基于小波降噪的压缩感知—循环平稳检测技术

认知无线电技术通过次级用户动态接入空闲频谱来提高空闲频谱资源的利用效率,是认知无线电的重要环节。在低信噪比环境下,如何快速精确地进行频谱感知是频谱感知面临的重大挑战。提出了一种基于小波降噪的压缩感知—循环平稳特征检测器来实现低信噪比环境下的频谱检测。采用压缩感知技术提高了频谱感知的效率,并进一步利用小波变换技术降低了压缩感知过程中引入的压缩噪声,提高了低信噪比环境下的频谱感知准确度。仿真结果证明,提出的基于小波降噪的压缩感知技术能够实现低信噪比环境下的频谱空洞检测。     

基于频谱预留的认知无线接入研究

动态频谱接入是认知无线电技术的一个重要应用,它利用通信过程中出现的频谱空洞机会式地接入频谱,能极大地提高频谱的利用率。文章基于马尔可夫理论提出了一种改进的频谱预留接入方案,即按照业务需求对用户进行用户模型改进,并根据优先级不同进行接入策略改进。仿真结果显示该方案能有效保障高优先级用户的服务质量。     

Cyclostationarity-Based Decision Reporting Scheme for Cooperative Spectrum Sensing

Cooperative spectrum sensing has emerged as a promising solution to the hidden terminal problem in cognitive radio networks (CRNs). It could significantly promote the sensing capability of CRNs by exploiting space diversity gains in a fading environment. However, owing to the decision reporting overhead, there exists a tradeoff between the system throughput and performance of cooperative spectrum sensing. In this paper, we propose a cyclostationarity-based decision reporting scheme for cooperative spectrum sensing in CRNs with cyclic delay diversity orthogonal frequency division multiplexing (CDD-OFDM). Decision information would be embedded into the CDD-OFDM signals in terms of cyclostationary signatures and shared among cognitive radio (CR) users along with data transmissions. As a result, satisfied system throughput could be achieved without additional spectral or temporal resources consumption when the number of cooperative users increases. Numerical results are presented to show the system throughput enhancement.     

Cooperative spectrum sensing for cognitive radio networks in the presence of smart malicious users

Cognitive radio (CR) signaling imposes some threats to the network. One of these common threats is commonly referred to as primary user emulation attack, where some malicious users try to mimic the primary signal and deceive secondary users to prevent them from accessing vacant frequency bands. In this paper, we introduce a smart primary user emulation attacker (PUEA) that sends fake signals similar to the primary signal. We assume a smart attacker, in the sense that it is aware of its radio environment and may choose different transmission strategies and then, we compare it to an always present attacker. In the proposed smart attacker strategy, the occurrence of fake signal is adjusted according to the primary user activity. First, we investigate the received signal at the CR users under such attackers. Then, we formulate and derive cooperative spectrum sensing (CSS) rules for a cognitive network operating in the presence of a PUEA and propose a new spectrum sensing scheme based on energy detection. Simulation results show that our proposed method can mitigate the destructive effect of PUEA in spectrum sensing, compared to conventional energy detection spectrum sensing.     

Coopetive Spectrum Trading – Creating Endogenous Spectrum Holes

Dynamic spectrum access has been proposed to address the spectrum scarcity. Based on this access mode, secondary users are allowed to opportunistically operate when primary users are absent. Secondary Users are, however, required to relinquish the spectrum upon return of the incumbent spectrum holder. Although it improves spectrum utility, opportunistic spectrum access limits secondary users’ access to exogenous spectrum holes vacated by primary users. We argue that spectrum holes should be endogenous, enabled by incentives to incumbent users to increase spectrum availability, while mitigating risk. To this end, we take a different perspective on dynamic spectrum sharing and propose a coopetive framework, whereby primary and secondary users engage in a cooperative, yet competitive, auction-driven spectrum sharing to enhance spectrum usage. In this framework, the primary users’spectrum is organized in three bands, namely exclusive usage, usage right and management right bands. Based on this structure, spectrum is traded as financial options to increase trading flexibility and capture the value primary and secondary users attach to spectrum access and usage. Spectrum trading between primary and secondary users is formalized as a utility maximization problem. Approximate solutions to this problem are derived and their performance is analyzed. The results show that spectrum trading, which combines revocable and exclusive leasing, provides higher utilities for both primary and secondary users.     

