Cognitive radios are currently presented as the solution to the ever-increasing spectrum shortage problem. However, their increased capabilities over traditional radios introduce a new dimension of security threats. Cooperative Spectrum Sensing (CSS) has been proposed as a means to protect cognitive radio networks from the well known security threats: Primary User Emulation (PUE) and Spectrum Sensing Data Falsification (SSDF). I demonstrate a new threat to trust-based CSS protocols, called the Rogue Signal Framing (RSF) intrusion. Rogue signals can be exploited to create the illusion of malicious sensors which leads to the framing of innocent sensors and consequently, their removal from the shared spectrum sensing. Ultimately, with fewer sensors working together, the spectrum sensing is less robust for making correct spectrum access decisions. The simulation experiments illustrate the impact of RSF intrusions which, in severe cases, shows roughly 40\% of sensors removed. To mitigate the RSF intrusion's damage to the network's trust, I introduce a new defense based on community detection from analyzing the network's Received Signal Strength (RSS) diversity. Tests show a 95\% damage reduction in terms of removed sensors from the shared spectrum sensing, thus retaining the benefits of CSS protocols.

The rapid growth of the Internet of Things (IoT) introduces an additional challenge to the existing spectrum under-utilisation problem as large scale deployments of thousands devices are expected to require wireless connectivity. Dynamic Spectrum Access (DSA) has been proposed as a means of improving the spectrum utilisation of wireless systems. Based on the Cognitive Radio (CR) paradigm, DSA enables unlicensed spectrum users to sense their spectral environment and adapt their operational parameters to opportunistically access any temporally unoccupied bands without causing interference to the primary spectrum users. In the same context, CR inspired Machine-to-Machine (M2M) communications have recently been proposed as a potential solution to the spectrum utilisation problem, which has been driven by the ever increasing number of interconnected devices. M2M communications introduce new challenges for CR in terms of operational environments and design requirements. With spectrum sensing being the key function for CR, this thesis investigates the performance of spectrum sensing and proposes novel sensing approaches and models to address the sensing problem for cognitive M2M deployments. In this thesis, the behaviour of Energy Detection (ED) spectrum sensing for cognitive M2M nodes is modelled using the two-wave with dffi use power fading model. This channel model can describe a variety of realistic fading conditions including worse than Rayleigh scenarios that are expected to occur within the operational environments of cognitive M2M communication systems. The results suggest that ED based spectrum sensing fails to meet the sensing requirements over worse than Rayleigh conditions and consequently requires the signal-to-noise ratio (SNR) to be increased by up to 137%. However, by employing appropriate diversity and node cooperation techniques, the sensing performance can be improved by up to 11.5dB in terms of the required SNR. These results are particularly useful in analysing the eff ects of severe fading in cognitive M2M systems and thus they can be used to design effi cient CR transceivers and to quantify the trade-o s between detection performance and energy e fficiency. A novel predictive spectrum sensing scheme that exploits historical data of past sensing events to predict channel occupancy is proposed and analysed. This approach allows CR terminals to sense only the channels that are predicted to be unoccupied rather than the whole band of interest. Based on this approach, a spectrum occupancy predictor is developed and experimentally validated. The proposed scheme achieves a prediction accuracy of up to 93% which in turn can lead to up to 84% reduction of the spectrum sensing cost. Furthermore, a novel probabilistic model for describing the channel availability in both the vertical and horizontal polarisations is developed. The proposed model is validated based on a measurement campaign for operational scenarios where CR terminals may change their polarisation during their operation. A Gaussian approximation is used to model the empirical channel availability data with more than 95% confi dence bounds. The proposed model can be used as a means of improving spectrum sensing performance by using statistical knowledge on the primary users occupancy pattern.

