59 research outputs found
Effects of Correlation of Channel Gains on the Secrecy Capacity in the Gaussian Wiretap Channel
Secrecy capacity is one of the most important characteristic of a wireless communication channel. Therefore, the study of this characteristic wherein the system has correlated channel gains and study them for different line-of-sight (LOS) propagation scenarios is of ultimate importance.
The primary objective of this thesis from the mathematical side is to determine the secrecy capacity (SC) for correlated channel gains for the main and eavesdropper channels in a Gaussian Wiretap channel as a function from main parameters (μ, Σ, ρ). f(h1, h2) is the joint distribution of the two channel gains at channel use (h1, h2), fi(hi) is the main distribution of the channel gain hi. The results are based on assumption of the Gaussian distribution of channel gains (gM, gE). The main task of estimating the secrecy capacity is reduced to the problem of solving linear partial differential equations (PDE). Different aspects of the analysis of secrecy capacity considered in this research are the Estimation of SC mathematically and numerically for correlated SISO systems and a mathematical example for MIMO systems with PDE.
The variations in Secrecy Capacity are studied for Rayleigh (NLOS) distribution and Rician (LOS) distribution. Suitable scenarios are identified in which secure communication is possible with correlation of channel gains. Also, the new algorithm using PDE has a higher speed and than analog algorithms constructed on the classical statistical Monte Carlo methods. Taking into account the normality of the distribution of system parameters, namely the channel gain (gM, gE), the algorithm is constructed for systems of partial differential equations which satisfies the secrecy criterion.
Advisor: H. Andrew Harm
Enhancing security of TAS/MRC-based mixed RF-UOWC system with induced underwater turbulence effect
Post commercial deployment of fifth-generation (5G) technologies, the consideration of sixth-generation (6G) networks is drawing remarkable attention from research communities. Researchers suggest that similar to 5G, 6G technology must be human-centric where high secrecy together with high data rate will be the key features. These challenges can be easily overcome utilizing PHY security techniques over high-frequency free-space or underwater optical wireless communication (UOWC) technologies. But in long-distance communication, turbulence components drastically affect the optical signals, leading to the invention of the combination of radio-frequency (RF) links with optical links. This article deals with the secrecy performance analysis of a mixed RF-UOWC system where an eavesdropper tries to intercept RF communications. RF and optical links undergo η−μ and mixture exponential generalized Gamma distributions, respectively. To keep pace with the high data rate of the optical technologies, we exploit the antenna selection scheme at the source and maximal ratio combining diversity at the relay and eavesdropper, while the eavesdropper is unaware of the antenna selection scheme. We derive closed-form expressions of average secrecy capacity, secrecy outage probability, and probability of strictly positive secrecy capacity to demonstrate the impacts of the system parameters on the secrecy behavior. Finally, the expressions are corroborated via Monte Carlo simulations
Enhancing physical layer security in wireless networks with cooperative approaches
Motivated by recent developments in wireless communication, this thesis aims to
characterize the secrecy performance in several types of typical wireless networks.
Advanced techniques are designed and evaluated to enhance physical layer security in
these networks with realistic assumptions, such as signal propagation loss, random node
distribution and non-instantaneous channel state information (CSI).
The first part of the thesis investigates secret communication through relay-assisted
cognitive interference channel. The primary and secondary base stations (PBS and SBS)
communicate with the primary and secondary receivers (PR and SR) respectively in the
presence of multiple eavesdroppers. The SBS is allowed to transmit simultaneously with
the PBS over the same spectrum instead of waiting for an idle channel. To improve
security, cognitive relays transmit cooperative jamming (CJ) signals to create additional
interferences in the direction of the eavesdroppers. Two CJ schemes are proposed to
improve the secrecy rate of cognitive interference channels depending on the structure of
cooperative relays. In the scheme where the multiple-antenna relay transmits weighted
jamming signals, the combined approach of CJ and beamforming is investigated. In
the scheme with multiple relays transmitting weighted jamming signals, the combined
approach of CJ and relay selection is analyzed. Numerical results show that both these
two schemes are effective in improving physical layer security of cognitive interference
channel.
In the second part, the focus is shifted to physical layer security in a random wireless
network where both legitimate and eavesdropping nodes are randomly distributed. Three
scenarios are analyzed to investigate the impact of various factors on security. In
scenario one, the basic scheme is studied without a protected zone and interference. The
probability distribution function (PDF) of channel gain with both fading and path loss
has been derived and further applied to derive secrecy connectivity and ergodic secrecy
capacity. In the second scenario, we studied using a protected zone surrounding the source
node to enhance security where interference is absent. Both the cases that eavesdroppers
are aware and unaware of the protected zone boundary are investigated. Based on the
above scenarios, further deployment of the protected zones at legitimate receivers is
designed to convert detrimental interference into a beneficial factor. Numerical results
are investigated to check the reliability of the PDF for reciprocal of channel gain and to
analyze the impact of protected zones on secrecy performance.
