WO2002061971A1 - Communication system using an aerostat tethered above the earth surface and linked to a base station by an optical fibre - Google Patents

Abstract

A communication system comprises a base station (27) connected to a communications network and arranged to provide a radio frequency communications signal to radio transmitting means (23) carried by an aerostat (1) tethered above the base station. The radio frequency communications signal is conveyed from the base station to the transmitting means by an optical fibre communications link (21) carried by the tether (20).

Description

COMMUNICATION SYSTEM USING AN AEROSTAT TETHERED ABOVE THE EARTH SURFACE AND LIN KED TO A BASE STATION BY AN OPTICAL FIBRE

Field of the Invention

This invention relates to a communications system using an aerostat as a platform for backhaul, broadcast or bidirectional wireless antennae. 5 Background to the Invention

A limitation in terrestrial communications is that, as higher radio frequencies are used to permit more information to be carried, the propagation characteristics of the radio signals tend more towards direct line-of-sight transmission, requiring, because of scattering and absorption from variations in the landscape topography and the presence

10 of man-made and natural obstacles, large numbers of transmitting and receiving stations to achieve satisfactory geographical coverage. Such stations are expensive to build and maintain, but the greatest problem in the more developed countries is finding suitable sites for the stations, especially in view of increasingly restrictive planning regulations. This is especially true for cellular radio telephone networks, whose growth is potentially

15 limited by the availability of suitable sites for transmitting and receiving masts.

Satellite communications systems, by contrast, can cover a large area of the Earth's surface. However, they spread the finite bandwidth available over this much larger area, resulting in a system which offers low performance to many users. They suffer from lack of power and low signal levels due to the great distances over which

20 they send and receive signals. They are also extremely expensive, due to the high build quality requirements and launch costs. The signal transmitted by the satellite is limited by the availability of electrical power, and so the signal received at the base station is necessarily very weak, requiring large and costly dish antennae for their reception, and a high degree of amplification.

25 The use of airships for transmitting data from satellites to ground stations, and from ground station to ground station, is disclosed in US-B-6 205 320, while Hase et al, "A novel broadband all-wireless access network using stratospheric platforms", 1 98 IEEE 1 191 , proposes using a large airship as a platform. Djunik et al "Establishing Wireless Communications Services via High-Altitude Aeronautical Platforms: A Concept

30 Whose Time Has Come", IEEE Communications Magazine, September 1997, page 128, discuss the use of various platforms such as aircraft and airships to serve as communications platforms. In each case, the cost in terms of fossil fuel to maintain the platform in a position which is stationary relative to the surface is prohibitive. Summary of the Invention Accordingly, the present invention provides a communication system comprising a base station connected to a communications network and arranged to provide a radio frequency communications signal to radio transmitting means carried by an aerostat tethered above the base station, characterised in that the radio frequency communications signal is conveyed from the base station to the transmitting means by an optical fibre communications link carried by the tether.

The optical fibre communications link may be an optical cable carried by the tether, either as part of the tether or simply supported thereby.

The aerostat may carry electrical power-generating means, which could be an engine-driven generator, but which is suitably a wind turbine and/or solar panels. Al- ternatively or additionally, the tether may carry an electrical power cable.

Another aspect of the invention provides a cellular communications network, comprising a base station connected thereto, an aerostat provided with a communications link to the base station, and cellular network transmitting and receiving antennae mounted on the aerostat and connected to the base station, and through the base sta- tion to the remainder of the network, via the communications link.

A plurality of antennae may be mounted on the aerostat and arranged to transmit signals to different areas on the ground constituting different cells within the network. For example, the antennae may comprise an antenna array, and control means are provided for controlling the signals applied to separate elements of the array such that the resultant radio beam transmitted from the array can be selectively directed towards a chosen location on the ground.

The invention also provides a method of controlling a cellular communications network, comprising providing a plurality of cellular network transmitting and receiving antennae on an aerostat positioned above and in communication with a ground base station connected to said network, and controlling the antennae and the communica- tions therewith in such a manner as to create and manage a plurality of different network cells on the ground beneath the aerostat.

