WO1997007609A2

WO1997007609A2 - High altitude cellular communication system platform

Info

Publication number: WO1997007609A2
Application number: PCT/IL1996/000074
Authority: WO
Inventors: Avi Gover; Raphael Kastner
Original assignee: Ramot University Authority For Applied Research & Industrial Development Ltd.
Priority date: 1995-08-11
Filing date: 1996-08-07
Publication date: 1997-02-27
Also published as: WO1997007609A3; AU6709396A

Abstract

A high altitude cellular communications system platform utilizing a multi-beam array antenna having an array of elements. A multi-beam array antenna and associated circuitry are mounted on a high altitude atmospheric/stratospheric platform positioned up to 150 kilometers high. A cellular communications processor controls setup and tear down of wireless communication links or calls with subscriber's personal communication devices. Vertical broadcasting provides sharp antenna radiation pattern boundaries between cells allowing for both high inter-cell resolution and small cell size. In an alternative embodiment, the cellular communications processor is located on the ground and communicates with the high altitude platform via a communications link. The system can operate independently or in conjunction with ground based cellular systems and Low Earth Orbit Satellite systems using common personal communication devices.

Description

HIGH ALTITUDE CELLULAR COMMUNICATION SYSTEM PLATFORM

FIELD AND BACKGROUND OF THE INVENTION

Today, the demand for wireless cellular communications, including personal communication services (PCS), is growing at a faster and faster rate. United States PCS usage alone is currently estimated at 31 million subscribers by the year 2003 and 150 million in the long run. The PCS international industry is expected to be worth 195 billion dollars by fhe end of the decade. High demand currently exists for both narrowband communications (i.e. voice, picture phone, paging and private radio) and wideband communications (i.e. video, multimedia and high bandwidth digital data). A limited frequency band, however, has been allocated to cellular communication. In the United States, the Federal Communications

Commission (FCC) has recently allocated at present 3 MHz centered around 900 MHz for narrowband communications and 120 MHz centered around 2 GHz for wideband communications.

Without cellular technology, a very limited number of calls could take place simultaneously in the same region. Cellular technology greatly expands the number of calls that can simultaneously be made. Regions with high demand for cellular telephone subscriptions are broken down geographically into smaller sized cells. A large city or densely populated region may be divided into a large number of cells. Each cell has a base station and is able to support a finite number of simultaneous calls. Base stations include transmission and receiving equipment for communicating with subscriber cellular equipment, such as telephones. Each base station maintains communication links with all other cells as well as maintaining access to the standard telephone network. Both base station transmitters and cellular communication devices transmit radio signals at a reduced power level. Transmitted power is reduced in order to ensure that communications taking place in one cell do not interfere with communications in other cells. It is this feature of cellular communications that makes it possible to reuse the limited frequency spectrum available in cells that are remote enough to avoid interference and crosstalk between them. The limitation of conventional cellular communication systems is that it is very difficult to limit the range of the base station radio transmitters and at the same time maintain a uniform level of radio transmission and reception within the cell, a necessary condition for mamtaining reliable communications. Radio waves emitted from the antennas of base station transmitters decay gradually, typically at a rate of 1/R⁴, R being the distance from the base station antennas, and do not maintain a uniform level within the cell. In addition, due to destructive scattering, reflections and wave interference (especially in areas with numerous structures) the radio signal level at each point in the cell is unpredictable and is susceptible to periodic fading and statistical variation. The situation becomes worse when considering hand held personal cellular or motor vehicle cellular communications, both of which are mobile. As the cells become smaller, this problem becomes more and more acute. This is due to the larger affect of statistical variation of the communication range. This is a major obstacle for increasing the volume of cellular communication service. With present day technology, often the entire available frequency spectrum has been used. The only way to increase the service volume is by reducing the size of the cells.

The difficulty in maintaining both a uniform broadcasting level within the cell and a sharp cutoff beyond the cell boundary, results in problematic and poor quality cellular commumcations. Due to the lack of a sharp cutoff in the power level across cell boundaries, the reuse rate of the spectral bandwidth is reduced. Typically, the reuse rate in a hexagonal lattice cell arrangement is less than 1:7. This means that each cell, in a group of seven neighboring cells, must use different portions of the available spectrum. Spectral band reuse can only take place in the adjacent group of seven cells. Crosstalk, however, still occurs, reducing the quality of the communications.

