# Flexible Antennas for a Sub-6 GHz 5G Band:
## 1 Introduction  
Wireless communication has witnessed a huge transformation over the past few decades. The recent advancements in wireless communication need a huge number of devices connected to the internet which cannot be managed by the existing 4G LTE (fourth generation long term-evolution) system [1]. The fifth-generation (5G) communication system is a promising technology that fulfills the increasing requirements of data rates and also enables its integration with various services [2]. 5G can solve the issues that existed in the previous generation, in terms of bandwidth, QoS (quality of service), latency rate, and data transmission rates. The connection of a huge number of devices with high data rates, a low latency, more connectivity, and efficiency is achieved with 5G. The 5G network will be beneficial to many industries, including improved health care, smart cities, 3-D imaging, the IoT (internet of things), and more. The entire spectrum of 5G is classified into mainly three frequency bands viz a low band ranging from 600 to 700 MHz, a sub-6 GHz band ranging from 3 to 5 GHz, and a millimeter-wave band ranging from 24 to 100 GHz [3; 4; 5; 6]. Table 1 gives an overview of the 5G sub-6 GHz mid-band spectrum utilized in different countries [7]. A sub-6 GHz (Mid band) has become the main focus ofresearchers due to the limitation of mm-waves. Since mm-waves are high-frequency waves, they can only cover very short ranges. So, the implementation of mm-wave devices is expensive. Moreover, the implementation of sub 6-GHz could be achieved with the already existing 4G LTE bands thus making it an appropriate choice for 5G communication. One of the technological advancements with the outbreak of the fifth generation is in the area of flexible industry. Flexible devices are adaptable, lightweight, inexpensive, and environmentally friendly. Such flexible devices need an antenna to operate in particular frequency bands in order to give a good wireless connectivity. The general characteristics of a flexible antenna are given in Figure 1.  
The market for flexible electronics is increasing due to the high demand for wearable devices, especially health monitoring systems. Therefore research on several flexible antennas has increased in recent years in biomedical applications [8; 9].Flexible and conformal antennas provide good mechanical properties that make them suitable for wireless body area networks (WBANs) since continuous bending of the antenna should not degrade the performance of the antenna [10; 11]. Moreover, the human body also has different \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Countries** & **1 to 3 GHz Band** & **3 to 4 GHz Band** & **4 to 5 GHz Band** \\ \hline Korea & 2.30 to 2.39 GHz & 3.40 to 3.70 GHz, 3.70 to 4.00 GHz & \\ \hline Japan & & 3.60 to 4.10 GHz & 4.50 to 4.90 GHz \\ \hline China &
\begin{tabular}{c} 2.50 or 2.60 GHz \\ (B41 or n41) \\ \end{tabular} & 3.30 to 3.60 GHz & 4.50 to 5.00 GHz \\ \hline EU & & 3.40 to 3.80 GHz & \\ \hline UK & & 3.40 to 3.80 GHz & \\ \hline Germany & & 3.40 to 3.80 GHz & \\ \hline France & & 3.46 to 3.80 GHz & \\ \hline Italy & & 3.60 to 3.80 GHz & \\ \hline USA & 2.50 or 2.60 GHz & 3.45 to 3.70 GHz, 3.70 to 3.98 GHz & 4.49 to 4.99 GHz \\ \hline Canada & & 3.47 to 3.65 GHz, 3.65 to 4.00 GHz & \\ \hline Australia & & 3.40 to 3.70 GHz & \\ \hline India & & 3.30 to 3.60 GHz & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Sub-6 GHz mid-band spectrum. Reprinted from ref. [7]. Figure 1: Characteristics of a flexible antenna. dimensions and curvatures in different parts of the body. As a result, a flexible antenna plays a key role in this regard, as it must bend to match the curve of the human body when positioned in various places of the body [12]. Recently, the flexible antenna has marked a significant role in military applications. An all-textile cavity-backed substrate integrated waveguide (SIW) antenna is proposed for military applications in [13].