Cognitive radio (CR) is a cutting-edge wireless communication technology that adopts several existing communication concepts in order to efficiently utilize the spectrum and meet the users demands of high throughput and real-time systems. Conventionally, high throughput demands are met through adopting broadband and multi-antenna technologies such as, orthogonal frequency division multiplexing (OFDM) and Multi-Input Multi-Output (MIMO). Whereas, real-time application demands are met by analyzing metrics which characterize the delay limited channels, such as, outage probability over block-fading channels. Being an environmental friendly technology, energy efficiency metrics should be considered in the design of a CR application. This thesis tackles the energy efficiency of CR system from different aspects, utilizing different measuring metrics and constrains. Under the single-input single-output (SISO) OFDM we minimized the energy per goodbit (EPG) metric subject to several power and Quality of Service (QoS) constraints. In this approach, the minimum EPG metric is optimized via proposing two optimal and sub-optimal resource allocation schemes. We consider several parameters as optimization variables, such as, power policy, sensing threshold, and channel quality threshold. We also captured the impact of involving the media access control (MAC) layers parameters, such as, frame length, in the minimization of a modified EPG metric. Also, a MAC protocol, i.e., hybrid automatic repeat request (HARQ), and the associated power consumption of the retransmission mechanism is considered in the formulation of the problem. In this context, the optimal power and frame length are derived to minimize the modified EPG while considering several spectrum-sharing scenarios, which depend on sensing information. In MIMO based CR system, we maximized capacity to power ratio (CPR) (as an energy efficiency (EE) metric) subject to several power and QoS constraints. In this context, the impact of sensing information with imperfect channel state information (CSI) of the secondary channel has been considered. To realize a CR system with real-time applications we minimized the outage probability over M block-fading channel with several long-term and short-term energy constrains. We derive the minimum outage region and the associated optimal power. Tractable expressions to lower and upper bound the outage probability are derived. We then analyze the impact of utilizing the sensing process of primary user activity.

Interruptions in power supply are inevitable due to faults in power system distribution network. These interruptions are not only expensive for the customers but also for the distribution system operator in the form of penalties. Increase in system redundancy or the use of component-specific sensors can help in reduction of interruptions. However, these options are not always economically feasible. Therefore, there is a need to check for other possibilities to reduce the risk of outages. The data stored in substations can be used for reducing the risk of outages by deriving component importance indices followed by ranking and predicting the outages. This thesis presents component importance indices derived by identifying the critical components in the grid and assigning index based on certain criterion. The model for predicting the faults is based on the weather conditions observed during the outages in the past. Component importance indices are derived and ranked based on the de-energisation time of components, frequency and impact of outages. This helps prioritize components according to the chosen criterion and adapt monitoring strategies by focusing on the most critical components. Based on categorical Naive Bayes, a model is developed to predict the probability of fault/failure, location and component type likely to be affected for a given set of weather conditions. The results from the component importance indices reveal that each component’s rank varies based on the chosen criterion. This indicates that certain components are critical with respect to specific criterion and not all criteria. However, some components are ranked high in all the methods. These components are critical and need focused monitoring. The reliability of results from component importance indices to a great extent depends on the time frame of the outage data considered for analysis. The prediction model can alert the distribution system operator regarding the possible outages in the network for a given set of weather conditions. However, the prediction of location and component type likely to be affected is relatively inaccurate, since the number of outages considered in the time frame is low. By updating the model regularly with new data, the predictions would be more accurate. / Avbrott i strömförsörjningen är oundvikliga på grund av fel i distributionsnätet för kraftsystemet. Dessa avbrott är inte bara dyra för kunderna utan också för distributionssystemoperatören i form av