Browsing by Subject "Cognitive radio"
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Item Open Access Adaptive decision fusion based cooperative spectrum sensing for cognitive radio systems(IEEE, 2011) Töreyin, B. U.; Yarkan, S.; Qaraqe, K. A.; Çetin, A. EnisIn this paper, an online Adaptive Decision Fusion (ADF) framework is proposed for the central spectrum awareness engine of a spectrum sensor network in Cognitive Radio (CR) systems. Online learning approaches are powerful tools for problems where drifts in concepts take place. Cooperative spectrum sensing in cognitive radio networks is such a problem where channel characteristics and utilization patterns change frequently. The importance of this problem stems from the requirement that secondary users must adjust their frequency utilization strategies in such a way that the communication performance of the primary users would not be degraded by any means. In the proposed framework, sensing values from several sensor nodes are fused together by weighted linear combination at the central spectrum awareness engine. The weights are updated on-line according to an active fusion method based on performing orthogonal projections onto convex sets describing power reading values from each sensor. The proposed adaptive fusion strategy for cooperative spectrum sensing can operate independent from the channel type between the primary user and secondary users. Results of simulations and experiments for the proposed method conducted in laboratory are also presented. © 2011 IEEE.Item Open Access Cognitive-radio systems for spectrum, location, and environmental awareness(IEEE, 2010) Celebi, H.; Güvenç, I.; Gezici, Sinan; Arslan, H.In order to perform reliable communications, a system needs to have sufficient information about its operational environment, such as spectral resources and propagation characteristics. Cognitive-radio technology has capabilities for acquiring accurate spectrum, location, and environmental information, due to its unique features such as spectrum, location, and environmental awareness. The goal of this paper is to give a comprehensive review of the implementation of these concepts. In addition, the dynamic nature of cognitive-radio systems - including dynamic spectrum utilization, transmission, the propagation channel, and reception - is discussed, along with performance limits, challenges, mitigation techniques, and open issues. The capabilities of cognitive-radio systems for accurate characterization of operational environments are emphasized. These are crucial for efficient communications, localization, and radar systems.Item Open Access Communication efficient channel estimation over distributed networks(IEEE, 2014) Sayın, Muhammed O.; Vanlı, N. Denizcan; Göze, T.; Kozat, Süleyman SerdarWe study diffusion based channel estimation in distributed architectures suitable for various communication applications such as cognitive radios. Although the demand for distributed processing is steadily growing, these architectures require a substantial amount of communication among their nodes (or processing elements) causing significant energy consumption and increase in carbon footprint. Due to growing awareness of telecommunication industry's impact on the environment, the need to mitigate this problem is indisputable. To this end, we introduce algorithms significantly reducing the communication load between distributed nodes, which is the main cause in energy consumption, while providing outstanding performance. In this framework, after each node produces its local estimate of the communication channel, a single bit or a couple of bits of information is generated using certain random projections. This newly generated data is diffused and then used in neighboring nodes to recover the original full information, i.e., the channel estimate of the desired communication channel. We provide the complete state-space description of these algorithms and demonstrate the substantial gains through our experiments.Item Open Access Decentralized dynamic rate and channel selection over a shared spectrum(IEEE, 2021-03-15) Javanmardi, Alireza; Qureshi, Muhammad Anjum; Tekin, CemWe consider the problem of distributed dynamic rate and channel selection in a multi-user network, in which each user selects a wireless channel and a modulation and coding scheme (corresponds to a transmission rate) in order to maximize the network throughput. We assume that the users are cooperative, however, there is no coordination and communication among them, and the number of users in the system is unknown. We formulate this problem as a multi-player multi-armed bandit problem and propose a decentralized learning algorithm that performs almost optimal exploration of the transmission rates to learn fast. We prove that the regret of our learning algorithm with respect to the optimal allocation increases logarithmically over rounds with a leading term that is logarithmic in the number of transmission rates. Finally, we compare the performance of our learning algorithm with the state-of-the-art via simulations and show that it substantially improves the throughput and minimizes the number of collisions.Item Open Access Error rate analysis of cognitive radio transmissions with imperfect channel sensing(IEEE, 2013) Ozcan G.; Gursoy, M. C.; Gezici, SinanIn this