A cluster-based selective cooperative spectrum sensing scheme in cognitive radio
© Nguyen-Thanh and Koo; licensee Springer. 2013
Received: 16 February 2013
Accepted: 11 June 2013
Published: 24 June 2013
Developing an effective cooperative spectrum sensing (CSS) scheme in cognitive radio (CR), which is considered as promising system for enhancing spectrum utilization, is necessary. In this paper, a cluster-based optimal selective CSS scheme is proposed for reducing reporting time and bandwidth while maintaining a certain level of sensing performance. Clusters are organized based on the identification of primary signal signal-to-noise ratio value, and the cluster head in each cluster is dynamically chosen according to the sensing data qualities of CR users. The cluster sensing decision is made based on an optimal threshold for selective CSS which minimizes the probability of sensing error. A parallel reporting mechanism based on frequency division is proposed to considerably reduce the time for reporting decision to fusion center of clusters. In the fusion center, the optimal Chair-Vashney rule is utilized to obtain a high sensing performance based on the available cluster’s information.
Cognitive radio (CR) has been recently proposed as a promising technology to improve spectrum utilization by enabling secondary access to unused licensed bands. A prerequisite to this secondary access is having no interference to the primary system. This requirement makes spectrum sensing a key function in cognitive radio systems. Among common spectrum sensing techniques, energy detection is an engaging method due to its simplicity and efficiency. However, the major disadvantage of energy detection is the hidden node problem, in which the sensing node cannot distinguish between an idle and a deeply faded or shadowed band . Cooperative spectrum sensing (CSS) which uses a distributed detection model has been considered to overcome that problem [2–12].
Cooperation among CR users (CUs) is usually coordinated by a fusion center (FC). For each sensing interval, CUs will send their sensing data to the FC. In the FC, all local sensing data will be combined to make a final decision on whether the primary signal is present or absent. An optimal data fusion rule was firstly considered by Chair and Varshney in . Despite a good performance, the requirement for knowledge of detection and false alarm probabilities at each local node is still a barrier to the optimal fusion rule.
CSS schemes require a large communication resource including sensing time delay, control channel overhead, and consumption energy for reporting sensing data to the FC, especially when the network size is large. There are some previous works [3–9] that considered this problem. In our previous work , we proposed an ordered sequential reporting mechanism based on sensing data quality to reduce communication resources. A similar sequential ordered report transmission approach was considered for reducing the reporting time in . However, the reporting time of these methods is still unpredictably long. In , the authors proposed to use a censored truncated sequential spectrum sensing technique for saving energy. On the other hand, cluster-based CSS schemes are considered for reducing the energy of CSS  and for minimizing the bandwidth requirements by reducing the number of terminals reporting to the fusion center . In , Chen et al. proposed a cluster-based CSS scheme to optimize the cooperation overhead along with the sensing reliability. In fact, these proposed cluster schemes can reduce the amount of direct cooperation with the FC but cannot reduce the communication overhead between CUs and the cluster header. A similar problem can be observed in the cluster scheme in , though the optimal cluster size to maximize the throughput used for negotiation is identified. Another consideration of the cluster scheme is to enhance sensing performance when the reporting channel suffers from a severe fading environment [10, 11].
In this paper, we propose a cluster-based selective CSS scheme which utilizes an efficient selective method for the best quality sensing data and a parallel reporting mechanism. The selective method, which is usually adopted in cooperative communications [14, 15], is applied in each cluster to implicitly select the best sensing node during each sensing interval as the cluster header without additional collaboration among CUs. The parallel reporting mechanism based on frequency division is considered to strongly reduce the reporting time of the cluster decision. In the FC, the optimal Chair-Vashney rule (CV rule) is utilized to obtain a high sensing performance based on the available cluster’s signal-to-noise ratio (SNR). In this way, the proposed cooperative sensing will be performed with an extremely low cooperation resource while a certain high level of sensing performance is ensured.
The remainder of this paper is organized as follows. In Section 2, some background on spectrum sensing and optimal fusion rule is described. In Section 3, we present system descriptions. The proposed system model and detailed descriptions of the proposed cluster-based selective CSS scheme are also given in Section 4. Simulation results are shown in Section 5. Finally, the conclusions are drawn in Section 6.
