Security-reliability trade-off for cognitive radio networks in the presence of eavesdropping attack
© Zhu; licensee Springer. 2013
Received: 13 May 2013
Accepted: 14 October 2013
Published: 6 November 2013
In this paper, we consider a cognitive radio network that consists of one cognitive base station (CBS) and multiple cognitive users (CUs) in the presence of an eavesdropper. In the cognitive radio network, CBS first detects whether there is spectrum hole through spectrum sensing and then communicates with CUs over the detected spectrum hole. Due to the broadcast nature of wireless transmission, the eavesdropper can overhear the cognitive transmissions between CBS and CUs and attempts to decode its overheard signals for interception purpose. In order to effectively defend against the eavesdropping attack, we propose a multiuser scheduling scheme for cognitive transmissions, where a CU with the highest instantaneous capacity to CBS is selected and scheduled to communicate with CBS. We analyze the security-reliability trade-off performance of proposed multiuser scheduling scheme for cognitive transmissions with the imperfect spectrum sensing over Rayleigh fading channels, where the security and reliability are evaluated in terms of the intercept probability and the outage probability, respectively. Numerical results illustrate that as the intercept probability requirement loosens, the outage probability of proposed multiuser scheduling scheme decreases accordingly, showing the trade-off between security and reliability. In addition, as the number of CUs increases, numerical intercept probability and outage probability of the multiuser scheduling scheme significantly improve, implying the security and reliability benefits through multiuser scheduling.
Cognitive radio (CR) is emerging as a means to improve the spectrum resource utilization and allows unlicensed users (also called secondary user or cognitive user) to access the licensed spectrum in an opportunistic way[1–3]. Nowadays, most of radio spectrum is already licensed to particular wireless systems, e.g., the very high frequency (VHF) band from 174 to 216 MHz is allocated to broadcast television systems in North America, the ultra-high frequency (UHF) band from 880 to 915 MHz is allocated to the Global System for Mobile Communications (GSM), and so on. There is no available spectrum suited for developing new wireless communication systems. However, it has been reported by Federal Communications Commission (FCC) that the utilization of licensed spectrum typically ranges from 15% to 85% with a high variance in time, showing that the licensed spectrum is underutilized. This provides us with an opportunity to develop a cognitive radio system that opportunistically utilizes the licensed spectrum. More specifically, if the licensed spectrum band is detected as idle, i.e., the band is not occupied by a licensed user (referred to as primary user), the cognitive radio system can reuse such idle licensed spectrum for data transmission. In view of the above observations, cognitive radio relies on the spectrum sensing that is responsible for detecting and identifying the idle spectrum (known as spectrum hole).
Spectrum sensing enables the cognitive users to be adapted to the radiofrequency (RF) environment by detecting spectrum holes, and the simplest way for spectrum hole detection is to detect whether the primary user is present (or absent). Specifically, the presence of primary user implies the unavailability of licensed spectrum and the absence of primary user implies an available spectrum hole. At present, there are three main signal detection approaches for the spectrum sensing, i.e., the energy detection, matched filter detection, and cyclostationary detection[8, 9]. In general, the energy detection is sensitive to the background noise and unintended interference, which, however, has the advantage of simpler implementation than the matched filter detection and cyclostationary detection. In contrast, the matched filter detection is robust to the unintended interference, but requires some prior knowledge about the primary user signal such as the modulation type and signal waveform which may not be available in practice. As an alternative, the cyclostationary detection is presented through exploiting the cyclostationary feature of modulated signals, which typically requires long observation time to extract certain signal features. In addition, in order to combat the wireless fading effect, cooperative spectrum sensing[10–12] emerges by allowing cognitive users to cooperate with each other in detecting the presence of primary user. It is shown that the detection performance can be significantly improved through cooperative spectrum sensing.
It is pointed out that most existing work on cognitive radio is focused on the spectrum sensing and less attention has been paid to the cognitive radio security against eavesdropping attack. As is known, in cognitive radio networks, CUs first perform the spectrum sensing to identify spectrum holes and then communicate with each other over the detected spectrum holes[2, 3]. However, due to the broadcast nature of wireless medium, an eavesdropping attacker within the transmit coverage of cognitive source node will be able to overhear the cognitive transmissions and may decode its overheard signals for the interception purpose. Traditionally, cryptographic techniques relying on secret keys are employed to protect the confidential transmissions from eavesdropping, which, however, is not secure when the eavesdropper is with sufficient computational capacity for exhaustive key search (also known as brute-force attack)[13, 14]. To this end, physical layer security is now emerging as a means to secure wireless transmissions against the eavesdropping attack[15, 16]. In, the authors have proposed the concept of security-reliability trade-off (SRT) for wireless communications and examined the opportunistic relay selection for improving SRT. It has been shown in that as the number of cooperative relays increases, the SRT performance of wireless communications can be significantly improved.