Em 1997, o conceito de rádio cognitivo foi proposto pela primeira vez e evoluiu significativamente até os dias de hoje, como solução para o problema da escassez de espectro eletromagnético. Nessa proposta, usuários oportunistas, através de acesso dinâmico ao espectro, fazem uso das faixas de frequências atribuídas a usuários licenciados, quando eles não as estão utilizando. Para que isso seja possível, sem interferir ou degradar os sinais dos usuários licenciados, é necessário atender a quatro requisitos essenciais de rádios cognitivos: Sensoriamento espectral, Decisão do espectro, Compartilhamento do espectro e Mobilidade espectral. Neste trabalho, o sensoriamento espectral é investigado com base na detecção de energia. Um algoritmo é desenvolvido para se determinar o número de canais ocupados e o número de amostras necessárias na detecção para se atingir probabilidades de detecção e falso alarme pré-estabelecidas. Resultados de simulações são apresentadas mostrando que a incerteza do ruído degrada o desempenho do sistema quando a relação sinal-ruído é baixa. O algoritmo desenvolvido permite também determinar o limite inferior para a relação sinal-ruído, quando há incerteza do ruído. O comportamento da probabilidade de detecção em função da probabilidade de falso alarme parametrizado para número de amostras e relação sinal-ruído é apresentado. As curvas resultantes são muitas vezes referidas como curvas ROC - Receiver Operation Characteristics na literatura. Em função do grande interesse sócio-político pela banda de TV, que o cenário das telecomunicações atualmente apresenta, a mesma foi escolhida para alguns exemplos deste estudo. / In 1997, the concept of cognitive radio was proposed for the first time and evolved significantly to the present days, as a solution to the problem of electromagnetic spectrum scarcity. In the proposed approach, opportunistic users utilize frequency bands originally assigned to licensed users through dynamic spectrum access when the licensed users are not using them. To make this possible, without interfering or degrading the signals from the licensed users, it is necessary to fulfill four essential requirements of cognitive radios: spectrum sensing, spectrum decision, spectrum sharing, and spectrum mobility. In this work, spectrum sensing based on energy detection was investigated. An algorithm was developed for the determination of channel occupation and the number of samples needed for the detection process to achieve pre-established probabilities of detection and false-alarm. Simulations results are presented showing that noise uncertainty degrade the performance of the system when the signal-to-noise ratio is low. The developed algorithm allows determining a lower threshold for the signal-to-noise ratio, when noise uncertainty exists. The detection probability behavior as a function of the false alarm probability having the number of samples and the signal-to noise ratio as parameters is presented. The resulting curves are often denominated ROC - Receiver Operation Characteristics in the literature. Due to the high social and political interest in the TV broadcasting band, that telecommunications scenario currently presents, this band was chosen for same examples in this study.

Em 1997, o conceito de rádio cognitivo foi proposto pela primeira vez e evoluiu significativamente até os dias de hoje, como solução para o problema da escassez de espectro eletromagnético. Nessa proposta, usuários oportunistas, através de acesso dinâmico ao espectro, fazem uso das faixas de frequências atribuídas a usuários licenciados, quando eles não as estão utilizando. Para que isso seja possível, sem interferir ou degradar os sinais dos usuários licenciados, é necessário atender a quatro requisitos essenciais de rádios cognitivos: Sensoriamento espectral, Decisão do espectro, Compartilhamento do espectro e Mobilidade espectral. Neste trabalho, o sensoriamento espectral é investigado com base na detecção de energia. Um algoritmo é desenvolvido para se determinar o número de canais ocupados e o número de amostras necessárias na detecção para se atingir probabilidades de detecção e falso alarme pré-estabelecidas. Resultados de simulações são apresentadas mostrando que a incerteza do ruído degrada o desempenho do sistema quando a relação sinal-ruído é baixa. O algoritmo desenvolvido permite também determinar o limite inferior para a relação sinal-ruído, quando há incerteza do ruído. O comportamento da probabilidade de detecção em função da probabilidade de falso alarme parametrizado para número de amostras e relação sinal-ruído é apresentado. As curvas resultantes são muitas vezes referidas como curvas ROC - Receiver Operation Characteristics na literatura. Em função do grande interesse sócio-político pela banda de TV, que o cenário das telecomunicações atualmente apresenta, a mesma foi escolhida para alguns exemplos deste estudo. / In 1997, the concept of cognitive radio was proposed for the first time and evolved significantly to the present days, as a solution to the problem of electromagnetic spectrum scarcity. In the proposed approach, opportunistic users utilize frequency bands originally assigned to licensed users through dynamic spectrum access when the licensed users are not using them. To make this possible, without interfering or degrading the signals from the licensed users, it is necessary to fulfill four essential requirements of cognitive radios: spectrum sensing, spectrum decision, spectrum sharing, and spectrum mobility. In this work, spectrum sensing based on energy detection was investigated. An algorithm was developed for the determination of channel occupation and the number of samples needed for the detection process to achieve pre-established probabilities of detection and false-alarm. Simulations results are presented showing that noise uncertainty degrade the performance of the system when the signal-to-noise ratio is low. The developed algorithm allows determining a lower threshold for the signal-to-noise ratio, when noise uncertainty exists. The detection probability behavior as a function of the false alarm probability having the number of samples and the signal-to noise ratio as parameters is presented. The resulting curves are often denominated ROC - Receiver Operation Characteristics in the literature. Due to the high social and political interest in the TV broadcasting band, that telecommunications scenario currently presents, this band was chosen for same examples in this study.

