Smart Transfer Planer with Multiple Antenna Arrays to Enhance Low Earth Orbit Satellite Communication Ground Links

Abstract

In this study, we propose a smart transfer planer equipped with multiple antenna arrays to improve ground links for low Earth orbit (LEO) satellite communication. The STP features a symmetrical structure and is strategically placed on both ends of a window, serving both indoor and outdoor environments. Using the window glass as a medium, energy transmission occurs through a coupling mechanism between the planers. The design focuses on large array antenna design, beamforming networks, and coupler design on both sides of the glass. Beamforming networks enable the indoor and outdoor antenna arrays to switch beams in various directions, optimizing high-gain antennas with narrow beamwidths. Through electromagnetic induction and filter couplers, a robust signal transmission channel is established between indoor and outdoor environments. This setup significantly enhances communication efficiency, particularly in non-line-of-sight environments.

1. Introduction

Satellite communication systems utilize satellites as repeaters to establish communication links between the Earth’s surface and remote regions. These communication satellites fall into three main categories: geostationary satellites, medium Earth orbit (MEO) satellites, and low Earth orbit (LEO) satellites. In this system, satellites play a crucial role as relays, enabling communication between the Earth’s surface and distant regions. Ground stations transmit wireless signals to satellites, which then relay the signals to receivers in the target area. This technology finds widespread application in satellite phones, internet connectivity, and data transmission.

LEO satellites can be utilized to offer fixed wireless access, achieving high-speed internet connectivity in regions where traditional fixed-line solutions might be challenging to deploy. In urban areas, LEO satellites can effectively serve as an overlay network, providing significant improvements to the existing communication infrastructure. Their capabilities extend to relieving traffic congestion by offloading data from densely populated networks, thereby contributing to the overall improvement of network performance. This approach takes advantage of the unique advantages of LEO satellites, such as low latency and widespread coverage, to address the challenges posed by high-density urban environments. Satellite communication operates in the Ku-band and Ka-band using high frequencies. However, due to the high propagation loss and low penetration of centimeter waves, communication links must operate in line-of-sight environments. High-gain large antenna arrays are needed for high-throughput broadband communication, posing challenges in system planning and large antenna array design.

This study extends the practical work based on the smart transfer planer concept proposed in [1], Figure 1, featuring multiple antennas suitable for both indoor and outdoor applications. The planer is symmetrically structured and is installed at both ends of a window, serving as a separator between indoor and outdoor environments. Communication between the planers occurs through the glass window via a coupling mechanism. The primary focus of this design lies in the large array antenna design, beamforming networks, and coupler design on both sides of the glass.

The beamforming networks enable indoor and outdoor antenna arrays to switch beams in various directions, offering a solution for high-gain antennas with narrow beamwidths. Using centimeter-wave electromagnetic induction and filter couplers, a signal transmission channel is established between indoor and outdoor environments. This configuration improves communication efficiency for signal transmission in non-line-of-sight environments.

2. Design Concept of Smart Transfer Planer

The STP architecture is divided into three blocks: signal transmission, wireless charging, and beamforming antenna array. These are based on references from the literature for antenna design, beamforming network design, and electromagnetic induction circuit design applicable to satellite communication bands. The purpose of signal transmission is to address the issue of communication link propagation suitable for line-of-sight environments. The Ku-band is chosen as the design foundation, and vertical coupling filters are used to achieve this. The beam selection switching is performed through active ICs to create transmission channels for outdoor and indoor signals. The wireless charging coil provides bias voltage required by outdoor active chips through 6.78 MHz (Qi wireless charging standard) electromagnetic induction, enabling wireless charging of indoor power to outdoor smart planers via the charging coil. After rectification, the electrical energy can provide the DC bias voltage required by outdoor smart planer active chips. Finally, the beamforming antenna array provides broadband communication with multiple paths and high throughput. Beamforming switching is performed using a 3 × 8 Rotman lens matrix circuit and transmission, and reception are carried out through an 8 × 8 array antenna. The functional diagram of the multi-antenna smart transfer planer is shown in Figure 2.

