Research On The Antenna And The Spatial Domain Of The Channel For Multi-antenna Communications

Multiple-antenna technologies, such as those applied in the antenna diversity, smart antenna system and the Multiple-Input Multiple-Output (MIMO) wireless systems, exploit the spatial resources to improve the reliability and capacity of the systems, and currently become an active area of research in wireless communications. Especially, the MIMO systems, which using multiple antennas at both the transmitting and receiving ends to increase the channel capacity considerably, have been considered one of the key techniques for the next generation wireless communications. This dissertation mainly focuses on the antenna subsystems and the spatial characteristic of wireless channel, which determining the spatial resource that can be utilized to improve the performance of communications. Most of the researches on multiple-antenna systems are mainly focusing on the communication signal processing, including the spatial-tempol signal processing, the equilibria and modulation, etc. Furthermore, the antenna subsystem and wireless channel are included in a baseband model for multiple-antenna systems. The baseband model is convenient for the analysis of signal processing. However, the physical phenomenon and physical restriction for the spatial resource are concealed by this model. In this thesis, the detail effects of antenna and the spatial characteristic of channel will be investigated using the theory and method of electromagnetic. In fact, this interdisciplinary character is a major theme and contribution of the thesis, since most of the literatures on the analyzed topics do not make this connection. In a sense, this thesis try to bridges the gap between the complicated wave propagation environment of radio signals and the idealized baseband models mostly used when analyzing multiple-antenna systems.Focusing on the two topics of antenna and wireless channel, four parts of research areas, as listed below, will be investigated systematically and thoroughly in this dissertation.Firstly, the mutual coupling between antennas and its effects on the multiple-antenna systems will be studied. A new mutual couping compensation method is proposed first. Then its rigorous derivation from electromagnetic boundary conditions, based both on reciprocity theorem and the concept of reaction, is presented. Using the proposing mutual coupling compensation method, the effects of mutual coupling on multiple-antenna systems are investigated and they show that the effects of mutual coupling on spatial correlation depend on the spatial channel parameters. Since the effects of the antenna, wireless channel and the RF front-end are coupled, a unify channel model is proposed in the last. This proposed model uses the moment methods to simulate the near-field environment accurately; and applies generalized matrix Ohm's law to connect the near field and circuit model; while employs the classical wireless channel model based on far-field approximation to connect the far-fields of the multi-antenna in transmitter and receiver respectively.Secondly, the polarization resource, which is also an important spatial resource, will be investigated thoroughly. The performance of polarization deverstiy in the small terminal is investigated first. The multi-polarization cases are investigated next and they show that the polarization domain can increase channel capacity significantly. Aiming to improve the power imbalance of polarization diversity, circular polarized antenna is proposed to be used in the MIMO communications. A diversity pre-processing scheme is proposed in the last. This method can be used to obtain circular polarization diversity using dual-polarized antennas.In the third part, the channel capacity given fixed transmission volumes is investigated. Firstly, the effects of antenna are neglected and the eigenmode method is applied to investigate the maximum spatial resource which the multiple-antenna system can be utilized. The connection between the conventional information theory and the eigenmode method is also presented. Then, the mutual coupling is considered due to its effects will become serious when the spacing of antennas is closer and closer.Finally, a novel reconfigurable microstrip antenna with the capability of switching among four polarization senses is proposed. Its applications in the multiple-antenna systems are also presented. An analysis framework for assessing the performance of antenna selection diversity reception system with the proposed reconfigurable antenna is presented for the first time. Based on this proposed reconfigurable antenna, a compact antenna selection scheme with low complexity is also proposed. Two antenna arrangements are considered respectively.These above researches provide powerful guarantee for precise analysis and design both of space-time coding/decoding and modulation/demodulation, and establish a solid theoretical basis for providing both important insights into multiple-antenna antenna design and physical restriction on the multiple antenna techniques.