# Efficient Implementation of Polymer Microwave Fiber Links Employing Non-Galvanic Coupling Mechanism
## 1 Introduction  
Modern communication concepts such as those required for IoT and fully autonomous vehicles demand high volumes of data transmission. The ever-improving semiconductor technology offers the opportunity for high-data-rate wireless and non-wireless systems utilizing higher carrier frequencies in the sub-THz frequency domain [1, 2, 3]. For wireline communication links, two major solutions exist: copper and optical. For very long-distance and very high-speed links, expensive optical fiber solutions prevail. For medium-long-range links and low to medium speeds, copper is the most cost-efficient solution.  
On the other hand, when it comes to low/medium distances and high data rates, the optical fiber becomes an expensive solution, whereas copper-based wired links are limited due to the increased impact of the skin effect at high frequencies [4]. Figure 1 clearly shows the limits of copper-based wired links, as a bit rate of 10 Gbps is impossible for a link longer than 5 m. Active copper cables can overcome the speed limitations [5]. However, they are hindered by energy efficiency limitations as the energy per bit rises over medium-distance links. The dielectric fiber waveguides proposed in recent years [6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22] provide an attractive and efficient solution, as shown in Figure 2.  
Polymer fibers are very cheap and easy to produce, while they exhibit dielectric losses that are significantly smaller than those of their copper counterparts for short- and medium-range distances. Operation in the sub-THz frequency band (0.1-0.4 THz) is feasible and efficient through the polymer waveguide, while the attenuation is in the 1-5 dB/m range [6]. The extremely high carrier frequency guarantees very broadband communication channels, and this, in combination with the low attenuation and dispersion properties of the polymerfibers, ensures high data rates (in the range of 10 Gbps and beyond). It is worth noting that PMF links are applicable not just in the field of telecommunications but also in remote sensing, as discussed in reference [20].  
A key issue in the implementation of a polymer fiber waveguide solution is the way in which the fiber is coupled to the signal source/sink, i.e., the IC RF frontend. Various solutions have been proposed for this, including a micromachined interface and a wireless interface [8; 9; 10; 11; 18]. In all cases, the setup is either complex or suffers from mechanical stability issues and/or poor electrical performance.  
In this paper, an overview of the performance of various polymer microwave fiber geometries is presented and evaluated first. Next, emphasis is placed on the wireless coupling solutions between the transceiver IC and the PMF. To this end, a compact, robust and very cost-efficient implementation is proposed, employing an antenna-in-package (AiP).  
This paper is organized as follows. Section 1 presents a short introduction regarding the need for polymer microwave fiber (PMF) links, as well as our proposed implementation. In Section 2, an overview of polymer microwave fibers is presented, where we discuss how the design factors of the PMF affect its performance. In Section 3, the design process of the proposed coupling structure is shown, along with the results of EM simulations.  
Figure 1: Bit rate vs. maximum cable length of a single copper wireline [5].  
Figure 2: Bit energy per meter versus the maximum reach of commercial and research electrical links (blue diamonds) and commercial optical links (red circles) [23].  
In Section 4, we demonstrate the measurements of the implemented coupling structure between the PMF and the IC. Lastly, Section 5 draws the conclusions.