A mechanism is proposed for operation with a standard point-to-point or point- to-multipoint system, as is known in the art, which allows it to be operated as part of a beamsteered or SDMA system. Both the case of a Time Division (TD) and Frequency Division (FD) system are considered.

In both cases a controller unit is placed after the output block of the standard hub which consists of two elements. The first element is a controller unit which stores details of end users and their location relative to the transmitting antenna (normally present on the aerostat). The bearing of the receiver is also stored locally as antenna weight factors which are used to direct the antenna beams electronically. This information is continuously updated either by using a Direction of Arrival (DOA) algorithm as is known in the art or by directly feeding information about the antenna's orientation; pitch, roll etc to the control unit which updates the weights accordingly to compensate for any changes.

The second block operates differently according to whether the system is a TD or FD system. For a TD system each individual user has an assigned time slot for transmission and reception of data. For this unit to operate as a beam steered antenna, the time signal is fed to the controller (described above) and ςross referenced for each time frame against the location of the user owning that time slot. Using this procedure the beam is steered with time.

A FD system assigns a small portion of the total spectrum to each user. To adapt a conventional hub to allow beam steering the signal needs to be filtered so that each individual user is isolated into a separate circuit. The controller unit described above then correlates each users location information and transforms each channel accordingly using the weights described above. Once weights have been applied for each user the separate channels are combined and transmitted to each user. For received signals the known weights are applied to each time slot or frequency slot as appropriate to di- rectionally receive from the required user Yet another aspect of the invention provides a communications system comprising a communications satellite in earth orbit, and a plurality of earth stations for transmitting communications signals to and receiving signals from the satellite whereby any of the earth stations may transmit communications signals to any of the other sta- tions, wherein at least one of the earth stations comprises a base station in communication with an aerostat maintained at a predetermined location relative to the base station, the aerostat carrying at least one antenna for transmitting signals to and receiving signals from the satellite and being maintained at an altitude above the earth's surface such that radio signal attenuation between the satellite and the aerostat is substantially smaller than that between the satellite and the ground at the same location.

The height of the aerostat above the earth's surface will be selected such that the signal attenuation between the satellite and the aerostat is substantially smaller than that between the satellite and the ground at the same location. Typically, the aerostat may be maintained at an altitude of up to 8km above the surface. The system of the invention permits provision of broadband communications and other global services, and could offer hybrid network solutions. For example, a network could start as a pure platform network, then integrate (acting independently, or integrated) terrestrial, fibre infrastructure and other networks with time. The system has the ability to offer dedicated high data rate bi-directional signals to a point(s). The aerial platform could also act as a backbone network to existing communications such as a mobile network backbone. This could happen independently or integrated into the platform other communication requirements.

The system of the invention could offer backbone and other services to current networks, or dedicated high data rate links, for example links to a business park could then potentially be broken down to allow access by multiple parties (via wireless or cabled access).

As the aerial platform can potentially sit above the majority of the rain, water vapour and oxygen absorption bands, there is potential to provide radio and/or free space optical links to and from satellites and to other networks. This would allow bi- directional very high data rates links from satellite to aerial platforms and other. ln utilising such radio links it would be possible to deploy a very high data rate optical laser network. The deployment of a LEO, MEO or GEO network, or a hybrid combination of networks, which communicate with other networks, would be possible, thus creating an integrated hybrid mesh network of satellites, platforms, terrestrial in- frastructure based communications and cabled networks.

A geo-stationary network of satellites orbiting the earth may be provided, which communicate with aerial platforms and then to other networks. The satellite network would require three or more orbiting geo-stationary satellites. This would provide the satellite section of a hybrid network. The links between the satellite and other net- works i.e. the aerial platform, can use time division multiple access, to allow aerial platforms in close proximity, the opportunity to access the channel without disruption caused by interference etc.

The network can also employ Space Division Multiple Access (SDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency Division Multiplexing (OFDM), Code Division Multiple Access (CDMA), DAMA and CF-DAMA. The network will also employ coding techniques, which could be one or more of the following coding techniques, space-time codes, turbo codes, multi-level coded modulation, concatenated codes and block codes.

