KR100373412B1 - 3-dimensional beam steering system - Google Patents

Abstract

The present invention relates to a three-dimensional millimeter wave beam steering system, comprising a unit antenna capable of electrically and mechanically steering a beam, an antenna array in which the unit antenna is arranged, and a three-dimensional beam steering element and an MMIC. Active circuits of this type consist of a semi-optically integrated three-dimensional millimeter wave subsystem. The unit antenna is controlled in real time by an electromagnetic driving method to move two-dimensionally in space. In other words, it is a three-dimensional beam steering system in which the beam of the antenna is electrically steered by a phase shifter and can be physically moved by mechanical driving. Three-dimensional beam steering antennas and devices are integrated on one substrate in a monolithic way using MEMS technology, and the remaining active circuits are mixers, power amplifiers (PAs), low loss amplifiers (LNAs), and VCOs. The back is implemented in the form of an MMIC active array. The system is integrated through a quasi-optical method for the interconnection of three-dimensional beam steering elements and active MMIC circuits.

According to the present invention, a new transmission scheme called three-dimensional beam steering and an ultra-small antenna structure that are electrically and mechanically moved have been introduced to solve the problem of low signal-to-noise ratio (SNR) due to low device output and high space loss in the millimeter wave band. In this regard, it is very innovative in that 3D imaging as well as broadband wireless communication in a Pico cell environment will be possible.

Description

3-dimensional beam steering system

The present invention relates to a wireless transmission system, and to a three-dimensional steering of a radio beam (beam) and focusing on a point in space, a wireless transmission method for maximizing signal transmission efficiency and an antenna structure for implementing the same and Related to this is an active MMIC circuit and system integration method.

A new frequency resource, the millimeter-wave (30-300 GHz) band, allows the use of a wider frequency band, thus enabling the transfer of large amounts of information. This can be seen from the fact that the frequency band of wireless systems such as LMDS (Local Multi-point Distribution System) and wireless LAN (WLAN), which are recently attracting attention, is aiming at the millimeter wave bands such as 30 GHz and 60 GHz. have. Another important application of millimeter waves is imaging. Conventional imaging techniques mainly use light (light) or microwave bands. The development of a new, more accessible concept of 3D imaging technology in the millimeter wave band would be of great help to many applications in medicine and engineering that require sophisticated resolution of 3D images.

The biggest scientific and technical problem with using these millimeter wave frequency bands is that the output of the millimeter wave generating device is extremely limited. As shown in Figure 1, the output of the device is inversely proportional to the square of the frequency. Because it obeys the law. In addition, there is a disadvantage in that the path loss is large even when transmitting the same distance because the wavelength is shorter than the microwave. This disadvantage means that the higher the frequency, the smaller the size of the received signal. Millimeter waves are vulnerable in terms of signal size as well as noise. Active devices used in transceivers increase the frequency and decrease the gain while increasing the noise figure. That is, the active device amplifies and adds noise, which means that the amount of noise added increases with frequency. As a result, the signal passing through the active element is inevitably worsened at higher frequencies. Signal-to-noise ratio (SNR) is the most important figure in a signal transmission system, and an SNR below a threshold indicates that the system is meaningless.