In the third part, physical layer security in the downlink transmission of cellular network
is studied. To model the repulsive property of the cellular network planning, we assume
that the base stations (BSs) follow the Mat´ern hard-core point process (HCPP), while
the eavesdroppers are deployed as an independent Poisson point process (PPP). The
distribution function of the distances from a typical point to the nodes of the HCPP is
derived. The noise-limited and interference-limited cellular networks are investigated
by applying the fractional frequency reuse (FFR) in the system. For the noise-limited
network, we derive the secrecy outage probability with two different strategies, i.e. the
best BS serve and the nearest BS serve, by analyzing the statistics of channel gains. For
the interference-limited network with the nearest BS serve, two transmission schemes are
analyzed, i.e., transmission with and without the FFR. Numerical results reveal that both
the schemes of transmitting with the best BS and the application of the FFR are beneficial
for physical layer security in the downlink cellular networks, while the improvement du
Non-Orthogonal Multiple Access for 5G: Design and Performance Enhancement
PhDSpectrum scarcity is one of the most important challenges in wireless communications
networks due to the sky-rocketing growth of multimedia applications. As the latest
member of the multiple access family, non-orthogonal multiple access (NOMA) has
been recently proposed for 3GPP Long Term Evolution (LTE) and envisioned to be
a key component of the 5th generation (5G) mobile networks for its potential ability on
spectrum enhancement. The feature of NOMA is to serve multiple users at the same
time/frequency/code, but with di erent power levels, which yields a signi cant spectral
e ciency gain over conventional orthogonal multiple access (OMA). This thesis provides
a systematic treatment of this newly emerging technology, from the basic principles of
NOMA, to its combination with simultaneously information and wireless power transfer
(SWIPT) technology, to apply in cognitive radio (CR) networks and Heterogeneous
networks (HetNets), as well as enhancing the physical layer security and addressing the
fairness issue.
First, this thesis examines the application of SWIPT to NOMA networks with spatially
randomly located users. A new cooperative SWIPT NOMA protocol is proposed, in
which near NOMA users that are close to the source act as energy harvesting relays in
the aid of far NOMA users. Three user selection schemes are proposed to investigate
the e ect of locations on the performance. Besides the closed-form expressions in terms
of outage probability and throughput, the diversity gain of the considered networks is
determined.
Second, when considering NOMA in CR networks, stochastic geometry tools are used to
evaluate the outage performance of the considered network. New closed-form expressions
are derived for the outage probability. Diversity order of NOMA users has been analyzed
based on the derived outage probability, which reveals important design insights regarding the interplay between two power constraints scenarios.
Third, a new promising transmission framework is proposed, in which massive multipleinput
multiple-output (MIMO) is employed in macro cells and NOMA is adopted in
small cells. For maximizing the biased average received power at mobile users, a massive
MIMO and NOMA based user association scheme is developed. Analytical expressions
for the spectrum e ciency of each tier are derived using stochastic geometry. It is
con rmed that NOMA is capable of enhancing the spectrum e ciency of the network
compared to the OMA based HetNets.
Fourth, this thesis investigates the physical layer security of NOMA in large-scale networks
with invoking stochastic geometry. Both single-antenna and multiple-antenna
aided transmission scenarios are considered, where the base station (BS) communicates
with randomly distributed NOMA users. In addition to the derived exact analytical
expressions for each scenario, some important insights such as secrecy diversity order
and large antenna array property are obtained by carrying the asymptotic analysis.
Fifth and last, the fundamental issues of fairness surrounding the joint power allocation
and dynamic user clustering are addressed in MIMO-NOMA systems in this thesis. A
two-step optimization approach is proposed to solve the formulated problem. Three
e cient suboptimal algorithms are proposed to reduce the computational complexity.
To further improve the performance of the worst user in each cluster, power allocation
coe cients are optimized by using bi-section search. Important insights are concluded
from the generated simulate results
Physical Layer Security for STAR-RIS-NOMA: A Stochastic Geometry Approach
In this paper, a stochastic geometry based analytical framework is proposed for secure simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted non-orthogonal multiple access (NOMA) transmissions, where legitimate users (LUs) and eavesdroppers are randomly distributed. Both the time-switching protocol (TS) and energy splitting (ES) protocol are considered for the STAR-RIS. To characterize system performance, the channel statistics are first provided, and the Gamma approximation is adopted for general cascaded κ-μ fading. Afterward, the closed-form expressions for both the secrecy outage probability (SOP) and average secrecy capacity (ASC) are derived. To obtain further insights, the asymptotic performance for the secrecy diversity order and the secrecy slope are deduced. The theoretical results show that 1) the secrecy diversity orders of the strong LU and the weak LU depend on the path loss exponent and the distribution of the received signal-to-noise ratio, respectively; 2) the secrecy slope of the ES protocol achieves the value of one, higher than the slope of the TS protocol which is the mode operation parameter of TS. The numerical results demonstrate that: 1) there is an optimal STAR-RIS mode operation parameter to maximize the secrecy performance; 2) the STAR-RIS-NOMA significantly outperforms the STAR-RIS-orthogonal multiple access
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