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- Johansson, Mattias Erik, 1980
(author)
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Development and Use of Optical Remote Sensing Methods for Studies of Megacity Air Pollution and Volcanic Gas Emissions
- 2007
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Licentiate thesis (other academic/artistic)abstract
- Optical Remote Sensing Techniques offers the opportunity to, in real-time, obtain line averaged measurements of gases in the lower and upper atmosphere. These techniques easily offer themselves to automatization, which increases their usability for e.g. risk-assessment of remotely situated volcanoes and routine monitoring of air pollution levels in urban areas.In 2007, a milestone in human history will be passed when half of the world’s population will be living in urban areas. Of these approximately 300 millions will be living in Megacities, defined as urban agglomerations with more than 10 million people. These large urban settlements cause problems with air pollution, problems worsened by the use of low-efficiency combustion techniques. At the same time, approximately 500 million people are living near active volcanoes. People strongly affected by the risk these volcanoes pose on their surroundings and by the emissions of sulphuric and halogenic gases.This thesis deals with the development and use of one Optical Remote Sensing technique, Differential Optical Absorption Spectroscopy (DOAS), for studying air pollution in Megacities, applied to México City and Beijing, and for monitoring of emissions of volcanic gases. A measurement technique for determining the speed and altitude of a gas plume using a DOAS-based instrument has been developed. Also, a method for creating tomographic reconstructions of the gas concentration in a cross section of a gas plume has been developed together with a novel instrument for performing fixed point automatic measurements on gas plumes.
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| 3. |
- Olsson, Mattias, 1977-
(author)
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Contributions to Frequency Offset and Time Delay Estimation
- 2006
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Licentiate thesis (other academic/artistic)abstract
- The demand for reliable high rate and efficient communication is ever increasing. In this thesis we look at two different problems in such systems, and their possible solutions.In recent years orthogonal frequency division multiplexing (OFDM) has gone from a promising data transmission technique to become a mainstream technique used in several current and future standards. The main attractive property of OFDM is that it is inherently resilient to multipath reflections because of its long symbol time. However, this comes at the cost of a relatively high sensitivity to carrier frequency offsets (CFOs).In this thesis we present a technique for CFO estimation in OFDM systems that is based on locating the spectral minimas within so-called null or virtual subcarriers embedded in the spectrum.~The spectral minimas are found iteratively over a number of symbols and is therefore mainly useful for frequency offset tracking or in systems where an estimate is not immediately required, such as in TV or radio broadcasting systems. However, complexity wise the estimator is relatively easy to implement and it does not need any extra redundancy beside a nonmodulated subcarrier. The estimator performance is studied both in a channel with additive white Gaussian noise and in a frequency selective channel environment.A goal for many years has been to be able to implement as much as possible of a radio system in the digital domain, the ultimate goal being so called software defined radio (SDR). One important part of an SDR receiver is the high speed analog-to-digital converter(ADC) and one path to reach this goal is to use a number of parallel, time-interleaved, ADCs. Such ADCs are, however, sensitive to sampling instant offsets, DC offset and gain offset.This thesis also discusses iterative time-delay estimators (TDEs) utilizing adjustable fractional-delay filters. The TDEs could for example be used to estimate and calibrate the relative delay between the ADCs comprising the time interleaved ADC. TDEs using a direct correlator and an average squared difference function are compared. Furthermore, an analysis of the effects of the batch length dependence is presented.
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