In order to solve the problems of the millimeter wave, many studies are currently being conducted around the world, mainly focused on lowering noise and increasing output at the device level. Another research direction is to reduce the intermediate interconnection loss between components and increase the gain of the antenna, such as a smart antenna system using a phased array of the antenna has been proposed. . However, lowering the device's noise figure at millimeter waves requires an expensive e-beam lithography process that reduces the gate length to 0.1-0.2 μm , and the resulting SNR improvement is 1-. It is less than 2dB. In the case of an antenna, in order to increase the gain, the physical size of the antenna must be large, which increases the price and causes many problems in terms of system configuration. Another way to increase gain is to reduce losses, such as using superconductors, but the additional gain that can be gained by this method does not exceed 1-2dB. The smart antenna system is a two-dimensional beam forming and steering system, which can be considered as the previous stage of the three-dimensional beam steering system proposed in this study. Many research teams around the world, such as the United States and Japan, are conducting research on smart antenna systems. In the case of antennas that detect only the direction of a received signal, low frequency systems have been commercialized and used in mobile communication systems. However, the current technical level has economic problems such as high unit cost of base stations adopting smart antenna system and technical problems that it is difficult to completely solve the call quality problem. This problem cannot be solved fundamentally with the existing smart antenna technology, which relies on two-dimensional orientation. As a result, current research methods do not significantly increase the millimeter-wave signal-to-noise ratio. This is because the power of the received signal is limited by the Friis formula, which is proportional to the gain of the transmit / receive antenna and inversely proportional to the square of the distance of the transmit / receive period.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a three-dimensional beam steering system capable of overcoming the low SNR limitation of the millimeter wave band and implementing a broadband wireless transmission system.

1 is a graph showing the ratio of frequency to output of microwave and millimeter wave sources,

2A is a perspective view of a three-dimensional beam focusing antenna according to an embodiment of the present invention;

2b is a perspective view of an antenna element constituting a three-dimensional beam focusing antenna according to an embodiment of the invention,

3 is a photograph of a micro mirror,

4 is a graph comparing the reception power of the 3D beam focusing method and the conventional faced array method according to the present invention;

5 is a conceptual diagram of an antenna array mechanically redirected in a three-dimensional beam focusing antenna according to an embodiment of the present invention;

FIG. 6 illustrates a case of two-dimensional beam steering in which the three-dimensional focusing is performed electrically and mechanically simultaneously, the three-dimensional focusing is performed only electrically, and the two-dimensional beam steering mechanically moves each unit antenna in the same direction. Is a graph comparing the intensity of the radiated electric field on the E-plane,

7 is a conceptual diagram of a semi-optically integrated three-dimensional millimeter wave subsystem,

8 is a conceptual diagram of a multidimensional monolithically integrated three-dimensional millimeter wave subsystem.

In order to solve this problem, the present invention provides a three-dimensional millimeter wave beam steering system in which at least two three-dimensional beam steering elements including a unit antenna for transmitting and receiving a signal and a driving unit for mechanically driving the unit antenna are arranged. .

At this time, a phase shifter for electrically adjusting the phase of the signal to be oscillated in the unit antenna is connected to each antenna unit element so as to electrically perform beam steering. The active circuit of the MMIC type that exchanges signals with the 3D beam steering element in a semi-optical manner is preferably integrated with the entire RF transmission / reception system including a VCO, a mixer, an amplifier, and the like. The beam steering element may be monolithically integrated by a quasi-optical method or a multi-level integration technique.

The theoretical background of the three-dimensional beam focusing antenna according to the present invention is as follows.

According to the fleece formula widely used in general signal transmission systems, the received power is proportional to the transmit power and the gain of each antenna, and inversely proportional to the square of the distance between the transmitter and the receiver. According to this, in order to increase the reception power 10 times at a certain distance, the transmission power should be increased 10 times or the gain of the antenna should be increased 10 times. As mentioned earlier, to increase the transmission power There are restrictions due to the law, and since the unit cost per unit power increases exponentially with frequency, there is little practicality in terms of commercialization. In addition, even though it is increased by a breakthrough method, the radio environment is degraded due to the consumption of a lot of energy. In addition, the use of a battery (battery) has the additional disadvantage of increasing the size of the terminal. In order to increase the gain of the antenna has a problem that the size of the antenna must be increased 10 times. Therefore, it seems impossible to increase the reception power more than the reception power by the fleece formula while using a constant transmission power and antenna size at a constant transmission and reception distance. However, this is because the two-dimensional signal transmission method of adjusting the beam of the antenna only in the direction does not escape. When the beam of the transmitting antenna is three-dimensionally focused on the position of the receiving antenna by fine phase control, a signal transmission result that does not conform to the Friis formula can be obtained.

