System design and performance analysis of asymmetrically and symmetrically clipped optical (ASCO)-OFDM for IM/DD optical wireless communications

As the quantity of mobile communication devices, such as cellphones, tablets, and laptops, dramatically increase, the demand for high speed wireless service has been growing. Optical wireless communications (OWCs), which offer unlimited transmission bandwidth, have received a lot of attention and been studied in recent decades. They can be an effective alternative to radio frequency communications (RFCs) for indoor high speed data transmission. Intensity modulation direct detection (IM/DD) is a simple way to realize the transmission of optical wireless signals in an indoor environment. Information data streams are modulated into the intensity of optical carriers and transmitted by light emitting diodes (LEDs). At the receiver, the instantaneous power of optical signals can be directly detected by photodiodes. Multipath distortion, especially in an indoor environment, caused by reflection from walls or furniture, severely affects the transmission quality of optical signals. Orthogonal frequency division multiplexing (OFDM) is a promising modulation technique and has been widely used to combat inter-symbol-interference (ISI) resulting from multipath propagation in RFCs. So far, the technique of OFDM has also been successfully applied into IM/DD optical wireless systems. In this dissertation, the author focuses on the system design and performance analysis of a novel power-efficient scheme based on OFDM for IM/DD OWCs. This dissertation is divided into four main sections. In the first part, a novel power-efficient scheme, called asymmetrically and symmetrically clipped optical (ASCO)-OFDM, for intensity modulation direct detection (IM/DD) optical wireless systems is proposed. The average bit rate versus (vs.) normalized bandwidth and the optical power per bit of this novel scheme are expressed by a closed form, respectively. The symbol error rate (SER) performance is investigated when optical signals are transmitted in a flat fading channel. Simulation results show that this proposed scheme can achieve better performances in terms of both power efficiency and symbol error rate (SER) when the optical power of transmitted signals is limited. In the second part, an improved receiving technique is applied into the conventional receiver of ASCO-OFDM to improve the SER performance. This technique can explore and reuse some useful information hidden in the received signals. The detection procedure is described in detail and the improved SER performances are presented for different constellation cases. In the third part, the information rates of ADO-OFDM and ASCO-OFDM are obtained for an additive white Gaussian noise (AWGN) channel with an average transmitted optical power constraint. In the last part, this novel power efficient scheme, ASCO-OFDM, is extended into two-dimensional (2D) IM/DD optical wireless systems. The theoretical analysis and simulation results show that this technique not only achieves high average bit rate, but reduces the Peak-to-average power ratio (PAPR) as well

Multi-scale, multidisciplinary analysis of mass-transport complex (MTC) emplacement and structure

Mass-transport complexes (MTCs) refer to seismically-chaotic deposits generated by large-scale gravity flows, and form one of the most sedimentologically- and seismically- distinctive depositional elements in deep-water depositional systems. This thesis aims to provide a novel approach to the study of multi-scale MTCs in (i) a salt-confined minibasin, and (ii) a salt-free, relatively un-confined basin, focusing on MTC morphology, kinematic indicators, petrophysical properties and flow process. To achieve these aims, I use high-quality seismic reflection datasets, and borehole data from the northern Gulf of Mexico and offshore NW Australia to investigate: (i) the geometries, spatial distribution and kinematic indicators of MTCs; (ii) the complex interactions between the occurrence of MTCs and salt halokinesis; (iii) the petrophysical properties and sealing potenials of MTCs; and (iv) the detailed formation process of giant coherent blocks in MTCs. The first technical chapter presents a generic model that emphasises the dynamic nature of minibasin evolution, and how MTC emplacement relates to halokinetic sequence development. The second technical chapter investigates the petrophysical properties of MTCs, especially their basal shear zones. The third technical chapter investigates a submarine spread deposit that contains giant coherent blocks in a low gradient, basin floor setting. For hydrocarbon exploration, the results contribute to understanding the sealing capacity of MTCs. This includes valuable methodologies that combine seismic reflection and petrophysical data to recognise and predict the most compacted and least permeable intervals (the basal shear zone) in MTC. The recognition of the basal shear zone could help the offshore drilling community to optimise well planning and borehole stability. In terms of geohazard prevention, a better understanding of the formation of the blocks in MTCs will provide crucial information for where future failure might occur and help to reduce the damage of associated with MTCs to the offshore infrastructures (e.g. pipelines and drilling rigs).Open Acces

