performance for the SUs who cooperate with the PUs. In addition, both the secure communications for PUs and SUs are considered in Ref. [25]. In contrast, Refs. [27,28,29] studied some transmission schemes to maximize secrecy rate or to minimize secrecy outage probability for the SUs in the underlay cognitive models, respectively. In addition, the user selection in cooperative transmission is also an efficient method to enhance the security performance for communication systems due to the multiuser gain. The security enhanced technologies of the SUs is investigated with the user selection in Refs. [21,30,31]. However, transmission protocols for improving the secrecy performance of the PUs are barely known. How to design the transmission protocol for protecting the PU’s security requirement remains a crucial issue in cognitive Internet of things model, where home terminal-to-terminal communication coexists with uplink or downlink of the femtocell station.

To improve the primary secrecy performance and secondary outage performance, we employ cooperative jammer and multi-user diversity technology in this paper. Namely, artificial noise is transmitted by selecting a secondary transmitter (ST), which can improve the outage performance of the primary system. Moreover, an ST has access to the primary spectrum if it can improve the outage performance of the secondary performance and satisfy the interference threshold. To encourage the secondary transmitter to act as a friendly jammer, the interference threshold for secondary system is relaxed by primary system in this paper. The main contributions are summarized as follows:

The remainder of the paper is organized as follows. The system model of cooperative jammer selection for primary systems is provided in Section 2. Section 3 analyzes the performances of transmission and security for our proposed protocol. Section 4 provides numerical simulations for the proposed protocol. Section 5 concludes this paper.

Notations: The channels coefficients over links $\mathrm{PS}\to \mathrm{PD}$, $\mathrm{PS}\to \mathrm{SR}$, $\mathrm{PS}\to \mathrm{E}$, ${\mathrm{ST}}_i\to \mathrm{PD}$, ${\mathrm{ST}}_i\to \mathrm{SR}$, ${\mathrm{ST}}_o\to \mathrm{E}$, and ${\mathrm{ST}}_i\to \mathrm{E}$ are denoted by $h_{\mathrm{P}}$, $h_{\mathrm{PS}}$, $h_{\mathrm{PE}}$, $h_{{\mathrm{S}}_i\mathrm{P}}$, $h_{{\mathrm{S}}_i}$, $h_{{\mathrm{S}}_o\mathrm{E}}$, and $h_{{\mathrm{S}}_i\mathrm{E}}$, respectively. $R_{\mathrm{P}}$ denotes the minimum rate of transmission for primary systems. We also use $R_{\mathrm{S}}$ to denote the minimum rate of transmission for secondary systems. The transmit power of ST and PT are denoted by $P_{\mathrm{P}}$ and $P_{\mathrm{S}}$, respectively. The expectation of a variable X is denoted by $E\left[X\right]$. The probability of a variable X is denoted by $\mathrm{Pr}\left\{X\right\}$.

In this section, we propose a ST cooperative transmission protocol and a selection scheme to determine the friendly jammer and secondary transmitter. Figure 1b shows The system configuration of our protocols. The system model comprises a primary pair (PS-PD), an eavesdropper (E), a secondary receiver (SR) and K secondary transmitters ${\mathrm{ST}}_i$, where $i\in I$, $I=\{1,\dots ,K\}$. In this transmission models, one secondary user is selected as a friendly jammer to interfere with eavesdropping at first, which is denoted by ${\mathrm{ST}}_o$, $o\in I$. The other one has access to the licensed spectrum if the secondary transmission cannot cause an outage over link (PS→PD), which is denoted by ${\mathrm{ST}}_i$, $i\in I$ and $i\ne o$. However, the transmitted information of primary users can be overheard by the eavesdropper. To prevent eavesdropping, ${\mathrm{ST}}_o$ transmits the artificial noise to interfere the eavesdropper. In the proposed model, PD and SR know the information of the artificial noise and the eavesdropper does not know the information. Therefore, PD and SR will not be affected by the artificial noise, which may disturb the eavesdropper. In the proposed protocol, on the one hand, ${\mathrm{ST}}_o$, which provides the most optimal security performance, is selected as a cooperative jammer. On the other hand, if the best outage performance of the secondary system is achieved by selecting a secondary user ${\mathrm{ST}}_i$, and the interference threshold of the primary system is satisfied for the secondary user ${\mathrm{ST}}_i$, then ${\mathrm{ST}}_i$ has access to the licensed spectrum. Furthermore, we study two criterions, which are used to select the cooperative jammer and secondary information transmitter, respectively. In addition, we assume that $h_v\sim \mathcal{CN}(0,{\sigma }_v^2)$, where $v\in \left\{\mathrm{P},{\mathrm{PS}}_i,\mathrm{PE},{\mathrm{S}}_i\mathrm{P},{\mathrm{S}}_i,{\mathrm{S}}_o\mathrm{E},{\mathrm{S}}_i\mathrm{E}\right\}$. We also assume that noises are Additive White Gaussian Noise (AWGN) with zero mean and variance $N_0$.

