The rapid rise in computational performance offered by computer systems has greatly increased the number of practical software defined radio applications. The addition of FPGAs to these flexible systems has resulted in platforms that can address a multitude of applications with performance levels that were once only known to ASICs. This work presents an embedded heterogeneous scalable cluster platform with software defined radio applications. The Xilinx Zynq chip provides a hybrid platform consisting of an embedded ARM general-purpose processing core and a low-power FPGA. The ARM core provides all of the benefits and ease of use common to modern high-level software languages while the FPGA segment offers high performance for computationally intensive components of the application. Four of these chips were combined in a scalable cluster and a task assigner was written to automatically place data flows across the FPGAs and ARM cores. The rapid reconfiguration software tFlow was used to dynamically build arbitrary FPGA images out of a library of pre-built modules. / Master of Science

This research addresses the performance degradation in receivers due to poor selectivity. Poor selectivity is expected to be a primary limitation on the performance of Dynamic-Spectrum-Access (DSA) and millimeter wave (mmWave) technologies. Both DSA and mmWave are highly desired technologies because they can address the spectrum-deficit problem that is currently challenging the wireless industry. Accordingly, addressing poor receiver selectivity is necessary to expedite the adoption of these technologies into the main street of wireless. This research develops two receiver design concepts to enhance the performance of poorly-selective receivers. The first concept is called cognitive RF front-end control (CogRF). CogRF operates by cognitively controlling the local-oscillator and sampling frequencies in receivers. This research shows that CogRF can fulfil the objective of pre-selectors by minimizing the effects of weak and moderately-powered neighboring-channel signals on the desired signal. This research shows that CogRF can be an alternative to high-performance pre-selectors, and hence, CogRF is a viable architecture to implement reliable DSA and mmWave receivers. The theoretical design and hardware implementation of a cognitive engine and a spectrum sensor of CogRF are reported in this dissertation. Measurement results show that CogRF significantly reduces the rate of communication outage due to interference from neighboring-channel signals in poorly-selective receivers. The results also indicate that CogRF can enable a poorly-selective receiver to behave like a highly-selective receiver. The second receiver design concept addresses very strong neighboring-channel signals. The performance of poorly selective receivers can easily suffer due to a strong, unfiltered neighboring-channel signal. A strong neighboring-channel signal is likely for a DSA radio that is operating in military radar bands. Traditionally, strong neighboring signals are addressed using an Automatic-Gain-Control (AGC) that attempt to accommodate the strong received signal into the dynamic range of the receiver. However, this technique potentially desensitizes the receiver because it sacrifices the Signal-to-Noise-Ratio (SNR) of the desired signal. This research proposes the use of auxiliary-receive path to address strong neighboring-channel signals with minimal penalty on the SNR of the desired signal. Through simulation based analysis, and hardware-based measurement, this research shows that the proposed technique can provide significant improvement in the neighboring-channel-interference handling capability of the receiver. / Ph. D.

With the advent of ubiquitous computing and Internet of Things (IoT), potentially billions of devices will create a broad range of data services and applications, which will require the communication networks to efficiently manage the increasing complexity. Cognitive network has been envisioned as a new paradigm to address this challenge, which has the capability of reasoning, planning and learning by incorporating cutting edge technologies including knowledge representation, context awareness, network optimization and machine learning. Cognitive network spans over the entire communication system including the core network and wireless links across the entire protocol stack. Cognitive Radio Network (CRN) is a part of cognitive network over wireless links, which endeavors to better utilize the spectrum resources. Core network provides a reliable backend infrastructure to the entire communication system. However, the CR communication and core network infrastructure have attracted various security threats, which become increasingly severe in pace with the growing complexity and adversity of the modern Internet. The focus of this dissertation is to exploit the security vulnerabilities of the state-of-the-art cognitive communication systems, and to provide detection, mitigation and protection mechanisms to allow security enhanced cognitive communications including wireless communications in CRNs and wired communications in core networks. In order to provide secure and reliable communications in CRNs: emph{first}, we incorporate security mechanisms into fundamental CRN functions, such as secure spectrum sensing techniques that will ensure trustworthy reporting of spectrum reading. emph{Second}, as no security mechanism can completely prevent all potential threats from entering CRNs, we design a systematic passive monitoring framework, emph{SpecMonitor}, based on unsupervised machine learning methods to strategically monitor the network traffic and operations in order to detect abnormal and malicious behaviors. emph{Third}, highly capable cognitive radios allow more sophisticated reactive jamming attack, which imposes a serious threat to CR communications. By exploiting MIMO interference cancellation techniques, we propose jamming resilient CR communication mechanisms to survive in the presence of reactive jammers. Finally, we focus on protecting the core network from botnet threats by applying cognitive technologies to detect network-wide Peer-to-Peer (P2P) botnets, which leads to the design of a data-driven botnet detection system, called emph{PeerClean}. In all the four research thrusts, we present thorough security analysis, extensive simulations and testbed evaluations based on real-world implementations. Our results demonstrate that the proposed defense mechanisms can effectively and efficiently counteract sophisticated yet powerful attacks. / Ph. D.