Another disadvantage of conventional cellular technology is that due to the need to maintain a minimum power level at cell boundaries, destructive wave interference sites and areas shadowed by building structures, the power of the base station transmitter must be high. In densely populated areas high transmitted power is a common source of concern with residents residing near the base station apprehensive about radiation hazards.

In addition, in order to maintain radio communication with subscribers at any geographical point in the cell at all times, the power of the subscriber's radio transmitter must be higher than essentially required. This is due to nonuniformity within the cell of the reception sensitivity of the base station receiver. This nonuniformity is similar in nature to the transmission nonuniformity described above. Reception sensitivity nonuniformity and the consequent need to increase the power of the subscriber's radio transmitter causes personalized cellular communications devices to be heavier and more expensive than necessary, in addition to generating complaints from radiation hazard conscious subscribers.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages and limitations of the prior art by providing a system of cellular communications employed at high altitude on a floating, flying or hovering platform that utilizes multi-beam array antenna technology. The platform consolidates many conventional base stations into one system, eliminating the need for redundant equipment.

A main advantage of utilizing a multi-beam array antenna, mounted on a high altitude platform, is that the array can be designed to have radiation lobes (i.e. beams) with sharp boundaries between cells. The ground can, therefore, be illuminated by a beam with a slowly varying distribution, greatly reducing the effects of diffractions from obstacles, multiple scattering and rapid variations and fading of the signal. This advantage is due mostly to the effects of communicating in the vertical direction, rather than in the horizonal direction. An added benefit is that in military applications, vertical communication provides better electronic intelligence (ELINT) security and better unmunity to j_--mπιing and other types of electronic warfare. In addition, the shape of each individual beam can be optimized, using aperture synthesis techniques, to increase insulation and reduce crosstalk between cells. This allows both the base station transmitter and the personal communications devices, to transmit and receive power at a reduced level resulting in lighter weight portable radio devices, longer battery life, lower cost, less health risk from radiation hazards, etc. The more efficient upwards looking antennas included in personal communication devices of such a system further reduce the required transmit power resulting in the same benefits in connection with reduced transmit power levels. The boundaries for the entire service area can be easily changed by transporting the high altitude platform to a new location. This mobility also allows cellular service to be setup quickly to provide coverage for new service areas, on a temporary basis over disaster areas (e.g., earthquakes, floods, fires or other events leading to outage of ground based communication or requiring major evacuation), special events and for military applications.

Use of multi-beam array antenna technology allows cells to be reassigned dynamically. Cell coverage (e.g., cell size reduction and expansion) can be electronically controlled and adjusted in response to, for example, changes in communication load distribution, emergencies, etc. In addition, reliability and stability are increased and the area of service coverage expanded by connecting, either physically or electronically, two or more floating platforms controlled by a central computer. Hence, there is provided according to the teachings of the present invention, a high altitude platform cellular communications system comprising a cellular communications processor for controlling a plurality of wireless radio communication links between a plurality of cellular communication devices and the system, multi-beam array antenna control circuitry coupled to the cellular communications processor, the multi-beam array antenna control circuitry for controlling radio frequency transmission from and reception to the high altitude cellular communications system, multi-beam array circuitry coupled to the multi-beam array antenna control circuitry, the multi-beam array circuitry for simultaneously generating a plurality of transmission signals and for simultaneously receiving a plurality of reception signals, a multi-beam array antenna coupled to the multi-beam array circuitry, the multi-beam array antenna for simultaneously generating a plurality of independent beams for transmission and reception of radio signals to and from assigned terrestrial cells, and a high altitude platform for supporting the high altitude cellular communications system at a predetermined altitude, the high altitude platform for approximately mamtaining the position of the high altitude cellular communications system at the predetermined altitude and horizontal position. While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference to the accompanying drawing, wherein:

FIG. 1 is a block diagram of a preferred embodiment of the present invention illustrating an airborne based cellular communications processor; and

FIG. 2 is a block diagram of a preferred embodiment of the present invention illustrating a ground based cellular communications processor.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is of a cellular communications system mounted on a high altitude platform and utilizing a multi-beam array antenna to broadcast a multitude of signals simultaneously to cells located on the ground.

The principles and operation of the present invention may be better understood with reference to the drawing and the accompanying description.