The necessary payload to provide the satellite network can be launched on an independent satellite or can be fitted to existing satellites. The satellite-to-platform transceivers for the platform can be mounted on the shell of the aerostat, typically the upper section or roof, within the body of the aerostat (inside gas filled shell) and/or on the payload mounting. Smart antennae, mounted as previously stated but typically on upper section, may also be used for transmitting radio signals to satellite. Optical trans- ceiver equipment mounted to a platform would also require pointing accuracy controls to propagate radio signal in the correct direct of other network such as other platforms and satellite networks. Points of reference are required for the adaptive platform-to- satellite pointing accuracy method. This would typically require the specific location of the satellite, sup-platform point and the location of the aerostat (typically determined using GPS or by ground reference signal and processing). Optical signals transmitted to the platform would thus be directed such as to compensate for platform movement and or stability.

With such a topology it is also possible to maximise existing type bi-directional satellite links. With satellite-to-platform communication, rather than satellite directly to ground receivers, and assuming similar system components (such as amplifiers and antennae), the platform receiver would receive significantly higher power signals. This increase in the radio link margin, for a given bandwidth signal, is due to the reduced radio path losses encountered. Therefore for a given frequency substantially higher data rates can be transmitted within the same bandwidth. Spectral efficiency and system ca- pacity can be increased. This is particularly useful for systems operating at high frequencies, due to the greater attenuation when transmitting through the atmosphere.

In regions with high attenuation of the radio link, typically due to weather conditions, satellite signals are often lost or degraded. By relaying signals via the tethered aerostat higher radio link availability can be provided (the extent is dependant on the platform height and geographical location) during such conditions.

The platform can be used for propagation planning purposes for a multiplicity of communications based services (i.e. 2G/3G and BFWA) and other radio requirements. The platform can be provided with a transceiver that communicates with another transceiver (fixed or mobile) on the ground to provide measurement data which can be in- terpreted into useful results. These results can help to define radio channel characteristics for various standards of radio requirements. The platform deployment and measurement facility, which could be mobile in nature (i.e. fitted to a vehicle), can be deployed at the appropriate location and potentially carry the necessary test and communications equipment. Processing of such data could be done locally, or transmit- ted/transported to an appropriate location. Backhaul communications for such a test facility could be via other networks, with the appropriate interface mounted to the ground/mobile unit.

Such a facility can be manned or un-manned, tethered or un-tethered and may be remote pointed or programmed in advance of its measurement locations/route. The platform could travel independently and transmit data to a remote (fixed or mo- bile) facility. The data could be fed via a tether to a ground unit, stored and processed later or transmitted via an existing network to a processing facility.

Typical channel effects and measurements may be propagation effects, losses at varying frequencies, fading, multi-path measurements, Doppler and other. Such ele- ments typically vary with physical/ geographical location, the plane of propagation (i.e. horizontal, slant or vertical plane) height, temperature, rain, air density, water vapour content, and the presence of other signals and interference. By using single or bidirectional links at a required frequency, we can evaluate the radio channel characteristics and propagation effects. Imaging used to predict line-of-sight propagation is particularly beneficial at high frequencies. The imaging could be done by using photography, video, and other types of spectral imaging (i.e. thermal is an example of spectral imaging).

The platform measurements and particularly the imaging can be use to produce 3-Dimensional models of the ground terrain, clutter, and the physical environment. These 3D models are used for a multiplicity of services such as propagation planning tools (such as Planet™), 3D imaging of buildings, web pages, producing virtual reality maps and models, mapping and modelling forms within the physical environment, aerial photography, video, laser radar, triangular mapping techniques and the appropriate software could be required for producing 3D profiles from imaging data. These models can then by applied to a multiplicity of requirements but are particularly applicable to propagation planning and the associated propagation planning tools.

Currently satellite newsgathering services (SNGs) are used extensively by the TV industry. This technology requires expensive mobile vehicles, multiple persons to operate equipment, and incurs running costs due to erecting equipment and for leasing satellite capacity. Use of the aerostat platform of the invention to relay data/pictures from a remote location back to a required point could substantially reduce costs. Already there are transceivers capable of operating in the BFWA frequency band, which could be fitted directly onto or separate to camera/unit housing. The requirement for current SNGs for areas could be removed within a platform coverage region. For the said BFWA example (or any such high frequency signal) physically small, simple trans- mitters could be used. These maybe built directly into the camera for example or installed into PCs/laptops for relaying data/images.

