Dynamic Spectrum Sharing in Cognitive Radio and Device-to-Device Systems

abstract: Cognitive radio (CR) and device-to-device (D2D) systems are two promising dynamic spectrum access schemes in wireless communication systems to provide improved quality-of-service, and efficient spectrum utilization. This dissertation shows that both CR and D2D systems benefit from properly designed cooperation scheme. In underlay CR systems, where secondary users (SUs) transmit simultaneously with primary users (PUs), reliable communication is by all means guaranteed for PUs, which likely deteriorates SUs’ performance. To overcome this issue, cooperation exclusively among SUs is achieved through multi-user diversity (MUD), where each SU is subject to an instantaneous interference constraint at the primary receiver. Therefore, the active number of SUs satisfying this constraint is random. Under different user distributions with the same mean number of SUs, the stochastic ordering of SU performance metrics including bit error rate (BER), outage probability, and ergodic capacity are made possible even without observing closed form expressions. Furthermore, a cooperation is assumed between primary and secondary networks, where those SUs exceeding the interference constraint facilitate PU’s transmission by relaying its signal. A fundamental performance trade-off between primary and secondary networks is observed, and it is illustrated that the proposed scheme outperforms non-cooperative underlay CR systems in the sense of system overall BER and sum achievable rate. Similar to conventional cellular networks, CR systems suffer from an overloaded receiver having to manage signals from a large number of users. To address this issue, D2D communications has been proposed, where direct transmission links are established between users in close proximity to offload the system traffic. Several new cooperative spectrum access policies are proposed allowing coexistence of multiple D2D pairs in order to improve the spectral efficiency. Despite the additional interference, it is shown that both the cellular user’s (CU) and the individual D2D user's achievable rates can be improved simultaneously when the number of D2D pairs is below a certain threshold, resulting in a significant multiplexing gain in the sense of D2D sum rate. This threshold is quantified for different policies using second order approximations for the average achievable rates for both the CU and the individual D2D user.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Capacity and Delay of Unmanned Aerial Vehicle Networks with Mobility

Unmanned aerial vehicles (UAVs) are widely exploited in environment monitoring, search-and-rescue, etc. However, the mobility and short flight duration of UAVs bring challenges for UAV networking. In this paper, we study the UAV networks with n UAVs acting as aerial sensors. UAVs generally have short flight duration and need to frequently get energy replenishment from the control station. Hence the returning UAVs bring the data of the UAVs along the returning paths to the control station with a store-carry-and-forward (SCF) mode. A critical range for the distance between the UAV and the control station is discovered. Within the critical range, the per-node capacity of the SCF mode is O(n/log n) times higher than that of the multi-hop mode. However, the per-node capacity of the SCF mode outside the critical range decreases with the distance between the UAV and the control station. To eliminate the critical range, a mobility control scheme is proposed such that the capacity scaling laws of the SCF mode are the same for all UAVs, which improves the capacity performance of UAV networks. Moreover, the delay of the SCF mode is derived. The impact of the size of the entire region, the velocity of UAVs, the number of UAVs and the flight duration of UAVs on the delay of SCF mode is analyzed. This paper reveals that the mobility and short flight duration of UAVs have beneficial effects on the performance of UAV networks, which may motivate the study of SCF schemes for UAV networks.Comment: 14 pages, 10 figures, IEEE Internet of Things Journa

Technology Implications of UWB on Wireless Sensor Network-A detailed Survey

In today’s high tech “SMART” world sensor based networks are widely used. The main challenge with wireless-based sensor networks is the underneath physical layer. In this survey, we have identified core obstacles of wireless sensor network when UWB is used at PHY layer. This research was done using a systematic approach to assess UWB’s effectiveness (for WSN) based on information taken from various research papers, books, technical surveys and articles. Our aim is to measure the UWB’s effectiveness for WSN and analyze the different obstacles allied with its implementation. Starting from existing solutions to proposed theories. Here we have focused only on the core concerns, e.g. spectrum, interference, synchronization etc.Our research concludes that despite all the bottlenecks and challenges, UWB’s efficient capabilities makes it an attractive PHY layer scheme for the WSN, provided we can control interference and energy problems. This survey gives a fresh start to the researchers and prototype designers to understand the technological concerns associated with UWB’s implementatio