påföljder. Ökad systemredundans eller användning av komponentspecifika sensorer kan hjälpa till att minska avbrott. Dessa alternativ är dock inte alltid ekonomiskt genomförbara. Därför är det nödvändigt att kontrollera om det finns andra möjligheter för att minska risken för avbrott. Data lagrade i transformatorstationer kan användas för att minska risken för avbrott genom att härleda komponentviktindex följt av rangordning och förutsäga avbrott. I denna avhandling härleds viktighetsindex genom att identifiera de kritiska komponenterna i nätet och tilldela index baserat på vissa kriterier. Felprognoserna gjordes baserat på de väderförhållanden som observerades under avbrott. komponentviktighetsindex härleds och rankas baserat på komponenternas urladdningstid, frekvens och påverkan av avbrott. Detta hjälper till att prioritera komponenter enligt det valda kriteriet och anpassa övervakningsstrategier genom att fokusera på de mest kritiska komponenterna. Baserat på kategoriska Naive Bayes utvecklas en modell för att förutsäga sannolikheten för fel / fel, plats och komponenttyp som sannolikt kommer att påverkas under en viss uppsättning väderförhållanden. Resultaten från komponentviktighetsindexen visar att varje komponents rang varierar beroende på det valda kriteriet. Vissa komponenter rankas dock högt i alla metoder. Dessa komponenter är kritiska och behöver fokuserad övervakning. Tillförlitligheten hos resultat från komponentviktindex beror till stor del på tidsramen för avbrottsdata som beaktas för analys. Prognosmodellen kan varna distributionssystemoperatören om möjliga avbrott i nätverket för en viss uppsättning väderförhållanden. Förutsägelsen av plats och komponenttyp som sannolikt kommer att påverkas är dock relativt felaktig, eftersom antalet avbrott som beaktas i tidsramen är lågt. Genom att uppdatera modellen regelbundet med nya data skulle förutsägelserna vara mer exakta.

The reliability and data rate of wireless communication have traditionally been limited by the presence of multipath fading in wireless channels. However, dramatic performance improvements can be obtained by the use of multiple transmit and receive antennas. Specifically, multiple antennas increase reliability by providing diversity gain, namely greater immunity to deep channel fades. They also increase data rates by providing multiplexing gain, i.e., the ability to multiplex multiple symbols in one signaling interval. Harvesting the potential benefits of multiple antennas requires the use of specially designed space-time codes at the transmitter front-end. Space-time codes introduce redundancy in the transmitted signal across two dimensions, namely multiple transmit antennas and multiple signaling intervals. In this work, we focus on linear space-time codes, which linearly combine the real and imaginary parts of their complex inputs to obtain transmit vectors for multiple signaling intervals. We aim to design optimum linear space-time codes. Optimality metrics and design principles for space-time codes are shown to depend strongly on the codes' function in the overall transmitter architecture. We consider two cases, depending on whether or not the space-time code is complemented by a powerful outer error-control code. In the absence of an outer code, the multiplexing gain of a space-time code is measured by its rate, while its diversity gain is measured by its raw diversity order. To maximize multiplexing and diversity gains, the space-time code must have maximum possible rate and raw diversity order. We show that there is an infinite set of maximum-rate codes, almost all of which also have maximum raw diversity order. However, different codes in this set have different error rate for a given input alphabet and SNR. Therefore, we develop analytical and numerical optimization techniques to find the code in this set which has the minimum union bound on error rate. Simulation results indicate that optimized codes yield significantly lower error rates than unoptimized codes, at the same data rate and SNR. In a concatenated architecture, a powerful outer code introduces redundancy in the space-time code inputs, obtaining additional diversity. Thus, the raw diversity order of the space-time inner code is only a lower limit to the total diversity order of the concatenated transmitter. On the other hand, we show that the rate of the space-time code places an upper limit on the multiplexing ability of the concatenated architecture. We conclude that space-time inner codes should have maximum possible rate but need not have high raw diversity order. In particular, the serial-to-parallel converter, which introduces no redundancy at all, is a near-optimum space-time inner code. This claim is supported by simulation results. On the receiver side, we generalize the well known sphere decoder to develop new detection algorithms for stand-alone space-time codes. These new algorithms are extended to obtain efficient soft-output decoding algorithms for space-time inner codes.

