A Dual Polarization 3-D Beamforming AiP

Abstract

This paper describes the implementation of an antenna-in-package (AiP) with a dual polarization function, supporting a three-dimensional (3D) beamforming operation. In order to implement 3D beamforming, a Yagi-type end-fire antenna supporting each of the x and y directions and a patch-type broadsided antenna supporting the z-direction were implemented. The broadside antennas have dual polarization functions so that they can be received in any direction. Each antenna was implemented in four array structures to support beamforming operations. The broadside antenna was designed in a 2 × 2 array structure, with a patch-type antenna and two linear dual polarization functions. The single antenna operated with a gain of 6 dBi, an E-plane beam width of ±45 degrees, and an H-plane beam width of ±50 degrees and had an antenna gain of 9~11 dBi as well as a vertical/horizontal forming operation with a radiation angle of ±22 degrees The end-fire antenna unit was designed in a 1 × 4 array structure with a Yagi-type antenna. The single antenna had a gain of 4 dBi, with an antenna gain of 8 dBi in the array structure, and it was improved to 11 dBi by adding a parasitic array director. The final end-fire antenna unit had a radiation angle of ±11 degrees and a beamforming coverage of ±45 degrees The vertical and horizontal design results were secured for reception in any direction, and all the array antennas had a return loss of 10 dB or less in the entire frequency band, from 57 to 66 GHz.

1. Introduction

Vehicle-to-pedestrian communication (V2P) has become important, and New Radio-Vehicle to Everything communication (NR-V2X) standards that support intelligent transport systems (ITS) are being created using 60 GHz [1].

As shown in Figure 1, V2X requires an omnidirectional beamformer installed on the rooftop of the vehicle to support all the 3D directions—left, right, and top to bottom—and the beam coverage in each direction must satisfy ±45 degrees to achieve 3D propagation. An end-fire radial antenna application package was set for front and rear/left coverage, and a broadside radial antenna application package was set for top–bottom coverage to support 3D radiation implementation in one package. When many adjacent cars communicate, an unrelated communication signal acts as an interference signal. To minimize this interference effect, a narrow beam is advantageous, and a narrow beam is implemented in an array antenna structure. However, the more arrays, the more antennas and beamformer semiconductors are required, resulting in a larger package size. It is necessary to define an array structure with this issue in mind.

The packages designed to receive signals in either the lateral or longitudinal direction also need to be designed to accommodate both vertical and horizontal polarization and to implement a high gain of antennas for long-distance transmission. We designed an array antenna that supports 2D beamforming implementation and has a dual polarization function while supporting a high gain to suit the V2X application. In order to support V2X communication, communication up to 100 meters was targeted. Since there is a limit to obtaining an output level in a silicon semiconductor, it was necessary to improve the antenna gain.

It was necessary to implement an additional high gain of the antenna for long-distance transmission so that the V2X applications could be sensed without difficulty even in climate and environmental changes, as shown in Figure 2. The structure of various high-gain end-fire antennas has been presented [2,3,4]. We additionally applied and designed a dummy array director to obtain additional gain along with the beamforming technology.

Many papers have presented beamforming technology with array antennas designed for the purpose of using mobile communication and supporting dual polarization for safe transmission and reception. Two studies [5,6] presented a dual polarization antenna used in 5G systems with mmWave, and another study [7] achieved a dual polarization technique in the 2 GHz band. In addition, since mmWave is used in various applications, dual polarization has also been discussed in this context [8]. The higher the frequency, the weaker the diffraction characteristics of the propagation signal and the stronger the line-of-sight transmission characteristics. Especially in mmWave, communication is possible only in a line-of-sight environment due to the nature of the propagation signal. For this reason, beamforming behavior should be possible in all directions of 3D coverage so that the optimal beam can always be found. We designed a dual polarization 3D beamforming antenna package applicable to NR-V2X, suitable for vehicle use. In addition, a study was conducted to achieve an additional gain margin that could overcome the rainfall attenuation environment. To show these results, Section 2 presents the antenna package structure and design contents; Section 3 shows the beam characteristics and 3D beamforming results. Finally, Section 4 summarizes the conclusions of the design results.

