JP5470578B2 - Derivation method for low sidelobe multi-beam excitation of defocused phased array fed reflector antenna - Google Patents

Description

  The present invention relates to a multi-beam antenna system that combines a large reflector and a multi-element phased array feeding unit placed at a defocused position to reduce the side lobe level of each beam. The present invention relates to a method for deriving an excitation distribution.

  For example, in a satellite mobile phone system using a geostationary satellite, a satellite-equipped multi-beam is indispensable. In order to realize this, a satellite-mounted antenna including a large reflector and a phased array feeding unit is used.

  Patent Document 1 (U.S. Pat. No. 3,364,490) discloses a method for adjusting a beam diameter by adjusting the phase of a radio wave to be fed when the radio wave collecting system is fed by radiating radio waves from a plurality of antennas. A beam diameter variable antenna that can be controlled is disclosed.

  Further, with respect to the radiation beam pattern of the phased array antenna, generally, a desired two-dimensional beam pattern can be formed by adjusting the excitation distribution for the beam forming network (the adjustment amount of the amplitude and phase of the plurality of feed element signals). Done.

  However, in the defocused phased array fed reflector antenna system (see Figs. 1 and 2), which combines a large reflector and a phased array feeding unit, the desired two-dimensional beam pattern is different from the direct radiation type phased array antenna. It is known that it is difficult to directly calculate the excitation distribution for the beamforming network to form.

  As the calculation method known so far, for example, there is a method described in Non-Patent Document 1. This is based on numerical calculation using a computer, and is based on a search algorithm that repeats beam pattern calculation while changing the excitation distribution little by little until a desired beam pattern is obtained. In this search algorithm, in order to obtain a desired gain pattern, a desired gain value is set at a plurality of positions (that is, constraint points) in the beam pattern, and a difference from the gain value calculated from the excitation distribution is calculated. As the error, an algorithm that minimizes the difference (for example, an algorithm that minimizes the sum of squared errors) is used.

  However, at present, in this search algorithm, the initial value of the excitation distribution and the method of determining the location of the constraint point are ad hoc. In other words, the number of constraint points and the set gain are determined by repeated adjustment through trial and error until a desired beam pattern is obtained. If the purpose is to form a pattern of one specific beam or reduce the side lobe level at a specific location in the beam pattern, it is possible to repeat the beam pattern calculation by setting the constraint point by trial and error. is there. However, in an antenna system that generates multiple beams (for example, 100 beams or more), in order to obtain an excitation distribution for reducing the side lobe level of each beam over a wide range to a desired value, a huge number of trials and errors are required. is necessary.

  More specifically, for example, Non-Patent Document 1, which assumes an antenna system based on a large reflector and a phased array feeding unit, uses antenna gains at a plurality of predetermined locations in a desired beam pattern as a constraint condition. This method is used to search for the optimum excitation distribution, and the excitation distribution determination method used in the technical test satellite VIII equipped with a similar antenna system is empirically restricted around the place where side lobe reduction is desired. Is a method for searching for an optimum value (Non-patent Document 2). All of these require artificial determination of the position of the constraining point, so enormous effort is required to reduce the sidelobe levels of a large number of beams with different directivity directions such as a multi-beam antenna. .

US Pat. No. 3,364,490

Tanaka Shoji, et al., "Examination of radiation pattern of phased array fed reflector antenna for broadcasting satellite", IEICE Technical Report, AP 2001-169, pp61-67, January 2002 Kenichi Hashio et al., "A Study on Beam Formation of Large Antenna in S-band Mobile Satellite Communication / Broadcast System", IEICE Technical Report, SAT-95-19, RCS95-55, pp.73--78, 1995 July

  As described above, enormous effort is required to reduce the side lobe levels of a large number of beams having different directivity directions such as a multi-beam antenna.

  Therefore, in the present invention, in a multi-beam antenna system that generates a large number of beams, it is possible to automate the excitation distribution search for various directional beams with the side lobe level reduced over a wide range.