To date, a system known as the most efficient way to use the beam of the antenna is a smart antenna system. This system is a type of phased array antenna system that spatially arranges multiple antennas and then gives each phase necessary for each antenna to transmit the beam direction only in the intended direction. The system is a two-dimensional beam steering system that adjusts only direction without space resolution capability in space. Current phased array antennas generally use antennas fixed on a plane. The reason for this is that in manufacturing a total phased array antenna including several hundred antennas, the manufacturing process is easy, mass production is easy, and the manufacturing cost is low. However, when using antennas that are physically fixed in direction, when viewed from the side of each antenna, the direction of the entire beam must be deviated from the direction having the maximum gain of each antenna. Therefore, electrical performance is inevitably deteriorated. Traditionally, they chose to cut prices in the trade-off of price versus performance. However, recent advances in rapid MEMS (Micro Electromechanical System) technology offer the possibility of producing thousands of mechanically rotatable antennas at low cost by a batch process. Therefore, the maximum gain can be obtained in all directions by mechanically changing the direction of each antenna in the direction of the beam to be sent.

Next, a 3D beam focusing antenna according to an exemplary embodiment of the present invention will be described with reference to the accompanying drawings.

2A is a perspective view of a small base station with a three-dimensional beam focusing antenna according to an embodiment of the invention.

An antenna element 10 constituting a three-dimensional beam focusing antenna is arranged on the front side of a transmitting and receiving device such as a cylindrical base station 1. In FIG. 2A, seven antenna elements 10 are shown arranged in a matrix form, but the number or arrangement of antenna elements 10 may vary. For example, ten antenna elements 10 may be arranged concentrically.

2B is an enlarged view of each antenna element of the three-dimensional focusing antenna according to the embodiment of the present invention.

Each antenna element may control a phase of a signal to be oscillated through a unit antenna 10 capable of three-dimensional rotation, a driver 12 for rotating the unit antenna 10 three-dimensionally, and a unit antenna 10. It consists of a phase shifter 11.

In this case, the unit antenna 11 and the driver 12 may be formed in a batch process using MEMS technology. This uses a driving micro mirror fabrication technology. 3 is a photograph of a driving micromirror. The micro mirror is an element for optically reflecting light, and is a device for inducing light by mechanically controlling the direction of the mirror according to a driving signal. Since the structure of the patch antenna according to the present invention is similar to the micro mirror, the micro mirror technology can be applied almost as it is. In addition, a two-dimensional mechanical steering antenna array can be implemented using the Micro-Elevator by Self-Assembly (MESA) process proposed by UCLA's M.C.Wu professor team.

High-precision angle and distance resolution are required for the steering of the three-dimensional RF beam, which is done electrically using the phase shifter 11.

The phase shifter 11 is the most important element in a phased array beam steering element and is generally implemented using an electrical element, a PIN diode or a Schottky diode. In this case, high frequency, in particular 30 GHz In the above millimeter wave band, a large loss is caused. Therefore, the present invention designs and implements a new type of low loss phase shifter using MEMS technology in order to reduce loss and maximize efficiency in the millimeter wave band. Unlike conventional methods using electrical devices such as diodes, MEMS technology is applied to realize micromachined phase shifters that cause phase shift by mechanically changing the dimensions of transmission lines. Since the mechanical phase shifter is implemented using only a conductor without using a semiconductor, the loss can be drastically reduced and the amount of phase shift can be continuously adjusted. Therefore, it is an optimal device for implementing a faced array capable of beam steering in any direction in the millimeter wave band.

Next, the principle that the 3D beam focusing antenna according to the present invention can exhibit excellent signal transmission efficiency will be described.