Thesis ( M.Sc. ( Computer Science)) -- University of Limpopo, 2018 / Dynamic spectrum access enabled by cognitive radio networks is envisioned to address the problems of the ever-increasing wireless technology. This innovative technology increases spectrum utility by allowing unlicensed devices to utilise the unused spectrum band of licenced devices opportunistically. The unlicensed devices referred to as secondary users (SUs) constantly sense the spectrum band to avoid interfering with the transmission of the licenced devices known as primary users (PUs).Due to some environmental challenges that can interfere with effective spectrum sensing, the SUs have to cooperate in sensing the spectrum band. However, cooperative spectrum sensing is susceptible to the spectrum sensing data falsification (SSDF) attack where selfish radios falsify the spectrum reports. Hence, there is a need to design a defence scheme that will defend the SSDF attack and guaranty correct final transmission decision. In this study, we proposed the integration of the reputation based system and the qout-of-m rule scheme to defend against the SSDF attack. The reputation-based system was used to determine the trustworthiness of the SUs. The q-out-of-m rule scheme where m sensing reports were selected from the ones with good reputation and q was the final decision, which was used to isolate the entire malicious nodes and make the correct final transmission decision. The proposed scheme was implemented in a Cognitive Radio Ad Hoc Network (CRAHN) where the services of a data fusion centre (FC) were not required. The SUs conducted their own data fusion and made their own final transmission decision based on their sensing reports and the sensing reports of their neighbouring nodes. Matlab was used to implement and simulate the proposed scheme. We compared our proposed scheme with the multifusion based distributed spectrum sensing and density based system schemes. Metrics used were the success probability, missed detection probability and false alarm probability. The proposed scheme performed better compared to the other schemes in all the metrics. / CSIR, NRF and, University of Limpopo research office

'It is not a lack of spectrum. It is an issue of efficient use of the available spectrum"--conclusions of the FCC Spectrum Policy Task Force. There is growing interest towards providing broadband communication with high bit rates and throughput, especially in the ISM band, as it was an ignition of innovation triggered by the FCC to provide, to some extent, a regulation-free band that anyone can use. But with such freedom comes the risk of interference and more responsibility to avoid causing it. Therefore, the need for accurate interference detection and identification, along with good blind detection capabilities are inevitable. Since cognitive radio is being adopted widely as more researchers consider it the ultimate solution for efficient spectrum sharing [1], it is reasonable to study the cognitive radio in the ISM band [2]. Many indications show that the ISM band will have less regulation in the future, and some even predict that the ISM may be completely regulation free [3]. In the dawn of cognitive radio, more knowledge about possible interfering signals should play a major role in determining optimal transmitter configurations. Since signal identification and interference will be the core concerns [4], [5], we will describe a novel approach for a cognitive radio spectrum sensing engine, which will be essential to design more efficient ISM band transceivers. In this thesis we propose a novel spectrum awareness engine to be integrated in the cognitive radios. Furthermore, the proposed engine is specialized for the ISM band, assuming that it can be one of the most challenging bands due to its free-to-use approach. It is shown that characterization of the interfering signals will help with overcoming their effects. This knowledge is invaluable to help choose the best configuration for the transceivers and will help to support the efforts of the coexistence attempts between wireless devices in such bands.

