JPH0366858B2 - - Google Patents

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to a satellite communication system, particularly when an antenna having a relatively wide beam width is used at an earth station, and the earth station is a ship or an aircraft. As can be seen, the present invention relates to a satellite communication system that can efficiently configure a satellite communication system by reducing the influence of reflected waves even when the influence of reflected waves from a reflecting body such as the sea surface cannot be ignored.

(Prior art and its problems) A conventional example of this type of satellite communication system is schematically shown in FIG. The antenna 2 mounted on the satellite 1 orbiting the earth and the antennas 4 and 4' provided on the mobile earth stations 3 and 3' both generate one type of circularly polarized wave (hereinafter described as right-handed circularly polarized wave). ) is being sent and received. Since the transmission and reception of electromagnetic waves by an antenna is a reversible phenomenon, the case where radio waves are transmitted from the satellite 1 to the mobile earth stations 3 and 3' will be explained below.

The radio waves sent from the satellite 1 are received as direct waves 5 by the earth station antennas 4 and 4', as well as
It is also received by the earth station antennas 4, 4' as reflected waves 6 that have passed through a reflector 7 such as the sea surface. In this way, since the earth station antennas 4 and 4' receive both the direct wave 5 and the reflected wave 6 at the same time, the reception level at the earth station antenna changes significantly depending on the phase relationship between the direct wave 5 and the reflected wave 6. , that is, fading occurs. When designing a satellite communication line where such fading occurs, it is necessary to allow for an amount of energy equivalent to the depth of fading (hereinafter referred to as "fading margin"), assuming that there is no fading. It is necessary to increase the transmit power and the gain of the transmitting and receiving antennas compared to the above.

As is clear from Figure 1, the earth station antenna 4
When the beam width of the satellite is relatively wide, that is, when the size of the earth station antenna 4 is not sufficiently large compared to the wavelength, or when the elevation angle (E1 in Fig. 1) when looking at the satellite 1 from the mobile earth station 3, 3' is small. In this case, the reflected wave 6 is strongly received, so a large fading margin is required. For example, in the 1.5 GHz band, in order to use a 40 cm antenna as the earth station antenna 4 and communicate with the satellite 1 at an elevation angle of 5 degrees, for 99% of the time,
It is necessary to allow for a margin of approximately 9 dB due to sea surface reflection fading. (Shiokawa et al., Institute of Electronics and Communication Engineers Antenna Propagation Research Fund AP83-10).

Several conventional methods for reducing this fading margin will be described below.

The first method is to shape the beam shape of the satellite antenna. As mentioned above, the required fading margin increases as the elevation angle decreases, so the transmission gain of the satellite antenna is increased for the region with a low elevation angle, that is, the peripheral region of the earth as seen from the satellite, as shown in Figure 2b. . In this figure, θ on the horizontal axis represents the angle at which the earth 8 is viewed from the satellite 1 shown in FIG. 2a, and the vertical axis represents the gain of the satellite antenna. In order to absorb the fading margin with this method, the difference in satellite antenna gain between the center and the periphery (Δ in Figure 2b) needs to be 9 dB in the above case, but it is necessary to shape the antenna beam in this way. is extremely difficult and has the disadvantage of requiring a large satellite antenna.

The second method is to make the earth station antenna less susceptible to reflected waves. (1) The beam shape of the antenna is modified; (2) Multiple antennas are used to constantly cancel reflected waves. and (3) those that change the polarization characteristics of the antenna according to the elevation angle of the opposing satellite.

Method (1) involves shaping the antenna beam shape in the earth station antenna so that the gain in the direction of the reflector is small, but this is not possible with a small antenna with a wide beam width like the one in question here. .

Method (2) has the drawbacks of requiring multiple antennas and a control device that can follow changes in reflectors such as the sea surface in real time.

The method (3) using polarization control (Shiokawa et al., Theoretical study of fading reduction method using polarization control, Institute of Electronics and Communication Engineers Antenna Propagation Research Fund AP81-144) is closely related to the present invention, so it will be explained in detail below. do.

