Studies On The Theory And Basic Applications Of Space-time Four-dimensional Antenna Arrays

Antenna arrays are widely used in modern wireless systems such as communication and radar systems, which have a significant impact on the overall performance of these wireless systems and even lead the development of them. Antenna arrays have many advantages, such as achieving low/ultra-low sidelobe, fast beam scanning, fast beam forming, multi-beam forming and adaptive nulling. However, conventional antenna arrays require precise amplitude and phase controlling in order to achieve the above functions. In actual engineering applications, the feed network of a conventional antenna array is very complicated, and it is not easy to realize precise amplitude and phase weights. These problems greatly increase the difficulty in designing and fabricating such a conventional antenna array, resulting in significantly higher cost.The conventional antenna array can be considered as a radiation source distributed in a three-dimensional space. The four-dimensional antenna array is a new type of antenna array, which is formed by introducing the time as the fourth dimension into a conventional antenna array. It can be used to achieve amplitude and phase weighting by time-modulation, without using attenuators or phase shifters as in a conventional antenna array, thus simplifying the design of the feed network. As compared to the conventional antenna array, the four-dimensional antenna array has many advantages, such as achieving low sidelobe patterns with uniform amplitude and phase excitations, beam scanning without phase shifters, multi-beam forming, direction-finding in a wide-angle range, directional modulation of the transmitted signal. In the hardware configuration, the four-dimensional antenna array is configured by adding a group of high-speed RF switches to a conventional antenna array. By controlling the switches, the radiation characteristics of the four-dimensional antenna array can be adjusted quickly and accurately, with great design flexibility.As a frontier technology in antenna arrays, the four-dimensional antenna array has received wide attention in the past decade. In this dissertation, the basic theory and application of the four-dimensional antenna array is studied, including time- and frequency-domain analysis of the four-dimensional antenna array, estimation of the array SNR performance, time modulation methods, pattern synthesis methods, and directional modulation technique based on the four-dimensional antenna array. The main content of this dissertation are summarized as the following five parts.1. The basic working principle of the four-dimensional antenna array is analyzed in the time- and frequency-domain. The radiation pattern, radiated signal, and directivity of a four-dimensional antenna array are analyzed, and the relationship between the results obtained in the two domains is explained. Combining the spectrum of a time-modulated signal, the bandwidth of a signal received or transmitted by the four-dimensional antenna array is analyzed. Meanwhile, a low cross-polarization broadband printed dipole antenna with parasitic strips is designed. Based on printed dipole elements, a splitter, phase shifters, high-speed RF switches, and a controlling circuit board, a four-dimensional antenna array system is built up. The measurement method of a four-dimensional antenna array is also introduced, and the above-mentioned analysis results in the time- and frequency-domain are verified by experiments.2. The signal-to-noise ratio performances of signals received by a four-dimensional antenna array are analyzed. A time-domain expression for calculating the directivity of a four-dimensional antenna array is also proposed. Considering the low efficiency of the feed network in a four-dimensional antenna array with SPST switches, a four-dimensional antenna array with SPDT swtches is proposed, with an improved efficiency of the feed network. The proposed method for calculating the signal-to-noise ratio is verified by BER simulation of BPSK signals received by the four-dimensional antenna array. Combining with the processing of a signal received by the four-dimensional antenna array, the effect of time modulation on the received signal spectrum is anylyzed. By comparing the spectrum of a signal received by an element and the spectrum of a signal received by the full array, the proposed method for calculating the signal-to-noise ratio is validated further.3. A flexible time modulation method, named as subsectional optimized time steps, is proposed. The proposed time modulation method is applied to the pattern synthesis in a four-dimensional linear array and two planar arrays, and low sidelobe and low sideband patterns are achieved when the elements are excited with uniform amplitudes and phases. As compared to any previous time modulation method, the proposed method has greater design flexibility. When synthesizing the same low sidelobe pattern, the proposed method can get much lower sideband level. The measured low sidelobe and low sideband pattern of a 16-element four dimensional antenna array demonstrates the feasibility of the proposed method.4. Two pattern synthesis methods of four-dimensional antenna arrays are proposed. The methods combine commercial full-wave electromagnetic simulation software with an optimization algorithm. Low sidelobe patterns of several actual array antenna models are synthesized. The first method optimizes time sequences using an optimization algorithm, then transforms optimized time sequences into amplidute and phase excitations, and finally transfers them into the simulation software through an interface program. The first method is verified by the numerical results on a linear array and a conformal array. The second method firstly gets quickly the active element pattern data of each element, and then synthesizes the pattern by summing up the element patterns with each factored their corresponding amplidute and phase excitations. The second method is verified by the numerical results of a plannar array that is also full-wave simulated.5. A directional modulation technique based on the four-dimensional antenna array is proposed. It can make the four-dimensional antenna array tranmit a correct signal in a desired direction, and transmit distorted signals in non-desired directions. The technique is validated through the simulation of analog/digital signals transmitted by the four-dimensional antenna array. A four-dimensional antenna array experimental system is built up, and the waveform and spectrum of the transmitted analog signal is tested to validate the proposed techique. In order to further enhance the effect of the directional modulation technique, random time modulation methods are proposed. Finally, the direction modulation angular range of the four-dimensional antenna array is analyzed and evaluated.