# Compact N-Band Tree-Shaped Multiplexer-Based Antenna Structures for 5G/IoT Mobile Devices
## 1 Introduction  
It is clear that the global tendency of exchanged data traffic is growing exponentially and, with it, the number of interconnected devices, pushing existing systems to their limits. In addition to the increasing traffic on each network, the variety of the services supported is increasing too, which naturally gives rise to great concerns about energy consumption. In this sense, 5G wireless systems must fulfil three main requirements: (i) have high throughput; (ii) serve many users simultaneously; and (iii) have less energy consumption [1], the latter being probably the biggest driving force of 5G. Associated with this, the high user mobility will force new antenna designs and new concepts to be implemented [2].  
In addition, migration to mmWaves becomes mandatory, because despite implying higher propagation issues, it is probably the most effective way to achieve the necessary bandwidth [3]. In these frequencies, new challenges arise, given the reduced dimensions, however, there is an inherent opportunity to produce compact solutions and an increasing demand for robust multipurpose dual-band or multi-band antenna systems for 5G applications. Today, mobile devices comprise a wide range of applications and features, most of them involving several communication frontends, and also multiple antennas, requiring space, which can create problems such as the coupling between them.  
A possible solution to combat the lack of available space resulting from this growing incorporation of communication systems, is to develop a single antenna, operating in each frequency band, and shared by the various communication systems. Figure 1 clarifies the scenario referring to a mobile device where a single antenna structure interacts with multiple radio-frequency frontends, which can be oneof the main characteristic nowadays, since even a common smartphone gathers multiple antennas, enabling the connection with the most varied services.  
There are several ways to design an antenna that meets these requirements. One is through the bandwidth, i.e. producing an ultra-wideband antenna, however, it this (obtaining a band that includes all the necessary frequencies) is an almost impossible condition, especially given the disparity of frequency bands used. In addition, isolation between bands and frontends would not be guaranteed, decreasing the efficiency.  
Another possibility is to develop antennas resonating in different bands, which are the commonly named multi-band antennas. This option consists of a process with a high degree of randomness, since in most cases this is done at the expense of deformations of the radiating structure, presenting little freedom to the control of the antenna's properties. There is also the group of reconfigurable antennas (in this case, in terms of frequency), although these antennas are much more complex because they are active and require additional control to select the operating frequency. In addition, these antennas use switches in the form of discrete elements, which consume energy. Due to all this, an alternative design is proposed.  
This paper is divided into 6 sections, starting with this introduction. Section 2 presents a careful revision of the related works, focusing on multi-band antennas and in frequency-selective feeding structures. Then, in Section 3 the design principle is presented, clarifying the concept proposed. Section 4 contains all methods and presents the structures developed, each in a different sub-section. The results, both measured and simulated can be found in Section 5, as well as the prototypes built. Finally, major conclusions can be found in Section 6.