The simulation results of the proposed protocols are provided in this section. The systems comprise a primary pair (PS-PD), an eavesdropper (E), a secondary receiver (SR) and K secondary transmitters ${\mathrm{ST}}_i$ ($i=1,\dots ,K$). Since the secondary user can serve as cooperative jammer, the primary user relaxes the interference threshold in return, which decreases the minimum achievable rate of primary user $R_{\mathrm{P}}$. Thus, we set $R_{\mathrm{P}}=1.5$ Bit/s/Hz and $R_{\mathrm{P}}=1$ Bit/s/Hz in the conventional model and the proposed model, respectively. If the parameters are not specified, the simulation parameters are settled as follow: $R_{\mathrm{S}}=1$ Bit/s/Hz; $r_1=10lg(P_{\mathrm{P}}/N_0)=10$ dB is the average transmit SNR of the primary user. In addition, ${\sigma }_{\mathrm{P}}^2={\sigma }_{\mathrm{SP}}^2={\sigma }_{\mathrm{PS}}^2={\sigma }_{\mathrm{PE}}^2=1$, ${\sigma }_{{\mathrm{S}}_o\mathrm{E}}^2=3$, ${\sigma }_{{\mathrm{S}}_i\mathrm{E}}^2=1/5$ and ${\sigma }_{\mathrm{S}}^2=4$.

The outage probabilities of the primary user versus $r_2$ in the conventional model and the proposed model are shown as Figure 2, where $r_2=10lg(P_{\mathrm{S}}/N_0)$. The special parameter is the number of STs, which is fixed as $K=3;4;9$. In Figure 2, the outage probability of primary system increases with increase of the secondary SNR. In the same protocol, the primary outage probability decreases with increase of the number of secondary users. This is because the diversity gain increases with increase of the number of secondary users. Furthermore, the outage probability of primary system in our proposed protocol is less than the conventional protocol, which is because that the secondary user is encouraged to serve as friendly jammer, which decreases the interference threshold.

The outage probabilities of the secondary user versus $r_2$ are shown in Figure 3, which is generated by using the same parameters as those in Figure 2. In Figure 3, in the same protocol, the secondary outage performances are improved when the number of STs becomes larger. Moreover, the outage probabilities of secondary decrease firstly, and increase with the increase of the average SNR for secondary in the two protocols. Furthermore, the increasing trend is due to that the interference threshold is always not satisfied by secondary user when the SNR of the secondary user is too large. In the small secondary average SNR range, the outage performance of secondary users in the conventional model is better than the performance in the proposed protocol. This performance is mainly determined by the multi-user diversity gain. In this case, the proposed protocol has a lower multi-user diversity gain than the conventional model due to one secondary transmitter acting as the cooperative jammer. In contrast, the proposed protocol can provide a better secondary outage performance in the high secondary average SNR range because the primary user relaxes the interference threshold.

Figure 4 is generated using the same parameters as those in Figure 2, which shows the intercept probabilities of the primary versus $r_2$ with different number of STs. In Figure 4, the primary security performance is improved significantly in the proposed protocol and is improved slightly in the conventional model as the number of STs becomes larger due to the multi-user diversity gain. Moreover, compared with the conventional protocol, our protocol can provide better primary security performance. The intercept probabilities of the primary system decrease with the increase of $r_2$ in the proposed protocol because the interference from ST to eavesdropper increases with the increase of $r_2$. However, the intercept probabilities of the primary system decrease firstly and increase with the increase of $r_2$ in the conventional protocol. In the small value range of $r_2$, the interference threshold is always satisfied, but the interference from ST to eavesdropper increases with the increase of $r_2$, which causes the decreasing phenomenon. In the large value range of $r_2$, the interference threshold is hard to satisfy. Thus, the access probability of the secondary transmission decreases and the interference from ST to eavesdropper is reduced with the increase of $r_2$. This is the cause of the latter increasing phenomenon. These numerical results can also be found in Figure 5, Figure 6 and Figure 7.

