A 77 GHz Transmit Array for In-Package Automotive Radar Applications

Abstract

A packaged transmit array (TA) antenna is designed for automotive radar applications operating at 77 GHz. The compact dimensions of the proposed configuration make it compatible with standard quad flat no-lead package (QFN) technology. The TA placed inside the package cover is used to focus the field radiated by a feed placed in the same package. The unit cell of the array is composed of two pairs of stacked patches separated by a central ground plane. A planar patch antenna surrounded by a mushroom-type EBG (Electromagnetic Band Gap) structure is used as the primary feed. An analytical approach is employed to evaluate the primary parameters of the suggested TA, including its directivity, gain and spillover efficiency. The final design has been refined using comprehensive full-wave simulations. The simulated gain is 14.2 dBi at 77 GHz, with a half-power beamwidth of 22°. This proposed setup is a strong contender for highly integrated mid-gain applications in the automotive sector.

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 ±40° to ±80° in azimuth and ±5° to ±12° 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 progressive phase 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.

The design procedure is based on an analytical model of a transmit array to optimize several antenna parameters like gain, but also the focal distance over aperture size (f/D). The final structure has been integrated into a 12 × 12 mm² QFN [13,15,16] open-cavity package. The proposed design approach allows us to place the transmit array elements very close to the feed (f/D = 0.5), allowing for a highly integrated structure compatible with QFN technology.

The high gain and integration properties of the proposed transmit array are promising for automotive radar applications. The preliminary results were presented by the authors in [17].

2. Transmit Array Design

The overall structure of the proposed transmit array is presented in Figure 1. A planar patch antenna surrounded by a mushroom-type EBG (Electromagnetic Band Gap) structure is used as the primary feed. The proposed configuration was designed to be integrated into a W × W (12 × 12 mm²) package using QFN technology. The feed is in the centre of the package, while the package cover is positioned at a distance of f = 2 mm. The following sections will provide a detailed description of the feed antenna and transmit array.

2.1. Antenna Feed

A planar antenna configuration is used as the primary feed to realize a low-cost, integrated, 77 GHz RF, front-end package for automotive applications, integrable into a transceiver module. The transmit array fed can be either monolithically integrated onto the chip [18] or printed on a standard substrate. In this paper, a standard printed antenna is employed (Figure 2). A conventional patch antenna with an inset feed has been designed to resonate at the central frequency of the band. The patch is printed on a 0.25 mm Rogers RO3003 substrate with DK = 3 and a dissipation factor of 0.0010.

One of the main problems associated with this type of configuration is the excitation of surface waves, which cause a deterioration of the radiation pattern. It is well known that the suppression of the surface waves improves the antenna performance, increasing the energy radiated in the broadside direction. To suppress the propagation of surface waves, the radiating patch has been surrounded by a mushroom-type EBG (Electromagnetic Band Gap) structure [19,20,21].

The EBG has been designed to provide a band gap able to cover the whole antenna bandwidth. In this manner, the surface waves launched from the antenna inside the substrate are efficiently suppressed. The mushroom-type EBG [22] is composed of metallic patches with central vias, as shown in Figure 3. This type of structure exhibits unique electromagnetic properties with a bandgap characteristic that is mainly determined by the patch and gap width, substrate permittivity, substrate thickness and metal vias’ radius.

To evaluate the band gap proprieties of the EBG, a dispersion diagram of the mushroom-type structure’s unit cell has been generated. The parameters have been tuned to create a band gap able to cover the band of interest. An LC filter array can be used for the explanation of the operation mechanism of the EBG structure. An increment in patch width Wp and decrement in gap (Dp-Wp), while keeping the height and permittivity of the substrate fixed, causes an increment in capacitance, which results in the decrement of the structure’s resonant frequency. Figure 3 shows the dispersion diagram obtained with Dp = 1.05 mm; Wp = 0.7 mm; and via a diameter of 0.254 mm. A large stop band is created on the periodic surface from 60 GHz to 82 GHz.

In order to improve the transmit array’s performance, a flat radiation pattern over a large angular range is preferable. To this end, the patch antenna has been surrounded by three rows of mushroom-type EBGs, as shown in Figure 2. The normalized radiation patterns of the patch antenna with and without EBGs are compared in Figure 4. A noticeable improvement in terms of the radiation pattern’s flatness is observed: the −3 dB beamwidth has been increased from 14° to about 90°. The smoother pattern is more suited to being a feed for the transmit array. The feeding patch has a gain of 7.5 dBi.

2.2. Transmit Array Unit Cell

A slot-fed stacked patch antenna [23] was used as a transmit array unit cell, as shown in Figure 5. From previous research [24], it is well known that a slot-fed stacked patch antenna can be used to enhance the directivity of a radiated field if properly dimensioned. Two pairs of stacked patches are separated by a central ground plane. The lower square patches are used as receiving elements while the upper radiators are used as transmitting elements. In the proposed configuration, the receiving patches face the focal source, while the transmitting patches face the free space. An aperture (slot) is used in the central ground plane to couple the receiving elements on their lower side, with the transmitting ones placed on the opposite side of the transmit array. The patches are printed on a 0.2 mm Arlon AD350A with a dielectric constant ${\epsilon }_r=3.55$ and a loss tangent of 0.0033. The four patches have the same dimensions. The phase response is controlled by changing the side lengths of all the stacked patches. Unit cells with different patch lengths can be arranged to provide a desired phase distribution across the array surface. The unit cell’s periodicity is W = 1.35 mm (0.35 λ₀ at 77 GHz) for both planes.

