Electrically Tunable Microwave Technologies With Ferromagnetic Thin Film: Recent Advances in Design Techniques and Applications

The rapidly growing wireless communication market has resulted in tremendous changes in the requirements and capabilities of radios to support numerous multiband wireless connections [1] . For example, the 850- and 1,900-MHz frequency spectrums are used to support the Global System for Mobile Communications (GSM) networks. New frequency bands of nearly 2,100 MHz have been adopted in 3G networks, and more frequency bands and spectrums are introduced in 4G systems, ranging from 600 MHz to 2.5 GHz [2] , [3] . Wireless communications have resulted in radio that, by 2010, had evolved from a single-mode, triple-band 2G system, to a triple-mode, nine-band (4 × GSM, 5 × Universal Mobile Telecommunications System with High-Speed Packet Access+), high-speed data-capable system. The trend continues today with rapid deployment of 5G and future 6G systems, driving new bands and modes of operation [4] . The frequency band of sub-6 GHz from 450 MHz to 6 GHz and the millimeter-wave spectrum from 24 to 52 GHz are employed in 5G systems to provide multifunctional services [5] . To support the continuous evolution of high data rates, as shown in Figure 1 , 5G mobile networks have been widely developed as an ideal on-demand connection solution for devices and users in urban areas [6] .