paper, error rate performance of cognitive radio transmissions is studied in the presence of imperfect channel sensing decisions. It is assumed that cognitive users first perform channel sensing, albeit with possible errors. Then, depending on the sensing decisions, they select the transmission energy level and employ MI × MQ rectangular quadrature amplitude modulation (QAM) for data transmission over a fading channel. In this setting, the optimal decision rule is formulated under the assumptions that the receiver is equipped with the sensing decision and perfect knowledge of the channel fading. It is shown that the thresholds for optimal detection at the receiver are the midpoints between the signals under any sensing decision. Subsequently, minimum average error probability expressions for M-ary pulse amplitude modulation (M-PAM) and MI×MQ rectangular QAM transmissions attained with the optimal detector are derived. The effects of imperfect channel sensing decisions on the average symbol error probability are analyzed. Copyright © 2013 by the Institute of Electrical and Electronic Engineers, Inc.Item Open Access Error rate analysis of cognitive radio transmissions with imperfect channel sensing(Institute of Electrical and Electronics Engineers Inc., 2014) Ozcan, G.; Gursoy, M. C.; Gezici, SinanThis paper studies the symbol error rate performance of cognitive radio transmissions in the presence of imperfect sensing decisions. Two different transmission schemes, namely sensing-based spectrum sharing (SSS) and opportunistic spectrum access (OSA), are considered. In both schemes, secondary users first perform channel sensing, albeit with possible errors. In SSS, depending on the sensing decisions, they adapt the transmission power level and coexist with primary users in the channel. On the other hand, in OSA, secondary users are allowed to transmit only when the primary user activity is not detected. Initially, for both transmission schemes, general formulations for the optimal decision rule and error probabilities are provided for arbitrary modulation schemes under the assumptions that the receiver is equipped with the sensing decision and perfect knowledge of the channel fading, and the primary user's received faded signals at the secondary receiver has a Gaussian mixture distribution. Subsequently, the general approach is specialized to rectangular quadrature amplitude modulation (QAM). More specifically, the optimal decision rule is characterized for rectangular QAM, and closed-form expressions for the average symbol error probability attained with the optimal detector are derived under both transmit power and interference constraints. The effects of imperfect channel sensing decisions, interference from the primary user and its Gaussian mixture model, and the transmit power and interference constraints on the error rate performance of cognitive transmissions are analyzed.Item Open Access An experimental setup for performance analysis of an online adaptive cooperative spectrum sensing scheme for both in-phase and quadrature branches(IEEE, 2011) Yarkan, S.; Qaraqe, K.A.; Töreyin, B.U.; Çetin, A. EnisSpectrum sensing is one of the most essential characteristics of cognitive radios (CRs). Robustness and adaptation to varying wireless propagation scenarios without compromising the sensing accuracy are desirable features of any spectrum sensing method to be deployed in CR systems. In this study, an online adaptive cooperation technique for spectrum sensing is proposed in order to maintain the level of reliability and performance. Cooperation is achieved by sensors which employ energy detection. These sensors send their output to a center where data fusion operation is carried out in an online and adaptive manner. Adaptation is realized by the use of orthogonal projections onto convex sets (POCS). In conjunction with the proposed method, an end-to-end methodology for a flexible experimental setup is also proposed in this study. This setup is arranged to emulate the proposed adaptive cooperation scheme for spectrum sensing and validate its practical use in cognitive radio systems. Comparative performance results for both inphase and quadrature branches are presented. © 2011 IEEE.Item Open Access An experimental validation of an online adaptive cooperation scheme for spectrum sensing(IEEE, 2011-05) Yarkan, S.; Töreyin, B. U.; Qaraqe, K. A.; Çetin, A. EnisCooperative spectrum sensing methods in the literature assume a static communication scenario with fixed channel and propagation environment characteristics. In order to maintain the level of sensing reliability and performance under changing channel and environment conditions, in this study, an online adaptive cooperation scheme is proposed. Energy detection data from each cooperating sensor are fused together by an adaptive weighted linear combination at the fusion center. Weight update operation is performed online through the use of orthogonal projections onto convex sets (POCS). Also, in this paper, an end-to-end methodology for a flexible experimental setup is proposed. This setup is specifically deployed to emulate the proposed adaptive cooperation scheme for spectrum sensing and validate its practical use in cognitive radio systems. © 2011 IEEE.Item Open Access Fully distributed bandit algorithm for the joint channel and rate selection