In Figure 6, the fields radiated from the unit cell with different angles of incidence are plotted in terms of magnitude and phase. The structure has highly stable behaviour for incidence angles up to 60°. This characteristic can be used to reduce the f/D of the proposed configuration.

3. Analytical Model

A transmit array analytical model based on the evaluation of power transfers [10] is used in this paper. Four parameters have been considered to compute the TA’s efficiency (Figure 7): P₁ is the power at the input of the focal source, while P₂ is the power radiated by the same source. P₃ is the incident power collected by the receiving side of the transmit array. Finally, P₄ is the power re-radiated from the transmission side of the array. The antenna efficiency is defined as follows:

$$\eta ={\displaystyle \frac{P_4}{P_1}}={\eta }_{FS}{\eta }_{SO}{\eta }_{IL}$$

where ${\eta }_{FS}$ is the realized efficiency of the focal array (P₂/P₁), ${\eta }_{SO}$ is the spillover efficiency (P₃/P₂) and ${\eta }_{IL}$ is the insertion loss of the unit cell (P₄/P₃). To enhance directivity and spillover efficiency, it is essential to examine the behaviour of the transmit array versus variations in its design parameters such as focal distance (f) and D, as well as the number and spacing of its elements. For simplicity, the feed’s radiation pattern is assumed to be equal to ${\cos }^q\Theta$ with q = 0.5. Figure 8 illustrates the TA’s performance in terms of spillover efficiency and gain for different f/D ratios, based on the previously discussed analytical model. The optimal performance is achieved when f/D equals 0.5. Additionally, the cell size is a crucial factor for a given f/D value. Figure 9 depicts how element spacing affects the transmit array’s directivity. As shown, the directivity increases as the spacing between the elements decreases, leading to a chosen spacing of λ/3 to maximize directivity.

4. Results

The proposed analytical model was utilized to examine the relationships between the dimensions of the array, the focal length and the number of elements, as well as the behaviour of the transmit array in response to these variations. After defining all the parameters, the proposed configuration was simulated using Ansys HFSS [25]. The effect of the package has been included in the final model. The system comprises an 8 × 8 element transmit array with a spacing of λ/3 between elements. Each TA element has been dimensioned based on the results shown in Figure 6a to compensate for the phase differences of the signals coming from the feeding patch. Figure 10 presents the simulated radiation pattern of the antenna feed both with and without the transmit array.

A prototype of the proposed transmit array configuration has been fabricated (Figure 11). To measure the antenna with WR10 waveguide equipment, a printed circuit board (PCB) launch connector (BN 533411, Spinner GmbH, Munich, Germany) has been used.

Figure 12 shows the simulated and measured matching of the fabricated prototype, while the radiation H-plane patterns are compared in Figure 13. the Simulated side lobes are lower than −20 dB. This is a value perfectly in line with those typically obtained from a high-performance transmit array. The E-plane patterns were not measured because of the limitations of the measurement setup. Minor discrepancies are visible in the measured beam, as well as a slightly higher side-lobe level. This can be attributed to several reasons like the PCB fabrication tolerances of the feeding patch or the Tas and the alignment of the lower patch to the upper TA and the transmit array to the feeding horn used in the high-frequency measurement setup (Figure 11c). However, the measured results agree quite well with the simulations. The measured gain is 14.2 dB at 77 GHz, with a 68% radiation efficiency and a First Null Beamwidth of 52.5°.

5. Discussion

In the literature, several array-based solution examples for automotive radar applications like those in [1,3,4,5] are available; however, they may suffer from larger feed network losses. If compared to other solutions based on a lens like the ones described in [6,7], our proposed configuration is more compact and lightweight, while the reflect array proposed in [9] suffers from the blockage effect. Solutions based on transmit arrays can be considered good solutions for increasing the directivity of the antenna systems while maintaining a low weight and low profile. In

Table 1. Transmit array comparison.

Ref.	Freq (GHz)	W (mm)	Eff (%)	Feed Type	QFN Compatible
[8]	76.5	40.35	5	Planar (SIW Slot)	NO
[26]	60	50	14.2	Not planar (Horn)	NO
[27]	77	46.55	19.7	Not planar (Horn)	NO
This Work	77	12	22	Planar (Patch)	YES

, the configuration proposed in this work is compared with other transmit arrays taken from the recent literature. The aperture efficiency of the proposed transmit array compares well with other solutions, while its compact side dimension (W = 12 mm) makes it compatible with QFN package integration.

6. Conclusions

A 77 GHz packaged transmit array antenna for automotive radar applications has been presented. The whole structure is compact and realized in planar technology, which make it compatible with standard quad flat no-lead package (QFN) technology. A planar feed and a transmit array are both integrated into a single package. The electromagnetic field radiated by a feed is focused by the transmit array antenna placed on the package cover. The unit cell of the array is composed of two pairs of stacked patches separated by a central ground plane. The primary feed is realized with a planar patch antenna surrounded by a mushroom-type EBG. The relevant parameters of the proposed TA, like directivity, gain and spillover efficiency, have been evaluated using an analytical model. The final configuration has then been optimized by means of full-wave simulations. The simulated gain is 14.2 dBi at 77 Ghz, while the half-power beamwidth is 22°. The gain and beamwidth are in line with medium- and short-range automotive radar requirements [1,28]. The proposed transmit array is a good candidate for highly integrated mid-gain automotive applications.