Exceptional Point of Degeneracy in Backward-Wave Oscillator with Distributed Power Extraction

We show how an exceptional point of degeneracy (EPD) is formed in a system composed of an electron beam interacting with an electromagnetic mode guided in a slow wave structure (SWS) with distributed power extraction from the interaction zone. Based on this kind of EPD, a new regime of operation is devised for backward wave oscillators (BWOs) as a synchronous and degenerate regime between a backward electromagnetic mode and the charge wave modulating the electron beam. Degenerate synchronization under this EPD condition means that two complex modes of the interactive system do not share just the wavenumber, but they rather coalesce in both their wavenumbers and eigenvectors (polarization states). In principle this new condition guarantees full synchronization between the electromagnetic wave and the beam's charge wave for any amount of output power extracted from the beam, setting the threshold of this EPD-BWO to any arbitrary, desired, value. Indeed, we show that the presence of distributed radiation in the SWS results in having high-threshold electron-beam current to start oscillations which implies higher power generation. These findings have the potential to lead to highly efficient BWOs with very high output power and excellent spectral purity

Exceptional Points of Degeneracy Induced by Linear Time-Periodic Variation

We present a general theory of exceptional points of degeneracy (EPD) in periodically time-variant systems. We show that even a single resonator with a time-periodic component is able to develop EPDs, contrary to parity-time- (PT) symmetric systems that require two coupled resonators. An EPD is a special point in a system parameter space at which two or more eigenmodes coalesce in both their eigenvalues and eigenvectors into a single degenerate eigenmode. We demonstrate the conditions for EPDs to exist when they are directly induced by time-periodic variation of a system without loss and gain elements. We also show that a single resonator system with zero time-average loss-gain exhibits EPDs with purely real resonance frequencies, yet the resonator energy grows algebraically in time since energy is injected into the system from the time-variation mechanism. Although the introduced concept and formalism are general for any time-periodic system, here, we focus on the occurrence of EPDs in a single LC resonator with time-periodic modulation. These findings have significant importance in various electromagnetic and photonic systems and pave the way for many applications, such as sensors, amplifiers, and modulators. We show a potential application of this time-varying EPD as a highly sensitive sensor

Evaluating the Assessment Process in the EFL Teaching Programs and the General Secondary Education Certificate English Exams for 1989-1993

The focal point in this paper is the evaluation of the current assessment process in our EFL teaching program in the general secondary school certificate education and the general secondary school certificate English exams (G.S.E.C.E.E.) being the only tool used in this respect. The purpose of such evaluation is to decides its effectiveness as an integrated part in the EFL teaching process since effective assessment makes effective teaching. The hypotheses tested in this study have been: a. The assessment process in our EFL program is not effective enough and it has harmful effects on the the EFL teaching process., b. The G.S.E.C.E.E. are note adequate enough as assessment tools. Yet, it is important to note that the present study is not intended to exclude the G.S.E.C.E.E. from the assessment process but to shed light on the other additional tools that can be used in this respect. In chapter one, the rationale of the study is presented. It includes the statement of the problem and its significance. It also covers the objectives of the present study and its hypotheses along with the definition of terms. The development of the assessment process in ESL/EFL programs all over the world and the current issues in language assessment have been topped in this chapter. In chapter two, the literature related to the assessment process in language teaching has been reviewed and classified. It has been classified into the following three categories: 1. Literature related to the ESL/EFL assessment in general; 2. Literature related to the current trends in this respect; and 3. Literature related to the portfolio assessment approach that is claimed to be the right alternative. In chapter 3 the methodology of the study has been presented and the development of the present study has been stated. The current study has developed according to the following stages: a. setting the criteria that can be used for the evaluation purposes of the present study; b. The criteria found effective for the purpose of the present study were subcategorized into: a. Criteria for language assessment in general; and b. Criteria for language test in particular. It is important to refer to the point that the criteria set have been based on the framework presented by L.J. Harp (1991) and others in language assessment. For more effective criteria, two questionnaires based on these two categories of criteria, were subjected to some ESL/EFL and evaluation professors in the States. In chapter 4, the current assessment process has been analyzed and evaluated according to the criteria set in chapter 3. The current assessment process and the G.S.E.C.E.E. have proved ineffective since they do not meet the criteria of effective assessment process or language tests. In chapter 5, the conclusions have been made along with the findings of the study. The portfolio, the alternative assessment approach has been presented in this chapter. The guidelines for planning a portfolio, its requirements have been shown in chapter 5. Also, recommendations, along with the suggsted proposals are included in chapter 5. The study appendices and bibliography have been included at the end of the present study. The findings of the current study can be stated in brief as follows: a. The present assessment process has not been effective enough to do its functions in the EFL teaching programs and it has harmful effects on the EFL teaching process. b. The G.S.E.C.E. Exams lack the criteria of the effective language exams and tests. They may be reliable but their validity and efficiency are inadequate. The assessment process needs a reform that can be the key to the educational reform we are looking forward to