2.1. Antenna Array

Microstrip antennas are renowned for their lightweight, thin and low profile characteristics, and their planer nature enables them to be utilized in multi-layer board designs [2,3,4,5], thus avoiding the issue of excessive overall volume. Additionally, microstrip antennas can achieve high gain and phase-shifted radiation patterns through array theory. Consequently, microstrip antennas find widespread application in various wireless communication fields. However, while individual unit antennas also possess gain and directivity, if it is necessary to enhance gain or achieve higher directivity to meet different design requirements, or to change the beam direction for wider beam scanning, designing array antennas is the most commonly used approach. In addition, antenna arrays are designed with various feeding methods to accommodate different needs and limitations. One common feeding method is parallel feed networks, which exhibit symmetry, aiding in balancing mutual coupling effects, and have the advantage of a wide bandwidth. However, due to the nature of the feed lines, this design is more challenging to arrange compactly. In contrast, the series feed [3], as depicted in Figure 3a, offers the advantages of a simple and compact structure and is commonly used to design antenna arrays with narrow beams and high gain. This design approach is adopted in this paper. Microstrip antennas are used as design units in the antenna array. The antenna design is conducted using the high-frequency structure simulator (Ansys HFSS).

The serial antenna design is implemented for broadband performance, with a center frequency of 13.375 GHz selected to cover both uplink and downlink frequencies. A unit microstrip patch antenna with dimensions of 5.3 × 7.6 mm² forms a 1 × 8 series sub-array antenna, with a distance of 1.6 mm between the antenna and the ground to achieve broadband performance. After designing the 1 × 8 series sub-array antenna, it is arranged into an 8 × 8 series antenna array, forming a balanced layout. This manufacturing process helps avoid board bending and reduce overall thickness, albeit with additional design constraints. Figure 3b illustrates the 2D radiation pattern of the 8 × 8 microstrip array antenna. When Theta equals 0°, the maximum Gain value is 23.87 dBi. By changing the feeding phase, beamforming effects can be achieved, with a gain of 21.2 dBi at 60° and −60°. The reflection coefficient of the microstrip array antenna is shown in Figure 3c. The reflection coefficient across the operating frequency range from 10.3 GHz to 15 GHz remains below −10 dB.

2.2. Beamforming Network

To meet the usage scenario, the reception of outdoor signals and the transmission of indoor signals were simplified into three directions: forward, positive lateral angle, and negative lateral angle. Accordingly, we planned beamforming circuits with three directions and selected a Rotman lens [6,7,8,9,10], for its high design flexibility. The principle behind the Rotman lens involves feeding any point on the circular focal arc, passing through the parallel plate cavity formed by the upper and lower metals, reaching the inner contour C1, and then through the delay lines to the outer contour C2 [6]. This ensures that the path lengths through the lens are equal, achieving scanning functionality.

The basic structure and derivation of the outer curve of the Rotman lens are presented in the references, along with several important design parameters, including the issue of different propagation constants between the Rotman lens cavity and transmission lines. We designed a Rotman lens with three input ports and eight output ports, where the geometrical shapes of the lens on the left and right are identical to achieve symmetrical offset of the radiation pattern. To reduce losses and interference, the design employs substrate-integrated waveguide (SIW) technology [11,12,13,14], Figure 4.

Initially, the calculated lens profile places the input and output ports along the focal positions, with the cavity structure of the Rotman lens in-between. To prevent multiple reflections of electromagnetic waves within the cavity from affecting the output and phase, dummy ports are designed on both sides of the cavity, and the optimal positions are determined by observing the angle of the dummy ports’ impact on the multiple reflections of electromagnetic waves [8]. Finally, by adjusting the feeding angles of the three input ports, individual phase differences are achieved in the output ports. By simulating and observing the reflection coefficients and phase differences, and using an 8 × 8 antenna array to observe the simulated results of the radiation pattern, the antenna radiation pattern offset can be determined in this step. The input ports are denoted as A1–A3; the dummy ports are located on both sides of the cavity; and the output ports are denoted as B1–B8. The reflection coefficients of each input port are observed through a simulation, as shown in Figure 5a, where the reflection coefficient of forward input A1 is below −10 dB between 11.2 GHz and 13.2 GHz, and that of lateral inputs A2 and A3 is below −10 dB between 11.7 GHz and 14.72 GHz, meeting the requirements for STP applications. Figure 5b shows the phase of the forward input A1 to each output port B1–B8, with phase differences between each output approaching $0^{°}$. Figure 5c,d show the phase of the lateral inputs A2 and A3 to each output port B1–B8, respectively, with phase differences between each output approaching $-{45}^{°}$ and ${45}^{°}$, respectively.