In general, the loss of electromagnetic waves can be divided into resistance loss due to the skin effect of metal, dielectric loss due to incompleteness of dielectric, and radiation loss due to radiation of radio waves. As the operating frequency is increased, the losses listed above are rapidly increased, and therefore, the existing transmission media such as coaxial cables and waveguides are not efficient transmission means. Also, the attenuation of radio waves given by conductor and dielectric losses is always form( Is given by the exponential decay of the distance traveled, so even with a small decay constant, α, the large decay cannot be avoided over long distances. In order to avoid this type of propagation attenuation and to transmit efficiently over long distances, a free-space propagation method using an antenna is used in long distance communication. In this case, the power of the radio waves is not an exponential function, assuming the transmission under ideal vacuum. form( Is attenuated by a distance), so that relatively little attenuation is possible in a long distance propagation. However, when the omni-directional antenna is used, since there are too many radio waves leaking in a direction other than the desired direction, the point-to-point link collects and transmits the beam to a high-gain antenna having a large gain. An RF system implemented by applying such a low loss free space transmission technique and a method of collecting beams and transmitting them as if they are light is called a "Quasi-Optical RF System". When the RF system is configured semi-optically in the millimeter wave band, relatively small interconnection loss can be obtained compared to the case of using a transmission line such as a conventional waveguide or coaxial cable, and thus an improvement in SNR can be expected.

The most basic formula used to calculate the general link budget in wireless communication is the following Friis formula.

P _r = P _t

P _r : Receive Power P _t : Transmit Power

G _t : gain of receiving antenna G _t : gain of transmitting antenna

: Wavelength of electromagnetic wave r: Distance between transmitter and receiver

The above formula is a formula in the far field (assuming focal length is infinite) and is a general formula that assumes a plane wave. The above equation shows that the received power decays according to the square of the distance, no matter how large the gain of the antenna. Using the three-dimensional beam steering proposed in this study, it is possible to focus not only in the transverse direction of the beam but also in the longitudinal direction of the beam. The Friis formula is insignificant and if the effective area of the receiving antenna is larger than the waste area of the beam, distance-independent receiving power can be obtained.

According to Gaussian beam theory, the waste of the beam at the focal point is expressed as follows assuming a Gaussian beam.

: Radius of lens

: Radius of waist at focus

: Distance to focus

In this equation, using a 10-cm-radius array antenna operating at 100 GHz and placing the receiving antenna at a distance of 10 m as a focal point can receive almost all beams with a receiving antenna of about 10 cm in radius. That is, when the 3D beam steering technique is used, the received power, which is a basic rule of wireless communication, We can overcome the limitation of Friis's formula to decrease.

In order to verify this theory, 25 patch antennas, which are simple cases, were arranged on a 22cm × 22cm plate in a 5 × 5 format, and the difference between the method of the present invention and the existing method was examined. That is, the change of the received power according to the distance when the beam is steered in three dimensions is simulated and compared with the conventional two-dimensional phased array method. At this time, the size of the receiving antenna is 8cm 8 cm and the calculation results are shown in FIG. 4.

In the smart antenna, in the case of the antenna array arranged on a plane, the direction of the beam can be changed within about 70 degrees from the wide side direction. However, because the system uses an array of antenna elements that are physically fixed in direction, the direction of the entire beam is inevitably deviated from the direction of the maximum gain of each antenna element when viewed from the side of each antenna element. In the case of a typical patch antenna, when the beam is sent in a direction inclined 60 degrees from the vertical, a gain loss of 6 dB or more is obtained. Therefore, if the physical direction of each antenna element can be changed in the direction of the beam to send away from the conventional stereotype that the arrangement of physically fixed antenna elements should be used, the maximum gain can be obtained regardless of the direction. will be. FIG. 5 shows the concept of the mechanically changing antenna arrangement in three dimensions. In order to change the direction of the antenna in real time, the antenna may be driven by electrostatic force or magnetostatic force using MEMS technology. Therefore, when the electrical method of phase adjustment and the method of mechanically changing the direction of the antenna element are simultaneously used for the 3D beam transmission, the received signal can be maximized. Since the antenna driving system of this concept basically allows each unit antenna to move independently, it becomes a technology that can be used in both microwave and millimeter wave band regardless of frequency.