Securing routing protocols in mobile ad hoc networks

A Mobile Ad Hoc Network (MANET) is more prone to security threats than other wired and wireless networks because of the distributed nature of the network. Conventional MANET routing protocols assume that all nodes cooperate without maliciously disrupting the operation of the protocol and do not provide defence against attackers. Blackhole and flooding attacks have a dramatic negative impact while grayhole and selfish attacks have a little negative impact on the performance of MANET routing protocols. Malicious nodes or misbehaviour actions detection in the network is an important task to maintain the proper routing protocol operation. Current solutions cannot guarantee the true classification of nodes because the cooperative nature of the MANETs which leads to false exclusions of innocent nodes and/or good classification of malicious nodes. The thesis introduces a new concept of Self- Protocol Trustiness (SPT) to discover malicious nodes with a very high trustiness ratio of a node classification. Designing and implementing new mechanisms that can resist flooding and blackhole attacks which have high negative impacts on the performance of these reactive protocols is the main objective of the thesis. The design of these mechanisms is based on SPT concept to ensure the high trustiness ratio of node classification. In addition, they neither incorporate the use of cryptographic algorithms nor depend on routing packet formats which make these solutions robust and reliable, and simplify their implementations in different MANET reactive protocols. Anti-Flooding (AF) mechanism is designed to resist flooding attacks which relies on locally applied timers and thresholds to classify nodes as malicious. Although AF mechanism succeeded in discovering malicious nodes within a small time, it has a number of thresholds that enable attacker to subvert the algorithm and cannot guarantee that the excluded nodes are genuine malicious nodes which was the motivation to develop this algorithm. On the other hand, Flooding Attack Resisting Mechanism (FARM) is designed to close the security gaps and overcome the drawbacks of AF mechanism. It succeeded in detecting and excluding more than 80% of flooding nodes within the simulation time with a very high trustiness ratio. Anti-Blackhole (AB) mechanism is designed to resist blackhole attacks and relies on a single threshold. The algorithm guarantees 100% exclusion of blackhole nodes and does not exclude any innocent node that may forward a reply packet. Although AB mechanism succeeded in discovering malicious nodes within a small time, the only suggested threshold enables an attacker to subvert the algorithm which was the motivation to develop it. On the other hand, Blackhole Resisting Mechanism (BRM) has the main advantages of AB mechanism while it is designed to close the security gaps and overcome the drawbacks of AB mechanism. It succeeded in detecting and excluding the vast majority of blackhole nodes within the simulation time

Security and Prioritization in Multiple Access Relay Networks

In this work, we considered a multiple access relay network and investigated the following three problems: 1- Tradeoff between reliability and security under falsified data injection attacks; 2-Prioritized analog relaying; 3- mitigation of Forwarding Misbehaviors in Multiple access relay network. In the first problem, we consider a multiple access relay network where multiple sources send independent data to a single destination through multiple relays which may inject a falsified data into the network. To detect the malicious relays and discard (erase) data from them, tracing bits are embedded in the information data at each source node. Parity bits may be also added to correct the errors caused by fading and noise. When the total amount of redundancy, tracing bits plus parity bits, is fixed, an increase in parity bits to increase the reliability requires a decrease in tracing bits which leads to a less accurate detection of malicious behavior of relays, and vice versa. We investigate the tradeoff between the tracing bits and the parity bits in minimizing the probability of decoding error and maximizing the throughput in multi-source, multi-relay networks under falsified data injection attacks. The energy and throughput gains provided by the optimal allocation of redundancy and the tradeoff between reliability and security are analyzed. In the second problem, we consider a multiple access relay network where multiple sources send independent data simultaneously to a common destination through multiple relay nodes. We present three prioritized analog cooperative relaying schemes that provide different class of service (CoS) to different sources while being relayed at the same time in the same frequency band. The three schemes take the channel variations into account in determining the relay encoding (combining) rule, but differ in terms of whether or how relays cooperate. Simulation results on the symbol error probability and outage probability are provided to show the effectiveness of the proposed schemes. In the third problem, we propose a physical layer approach to detect the relay node that injects false data or adds channel errors into the network encoder in multiple access relay networks. The misbehaving relay is detected by using the maximum a posteriori (MAP) detection rule which is optimal in the sense of minimizing the probability of incorrect decision (false alarm and miss detection). The proposed scheme does not require sending extra bits at the source, such as hash function or message authentication check bits, and hence there is no transmission overhead. The side information regarding the presence of forwarding misbehavior is exploited at the decoder to enhance the reliability of decoding. We derive the probability of false alarm and miss detection and the probability of bit error, taking into account the lossy nature of wireless links

L-based numerical linked cluster expansion for square lattice models

We introduce a numerical linked cluster expansion for square-lattice models whose building block is an L-shape cluster. This expansion exhibits a similar or better convergence of the bare sums than that of the (larger) square-shaped clusters, and can be used with resummation techniques (like the site- and bond-based expansions) to obtain results at even lower temperatures. We compare the performance of weak- and strong-embedding versions of this expansion in spin-1/2 models, and show that the strong-embedding version is preferable because of its convergence properties and lower computational cost. Finally, we show that the expansion based on the L-shape cluster can be naturally used to study properties of lattice models that smoothly connect the square and triangular lattice geometries.Comment: 13 pages, 16 figures, pre-print versio