No abstract

Fractal structure is a unique geometry that can be seen in many objects in nature, such as clouds, coastlines, DNA, trees, and even pineapple. This structure has manifold geometries, self-similarities, and space-filling properties. Due to these properties, fractal geometries are preferred to miniaturize an antenna in wireless communications. There are many cases that require a small compact antenna, including in-body communications. In this article, we present a review of the recent trends and advancements in fractal antenna research, especially in the miniaturization of implantable antennas for in-body communications. The review is derived from articles that are gathered from online libraries such as IEEE, PubMed, Nature, MDPI, Elsevier, and Google Scholar. As a result, we have collected more than 60 articles related to fractal-implantable antenna and in-body communications. Indeed, many researchers have proposed an implantable compact antenna with fractal geometries in the last decades. Fractal geometry allows a longer electrical length to be routed in a smaller area of the antenna. However, several things remain challenging in designing a fractal antenna, including bandwidth, fabrication complexity, and intercell interference.

Intelligent Reflecting Surfaces (IRS), a planar array consisting of a large number of low-cost and low-complexity metal patch elements, is predicted to be among the key technologies in future wireless communications. This is mainly due to its capability to improve wireless energy and spectrum efficiency by reconfiguring the wireless environment in a smart and flexible way. IRS is capable of shifting the phase, amplitude, frequency, or even polarity of the impinging electromagnetic (EM) wave. However, only the phase-shift type of IRS is widely adopted in most scenarios. In this paper, we exploit the property of such IRS to directly modulate an incident EM wave with phase shift keying (PSK) modulation. We consider one transmit antenna that transmits a continuous single-tone electromagnetic (EM) wave, which is then modulated by the IRS and reflected toward the receiver. In addition to the theoretical explanation, we present simulation results with different modulation orders. Moreover, we validated the simulation results by experiment. Furthermore, we also proposed a beam scanning method to obtain IRS patterns to steer the beam towards the desired direction in an adaptive manner. One optimal pattern that maximizes the received power is then selected from the beam scanning codebook. By doing so, a wireless path strength can be significantly improved.