2. Design of the Dual Polarization 3D Package

We designed a V2X applicable mmWave antenna package that supports 3D radiation coverage. To this end, 1 × 4 array antennas covering ±45 degrees on the left and lower edges of the package were designed to have 180 degrees of coverage, and two packages could be used to completely cover 360 degrees in all vertical and horizontal directions. In addition, 2 × 2 array antennas with broadside radiation characteristics were implemented on the top and bottom to enable complete 3D omnidirectional support. Broadside radial antennas were designed to accommodate dual polarization so that transmission/reception could be performed without any problem at any communication angle. In addition, an additional parasitic dummy array director design was applied to double the package transmission distance so that the package operation accommodating 3D would be supported without problems, even in rainfall attenuation.

2.1. Package for the x, y Direction

Figure 3 shows a conceptual diagram of the structure that covers all directions.

In one package, the radiation signal that covers 180 degrees horizontally had an end-fire radiation pattern by arranging an antenna at the package edge so that a signal may be transmitted at the package edge. The antennas at each of the two edges of the package were implemented by applying a 90-degree radiation angle.

Each edge antenna was designed in a 1 × 4 array antenna structure to operate omnidirectionally, covering 360 degrees with two packages, and the beam width of the antenna signal emitted through the antenna was implemented with a small pencil beam of ±11 degrees and covered 90 degrees, with nine beams tilting in 7 degrees. The pencil beam through the array antenna provided excellent performance for V2X communication, operating robustly in interference environments while increasing frequency space utilization.

A Yagi-type antenna with end-fire beam formation was designed in a package, and a 1 × 4 array structure was designed to implement a higher gain from narrow beam characteristics. In order to accommodate broadband behavior, the thickness of each layer of antenna, feeder, and GND was implemented at 140 um intervals. To this end, the intermediate metal was removed from two layers, with a thickness of 70 um. In the case of a thickness of 70 um for each layer pattern, the 50 Ohm pattern became thinner and exhibited narrowband characteristics when connected to the antenna. In addition, there was a concern that the frequency operation characteristics of the thin pattern could vary, with an error rate of 10% that can appear in the process. Layer 1 implemented an antenna pattern, Layer 2 had a dielectric form without a pattern, and Layer 3 had a feeding pattern. Layer 4 had a dielectric without a pattern, and Layer 5 was designed as a GND pattern implementation layer. The designed 1 × 4 Yagi-type antenna results are shown in Figure 4. An antenna-in-package (AiP) was designed with a 1 × 4 array Yagi antenna in the package, and the beamformer semiconductor was mounted by the flip-chip method on the package surface. As shown in Figure 4, the 1 × 4 array antenna was connected through the feeder line from the output/input bumps on the 1 × 4 beamformer semiconductor. In order to ensure accurate 1 × 4 array antenna operation, the feeder line was designed to have an electrically identical phase length.

The goal was to support a transmitter/receiver transmission distance of 100 m. In addition, in order to compensate for the reduction in the support distance due to the characteristics of rainfall attenuation, an additional design was created to support transmissions up to 200 m, double the distance. To achieve this goal, the additional gain was added to the end-fire array antenna using the method of adding a parasitic dummy array director to the existing package. Since the method was implemented through additional work on the mounted board, the antenna gain was improved while utilizing the existing package.

The 1 × 4 array beamformer semiconductor operated with an output level of 18 dBm (P1dB) in a single path. The equivalent isotropically radiated power (EIRP) obtained from the 1 × 4 array antenna package structure was 31 dBm using Equation (1).

Using Equation (2), the power combined gain (PG) and air gain (AG) were defined, where the PG refers to the power level at which an output level of 1 dB gain compression point (P1dB) for a single path becomes a 1 × 4 array output structure in a 1 × 4 array semiconductor structure; the AG increases as the antenna increases.

In the four-array structure, the PG and AG were ideally 6 dB; however, they were calculated at 5 dB, considering the 1 dB combined loss.

Therefore, the 18 dBm of P1dB for each path was 23 dBm from Equation (3), and there was an 8 dBi antenna directional gain (ADG), which is the total array antenna gain obtained through the array antenna, calculated using Equation (4).

Based on this, the EIRP obtained from the 1 × 4 array antenna package was 31 dBm.

$$EIRP=Pout+ADG$$

(1)

$$PG=AG+10\mathit{log}\left(N\right)$$

(2)

$$Pout=P1dB+PG$$

(3)

$$ADG=single antenna gain+AG$$

(4)

Here, $Pout$ is the output power level, ADG is the antenna’s directional gain, PG is the power combined gain, AG is the air gain, N is the number of the antennas in the array structure, and P1dB is the output level of a 1 dB gain compression point. All equations are expressed on a dB scale.