The intercept probabilities of primary users versus $r_2$ with different values of ${\sigma }_{\mathrm{SE}}^2$ are shown in Figure 5. Namely, the special parameter is the channel coefficient ${\sigma }_{\mathrm{SE}}^2$, which equals 3, 3.5 or 4. As described in Figure 5, the primary security performance is improved significantly in the proposed protocol and is improved slightly in the conventional model as the value of ${\sigma }_{\mathrm{SE}}^2$ becomes larger. Compared with the conventional protocol, our protocol can provide the better primary security performance because the larger value of ${\sigma }_{{\mathrm{S}}_o\mathrm{E}}^2$ represents the better channel conditions for links ${\mathrm{ST}}_o\to \mathrm{E}$. In other words, the interference from ${\mathrm{ST}}_o$ to eavesdropper increases with the increase of ${\sigma }_{{\mathrm{S}}_o\mathrm{E}}^2$. In addition, the interference to eavesdropper from ${\mathrm{ST}}_o$ is greater than that from ${\mathrm{ST}}_i$. In proposed protocol, both ${\mathrm{ST}}_o$ and ${\mathrm{ST}}_i$ interfere with the eavesdropping. However, the interference to eavesdropper just comes from ${\mathrm{ST}}_i$ in the conventional model and the probability that i is equal to o is $1/K$.

The intercept probabilities of primary users versus $r_2$ with different values of ${\sigma }_{\mathrm{PE}}^2$ as shown in Figure 6. Namely, the special parameter is the channel coefficient ${\sigma }_{\mathrm{PE}}^2$, which equals to 1.2, 1 or 0.8. In Figure 6, a smaller value of ${\sigma }_{\mathrm{PE}}^2$ can lead to a good primary security performance in the same protocol because a larger value of ${\sigma }_{\mathrm{PE}}^2$ represents the better channel conditions for links $\mathrm{PS}\to \mathrm{E}$. In other words, the instantaneous capacity of $\mathrm{PS}\to \mathrm{E}$ increases with the increase of ${\sigma }_{\mathrm{PE}}^2$. In the proposed protocol with the larger values of ${\sigma }_{\mathrm{PE}}^2$, the primary security performance is enhanced significantly in small value range of $r_2$ and is enhanced slightly in large value range of $r_2$. Compared to the conventional protocol, the proposed protocol can provide the better primary security performance. These numerical results are consistent with those in Figure 4 and Figure 5.

The intercept probabilities of primary users versus $r_2$ with different values of $r_1$ are shown in Figure 7. Namely, the special parameter is the average SNR of the primary user, which is set as $r_1=10lg(P_{\mathrm{P}}/N_0)=5$, 10 or 15 dB. In Figure 7, the primary security performance in the proposed protocol is improved as the value of $r_1$ becomes larger. On the contrary, the primary security performance in the conventional model is reduced as the value of $r_1$ becomes larger. The valid primary information received by eavesdropper and the interference to eavesdropper are the two main factors related to the security performance of primary system. The more valid primary information is received by the eavesdropper, the worse is primary security performance achieved, and the more interference to he eavesdropper, the greater is primary security performance achieved. In the proposed protocol, the smaller value of $r_1$ causes the less valid primary information received at eavesdropper, so the smaller intercept probability of the primary system is obtained. In the conventional model, the interference threshold is hard to satisfy with the smaller $r_1$ and the interference caused by ${\mathrm{ST}}_i$ to eavesdropper is very little, so the larger intercept probability of the primary system is obtained. All of the above numerical results are consistent with the theoretical results in Section 3.

In this paper, we have investigated the physical-layer security for a cognitive Internet of things model, which is composed of a primary pair (PS-PD), a secondary receiver (SR), K secondary transmitters and an eavesdropper. To protect the information of primary users against eavesdropping, we have proposed the ST cooperative jammer selection transmission protocol. In return, for the cooperation of ${\mathrm{ST}}_o$, interference threshold for secondary user is relaxed by the primary system compared with the non-security management model. When this interference threshold is satisfied and the best outage performance of secondary users is obtained by selecting ${\mathrm{ST}}_i$, then the secondary user ${\mathrm{ST}}_i$ has access to the licensed spectrum. Due to the cooperation of ${\mathrm{ST}}_o$, the security performance of primary users are enhanced. Due to the cooperation of ${\mathrm{ST}}_o$ and the selection of ${\mathrm{ST}}_i$, the outage performance of secondary users are enhanced in high secondary transmit SNR region. Furthermore, the intercept probability and outage probability of the primary system have been derived. The outage probability of the secondary system has also been obtained. For comparison purposes, the conventional non-security management was also investigated as a baseline. The numerical results have shown that our protocol has better primary secrecy performance than the non-security management model. In addition, the proposed protocol also has better secondary and primary transmission performance than the conventional model.