The cognitive radio network (CRN) concept has been proposed as a solution to the growing demand and underutilization of the radio spectrum. To improve the radio spectrum utilization, CRN technology allows the coexistence of licensed and unlicensed systems over the same spectrum. In an underlay spectrum sharing system, secondary users (SUs) transmit simultaneously with the primary users (PUs) in the same frequency band given that the interference caused by the SU to the PU remains below a tolerable interference limit. Besides the transmission power limitation, a secondary network is subject to distinct channel impairments such as fading and interference from the primary transmissions. Also, CRNs face new security threats and challenges due to their unique cognitive characteristics.This thesis analyzes the performance of underlay CRNs and underlay cognitive relay networks under spectrum sharing constraints and security constraints. Distinct SU transmit power policies are obtained considering various interference constraints such as PU outage constraint or PU peak interference power constraint. The thesis is divided into an introduction and two research parts based on peer-reviewed publications. The introduction provides an overview of radio spectrum management, basic concepts of CRNs, and physical layer security. In the first research part, we study the performance of underlay CRNs with emphasis on a multiuser environment.In Part I-A, we consider a secondary network with delay-tolerant applications and analyze the ergodic capacity. Part I-B analyzes the secondary outage capacity which characterises the maximum data rate that can be achieved over a channel for a given outage probability. In Part I-C, we consider a secondary network with delay constrained applications, and derive expressions of the outage probability and delay-limited throughput. Part I-D presents a queueing model that provides an analytical tool to evaluate the secondary packet-level performance with multiple classes of traffic considering general interarrival and service time distributions. Analytical expressions of the SU average packet transmission time, waiting time in the queue, andtime spent in the system are provided.In the second research part, we analyze the physical layer security for underlay CRNs and underlay cognitive relay networks. Analytical expressions of the probability of non-zero secrecy capacity and secrecy outage probability are derived.Part II-A considers a single hop underlay CRN in the presence of multiple eavesdroppers (EAVs) and multiple SU-Rxs. In Part II-B, an underlay cognitive relay network in the presence of multiple secondary relays and multiple EAVs is studied.Numerical examples illustrate that it is possible to exploit the physical layer characteristics to achieve both security and quality of service in CRNs while satisfying spectrum sharing constraints.

Recently, wireless access has become an essential part of modern society. Consequently, the demand for new wireless applications and services, as well as the number of wireless users, are gradually increasing. Given that this amount of expansion is eventually controlled by the available radio frequency spectrum, government regulatory agencies have recently adopted a strict approach to the licensing of limited amounts of spectrum to different entities (e.g., public safety, military, service providers, unlicensed devices, and TV). All of them possess exclusive transmissions to their assigned frequency channels. A new study on spectrum efficiency revealed big geographic and temporal variations in spectrum utilisation, ranging from 15-85% in the bands below 3GHz. These variations were less at frequencies above this figure. Recently, the Cognitive Radio (CR) has risen as an encouraging piece of technology to improve spectrum efficiency and to solve the problem of spectrum scarcity. This is because CR allows the secondary (unlicensed) users to occupy unused licensed spectrum bands temporarily, given that the interference of the primary (licensed) users is prohibited or minimised. In this thesis, various spectrum handoff management schemes have been proposed in order to improve the performance evaluation for CR networks. The proposed spectrum handoff schemes use the Opportunistic Spectrum Access (OSA) concept to utilise available spectrum bands. The handoff Secondary Users (SUs) have a higher priority to occupy available spectrum channels in the licensed and unlicensed spectrum bands without interfering with the legacy spectrum owner, i.e. primary users (PUs). However, existing spectrum handoff management schemes in CR networks do not provide high transmission opportunities for handoff secondary users to utilise the available radio spectrum resources. The first part of this thesis addresses the issue of spectrum handoff management in a licensed spectrum band environment. In this case, both reactive and proactive spectrum handoff schemes are proposed. Queuing theory or/and simulation experiments have been used to evaluate the performance of the proposed schemes and compare them with other existing schemes. Handoff delay has mainly been used to investigate the impact of successive handoff operations on the performance of the proposed CR networks. Implemented models have shown an improvement in the adopted performance measures. According to the achieved results, the improvement of the proposed, prioritised handoff schemes in some cases is approximately 75% when compared with existing schemes. On the other hand, the second part of this research proposed a prioritised spectrum handoff scheme in a heterogeneous spectrum environment, which is composed of a pool of licensed and unlicensed spectrum channels. In general, the availability of substantial numbers of the licensed spectrum channels is the key benefit of using this type of radio spectrum channel. Whereas, accessing with equal rights for all types of users is the main advantage of using unlicensed spectrum channels. In this respect, no transmission interruptions occur once a user obtains a channel. In addition, the proposed schemes use only the unlicensed spectrum channels as their backup channels. This enables the user to resume interrupted transmission in the case of the spectrum handoff operation (mainly; due to the appearance of the primary users), and thus facilitates a SUs communication. The proposed principle is investigated using a retrial queuing theory as well as extensive simulation experiments, and is compared with another non-prioritised scheme which do not give any preference to handoff SUs over new SUs. The results indicate that the proposed model has improved on current average handoff delay. This thesis contributes to knowledge by further enhancing the efficient utilisation of available radio spectrum resources and therefore subsequently provides an improvement in the spectrum capacity for wireless cognitive radio networks.