When a right-handed circularly polarized wave is incident, the reflected wave 6 from a reflector such as the sea surface depends on the elevation angle E1, the state of the reflector 7, and the frequency of the reflected wave 6, but generally it becomes a left-handed elliptically polarized wave that is flat in the horizontal direction. . Further, the elliptical polarization rate of this elliptically polarized wave depends almost only on the elevation angle E1 of the antenna. As an example of a circularly polarized antenna, Figure 3a
Consider a cross dipole antenna 10 installed at an angle of 45 degrees with respect to the horizontal plane. When received radio waves from two dipole antenna elements are combined by a combiner 12 with a phase difference of 90° via a 90° phase shifter 11 and received by a receiver 13, this antenna receives right-handed circularly polarized waves. do. In contrast, a variable phase shifter 14 is inserted as shown in FIG. 3b, and the amount of phase shift is
When changing from 0° to 90°, according to the change of the phase shifter,
Receive polarization varies from perfect right-handed circular polarization to vertical linear polarization. In the polarization of radio waves, for any given polarization, there is a polarization orthogonal to it. When received by an antenna with polarization characteristics orthogonal to the polarization of the arriving radio wave 5, the receiving sensitivity of the arriving radio wave 5 becomes zero. As described above, the polarized wave orthogonal to the polarization characteristic of the reflected wave 6 is a vertically elongated right-handed elliptically polarized wave. Therefore, by adding an appropriate insertion phase shift amount to the variable phase shifter 14 and changing the polarization characteristic of the antenna to a vertically elongated right-handed elliptical polarization, the reception sensitivity to the reflected wave 6 can be made zero.
The amount of insertion phase shift required to control the polarization characteristics of the antenna is uniquely determined once the angle of elevation E1 is determined, so if this amount of phase shift is controlled using the signal from the sensor 16 that obtains the angle of elevation E1, the reflected wave 6 can be canceled out. In this way, by shifting the polarization characteristics of the earth station antenna from perfect circular polarization by the elevation angle E1, the reception level of the direct wave 5 also decreases by the amount due to its elliptical polarization (polarization loss), but the reflection Since the reception level of wave 6 is significantly reduced, deep fading due to interference between the two does not occur, and the effect is that the fading margin can be reduced by 4 to 5 dB. However, since this method needs to be implemented at all earth stations, it is inconvenient when there are a large number of earth stations. In addition, variable phase shifter 1
4. There is a drawback that an elevation angle sensor 16 and a control system 15 for controlling the variable phase shifter 14 using the elevation angle E1 are required.

(Object of the Invention) An object of the present invention is to solve the problems of the conventional methods described above and to provide a satellite communication system with a small fading margin.

(Structure and operation of the invention) The present invention will be explained in detail below.

Let us take as an example a satellite communication system that uses an antenna that transmits and receives one type of circularly polarized wave as an earth station antenna (hereinafter, it will be explained assuming that it receives right-handed circularly polarized wave).
In a conventional satellite communication system of this kind, the satellite antenna 2 corresponds to the earth station antenna 4 and uses an antenna that transmits and receives right-handed circularly polarized waves. That is, as shown in FIG. 4b, the polarization characteristics of the satellite antenna 2 have been such that an antenna that emits substantially right-handed circularly polarized waves in any direction within the service target area has been used.

In contrast, in the satellite communication system according to the present invention, the polarized waves used on the transmitting side (satellite) are different from the polarized waves on the receiving side (earth station) as follows. That is, the feature is that a satellite antenna with specially shaped polarization characteristics is used. As shown in FIG. 4c, the satellite antenna 2 emits right-handed circularly polarized waves in a direction looking toward the center of the earth. On the other hand, in the direction looking toward the Earth's periphery, a right-handed elliptically polarized wave that is flat and long in the radial direction from the periphery to the central beam direction is emitted. By using the satellite antenna 2 having such polarization characteristics, the earth station antenna 4 receives right-handed circularly polarized waves, and the reflected waves 6 from the sea surface, etc., tend to become left-handed circularly polarized waves. 6 will no longer be received, and fading will no longer occur. In this case, since the polarization characteristics of the earth station antenna 4 do not match the direct wave 5, a polarization loss occurs, but this is sufficiently small compared to deep fading that reaches 9 dB or more.

Next, "flat elliptically polarized waves long in the radial direction" will be explained. Let i be the angle of incidence of a ray of light from the satellite 1 as shown in FIG. 5 toward the earth's surface looking toward the earth's periphery. When the radiated electric field of the satellite antenna 2 in the θ direction is expressed by the X direction component and the Y direction component as follows, 〓=〓＾E _x +〓＾E _y ……(1) The polarization characteristics in that direction are It can be expressed as the ratio of E _x and E _y .