A block diagram of an embodiment of the present invention is shown in Figure 1. This embodiment includes an airborne based cellular communications processor 26 at the core of cellular communications system 10. Coupled to airborne cellular processor 26 is multi-beam array (MBA) antenna control circuitry 30. Coupled to MBA antenna control circuitry 30 is multi-beam array (MBA) radio frequency (RF) antenna circuitry 24. Coupled to MBA RF antenna circuitry 24 is an MBA antenna 14. MBA antenna 14 includes a plurality of array elements 16. The entire system is mounted on a high altitude platform 12 that can either float, hover or fly over a certain area on the ground. Platform 12 may be a blimp, drone, airplane, remotely piloted vehicle (RPV), helicopter or the apparatus for supporting an unmanned airborne vehicle disclosed in U.S. Patent No. 5,074,489, issued to Gamzon, entitled Method And System For Supporting An Airborne Vehicle In Space. A preferred embodiment includes high altitude platform 12 disclosed in U.S. Patent No. 5,074,489. System 10 maintains a connection to an external communications network 40, which may include the standard telephone 5 network, a video network, a fiber optic network, a computer data network and/or other types of communications networks. A ground station functions in mamtaining the link between system 10 and external communications network 40. A ground link controller located on platform 12 controls commumcations between platform 12 and the ground station. 0 A satellite link controller located on platform 12 controls communications between platform 12 and one or more satellites 46.

As previously described, conventional cellular technology works by dividing a geographical area to be covered into small sized cells. Within each cell is placed a base station transmitter and receiver. Both cellular 15 phone transmitters and base station transmitters transmit at a reduced power level suitable for the particular cell size. This scheme allows the available frequency spectrum to be reused by distant cells. A processor controls the setup and tearing down of each call made through subscriber cellular equipment. 0 Instead of having a multitude of land based transmitters and receivers, spread out over a large geographic area, system 10 joins them together into one central location. MBA antenna 14, when connected to MBA RF antenna circuitry 24, is capable of generating many independent antenna radiation beams 20 simultaneously. MBA RF Antenna circuitry 25 24 is an RF beam forming network, typically including a Rotman lens or a Butler RF matrix, and an RF switching matrix. RF antenna circuitry 24 is capable of simultaneously generating all beams 20 of MBA antenna 14. The aperture distributions across all array elements 16 for all beams are superimposed on each other, making use of the same physical aperture 30 which is common to all beams 20. Each beam 20 is assigned a transmit/receive port in RF antenna circuitry 24. Each transmit/receive port is connected to a corresponding port of MBA control circuitry 30. MBA control circuitry 30 controls MBA antenna circuitry 24 by channeling signals to and from transmit/receive ports of MBA antenna circuitry 24. Each beam 20 covers a single cell 22. Thus, each beam replaces one terrestrial base station. RF antenna circuitry 24 with many beams enables MBA antenna 14 to cover an area with many cells. In addition, coverage can be expanded by joining multiple platforms 12 together. Each platform 12 is able to communicate with other platforms, satellites and ground stations, forming a network.

In a preferred embodiment, shown in Figure 1 , airborne cellular communications processor 26 is located on high altitude platform 12, preferably up to 150 kilometers high. Ground station 42 communicates with a ground link controller 56 located on high altitude platform 12 via a suitable communications link such as RF, optical, microwave, etc. In addition, an optional satellite link controller 54 communicates with one or more satellites 46. Airborne cellular communications processor 26 controls the establishment and breakdown of calls. Airborne processor 26 communicates with ground station 42, though ground link controller 56. Ground station 42 maintains communication links with external communications network 40. External communications network 40 may include the standard telephone network or other types of networks such as, video, fiber and/or data.

Similarly to maintaining communication links with satellites, high altitude platform may include an auxiliary radio link for communicating with planes, ships or boats. Alternatively, MBA antenna 14 may be used to communicate with planes, ships or boats. The auxiliary radio link would be coupled to communications processor 26.

In a preferred embodiment, shown in Figure 2, cellular commumcations processor 48 is ground based, communicating with airborne cellular communications processor 58 through ground link controller 52. Optional satellite link controller 50 maintains communications with one or more satellites 46.

The laws of diffraction can be utilized to estimate the number of cells that be can be serviced by MBA antenna 14. The following definitions apply:

N = Number of cells D = Cell diameter d = Antenna array diameter h = Platform height

F = Factor < 1 λ = Radio wavelength

Aθ = Angular extent of the entire service area (including N cells) from the high altitude platform D = Fλh/d

N = (Δ0d/Fλ)² As an example, assume λ = 15 cm (for a frequency of 2 GHz), d = 5 m, F = 1 and Aθ = 1 rad (considering viewing angles smaller than 30° relative to the vertical direction on all cells). The number of cells, N, is approximately 1,000 (i.e. a grid of cells approximately 32 x 32).

