## 1 Introduction  
With the dramatic development of wireless communication technology, interest in vehicle communication systems has been increasing considerably in various applications, such as autonomous driving [1; 2; 3], telematics service [4; 5; 6], and vehicle-to-everything (V2X) communications [7; 8; 9; 10; 11]. In recent years, these systems require 5G wireless communication technology for high-throughput and low-latency in an ever-changing communication environment. In particular, for vehicle applications, it is significantly important to employ high-gain 5G antennas to maintain a stable 5G communications link while driving. Many studies have investigated the use of conventional antenna structures, i.e., patch antennas [12], dipole antennas [13; 14], and dielectric resonator antennas [15] in 5G vehicle antenna designs. In some cases, research has attempted to apply such conventional antennas to actual vehicles using additional structures; for example, the vehicle roof is re-designed and modified to construct a cavity structure for mounting a 5G inverted-F monopole antenna [16]. Although these antennas have shown high antenna performances, the need to modify the vehicle structure or add new structures to the vehicle creates a disadvantage for their adoption as 5G wireless vehicle communication systems. To resolve these problems, a number of antenna designs that directly print the antennas onto the vehicle window glass have been reported, for example, mesh-grid strip line patterns [17], horizontal strip line patterns [18], and printed inverted-F antenna [19]. However, in general, on-glass antennas have been employed for low-frequency band applications below 100 MHz, such as amplitude modulation broadcasting (AM), frequency modulation broadcasting (FM), and digital multimedia broadcasting (DMB). In addition, the linear polarization (LP) characteristic is essential for 5G vehicle communications. Thus, there needs to be more in-depth research to develop glass-printed antennas with monopole radiators to achieve the LP property.  
In this paper, we propose a printed 5G monopole antenna on the vehicle window glass using parasitic elements and a lattice-structure reflector for gain enhancement. The proposed antenna consists of a coplanar waveguide (CPW), a monopole radiator, parasitic elements, and a lattice-structure reflector. The CPW is used as a feeding line instead of other transmission lines because guided fields are not induced inside the thick vehicle window glass. The monopole radiator is directly connected to the CPW inner line, and then the multiple rectangular parasitic elements surrounding the monopole are designed to enhance the bore-sight gain. The lattice-structure reflector, composed of multiple patches, is printed onto the opposite side of the monopole radiator to minimize pattern distortion. Herein, the pattern distortions resulting from thick vehicle window glass can be adjusted by the number and dimension of the reflector elements. The proposed antenna is modeled and optimized using the CST Studio Suite full EM simulation software [20]. To verify the feasibility, the proposed antenna is fabricated, and measured in a full anechoic chamber to obtain its performances, such as gains, reflection coefficients, and radiation patterns. The results confirm that the proposed antenna printed on thick vehicle window glass is suitable for 5G vehicle applications with improved gain and low pattern distortion.