Study Of Reconfigurable Antennas And Their Applications In MIMO Systems

Reconfigurable antennas (RAs) are the frontier of the current antenna researchand excellent antenna candidates for future advanced wireless communicationapplications. The basic structure of a RA is similar to a conventional antenna; however,by using electrical, mechanical or other means to change the fundamental operatingmechanism of an individual radiator, the frequency and radiation characteristics of theRA can be varied significantly. This makes a single RA equal to multiple antennas withfixed functionalities. Due to the performance diversity of the RAs, they are capable ofnot only adapting with the usually fast changing communication environments, butalso reducing the number of antennas in a single wireless platform. Nevertheless, it is agreat challenge to reconfigure one characteristic of an antenna, such as frequencyresponse, without affecting others (polarization or radiation pattern), since thefrequency response and the radiation characteristic of an antenna are always highlyrelated. Furthermore, for the RAs using electronic components to achievereconfigurability, the design of an effective dc bias network that has limited influenceon the antenna performance is another issue for antenna engineers.In this dissertation, a thorough research is presented on RAs in terms of frequency,polarization, pattern, and compound frequency and polarization RAs. Several novelRAs and dc bias networks are proposed, prototyped and measured. In addition, aliterature review reveals that although a considerable number of papers on RAs havebeen published, much fewer reports on the application of RAs in wirelesscommunication systems have been shown. In order to examine the practical applicationof RAs, the proposed polarization and pattern RAs are incorporated separately in a2×2multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing(OFDM) system to demonstrate the improvement in channel capacity.Specifically, the major contribution of this dissertation is summarized as follows:1. Motivated by the lack of wide band frequency tunable antennas, a method offrequency reconfigurability based on quasi-Yagi folded dipole or dipole antenna isproposed. This method can realize wide band frequency agility while keep theradiation characteristics stable across the entire tunable frequency range. In addition,the folded dipole allows the dc biasing network to be located far away from theradiating elements of an antenna. Therefore, the interference from the bias lines on the antenna performance can be greatly reduced. This biasing method can bring significantbenefits for the millimeter wave frequency reconfigurable antenna design since atmillimeter wave band the biasing circuits around the radiating elements will affect theantenna performance considerably. Based on this method, two frequency RAs in themicrowave band including one frequency continuous tuning antenna and onefrequency discrete tuning antenna are designed. Moreover, a frequency quasi-Yagifolded dipole RA in millimeter wave band and a frequency reconfigurable quasi-Yagidipole array are developed.2. A polarization reconfigurable U-slot microstrip patch antenna is proposed.Most of the previous work for polarization reconfigurability concentrates on switchingbetween right-hand circular polarization (RHCP) and left-hand circular polarization(LHCP). Relatively few antennas have been presented that can switch between linearand circular polarization with a wide overlapping frequency bandwidth and a3dBaxial ratio bandwidth. In order to solve this problem, a polarization reconfigurableU-slot antenna is proposed that has the capability to switch between circular and linearpolarizations. The antenna is compact and employs a very simple bias network. It hasthe further advantages of wide frequency and polarization bandwidth.3. A pattern reconfigurable U-slot patch antenna using PIN diodes is proposedfor MIMO communication systems. There are two requirements for patternreconfigurable antennas that serve for MIMO systems. The first is the number ofradiation patterns that can be reconfigured should be as many as possible. The secondis the correlation coefficient between different patterns should be as low as possible.The proposed pattern reconfigurable antenna can switch between three complementaryradiation patterns, and the correlation coefficients between them are very small.Moreover, the overlapping frequency bandwidth is wide and the antenna does not needany bias network to control the PIN diodes.4. A reconfigurable microstrip patch antenna with compound frequency andpolarization agility is proposed. The ultimate goal of reconfigurable antenna design isthe ability to independently reconfigure the operating frequency, polarization, andradiation pattern of an antenna. However, relatively few papers have been published onrealizing the compound frequency and polarization agility. A singly-fed compoundpolarization and frequency reconfigurable microstrip patch antenna with a simplestructure is proposed. It can switch between the horizontal, vertical or45°linearpolarization, with independent frequency tuning for each polarization state. The tuningratio is1.67for either the horizontal or vertical polarization, and it is1.4for the45° linear polarization.5. In order to demonstrate the RA’s capability of enhancing the MIMO systemcapacity, the polarization RA proposed in the second part and the pattern RA proposedin the third part are incorporated separately in a2×2MIMO-OFDM system. Twodifferent channel matrix normalization methods are employed to separate the effect ofsubchannel correlation on the system capacity from the effect of antenna gain.Therefore, the enhancement of the system capacity derived only from the polarizationor pattern diversity can be explicitly shown. For the MIMO-OFDM system withcircular polarized reconfigurable antennas, the capacity of the system with hybridpolarized antennas is compared to that with the same polarized antennas. For theMIMO-OFDM system with pattern reconfigurable antennas, the capacity of the systemwith the proposed reconfigurable antennas is compared to that with omnidirectionalantennas. In a real time MIMO-OFDM channel measurement, the improvement of thechannel capacity by using RAs is described and analysed, and the factors that canaffect the extent of the enhancement are investigated.