Electrical and Electronic Engineering - Theses
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ItemResource optimization for future wireless communications and energy harvesting systems with coordinated transmission( 2019)Dense-cell deployment with coordinated multiple point transmission has been widely investigated to minimize inter-cell interference. Depending on the knowledge of channel state information and whether joint coding and signal processing are performed at the cooperative transmitters, coordinated transmission can be divided into coherent and non-coherent transmission. In the first half of the thesis, we study optimal power allocation for capacity maximization with coherent and non-coherent transmission, in which K coordinated transmitters coherently/non-coherently allocate power across N subchannels under joint total and individual power constraints. This allows the system to limit the overall energy consumption for cost and/or green factors, while also preventing individual transmitters to overdrive their high-powered amplifiers. For coherent coordinated transmission, we derive a new optimal co-phasing power allocation which shows that the optimal power allocation must follow a particular proportional rule. This result highlights that the optimal power allocation for transmitters with individual power constraints is different from waterfilling, as more power is not necessarily allocated to the subchannels with better channel conditions. In the non-coherent coordinated transmission case, we show that the optimal power allocation solution has an interesting sparse feature that among N subchannels, at most K-1 subchannels can be allocated power for joint transmission by multiple transmitters, and the rest of the subchannels must be served by a single transmitter. As wireless devices (e.g., Internet of things device and wireless sensor) become more pervasive, there is an ever-increasing interest for powering electronic devices wirelessly. In order to avoid the high radiation intensity and expand coverage, distributed but coordinated wireless power transfer (WPT) using energy beamforming is considered as a promising technology to address the energy scarcity problem. In the second half of the thesis, we study an optimal distributed energy beamforming strategy for total harvested power maximization, where K coordinated energy transmitters (CETs) coherently transmit energy over N subchannels. Under joint total and individual antenna power constraints, we derive the optimal power allocation rule which reveals that all K CETs will participate in energy beamforming with T < K CETs transmitting with their maximum individual powers due to the total power constraint. Nevertheless, the optimal WPT strategy is that no more than T+1 subchannels are selected for power allocation regardless of the channel conditions. Finally, we analyse a distributed multi-antenna WPT system, where each CET k is equipped with M antennas and has a transmit power constraint Pk. We show that the optimal power allocation has similar properties as coherent wireless information transmission. However, the optimal WPT strategy is that no more than K subchannels are selected for power allocation regardless of the channel conditions or the number of antennas in each CET.
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ItemOptimal and Game-theoretic Resource Allocations for Multiuser Wireless Energy-Harvesting and Communications Systems( 2019)The fifth generation (5G) of wireless cellular networks will see a paradigm shift towards extreme base station densification with massive amounts of data transmissions, massive number of mobile users, and massive number of antenna systems. To support this, network resources such as power and bandwidth will need to be efficiently allocated to multiple nodes with consideration for energy-efficiency, fairness, security, and scalability. The advancements in wireless power transfer have enabled high power conversion efficiency over practical transmission ranges. This can result in overcoming the energy constraints of wireless nodes such as mobile users, sensors, and IoT, and cutting the wires to recharging stations. As such, this results in a new dimension to resource allocation for traditional information-based communication systems due to extra consideration of energy harvested. The focus of this thesis is to design new resource allocation frameworks based on optimization-techniques and game-theory for future wireless energy-harvesting and communications systems. We consider the two main wireless power transfer (WPT) and communications technologies namely 1) Simultaneous wireless information and power transfer (SWIPT) which transfer power and information from the same access point, and 2) wireless-powered communications (WPC) with separate energy access points (EAPs) for power signals and data access points (DAPs) for information signals. For multi-user SWIPT systems with fairness constraints, we developed a max-min energy harvesting solution while satisfying the sum power budget and minimum user rate. By using the max-min energy harvesting solution we solved the dual problem which is the max-min rate satisfying minimum user energy-harvesting levels and sum power constraints. All these problems are NP-hard in nature, thus, we decompose the problem into distinct stages and developed efficient algorithms to tackle them. Furthermore, we provided insights on the total transmit power, channel bandwidth and minimum required rate considerations for the practical implementation and feasibility of energy harvesting in SWIPT systems. Security is a key concern in SWIPT systems due to the broadcast transmission of energy and information signals. Towards this end, we have developed a new optimization algorithm for secure SWIPT in OFDMA networks with multiple legitimate users communicating in the presence of an eavesdropper. The objective of our optimization framework is to maximize the total harvested-power satisfying a minimum secrecy capacity constraint for each legitimate user. We also optimized the power splitting ratio between the information and power transmission for legitimate users. To obtain deeper insights, we investigated novel game-theoretic formulations to facilitate individual user utility maximization when the users are rational players. We designed a Stackelberg game with users as multiple leaders deciding power splitting ratios as their strategy, and the base station as the follower deciding transmit power. For WPC, we proposed a new model where the EAP also acts as a relay for information signals between the users and the DAP. Specifically, we design an energy trading game between multiple relay-energy access points (REAP) and DAPs for buying energy for users. The same REAPs can be used for relaying information to the DAP based on the channel states. We exploited the cooperative communication advantages for information transfer in energy harvesting. Numerical examples for showing the effectiveness of all the proposed algorithms are given.
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ItemReal-time signal processing for coherent optical OFDM system( 2011)The principle of Orthogonal Frequency-Division Multiplexing (OFDM) modulation has been proposed for several decades. OFDM technology has moved out of laboratories into practice in modern wireless communication systems since early 1990s. Recently, OFDM is applied to the optical domain, which has shown a multitude of benefits such as immunity to chromatic dispersion, polarization-mode dispersion and high spectral efficiency. However, most of these research works are conducted in off-line processing where only a limited length of received signal is sampled and processed. This is useful to verify signal processing algorithms, but does not allow long-term performance verification. To solve this problem, real-time experiments are imperative. In this thesis, we conduct experiments on real-time implementation of coherent optical OFDM (CO-OFDM) systems, demonstrate up to 110-Gb/s real-time reception for over 600-km standard single mode fibre (SSMF) transmission, and perform real-time I/Q imbalance calibration for CO-OFDM transmission. We investigate the optimum design of real-time signal processing algorithms for both single-input single-output (SISO) and multiple-input multiple-output (MIMO) CO-OFDM receivers. By using high-speed Analog-to-Digital Convertors (ADCs), Field-Programmable Gate Arrays (FPGAs) and efficient OFDM signal processing at the receiver, we have achieved record data rates for both single-band and multi-band CO-OFDM transmissions. In particular, the first multi-gigabit real-time CO-OFDM reception at 3.1 Gb/s is successfully demonstrated. A 110-Gb/s multi-band real-time receiver after 400-ps Differential-Group-Delay (DGD) and 600-km transmission through SSMF without optical dispersion compensation is also provided. Several experiments on real-time signal processing confirm the feasibility of applying CO-OFDM to practical communication applications. We also carry out experiments to demonstrate the effect of hybrid in-phase/quadrature (I/Q) imbalance compensation for CO-OFDM transmission. Since direct up/down conversion is applied, I/Q imbalance at the transmitter and receiver need to be compensated. We demonstrate the effectiveness of the adopted compensation algorithms by applying them both in off-line and real-time experiments. They perform well under a wide range of conditions in the presence of carrier frequency offset. The hybrid scheme mitigates the transmitter and the receiver I/Q imbalance in optical OFDM transmission.