Essays on Economic Development

My thesis consists of three chapters on economic development. Chapter 1: The research on the catch-up process of a developing economy focuses on the role of foreign technology transfer and the importance of domestic technology transfer lacks study. We study the trends of expenditures on innovation, foreign technology transfer, and domestic technology transfer. During the transition period of China from 1998 to 2007, the expenditures on innovation and domestic technology transfer of Chinese firms in the manufacturing sector grow two times faster than the expenditure on foreign technology transfer. Furthermore, the estimated productivity at the firm level shows that rapid productivity growth is accompanied by decreasing productivity dispersion. The productivity dispersion has decreased by 39% in the same period. I document several empirical facts at the level of the industry. First, innovation is positively correlated with relative productivity. Second, the expenditure on domestic technology transfer increases in relative productivity and the growth rate of relative productivity is positively correlated to the expenditure on domestic technology transfer. Chapter 2: I develop a theory in which firms endogenously choose one of three mutually exclusive methods to increase productivity: innovation, foreign technology transfer, and domestic technology transfer. Domestic technology transfer offers firms with low productivity a chance to become highly productive by meeting highly productive domestic peers. Domestic technology transfer leads to faster growth of productivity and a greater number of firms with high productivity. The productivity growth in China makes more Chinese firms choose to innovate or learn from domestic peers in a dynamic environment. Thus, the expenditures on innovation and domestic technology transfer increase faster. In our model, firms with low productivity adopt foreign or domestic technology and grow faster than highly productive firms. This results in the decreasing productivity observed in the data. We use the simulated method of moments to estimate the key parameter values of the transition model. Our model fits data well. After checking the model fit, we conduct two experiments to answer the two questions mentioned at the beginning of our talk. In one experiment, domestic technology transfer is not allowed, and I find that domestic technology transfer contributes 30% of productivity growth and 31% of relative innovation expenditure growth. In the other experiment, we improve the domestic intellectual property and the policy changes significantly reduce both productivity growth and expenditure on innovation.Chapter 3: Our economic geography model features cross-country productivity, human capital, amenity and population differences, international trade, migration cost, and heterogeneous working and entrepreneurial skills. We compare welfare under baseline parameterization with a migration autarky counterfactual and the welfare gains of the US native residents from migration reform. The gains from migration are substantial, as high as trade gains, and natives in countries that received a lot of migration are much better off, at about 5% to 15%. Both the native entrepreneurs and workers benefit from migration while the entrepreneurs tend to gain twice as large as the workers. The welfare of the US native entrepreneurs and workers can increase by 5.1% and 1.6% by optimizing migration frictions and the population of the US increases by 14.7% under optimization

Orthogonal space time coding for IM/DD optical wireless communication with single carrier frequency domain equalization

Recently the development of optical wireless communication has received growing attention because of its unlimited bandwidth and simple implementation as compared to radio frequency communication application for indoor wireless data transmission. The objective of this thesis is to improve the performance of intensity-modulated direct- detection (IM/DD) optical wireless communications with single carrier frequency domain equalization (SCFDE) by using Orthogonal Space Time Code (OSTC). More attention has been paid to three SCFDE schemes due to their outstanding performances in practical cases. It is demonstrated that OSTC can be modified to be suitable to optical systems and provide an excellent enhancement to system performances. By applying 4x4 OSTC that was formally used extensively in radio frequency communications, the performance of optical wireless systems with SCFDE is to be improved remarkably. It is pointed out that there exist some differences of the application of OSTC in between radio frequency communications and optical wireless communications. The performances, such as Peakto-average Power ratio, complexity and Bit-error rate, are compared and discussed. Finally the optical system, which has the best performance, is proposed and extended with multiple LEDs and Photodiodes