Theoretical Analysis And Study On The Signal Processing In Space-time Four-dimensional Antenna Arrays

With the rapid development of modern electronic technology, antenna arrays which act as the sensors for various wireless systems usually operate in very complex electromagnetic environment. In order to effectively suppress interferences received by the sidelobes of antenna arrays, low sidelobe levels (SLLs) are generally required in wireless systems. However, it is difficult for conventional antenna arrays to achieve low/ultra-low sidelobes due to the quite high dynamic range ratios of amplitude excitations and stringent requirements on various error tolerances. If an additional degree of design freedom, time, is introduced into conventional phased arrays, thus forming the four-dimensional (4-D) antenna arrays, the low/ultra-low sidelobes can be realized with lower dynamic range ratios of amplitude excitations by the time modulation. Furthermore, due to the introduction of time freedom, there are some new properties in 4-D antenna arrays which can not be obtained in conventional antenna arrays. The studies of this thesis are mainly concentrated on the theoretical analysis, design and optimization, signal analysis and processing, and applications in radar systems of the 4-D antenna arrays. The thesis is composed of the following five parts.1. The research background of the 4-D antenna arrays is reviewed. First, the methods of synthesizing low sidelobe patterns in conventional antenna arrays are introduced, which is to illuminate the difficulties and limitations of conventional antenna arrays on the suppression of SLLs. Next, basic theories of the 4-D antenna arrays are reviewed, including the"unidirectional/bidirectional phase center motion"and"variable aperture sizes"time modulation modes, the methods of synthesizing low/ultra-low sidelobe patterns, shaped patterns, and the suppression of sideband radiation in the 4-D antenna arrays.2. The time modulation technology is extended and applied to unequally spaced linear arrays and rectangular planar arrays to suppress the SLLs with lower dynamic range ratios of amplitude excitations. Moreover, the differential evolution algorithm is adopted to minimize the sideband radiation in the 4-D antenna arrays. 3. Based on the theory of the 4-D antenna arrays, the influences of time modulation on amplitude and phase weighting are studied, and a type of phase weighting technique in the 4-D antenna arrays is proposed. By appropriately designing time modulation parameters, the electronic beam steering can be realized in 4-D antenna arrays without using phase shifters. In addition, various shaped patterns such as flat-top pattern and cosecant-squared pattern can also be synthesized by the proposed technique. Due to the fact that phase shifters are not used in the antenna arrays, this technique can be used to simplify the complexity of feed network and reduce the cost of antenna arrays.4. The array signal processing technologies which are widely used in conventional antenna arrays are applied to the 4-D antenna arrays to realize the estimation of source numbers, direction of arrival of far-field signals and adaptive beamforming. Firstly, multiple beams can be obtained in 4-D antenna arrays by using the time modulation technique. Based on the data received by the multiple beams, the source number and the direction of arrival can be determined by relative array signal processing methods, which show better performances as compared to those of conventional antenna arrays. Secondly, by using time weighting instead of amplitude/ phase weighting or using the beamspace methods, adaptive beamforming algorithm can be applied to 4-D antenna arrays to suppress interference signals and noises effectively.5. The signal transimission in 4-D antenna arrays is studied, and the application of the 4-D antenna arrays to the pulsed Doppler radar is proposed. With the operation system of the pulsed Doppler radar, the waveform in time domain, spectrum, and average power losses caused by the time modulation are studied.The design and signal processing in the 4-D antenna array are studied systematically and deeply in this thesis. The simulated and some measured results validate the methods of pattern synthesis or signal processing in the 4-D antenna arrays, which show better performance as compared to conventional antenna arrays. The studies of this thesis demonstrate that the 4-D antenna array has great predominance and potential in practical engineering, and it will be extended in future research and engineering application.