Experimentally, it was confirmed that the gain of a single Yagi antenna was 3 dBi, and the 1 × 4 Yagi array antenna was measured at 8 dBi by adding the gain of 5 dB of the AG for a 1 × 4 array structure.

The sensitivity of the receiver semiconductor was −69 dBm, and the reception sensitivity ($Sensitivit{y}_{rx}$) through the 1 × 4 array antenna with an antenna gain of 8 dBi was −77 dBm. Equation (5) shows that the path loss of the transmitter/receiver was allowed up to 108 dB.

$$Admitted path loss \left(APL\right)=EIRP-Sensitivit{y}_{rx}$$

(5)

Since the path loss (PL) can be expressed as in Equation (6), the possible transmission distance for 108 dB of the admitted path loss at 60 GHz is 100 m [9].

$$PL=20\log \left(\frac{4\pi d}{\lambda }\right)$$

(6)

where d is the transmission distance of the signal through the transceiver, and λ is the wavelength of the signal.

In addition, in order to compensate for the distance characteristics that are reduced due to the actual environment of rainfall attenuation, the 1 × 4 array antenna was improved by 3 dB to secure double the distance support.

Three parasitic dummy array directors were added to each of the 1 × 4 array antennas, aiming to obtain an 11 dBi gain by improving the gain of an array antenna of 8 dBi by 3 dB. More than three directors only slightly improved the gain and had the disadvantage of increasing the area.

The director size was set to 1.4 mm in the length of a 1/4 wavelength, and the space between parasitic dummy directors was set to 0.6 mm to obtain the maximum direction gain. The spacing between antennas was set to 2.5 mm to match the 1 × 4 Yagi array antenna implemented in the package. The design drawings of the parasitic dummy array director are shown in Figure 5.

Figure 6 shows the return loss and realized gain of the manufactured 1 × 4 array antenna package.

The return loss characteristics of 15 dB or less were obtained in all bands, including and without the parasitic dummy array directors, and the application of the parasitic dummy array directors improved to an 11 dBi gain from 8 dBi. In the return loss, the #3 and #4 ports of the 1 × 4 array antenna were implemented symmetrically with the #1 and #2 ports, so the measurement results are omitted.

2.2. Dual Polarization Package for the z-Direction

A 2 × 2 beamforming package with a dual polarization function was designed to support the z-axis direction. It was designed as a patch-type antenna to form a broadside beam on the front of the package; a 2 × 2 array structure was introduced to enable obtaining the high gain and the function of beamforming vertically or horizontally on the front of the package, and a director pattern was added around the patch. To implement dual polarization, an antenna layer, a feeding line layer, and a GND layer were separately designed.

For transmission and reception without problems in any environment, a 2 × 2 array antenna implementation layer structure with a dual polarization function that supported both vertical and horizontal polarization was established. It was designed to support all polarization by placing the feeding pattern in the vertical and horizontal directions and selectively using it. Figure 7 shows the results of the designed antenna and feeding line.

Figure 8 shows the S-parameter results of the return loss and antenna gain for each frequency of the antenna manufactured using the material of low-loss FR4. In the entire frequency band, the return loss of all the antennas was set to 10 dB or less, and it was confirmed that the antenna gain was 9 to 11 dBi.

2.3. Design of the Asymmetric 1 × 4 Divider

One package consisted of two types of end-fire 1 × 4 array antennas that radiated in each direction of x and y and had 180 degrees of coverage, along with broadside 2 × 2 array antennas that had a dual polarization function to cover radiation in the z-direction. Two 1 × 4 beamformer semiconductors supporting two kinds of end-fire 1 × 4 array antenna and two 1 × 4 beamformer semiconductors supporting the broadside 2 × 2 array antenna with a dual polarization function were applied to the package. That is, four 1 × 4 beamformer semiconductors were flip-chipped to a package containing four array antennas and connected to each antenna.

The V2X AiP supporting any direction had four 60 GHz signals from the modem connected to the four 1 × 4 beamformers. To achieve this, we designed a four-way divider that supported 60 GHz on the package substrate.