An optimization technique for simultaneous reduction of PAPR and out-of-band power in NC-OFDM-based cognitive radio systems

Noncontiguous orthogonal frequency division multiplexing (NC-OFDM)-based cognitive radio (CR) systems achieve highly efficient spectrum utilization by transmitting unlicensed users' data on subcarriers of licensed users’ data when they are free. However, there are two disadvantages to the NC-OFDM system: out-of-band power (OBP) and a high peak-to-average power ratio (PAPR). OBP arises due to side lobes of an NC-OFDM signal in the frequency domain, and it interferes with the spectrum for unlicensed users. A high PAPR occurs due to the inverse fast Fourier transform (IFFT) block used in an NC-OFDM system, and it induces nonlinear effects in power amplifiers. In this study, we propose an algorithm called “Alternative Projections onto Convex and Non-Convex Sets” that reduces the OBP and PAPR simultaneously. The alternate projections are performed onto these sets to form an iteration, and it converges to the specified limits of in-band-power, peak amplitude, and OBP. Furthermore, simulations show that the bit error rate performance is not degraded while reducing OBP and PAPR.     

Recognition and elimination of SSDF attackers in cognitive radio networks

The nature of cognitive radio (CR) technology creates a lot of opportunities for attackers. When an attack occurs, the function of the primary network is affected and thus the overall system performance will be reduced. In the present paper, we introduce and simulate a novel method for identifying spectral sensing data falsification (SSDF) attack and recognizing the malicious users (MU), which we refer to as “Recognition and Elimination of SSDF Attackers”. Our proposed scheme uses the generalized likelihood ratio test (GLRT) approach for solving the MUs detection problem. In this method, we do not need previous information about the network and number of the MUs and secondary users (SUs). In addition to detecting the occurrence of an attack, our method can recognize attackers. By recognizing the MUs, their negative effect will be eliminated and the cognitive radio network (CRN) performance will return to normal condition. Consequently, our scheme can save resources by identifying the strategy of the known attackers. Simulation results reveal that our detection and recognition scheme is better than some of methods available.

     

基于因子图的分布式变分稀疏贝叶斯压缩感知简

提出了一种基于因子图的分布式变分稀疏贝叶斯压缩感知算法。该算法利用因子图和变分方法将全局感知问题分解为简单的局部问题,通过认知用户邻居间的置信传播实现“软融合”,使每个认知用户能够获得全局最优估计。且充分利用邻居间传递的信息所具有的时间和空间二维相关性,提高认知用户在低信噪比下的感知性能。同时,算法在迭代过程中自适应地删除不收敛的超参数及对应的基函数,降低通信负载。实验结果表明：该方法在低采样率和低信噪比下有较好的感知性能。     

Bio-Inspired Algorithms for Dynamic Resource Allocation in Cognitive Wireless Networks

Regulation will experience enormous changes in the near future resulting in seamless connectivity by spectrum borders. A promising approach in this context is dynamic spectrum allocation which leads to a more flexible access to spectral resources by employing intelligent radio devices called cognitive radios. This paper is concerned with bio-inspired approaches that exploit distribution in multi-radio environments where many users have to share a finite resource harmoniously. Three applications of bio-inspired techniques are described. The first one deals with the detection of spectrum holes whereas the second one describes resource allocation in orthogonal frequency division multiple access based systems. The third one is concerned with distributed resource auctioning.

基于HMM和平衡二叉树递归搜索的宽带频谱感知

为了使得二级或无许可证用户在给定宽带上获取空闲的子带以供使用,针对认知无线电中的宽带频谱感知技术进行研究,提出一种有效的宽带频谱感知算法。算法采用隐马尔可夫模型(HMM)对初级用户的动态行为进行建模,以克服目前宽带感知技术的局限性;其次,利用现有窄带感知技术,将感知频带划分为较小的频道,将其建模为一棵平衡二叉树,并对频谱孔洞进行递归搜索。如果检测到有孔洞在频率上相邻,则将它们合并成一个单一的频谱孔洞,使得认知二级用户的容量在整个频带上最大化。仿真实验结果表明,与现有宽带频谱感知方法相比,提出的宽带频谱感知算法具有更好的感知性能增益和更强的鲁棒性。     

