A decoding-based fusion rule for cooperative spectrum sensing with nonorthogonal transmission of local decisions
© Bokharaiee et al.; licensee Springer. 2013
Received: 4 April 2013
Accepted: 11 June 2013
Published: 8 July 2013
Cooperative spectrum sensing in cognitive radio (CR) networks is studied in which each CR performs energy detection to obtain a binary decision on the absence/presence of the primary user. The problem of interest is how to efficiently report and combine the local decisions to/at the fusion center under fading channels. In order to reduce the required transmission bandwidth in the reporting phase, the paper examines nonorthogonal transmission of local decisions by means of on-off keying. Proposed and analyzed is a novel decoding-based fusion rule that essentially performs in three steps: (1) estimating minimum mean-square error of the transmitted information from cognitive radios, (2) making hard decisions of the transmitted bits based on the estimated information, and (3) combining the hard decisions in a linear manner. Simulation results support the theoretical analysis and show that the added complexity of the decoding-based fusion rule leads to a considerable performance gain over the simpler energy-based fusion rule when the reporting links are reasonably strong.
Cognitive radio (CR) is an attractive technology to deal with the spectrum scarcity issue as the number of wireless applications and systems grows quickly. The main principle behind cognitive radio is to provide wireless access to potential users by opportunistically detecting the unused licensed bands, originally allocated to some primary users. The key for enabling such an opportunistic access lies in a reliable spectrum sensing technique. The technique of distributed spectrum sensing, in which the observations of CR nodes are collected and transmitted to a fusion center (FC) for a final sensing decision, has received a great interest in recent years. The fusion center aggregates the information pieces transmitted from the CRs and combines them according to some fusion rule in order to make a final decision about the absence (denoted by) or presence (denoted by) of the primary user in the band of the interest.
Since the transmission of the local sensing data to the FC can be costly in terms of bandwidth requirement, particularly for large-scale distributed CR networks, some form of local data compression is preferred in which each CR sends to the FC only one or a few bits of data, representing its local sensing result. To further reduce bandwidth consumption while maintaining simple fusion processing, nonorthogonal transmission of local decisions can be employed by means of on-off keying (OOK). In such a transmission technique, the CRs are allocated with nonorthogonal (correlated) signature vectors (SVs). If the length of the signature vectors is substantially less than the number of CRs, the bandwidth efficiency can be significantly improved[1, 2].
Our earlier work in proposes a collaborative weighted energy-based fusion rule with noncoherent transmission of 1-bit decisions and when the sensing results are reported to the FC over orthogonal channels. The main focus of the work in is to optimize the sensing thresholds at the local CRs, the combining gains at the fusion center, and the sensing time to maximize the secondary throughput of a CR network. However, for large-scale CR networks, assigning orthogonal channels to all CRs might lead to an unaffordable bandwidth expenditure.
This paper adopts the same nonorthogonal transmission framework of for the reporting phase. Different from, our main contribution is to develop a low-complexity decoding-based fusion rule as an alternative to the energy-based fusion rule in order to efficiently suppress the noise in the received signal at the fusion center and achieve a better sensing performance when the reporting channels are strong. The performance of the proposed fusion rule is analyzed and compared with the performance of the energy-based fusion rule under different scenarios.
The remaining of the paper is organized as follows. Section 2 introduces the model of cooperative spectrum sensing with nonorthogonal transmission of local decisions. Section 3 summarizes the energy-based fusion rule, whereas the decoding-based fusion rule is developed and analyzed in Section 4. Simulation results are presented and discussed in Section 5. Finally, Section 6 draws conclusions.
2 System model
During the sensing period, each CR collects its observations from the primary user’s signal in order to make a local decision on the binary hypothesis or. Due to the presence of Rayleigh fading channels between the primary user and CRs, the local observations at CRs can be treated as independent and identically distributed (i.i.d.) random variables. For processing the observations at each CR, an energy detector (which is known as an optimal detector for i.i.d. signals) is implemented. In particular, local binary decisions are obtained by comparing the energy of the collected signals to a sensing threshold.