An antenna with low-loss characteristics and broadband characteristics was designed around the top surface of the AiP; one to five layers were utilized to design the antennas and feeding structures, and a 1 × 4 divider was designed at 10 layers.

The bottom surface, where the 1 × 4 divider was designed, had five chips and a solder ball placed around the chip, and the signal line was connected to the input/output port of the chip without loss. Furthermore, for the design of a 1 × 4 divider between chips, it was not possible to place a 1/4 length of wavelength and a resistance of 100 ohm to secure the isolation of each port as in the existing Wilkinson type [10,11,12].

In the NR-V2X system targeted in this paper, all 16 antennas do not operate simultaneously. Only one of the four beamformers operates, and the other three beamformers are off. Thus, since each beamformer operates without the influence of the signal, isolation characteristics are not important, and the concordance of each port and the signal loss from the MODEM to the beamformer are important design factors. An asymmetric structure was implemented for the divider implementation with minimal transmission lines (TL), and the long TL side was designed to have a minimal signal loss, with the long TL side being matched before the short TL side. Figure 9a shows the designed 1 × 4 divider, and Figure 9b shows the concept of the package bottom layer applied with the asymmetric 1 × 4 divider.

Figure 10 shows the results of the measurement of a 1 × 4 divider designed on the substrate bottom face around the chip.

From the measurement, the return loss of the beamformer port in the direction supporting top and right was less than −10 dB in the entire band, but the return loss in the port supporting the bottom and left directions was in the range of 3 to 12 dB. On the other hand, in terms of insertion losses, the top and right directions were 7 to 10 dB, and the losses in the left and bottom directions were 8 to 12 dB.

The asymmetric structure of the 1 × 4 divider was designed to minimize the insertion loss of the long signal line by minimizing the return loss of the long signal line at the expense of the return loss on the short signal line side.

The measurement results from the SIW technology [13] are compared and shown in

Table 1. Comparison of strengths and weaknesses.

Parameters	SIW [13]	This Work	Remarks
Size	Large (33 × 27 mm)	Small (5.7 × 1.6 mm)	Benefits for pkg implementation (1/81))
Additional element	100 Ω	none	No additional process required
Insertion Loss	Min. 10 dB (including chip bump loss 2 dB)	Min. 12 dB (Meas.)	Long TL First Match
Return Loss	Max. 10 dB	Max. 3 dB
Isolation	-	Max. −5 dB	Independent Behavior

. Compared to [13], the losses were designed to be 2 dB larger, but they had the advantage of being 1/81 times smaller in size to enable the package design.

3. AiP Measurement

The measurement results were evaluated by dividing them into package results and beamforming results. In addition, the parasitic dummy array director was applied to show the improved results.

The PCB of the package used a material (MCL-E-770G) with a low tangent loss (dissipation factor 0.01 @ 60 GHz, dielectric constant 4.1) to minimize the 60 GHz signal loss, and it was flip-chipped with four beamformer chips and one modem chip, along with a 16-array antenna on the package substrates. These semiconductors were mounted on the bottom surface of the PCB.

The parasitic dummy array director was made of Teflon, a Taconic product with a low tangent loss (dissipation factor of 0.005 @ 60 GHz, dielectric constant of 2.1). Figure 11 shows the result of applying the parasitic dummy array director to the manufactured package.

3.1. Beamforming Result for the x, y Direction

Figure 12 shows the environment for measuring the beam pattern of the array package. The tilting angle of each beam was implemented by adjusting the phase value of the 1 × 4 beamformer semiconductor. The beamformer allocated 4 bits for each path so that each bit could be allocated 22.5, 45, 90, and 180 degrees to change the phase value in a unit of 22.5 degrees. Since the beam could be tilted in 7–8-degree units experimentally by changing the phase difference in 22.5-degree units, the performance was reviewed while measuring nine beam patterns tilted in 7–8-degree units.

As shown in Figure 12, the device under test (DUT) was placed on the test-bench board. A manual switch was applied together to change the phase of each path of the beamformer on the test-bench board. By setting the phase of each path of the beamformer through a manual switch, a tilting angle of the beam was defined, and the corresponding beam pattern was measured. The beam pattern was measured using a 20 dBi horn antenna as a reference antenna.