The news gathering services could exploit aerial platforms to provide such data services or deploy a tailored aerial platform for their own purposes, thus producing a local area network. This approach would also be suitable for use with sporting and other large public events.

The system of the invention may be used to provide image processing services, such as surveillance, traffic monitoring, filming and photography, early warning systems (i.e. fire and flood detection), in-premises monitoring (sensors, video and communica- tion), home care for the elderly, and security.

Another aspect of the invention provides for the use of the system for propagation planning. A key time-consuming and costly element associated with the roll-out and planning of any radio based system is the need to plan the location of the transmitters accurately to optimise performance. Although software tools can be used to aid this process, it is still necessary to make measurements to establish whether a given site meets the requirements of the system. A tethered aerostat makes an ideal tool for this job as it can be easily moved to the location of the test, inflated and flown to the required altitude with a suitable transmitting/ receiving test payload. Operatives may then proceed to make mobile measurements in the locality so as to establish the suitability of the site.

Brief Description of the Drawings

In the drawings, which illustrate diagrammatically exemplary embodiments of the invention:

Figure 1 shows a communications network in accordance with the invention; Figure 2 shows an aerostat and its base station;

Figure 3 illustrates the means for handing off between fixed antennae in the system of Figure 2;

Figure 4 shows the downlink part of the system;

Figure 5 shows the uplink part of the system; Figure 6 shows an alternative arrangement of the downlink system; Figure 7 shows an alternative arrangement of the uplink system; Figure 8 shows yet another alternative arrangement of the downlink system; and

Figure 9 shows the uplink system corresponding to the downlink system shown in Figure 8.

Detailed Description of the Illustrated Embodiments

Figure 1 illustrates the scope of the network containing the tethered aerostat platform 1 and shows the interconnectivity of the unit to other services. Beneath the platform 1 terrestrial wireless techniques 2, including G, 3G, 4G, LMDS and MMDS, operate within the confines of their respective propagation characteristics and bandwidth, optical cables, waveguides and coaxial cables 3 provide services to areas where the last-mile cost can be recovered and terrestrial telephone lines 4 offer analogue, ISDN and xDSL services to the majority of the population according to their location. In the plane of the tethered platform 1 communication may occur via the radio or optical spectrum to neighbouring platforms creating fast inter-city data links.

Upward integration is also used to extend the applicability of the system with radio and optical links to High Altitude Platforms (HAPs) 5 at such time as they become feasible, Low Earth Orbit satellites (LEOs) 6 and Geo-stationary Earth Orbit satellites (GEOs) 7. Referring now to Figure 2, the aerostat 1 provides an elevated platform which gives an extremely large field of view to its onboard antenna systems, extending the potential range of communications services and enhancing the ground area over which line of site (LOS) paths can be achieved. The aerostat 1 is constructed from a high strength gas retaining fabric which is inflated with a lighter-than-air gas which is suitably Helium or Hydrogen, although other gases may be used. The aerostat is tethered to the ground via a single or multiple tether 20. The role of the tether is to fix the location of the aerostat in space above the Earth's surface and to reduce its ability to pitch and roll. The tether 20 consists of one or more lines designed to stabilise the craft. They can be constructed from steel or modern fibres such as Kevlar, Zylon or Spectra as is known in the art. Attached to the tether, or incorporated into its structure, are a data link 21 , described hereinafter, and a power link 22 to power the craft or recover energy in cases where a renewable source running on the aerostat is running at over-capacity. Supplying power to the aerostat and communications payload is potentially a problem. A liquid or gas driven electrical generator or a battery or 240v mains supply can be used on the ground. It is envisaged that the aerostat and payload could be powered from the ground through a set of electrical cables running up the tether. If the wrong system is used the efficiency of the tether cable supply could be very low and a great deal of electrical heating could take place along the tether electrical supply cables. Several alternative systems may be employed for reducing this loss and increasing power transfer efficiency. The first solution is to have a small alternating current substation on the ground that transforms up the main supply to approaching, if not exceeding, 1000 volts a.c. (this could be several thousand volts). The transferred power would then be down-converted on the aerostat to provide a.c. power to motors and regulated d.c. power for the communications payload. This high voltage transfer solution means that only low current needs to flow in the tether power cables and hence the l²R cable heating is reduced and efficiency increased. This is how electrical power is transferred throughout a typical mains national grid.