Performance analysis of Unmanned Aerial Vehicles-enabled Wireless Networks

University of Technology Sydney. Faculty of Engineering and Information Technology.As an indispensable part of mobile communication systems, Unmanned Aerial Vehicles (UAVs) can be leveraged to complement terrestrial networks by providing coverage to areas where infrastructures are scarce. Equipped with self-navigation and strong automation, UAVs have extensive applications to environmental monitoring, disaster recovery, search and rescue, owing to their excellent agility and autonomy. As a result, an increasing demand arises for ubiquitous connectivity and reliable communication for data exchange between UAVs, and between UAVs and ground stations. Since UAVs operate in three-dimensional (3D) space with strong manoeuvrability, random trajectories and wireless propagation environment can pose significant challenges to the study on coverage and capacity of UAV networks. On the other hand, UAVs are increasingly posing threats to information security. UAVs can be potentially used to eavesdrop and jam wireless transmissions between legitimate terrestrial transceivers. It is of practical interest to understand the robustness of terrestrial wireless communications under exposure to new threats from aerial adversaries. This thesis studies the coverage and capacity, including secure coverage and secrecy capacity, of UAV-enabled wireless networks with UAVs flying under 3D random trajectories based on stochastic geometry and measure convergence theory. The detailed contributions of this thesis are summarised as: • Capacity analysis of UAV networks under random trajectories. We geometrically derive probability distributions of UAV-to-UAV distances and closed-form bounds for the capacity can be obtained by exploiting the Jensen's inequality. We extrapolate the idea to dense UAV networks and analyse the impact of network densification and imperfect channel state information on the capacity. • Connectivity analysis of uncoordinated UAV swarms. New closed-form bounds are derived for the outage probability of individual UAVs, and broadcast connectivity of each UAV which evaluates the reliability of broadcast across the swarm. The qualifying conditions of the bounds on 3D coverage and impact of ground interference on the outage are identified. • Secure connectivity analysis in UAV networks. We propose a trust model based on UAVs’ behaviour and mobility pattern and characteristics of inter-UAV channels. We derive analytical expressions of both physical and secure connectivity probabilities with/without considering Doppler shift. • Secrecy capacity analysis against aerial eavesdroppers. We analyse ergodic and ϵ-outage secrecy capacities of ground link in the presence of cooperative aerial eavesdroppers. The “cut-off” density of eavesdroppers under which the secrecy capacities vanish is identified. By decoupling the analysis of random trajectories from random channel fading, closed-form approximations with almost sure convergence to the secrecy capacities are devised

Analysis and design of energy harvesting wireless communication systems

Wireless-powered communication is an emerging technology for powering the large number of miniature devices of the future. In a wireless-powered communication system, low-power sensors extract energy from the incident wireless signals to power their operations such as information transmission, sensing or reception. Due to sporadic energy availability, however, such a system is fundamentally different from a traditionally-powered communication system. This dissertation investigates three distinct aspects of wireless-powered communications to get insights on the system operation. First, leveraging concepts from finite-length information theory, an analytical framework is developed for examining wireless-powered communications with short packets, i.e., in the finite blocklength regime. This is relevant as remotely-powered communications may entail short packets due to small payloads, low-latency requirements, or limited energy to support a longer transmission. Second, using a stochastic geometry framework, an analytical model is developed for characterizing the performance of wireless-powered communications in the millimeter wave (mmWave) band. The proposed model incorporates the key features of mmWave systems such as directional beamforming and sensitivity to building blockages. Finally, the power transfer efficiency and the energy efficiency of a wireless-powered communication system aided by massive MIMO is characterized. The broad goal of this dissertation is to better understand wireless-powered communications in the context of the emerging technologies for 5G.Electrical and Computer Engineerin