WO2002073739A1

WO2002073739A1 - Multibeam spherical antenna system for fixed microwave wireless network

Info

Publication number: WO2002073739A1
Application number: PCT/AM2001/000007
Authority: WO
Inventors: Souren Guerouni
Original assignee: Souren Guerouni
Priority date: 2001-03-13
Filing date: 2001-03-13
Publication date: 2002-09-19

Abstract

Multibeam microwave Base Antenna Station for the creation of fixed wireless Network of urban scale and topology of 'star' type for the frequency range of 30 GHz and higher. With the purpose of providing narrow, address or group connection with a great number of Network subscribers situated at different positions in azimuth and elevation, including subscribers situated at different elevation angles of one and the same bearing, there is used an immovable mirror in the shape of a hemispherical belt. One mirror may provide multibeam coverage of the Network area in the angle sector of 120 degrees in azimuth and of, for example, 30 degrees in elevation.The assembly of three mirrors fastened to a common mast with their rear sides provides circular multibeam coverage of the area. The assembly with the fourth mirror added in the upper tier represents the Network subscribers access into satellite communications.

Description

MULTIBEAM SPHERICAL ANTENNA SYSTEM FOR FIXED MICROWAVE WIRELESS NETWORK

1. FIELD OF THE INVENTION

The invention relates to the technique of microwave multibeam mirror antennas and may be used as a peripheral or central Base Antenna Station to create fixed Wireless Local Area Network for hundreds of subscribers, or integrated Wireless Network on urban scale for thousands of subscribers, with the access possibility to satellite communications and other ground Networks.

2. PRIOR ART

There is known a multibeam mirror antenna in the shape of spherical segment with N feeds placed with the possibility of moving along one concentric guide ("Antenna System". PCT-application JY° PCT/AM94/00001, PCT-publication JV° WO 95/06963, PCT GAZETTE- SECTION 1, 1/1995).

However, an Antenna System known has a limitation and cannot be used to create a full-value Wireless Network since its feeds have the possibility of positioning their axes (i.e. forming their beams) only in azimuthal section of the mirror, in angles corresponding to the direction towards the positions of geostationary satellites in the equatorial orbital plane. And given antenna system is not adapted for simultaneous forming multiple beams in one or several vertical elevations of mirror sections.

There is also known an antenna assembly of three mirrors in the shape of parabolic cylinder with N feeds placed in one horizontal plane ("Multimirror Cylindrical Paraboloidal Antenna for Omnidirectional Coverage". USA Patent JY° 3,317,912 from 2 May, 1967). However, such an antenna assembly in principle has no possibility to form numerous beams in elevation, since in the vertical section its mirrors have the shape of parabola. And, as it is known from the parabola properties, if the feed is displaced from its focus for an angle of more than several degrees the degradation of pattern and Gain Factor occurs.

Thus, both known antennas form one-directional row of beams in the azimuthal plane and are not fitted or have no possibility to simultaneously form multiple beams in vertical elevation planes of the mirror.

At the same time in a real urban Network a great number of subscribers may be at different elevation angles, in perceptible range of angles in the vertical, and, with this, find themselves in one azimuthal bearing. 3. OBJECT OF THE INVENTION.

Technical task and object of the present invention is to create a simple and cheap multibeam mirror antenna system and variants of its assembly which will permit to create a fixed microwave Wireless Local Area Network or a Network on urban scale of cellular type with the possibility of simultaneous forming and pointing of numerous narrow beams onto subscribers being at both different azimuths, and different elevation angles, while many of the latter may be in one azimuthal direction. Here, it is desirable that the Network subscribers may have the possibility of realizing, via Base Station offered, the access to the satellite communication systems and other ground Networks, too.

4. DISCLOSURE OF THE INVENTION

It is offered to realize the task put using hemispherical mirrors truncated in the shape of a belt, which in their immovable state will ^"provide, without changing their characteristics, an angle coverage of 120° in their horizontal-azimuthal section and designed angle coverage in their vertical-elevation section. The mirrors may be installed at the town periphery and in the line-of- sight of each other. Combining their Local Area Networks over the beams allocated for it,

"peripheral" Base Stations create an integrated Network of urban scale. Depending on the configuration of urban quarters location, it may appear necessary to put at one point, along with

"peripheral" stations, an assembly of two mirrors with total coverage of 240° in azimuth.