The distribution system which forms the final connection between customers and power source plays a vital role in an electrical network. Different studies show that substantial proportion of the customer interruptions occurs due to the failures on distribution network. The ongoing privatization process of the electrical distribution services in Turkey raises the importance of reliable and continuous electricity supply significantly. The new regulations come up with this privatization process and the electrical distribution companies are strictly required to comply with these regulations to ensure the reliability of the distribution network. The legal framework and severe punishments applied to the electrical distribution companies exceeding the continuity of supply indices force them to invest on their network in order to increase the reliability of their system. As the reliability of electricity supplied increases, investment cost also increases. However, low system reliability causes higher outage frequency and duration which will increase the damage of these outages to customers and also increases the cost of the distribution company as a result of the penalty payments. This tradeoff between Outage Cost and Utility Cost requires consideration of an optimization when determining the optimal reliability level. In rural areas where electrical distribution network consists of long radial overhead lines in arborescent structure, continuity of supply is a major problem due to the high failure rates. The implementation of protection devices having reclosing capability and automated sectionalizing switches enhances the continuity of supply on rural networks substantially. The balance between the cost associated with installation of switches and the reduction on Outage Cost is an important optimization issue for distribution network operators. In this thesis study an algorithm is developed in order to determine the optimum number and locations of the sectionalizing switches on a rural electrical distribution network in Turkey which gives an optimum investment level with an optimum Outage Cost.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2017-03-08T12:29:03Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) JOAO VICTOR DE CARVALHO EVANGELISTA_DISSERTACAO_VERSAO_FINAL_2016.pdf: 4051425 bytes, checksum: c53a5039b8aa3054c77f2ee82a10849f (MD5) / Made available in DSpace on 2017-03-08T12:29:03Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) JOAO VICTOR DE CARVALHO EVANGELISTA_DISSERTACAO_VERSAO_FINAL_2016.pdf: 4051425 bytes, checksum: c53a5039b8aa3054c77f2ee82a10849f (MD5) Previous issue date: 2016-08-16 / Message authentication, which ensures that a received message comes from its acclaimed sender, is of fundamental importance for secure communication systems. We consider in this work a physical layer authentication system employing tag signals embedded in the message to provide a robust authentication method. This work diverges from previous work in the area when it comes to the tag generation method. While the previous works use methods based on cryptographic hash functions or on the channel side information our system employs unidimensional chaotic maps to generate these tags. Due to the loss of information about the initial condition of chaotic maps, we show that they are strong candidates for the tag generation process. We prove that chaotic tags provide a positive lower bound on the unconditional security of the system. Additionally, we calculate the probability of success for three possible attacks to the authentication system: impersonation, substitution and replay.Finally, we analyze how the system parameters affect these probabilities and some performance metrics (bit error rate, outage probability, probability of false negative) and explore the tradeoff between security and performance in order to provide guidelines to design the system. / A autenticação de mensagem, o que garante que uma mensagem recebida vem de seu aclamado remetente, é de fundamental importância para sistemas de comunicação seguros. Neste contexto, considera-se neste trabalho um sistema de autenticação em camada física empregando tags embutidos nas mensagens proporcionando um robusto método de autenticação. Este trabalho diverge de trabalhos anteriores na área no que se refere ao método de geração de tags. Enquanto os trabalhos anteriores utilizam métodos baseados em funções criptográficas de hash e na informação do estado do canal, nosso sistema emprega mapas caóticos unidimensionais para gerar os tags. Devido ao fato de que a informação sobre a condição inicial se perde ao longo de uma órbita caótica mostraremos que elas são fortes candidatas para o processo de geração de tags. Provamos que tags caóticos garantem um limitante inferior positivo na segurança incondicional do sistema. Adicionalmente, nós calculamos a probabilidade de sucesso de três tipos de ataque: de personificação, de substituição e de repetição. Para finalizar, analisamos como os parâmetros do sistema afetam essas probabilidades e algumas métricas de performance (taxa de erro por bit, probabilidade de interrupção e probabilidade de falso negativo) e os compromissos entre segurança e performance para prover um guia de projeto do sistema.

