# Compact N-Band Tree-Shaped Multiplexer-Based Antenna Structures for 5G/IoT Mobile Devices
## 2 Related Work  
In this way, several authors have tested the feasibility of reducing the number of antennas in each equipment and thereby saving space. In [2] a structure based on a dual-band slot is proposed and is designed to operate at 28/38 GHz. Along with the operating frequencies, this antenna exhibits adequate bandwidth values, as well as adequate gains for incorporation in a 5G mobile device. Despite the interesting results, this work lacks measures and the authors end up defending their proposal by comparing CST with HFSS. In [4] another dual-band antenna in a massive MIMO arrangement is described. This structure is based on a series-fed antenna array and one can denote high gains (of more than 12 dB at each band) and good radiation patterns, however, once again, only simulation values are shown. Figure 1: Single N-Band Antenna Shared by Multiple Frontends. On the contrary, in [5] the measurement results support the work on a substrate integrated waveguide (SIW) antenna array for the Ka-band. There, authors implemented a linear array to improve the overall antenna's performance and they were able to achieve satisfactory bandwidths (lower than 5%) and proper gain measurements (around 5 dBi). The authors claim that by using the presented structure, multi-band antennas can be designed, but no implementation is shown to prove this statement. Additionally, a dual-band dual-circularly-polarized antenna operating in the Ka-band is presented in [6]. The work is supported by simulated and measured results, however, a mediocre correspondence can be denoted between simulations and measurements. The gain, both in the uplink and downlink bands, is improved when the radiating element is implemented into an array, however measurements regarding axial ratio are poor, the complexity is huge and both the antenna's scalability and the robustness are questionable. When it comes to tri-band antennas, neither the microstrip patch antenna presented in [7] nor the antenna array with defected ground seen in ref. [8] show any measures confirming their respective simulations. Both structures have resonances in the mmWaves region, along with suitable bandwidths and gains. In [7] the efficiency values shown are quite high, whereas in [8] these values are not mentioned. Additionally, a scalable structure can be found in [9]. It proposes a compact dual-band and small slot antenna without compromising its performance. Measurements show good correspondence with simulations, as well as good gain, considering the typical values for slot antennas. Apart from these good results, one could argue that the major contribution of the work is its scalability, since authors defend that by adding more slots to the structure the desired multi-band behavior can be achieved. Nevertheless, the structure proposed operates at 2.4/5.2 GHz, very low frequencies for the 5G context. Recently, an aperture-sharing integration methodology implementing a 3.5/28 GHz antenna with mmWave beam steering capability was proposed [10]. The main concept is to share the aperture of a linear 28 GHz array, comprised by four separately fed dipoles, with a 3.5 GHz dipole antenna. Favorable results were obtained regarding a stable mmWave beam at different scanning angles, meanwhile with broad impedance bandwidths in both operating bands (over 20%). Nevertheless, only one of the resonating frequencies suits the mmWave spectrum region and the overall size of the structure is close to the size of a single 3.5 GHz dipole antenna, which, in many of the future applications may be inappropriate. In [11] an interesting approach is conducted on using the half-mode substrate integrated waveguide (HMSIW) technique to design low-profile cavity-backed multi-band antennas. Authors designed single, dual and triple band structures to validate the multi band responses which indicated favorable results on the radiation patterns' stability and the front-to-back ratio.