In arranging a Base Station in the center of the town one may apply the variant of "central" antenna assembly in the shape of three mirrors placed at a point and creating in common a full circular coverage of 360°.

Given variants of the Base Station are convenient to put on the roofs of high-rise buildings and other advantages heights.

Main axes of these mirrors should be oriented over the angles below the horizon to serve ground subscribers. Let's call these antennas "lower tier" of the assembly.

To provide the Network subscribers' access to other ground Networks there may be organized positioning of one of the assembly feeds to the terminal of, for example, some communication system radiolink.

To provide the Network subscribers' access to the satellite communication channels it is necessary one more mirror with similar characteristics, aimed upward with its main axis into equatorial orbital plane where geostationary satellites are placed. Let's call this mirror the antenna of "the top tier" of the assembly.

To realize radio link between subscribers on the local area Network or urban scale at the distances of, for example, 5-10 km it is enough handle small power values of the transmitting part of electronic network equipment (particularly, the equipment of broadband technology uses powers of the order of 5-10 mW). However, to transmit signals to the geostationary satellite there are used powers of higher order ( for example, in the space communications systems of NSAT ( Very Small Aperture System) type with the mirror diameter of 2-3 m there are used signal powers of 5-10 W). To separate antennas of the assembly lower tier from the influence of the top tier antenna it is offered in the present application to place an isolating shield in the shape of a metallic disk between them.

The spherical mirror in its nature is highly broadband. For different reasons it is expedient to apply the multibeam antenna suggested to create the nets at the frequencies above 30 GHz. Note that the mirror may simultaneously function at different frequencies which enlarges the Network possibilities, including the possibility of integrating with other Networks. The multifeed principle consists of that each feed "illuminates" only a part of the mirror surface, creating its own local aperture. Multiple of such local apertures, created by a great number of feeds, can coexist in one mirror being superimposed on each other, but not preventing each other. The least angular discrete of superposition both in the horizontal and in the vertical is dictated by the smallest distance to which the two adjacent feeds can be pushed to each other. In practice, the feeds can be pushed together till the physical contact of their fastening parts. Calculations show that in the aperture of one hemispherical mirror of 2 m x 1.2 m in size it is possible, without special fears, to mount more than hundreds of feeds prepared for the frequency of 40 GHz. These may be compact feeds, for example, in the shape of open end circular waveguide with its aperture diameter of 15 mm. Each feed may create a local zone/spot of, for example, 0.5 m in diameter at a common mirror. The pattern width from such a local aperture is 1°. Gain Factor (GF) of local aperture is 42 dB. The range is 50 km. With the feeds' number of 100 the mirror shadowing factor may increase up to 12%. However, in view of big GF reserve this shadowing does not prevent from the main goal of the Base Station, that is, from forming wireless Network with the cell radius of 5-10 km which is quite enough for the town. As it was noted in the section 2, subscribers of the urban wireless Network can be situated at very different azimuthal angles and different elevations, for example, at different floors of a high rise building or at different relief heights and different distances from the Base Station, but in one azimuthal bearing. To provide numerous subscribers with individual narrow beams it is necessary to synthesize an appropriate two-dimensional matrix of mirror feeds. If in the service area of one "address" beam there appear several subscribers, all of them will also be full members of the Network since current technologies of coding and signal processing - for example, the technology with time, division of signals - permit to conduct simultaneous and independent work with different subscribers situated in the service area of one and the same beam. In the vertical elevation plane, in practice, there is no need in such a wide range of coverage angles which is provided by a hemispherical mirror. Simple calculation shows that angle sector, equal to 30° provides at the distance of 5 km from the Base Station the coverage of heights with an overfall of 2.5 km which is quite enough for a town with an abrupt overfall of relief. Thus, if the coverage of subscribers inside the angle sector of 120° in azimuth and in elevation of, for example, 30° is to be provided it is sufficient to use a spherical mirror opened in its horizontal up to the size of a hemispherical and symmetrically truncated in its vertical size to the designed value which provides the coverage of specified elevation angles sector.