problem in heterogeneous cognitive radio networks(Bilkent University, 2020-12) Javanmardi, AlirezaWe consider the problem of the distributed sequential channel and rate selection in cognitive radio networks where multiple users choose channels from the same set of available wireless channels and pick modulation and coding schemes (corresponds to transmission rates). In order to maximize the network throughput, users need to be cooperative while communication among them is not allowed. Also, if multiple users select the same channel simultaneously, they collide, and none of them would be able to use the channel for transmission. We rigorously formulate this resource allocation problem as a multi-player multi-armed bandit problem and propose a decentralized learning algorithm called Game of Thrones with Sequential Halving Orthogonal Exploration (GoT-SHOE). The proposed algorithm keeps the number of collisions in the network as low as possible and performs almost optimal exploration of the transmission rates to speed up the learning process. We prove our learning algorithm achieves a regret with respect to the optimal allocation that grows logarithmically over rounds with a leading term that is logarithmic in the number of transmission rates. We also propose an extension of our algorithm which works when the number of users is greater than the number of channels. Moreover, we discuss that Sequential Halving Orthogonal Exploration can indeed be used with any distributed channel assignment algorithm and enhance its performance. Finally, we provide extensive simulations and compare the performance of our learning algorithm with the state-of-the-art which demonstrates the superiority of the proposed algorithm in terms of better system throughput and lower number of collisions.Item Open Access Fundamental limits on time delay estimation in dispersed spectrum cognitive radio systems(IEEE, 2009) Gezici, Sinan; Celebi, H.; Poor, H. V.; Arslan, H.In this paper, fundamental limits on time delay estimation are studied for cognitive radio systems, which facilitate opportunistic use of spectral resources. First, a generic Cramer-Rao lower bound (CRLB) expression is obtained in the case of unknown channel coefficients and carrier-frequency offsets (CFOs) for cognitive radio systems with dispersed spectrum utilization. Then, various modulation schemes are considered, and the effects of unknown channel coefficients and CFOs on the accuracy of time delay estimation are quantified. Finally, numerical studies are performed in order to verify the theoretical analysis.Item Open Access Interference mitigation and awareness for improved reliability(Cambridge University Press, 2011) Arslan, H.; Yarkan S.; Şahin, M. E.; Gezici, SinanWireless systems are commonly affected by interference from various sources. For example, a number of users that operate in the same wireless network can result in multiple-access interference (MAI). In addition, for ultrawideband (UWB) systems, which operate at very low power spectral densities, strong narrowband interference (NBI) can have significant effects on the communications reliability. Therefore, interference mitigation and awareness are crucial in order to realize reliable communications systems. In this chapter, pulse-based UWB systems are considered, and the mitigation of MAI is investigated first. Then, NBI avoidance and cancelation are studied for UWB systems. Finally, interference awareness is discussed for short-rate communications, next-generation wireless networks, and cognitive radios.Mitigation of multiple-access interference (MAI)In an impulse radio ultrawideband (IR-UWB) communications system, pulses with very short durations, commonly less than one nanosecond, are transmitted with a low-duty cycle, and information is carried by the positions or the polarities of pulses [1-5]. Each pulse resides in an interval called frame, and the positions of pulses within frames are determined according to time-hopping (TH) sequences specific to each user. The low-duty cycle structure together with TH sequences provide a multiple-access capability for IR-UWB systems [6].Although IR-UWB systems can theoretically accommodate a large number of users in a multiple-access environment [2, 4], advanced signal processing techniques are necessary in practice in order to mitigate the effects of interfering users on the detection of information symbols efficiently [6]. © Cambridge University Press 2011.Item Open Access On the interplay between channel sensing and estimation in cognitive radio systems(IEEE, 2011) Gursoy, M.C.; Gezici, SinanCognitive radio transmissions in the presence of channel uncertainty are considered. In practical scenarios, cognitive secondary users need to perform both channel sensing in order to identify whether the channel is being occupied by the primary users or not, and also channel estimation in order to learn the channel fading coefficients. Generally, errors occur in both channel sensing and estimation, and this leads to a coupling between the two. More specifically, imperfect sensing affects both the structure and the performance of channel estimation schemes. With this motivation, the interactions between channel sensing and estimation are studied in this paper. In particular, different channel estimation schemes including minimum mean-square error (MMSE), linear MMSE, and mismatched MMSE estimations are analyzed, and their dependence on sensing decisions and their performances are investigated. © 2011 IEEE.Item Open Access Optically reconfigurable planar monopole antenna for cognitive radio application(Wiley Periodicals, Inc., 2019) Ali, A.; Topalli, K.; Ramzan, M.; Alibakhshikenari, M.; Khan, Talha Masood; Altıntaş, Ayhan; Colantonio, P.Frequency reconfigurable antenna is one of the important elements needed for cognitive radio application. Such antenna can be designed using highly resistive (HR) silicon (Si) operating as an optical switch. This letter presents a novel frequency reconfigurable planar monopole antenna suitable for cognitive radio application. The antenna is designed using HR Si working as an optical switch. The main idea behind the design of antenna is the redistribution of surface current on the antenna while changing the state of Si switches optically from high resistance to low resistance. The antenna is highly compact and uses only two switches for multiband reconfiguration. It is switchable on 1.9 GHz, 2.75 GHz, 3.7 GHz, 4.1 GHz, 4.6 GHz, 4.8 GHz, and 7.6 to 11 GHz frequency bands. Simulated and measured results are presented for the antenna. To the best of authors knowledge, this is the first multiband optically reconfigurable planar monopole antenna.Item Open Access Optimal detector randomization in cognitive radio receivers in the presence of imperfect sensing decisions(Bilkent University, 2013) Sezer, Ahmet DündarIn cognitive radio systems, spectrum sensing is one of the crucial tasks to be performed by secondary users in order to limit the interference to primary users. Therefore various spectrum sensing methods have been proposed in the literature. Once secondary users make a sensing decision, they adapt their communication parameters accordingly, which means that they perform communications when the channel is sensed as idle whereas they either do not transmit at all or transmit at a reduced power when the channel is sensed as busy. However, in practical systems, sensing decisions of secondary users are never perfect; hence, there can be cases in which the sensing decision is idle (busy) but primary user activity actually exists (does not exist). Therefore, the optimal design of secondary systems requires the consideration of imperfect sensing decisions. In this thesis, optimal detector randomization is developed for secondary users in a cognitive radio system in the presence of imperfect spectrum sensing decisions. Also, suboptimal detector randomization is proposed under the assumption of perfect sensing decisions. It is shown that the minimum average probability of error can be achieved by employing no more than four maximum a-posteriori probability (MAP) detectors at the secondary receiver. Optimal and suboptimal MAP detectors and generic expressions for their average probability of error are derived in the presence of possible sensing errors. Numerical results are presented and the importance of taking possible sensing errors into account is illustrated in terms of average probability of error optimization.Item Open Access Optimal detector randomization in cognitive radio systems in the presence of imperfect sensing decisions(2014) Sezer, A. D.; Gezici, Sinan; Gursoy, M. C.In this study, optimal detector randomization is developed for secondary users in a cognitive radio system in the presence of imperfect spectrum sensing decisions. It is shown that the minimum average probability of error can be achieved by employing no more than four maximum a-posteriori probability (MAP) detectors at the secondary receiver. Optimal MAP detectors and generic expressions for their average probability of error are derived in the presence of possible sensing errors. Also, sufficient conditions are presented related to improvements due to optimal detector randomization.Item Open Access Performance limits on ranging with cognitive radio(IEEE, 2009-06) Dardari, D.; Karisan, Yasir; Gezici, Sinan; D'Amico, A. A.; Mengali, U.Cognitive radio is a promising paradigm for efficient utilization of the radio spectrum due to its capability to sense environmental conditions and adapt its communication and localization features. In this paper, the theoretical limits on time-of-arrival estimation for cognitive radio localization systems are derived in the presence of interference. In addition, an optimal spectrum allocation strategy which provides the best ranging accuracy limits is proposed. The strategy accounts for the constraints from the sensed interference level as well as from the regulatory emission mask. Numerical results are presented to illustrate the improvements that can be achieved by the proposed approach. © 2009 IEEE.Item Open Access Power adaptation for cognitive radio systems under an average sinr loss constraint in the absence of path loss information(Kluwer Academic Publishers, 2014) Dulek, B.; Gezici, Sinan; Sawai, R.; Kimura, R.An upper bound is derived on the capacity of a cognitive radio system by considering the effects of path loss and log-normal shadowing simultaneously for a single-cell network. Assuming that the cognitive radio is informed only of the shadow fading between the secondary (cognitive) transmitter and primary receiver, the capacity is achieved via the water-filling power allocation