First, the defocused phased array fed reflector antenna low-sidelobe multi-beam excitation distribution derivation method of the present invention is a combination of a large reflector that is a radio wave condensing system and a phased array feeding unit placed at the defocused position. This is applied to the beam antenna system, and the following processing is performed in order.
(1) The phase and amplitude of the radio wave applied to each feeding element of the phased array feeding unit for maximizing the peak gain in the specified service area direction is determined using the maximum ratio combining method, and this is the initial value. And
(2) For the main lobe beam of the beam pattern based on the excitation distribution with the initial value , (a) one central constraint point is set in the beam center direction designated when using the maximum ratio combining method, and (b) the above Outside the service area , four area end restraint points are selected whose azimuth and elevation angles from the specified beam center direction are ± ΔAz and ± ΔEL, respectively.
(3) The gain condition at the center constraint point is set to be equal to or greater than the peak gain of the beam pattern according to the initial value, and the respective gain conditions at the area end constraint point are set to be equal to or less than the minimum gain in the predetermined service area. .
(4) The phase and amplitude of the radio wave applied to each of the power supply units is set to the initial value of the excitation distribution according to the above (1) so that all gain conditions at the center constraint point and the area edge constraint point are satisfied. Is corrected by the optimization means.
(5) Check the presence or absence of sidelobe peaks occurring outside the service area from the beam pattern obtained by the correction, and if any,
(6) An off-axis constraint point is set for each of the side lobe peaks, and a gain condition at the off-axis constraint point is set to be equal to or less than a desired side lobe gain.
(7) A process of recorrecting the phase and amplitude of the radio wave applied to each of the power feeding units is performed so as to satisfy all the gain conditions at the center constraint point, the area end constraint point, and the off-axis constraint point.
(8) In the gain pattern obtained using the excitation distribution, when a new sidelobe peak occurs in a place other than the center constraint point and the area end constraint point, the process returns to ( 5 ) above. If the side lobe peak cannot be suppressed even if the process of returning to ( 5 ) is performed a predetermined number of times, the generation of a beam with a low side lobe for the beam in this direction is abandoned.
The multi-beam antenna system can reduce the side lobe of the defocused phased array-fed reflector antenna by performing the processes (1) to (8) for all beams in different directions of the multi-beam antenna system. The excitation distribution is derived.

  The above processing can also be performed using an actual antenna system or its scale model, but the above processing (1) to (7) is performed by a simulator using a computer and applied to each of the power feeding units. By determining the phase and amplitude of the radio wave, the above-described excitation distribution can be derived in a short period of time.

  This method makes it possible to automate the operation of deriving an excitation distribution that generates a low sidelobe multi-beam (eg, 100 beams or more), and has been performed by trial and error until now. The working time for reducing the side lobe level can be shortened.

It is a figure which shows the defocusing type satellite mounting antenna system comprised from the parabolic mirror surface and the phased array electric power feeding part arrange | positioned a little from the focus to the mirror surface side. It is a figure which shows the structure of the phased array electric power feeding part and beam forming network in the case of using for a receiving side (a) and a transmission side (b). It is a figure which shows the beam pattern calculation example (Az = 1.04 degree | times, El = 1.4 degree | times as an example of a beam direction) by the antenna system mounted in a satellite, (a) Beam pattern by excitation distribution of peak gain maximization (B) As a result of obtaining an excitation distribution by setting a center constraint point and an area end constraint point, (c) As a result of obtaining an excitation distribution by adding an off-axis constraint point to a side lobe, (d) (c) ) Shows a result of further adding an off-axis restraint point to the side lobe newly generated in FIG. 6 is a flowchart illustrating an example of an automatic generation algorithm. It is a figure which shows the example of automatic generation of the multi-beam which made low side lobe.

  The apparatus configuration to which the present invention is applied includes a large reflector 1 which is a radio wave condensing system, a phased array power feeding unit 2 placed at a defocused position shifted from the focal point 3, and the amplitude and phase of a transmission / reception signal connected to the power feeding unit. This is an antenna system in which elements are combined in a beam forming network 4 for controlling (excitation distribution). FIG. 1 shows a defocused satellite-mounted antenna system composed of a parabolic mirror surface which is a reflecting mirror 1 and a phased array power feeding unit 2 which is arranged slightly shifted from the focal point 3 toward the mirror surface side.

