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- Li, Hui
(författare)
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Decoupling and Evaluation of Multiple Antenna Systems in Compact MIMO Terminals
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Research on multiple antenna systems has been a hot topic in recent years due to the demands for higher transmission rate and more reliable link in rich scattering environment in wireless communications. Using multiple antennas at both the transmitter side and the receiver side increases the channel capacity without additional frequency spectrum and transmitted power. However, due to the limited space at the size-limited terminal devices, the most critical problem in designing multiple antennas is the severe mutual coupling among them. The aim of this thesis is to provide compact, decoupled and efficient multiple antenna designs for terminal devices. At the same time, we propose a simple and cost effective method in multiple antenna measurement. All these efforts contribute to the development of terminal devices for the fourth generation wireless communication. The background and theory of multiple antenna systems are introduced first, in which three operating schemes of multiple antenna systems are discussed. Critical factors influencing the performance of multiple antenna systems are also analyzed in details. To design efficient multiple antenna systems in compact terminals, several decoupling methods, including defected ground plane, current localization, orthogonal polarization and decoupling networks, are proposed. The working mechanism and design procedure of each method are introduced, and their effectiveness is compared. Those methods can be applied to most of the terminal antennas, reducing the mutual coupling by at least 6dB. In some special cases, especially for low frequency bands below 1GHz, the chassis of the device itself radiates like an antenna, which complicates the antenna decoupling. Thus, we extend the general decoupling methods to the cases when the chassis is excited. Based on the characteristic mode analysis, three different solutions are provided, i.e., optimizing antenna locations, localizing antenna currents and creating orthogonal modes. These methods are applied to mobile phones, providing a more reliable link and a higher transmission rate, which are evaluated by diversity gain and channel capacity, respectively. In order to measure the performance of multiple antenna systems, it is necessary to obtain the correlation coefficients. However, the traditional measurement technique, which requires the phase and polarization information of the radiation patterns, is very expensive and time consuming. In this thesis, a more practical and convenient method is proposed. Fairly good accuracy is achieved when it is applied to various kinds of antennas. To design a compact and efficient multiple antenna system, besides the reduction of mutual coupling, the performance of each single antenna is also important. The techniques for antenna reconfiguration are demonstrated. Frequency and pattern reconfigurable antennas are constructed, providing more flexibility to multiple antenna systems.
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| 2. |
- Li, Hui
(författare)
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Dynamic Fetal and Placental Circulatory Changes during Uterine Contractions
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The best mode of delivery for fetuses with intrauterine growth restriction (IUGR) and umbilical artery (UA) blood flow changes is not well elucidated. Uterine artery, fetal UA, middle cerebral artery (MCA), and venous blood flow changes during uterine contractions during an oxytocin challenge test (OCT) have not been observed systemically. AIMS: To evaluate outcome in IUGR with UA blood flow changes planned for vaginal delivery after a negative OCT, and to investigate uterine artery, fetal UA, MCA, and venous blood flow during OCT with Doppler ultrasound. RESULTS: 1. A positive OCT occurred twice as often in IUGR with abnormal UA blood flow as in IUGR with normal flow. In OCT negative cases allowed a trial of labor, the vaginal delivery rate was not different between the two flow groups. 2. Doppler velocimetry of blood flow velocity waveforms (FVWs) during uterine quiescence, contractions and relaxations in the uterine arteries identified 11 different waveform shapes when recordings were systemically classified with regard to the mean flow velocity over the pulse cycle. 3. No different uterine artery blood flow changes during the OCT were found in cases with or without operative delivery for fetal distress (ODFD) or birth asphyxia. 4. The UA vascular flow resistance was significantly higher in OCT positive cases compared with negative cases during uterine contractions and relaxations, but not during basal measurements; No different UA blood flow changes during the OCT were found in cases with or without ODFD or birth asphyxia. 5. The vascular flow resistance in fetal MCA decreased during uterine contractions and relaxations compared with basal measurements both in OCT positive and negative cases, but OCT positive cases showed a more pronounced decrease. 6. Increased blood flow velocities and decreased impedance to flow occurred in fetal cerebral veins parallel to a decrease of vascular flow resistance in the fetal MCA during uterine contractions. CONCLUSIONS: OCT was just as good in the abnormal flow group as in the normal flow group to select suitable candidates for a trial of vaginal delivery. The proposed qualitative classification system may accomplish to cover a complete spectrum of different uterine artery FVWs, but uterine artery blood FVWs during the OCT seems have a limited clinical value in predicting pregnancy outcome. A vascular pathophysiological mechanism operating in OCT positive cases was disclosed only during the OCT, but an ‘OCT Doppler velocimetry?of UA blood flow seems to have a limited predictive value on neonatal outcome. A redistribution of the fetal circulation with a ‘brain-sparing?flow is a physiological phenomenon occurring during acute hypoxemia provoked by uterine contractions.
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