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As multimedia applications become more popular, wireless communication systems are expected to reliably provide increased data rates. Multiple Input Multiple Output (MIMO) technologies can meet this demand without using additional bandwidth or transmit power. MIMO is part of modern wireless communication standards. Another critical aspect of communications is to secure the confidentiality of data transmission. Cryptography accomplishes this at the upper layers of the protocol stack. At the physical layer, data travels unencrypted and can be secured by using the channel characteristics to ``hide'' data transmission from potential eavesdroppers. We consider a Gaussian MIMO wiretap channel and are looking for the maximal rate at which data can be transmitted both reliably and securely to the intended receiver: the secrecy capacity. This quantity is difficult to find analytically and is known precisely in only a few cases. This thesis proposes several numerical optimization methods, both stochastic and deterministic, to evaluate the secrecy capacity and to find the optimal transmit covariance matrix. The stochastic approaches are based on Monte-Carlo and on Differential Evolution (a genetic algorithm). The deterministic approaches are based on successive linear approximation. The accuracy of the results obtained with these methods is, in general, better than the one offered by popular numerical optimization tools such as CVX or YALMIP.

For wireless communication systems, due to broadcasting nature of wireless medium, how to keep eavesdroppers from wiretapping messages is worth investigated. In addition to encryption techniques applied in application layer, physical layer secrecy techniques have been studied in literature. Under the premise that eavesdropper cannot steal any information, physical layer secrecy focus on maximizing the capacity of legal transmission, and make it more reliable by using physical properties of wireless channel. This thesis considers an amplify-and-forward (AF) multiple-input multiple-output (MIMO) cooperative communication network with an untrusted relay (UR), and linear precoders are employed at source, relay, and destination. The relay here serves as a bridge of transmission 1 between the source and the destination. However, assume that the untrusted relay may wiretap information from the source without authorization. In order to prevent relay from wiretapping information, the destination generates artificial noise (AN) to interfere the relay, when the relay is receiving information from the source. Since AN is generated by the destination, the destination can eliminate AN by itself after receiving signal from the relay without corrupting signals of legal transmission. We propose precoder design for source, relay and destination to maximize secrecy capacity under the power constraint of three nodes. By utilizing singular value decomposition (SVD) of all channel matrices and Hadamard inequality, we simplify the optimization problem of precoding matrices to scalar optimization problem, and optimization can be accomplished recursively.

Information security has been a very important issue in wireless networks. With the ever-increasing amount of data being exchanged over wireless networks, the confidentiality of information needs to be protected from unauthorized users called eavesdropper. Due to the broadcast nature of the wireless medium, the transmissions between legitimate users maybe overheard and intercepted by the unauthorized parties, which makes wireless transmission vulnerable to potential eavesdropping attacks. The security of wireless communications plays an increasingly important role in the cybercrime defense against unauthorized activities.     We consider the wireless physical layer security which has been explored for the sake of enhancing the protection of wireless communications against eavesdroppers. We consider the problem of secret communication through Rayleigh fading channel in the presence of an eavesdropper in which the transmitter knows the channel state information of both the main and eavesdropper channel. Then, we analyze the average capacity of the main channel and eavesdropper channel from which an expression of secrecy capacity is derived based on the cumulative distribution function and probability density function of the signal to noise ratio. We also analyze an expression for the symbol error rate of the main channel, and the outage probability is obtained for the main transmission. Finally, we perform the numerical results in MATLAB.

This paper is persuaded to understand the physical layer security in wireless commu-nications utilizing NOMA (Non Orthogonal Multiple Access) concepts in the presence of an eavesdropper. Physical layer security maintains the confidentiality and secrecyof the system against eavesdroppers. We use the power domain in this paper, where NOMA allows many users to share resources side by side. Power allocation concern-ing channel condition is taken into consideration where user whose channel condition is weak is allocated with eminent power to directly decode the signal, whereas theuser with better channel condition applies successive interference cancellation (SIC)to decode the signal. Here, the base station communicates with the users and sends data signals while the eavesdropper secretly eavesdrops on the confidential informa-tion simultaneously. In this thesis, to improve the physical layer security, jamming method was usedwhere users are assumed to be in full duplex, send jamming signals to degrade the performance of the eavesdropper. Analytic expressions of CDF, PDF, outage proba-bility and secrecy capacity are obtained from analyzing the NOMA jamming scheme. The numerical results are evaluated with the simulations results and analysed theeffect of jamming on improving the performance of the NOMA system in presenceof an eavesdropper.