A second solution is to simply transfer the power at mains voltage level and frequency, as this allows a simple ground system. The voltage would then be down- converted on the aerostat and regulated for those systems that require a clean d.c. input. This system would be less efficient than the first proposed solution but less complex and costly.

A third system is to increase either or both the power supply voltage and frequency. As the frequency increases, the up-converting and down-converting trans- formers used become smaller in size and cheaper in cost. If solid state up and down conversion is used, with a switched capacitor converter type, then capacitor size, weight and cost can all be reduced. Unfortunately the supply switching frequency increases, the skin effect in the tether supply cables becomes more of a problem and current densities will be increased. Therefore l²R heating becomes larger and efficiency is again reduced. This can be counteracted some way by not using very high switching frequencies and using multistrand cable instead of a solid core cable, as this increases the effective cable surface area and hence reduces current density. A good oxygen free copper cable will have lower resistivity and hence reduced loss. Silver cable could be considered, as its conductivity is 17% higher than copper or silver plated cable. Currently, altitudes of up to approximately 8 km are achievable using tether technology, although higher altitudes are potentially possible in the future. The presence of the tether allows the provision of power, and data directly from the ground overcoming the two major problems associated with the use of non-tethered or free flying craft for the purpose of wireless communications. The tether system also allows the location of most of the processing systems and communication systems to be on the ground rather than on the craft allowing them to be housed in a safe environment and maintained while the aerostat is still in flight without loss of service. This results in the aerostat being a 'bent pipe' system with a simple antenna payload 300 in the air and minimum support equipment. The antenna subsystem 23 sends signals to the ground based subscriber unit

(CPE) and in turn receives signals back and routes them to the ground and out to the rest of the world, the base station beneath the aerostat behaving as a gateway. The role of the antenna(s) is to provide an effective means of relaying the data to multiple users, stationary or mobile, on the ground. To achieve this, a single antenna or multiple anten- nae each with a single or many independent data feeds may be used, creating a fixed or dynamic pattern on the Earth's surface. To ensure maximum efficiency of the system, a frequency reuse pattern may be used whereby neighbouring antennae broadcast and receive different frequency bands. It is further possible that polarisation, for example linear, co-polar, cross-polar, circular or elliptical polarisation or any combination of these may be used to maximise the throughput of the system.

The antenna subsystem 23 may consist of antennae creating cellular patterns, or concentric rings on the Earth's surface or may roam across the Earth's surface or may create an umbrella cell covering a large area or may have each of the above present

Antennae used for the purpose of sending and receiving signals may form a sin- gle transmit/ receive unit or may split the transmit and receive roles into separate units to reduce signal interference or complexity and cost. Antennae may be of any conventional type including: rectangular or cylindrical horns; rectangular or cylindrical single or dual ridged horns; lensed horns of any of the above types; scalar horns; fractal antennae; z-transform antennae; logarithmic antennae; monopole or dipole antennae; patch antennae of any geometry; dish antennae including cassegrain; offset feed antennae or elliptically shaped antennae; spiral or helical antennae or reflector/collector designs. Another possible antenna type is the quad ridged horn.

Delivery of the signals to the ground based user irrespective of movement of the aerostat platform may be achieved by a number of means including hand-off, gim- baling of the entire antenna system, deployment of beam steering antennae or by the use of smart antennae or of mechanically adaptive antennae.

