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The past few decades have witnessed the acceleration in the development of groundbreaking information technology applications, such as smartphones/tablets, Internet of Things devices, virtual reality devices, etc. These new applications require a significant amount of data traffic to be conveyed wirelessly. Furthermore, the growing number of devices imposes a big challenge of providing seamless and low-delay communication. To satisfy these requirements, visible light communication (VLC) systems have gained considerable popularity thanks to its inherent advantages, such as unlicensed wide bandwidth, high area spectral efficiency, and high security. At the same time, in VLC systems, network security remains a significant challenge. Even with the inherent wireless communication security of VLC systems, there is still a possibility that an eavesdropper can wiretap important/private information in large open spaces. Therefore, this research investigates physical layer security (PLS) to secure the VLC indoor transmission in the presence of randomly located eavesdroppers, proposing three different beamforming and jamming strategies for different VLC system scenarios. PLS is a set of techniques that enable a transmitter and a legitimate receiver to securely transmit and receive important data, eliminating the possibility of eavesdropping by utilizing the randomness of a channel between transmitter and receiver. Dealing with the random locations of eavesdroppers by using tools from stochastic geometry, the proposed PLS schemes do not require precise locations or channel state information of eavesdroppers. Numerical and Monte Carlo simulation results are provided to verify the performances of the proposed schemes.

The performance of four wireless network scenarios, focusing on reliable device-to-device (D2D) communications and secure physical layer communications, is analyzed based on stochastic geometry in this thesis. The analyses and methodologies in this thesis constitute a comprehensive framework for system-level analysis and design of reliability and security in future wireless networks. Since reliability is the cornerstone of security in wireless communications, the first scenario focuses on enhancing the reliability of concurrent D2D transmissions in a Poisson clustered out-band D2D network. Two convex optimization schemes are proposed to maximize the joint coverage probability and minimize the power consumption for reliable concurrent transmissions. Meanwhile, the second scenario focuses on route selection for end-to-end reliability, which is jointly characterized by coverage, delay and trust criteria, in the multi-hop D2D based mission-critical communication systems. The proposed algorithm ensures that the candidate links meet the given requirements of the criteria while selecting the best path based on end-to-end coverage, timeliness or trust probabilities. Apart from the reliability issues discussed above, wireless communication networks including D2D networks are vulnerable to eavesdropping attacks. Thus, the third scenario moves on to study the meta distribution of the secrecy rate for a legitimate link in the presence of randomly located eavesdroppers (EDs) modeled by a Poisson point process. The meta distribution characterizes the fraction of the realizations of EDs that yield a target secrecy rate. Finally, the fourth scenario studies secure wireless communications in the presence of unmanned aerial vehicle (UAV) jamming and randomly located UAV EDs modeled by a uniform binomial point process. A fitting function on the line-of-sight probability is proposed so that a tractable expression is derived for the secure connection probability (SCP). The trends of the SCP on the transmit signal to jamming power ratio, UAV jammer location and UAV height are analyzed.

The performance of four newly proposed orthogonal frequency-division multiplexing (OFDM)-based relay systems with different application scenarios are analyzed in this thesis. The first scenario that is studied relates to the bulk and per-subcarrier relay selections, which can be regarded as two extremes of the performance-complexity trade-off in multi-carrier relay selection. Specifically, the outage performance yielded by these two selection schemes is analyzed in spatially random networks where relays are randomly distributed. Subsequently, a novel selection scheme for OFDM-based relay systems is proposed by combining conventional bulk and per-subcarrier selections. The asymptotic outage performance of the combined selection scheme in the high signal-to-noise (SNR) region is analyzed, and it is found that the combined selection can achieve the optimal outage probability without using the full set of available relays for selection. Hence, a satisfactory solution to the performance-complexity trade-off can be obtained. Apart from conventional application scenarios, OFDM-based relay systems with relay selections are also analyzed in more practical scenarios, e.g., device-to-device (D2D) communications. However, due to the adoption of underlay D2D communication protocol, power control mechanisms are thereby necessary to be applied to mitigate the interference to conventional cellular communications, which lead to a series of new analytical challenges. Meanwhile, to incorporate with new modulation techniques, OFDM-based relay systems are also combined with index modulation (IM). Then, an adaptive OFDM-IM is proposed for two-hop relay networks to enhance the system reliability when there is only one relay, and relay selection is thereby infeasible. Specifically, in this proposed adaptive OFDM-IM, the mapping schemes between a bit stream and indices of active subcarriers are adaptively selected by a certain criterion. All these analyses for a variety of cases given in this thesis constitute a comprehensive analytical framework for OFDM-based relay systems in next generation networks.

This thesis examines the effects of resource constraints on amplify-and-forward (AF) networks. Chapters 3 and 4 are the first research chapters. Chapter 3 studies the outage probability performance of a two-hop two-way AF peak power constrained orthogonal frequency division multiplexing (OFDM) network. Its performance is then optimized. Chapter 4 focuses on the one-way special case of the system studied in Chapter 3. It begins with an analysis of the network when nonlinear distortion produced by signal clipping dominates the additive noise in the system. To conclude Chapter 4, the theoretical study performed throughout Chapters 3 and 4 is used to optimize the performance of a one-way real world test bed. Chapter 5 studies the n-hop multiple-input multiple-output (MIMO) AF relay network. Novel techniques are developed using random dynamical system (RDS) theory and Lyapunov exponents to establish capacity and power scaling laws for the network as n grows large. One of the main conclusions is that the average transmit power must grow at an exponential rate if capacity decay across the network is to be avoided. Chapter 6 constitutes the final research chapter. In it, the techniques used to study peak-power constrained OFDM-based networks are combined with those developed in Chapter 5, which were used to study capacity and power scaling for multihop AF networks. The conclusion of this is that incorporating OFDM into peak-power constrained multihop AF relay networks will cause the capacity along each of the network's eigenchannels to decay exponentially. Finally, we show that the effects of distortion can be circumvented by ensuring the number of antennas at each node scales at a super-linear rate with the number of hops within the network.