In the reporting phase, the local decisions are transmitted to the fusion center over Rayleigh fading channels. For such transmission, the same framework presented in is adopted. The local decision at the k th CR is multiplied (i.e., modulated) with a unique signature vector g k whose length is M < K. All the K ‘modulated’ signature vectors are then transmitted simultaneously in M chip intervals to the fusion center. As mentioned before, the main reason for having M < K is to reduce the transmission bandwidth when compared to the case of M = K, i.e., orthogonal transmission. The latter has been studied in.
where x(t) denotes the low-pass equivalent of the transmitted signal from the primary user, E0 is the average transmitted symbol energy of the source, and denotes the coefficient of the fading channel between node 0 (primary user) and CR node i. The notation refers to a complex Gaussian random vector (or variable) with mean vector m and covariance matrix Σ. Also, n0,i(t) is the filtered additive white Gaussian noise process.
where n0,i(n T s ) are the samples of n0,i(t), which can be shown to be i.i.d. complex Gaussian random variables with mean 0 and variance[4–6]. Here, N0 is the two-sided power spectral density (PSD) of the white noise before the band-pass filter, and W is the bandwidth of the band-pass filter.
In essence, the above test statistics is a measure of the average energy of the band-limited signal at each CR node over a duration of τ seconds where and f s is the sampling rate. When the number of collected samples, N, is large, the central limit theorem can be applied to model y i under both hypotheses with Gaussian distributions[7–10]. That is,, and. Here, the notation means a real Gaussian random vector (or variable) with mean vector m and covariance matrix Σ.
After making a local binary decision, if the CR decides (i.e., u i = 1), the signal vector to be transmitted to the fusion center is obtained as the product of a i and the M × 1 signature vector g i . All the signature vectors have unit energy, i.e., ∥g i ∥2 = 1, i = 1,…,K. On the other hand, if a node decides (u i = 0), it remains silent and does not send a signal to the fusion center. Equivalently, the transmission scheme can also be viewed as a censoring scheme, where only the CRs with nonzero decisions transmit. The transmitted signal from the i th CR can simply be expressed as v i g i = (a i u i )g i , where v i = a i u i . It should be noted that the parameter a i sets the average transmitted power of the i th CR. Here, we assume that all CRs are similar, and without loss of generality, they can transmit with an average gain of a i = 1. For the case of M = K, one can choose g i = e i , where e i is a column vector of length K with the i th element equal to 1 and all other elements equal to 0. Obviously, the choice leads to orthogonal transmission of OOK modulated signals, which has been treated in. In contrast, the main focus of this paper is the case when M < K and the SVs cannot be made orthogonal. The key benefit of using shorter SVs is that the transmission in the reporting phase can be conducted with a smaller bandwidth.
The next sections examine two fusion rules, namely, the energy-based fusion rule and the decoding-based fusion rule. In fact, the simple energy-based fusion rule was also discussed in. However, its analysis does not explicitly take into account the signal processing at the CRs and cannot be used for parameter optimizations. On the other hand, the decoding-based fusion rule presented in this paper is novel and offers an attractive performance-complexity tradeoff when compared to the simple energy-based fusion rule or the optimum fusion rule in. It should also be pointed out that the optimum fusion rule in not only has the complexity that is exponential in the number of CRs, but is also difficult to analyze for the purpose of parameter optimizations. Similar to and, the objective of optimizing parameters for a fusion rule is to maximize the secondary throughput while maintaining the probability of detection equal or above a target value. For a given sensing time, maximizing the throughput function for a target is equivalent to minimizing the probability of false alarm.
3 Energy-based fusion rule
where λ(E) is the decision threshold at the FC.
It is noted that only the conditional variances depend on the set of SVs. Furthermore, for the special case of orthogonal transmission, one can easily verify that the means and variances reduce to the expressions given in.
Furthermore, one can also try to find the set of signature vectors to further minimize. Unfortunately, such an optimization problem in its general form appears to be very complex, and finding a closed-form solution for optimal G seems intractable. Nevertheless, a good set of signature vectors can be found by generating a large set of random signature vectors and picking the one that maximizes the expression in (20).
For the simple case when the sensing channels as well as the reporting channels have the same average SNRs, i.e., γ i = γ and ξ i = ξ, it can be shown that the so-called Welch-bound equality (WBE) sequences yield the optimal signature vectors. The proof is as follows: Observe from (16) and (17) that both ν0 and ν1 are functions of the total squared correlation (TSC), namely. Then, in order to maximize, it is clear that has to be minimized. It is well known that the WBE sequences minimize the TSC.
4 Decoding-based fusion rule
As can be seen, z H z is actually a weighted sum of the local decisions and noise terms. This suggests that one might decode the received signal first and then combine the hard decisions in the hope of achieving a better performance due to better noise reduction under certain channel conditions.