First, the beam pattern and gain before and after the application of the parasitic dummy array director to the Yagi antenna of the manufactured AiP were measured to confirm the improvement of the beam pattern according to the antenna gain. The measurement results are shown in Figure 13. As can be seen from the measurement results, the gain of the E-plane and H-plane were improved by 3 dB from 8 to 11 dBi under all conditions of beam tilting. This means that the transmission distance was doubled from the normal state. In addition, in the E-plane, the beam angle before and after applying the parasitic dummy array director had a similar value; however, in the H-plane, the beam angle decreased from ±70 degrees to ±45 degrees.

In the end-fire 1 × 4 array structure, beam patterns were measured in units of phase resolution of 22.5 degrees in each array path; the results of each measured beam pattern are shown in Figure 14.

Each of the nine beams had a tilting angle of 7-to-8-degree units; each beam had a beamwidth of ±11 degrees, and the beam coverage was ±45 degrees, with a total of nine beams.

3.2. Beamforming Result for the z-Direction

The broadside type 2 × 2 array antenna designed for the top surface of the AiP was measured using the measurement environment shown in Figure 12. Figure 15 shows the measured E/H-plane beam pattern result for each single antenna.

Since the antenna gain and radiation pattern were similar to the simulation value, it was confirmed that the production was well performed. The peak gain was 6 dBi in a single antenna, the radiation angle in the E-plane was ±45 degrees, and the radiation angle in the H-plane was ±50 degrees. The beam patterns for each frequency in the 2 × 2 array antenna structure consisting of a single measured antenna are shown in Figure 16.

The radiation pattern was well formed at all frequencies, the gain in accordance with the frequency was 9 to 11 dBi, and the radiation beam width was ±22 degrees.

Figure 17 shows the beam pattern and tilting behavior for each E/H polarization. Each beam pattern was measured by changing the path phase of the beamformer to 45-degree units, and it was found that the beam pattern operated by tilting at 14-degree intervals.

As shown in Figure 17, the beam pattern of the E-plane was measured under the V/H-polarization operation condition in the structure of the 2 × 2 array patch antenna, and it was confirmed that the beam tilting was performed at +28 degrees, +14 degrees, 0 degrees, and −14 degrees. While being tilted in a unit of 14 degrees, the overall V-polarization operated with a beam coverage of ±45 degrees, and the H-polarization operated with a beam coverage of ±50 degrees.

4. Conclusions

The end-fire 1 × 4 array antenna of the package was designed to tilt at ±11 degrees of beamwidth in units of 7~8 degrees, with a peak gain of 8 dBi, and it had ±45-degree coverage with nine beams. In addition, in order to improve the signal gain of 8 dBi, a parasitic dummy array director was applied to secure an excellent gain result of 11 dBi so that it could support double the transmission distance of the previous one.

The broadside 2 × 2 array antenna supported a dual polarization with a peak gain of 11 dBi, a beamwidth of ±22 degrees, and tilting in units of 14 degrees to operate with a beam coverage of ±45 degrees in the case of V-polarization and a beam coverage of ±50 degrees in the case of H-polarization.

Two end-fire 1 × 4 array antennas were applied to each edge in the two directions of the AiP, and a broadside 2 × 2 array antenna supporting dual polarization was applied to the top surface.

Using the two AiPs, 3D beamforming that enabled optimal communication in any direction was implemented.

The results were compared with other papers implementing dual polarization, as shown in

Table 2. Comparison with others.

Parameters	[6]	[7]	[8]	This Work
Freq [GHz]	28	2	40	60
Polarization	Dual polarization
Beam pattern	End-fire 1 × 4 array	Broad side 1 × 3 array	Broad side 2 × 2 array	Broad side 2 × 2 array with two end-fire 1 × 4 arrays
Gain	9 dBi	13 dBi	9 dBi	10 dBi
Beam coverage (HPBW)	±41 degrees (±11 degrees)	±35 degrees (±9 degrees)	-	±50 degrees (±22 degrees)
Result	1-D BF	1-D BF	2-D BF	3-D BF

. The beam coverage was implemented most widely while performing 3D beamforming, and competitive gain was secured.

Using this result, which should be applied to all four sides of the vehicle to secure the omnidirectional communication of the vehicle, two AiPs can be installed on the rooftop for a V2X application that can support 3D omnidirectional communication direction.