Resource allocation and optimization techniques in wireless relay networks

Relay techniques have the potential to enhance capacity and coverage of a wireless network. Due to rapidly increasing number of smart phone subscribers and high demand for data intensive multimedia applications, the useful radio spectrum is becoming a scarce resource. For this reason, two way relay network and cognitive radio technologies are required for better utilization of radio spectrum. Compared to the conventional one way relay network, both the uplink and the downlink can be served simultaneously using a two way relay network. Hence the effective bandwidth efficiency is considered to be one time slot per transmission. Cognitive networks are wireless networks that consist of different types of users, a primary user (PU, the primary license holder of a spectrum band) and secondary users (SU, cognitive radios that opportunistically access the PU spectrum). The secondary users can access the spectrum of the licensed user provided they do not harmfully affect to the primary user. In this thesis, various resource allocation and optimization techniques have been investigated for wireless relay and cognitive radio networks

Co₃O₄ Nanosheets with In-Plane Pores and Highly Active {112} Exposed Facets for High Performance Lithium Storage

Recently, two-dimensional transition metal oxide nanomaterials have been extensively investigated as promising candidates for the lithium-ion battery anode materials due to their elastic volume change, efficient ion/electrical pathways, and additional interfacial lithium storage sites. Herein, we report a simple wet-chemical method followed by thermal treatment to synthesize Co₃O₄ nanosheets with the in-plane pores. The as-prepared nanosheets are found to selectively expose the highly active {112} facets as the dominant surfaces. When fabricated into the anode configuration, a specific capacity of 1717 mA h g^–1 can be reliably retained after 100 cycles at a current density of 200 mA g^–1. While increasing the current density to 1 A g^–1 and prolonging the cycle life to 400 cycles, the nanosheets can still deliver a capacity of 1090 mA h g^–1 with a Coulombic efficiency of 99.5%. This excellent electrochemical performance can be attributed to the unique morphological structures of our porous nanosheets for the shortened lithium ion diffusion pathway, alleviated volume expansion, and enhanced active sites, indicating the technological potency of the nanosheets for high-performance lithium storage

Construction of Hierarchical MoSe₂ Hollow Structures and Its Effect on Electrochemical Energy Storage and Conversion

Metal selenides have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including metal-ion batteries and water splitting. However, their practical application is greatly hindered by collapse of the microstructure, thus leading to performance fading. Tuning the structure at nanoscale of these materials is an effective strategy to address the issue. Herein, we craft MoSe₂ with hierarchical hollow structures via a facile bubble-assisted solvothermal method. The temperature-related variations of the hollow interiors are studied, which can be presented as solid, yolk–shell, and hollow spheres, respectively. Under the simultaneous action of the distinctive hollow structures and interconnections among the nanosheets, more intimate contacts between MoSe₂ and electrolyte can be achieved, thereby leading to superior electrochemical properties. Consequently, the MoSe₂ hollow nanospheres prepared under optimum conditions exhibit optimal electrochemical activities, which hold an initial specific capacity of 1287 mA h g^–1 and maintain great capacity even after 100 cycles as anode for Li-ion battery. Moreover, the Tafel slope of 58.9 mV dec^–1 for hydrogen evolution reaction is also attained