If the reflected wave 6 becomes a left-handed circularly polarized wave, the earth station will no longer receive the reflected wave 6. For this purpose, the incident polarization (polarization in the θ direction of the satellite antenna 2) needs to be as follows.

Here, n is the equivalent complex refractive index of a reflector such as the sea surface. For example, using the complex refractive index of the sea surface in the 1.56GHz band n=9.12−j3.62, the incident angle is i=
If it is 80°, the polarization of the satellite antenna 2 according to equation (2) is |E _y /E _x |=2.99 〓〓E _y /E _x = −58.1°. This represents a right-handed elliptically polarized wave that is long in the y direction. The satellite communication system according to the present invention uses radiation angle characteristics of polarized waves substantially defined by equation (2) as polarized waves used on the satellite side. This has the advantage of not requiring a control device to reduce fading, and has the effect of not requiring a large fading margin.

Embodiment 1 In order to realize the satellite communication system according to the present invention, it is necessary to use a satellite antenna having polarization characteristics as shown in FIG. 4c. A specific example of this type of antenna will be described next.

Consider an array antenna in which a large number of cross dipole antennas (see FIG. 3) are arranged in a plane as shown in FIG. An X-Y coordinate system and a ρ-ψ coordinate system are taken within the plane. The cross dipole is composed of an X-direction element and a Y-direction element. In addition,
The antenna element of such an array antenna is not limited to a cross dipole antenna, but can have a similar configuration as long as it can radiate two orthogonal polarized waves.

In order to make the antenna beam rotationally symmetrical with respect to the center direction (the y-axis direction in FIG. 4a) and radiate right-handed circularly polarized waves as shown in FIG. 4b with this type of antenna,
Excitation coefficient of the X-direction antenna located at (ρ, ψ)
A _x (ρ, ψ) and excitation coefficient of Y-direction element antenna
A _y (ρ, ψ) was generally determined as follows.

A _x (ρ, ψ)=A ₀ (ρ) ……(3) A _y (ρ, ψ)=−jA ₀ (ρ) Here, A ₀ (ρ) is the rotationally symmetrical beam intensity distribution E _d
(θ) as shown in the following equation.

E _d (θ)=K ₁ ∫〓 ^m ₀ A ₀ (ρ)J ₀ (kρsinθ)dρ ……(4) Here, J ₀ is the zero-order Betzel function, k is the wave number of the electromagnetic wave, and ρm is the value of this array antenna. The effective aperture radius of , K ₁ is a constant, and θ is θ in FIG.

Equation (3) is based on the X-direction element of each cross dipole and the Y
The directional elements are excited with the same amplitude and a 90° phase difference,
That is, it shows that each cross dipole emits circularly polarized waves. As a result, the array antenna emits circularly polarized waves in an axially symmetrical manner.

Now, in order to obtain the polarization characteristics as shown in FIG. 4c, the excitation coefficient of each element antenna may be changed as follows.

A _x (ρ, ψ) = A ₀ ′ (ρ) + A ₂ (ρ) (cos2ψ
−j sin2ψ) A _x (ρ, ψ) = A ₀ ′ (ρ) + A ₂ (ρ) (cos2ψ
−j sin2ψ) A _y (ρ, ψ) = −jA ₀ ′ (ρ) + A ₂ (ρ) (j cos2ψ
+sin2ψ)...(5) Here, A ₀ (ρ) and A ₂ (ρ) are the rotationally symmetrical beam intensity distribution E _d (θ) and the function W (θ ) and the following related functions.

W(θ) is right-handed circularly polarized wave when W(θ)=1, W
When (θ)=0, it represents linear polarization. When W(θ) is between 0 and 1, it represents a right-handed elliptically polarized wave that is flat in the radial direction. By exciting each cross dipole as shown in equation (5), it is possible to create a beam that essentially has polarization characteristics as shown in FIG. 4c and whose intensity distribution is rotationally symmetric.

Example 2 Next, a second example of the present invention will be described.

This embodiment is a multi-beam satellite communication system in which a satellite antenna 2 emits a plurality of beams, and the plurality of beams cover an area where the earth is viewed from the satellite.
This is an application of the present invention.

When the earth station antenna 4 transmits and receives one type of circularly polarized wave (the case of receiving right-handed circularly polarized waves will be explained below), in a conventional system of this type, each beam of the satellite antenna 2 is The polarization of the wave was configured to have a substantially right-handed circular polarization. That is, the same polarization was used on the transmitter and receiver sides.