Gimbaling employs a system isolating the antenna array from the body of the aerostat. This unit can be mechanically moved so as to compensate for motion of the aerostat. Such systems are known in the art. Hand-off assumes a system where the antenna beam pattern may move across the Earth's surface or the subscriber may move such that he moves out of the operating area of one beam and into the scope of another. A further hand-off scenario may be applicable when the user moves out of the range of the platform but this is not considered here. Hand-off techniques are known in the art and will not be elaborated here. However a novel system is suggested which allows the deployment of standard hub equipment which may have no inherent hand-off capability. This system is shown in Figure 3 and is described hereinafter.

Tracking antennae which may or may not be electronically controlled smart antennae, or mechanically steerable or adaptive reflector designs can be employed to elec- tronically redirect the beam to the mobile or stationary ground based user independent of the motion of the aerostat and/ or the subscriber.

In the first version of the system the orientation of the aerostat is used to control the direction of the antenna's beam effectively compensating for motion of the craft. A feedback circuit is employed which can take data from the control systems de- scribed in this document and feed them to the control unit of the antenna. Using algo- rithms known in the art the antenna beam can be held stationary on the ground or may track a mobile user.

When smart antennae are employed a far higher degree of control can be achieved as the system detects the direction of arrival (DOA) of the subscriber unit and stores this information. When sending data to the subscriber, the Smart antenna uses the DOA information to redirect the data to the subscriber. Such a system operating at a rate faster than the motion of the aerostat and the subscriber can effectively maintain the connection under a wide degree of conditions.

An alternative system may involve the platform registering the initial state of us- ers and detecting which cell they are located in. As the platform then moves, the tacking antennae apply a target hold algorithm to maintain the communications channel.

Wide area umbrella antennae can be used to illuminate a large area of the Earth's surface below the airship affording a means of providing communication irrespective of motion of the craft. They are useful for backing up the services provided by the highly directional and tracking antennae.

Shaped reflector, patch or shaped lens antennae are particularly suitable for this application as they provide a means of creating an extremely wide beam profile. Other antenna designs are also usable.

The control system 24 enables the aerostat to operate in harsh weather condi- tions beyond those of a conventional aerostat. Such systems have not been essential in the past as generic aerostat applications do not require high stability of the craft to the extent required for communications purposes. The system may include:

• powered control systems such as motors and ion engines to adjust the location and orientation of the craft and to counter external environmental effects in- eluding turbulence and wind gusts;

• weather front detection either on board, below the craft or from third party sources including satellites which can be fed back to the control systems allowing the aerostat to be prepared for incoming events. This system also allows the aerostat to be recovered prior to the arrival of incoming catastrophic weather events;^• Reactive torque-controlled winches which can be employed to react to strains on the aerostat caused by variations in local pressure. These can improve system control and reduce the possibility of system failure due to cases of extreme turbulence; and • A 'safety chain link' mechanism can be incorporated into the tether as a means of saving the craft if it exceeds its permissible load. Instead of the tether failing catastrophically, the safety chain will release protecting the aerostat and allowing for the craft to be returned to earth.

Power augmented controls for motors may permit stabilisation or positioning of the platform and alleviate the effects of downdrafts and kiting. The tether may employ damping mechanisms to control undesired movements, weak links to release portions of the tether selectively in extreme conditions, and torque control winches to assist in position stabilisation. Sensors may be employed on the aerostat or elsewhere to detect adverse weather such as high winds, thunder storms, changes in pressures, lightning, as well as conditions such as parted tether, kiting, tether stress and damage, and helium leakage. They may also warn of approaching aviation. The information provided by the sensors may be employed in intelligent processing in a processor on board and/or located on the ground utility and/or from a remote location and fed via link options.

Various different communications options are available in accordance with the invention: Asymmetric Configuration

The full duplex high data rate service made possible by the network topology described above is over-specified for the home user, the SoHo (Small Office Home Office) user or residential users, as in these cases the data rate in the direction of the user normally exceeds that required from the user. In these cases an asymmetric system can be deployed whereby the user still receives signals from the aerostat to a CPE as described above. However data is not transmitted to the aerostat; rather it is channelled via conventional means such as via a telephone with a modem or using a conventional ISDN channel. In an alternative asymmetric arrangement, radio is used for sending data from the user to the aerostat, but at a lower data rate than that from the aerostat to the user.