Las redes inal´ambricas actuales exhiben caracter´ısticas heterog´eneas de acceso m´ultiple mediante el despliegue, la coexistencia y la cooperaci´on de varias Tecnolog ´ıas de Acceso Radio (RAT2). En este escenario, la prestaci´on de servicios multimedia garantizando una cierta calidad de servicio (QoS3) es obligatoria. El objetivo global de las redes heterog´eneas de acceso inal´ambrico consiste en sustentar la realizaci´on del concepto ABC (del ingl´es Always Best Connected), en el que un usuario est´a siempre conectado a la RAT que mejor satisface sus necesidades de servicio en cualquier momento, en cualquier lugar, de cualquier modo. En este sentido, las estrat´egias de gesti´on de recursos radio comunes [del ingl´es, Common Radio Resource Management (CRRM)] se dise˜nan para proporcionar una utilizaci´on eficiente de los recursos radio y de espectro radioel´ectrico dentro de la red heterog´enea, ofreciendo un mejor rendimiento en comparaci´on con la realizaci´on independiente de RRM en cada RAT. Adem´as, los recursos de espectro asignados a cada una de las RATs deben ser utilizado de manera eficiente, ya que se trata de un recurso escaso y costoso. En este sentido, conceptos y metodolog´ıas de radio cognitiva (del ingl´es Cognitive Radio o CR) se han aplicado a la gesti´on del espectro, permitiendo una compartici´on dinamico-oportunista del mismo. En estos casos, el espectro sujeto a licencia se abre hacia el acceso de usuarios sin licencia siempre que no perjudiquen y que el funcionamiento libre de interferencias est´e garantizado. Esta tesis analiza estrategias de gesti´on de recursos radio y de espectro para ofrecer un uso mayor y eficiente de los escasos recursos radio y de espectro con el objetivo final de aumentar al m´aximo la capacidad de usuario, garantizando los requerimientos de QoS. En concreto, estas tesis se centra primero en como seleccionar una RAT al inicio de una llamada/sesi´on (en adelante, selecci´on inicial de RAT) en una red de acceso heterog´enea. Un modelo de Markov ha sido desarrollado para definir la asignaci´on de m´ultiples servicios (multi-servicio) en m´ultiples RATs (multi-acceso). En este marco, varias pol´ıticas de selecci´on de RAT son propuestas y evaluadas, gen´ericamente clasific´andose en pol´ıticas basadas en servicio (SB4) y basadas en balanceo de carga (LB5). Adem´as, el rendimiento de las pol´ıticas de selecci´on de RATs en escenarios de acceso limitado debido a la deficiente cobertura radio, la falta de disponibilidad de terminales multi-modo y la incompatibilidad entre RAT y servicios tambi´en es evaluada. Principios espec´ıficos para la asignaci´on de servicios a RATs ser´an provistos en los escenarios antes mencionados con el objetivo general de aumentar la capacidad de usuarios, garantizando los requisitos m´ınimos de calidad de servicio. Finalmente, la congesti´on en el acceso radio tambi´en se trata en este escenario multi-acceso/multi-servicio y el impacto de la selecci´on de RAT evaluado. Los principios para la asignaci´on inicial de RAT con tal de evitar la congesti´on radio ser´an tambi´en proporcionados. En segundo lugar, esta tesis investiga sobre la forma de maximizar el uso eficiente del espectro sujeto a licencia (o licenciado) por medio del acceso din´amicooportunista de espectro a usuarios sin licencia. En este sentido, se concibe un modelo de Markov para captar el problema del uso compartido de espectro entre usuarios con y sin licencia. Un modelo basado en sensado de espectro se propone con el fin de detectar porciones de espectro no utilizados (en ingl´es white spaces) que pueden ser usados por los usuarios sin licencia mientras este siga libre. En este marco, los beneficios obtenidos de la compartici´on del espectro son investigados y las ventajas que implican evaluadas. En concreto, se eval´ua el rendimiento obtenido al ajustar el punto de funcionamiento (en ingl´es operating point ) del mecanismo de sensado, el cual determina los errores de no-detecci´on y falsa-alarma. Por otra parte, sistemas de canalizaci´on de espectro fijos versus adaptativos ser´an propuestos y analizados bajo dos disciplinas de servicio diferentes, cuya duraci´on (o tiempo de permanencia en el sistema) esta basada en tiempo y en contenido respectivamente. / Current wireless networks exhibit heterogeneous multi-access features by means of the coexisting and cooperative deployment of several Radio Access Technologies (RATs). In this scenario, the provision of multimedia services with ensured Quality of Service (QoS) is mandatory. The overall goal of heterogeneous wireless access networks is to enable the realization of the Always