FIG. 6 shows that when the beam is three-dimensionally focused at the same time electrically and mechanically with respect to the general patch arrangement (a), and only three-dimensionally electrically (b), the unit antennas are mechanically moved in the same direction. The intensity of the radiated electric field was shown by comparing the two-dimensional beam steering (c) and the two-dimensional method (d) used in the conventional phased array. According to FIG. 6, it can be seen that the case (a) according to the present invention is improved by 3 dB to 10 dB or more depending on the direction compared to the conventional methods (b) and (d), and also when the beam is mechanically steered in two dimensions. (c) also shows much better characteristics than the two-dimensional method (d) used in the conventional faced array. As a result, it can be seen that a system that moves each unit antenna mechanically is better than the case where the conventional unit antenna is fixed regardless of the beam steering dimension (1, 2 or 3).

FIG. 7 shows an example of a subsystem for implementing the interconnection of a three-dimensional beam steering element with an active MMIC circuit by a quasi-optical integration method.

A carrier signal is generated from a voltage controlled oscillator (VCO) wafer corresponding to a signal generator and transmitted to a mixer wafer through a guiding wall, and the mixer wafer modulates a baseband signal. To generate a mudulated signal. The modulated wave is transmitted to the amplifier through the guiding wall and amplified. The amplified signal wave is transmitted to the three-dimensional beam steering wafer and then radiated to the outside through an antenna. As the guiding wall of FIG. 7, a photonic bandgap material or a metal wall may be used.

8 is a conceptual diagram of a three-dimensional millimeter wave sub-system monolithically integrated in a multi-level. This suggests a new vertical integration method using bulk micromachining technology.

According to the present invention, a novel three-dimensional radio transmission technique is proposed by solving the problem of low signal-to-noise ratio (SNR) due to low device output and high space loss in the millimeter wave band using a novel radio transmission scheme and an antenna system. It was. The beam focusing effect increases the size of the received signal, and eliminates fading along the direction that can be obtained from a conventional smart antenna, as well as interference and noise by transmitters at different distances in the same direction. This can lower the overall noise level. Therefore, it is possible to expect a significant improvement in the signal-to-noise ratio. The proposed antenna array in which the two-dimensional unit antenna moves in this study is fundamentally applicable to both microwave and millimeter bands because it can steer beams in one, two, and three dimensions. Solving the low signal-to-noise ratio problem, which is the biggest obstacle to millimeter-wave frequency pioneering, will contribute to the pioneering research field of wireless transmission combining optical technology and microwave technology. It will have a big ripple effect in all fields of science and technology using millimeter waves. In the present invention, there is no need for devices and amplifiers that require high outputs as of today, and the use of MEMS technology for mass production of antennas and required passive devices opens up the market for communication components and systems in the millimeter wave band at a low price. Will be very competitive.

Claims (9)

A beam steering system comprising at least two beam steering elements comprising a unit antenna and an independent drive for electrically and mechanically steering the unit antenna. In claim 1, And the driving unit mechanically steers the unit antenna one-dimensionally and two-dimensionally to steer the beam in one, two, and three dimensions. In claim 1, The beam steering system operating in microwave and millimeter wave bands. In claim 1, And a MMIC type active circuit for exchanging signals with the beam steering element. In claim 4, The MMIC type active circuit includes a VCO, a mixer, an amplifier, and the like. The method of claim 4, wherein The MMIC type active circuit and the beam steering element are monolithically integrated by a quasi-optical method or a multi-level integration technique. In claim 1, And a phase shifter connected to the beam steering element and electrically adjusting a phase of a signal to be oscillated in the unit antenna. In claim 1, The beam steering device is a beam steering system, characterized in that to use the electric method for adjusting the phase of each unit antenna and the mechanical method for physically moving the direction of the unit antenna for beam steering at the same time. The method of claim 1, A beam steering system, characterized in that the components are fabricated by micromaching techniques.