# Low-Profile Dual-Polarized Antenna Integrated with Horn and Vivaldi Antenna in Millimeter-Wave Band
## 1 Introduction  
Over the past few decades, 5G has become a topic that people are discussing and interested in. As 4G systems and technologies mature, the improvements people can make and the spectrum people can choose on 4G are becoming less. However, with the advances of video streaming media and mobile phones, wireless data will increase rapidly, and data explosion will be produced. Evidently, 4G networks cannot meet the demand which is brought by immense numbers of data. In addition to the huge number of data, the number of devices and data transfer rates will also show exponential growth. This requires engineers to come up with new directions, of which super-densification, millimeter wave, and multiple input multiple output (MIMO) are the most anticipated directions [1, 2]. The purpose of chasing extreme densification is to enhance the area spectral efficiency, which can accommodate more active nodes in a smaller area. Attaining this goal will require making smaller cells [3, 4]. The first generation of the cells, which have the size of hundreds of square kilometers, appeared in the early 1980s. After several iterations, the cell sizes are now down to a few hundred meters. The reduction in cell size allows the reuse of the spectrum, thereby reducing the number of users per base station (BS). Therefore, it is to be expected that the cell size will be further reduced. The progress of MIMO can increase spectral efficiency, as well. MIMO represents the spatial dimension of the communication. When multi-antennas are used in BSs and mobile devices, the spatial characteristics will come out. Since the late 1990s, MIMO technology has been widely studied, and it was introduced into the WiFi system in 2006 and used to build 3G communication systems [5, 6]. MIMO has been an inherent component when long-term evolution becomes mature. There are at least two antennas in a mobile device, and up to eight antennas per BS section. In [7], the effect of increasing the number of antennas is described in detail, and it has three significant advantages. Firstly, spectral efficiency can be enhanced enormously [8]. Secondly, channel responses can become smooth on account of the large spatial diversity. At last, due to the quasiorthogonal character between each BS, simple structures can be used to transmit and receive signals. However, regardless of the development of densification and MIMO, a wider band is required [9]. At present, the microwave frequency band has been almost completely occupied. So, in order to use more new bands, enhancing the frequency and developing millimeter wave is one direction. There are a lot of free spectrums in the millimeter wave band, and there is also some free spectrum in the 20-30 GHz range. However, millimeter wave bands are limited due to high path loss, atmospheric absorption, and high equipment costs in the past, and the focus has been on WiFi and fixed wireless bands, such as 28 GHz, 38 GHz, 71-76 GHz, and 81-86 GHz. However, as semiconductors mature, the cost and power consumption of building millimeter wave systems are falling rapidly. Other obstacles are also thought to be surmountable [10; 11; 12]. The millimeter wave spectrum can meet the bandwidth requirements for 5G development, as well Therefore, the millimeter wave is an important direction for the development of communication systems. As a key component of a communication system, an antenna's performance is closely related to the whole communication system. Therefore, it is important to propose different millimeter wave antennas to meet different demands. With the development of millimeter wave antennas, a variety of antennas with different performance have been published, such as the multiband antenna, dual-frequency antenna, dual-polarized antenna, circularly polarized antenna, and so on. The dual-polarized antenna is one of the key technologies in wireless communication. Using a dual-polarized antenna can realize polarization diversity and improve the capacity and reliability of a wireless communication link.