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.