2.1 Local spectrum sensing
where H0 and H1 correspond, respectively, to hypotheses of absence and presence of the PU signal, x i (t) represents received data at CU i , h i denotes the gain of the channel between the PU and the CU i , s(t) is the signal transmitted from the primary user, and n(t) is additive white Gaussian noise. Additionally, channels corresponding to different CUs are assumed to be independent, and further, all CUs and PUs share a common spectrum allocation.
where x j is the j th sample of the received signal and N = 2T W in which T and W correspond to detection time and signal bandwidth in hertz, respectively.
where γ i is the SNR of the primary signal at the CU.
respectively, where Q(.) is the Marcum-Q function, i.e., .
2.2 The optimal fusion rule for global decision
Chair and Varshney provided the optimal data fusion rule in a distributed local hard decision detection system . This optimal rule is in fact the sum of weighted local decisions where the weights are functions of probabilities of detection and false alarm.
Local false alarm probability p f i and local detection probability p d i are defined in (6) and (7), respectively.
3 System description
How can the scheme efficiently select the cluster header, which is the node with the best quality for sensing data, for each sensing interval without any extra overhead among nodes in the cluster?
How can the cluster header optimally make the cluster decision?
What is the method for reporting the cluster decision to the FC?
The answers to these questions are given in the following section.
4 The proposed cluster-based selective CSS scheme
4.1 Selective CSS mechanism
It is obvious that the reliability of the sensing data will be higher if the absolute value of the normalized LLR is larger. We propose utilization of the absolute value of the normalized LLR as the reliability coefficient for selecting the cluster header as well as the selective cluster data.
where is equal to the normalized LLR with highest absolute value and is the cluster threshold. Next, we consider the problem of choosing the optimal cluster threshold.
4.2 Cluster threshold determination
In order to make a controllable cluster decision that follows a certain criterion such as the Neyman-Pearson criterion or minimum error probability criterion, one factor to consider is the probability density function of the cluster’s selective sensing data which is utilized to make the cluster decision. In this subsection, we will formulate this requirement.
where the conditional PDF’s of the LLR under H0 and H1 are determined in  as follows:
where a = [N2/4 + N log(2γ + 1)/γ](2γ + 1) and b = 2N(2γ + 1)/γ.
Otherwise, , , , and .
The derivation of (22) can be found in the Appendix. Similarly, the conditional PDF of the n0th absolute order sample under the H j hypothesis, j = 0,1, can be achieved.
4.3 Parallel report mechanism
In this method, the problems of strict synchronization and contention collision, which can occur with the previous method, are completely resolved. Indeed, with this parallel contention and reporting mechanism, the synchronization among CUs can be looser since there is only one contention time that is identical to the reporting time. No collision between two cluster reports will occur since these cluster decisions are transmitted at different frequencies. Even in the case that two CUs in a cluster have the same value of the most reliable sensing data, a collision still will not occur since the two nodes will transmit the same frequency, and at the receiver side, two transmitted frequencies can be considered as two versions of a multipath signal. The remainder problem with this parallel reporting method is that the FC needs to be equipped with parallel communication devices such as an FFT block, which is usually used in an OFDM receiver, or a filter bank block to detect multiple reporting frequencies. However, this requirement is not a big issue.
5 Simulation results
The simulation of the proposed cluster-based selective CSS scheme is conducted under the following assumptions:
The LU signal is a DTV signal as in .
The bandwidth of the PU signal is 6 MHz, and the AWGN channel is considered.
The local sensing time is 50 μ s.
The probability of the presence and absence of PU signal is 0.5 for both.
The network has n0 nodes and can be divided into n c clusters. Each cluster includes n0 nodes.
respectively. The energy consumption efficiency (EE) and the reporting time-saving efficiency (TE) of the conventional cluster and the proposed CSS schemes compared with the direct CSS scheme can be easily obtained by EE ∗ = 1 - E∗/EDIR and TE ∗ = 1 - T∗/TDIR, respectively, where the asterisk (*) can be replaced by CON or PROP.
In this paper, we have proposed a cluster-based CSS scheme which includes the selective method in the cluster and the optimal fusion rule in the FC. The proposed selective combination method can dramatically reduce the reporting time and energy consumption while achieving a certain high level of sensing performance especially when it is combined with the proposed frequency division-based parallel reporting mechanism.
Derivation of Equation 22
By replacing Y with and substituting k = n0 into (30), Equation 22 can be obtained.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2012R1A1A2038831 and NRF-2013R1A2A2A05004535).
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