Platform height can also be estimated given the cell diameter. The platform height is primarily limited by the cell diameter and antenna array size, as shown in the following relationship: h = Dd/(Fλ) Lower heights or larger antenna arrays will be required for smaller cell sizes. If high altitude platform 12 is at a height h = 20 km, cell size 22 dimension D is calculated to be 600 m.

The example above is realistic for the following reasons.

Generally, global winds are at a minimum between 17 and 22 km high, making it easier to maintain the position and orientation of high altitude platform 12. The density of air at this altitude is low but still high enough to support a number of practical platforms 12 enumerated previously. In addition, the distance of 20 km is close enough to the earth, making it possible to energize the platform by power transmission or other means. As cell size is reduced, the uniformity of transmission/reception levels inside cells 22 and the low inter-cell interference cause the general performance of system 10 to remain high. This is in direct contrast to the negative effects of reduced cell size associated with conventional cellular technology. Note that because of practical limitations on the size of antenna array 14, platforms that are located at heights much larger than that of the example will generate cells that are much larger than 600 meters and will not match the increasing need for lower cell sizes.

In a preferred embodiment, cellular communications system 10 is deployed together with specialized personal communication devices (i.e. personal cellular telephones or computer or video devices) distributed to subscribers. The high altitude platform 12 is positioned approximately vertical (i.e. 45° to 90° off the horizon) relative to most cells 22. MBA antenna 14 transmits and receives using circularly polarized radiation. The personal communication devices are equipped with an upwards looking antenna which can receive circularly polarized radiation and an optional sideways looking antenna which can receive linear polarization. The ability to receive linearly polarized radiation, in addition to circularly polarized radiation, allows the personal communications devices to also be used with conventional ground base stations. The feature of an upwards looking antenna made available to subscribers of the high altitude platform cellular service allows them to be used with satellite global communications systems, such as Motorola's Iridium system or Teledesic's Global Wireless Broadband Network, thereby enhancing the services available to subscribers with the same radio communication device they already own. In another embodiment, the number of available calls or channels can be doubled by simultaneously utilizing both clockwise and counterclockwise circular polarization. Half the personal communication devices would be outfitted with a clockwise circularly polarized antenna and the other half with a counter clockwise circularly polarized antenna. Signals having clockwise circular polarization, for example, cannot be received by phones having counter clockwise circularly polarized antennas, and vice versa.

Another application of the system 10 is as an add-on to a global satellite system. Typically, global satellite systems have very large cells in sparsely populated areas. However, they utilize personal communication devices having upwards looking antennas. Thus, high altitude platform 12, positioned at a relatively low altitude compared to global satellite systems, can use the same phones already owned by subscribers, such as of low earth orbiting satellite systems, to provide higher resolution cell coverage in specifically chosen high traffic areas.

As stated previously, current cellular communications services cannot provide wideband services, such as video communication (e.g., traffic control of road junctions, picturephone, video on demand, etc.), computer communication, multimedia services, etc. Such wide bandwidth services would consume the available spectrum unless very small cells with high reusability of the spectrum were employed. Cellular communications system 10 can provide such wideband services due to its ability to provide small size cells having sharp beam cutoffs between cells. An advantage of this feature is that system 10 can be employed in delivering wideband communications services. These services may be integrated in present state of the art communications networks (e.g., the Internet), making it possible to provide a large number of subscribers with enhanced quality and high speed transmission of pictures, real time video and large computer data files. Such services can also be provided through other video and multimedia vendors, including cable TV, or video networks (which can provide personal movie order transmissions), picturephone networks, etc. In addition, it may be possible that the orthogonality between the polarizations of the vertically transmitting system and the standard horizontally transmittmg system, may allow the reuse of the entire frequency bandwidth allocated for ground based services (including the telephone service bands). In addition, system 10 can be employed to respond quickly to the demand for new service areas.