Yes / We study transmission over multiple-input multiple-output (MIMO) block-fading channels with imperfect channel state information (CSI) at both the transmitter and receiver. Specifically, based on mismatched decoding theory for a fixed channel realization, we investigate the largest achievable rates with independent and identically distributed inputs and a nearest neighbor decoder. We then study the corresponding information outage probability in the high signal-to-noise ratio (SNR) regime and analyze the interplay between estimation error variances at the transmitter and at the receiver to determine the optimal outage exponent, defined as the high-SNR slope of the outage probability plotted in a logarithmic-logarithmic scale against the SNR. We demonstrate that despite operating with imperfect CSI, power adaptation can offer substantial gains in terms of outage exponent. / A. T. Asyhari was supported in part by the Yousef Jameel Scholarship, University of Cambridge, Cambridge, U.K., and the National Science Council of Taiwan under grant NSC 102-2218-E-009-001. A. Guillén i Fàbregas was supported in part by the European Research Council under ERC grant agreement 259663 and the Spanish Ministry of Economy and Competitiveness under grant TEC2012-38800-C03-03.

This thesis presents new methodologies to improve performance of wireless cooperative networks using the Golden Code. As a form of space-time coding, the Golden Code can achieve diversity-multiplexing tradeoff and the data rate can be twice that of the Alamouti code. In practice, however, asynchronism between relay nodes may reduce performance and channel quality can be degraded from certain antennas. Firstly, a simple offset transmission scheme, which employs full interference cancellation (FIC) and orthogonal frequency division multiplexing (OFDM), is enhanced through the use of four relay nodes and receiver processing to mitigate asynchronism. Then, the potential reduction in diversity gain due to the dependent channel matrix elements in the distributed Golden Code transmission, and the rate penalty of multihop transmission, are mitigated by relay selection based on two-way transmission. The Golden Code is also implemented in an asynchronous one-way relay network over frequency flat and selective channels, and a simple approach to overcome asynchronism is proposed. In one-way communication with computationally efficient sphere decoding, the maximum of the channel parameter means is shown to achieve the best performance for the relay selection through bit error rate simulations. Secondly, to reduce the cost of hardware when multiple antennas are available in a cooperative network, multi-antenna selection is exploited. In this context, maximum-sum transmit antenna selection is proposed. End-to-end signal-to-noise ratio (SNR) is calculated and outage probability analysis is performed when the links are modelled as Rayleigh fading frequency flat channels. The numerical results support the analysis and for a MIMO system maximum-sum selection is shown to outperform maximum-minimum selection. Additionally, pairwise error probability (PEP) analysis is performed for maximum-sum transmit antenna selection with the Golden Code and the diversity order is obtained. Finally, with the assumption of fibre-connected multiple antennas with finite buffers, multiple-antenna selection is implemented on the basis of maximum-sum antenna selection. Frequency flat Rayleigh fading channels are assumed together with a decode and forward transmission scheme. Outage probability analysis is performed by exploiting the steady-state stationarity of a Markov Chain model.

Efficiently allocating the scarce and expensive radio resources is a key challenge for advanced radio communication systems. To this end, cognitive radio (CR) has emerged as a promising solution which can offer considerable improvements in spectrum utilization. Furthermore, cooperative communication is a concept proposed to obtain spatial diversity gains through relays without requiring multiple antennas. To benefit from both CR and cooperative communications, a combination of CR networks (CRNs) with cooperative relaying referred to as cognitive cooperative relay networks (CCRNs) has recently been proposed. CCRNs can better utilize the radio spectrum by allowing the secondary users (SUs) to opportunistically access spectrum, share spectrum with primary users (PUs), and provide performance gains offered by cooperative relaying. In this thesis, a performance analysis of underlay CRNs and CCRNs in different fading channels is provided based on analytical expressions, numerical results, and simulations. To allocate power in the CCRNs, power allocation policies are proposed which consider the peak transmit power limit of the SUs and the outage probability constraint of the primary network. Thus, the impact of multiuser diversity, peak transmit power, fading parameters, and modulation schemes on the performance of the CRNs and CCRNs can be analyzed. The thesis is divided into an introduction and five research parts based on peer-reviewed conference papers and journal articles. The introduction provides fundamental background on spectrum sharing systems, fading channels, and performance metrics. In the first part, a basic underlay CRN is analyzed where the outage probability and the ergodic capacity of the network over general fading channels is derived. In the second part, the outage probability and the ergodic capacity of an underlay CRN are assessed capturing the effect of multiuser diversity on the network subject to Nakagami-m fading. Considering the presence of a PU transmitter (PU-Tx), a power allocation policy is derived and utilized for CRN performance analysis under Rayleigh fading. In the third part, the impact of multiple PU-Txs and multiple PU receivers (PU-Rxs) on the outage probability of an underlay CCRN is studied. The outage constraint at the PU-Rx and the peak transmit power constraint of the SUs are taken into account to derive the power allocation policies for the SUs. In the fourth part, analytical expressions for the outage probability and symbol error probability for CCRNs are derived where signal combining schemes at the SU receiver (SU-Rx) are compared. Finally, the fifth part applies a sleep/wake-up strategy and the min(N; T) policy to an underlay CRN. The SUs of the network operate as wireless sensor nodes under Nakagami-m fading. A power consumption function of the CRN is derived. Further, the impact of M/G/1 queue and fading channel parameters on the power consumption is assessed.