In this paper, we investigate the security-reliability trade-off for cognitive radio transmissions with imperfect spectrum sensing, where mutual interference between primary and cognitive users should be taken in account in characterizing the SRT performance. This differs from where no interference is considered in performing the SRT analysis. We consider a cognitive radio network consisting of one cognitive base station (CBS) and multiple cognitive users (CUs) in the presence of an eavesdropper, where CBS communicates with CUs over detected spectrum holes and the eavesdropper attempts to intercept the cognitive transmission. We propose a multiuser scheduling scheme to improve the security of cognitive transmission against eavesdropping attack and conduct the SRT analysis of proposed multiuser scheduling scheme in Rayleigh fading channels. Numerical results show that with an increasing number of CUs, the SRT performance of cognitive radio transmissions significantly improves, showing the advantage of proposed multiuser scheduling scheme against eavesdropping attack.
The rest of this paper is organized as follows. Section 2 presents the conventional direct transmission for cognitive radio in the presence of an eavesdropper and proposes the multiuser scheduling scheme to improve the cognitive transmission security against the eavesdropping attack. In Section 3, we perform the SRT analysis of traditional direct transmission and proposed multiuser scheduling schemes over Rayleigh fading channels. Next, in Section 4, the numerical SRT performance of proposed multiuser scheduling scheme is provided and compared with the traditional direct transmission. Finally, we give some concluding remarks in Section 5.
2 Proposed multiuser scheduling scheme in cognitive radio networks
In this section, we first describe the traditional direct transmission for cognitive radio in the presence of an eavesdropper and then propose the multiuser scheduling scheme to defend against eavesdropping attack.
2.1 Direct transmission
where subscript 'w’ denotes the wiretap channel from CBS to Eav.
2.2 Multiuser scheduling scheme
So far, we have presented the conventional direct transmission for cognitive radio in the presence of eavesdropping attack and also proposed the multiuser scheduling scheme to enhance the cognitive transmission security.
3 Security-reliability trade-off analysis for cognitive transmissions over Rayleigh fading channels
In this section, we present the SRT analysis for the traditional direct transmission and proposed multiuser scheduling schemes in cognitive radio networks.
3.1 Direct transmission
One can see that the outage probability and intercept probability of direct transmission are, respectively, given by Equations 21 and 27, which characterize the trade-off between security and reliability.
3.2 Proposed multiuser scheduling scheme
which is the same as the intercept probability of conventional direct transmission. As shown in Equations 34 and 38, we have derived the closed-form expression of outage probability and intercept probability of proposed multiuser scheduling scheme in Rayleigh fading channels. So far, we have completed the SRT analysis of conventional direct transmission and proposed multiuser scheduling schemes in cognitive radio networks.
4 Numerical results and discussion
In the section, we present the numerical results of outage and intercept probabilities to show the trade-off between security and reliability for cognitive radio networks in the presence of eavesdropping attack. In cognitive radio networks, the detection probability and false alarm probability should be guaranteed to be above certain requirements in order to protect the QoS of primary transmissions. According to the IEEE 802.22 standard, the detection and false alarm probabilities are specified to P d = 0.9 and P f = 0.1 throughout this paper. Moreover, the probability of licensed spectrum being idle and available is set to P a = 0.8.
In this paper, we have investigated the security-reliability trade-off for cognitive radio networks in the presence of an eavesdropper. We have proposed a multiuser scheduling scheme to improve the SRT performance of cognitive radio transmission against eavesdropping attack. We have derived closed-form expressions of intercept probability and outage probability of the proposed multiuser scheduling scheme in cognitive radio networks with imperfect spectrum sensing. For the comparison purpose, we have also analyzed the SRT performance of traditional direct transmission in Rayleigh fading channels. Numerical results have shown that the proposed multiuser scheduling scheme strictly outperforms the conventional direct transmission in terms of SRT performance. In addition, with an increasing number of cognitive users, the SRT performance of proposed multiuser scheduling scheme significantly improves, showing the security and reliability benefits through multiuser scheduling.
This completes the calculation of Equation 20.
This work was supported in part by the National Natural Science Foundation of China (grant no. 61302104) and the Scientific Research Foundation of Nanjing University of Posts and Telecommunications (grant no. NY213014).
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