Public Network option The overall availability of the system can increased by allowing a backup data transfer system using a conventional wired technology such as ISDN or telephone line modem. This service would kick in should service from the platform be unavailable for any period of time. Buffering at the receiver can also be used for this purpose Adaptive Spot Tracking beams A high precision steerable antenna can be used to augment the performance of the standard antenna pattern under exceptional circumstances. This steerable antenna can be operated electronically i.e. a Smart antenna, or may be mechanically steered or may be an adaptive reflector antenna as known in the art.

The purpose of this antenna is to provide backup to the normal antenna systems in cases where the radio link may be broken by for examples adverse weather / rain events or by extreme short term coverage requirements in a particular area, for example at a football match or some such gathering of people.

Ideally the adaptive spot tracking antenna will allow for the location and the beam width of the beam to be controlled so as to perform its roll. By using such a system it is possible to make other components of the system less specialised and thus cheaper such that for example a system with a performance of say 99% could operate to a much higher reliability level, say 99.99%. These reliabilities are only indicative.

An airborne refuelling system 25 or parts thereof may also be located on the aerostat and help to increase the aerostat flight time and reliability. Lighter-than-air vehicles have a limited flight time which is caused by the continuous loss of the lighter- than-air gas through the craft's fuselage. As the gas is often helium or hydrogen, which are extremely small molecules, loss is inevitable, as no material can be made through which these particles are unable to diffuse. For most aerostats this is not a significant problem and they can be returned to the ground, their on-board gas supply replenished and once again they may be re-launched. With the advent of high reliability continuous services from both tethered and untethered platforms, including HAPs, the situation whereby the aerostat is returned to the ground for gas replenishment is unacceptable. For a HAP recovering and re-launching the craft from an altitude of 20 km will take a great deal of time and a consequent loss of service. At present this can only be mitigated by cycling craft, which requires the presence of a spare craft.

For a tethered platform, the possibility of using the tether to replenish the craft's gas supply appears hopeful. However, to inject the gas into the craft it needs to be applied to the inlet valve at over-pressure. Further, the geometry of the feed pipe is extremely unfavourable for the delivery of lighter than air gases. The extremely large surface area to volume ratio of a tube results in extremely large gas losses along its length and makes this method impractical.

One aspect of the invention proposes the use of a second craft, probably smaller than the craft receiving the gas. This may be a small aerostat, but may be any aircraft. The refuelling craft as it shall be know is flown to the altitude and location of the aerostat with the aid of cameras and other suitable proximity sensors. For the case of the tethered platform the refuelling craft may be guided up the tether to the service craft

Once the refuelling craft gets close to the service craft, a flexible hose can be guided so as to mate with the valve on the service craft, and lighter-than-air gas can then be pumped so as to replenish the gas to the required level. The gas may be stored as pressurised gas, or in liquid or solid form when possible or in the case of the refuelling craft being a UAV it could refill the service craft from its envelope.

Elements 26 serving the purpose of system failure handling or fail-safe systems may also be included. At the base station 27 beneath the craft a commercial switching hub connects the aerostat system to the rest of the world via the dynamic router if necessary. To provide for the wide variety of possible backhaul technologies a universal adaptor 28 is included at this point which allows for the signal to be sent received directly using a waveguiding structure such as an optical fibre or a hollow waveguide or a coaxial transmission system. Provision is also made for the use of a terrestrial point-to-point microwave link as is known in the art or by means of a satellite link via an external antenna.

The base station may include a cache, whereby network traffic such as web pages, audio files etc can be stored on a storage medium, e.g. a hard disk, at the loca- tion of the network gateway. When such a stored document is accessed by a user, rather than rereading the data from the outside world the local copy is returned to the originator of the request, thus avoiding use of the backhaul link.

The system may be powered by means of a cabled mains supply 29 which will preferably run underground immediately beneath the aerostat to prevent major disrup- tion in the case of a failed tether. A local generator or storage battery is also possible.

