Cooperative Detection for Primary User in Cognitive Radio Networks
© Jia Zhu et al. 2009
Received: 26 May 2009
Accepted: 5 November 2009
Published: 2 February 2010
We propose two novel cooperative detection schemes based on the AF (Amplify and Forward) and DF (Decode and Forward) protocols to achieve spatial diversity gains for cognitive radio networks, which are referred to as the AF-CDS, (AF-based Cooperative Detection Scheme) and DF-CDS (DF-based Cooperative Detection Scheme), respectively. Closed-form expressions of detection probabilities for the noncooperation scheme, AND-CDS (AND-based Cooperative Detection Scheme), AF-CDS and DF-CDS, are derived over Rayleigh fading channels. Also, we analyze the overall agility for the proposed cooperative detection schemes and show that our schemes can further reduce the detection time. In addition, we compare the DF-CDS with the AF-CDS in terms of detection probability and agility gain, depicting the advantage of DF-CDS at low SNR region and high false alarm probability region.
Cognitive radio (CR), built on software-defined radio, has been proposed in  as a means to promote the efficient use of the precious radio spectrum resources. It is defined as an intelligent wireless communication system  that is aware of the surrounding environment and utilizes the methodology of understanding-by-building to learn from the environment. Spectrum detection technique (also referred to as spectrum sensing) enables CR networks to adapt to the environment by detecting spectrum holes, and the most efficient way to detect the spectrum holes is to detect the presence of primary users . In reality, however, it is difficult for a cognitive radio to have a direct measurement of the channel between a primary receiver and a transmitter. Therefore, the most recent work focuses on the primary transmitter detection based on local observations of secondary users (see [4–7]). Generally speaking, the spectrum sensing schemes proposed in recent years can be classified as noncooperative detection and cooperative detection.
At present, three noncooperative transmitter detection methods, namely, the matched filter detection, the energy detection and the cyclostationary feature detection, have been presented for CR networks. In , Sahai et al. have investigated the matched filter detector that can achieve high processing gain by employing coherent reception. In , energy detector has been put forward as an optimal method for the occasion where the secondary users cannot gather sufficient information about the primary user signal such as the modulation type, the pulse shape and so on, but it cannot differentiate signal types, thus inclining to false detection triggered by some unintended signals. The cyclostationary feature detection, as an alternative method, has been further presented in [6, 7], which can differentiate the modulated signal from the additive noise. However, this scheme is computationally complex and requires long observation time.
As to the cooperative detection, a collaborative spectrum sensing method has been proposed by Ghasemi and Sousa in , where the hard decision about the presence of the primary user from each secondary user is pooled together to determine the presence of primary user by utilizing a majority logic rule without the consideration of cooperative technology that has been proven as an effective means to combat Rayleigh fading [9–12]. More recently, Ganesan and Li [13, 14] have applied the AF protocol to the detection of primary users, and shown that by allowing the secondary users to cooperate with each other the detection time can be reduced. However, this cooperative scheme needs a centralized controller to manage all secondary users, which is some unreasonable for a practical wireless communication system. Besides, how to detect the presence of primary users through cooperation in DF-based CR networks, where the cooperative user has the ability to decode its received signal, is an open challenge.
In this paper, we address the above mentioned issues and present a more practical AF-based cooperative detection scheme without the assumption of centralized controller and a totally new DF-based cooperative detection scheme for the occasion where the cooperative relay has decoding ability. Our main contributions can be described as follows. Firstly, we propose two new cooperative detection schemes, namely, AF-CDS and DF-CDS, to detect the presence of primary users more quickly and accurately. Secondly, we develop the closed-form expressions of detection probability and detection time for both AF-CDS and DF-CDS over Rayleigh fading channels. Thirdly, the performance analysis for the noncooperation scheme and the AND-CDS (AND-based cooperative detection scheme) is also presented for the purpose of comparison with our schemes.
The remainder of this paper is organized as follows. Section 2 describes the system model used throughout this paper and proposes two new cooperative detection schemes (i.e., AF-CDS and DF-CDS) to improve the detection performance of cognitive radio network. In Section 3, we derive the closed-form expressions of detection probabilities and agility gains for the AF-CDS and DF-CDS as well as the traditional noncooperation scheme and AND-CDS, followed by numerical results analysis in Section 4, where we show the superiority of the proposed AF-CDS and DF-CDS schemes in terms of detection performances. Finally, we make some concluding remarks in Section 5.
2. Proposed Cooperative Detection Schemes
In this section, we first describe the system model used in the paper, and then propose AF-CDS and DF-CDS to improve the detection performance of CR networks.
2.1. System Model
where the subscripts and denote the transmission from the primary user to and that from the primary user to respectively. Besides, , and are the fading coefficients of the wireless channel from the primary user to , from the primary user to , and from to , respectively. Note that the variances of the three random variables (RVs) , and are and respectively. Throughout the paper, we make following assumptions: All wireless channels are independent from each other in space; The relaying protocols (i.e., the AF and the DF) employ full duplex mode, meaning that the relay can perform signal reception when transmits.
2.2. AF-Based Cooperative Detection Scheme
An important requirement of a cognitive radio network is to detect the presence of primary users as quickly as possible. Suppose that the primary user starts using the spectrum band. Then, the two secondary users need to sense the unavailability of the band as soon as possible to avoid collision with primary user. However, when the wireless link between the primary user and encounters shadowing fading, the signal received by from the primary user is so weak that takes a long time to detect its presence. We show that by cooperation with the detection probability of can be increased, thus reducing the overall detection time of the CR network.
We will use (11)–(13) listed above to analyze the detection probability and detection time for the proposed AF-CDS in Section 3.
2.3. DF-Based Cooperative Detection Scheme
Now, we have described the system model and formulated the primary user detection problems for AF-CDS and DF-CDS, based on which a detailed performance analysis will be presented in the following.
3. Performance Analysis of the Proposed Schemes over Rayleigh Fading Channels
where the parameters and are given in (29) and (40), respectively. In the following, we focus on deriving the closed-form expressions of detection probabilities and agility gains for the AF-CDS and DF-CDS.
3.1. Detection Performance of AF-CDS
3.2. Detection Performance of DF-CDS
4. Numerical Results and Analysis
In this paper, we have presented two novel cooperative detection schemes (i.e., AF-CDS and DF-CDS) to improve the detection performance of cognitive radios. We have developed closed-form expressions of detection probability and agility gain for both AF-CDS and DF-CDS over Rayleigh fading channels. For the purpose of comparison, we have also analyzed the detection performances for the noncooperation and the AND-based cooperation schemes. Through conducting numerical experiments, it has been shown that both AF-CDS and DF-CDS are superior to the noncooperation and the AND-base cooperation schemes in terms of the detection probability and the agility gain. Furthermore, we have shown that DF-CDS outperforms AF-CDS at low SNR region or high false alarm probability region.
A. Calculation of (40)
B. Proof of Equation (47)
and this is (47).
C. Proof of Equation (49)
and this is (49).
This work was partially supported by the Postgraduate Innovation Programs of Scientific Research of Jiangsu Province (no. CX09B_150Z, CX08B_080Z), the Key Project of Nature Science Funding of Jiangsu Province (no. BK2007729), the National High Technology Research Development Plan (no. 2009AA01Z241), and the Major Development Program of Jiangsu Educational Committee (no. 06KJA51001).
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