Injection-induced terahertz electroluminescence from silicon <Emphasis Type="Italic">p–n</Emphasis> structures

Injection-induced terahertz electroluminescence from silicon p ⁺–n structures is observed at helium temperatures. Structures fabricated by the diffusion of boron into a phosphorus-doped n-Si substrate are studied. Relatively narrow luminescence lines are observed in the luminescence spectra against a broad smooth background. The spectral position of a number of emission lines corresponds to optical transitions between excited donor states and the ground state of phosphorus donors. The intracenter optical transitions of electrons at phosphorus donors are excited as a result of recombination processes occurring in the n-type region of the structure under the injection of nonequilibrium holes. A number of other lines in the terahertz emission spectra are associated with intracenter transitions at acceptor centers, which are also excited as a result of injection. The structureless background in the electroluminescence spectra may be due to terahertz emission upon the intraband energy relaxation of “hot” carriers having an effective temperature exceeding the lattice temperature. These “hot” carriers appear in the structure under injection conditions.     

BLSBA‐ED‐SSO: Bivariate Lévy‐stable bat algorithm‐based energy detection for spectrum sensing optimization in cognitive radio networks (CRNs)

The vacant licensed spectrum can be employed by the secondary users in cognitive radio networks. Nevertheless, this process of identification is frequently agreed with shadowing, multipath fading and receiver uncertainty problems. The primary contributions of the spectrum sensing techniques adopted in this work mainly concentrate on energy detection for spectrum sensing as well as to recognize the optimal spectral estimation according to the multitaper spectral estimation. The algorithm proposed in this work is based on bivariate Lévy‐stable bat algorithm (BLSBA), energy detector (ED), and with the help of K out of M fusion rule; it is analysed for the single user as well as cooperative multiple users. In the BLSBA algorithm, a modified search equation with more helpful information from the search experiences is brought‐in to create an optimal energy solution and bivariate Lévy‐stable random walk is associated with BLSBA to remove the trapping process into local optima. The energy detection is defined numerically from this optimal detection. At last, a multi‐taper spectral estimator (MSE) is proposed to cognitive radio detection for a huge network. The simulation results in both cases are computed and checked with the help of a BLSBA optimizer. For an individual secondary user scenario, the advancement of MSE to the ED is broadly described. Experimental result indicates that the objective false alarm likelihood is minimized and the demanded signal‐to‐noise ratio is accomplished to the extent.     

基于功率谱熵的频谱感知算法研究

针对低信噪比条件下认知无线电频谱感知问题,提出了一种基于功率谱熵的频谱检测算法。在分析主用户信号空闲与占用两种不同条件下观测信号功率谱熵差异的基础上,将其作为检验统计量,并确定了相应的判决门限,以实现对主用户信号频谱是否空闲的判决。计算机仿真结果表明,本算法无需信号的先验信息,可在较低信噪比条件下实现对常用调制信号的频谱感知,与盒维数频谱感知方法相比,检测性能约有8 dB的改进。     

认知无线电应用讨论

讨论了当利用灰色频谱进行认知无线电通信时,第二用户对授权用户频谱所造成的干扰,并对这种干扰进行了计算。进一步分析中,以授权用户的信号干扰噪声比余量为基础,讨论并仿真了第二用户在保证授权用户正常通信时自身的信噪比受到来自授权用户的影响。结果表明,在灰色频谱区域进行认知无线电通信时,为改善第二用户信号干扰噪声比,其发射功率与通信距离将会是动态调整的。     

Maximizing the lifetime of energy‐constrained cooperative spectrum sensing sensor network

Adding the cognitive capability to wireless sensor networks allows the sensors to monitor the spectrum and identify the spectrum holes to operate in different frequencies according to the radio environment which result in better spectrum utilization. Spectrum sensing is a main component in any cognitive radio network. In this paper, we propose a new cooperative sensing scheme based on energy detection for cognitive sensor networks which is constrained by the energy limitation of the wireless sensor elements. The proposed scheme minimizes the sensing energy for individual sensor and carefully selects the suitable participant sensors in each cooperative sensing process. This results in maximizing the lifetime of energy‐constrained wireless sensor networks. The proposed scheme also takes into consideration the constraints on the detection accuracy. The simulation results show that the proposed scheme prolonged the lifetime of the cognitive network, makes efficient usage of available spectrum by secondary users, and satisfy the target detection performance.     

