Contents I. Introduction
As demands grow for faster data transmission, multiple high-band spectrums, such as 24, 28, and 39 GHz, are added to the fifth generation, i.e., 5G, mobile network to take an advantage of having a wider bandwidth than previous cellular technologies [1], [2]. Despite this advantage, higher path loss due to their millimeter wavelength is inevitable, which results in a short communication range [3]. To compensate for the huge path loss, mobile phones require a large number of antenna elements for higher directivity toward the direction of base stations [4]. However, there is not enough space for additional antennas because of following technical issues: first, most available spaces are already occupied by multiple antennas for various wireless communications technologies, such as the global system for mobile communication (GSM), long-term evolution (LTE), Wi-Fi, WiMAX, and Bluetooth [5]. Second, as the screen size becomes larger, the space for antennas is getting smaller because of slimmer bezels. Moreover, the radiation efficiency and matching bandwidth are significantly degraded, especially when the antennas are surrounded by a metallic enclosure [6]. As a solution to these issues, Park et al. [7] have proposed an approach of integrating antenna elements into a display panel by making the antenna structure optically invisible. Herein, we denote this approach as “display-integrated antennas” throughout this article for convenience. Since the screen area is usually greater than at 28 GHz ( in), we can expand territory for massive antenna integration by ensuring the optical transparency of over 80%. To provide more accurate and flexible design rules of the display-integrated antenna, empirical formulas have been proposed in [8] for characteristic impedance adjustment. In [9]–[15], several ways of fabricating optically invisible antennas have been discussed from various standpoints, and it is concluded that the approach with thin-metal mesh lines is the best solution for transparency and conductivity perspectives. Then, the remaining consideration is in the realization of broad bandwidth for display-integrated antennas to take the benefit of high-speed transmission. Desai et al. [16] have presented the design of optically transparent antenna patterned by indium tin oxide (ITO) on a thick glass substrate for target bands of 24, 28, and 39 GHz. Although it is well-known that the bandwidth can be improved with the use of thicker substrates, unfortunately, mobile phone manufacturers set a strict restriction that the increased thickness due to the antenna layer must be less than several hundreds of micrometers. More specifically, the thickness increase is restricted to be less than 200 for a 10-dB matching bandwidth of greater than 10.7%, i.e., Bandwidth GHz at 28 GHz. One of the popular solutions to the broad bandwidth without a compromise of substrate thickness is the use of artificial magnetic conductor (AMC) that exhibits multiband properties. Abbasi and Langley [17] have presented a multiband antenna associated with single-loop AMC structure that is designed to induce higher order currents for bandwidth improvement. In [18], multiband AMCs based on fractal unit cells have been studied to explore higher order resonances of the AMCs in detail. Despite their successful implementation, such higher order concepts are usually valid when the target resonances are integer multiples of the fundamental resonance, which is inapplicable in our approach.