Best Connected concept in which a user is seamlessly connected to the RAT best suiting its service requirements anytime, anywhere, anyhow. In this sense, Common Radio Resource Management (CRRM) strategies are devoted to provide an efficient utilization of radio resources within the heterogeneous network offering improved performances as opposed to performing stand-alone RRM in each RAT. In addition, allocated spectrum resources to each RAT must be efficiently utilized since it is a scarce and expensive resource. In this respect, cognitive radio concepts and methodologies have been applied to spectrum management by enabling dynamic/opportunistic spectrum sharing. In these scenarios, licensed spectrum is opened towards unlicensed access provided a non-harmful operation is guaranteed. This dissertation discusses both radio resource and spectrum management strategies to provide an utmost and efficient use of scarce radio/spectrum resources with the overall goal of maximizing user capacity while guaranteeing QoS constraints.Specifically, the thesis is first focused on how to select an appropriate RAT upon call/session initiation (henceforth, initial RAT selection) in a heterogeneous access network. A Markovian framework is developed to such extent supporting the allocation of multiple service-type users (multi-service) on multiple RATs (multi-access). Under this framework, several RAT selection policies are proposed and evaluated, broadly categorized into service-based (SB) and load-balancing (LB). In addition, the performance of RAT selection policies in access-limited scenarios due to poor radio coverage, non multi-mode terminal availability and RAT-service incompatibility is also evaluated. Specific guiding principles for the allocation of services on several RATs are provided in the abovementioned scenarios with the overall goal of increasing user capacity while guaranteeing minimum QoS requirements. Finally, radio access congestion is also addressed in this multi-access/multi-service scenario and the impact RAT selection assessed. Suitable allocation principles avoiding congestion are also provided.Secondly, this dissertation investigates on how to efficiently maximize the use of licensed spectrum by means of dynamic/opportunistic unlicensed spectrum access. Hereof, a Markovian framework is also devised to capture the problem of licensed spectrum sharing towards unlicensed users. A sensing-based spectrum awareness model is proposed in order to detect unused spectrum (so-called white spaces) which may be accessed by unlicensed users while remaining unused. Under this framework, the benefits of spectrum sharing are investigated and the involved gains assessed. Specifically, the sensing-throughput tradeoff and the adjustment of the sensing mechanism’s operating point, which tradeoffs missed-detection and false-alarm errors, is evaluated. Moreover, fixed vs. adaptive spectrum channelization schemes are proposed and analyzed under two different service disciplines considering time-based and volume-based content delivery.

The objective of the research is to develop analog spectrum processing techniques for cognitive radio (CR) applications in CMOS technology. CR systems aim to use the unoccupied spectrum allocations without any license when the primary users are not present. Therefore, the successful deployment of CR systems relies on their ability to accurately sense the spectrum usage status over a wide frequency range serving various wireless communication standards. Meanwhile, to maximize the utilization of the available spectrum segments, the bandwidth of the signal has to be highly flexible, so that even a small fraction of spectrum resources can be fully utilized by CR users. One of the key enabling technologies of variable bandwidth communication is a tunable baseband filter. In this research, a reconfigurable CR testbed system is presented as groundwork for the researches related with CR systems. With the feasibility study on the multi-resolution spectrum sensing (MRSS) functionality, a method for determining sensing threshold for MRSS functionality is presented, and a fully integrated MRSS receiver in CMOS technology is demonstrated. On the other hand, a reconfigurable CMOS analog baseband filter which can change its bandwidth, type and order with high resolution for CR applications is presented. In sum, an analog spectrum sensing method as well as a highly flexible analog baseband filter architecture is established and implemented in CMOS technology. Both designs are targeting the utilization of the analog signal processing capability with the aid of the digital circuits.

