# A 77 GHz Transmit Array for In-Package Automotive Radar Applications
## 1 Introduction  
Currently, in the automotive industry, vehicles are controlled using different systems like pedestrian detection, adaptive cruise control, radars for lane keeping, blind spot detection and collision alerts. In particular, the increasing interest in autonomous driving vehicles requires reliable radar sensors. For this reason, it is predictable that the demand for automotive radars will keep rising in the near future. In recent years, the industry has focused its attention on 79 GHz radar systems, which extend the range of the previous 77 GHz radars (76-77 GHz) to include the 77-81 GHz range [1]. Automotive radar systems, like collision avoidance radars or intelligent cruise control (ICC), are based on antennas with high directivity. This characteristic is fundamental to increasing their capability of distinguishing targets in a specified field of view. Generally, for medium- and short-range radars, 3 dB beamwidths of \(\pm\)40\({}^{\circ}\) to \(\pm\)80\({}^{\circ}\) in azimuth and \(\pm\)5\({}^{\circ}\) to \(\pm\)12\({}^{\circ}\) in elevation are required in antennas [1]. Another important requirement of modern automotive radar systems is related to the antenna's ease of integration [2]. Radiators need to provide a low profile, be lightweight and have low manufacturing costs. In this context, printed antenna arrays appear to be a natural choice [1, 3, 4, 5]; however, they may suffer from larger feed network losses. Free-space beam-forming techniques based on substrate lenses [6, 7], transmit arrays [8] or reflect arrays [9] have been proposed in the literature to address this problem. It is well known that the performances of reflect arrays are limited by the blockage of the feed source, while substrate lenses are typically bulky and heavy. On the other hand, solutions based on transmit arrays can be considered a good solution for increasing the directivity of antenna systems while maintaining low weight and a low profile. A transmit array is a phase-shifting surface consisting of unit cells. These can be used to focus the waves radiated by a feeding antenna into a narrower beamwidth. A progressivephase shift is applied across the aperture of the transmit array, and in this manner the beam can be controlled and focused/steered towards a direction away from the boresight. Several examples of focusing transmit arrays are available in the literature. For example, in [10; 11; 12], focusing is obtained using arrays of transmit/receive antennas connected through transmission line sections of appropriate lengths. This work uses a transmit array to focus the beam radiated by a package-integrated antenna. PCB space and low-cost assembly are critical for automotive applications; for this reason, package-integrated antennas can be an ideal choice for these space-constrained applications. In particular, the quad flat no-lead (QFN) packaging technique offers easier component placement as well as improved strength, reliability and thermal characteristics. The QFN packaging technique is a lead frame (LF)-based plastic package that has been widely used in the semiconductor industry since its introduction. This technique has several advantages, such as a cost-efficient packaging process with high heat dissipation properties and a simple production process [13; 14; 15]. The solution presented in this work is compact and perfectly compatible with QFN packaging technology for 77 GHz applications [4]. The proposed configuration demonstrates how a standard antenna with low directivity can be used as the feed for a transmit array (TA) with all components integrated into a standard QFN package. The antenna is incorporated into the package cover. This setup allows for the implementation of various integration methods, such as using an on-chip integrated feed or a feed array structure. Compared to other solutions available in the recent literature, like the ones described in [6; 7], the proposed solution is more compact and lightweight, while the solution proposed in [9] suffers from the blockage effect.