To accomplish the goal placed in the given invention there are introduced N pieces of elevation concentric relative to the spherical mirror guides installed in the mirror vertical sections, on which there are fastened N pieces of rows of feeds with the possibility of moving up/down and with the possibility of their fixing. These elevation guides are fastened in their ends to the two, also concentric relative to the mirror, azimuthal guides installed in the mirror horizontal sections. The second azimuthal guide is introduced here with the purpose of increasing the accuracy of feed axes positioning. Elevation guides have the possibility of moving to the left/to the right and being fixed along azimuthal guides which, in their turn, are rigidly fastened to the mirror.

Thus, two-dimensional azimuth-elevation matrix of feeds is completed, which in common with a hemispherical mirror truncated in its height forms a great number of narrow beams. Pointing of the Base Station beams to the subscribers' positions is carried out manually over the signal maximum level transmitted from a subscriber antenna in the direction of the Base Station. Feeds and, correspondingly, antenna beams can be easily re-positioned in connection with, for example, the changes in subscribers dislocation.

In case of double-tier antenna assembly with a multibeam antenna designed to enter different satellite communications, the latter is oriented with its main axis above the horizon and on 180° in bearing ( the direction to the south for the Northern hemisphere). The elevation angle of the axis is λ° - 90° - φ°, where φ is geographical latitude of the antenna installation place. Application of one hemispherical mirror permits it to provide, in its immovable state, the coverage of almost the wholeΛάsible part of the orbit with tens of from middle latitudes and higher.

5. BRIEF DESCRIPTION OF DRAWINGS

Fig.l. - Axonometric view of multibeam spherical Base Antenna Station of "peripheral" type. Fig.2. - Sketch illustrating the principle of feeds mounting and correction of their axes. Fig.3. - Axonometric view of the Base Station antennas assembly of "central" type with an access to satellite channels.

6. DETAILED DESCRIPTION OF PREFFERED EMBODIMENT

In Fig.l there is given an axonometric view of a multibeam reflector.

Mutibeam spherical antenna comprises: a mirror in the shape of a hemispherical belt 1, two horizontally placed concentric guides 2, N pieces of vertically placed guides 3, M pieces of feeds

4, M pieces of correction angle compensators 5.

Description of the structure. The mirror 1 represents a cutting-out of spherical surface opened in its horizontal size up too a hemisphere, and in its vertical size by the amount optimal for the coverage of elevation angles of the Network subscribers.

Azimuthal guides 2, placed in the horizontal section of the mirror and concentric to it, are opened in their size up to not less than 120°. Elevation guides 3, placed in the vertical sections of the mirror and concentric to it, too, are opened in their size up to the designed value of the angle, which provides optimal angle of coverage of the Network subscribers' elevation angles.

The guides 3 are fixed in their ends to the guides 2 with the possibility of moving along them and being fixed on them. The guides 2 themselves are rigidly fastened in their ends to the mirror 1.

The feeds 4 are fastened on the guides 3 with the possibility of moving along them.

In Fig.2 there is shown the principle of feeds mounting and their axes correction.

The feeds 4 axes should be installed so as to intersect without fail the 0 point in the sphere center generated by the spherical mirror 1. In the vertical section of the mirror this condition is accomplished by providing perpendicularity of the feed 4 axis to the concentric guide 3 in each point of it. However, in the horizontal section of the mirror the feed 4 axis turns out to be displaced from the sphere center to the left or to the right side depending from which side - left or right- it is fixed to guide 3.

Linear value of this displacement is equal to a half amount of thickness values of the guide 3 and external diameter of the feed 4.

To this horizontal linear value of the displacement there corresponds a definite azimuthal angular value γ which should be compensated.

For this purpose a compensator 5 is introduced between the guide 3 and feed 4. Note here an important detail: with constant dimensions of the guide 3 thickness and feed 4 diameter, nevertheless, an angle Y turns out to be different for different values of the mirror curvature radius, since in this case there changes the distance between the feed aperture and the sphere center. In Fig.2 there are also shown local apertures 6 generated by separate feeds 4.

Operation principle of the multibeam antenna is the following.

The Base Station Antenna is installed at a convenient site which has an advantageous height and good coverage of the local area Network territory. The main axis of the mirror is oriented to the point which is approximately the center of both azimuthal sector of coverage angles of subscriber and elevation sector of coverage angles of the Network subscribers. Then mutual pointing of beams of subscribers' antennas and of the Base Station is carried out.