2.3. Smart Transfer Planer Fabrication

In the multi-layer board integration design (Figure 6), there are usually two ways to achieve vertical signal transmission between the upper and lower layers. One is to use metal vias, and the other is to use slot coupling in the middle metal layer [11,15,16]. Both methods have very low insertion losses in the high-frequency band. Considering the reduction in space usage, this paper plans the integration of multi-layer board circuits and adopts the slot-coupling method between the Rotman lens and the antenna array, following the integration method in reference [16]. Three slots are opened in the lower metal layer of the lens as inputs. By calculating impedance matching, the maximum coupling amount can be achieved. The Romand lens made of SIW has a large metal plane, which can serve as a reference ground plane for the microstrip antenna array. Radiation patterns for the three input ports excited at 12.5 GHz are shown, demonstrating that the lens effectively generates relative scanning angles in the radiation patterns. Figure 7 shows the radiation pattern diagram of the three input terminals, excited at 12.5 GHz. It can be seen that the lens effectively causes the field pattern to produce a relative scanning angle. The field pattern diagram can verify the main beam offset of the side feed to about ±24°, and the main beam gain in three directions is approximately 11.1 to 11.6 dBi.

2.4. Signal Transform

Parallel coupled transmission lines can be designed as microwave filters [17]. In this scenario, a filter operating in the Ku-band is coupled from outdoors through window glass as a dielectric. First, the inductance and capacitance of the prototype circuit of the bandpass filter at the cutoff frequency are calculated, and alternative transmission line lengths for the inductance and capacitance are identified. These alternative segment combinations form a stepped LC filter. Knowing that indoor and outdoor structures are identical and symmetrical, where the glass thickness and dielectric constant are both variable within a range, it is observed from the simulation process that thicker glass leads to greater transmission loss in the coupler and there is significant variation in the dielectric constant of different glass types. The glass dielectric constant used in this design simulation is 3.7. Figure 8 depicts a schematic diagram of the filter coupler, designed in the form of a vertical coupled microstrip line stepped filter. The size of the single-sided stepped filter is calculated according to reference [18]. The outdoor and indoor sides of the filters are simulated in HFSS. As shown in Figure 8, the left side of the outdoor area is set as the input port, and the right side of the indoor area is set as the output port, while the remaining two ports are open-circuit, forming a bandpass filter. Figure 9 presents the S-parameter results of the filter coupler, where Port 1 represents the outdoor end filter and Port 2 represents the indoor end filter. Due to the symmetrical structure, the outdoor and indoor structures can be interchanged. By observing the simulation results from Port 1 to Port 2, it is observed that the reflection coefficient is below −10 dB within the operating frequency band of 12 GHz to 13 GHz, and the penetration coefficient can be considered as the coupling amount to the other side, controlled between −2 dB and −3 dB within the operating frequency band.

2.5. Switch Circuit

The functional blocks are integrated into a single substrate, including control ICs and Qi charging coils, to create a complete outdoor and indoor smart transfer planer for the multi-antenna array. This planer is divided into outdoor and indoor planers, with the only difference being the path of the beam selection IC. The rest of the structure is symmetrical, referring to the symmetry of the outdoor and indoor antenna arrays, Rotman lenses, and filter couplers. The circuit layout is shown in Figure 10. Upon selecting the appropriate IC, the signal received by the outdoor array antenna is determined through a Coupler Detector IC to identify the path with the highest gain among the three directions of the Rotman lens. Subsequently, the signal of that path is selected by a Switch IC and then finally directed to the filter coupler, coupling the signal indoors.

3. Experimental Results

After the design is completed, each functional block is integrated onto the circuit board in a multi-layered manner for verification and measurement. The multi-layer board is divided into three pieces for fabrication. These individual boards are then pressed together and combined using a coupling method. The finished product after splitting the multi-layer board into three pieces is shown in Figure 11, representing the bottom layer; Figure 11a shows the filter circuit; the middle layer with the Rotman lens is shown in Figure 11b; and top layer with the array antenna is shown in Figure 11c. The measurement environment setup is illustrated in Figure 12, with the outdoor and indoor spaces simulated by a glass panel. A network analyzer (Keysight (Santa Rosa, CA, USA) N5227A 2-Port PNA Network Analyzer) is used, with Port 1 connected to the transmitting standard antenna array set up outdoors and Port 2 connected to the receiving standard antenna array set up indoors.