A research report submitted to the Faculty of Humanities, University of the Witwatersrand in fulfilment of the requirements for the degree of Masters of Arts by Research (Development Studies), August, 2017 / In this master’s dissertation report, I explore how the National Union of Small-scale Farmers of Mozambique (UNAC) transnationalised its activism through its membership to La Via Campesina (LVC), covering the period between 2004 and 2014, to understand the nature of transnational activism of national peasant movement organisations. I identify the factors behind national peasant movement organisations embarking on transnational activism, the structure put in place to secure their transnational activism and the possible opportunities and threats transnational activism poses to their national mandate. I applied ethnography and more concretely Michael Burawoy’s approach to the extended case method to reconstruct (by extending) neo-Marxist approaches to the study of peasantry, peasant movements and social movements in general. Based on a qualitative approach to the data collected, the research conducted documentary analysis and in-depth interviews with relevant actors. Results suggest that peasants’ national movement organisations transnationalise part of their activism through joining an already transnational peasant’s movement organisation by adopting and campaigning for its political themes, building transnational peasant activists, and by practising transnational solidarity. Results also suggest that national peasant movement organisations transnationalise their activism precisely because they share a common interest of fighting capitalism with the movement organisation they affiliate with, they seek to expand their political messages and because they are looking for international solidarity. Added to this, results show that this type of transnational activism does not necessarily require specific administrative structures and does not pose major threats to the national mandate of national peasant movement organisations. More importantly, results suggest re-theorising the neo-Marxists’ approach to social movements by accommodating transnational agency of national peasant movement organisations. / XL2018

Recently, with the fast development of wireless communications, the radio spectrum becomes a precious natural resource. Many researches and reports reveal the problems of inefficient spectrum utilization. Cognitive Radio (CR) technology is now developing for solving this critical problem. This technology will enable various kinds of wireless systems to look for and connect radio frequency spectrum that the locality leave unused by oneself, to offer the best service to user. The CR will pass in and out the idle frequency band according to the demand while receiving and dispatching the signal, avoid the frequency band that has been already used. In CR network, the objective is to maximize the throughput of secondary users while limiting the probability of colliding with primary users below a prescribed level. In this paper, we consider a distributed secondary networks model where users seek spectrum opportunities independently that overlaying the primary networks to analyze the system performance and the effect to the primary users with the existence of both primary users and secondary users under the cognitive radio networks. In the cognitive system, due to the existence of noise and fading effect, error detection cannot be avoided. Therefore, we made a comparison to the difference of the efficiency among environments of different probability of miss detection. We also propose a random hopping method for all secondary users in system will re-sensing after a random period of time. Hereby, efficiently decreases the ratio of time that influences the primary users by the secondary users, and further research the factor that influences its efficiency.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Akyildiz, Ian; Committee Member: Ingram, Mary Ann; Committee Member: Blough, Douglas; Committee Member: Dovrolis, Konstantinos; Committee Member: Li, Ye. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Includes bibliographical references (p. 124-131).

Radio spectrum is becoming scarce resource due to increase in the usage of wireless communication devices. However studies have revealed that most of the allotted spectrum is not used effectively. Given the demand for more bandwidth and the amount of underutilized spectrum, DSA (Dynamic Spectrum Access) networks employing cognitive radios are a solution that can revolutionize the telecommunications industry, significantly changing the way we use spectrum resources, and design wireless systems and services. Cognitive radio has improve the spectral efficiency of licensed radio frequency bands by accessing unused part of the band opportunistically without interfering with a license primary user PU. In this thesis we investigate the effects on the quality of service (QoS) performance of spectrum management techniques for the connection-based channel usage behavior for Secondary user (SU). This study also consider other factors such as spectrum sensing time, spectrum handoff and generally distributed service time and channel contention for different SUs. The preemptive resume priority M/G/1 queuing theory is used to characterize the above mentioned effects. The proposed structure of the model can integrate various system parameters such spectrum sensing, spectrum decision, spectrum sharing and spectrum handoff. / Sajid Mahmood 0046-762788990 Mujeeb Abdullah 0046-760908069