In the satellite communication system according to the present invention, as shown in Figure 7b, the polarization of the beam that irradiates the center of the earth is right-handed circularly polarized, and the polarization of the beam that irradiates the periphery is in the radial direction from the periphery to the center. It is characterized by the use of long flat right-handed elliptically polarized waves.

The polarization characteristics (elliptic polarization) of the beam at the edge are:
It is substantially defined by equation (2) with respect to the representative direction of the peripheral edge (for example, the direction of the center of the beam).

By using different polarizations on the transmitting side and the receiving side and setting the elliptical polarization coefficient as shown in equation (2), the polarization of the reflected wave from a reflecting object such as the sea surface is almost left-handed circularly polarized. As a result, the reflected wave 6 is hardly received by the earth station antenna 4 which receives the right-handed circularly polarized wave. As a result, there is an effect that a large fading margin is not required.

FIG. 8 shows a configuration of a satellite antenna 2 for realizing a communication system according to a second embodiment of the present invention.
FIG. 8 shows a conventionally widely used multi-beam antenna using multi-feeds. The radio waves radiated from the feeding horn 21 are radiated through the reflecting mirror 20. Radio waves emitted from different horns travel in different directions and form beams. Consider the case where the OMT 22 is used to excite from two orthogonal ports 23 and 24 as shown in the figure. Figure 7
In order for each beam to emit circularly polarized waves as shown in a, the orthogonal ports 23 and 24 of each horn are
should be excited with the same amplitude and 90° phase difference. on the other hand,
In order to realize the polarization characteristics as shown in FIG. 7b used in this embodiment, the excitation amplitude ratio and phase difference of the orthogonal ports 23 and 24 of each feeding horn 21 must be adjusted according to the required polarization factor of each beam. It's fine if you take it as something like that.

(Effects of the Invention) As explained in detail above, according to the present invention,
By using circularly polarized waves for the central part of the polarization characteristic of the antenna mounted on the satellite, and using elliptical polarized waves with a long axis in the radial direction for the peripheral parts, it is possible to transmit signals to existing earth stations. However, the influence of reflected waves, which are unnecessary waves, can be eliminated without making any equipment changes, and an efficient satellite communication system can be realized. In particular, it is practical and can provide economical effects to mobile satellite communication systems with a large number of earth stations.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing the configuration of a satellite communication system to which the present invention is applied, and FIGS. 2a and 2b are schematic diagrams, characteristic diagrams, and diagrams for explaining the first conventional method for reducing the fading margin. 3a and 3b are block configuration diagrams for explaining a polarization control method conventionally performed on the earth station side in order to reduce the fading margin. A coordinate system diagram and a polarization characteristic diagram to explain the angular characteristics of polarized waves used by satellite antennas. Figure 5 is a diagram showing the coordinate system necessary to define the polarization rate of elliptically polarized waves. Figure 6 is a diagram from this book. First invention
FIG. 7 is a diagram showing an example of the polarization characteristics of each beam used by the satellite antenna in the second embodiment of the present invention. FIG. FIG. 2 is a perspective view showing an example of the configuration of a satellite antenna necessary for realizing a second embodiment of the present invention. 1...Satellite, 2...Satellite antenna, 3...Earth station, 4...Earth station antenna, 5...Direct wave, 6...
...Reflected wave, 7...Reflector, 8...Earth, 10...
Earth station antenna with cross dipole, 11...
... 90° phase shifter, 12 ... Combiner, 13 ... Transmitter/receiver, 14 ... Variable phase shifter, 15 ... Phase control device, 16 ... Elevation angle detector, 20 ... Reflector, 21
...Horn, 22...OMT, 23, 24...Power supply port.

Claims (1)

[Claims] 1. In a satellite communication system using a communication satellite placed in a satellite orbit, the polarization characteristics of the antenna of the communication satellite are circularly polarized for the center of the earth and circularly polarized for the periphery of the earth. On the other hand, a satellite communication system is characterized by elliptically polarized waves having a long axis in the radial direction. 2. The satellite communication system according to claim 1, wherein the antenna is an array antenna in which antenna elements capable of radiating two orthogonal polarized waves are arranged. 3. The satellite communication system according to claim 1, wherein the antenna is a multi-beam antenna using a multi-feed.