strategy under an average primary signal to secondary interference plus noise ratio loss constraint. Contrary to the perfect channel state information requirement at the secondary system (SS), the transmit power control of the SS is accomplished in the absence of any path loss estimates. For this purpose, a method for estimating the instantaneous value of the shadow fading is also presented. A detailed analysis of the proposed power adaptation strategy is conducted through various numerical simulations.Item Open Access Range estimation in multicarrier systems in the presence of interference: performance limits and optimal signal design(IEEE, 2011) Karisan, Y.; Dardari, D.; Gezici, Sinan; D'Amico, A. A.; Mengali, U.Theoretical limits on time-of-arrival (equivalently, range) estimation are derived for multicarrier systems in the presence of interference. Specifically, closed-form expressions are obtained for Cramer-Rao bounds (CRBs) in various scenarios. In addition, based on CRB expressions, an optimal power allocation (or, spectrum shaping) strategy is proposed. This strategy considers the constraints not only from the sensed interference level but also from the regulatory emission mask. Numerical results are presented to illustrate the improvements achievable with the optimal power allocation scheme, and a maximum likelihood time-of-arrival estimation algorithm is studied to assess the effects of the proposed approach in practical estimators.Item Open Access Restricted Neyman-Pearson approach based spectrum sensing in cognitive radio systems(Bilkent University, 2012) Turgut, EsmaOver the past decade, the demand for wireless technologies has increased enormously, which leads to a perceived scarcity of the frequency spectrum. Meanwhile, static allocation of the frequency spectrum leads to under-utilization of the spectral resources. Therefore, dynamic spectrum access has become a necessity. Cognitive radio has emerged as a key technology to solve the conflicts between spectrum scarcity and spectrum under-utilization. It is an intelligent wireless communication system that is aware of its operating environment and can adjust its parameters in order to allow unlicensed (secondary) users to access and communicate over the frequency bands assigned to licensed (primary) users when they are inactive. Therefore, cognitive radio requires reliable spectrum sensing techniques in order to avoid interference to primary users. In this thesis, the spectrum sensing problem in cognitive radio is studied. Specifically, the restricted Neyman-Pearson (NP) approach, which maximizes the average detection probability under the constraints on the minimum detection and false alarm probabilities, is applied to the spectrum sensing problem in cognitive radio systems in the presence of uncertainty in the prior probability distribution of primary users’ signals. First, we study this problem in the presence of Gaussian noise and assume that primary users’ signals are Gaussian. Then, the problem is reconsidered for non-Gaussian noise channels. Simulation results are obtained in order to compare the performance of the restricted NP approach with the existing methods such as the generalized likelihood ratio test (GLRT) and energy detection. The restricted NP approach outperforms energy detection in all cases. It is also shown that the restricted NP approach can provide important advantages over the GLRT in terms of the worst-case detection probability, and sometimes in terms of the average detection probability depending on the situation in the presence of imperfect prior information for Gaussian mixture noise channels.Item Open Access Secrecy capacity results for a secure NOMA-based cognitive radio network with an external eavesdropper(Elsevier, 2020) Mehr, K. Adli; Niya, J. M.; Seyedarabi, H.; Nobar, S. K.In this paper, we investigate a secure cognitive radio network (CRN), which deploys non-orthogonal multiple access (NOMA) to deliver a mixed multicast and unicast traffic to the intended receivers, while keeping them secret from the eavesdroppers. This model represents a cognitive interference channel with an external eavesdropper (CIC-EE). In this model, there are one pair of primary nodes, one pair of secondary nodes, and an external eavesdropper. The primary transmitter multicasts a confidential message to both primary and secondary receivers, while trying to keep it secret from the eavesdropper. The secondary transmitter helps the primary user to deliver its message in exchange of transmission opportunity. The secondary message is unicasted to the secondary receiver, while concealing it from both primary receiver and the eavesdropper. This scenario models a NOMA-based overlay cognitive radio paradigm with an external eavesdropper. For this scenario, the achievable rate-equivocation region is obtained and its optimality is shown for a class of degraded channels. Then, the results obtained for the discrete memoryless channel are extended to the Gaussian channel model. Furthermore, by deploying numerical examples, a comparison is made between the proposed secure NOMA-based scheme, its orthogonal multiple access (OMA) based counterpart, and a cognitive interference channel without an external eavesdropper. It is shown that the NOMA-based method achieves significantly higher rates than its OMA based counterpart.