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Theory, Computation and Application of Characteristics Modes

Periodic Reporting for period 1 - TECAM (Theory, Computation and Application of Characteristics Modes)

Reporting period: 2018-03-26 to 2020-03-25

In today's wireless communication systems, the antenna is one of the most critical components, as it enables the transmission and reception of signals in free-space. Antenna design is a key technique for system realization and marked improvement in the overall system performance. Conventional methods based simply on experience (intuition and smart guesses) and antenna theory (analytical models of antennas) become more and more unreliable. As a result, the common antenna design approach today is to choose a starting design from experience, which is then optimized via a time-consuming process of running many numerical simulations to adjust different design parameters without understanding physical insights of how the antenna operates.
Antenna analysis based on the Theory of Characteristic Modes (TCM) provides a set of feed-independent characteristic modes (CMs) with characteristic currents orthogonal to each other at the source region (nearfields) and characteristic fields orthogonal to each other at infinity (far-fields). Orthogonality allows the modes to be considered as separate or uncoupled, greatly simplifying both the mathematics and the concepts underlying antenna radiation. These modes can be controlled by setting a suitable feeding position, amplitude, and phase. Adjustment to antenna structure or feeding type controls the modes’ response, hence this information guide antenna parameter adjustment and optimization. The Theory of Characteristic Modes (TCM) now is the most popular and reasonably general antenna design methodology.
While the methodology promises to revolutionize antenna design, many of the key issues in the application have not been solved to realize the ideal vision. These are the issues that the project addresses:
Theoretical aspects – Relatively little attention was given to develop TCM for real materials compared to that for perfect electric conductors (PEC). However, many antenna-related problems involve materials that are both penetrable (dielectric objects and magnetic objects) and lossy. Yet, researchers have not successfully applied integral methods in extracting the CMs of inhomogeneous anisotropic dielectrics. The development of TCM based on integral methods in analyzing practical inhomogeneous anisotropic material is a primary and unfinished task, but this kind of material is indeed used in industrial antenna design.
Computational aspects –Regarding computational complexity, current strategies of CM analysis can only deal with small-scale problems. The capabilities of TCM in solving electrically larger problems are limited. However, when the antenna system is multi-scaled with multi-scale structures, the acceleration purely on mathematics regardless of the actual physical structure is less effective. Also, to restrict the condition number of the matrix, the minimum mesh size is used as the standard which leads to the sharp increase of the number of unknowns. Hence, to reduce the computational load in extracting CMs under multi-scale conditions is an urgent task.
Application aspects – In the massive multi-input multi-output (MIMO) technology for 5G cellular systems, base stations are equipped with hundreds to thousands of antennas to satisfy the need for high-speed and reliable wireless communication. One major challenge in designing massive MIMO systems is the design for a single element with the best performance. The lack of a suitable design and calculation method makes the implementation very costly. The development of TCM for application is, therefore, an urgent, critical and extremely promising task.
The project successfully developed a rigorous theory for inhomogeneous anisotropic materials by selecting appropriate basis functions and expanding the unknowns to construct and solve volume integral equations; developed and verified a new computational method of sub-structure CMs based on actual physical structures to achieve high efficiency and accuracy in addressing various multi-scale antenna problems; developed the application of TCM on the highly efficient design of multi-mode wideband antennas and multi-port MIMO antennas. So far one conference paper regarding the sub-structure CMs is accepted; one conference paper regarding inhomogeneous anisotropic materials is under review; two journal papers regarding multi-mode wideband antennas and multi-port MIMO antennas are under review; two journal papers regarding the application of sub-structure CMs are under draft revision.
Theoretical aspects: Low-ordered pulse functions and point matching method are successfully realized to construct the volume-integral-equations-based impedance matrix needed for the characteristic mode analysis for inhomogeneous anisotropic dielectric bodies. Taking the results given by the conventional method of moments (MoM) as a benchmark, the proposed method exceeds the conventional one in speed. Meanwhile, the extraction accuracy of the characteristic eigenvectors and eigenvalues can also be guaranteed and spurious modes can be effectively avoided. The indoor code package is eventually realized.
Computational aspects: The rationality of the sub-structure CM theory is demonstrated. A comparative study of sub-structure CMs and full-structure CMs reveals that the similarities and differences between the two types of CMs in specific conditions are largely dependent on the strength of the coupling between the sub-structure and the background. Further, the focus was on investigating the extent to which the sub-structure CM method can be used for multi-scale and large-scale practical antenna design problems. We concluded that the sub-structure CM method enables CM analysis of multi-scale and large-scale practical design problems, which could not be handled by the conventional full-structure method.
Application aspects: A wideband dual-resonance monopole-like patch antenna and a low-profile tri-polarized MIMO antenna are designed based on CM analysis, which is suitable for indoor wireless communications and highly integrated MIMO systems, respectively. The performance achieved by these antennas is currently the best by some measures.
After solving these problems, this TCM technology can be truly and more conveniently applied in industrial design and scientific research, including any existing wireless communication systems, such as personal mobile communication systems, satellite communication systems, and vehicular communication systems. The research of this project will bring great benefits, including but not limited to: the design process will be greatly accelerated, the design accuracy will be greatly improved, the design cost will be greatly reduced. At the same time, this methodology can be designed as software, bring business benefits, and provide valuable assistance in industrial work. More importantly, the innovation of methodology will bring new thinking and can be applied to new scenarios, 5G or even 6G hardware implementation. Manufacturers of the Internet of things, automobiles, telecommunications equipment, mobile terminals, medical electronics, satellite remote sensing, and other devices that involve the design of wireless devices will all benefit from the project.
Electric field distributions of the modes, and front and back views of the fabricated antenna
Mode current of the PEC bulk with two top subregions at three different frequencies
Modal current at the resonant frequencies of the two significant modes, and front and back views