The beams (axes) of subscriber antennas are pointed as accurately as possible to the sphere center of the Base Station antenna. The feeds of the Base Station mirror are positioned onto the appropriate subscribers' coordinates. Selection of the pair "feed-subscriber" is carried out according to the principle of optimum, i.e. for a concrete subscriber out of the two-dimensional matrix of feeds there is selected the feed which is the closest to the given azimuthal-elevation direction to the subscriber. More exactly, feed positioning is carried out over the signal maximum level.

Construction principle of double-tier antenna assembly.

In Fig.3 there is given an axonometric view of a double-tier assembly of four multibeam hemispherical antenna mirrors for application as a "central" Base Station of an urban microwave

Network with satellite communications output.

Lower tier represents an assembly of three mirrors in the shape of a hemispherical belt, fixed to each other with their rear sides around the common assembly mast, and in such a configuration that their main axes form a symmetrical "trilete star". Total angle of azimuthal coverage of the lower tier antennas is 360°. The structure of antenna mounting mechanism should provide for the possibility of the mirror main axes tilting by small angles below the horizon. The fourth mirror 7 is installed above the lower assembly creating the second, upper tier of the assembly. This mirror, having the shape of a hemispherical belt, is fastened on the common mast 8 of the assembly, with the possibility of adjusting its main axis position in an elevation angle in the plane of local meridian. The mirror is provided with a number of feeds equal to the number of required for two-way communication and/or for viewing teleprograms via them. In the middle between the two tiers there is coaxially fastened to the assembly post 8 a metallic disk 9 acting as a screen of electromagnetic emissions between the antennas of lower and upper tiers. The screen in the shape of a disk also performs the. function of a site for the upper tier antenna maintenance. The disk-screen diameter has dimensions not less than external dimensions of the assembly. Since antennas of microwave band are sensitive to the atmospheric precipitations and dust and to protect the mirrors against wind loads, it is expedient to arrange the Base Station inside a radio transparent radome with optimal dimensions and configuration. In case of double-tier antenna assembly an assembly of radomes consisting of the upper hemispherical part and lower cylindrical part is optimal. Hemispherical radome 10 covers the mirror 7 and is coaxially fixed in its base to the disk-screen 9. Cylindrical radome 11 covers the lower assembly mirror. The disk-screen 9 is also coaxially fastened on the upper perimeter of the radome 11. The radii of the hemispherical and cylindrical radomes as well as of the disk-screen have equal values.

7. INDUSTRIAL APPLICABILITY

The antenna offered and its assemblies may be used as a Base Station for joint operation with electronic and microwave equipment of different wireless network technologies, both already existing ones and perspective ones. Its cost turns out small since the mirror fragments and other units easily yield to pressing. The assembly has compact dimensions, light weight and urban style.

Claims

CLAIMSI claim:Multibeam Antenna System comprising a spherical mirror, a number of feeds and an azimuthal concentric guide, d i ff e r i n g in that

1. There are introduced N pieces of concentric elevation guides fastened in their ends to the first and second introduced concent ric azimuthal guides with the possibility of moving and being fixed on them, there are also introduced N rows of feeds mounted with the possibility of moving and being fixed on the introduced concentric elevation guides, by the number of feeds there are introduced correction compensators of azimuthal angles of feeds' axes, which are installed between the feeds and their concentric elevation guides, on the left or on the right side of them, herein spherical mirror profile being specified in the shape of a belt, opened in its width up to the size of a hemisphere and in its height by an angle providing coverage of elevation angles of all the Network subscribers.

2. Antenna assembly comprising four antenna modules according to Claim 1, three of which are fastened with their rear sides around a common mast in the configuration of a symmetric trilete star, with the possibility of their main axes tilting by small angles below the horizon, the fourth mirror is fastened to the common mast with the possibility of adjusting its main axis position in an elevation angle above the horizon, herein in the middle of the interspace between upper mirror and lower three mirrors there is introduced a plane screen, coaxially fastened to the common mast, in the shape of a disk with the diameter not less than the dimensions of the whole assembly.

3. Upper antenna assembly according to Claim 2 is mounted inside the introduced hemispherical radio transparent radome coaxially fixed in its base to the disk-screen, the latter being also coaxially fastened on the upper perimeter of the introduced cylindrical radome inside which lower tier antennas of the assembly are installed, herein the radii of hemispherical radome, cylindrical radome and of disk-screen have equal values.