Another application of cellular communications system 10 is as a complement to conventional existing ground based cellular networks. Ground based cellular communications systems can benefit from the flexible cell definition of system 10. Service in congested areas is improved by the ability to better defme cell boundaries and to create smaller cells. In addition, new service areas can quickly be covered by system 10, on a temporary basis, until permanent base station equipment is installed. System 10 can also provide service to cells in which it would be difficult to locate a base station because of real estate problems or because of objections by local residents. Ground base stations that develop problems and cease to operate can be taken offline and temporarily replaced by system 10 until fixed and placed back hi service.

In applications where system 10 complements conventional ground base stations, MBA antenna 14 must transmit and receive vertically polarized radiation in an almost horizontal direction. This makes it necessary to position floating platform 12 at some distance from the service area (i.e. less than 45° above the horizon) and to cause antenna array 14 transmit and receive nearly vertically polarized radiation. This allows system 10 to communicate with conventional personal commumcations devices. This will, however, reduce the number of available cells that can be serviced by system 10. Alternatively, the personal communications devices may be equipped with both horizontal and vertical looking antennas and platform 12 may still be placed approximately above the service area.

While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.

Claims

WHAT IS CLAIMED IS:

1. A high altitude platform cellular communications system comprising: a cellular communications processor for controlling a plurality of wireless radio communication links between a plurality of cellular communication devices and the system; multi-beam array antenna control circuitry coupled to said cellular communications processor, said multi-beam array antenna control circuitry for controlling radio frequency transmission from and reception to the high altitude cellular communications system; multi-beam array circuitry coupled to said multi-beam array antenna control circuitry, said multi-beam array circuitry for simultaneously generating a plurality of transmission signals and for simultaneously receiving a plurality of reception signals; a multi-beam array antenna coupled to said multi-beam array circuitry, said multi-beam array antenna for simultaneously generating a plurality of independent beams for transmission and reception of radio signals to and from previously assigned terrestrial cells; and a high altitude platform for supporting the high altitude cellular communications system at a predetermined altitude, said high altitude platform for approximately maintaining the position of the high altitude cellular communications system at said predetermined altitude and horizontal position.

2. The cellular communications system of claim 1 , wherein said transmission and reception signals include circularly polarized radiation broadcast to and from said cellular communication devices in a vertical direction.

3. The cellular communications system of claim 1 , wherein said transmission and reception signals include both clockwise and counterclockwise circularly polarized radiation broadcast to and from said cellular communication devices in a vertical direction.

4. The cellular communications system of claim 1 , wherein said transmission and reception signals include linearly polarized radiation broadcast to and from conventional cellular communication devices and said cellular communication devices in a nearly horizontal direction.

5. The cellular communications system of claim 1 , wherein said cellular communication devices receive and transmit clockwise circularly polarized radiation broadcast to and from said multi-beam array antenna in a vertical direction.

6. The cellular communications system of claim 1 , wherein said cellular communication devices receive and transmit counterclockwise circularly polarized radiation broadcast in a vertical direction.

7. The cellular communications system of claim 1 , wherein said cellular communication devices receive and transmit linearly polarized radiation broadcast in a nearly horizontal direction.

8. The cellular communications system of claim 1 , wherein said cellular communications devices receive and transmit circularly polarized radiation and linearly polarized radiation.

9. The cellular communications system of claim 1 , wherein said high altitude platform maintains an altitude range up to 150 kilometers.

10. The cellular commumcations system of claim 1 , wherein said high altitude platform includes a drone.

11. The cellular communications system of claim 1 , wherein said high altitude platform includes an aerostat.

12. The cellular communications system of claim 1 , wherein said high altitude platform includes an unmanned airborne vehicle.

13. The cellular communications system of claim 1 , wherein said cellular communications processor includes an airborne cellular communications processor located on said high altitude platform.

14. The cellular communications system of claim 13, further comprising a ground station for maintaining communications with an external communications network, said ground station communicating with said airborne cellular communication processor through a ground link controller.

15. The cellular communications system of claim 1 , wherein said cellular communications processor includes a ground portion and an airborne portion located on said high altitude platform, said ground portion communicating with said airborne portion through a ground link controller.

16. The cellular communications system of claim 15, further comprising a ground station for mamtøi-ning communications with an external communications network.

17. The cellular communications system of claim 1, further comprising a satellite link controller for communicating with one or more satellites.

18. The cellular communications system of claim 1, further comprising communications links with airplanes, ships and boats utilizing said MBA array antenna.

19. The cellular communications system of claim 1, further comprising an auxiliary radio link for providing communications with airplanes, ships and boats.