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In this paper, we propose to use a hybrid relay-reflecting intelligent surface (HR-RIS) to enhance the performance of a covert communication system. Specifically, the HR-RIS consists of passive reflecting elements and active relay elements to manipulate the wireless signals from a transmitter to a desired receiver while ensuring the covertness of the transmission via avoiding such signals being detected by a warden. To fully explore the benefits offered by the HR-RIS, we first formulate the joint design of the transmit power and relay/reflection coefficients of the HR-RIS as an optimization problem to maximize the covert rate subject to a covertness constraint. To tackle the solution to this optimization problem, we then derive a closed-form expression for an upper bound on covert rate, based on which we develop an alternate algorithm to solve the formulated optimization problem. Our examination shows that the HR-RIS outperforms the traditional RIS in term of achieving a higher covert rate. Interestingly, we also observe the major part of the performance gain brought by the HR-RIS can be obtained by a small number of active relay elements (e.g., 5) and further increasing this number does not improve the covert communication performance.

Random frequency diverse array (RFDA) based directional modulation (DM) was proposed as a promising technology in secure communications to achieve a precise transmission of confidential messages, and artificial noise (AN) was considered as an important helper in RFDA-DM. Compared with previous works that only focus on the spot of the desired receiver, in this work, we investigate a secrecy region around the desired receiver, that is, a specific range and angle resolution around the desired receiver. Firstly, the minimum number of antennas and the bandwidth needed to achieve a secrecy region are derived. Moreover, based on the lower bound of the secrecy capacity in RFDA-DM-AN scheme, we investigate the performance impact of AN on the secrecy capacity. From this work, we conclude that: 1) AN is not always beneficial to the secure transmission. Specifically, when the number of antennas is sufficiently large and the transmit power is smaller than a specified value, AN will reduce secrecy capacity due to the consumption of limited transmit power. 2) Increasing bandwidth will enlarge the set for randomly allocating frequencies and thus lead to a higher secrecy capacity. 3) The minimum number of antennas increases as the predefined secrecy transmission rate increases.

This thesis focuses on the study of Heterogeneous Networks (HetNets), analyzing the system performance and the allocation of available resources, such as serving BSs, spectrum, power supply and storage. Specifically, four main research focuses are included. The first focus is load balancing problem in HetNets. First, a factor graph model is developed to represent the user association problem and decompose the network-wide objective into local utility functions. Then, a distributed belief propagation (BP) algorithm is proposed to find out the optimal user association. Furthermore, several properties of the factor graph relevant to the performance of the BP algorithm are analyzed using stochastic geometry. The second focus is inter-cell interference in HetNets. To mitigate the inter-cell interference and maximizing the system sum rate, a BP algorithm to jointly optimize the user association, sub-channel assignment and power allocation is proposed. Importantly, a novel factor graph model is developed that not only decomposes the network-wide objective but also transforms the constraints into local functions. The third focus is probabilistic small-cell caching in HetNets, where each SBS caches a subset of the popular content with a specific caching probability. The theoretical results of the successful download probability (SDP) are derived using stochastic geometry theory, i.e., the probability that users can successfully download required content from SBSs. Importantly, I optimize the caching probabilities in two network architectures by maximizing the SDP. The fourth focus is cooperative inter-working of LTE and wireless fidelity (Wi-Fi). A new framework is proposed that considers both systems, downlink and uplink transmissions, and interference in both time and spatial domains. Based on this framework, I theoretically study the performance of a large-scale Wi-Fi network, and the network performance of several inter-working strategies.

Data missing was always when scanning plant organs by using 3D laser scanner, and this could lead to holes appearing in the mesh surface generated from the scanned point cloud. Basing on the analyzing to the geometrical features of different types of plant organs, a hole-filling algorithm was proposed which based on the normal vector of the mesh to make the repair area as close as possible to the original model of the organs mesh. The algorithm firstly extracts the hole boundary in the surface mesh, and compute the positions of new points based on the normal vector and normal curvature of the mesh boundary, then connects the new points and boundary points to generate a new triangular mesh. Experiments on different plant organs of geometric shapes were taken and new triangular mesh qualities were evaluated. Results show that the algorithm can fill holes in several kinds of scanned plant organs mesh quickly and efficiently.