Over time, wireless technology advancements in the field of communications have been attracting every individual to turn into a wireless user. To accommodate the increasing number of users and to avoid the problem of spectrum scarcity, the concept of Cognitive Radio Network (CRN) has been developed. Cognitive Radio (CR) is an intelligent radio which efficiently detects and allocates the spectrums of primary licensed users (PUs) to the secondary unlicensed users (SUs). The SUs can utilize these spectrums as long as they do not cause harmful interfere to the PUs. Interference may occur because of the following reasons: misdetection of spectrum availability, the high transmission power of SU when both SU and PU are present in the same channel at the same time. In order to avoid interference, the radio has to have a very accurate spectrum sensing method, transmit power at SU should be constrained by the peak interference power of PU and the CR should continuously sense the presence of PUs. To increase the wireless coverage area and reliability of CRN, a new technology called Cooperative Cognitive Radio Network (CCRN), which is a combination of CRN and cooperative communications was developed. A CCRN not only increases the reliability and wireless coverage area of CR but also improves the overall performance of the system. In this context, the main objective of this research work is to evaluate the outage performance of a CCRN in an environment where fading and shadowing also come into the picture and to study the importance of relay networks in CRN. To fulfill the objectives of this research work, a two-hop decode-and-forward CCRN is considered. The recently introduced κ − µ shadowed fading channel is employed over the CCRN to generate a realistic environment.  In order to implement such system as a whole, a deep literature study is performed beforehand. Analytical expressions for the Probability Density Function (PDF) and Cumulative Distribution Function (CDF) of the Signal-to-Noise Ratio (SNR) are obtained. The analytical expressions and simulation results for Outage Probability (OP) are obtained and compared under different fading parameters. The importance of a multiple-relay system in CRN is presented. From the results obtained in this research work, we can conclude that the OP decreases with increase in allowable peak interference power at the PU. The transmit power at SU should always be constrained by the peak interference power at the PU to avoid interference. The overall system performance increases with increasing number of relays.

With the rapid growth of wireless communications, the demand for the various multimedia services is increasing day by day leading to a deficit in the frequency spectrum resources. To overcome this problem, the concept of cognitive radio technology has been proposed which allows the unlicensed secondary user (SU) to access the licensed spectrum of the primary user (PU), thus improving the spectrum utilization. Cooperative communications is another emerging technology which is capable of overcoming many limitations in wireless systems by increasing reliability and coverage. The transmit and receive diversity techniques such as orthogonal space–time block codes (OSTBCs) and selection combining (SC) in multiple-input multiple-output (MIMO) cognitive amplify and forward relay networks help to reduce the effects of fading, increase reliability and extend radio coverage.   In this thesis, we consider a MIMO cognitive cooperative radio network (CCRN) with multiple relays. The protocol used at the relays is an amplify and forward protocol. At the receiver, the SC technique is applied to combine the signals. Analytical expressions for the probability density function (PDF) and cumulative distribution function (CDF) of the signal-to-noise ratio (SNR) are derived. On this basis, the performance in terms of outage probability is obtained. Mathematica has been used to generate numerical results from the analytical expressions. The system model is simulated in MATLAB to verify the numerical results. The performance analysis of the system model is hence done in terms of outage probability.