In general, the MMSE is a useful performance measure for parameter estimation. In many research papers concerning the detection performance over overloaded code-division multiple access systems, signature vectors are obtained by minimizing MMSE-related metrics, and WBE sequences (or weighted WBE sequences) turn out to be the optimum signature vectors[2, 12]. However, in the spectrum sensing problem, the main performance measures are mainly related to the probability of false alarm and probability of detection, and the WBE sequences are not necessarily the ones that optimize the performance.
Note that and.
where and are the probabilities of detection and probabilities of false alarm associated with the decoded bits. They are shown in to be given as ℘ d i = p d i ϖ i + (1−p d i )ϑ i and ℘ f i = p f i ϖ i + (1−p f i )ϑ i .
where λ(D) is the threshold, while the weights are and. Note that the above fusion rule is simply a weighted linear combination of the hard-decision bits. Moreover, the weights are inherently adjusted according to both the decision of the i th CR and the quality of the reporting channel.
Recall that an approximation of the optimal sensing thresholds used at the cognitive radios for the energy-based fusion rule is given in (21). The same result applies to the decoding-based fusion rule presented in this section. The proof of this follows the same steps in Appendix C of.
The next section compares the performance of the energy-based and decoding-based fusion rules and also verifies the accuracy of our analysis. It is pointed out that the simple expressions of the probability of false alarm and the probability of detection (Equations (18) and (19) for the energy-based fusion rule, or (36) and (37) for the decoding-based fusion rule) are very convenient not only in determining the threshold at the fusion center (λ(E) or λ(D)) for a given target probability of false alarm (or probability of detection), but also in evaluating the performance of different sets of signature vectors used in nonorthogonal transmission of local decisions.
5 Simulation results
Each point in the simulations results is obtained by averaging over 104 random realizations for primary transmitted signal, fading channels, and noise. For the sake of simplicity, it is assumed that. Unless otherwise stated, the number of the CRs is set to K = 30, the number of samples taken is N = 500, and the set of WBE signature sequences is assigned to CR users.
On the other hand, for the proposed decoding-based fusion rule, Figure3 shows that the sensing performance is significantly improved not only when increasing M from 5 to 15, but also by further increasing M to 30. Such an improvement can be explained by the fact that detection of the transmitted bits from CRs strongly depends on the interference caused by nonorthogonal transmission. As the interference reduces with longer signature vectors (larger M), the detection performance is greatly enhanced when all the reporting channels are fairly strong. The figure also shows that the theoretical result (with M = 15) follows the simulation result closely. Another observation from Figure3 is that with random signature vectors, the sensing performance is slightly better than the performance with WBE sequences. As pointed out before, it is difficult to obtain the optimal set of signature vectors for the proposed decoding-based fusion rule. The results in Figure3 suggest that either randomly generated or WBE sequences can be used for the proposed decoding-based fusion rule.
As can be seen in Figures5 and6, both figures show performance improvement whenM increases from 15 to 30. However, for the first scenario with the weak reporting links, the improvement is marginal. This is because the sensing performance in this scenario is mostly affected by the noise at the FC rather than the interference caused by shorter signature vectors. Another important observation is that the energy-based fusion rule outperforms the decoding-based rule. Thus, it can be concluded that for low channel SNRs, the energy-based fusion rule is preferred over the decoding-based fusion rule due to its lower complexity while at the same time delivering equal or better performance.
For the second scenario, when the channels are fairly strong, the decoding-based detection significantly outperforms the energy-based detection. Also, as M increases, the performance of the decoding-based fusion rule quickly improves due to the reduction of interference. However, for the energy-based fusion rule, interference reduction does not play a significant role in improving the sensing performance.
This paper has proposed a low-complexity decoding-based fusion rule for cognitive radio networks with nonorthogonal transmission of local decisions in the presence of channel impairments and noise. The proposed fusion rule first performs the MMSE estimation of the transmitted information, makes decisions on the individual bits sent by the cognitive radios, and then combines these hard-decision bits in a linearly weighted manner. Performance comparison with the energy-based fusion rule shows the superiority of the proposed fusion rule when the reporting channels are reasonably strong. The excellent match between simulation and analytical results verify the accuracy of the performance analysis of the proposed fusion rule.
a To avoid having too many curves on the same figure, only the analytical result for the case of M = 15 is shown.
This work was supported by an NSERC Discovery Grant.
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