FIG. 2A is a block diagram showing the configuration of the beam forming network 4 _R when the phased array power feeding unit 2 is on the receiving side, and the radio wave input to the amplifier 5 _R which is a low noise amplifier is transmitted to the amplitude adjuster 6. _The amplitude and phase are adjusted by _R and the phase adjuster 7 _R , respectively, and output as a combined output.

FIG. 2B is a block diagram showing the configuration of the beam forming network 4 _T when the phased array power feeding unit 2 is on the transmission side, and the phase and amplitude are adjusted by the phase adjuster 7 _T and the amplitude adjuster 6 _T. After adjusting and amplifying by the amplifier 5 _T which is a power amplifier, power is supplied from the phased array power supply unit 2.

  In order to automate the search for the excitation distribution of beams in various directional directions with a reduced sidelobe level over a wide range, the present invention uses trial and error to set the initial values and constraint points in the search algorithm for obtaining the excitation distribution. Decide regardless. As a result, in a multi-beam antenna system that generates a large number of beams, it is possible to automate the search for the excitation distribution of beams in various directional directions with the sidelobe level reduced over a wide range. For this purpose, the following processing is performed in order.

[1] In such a configuration, the phase and amplitude of a radio wave applied to each power feeding unit for generating a beam in a designated direction is determined using a maximum ratio combining method, and is used as an initial value.

  Here, the maximum ratio combining method is a well-known method for diversity using a plurality of antennas, and receives the signal components from the points where the feed signal received signal components are combined (in the case of a receiving system) or from each feed device. At a far reception point (in the case of a transmission system), a signal from each element is synthesized by multiplying a signal of an element having the same phase and a high received signal intensity by a weight proportional to the intensity. As a result, the beam gain of the antenna system including the large reflector and the phased array feeding unit can be maximized.

Next, how to obtain the excitation distribution that maximizes the reception gain of the phased array feed antenna system will be described. FIG. 2A shows a configuration of an M beam-compatible phased array receiving and feeding unit including an N element feeding element and a beam forming network that adjusts and synthesizes the amplitude and phase of a signal. When this power feeding unit receives a signal transmitted at a location far away from the antenna, the equivalent low-frequency signal (S _n ) received by the nth power feeding element can be expressed by the following _equation ( ₁ ).

Here, j is an imaginary number, and A _n and φ _n are the amplitude and phase of the received signal, respectively, which are determined by the geometrical arrangement such as the direction of the transmission point, the antenna mirror shape, and the feed element position, and are calculated in advance for each element. I can do it. The signal of each element is amplified by a low noise amplifier (LNA), and then synthesized by shifting the amplitude by G _n and shifting the phase by ψ _n . At this time, if the amplitude and phase are adjusted so that the combined signal power to LNA noise power ratio (γ) is maximized, the antenna gain in the direction of the transmission point is maximized. That is, G _n and ψ _n may be determined so as to maximize the following formula 2.

ここで、Ｎは素子数、Ｐ_noiseはＬＮＡが発生する熱雑音電力で全てのＬＮＡで等しい値とする。

Here, N is the number of elements, and P _noise is the thermal noise power generated by the LNA and is set to an equal value for all the LNAs.

ここでｒは０〜Ｎ−１の数から適当に選んだ基準となる素子番号である。 The phase term can be obtained using in-phase addition conditions, that is, Equation 3.

Here, r is a reference element number appropriately selected from 0 to N-1.

  As for the amplitude term, a maximum ratio combining method well known in the communication field or the like may be used as follows.

ここでαは全てのｎに共通な任意の正の値である。

Here, α is an arbitrary positive value common to all n.

With regard to maximization of the transmission antenna gain, the same result can be obtained if the ground station received signal of the feed element number n is set to _An exp (jφ _n ). In this case, φ _n and _An are the phase and amplitude of the ground station received signal of the n-th feed element signal determined by the reflecting mirror surface and the geometrical arrangement of each feed element when a flat excitation distribution is given. (See FIG. 2 (b)).