基于D-S证据理论的循环平稳频谱协作感知

为了避免干扰授权用户的正常通信,认知无线电系统需要较好的频谱检测性能。提出了基于循环平稳特征和D-S证据理论的多天线多用户频谱协作感知方法。分析了多天线接收信号的谱相关函数,在频域按照最大比合并的方法合并各天线接收信号,得到合并信号的谱相关函数。依据谱相关函数的能量,在多用户中采用D-S证据理论做协作频谱检测。仿真分析表明,基于循环平稳特征和D-S证据理论的多天线多用户频谱协作感知方法可有效改善认知无线电系统检测性能。     

Cooperative Spectrum Sharing based on Amplify and Forward Relaying in Cognitive Radio Networks

In this paper, we consider a cognitive radio system in non-ideal fading wireless channels and propose a geometric approach based on cooperative amplify and forward relaying for the secondary users, which serves as an alternative way to ensure the primary user??s transmission and realize spectrum sharing. The whole system consists of a pair of primary transmitter and receiver and a pair of secondary transmitter (ST) and receiver. The key feature of the proposed approach is that ST acts as a half-relaying for the primary system by linearly combining the primary signal with the secondary signal and allocating fractions ?? and 1 ? ?? of the transmit power to the primary and secondary signals separately. We show that for a fixed ??, we can find a critical region for ST restricted in two-dimension where outage probability of the primary system is kept or lower than the case without spectrum sharing. The outage performance achieved by the secondary system is also derived. We get the optimal ?? and location for the secondary system when the secondary parameters are changeable. Finally, the bit error rate for both primary and secondary systems are derived in closed-form expressions. Simulation results illustrate that the proposed scheme can significantly realize spectrum sharing.     

认知无线电网络中抗追踪用户攻击的隐式信道选择方案(英文)

For the discontinuous occupancy of primary users in cognitive radio networks(CRN),the time-varying of spectrum holes becomes more and more highlighted.In the dynamic environment,cognitive users can access channels that are not occupied by primary users,but they have to hand off to other spectrum holes to continue communication when primary users come back,which brings new security problems.Tracking user attack(TUA) is a typical attack during spectrum handoff,which will invalidate handoff by preventing user accessing,and break down the whole network.In this paper,we propose a Channel Selection Information Hiding scheme(CSIH) to defense TUA.With the proposed scheme,we can destroy the routes to the root node of the attack tree by hiding the information of channel selection and enhance the security of cognitive radio networks.     

Cooperative spectrum sensing using amplify-and-forward relaying with partial relay selection in cognitive radio networks

Cooperative spectrum sensing has been shown to be an effective approach to improve the detection performance by exploiting the spatial diversity among multiple cognitive nodes. By using the amplify-and-forward relaying with partial relay selection, this paper proposes a novel cooperative spectrum sensing scheme, which provides higher detection performance and is interesting in distributed cognitive radio networks. In the proposed sensing scheme, the “best” cognitive relay by means of partial relay selection technique amplifies and forwards the signals transmitted from the primary user (PU) to the cognitive user (CU). Then the CU detects PU’s states (i.e., presence or absence) via an energy detector. Moreover, the average missed-detection probability of proposed sensing scheme is studied over Nakagami-m fading channels, where m is a positive integer. In particular, the tight closed-form lower bounds of the average missed-detection probability are presented for the convenience of performance evaluation in practice. Finally, numerical results are provided to validate the derived closed-form lower bounds and the influence of the number of cognitive relays on the detection performance is also discussed.     

认知超宽带无线电波形设计和频谱整形技术

认知超宽带无线电（CUWBR）系统实现的关键问题是当CUWBR感知到频谱空洞时,怎样设计合适的发送波形和频谱整形使其对合法用户造成的干扰最小。针对这一问题,文中简要介绍了CUWBR系统中基于检测与躲避的抗干扰原理,综述了CUWBR抗干扰发送波形设计和频谱整形技术的研究状况和最新进展,并对未来的发展动态进行了分析。解决这一问题能够提高现有频谱的利用率,缓和频谱供需矛盾,对UWB的实用化和CR的实现有积极的推动意义。