During measurements, the angle of the receiving antenna on the indoor side remains constant (0°), while the transmitting antenna on the outdoor side emits signals toward the STP and glass at three different angles, 0 °, +24 ° and −24 °, as shown in Figure 13. Two sets of data are compared between without activating the STP, and with the STP being activated. The measured data for both cases are as follows, primarily observing changes in the penetration coefficient to understand whether the STP functions as expected. According to the simulation results of the integration of the Rotman lens and array antenna in Section 3, the gain of the array antenna is approximately 10 dB. Each set of STPs has an ideal gain of 20 dB, with an approximately 3 dB loss deducted from the simulation results of the filter coupler in Section 2. It is expected that the planer’s operational condition can improve the gain by approximately 17 dB compared to the non-operational condition.

Comprehensive simulation results show that for angle $-{24}^{°}$, the double-planer gain is 18.98 dB, with an expected gain of approximately 15.98 dB after subtracting the filter coupler loss. For angle $0^{°}$, the double-planer gain is 20.3 dB, with an expected gain of approximately 17.3 dB after subtracting the filter coupler loss. For angle $+{24}^{°}$, the double-planer gain is 19.94 dB, with an expected gain of approximately 16.94 dB after subtracting the filter coupler loss.

Without activating the STP, the angle of the receiving antenna on the indoor side remains constant ($0^{°}$), and the transmitting antenna on the outdoor side emits signals towards the STP and glass at three angles ($-{24}^{°}$, $0^{°}$ and $+{24}^{°}$). By observing the S-parameters through the network analyzer, it can be seen that after spatial loss, the penetration coefficient at the operating frequency of 12.5 GHz is $-52.2\mathrm{dB}$ for the $-{24}^{°}$ angle, $-53.9\mathrm{dB}$ for the $0^{°}$ angle, and $-50.6\mathrm{dB}$ for the $+{24}^{°}$ angle, as shown in Figure 13a–c, respectively. With the STP activated, the angle of the receiving antenna on the indoor side remains constant ($0^{°}$), and the transmitting antenna on the outdoor side emits signals toward the STP and glass at three angles ($-{24}^{°}$, $0^{°}$, and $+{24}^{°}$). Through the network analyzer, it can be observed that after a spatial loss and with an improvement in transmission gain due to the STP, the penetration coefficient at an operating frequency of 12.5 GHz is $-43.7\mathrm{dB}$ for the $-{24}^{°}$, $-40\mathrm{dB}$ for the $0^{°}$, and $-40.2\mathrm{dB}$ for the $+{24}^{°}$, as shown in Figure 13d–f, respectively.

The measurement results validate (

Table 1. Measurement result of the smart transfer planer.

Receiver Angle	$-{24}^{°}$	$0^{°}$	$+{24}^{°}$
Received power without STP (dB)	−52.2	−53.9	50.6
Received power with STP (dB)	−43.7	−40	−40.2
Increased received power (dB)	9.1	10.1	10.4

) that at an operating frequency of 12.5 GHz, the improvement is 9.1 dB for the $-{24}^{°}$ direction, 10.1 dB for the $0^{°}$ direction, and 10.4 dB for the $+{24}^{°}$ direction. Adding the STP indeed improves losses along the path. However, there are discrepancies between actual measurement results and simulation results due to factors such as the loss incurred by splitting the board, as well as errors in controlling the distance and angle during measurements.

The purpose of the STP is to address the issue of high propagation loss in high-frequency applications. First, high-gain large array antennas are used to achieve high-throughput broadband communication performance. Secondly, a Rotman lens is utilized to realize beamscanning and generate different directional main beams, thereby improving the directivity of signal reception in different directions. Lastly, a filter coupler is employed as a relay device to effectively transmit high-frequency signals propagated from outdoor to indoor environments under non-line-of-sight conditions, ultimately achieving intelligent relay control through ICs.

4. Conclusions

The proposed smart transfer planer offers a promising solution to enhance network resilience and address the challenges associated with satellite communication. By serving as a signal transmission channel between indoor and outdoor environments, the STP improves the communication performance of nonlinear sight waves and mitigates the low penetration characteristics of centimeter waves. The symmetrical architecture and electromagnetic coupling mechanism of the STP enable efficient power transmission and signal reception. With the ability to change beam directions, the STP facilitates the reception of centimeter-wave signals with indoor terminal devices. This innovation contributes to the advancement of satellite communication technologies, particularly in the Ku-bands, and offers a potential solution for enhancing broadband applications in the future.