# Compact N-Band Tree-Shaped Multiplexer-Based Antenna Structures for 5G/IoT Mobile Devices
## 2 Related Work
Moreover, the radiation efficiency being higher than 80% at the operating frequencies is a quite encouraging result to test this concept of introducing U-shaped strips outside the aperture of an HMSIW cavity in the mmWaves region of the electromagnetic spectrum, since the tests made were at the C-band. In [12], an architecture whose main objective is to solve the bandwidth limitations of phased arrays was proposed. The suggested design includes five printed quasi-Yagi antennas, which should be placed in the upper edge of a mobile device. Their placement, and the orientation of the active element and the directors are crucial to solve such bandwidth limitations. With the suggested configuration and by not using phase shifters and simply switching the feeding to one of the quasi-Yagi elements it is possible to scan the desired areas. However, this switchable antenna system results in a physically larger setup, since only one antenna is used at a time, which when compared to the phased array is a disadvantage, as in these structures, antenna area can be saved, as the whole aperture is exploited. Another structure which explores alternative designs is presented in [13]. Considering the advantages of omnidirectional radiation patterns in communicating regardless of direction, a modified fork-shaped microstrip monopole antenna with a probe feed line shows wideband and multi-band characteristics. Here, the impedance bandwidth is improved by designing a dual-triangle portion of the ground plane, yet the resonant frequencies are quite low, suitable for example for the GSM band. Regarding the emerging MIMO systems, in [14] a MIMO antenna system for multi-band 5G (mmWave) and wideband 4G application is shown. This structure works at triple bands (28, 37 and 39 GHz) for 5G and the wideband (1.8-2.6 GHz) for 4G. Each one of the MIMO elements consists of a slot in the ground plane and two microstrip feeding ports in the top layer (the isolation is also enhanced by using a low pass filter). Indeed, this design can work as a tapered slot antenna for 5G, covering 27.5-40 GHz or as an open-ended slot antenna for 4G covering 1.8-2.6 GHz. However, as noted, in the 5G band one denotes wideband operation, instead of multiple resonances at the frequencies of interest. One of the major challenges of a multi-band antenna structures is its efficiency, a specification that gains further importance when operating in the mmWave region. The major contribution of this work is the microstrip feeding arrangement, which allows to section the antenna into \(n\) frequency-selective parts. In the literature, other techniques can be found to improve the feeding network's efficiency. In [15], an antenna array consisting of five radiating elements is designed and measured, operating at the 2-4 GHz range, achieving a beamwidth around 24 degrees. The frequency selective feeding network delivers the signal to the selected elements at \(2f_{0}\) and gradually switches the signal between elements as the frequency decreases to \(f_{0}\). The major advantage of this strategy is an almost constant beamwidth over a broad frequency range. A similar concept is presented in [16] where a six-element antenna array operating in 1.75-3.5 GHz frequency range is seen. In this report, the feeding network uses a directional filter where the adjustment of coupled-line sections allows for the flexible selection of transmission coefficients. This structure permits a constant beamwidth in an octave frequency range the signal is redirected from the center elements to the outer elements. In the end, this feeding network's achievements are reached at the expense of a single directional filter and equal split power dividers. More recently in [17] an interesting approach on multi-band filtering slot antennas is proposed. There, authors realize a duplexing and filtering antenna by integrating a multi-band antenna and the multimode resonator. The work is validated since three antennas were designed, fabricated and measured.