Cognitive radio (CR) technology has recently been introduced to opportunistically exploit the spectrum. We present a robust and cost-effective design to ensure the improvement of spectrum efficiency with CR. We first propose probability-based spectrum sensing by utilizing the statistical characteristics of licensed channel occupancy, which achieves nearly optimal performance with relatively low complexity. Based on the statistical model, we then propose periodic spectrum sensing scheduling to determine the optimal inter-sensing duration and vary the transmit power at each data sample to enhance throughput and reduce interference. We further develop a probability-based scheme for combination of local sensing information collected from cooperative CR users, which enables combination of both synchronous and asynchronous sensing information. To satisfy the stringent bandwidth constraint for reporting, we also propose to simultaneously send local sensing data to a combining node through the same narrowband channel. With proper preprocessing at individual users, such a design maintains reasonable detection performance while the bandwidth required for reporting does not change with the number of cooperative users. To better utilize the spectrum and avoid possible interference, we propose spectrum shaping schemes based on spectral precoding, which enable efficient spectrum sharing between CR and licensed users and exhibit the advantages of both simplicity and flexibility. We also propose a novel resource allocation approach based on the probabilities of licensed channel availability obtained from spectrum sensing. Different from conventional approaches, the probabilistic approach exploits the flexibility of CR to ensure efficient spectrum usage and protect licensed users from unacceptable interference.

Wireless communications systems underwent an evolution as the voice oriented applications evolved to data and multimedia based services. Furthermore, current wireless technologies, regulations and the un- derstanding of the technology are insufficient for the requirements of future wireless systems. Along with the rapid rise at the number of users, increasing demand for more communications capacity to deploy multimedia applications entail effective utilization of communications resources. Therefore, there is a need for effective spectrum allocation, adaptive and complex modulation, error recovery, channel estimation, diversity and code design techniques to allow high data rates while maintaining desired quality of service, and reconfigurable and flexible air interface technologies for better interference and fading management. However, traditional communications system design is based on allocating fixed amounts of resources to the user and does not consider adaptive spectrum utilization. Technologies which will lead to adaptive, intelligent, and aware wireless communications systems are expected to come up with consistent methodologies to provide solutions for the capacity, interference, and reliability problems of the wireless networks. Spectrum sensing feature of cognitive radio systems are a step forward to better recognize the problems and to achieve efficient spectrum allocation. On the other hand, even though spectrum sensing can constitute a solid base to achieve the reconfigurability and awareness goals of next generation networks, a new perspective is required to benefit from the whole dimensions of the available electro hyperspace. Therefore, spectrum sensing should evolve to a more general and comprehensive awareness providing a mechanism, not only as a part of CR systems which provide channel occupancy information but also as a communication environment awareness component of dynamic spectrum access paradigm which can adapt sensing parameters autonomously to ensure robust identification and parameter estimation for the signals over the monitored spectrum. Such an approach will lead to recognition of communications opportunities in different dimensions of spectrum hyperspace, and provide necessary information about the air interfaces, access techniques and waveforms that are deployed over the monitored spectrum to accomplish adaptive resource management and spectrum access. We define multidimensional signal analysis as a methodology, which not only provides the information that the spectrum hyperspace dimension in interest is occupied or not, but also reveals the underlaying information regarding to the parameters, such as employed channel access methods, duplexing techniques and other parameters related to the air interfaces of the signals accessing to the monitored channels and more. To achieve multidimensional signal analysis, a comprehensive sensing, classification, and a detection approach is required at the initial stage. In this thesis, we propose the multidimensional signal analysis procedures under signal identification algorithms in time, frequency. Moreover, an angle of arrival estimation system for wireless signals, and a spectrum usage modeling and prediction method are proposed as multidimensional signal analysis functionalities.