  FIG. 3A shows an example of a beam pattern based on an excitation distribution with a maximum peak gain when an assumed antenna system is mounted on a geostationary satellite.

[2] Next, for the beam pattern near the point of the service area of the beam according to the initial value, the constraint points for reducing the side lobes are determined as follows (a) and (b).
For the main lobe beam of the beam pattern by the above initial value,
(A) One constraint point (center constraint point) at the center of the beam,
(B) Area edge restraint points are provided at four points (center direction ± ΔAz, center direction ± ΔEL) slightly wider than the desired service area.
This beam center is the beam direction designated when the maximum ratio combining method is performed. Here, ΔAz and ΔEL are azimuth angles and elevation angles from the beam center direction as viewed from the antenna.

[3] Next, the gain at the center constraint point is set to be slightly higher than the peak gain of the beam pattern according to the initial value, and the gain at the area end constraint point is set to be equal to or less than the desired minimum gain within the service area.

[4] Next, the phase and amplitude of the radio wave applied to each power feeding unit are corrected from the initial values so as to satisfy all the gain conditions at the constraint points. FIG. 3B shows a beam pattern obtained from the above correction result. This is a result of obtaining an excitation distribution by a search algorithm such as square error minimization under the above-described gain condition at the center constraint point and the area edge constraint point. For this correction, there is a method known as optimization of a multivariable function, for example, an optimization method known as a fastest descent method or a Marquardt method. As the computer program, for example, an optimization problem solving program of Masmatica (registered trademark, MATEMATICA: Wolfram Resatch Inc.) can be used.

[5] The presence or absence of side lobe peaks exceeding the desired value generated outside the main lobe is confirmed.

[6] When there is the sidelobe peak, (c) a constraint point (hereinafter referred to as an off-axis constraint point) is set for each sidelobe pattern, and the gain condition at the off-axis constraint point is set to a desired value. Set below sidelobe gain.
This off-axis restraint point is optimized in addition to the above restraint points. If there are a plurality of side lobe patterns, a plurality of off-axis constraint points are added accordingly. The set gains of these constraint points are set to values slightly lower than the desired side lobe gain. When the least square search algorithm is used, the error can be set to 0 when the gain calculation result at the place of the constraint point is lower than the set gain.

[7] Re-correct the phase and amplitude of the radio wave applied to each of the power supply units so as to satisfy all gain conditions at the center constraint point, the area end constraint point, and the off-axis constraint point. FIG. 3C shows a beam pattern using the phase and amplitude obtained by this re-correction.

[8] In the gain pattern obtained using the excitation distribution, when a new sidelobe peak occurs in a place other than the constraint point, the process returns to [ 5 ] above. If the side lobe peak cannot be suppressed even after the process of returning to [ 5 ] is performed a predetermined number of times, the generation of a beam with a low side lobe is abandoned for the beam in this direction.

  By performing processing for reducing the side lobe level for each beam of the multi-beam antenna system, an excitation distribution for a low side lobe multi-beam of a defocused phased array fed reflector antenna can be obtained.

  It should be noted that some trial and error is required for the setting gain of the center constraint point and the area end gain setting in the above [2] and the setting gain of the off-axis constraint point in the above [6]. If only one beam is determined, processing can be performed by using the same value for beams in other directivity directions.

  FIG. 4 is a flowchart showing an example of the automatic generation algorithm shown in the above [1] to [8]. The above [1] to [8] correspond to [1] to [8] in FIG. The above constraint point setting method is incorporated into the above algorithm, and a beam with low side lobe for each beam is specified by specifying a number of beam directions in a triangular arrangement with an interval of ± 4 degrees as seen from the antenna at intervals of 0.4 degrees. FIG. 5 shows an example of a multi-beam obtained by performing the automatic generation calculation. A 160 beam with a low side lobe was automatically generated within a range of a separation angle of ± 2 degrees. In this example, when the off-axis constraint point is added 10 times or more and the side lobe does not fall below the desired value, beam generation in that direction is impossible.