Wireless communications has gone through tremendous growth in the past decades. There has been a shift in wireless network research from spectral efficiency and quality of service (QoS) constraints to energy efficiency and green communications to reduce power consumption. Green energy resources such as solar, wind, thermal and mechan- ical vibrations can be employed to increase the energy efficiency of energy-constrained networks such as wireless sensor networks. Converting the available energy in the sur- rounding area into electricity, energy harvesting (EH), has been the subject of recent research. EH from radio frequency (RF) signals can be utilized to prolong the lifetime of devices in energy-constrained systems. Wireless power transmission (WPT) for EH is a promising solution to provide a reliable source of energy for devices which are di cult to service due to mobility and/or hard to reach locations. The integration of relaying into conventional wireless networks is promising to increase the coverage area and reduce power consumption. However, the extra power consumed to relay signals may be a problem that can be mitigated by WPT. WPT has made it possible for relays to power themselves by capturing ambient energy wirelessly. The received signal at the relay can be utilized to both forward information and harvest energy. This dissertation focuses on practical energy harvesting schemes in wireless com- munication networks. Further, the broadcast nature of wireless systems makes wire- less transmissions more vulnerable to eavesdropping compared to wired signals. The goal of this work is to develop EH schemes that are capable of supplying sustainable energy to the relays and overcoming the secrecy hazards from potential eavesdroppers. Power splitting (PS) and time switching (TS) are studied in communication networks to prolong the lifetime of an energy-constrained relay. First, a dual hop system with an amplify and forward (AF) relay employing wireless information and power transfer (WIPT) via power splitting is studied. Optimal transmit antenna selection that max- imizes the end-to-end signal to noise ratio (SNR) at the destination is considered and the outage probability is derived. It is shown that the outage probability increases with the number of transmit antennas but this also increases the system complexity. Since the spectral efficiency with two-way relaying is higher than with one-way relaying, a two-way EH-based relay network with an eavesdropper is investigated. The secrecy capacity at the users is derived for two diversity combining cases at the eavesdropper, selection combining (SC) and maximal ratio combining (MRC). A friendly jammer is introduced to increase the secrecy capacity of the users by reducing the received signal to noise ratio at the eavesdropper since the signal of the jammer is considered as noise at the eavesdropper. The corresponding optimization problem is reformulated using the single condensation method (SCM) and geometric programming (GP) into a convex optimization problem. Then, GP is used to jointly optimize the power splitting factor of the relay and transmit powers of the two users and jammer to maximize the secrecy capacity of the system. Imperfect cancellation of the jamming signal at the relay is assumed. It is shown that increasing the power allocated to the jammer decreases the secrecy capacity at the users. However, when perfect jamming signal cancellation is assumed, increasing the power allocated to the jammer increases the secrecy capacity at the users. The secrecy capacity is also shown to be greater with a jammer than without a jammer. Channel state information uncertainty at the eavesdropper is also considered as an extra noise source. TS at the relay of a two-way EH-based relay network was also considered. GP is used to jointly optimize the time switching ratio of the relay and transmit powers of the two users and jammer to maximize the secrecy capacity of the system. It is shown that PS two-way relaying achieves a better secrecy capacity than TS two-way relaying. / Graduate

The usage of wireless communication has increased over the past few years. Most wired communications are replaced by wireless communication for ease of use. Wireless communication transfers confidential information like personal information and credentials between two entities. We can't probably say that it is safe to send this information via wireless communication. As more data is sent, more attacks happen to steal the data. Hence, it is necessary to implement secure methods to transfer the data between source and receiver. In this communication channel, we use secrecy capacity as a parameter to measure how data is sent securely between source and destination. Generally, to achieve high system performance, the information is sent with low power, but this reduces the signal efficiency at the receiver. So, in this thesis, we have implemented cooperative transmission to increase the efficiency of low power signals by adding the relays between source and destination. This thesis consists of two relays. The relay that obtains the maximum signal-to-noise ratio is selected for the primary communication link. The other relay sends the signal to the eavesdropper to confuse the eavesdropper. In this thesis, we have derived the mathematical expression for SNR at receiver eavesdropper, and also we have derived a word for outage probability and secrecy capacity. Then, we simulated the Matlab code to obtain results on how the secrecy capacity affects by changing the various parameters like path loss exponent and fading severity parameter and suggests which environment is better to maintain high secrecy capacity. We also analysed the system performance and secrecy capacity in the presence of eavesdropper as well.

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The physical-layer security of a line-of-sight (LOS) free-space optical (FSO) link using orbital angular momentum (OAM) multiplexing is studied. We discuss the effect of atmospheric turbulence to OAM-multiplexed FSO channels. We numerically simulate the propagation of OAM-multiplexed beam and study the secrecy capacity. We show that, under certain conditions, the OAM multiplexing technique provides higher security over a single-mode transmission channel in terms of the total secrecy capacity and the probability of achieving a secure communication. We also study the power cost effect at the transmitter side for both fixed system power and equal channel power scenarios.