As described above, by using the present invention, it is possible to reduce a great amount of labor related to trial and error by the above-described automated processing, and it is possible to contribute to the following fields.
1) It is possible to reduce the calculation effort and the calculation time of the excitation distribution for the beam forming network in the antenna system combining the large reflector and the phased array feeder.
2) By incorporating this calculation method into commercially available general-purpose antenna analysis simulator software, advanced antenna design such as multi-beam antennas with 100 beams or more becomes easy.
3) Reduction of interference due to sidelobe level between same frequency beams in multi-beam antenna system that reuses frequency (for example, multi-beam antenna system with large reflector mounted on mobile communication satellite and phased array feeder) This contributes to an increase in the number of user accommodation stations in the mobile satellite communication system.
4) It becomes possible to reduce the interference due to the sidelobe level between the ground and satellite systems in the terrestrial / satellite integrated mobile communication system using the same frequency in the terrestrial mobile phone system and the satellite mobile phone system, and the terrestrial system and the satellite system coexist. This contributes to an increase in the number of user accommodation stations in the system.

  The present invention can be applied to the case of adjusting the multi-beam excitation distribution using an actual defocused phased array fed reflector antenna when a sufficient distance from the antenna system can be obtained. In addition, even when a scale model is used, it is clear that the multibeam excitation distribution can be adjusted by using radio waves having a wavelength corresponding to the scale. Moreover, the excitation distribution derivation can be performed in a short period of time by performing the above processing with a simulator using a computer and determining the phase and amplitude of the radio wave applied to each of the power feeding units.

  In the above description, the multi-beam antenna system has been described. However, it is obvious that the present invention can also be applied to a scan beam antenna system.

1 reflector 2 phased array feed unit trifocal 4 _R, 4 _T beamforming network 5 _R, 5 _T amplifier 6 _R, 6 _T amplitude adjuster 7 _R, 7 _T phase adjuster

Claims (3)

In a multi-beam antenna system that combines a reflector that is a radio wave condensing system and a phased array feeding unit placed at its defocus point,
(1) The phase and amplitude of the radio wave applied to each feeding unit of the phased array feeding unit for maximizing the peak gain in the specified service area direction is determined using the maximum ratio combining method, and this is the initial value. age,
(2) For the main lobe beam of the beam pattern by the excitation distribution by the initial value ,
(A) One center constraint point is set in the beam center direction designated when using the maximum ratio combining method, and
(B) Outside the service area, select four area end restraint points whose azimuth and elevation angles from the designated beam center direction are ± ΔAz and ± ΔEL,
(3) The gain condition at the center constraint point is set to be equal to or greater than the peak gain of the beam pattern according to the initial value, and the respective gain conditions at the area end constraint point are set to be equal to or less than the minimum gain in the predetermined service area. ,
(4) The phase and amplitude of the radio wave applied to each of the power supply units is set to the initial value of the excitation distribution according to the above (1) so that all gain conditions at the center constraint point and the area edge constraint point are satisfied. Corrected by optimization means,
(5) Check the presence or absence of sidelobe peaks occurring outside the service area from the beam pattern obtained by the correction, and if any,
(6) An off-axis constraint point is additionally set for each of the side lobe peaks, and a gain condition at the off-axis constraint point is set to be equal to or less than a desired side lobe gain.
(7) A process of recorrecting the phase and amplitude of the radio wave applied to each of the power feeding units so as to satisfy all gain conditions at the center constraint point, the area end constraint point, and the off-axis constraint point,
A method for deriving an excitation distribution for a low-sidelobe multi-beam of a defocused phased array fed reflector antenna, comprising performing steps (1) to (7) for each beam of the multi-beam antenna system. If a new sidelobe peak occurs at a place other than the center restraint point, the area end restraint point, and the off-axis restraint point as a result of the process of ( 7 ), such a new sidelobe peak occurs. 2. The method of deriving an excitation distribution for a low-sidelobe multi-beam of a defocused phased array fed reflector antenna according to claim 1, wherein the steps (5) to (7) are repeated until no more.   The processing of claim 1 or 2 is performed by a simulator using a computer to determine the phase and amplitude of a radio wave applied to each of the power feeding units. Side lobe multi-beam excitation distribution derivation method.

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