According to environmental conditions, solar panels will be used on the craft itself or as part of the base station. The attraction of locating the panels on the craft is that large losses associated with transmission along the tether are avoided. Wind energy may also be used by locating the wind generating mechanism on the craft. Referring now to Figure 3, the system provides a means of handing off between fixed antennae, located on the aerostat, whose antenna patterns move over the Earth's surface as result of variation in location and orientation of the aerostat For the tethered solution location is not a major problem as the aerostat's station can be kept down to as little as 10 m or so at an altitude of 1 km. The system comprises units 31, a dy- namic router or switch which receives continuously updated information on the current location and orientation of the aerostat via unit 33. The routing switch performs the task of keeping a look up table of which ground based user is associated with which fixed antenna on the aerostat and hence which hub signals need to be directed to in order to complete the circuit between the user and the rest of the world.

The units 32 are commercially available wireless hubs. Without the use of the dynamic routing element it would not be possible to perform signal hand off for a generic hub. It is envisaged that under certain circumstances the roles of units 31 and 32 may be combined to give a combined dynamic routing hub which can provide system hand-off features. Figure 4 illustrates one form of the downlink part of the system of the invention. The system comprises a base station 41 which includes, if necessary, a base station to system interconnector/ adapter, allowing the BFWA base station to mate with the proposed BFWA communications link. The adapter consists of a waveguide to K, 2.4mm or V connector adapter such as the commercially available Flann Microwave 22094- KF20. Similar adapters can be used for 2G, 3G, MMDS, LMDS communication systems. An attenuator 42 may be required to convert the high output power of the BFWA base station 41 to an acceptable level for the optical transmitter 44. This device could sit before or after the base station adapter and be produced in waveguide or have K, 2.4mm or V connectors or any other type of microwave/mm-wave connectors that operate up to and/or above 40GHz. Such a commercially available device is: Flann 22580 (fixed) or 22113 (variable)

A stabilised DC power supply/bias 43 (required if an external optical modulator with no internal DC bias circuitry) is used in order to set the modulation depth and hence determine how linear the RF modulation on to the optical carrier is. An RF bias tee is required to allow superposition of the DC and AC signals on to the same microwave input such a device is the 8810EF from Inmet Corporation.

The optical transmitter 44 allows the modulation of, in the case of BFWA, a data modulated 28GHz or 40GHz carrier on to an optical carrier. In the case of UMTS or MMDS, this data carrier will be considerably lower in frequency. The optical carrier could be any feasible optical frequency, with the current standard communication wavelengths being between 800nm and 1600nm. This device could take the form of a directly modulated laser diode, with an integrated electro-absorptive modulator such as the Mitsubishi ML9XX14, although current state of the art technology offers an upper modulation frequency limit of around 10GHz.

An alternative and preferred optical generation scheme is a lower cost laser diode such as the Mitsubishi ML60116R and an external optical modulator such as a Lithium Niobate Mach-Zender type like that of the IOAP-MOD 9203 from SDL or an electro-absorptive modulator or a single packaged laser diode, drive circuitry and modulator like that of Marconi's 40Gbps optical transmitter. The power supply unit is a dedicated unit required to drive the laser diode, if an external laser diode is used.

Fibre optical cable 45 is required for the RF modulated optical carrier transmission to the receiver. The cable runs up the aerostat's tether and may be armoured, or not armoured if weight is a problem or the environment is not too harsh. If a steel armoured fibre optic cable is used, then the cable could also be used as the aerostat's tether, as the fibre will stretch more than the steel before breaking. Such a cable is produced by Pinacl Communication Systems. If an armoured fibre optic cable is used, then several fibres can be run up an individual armoured sheath and therefore several com- munications payloads can be fed with a data modulated RF signals. Either monomode cable or multi-mode cable can be used dependant on the characteristics of the system and attenuation and dispersion requirements, which will be distance dependant

The optical receiver 46 is required for reception of the RF modulated optical carrier. Demodulation of the optical carrier is also performed within the receiver, i.e. the signal output from the receiver should be the same data modulated RF carrier that was injected into the optical transmitter. The optical receiver 46 could consist of a high speed photodiode again operating at optical wavelength around the standards used at this current time (800nm-1600nm) and or considerably lower and/or higher in frequency. The photo diode could have a low RF bandwidth of around 3GHz for MMDS and UNTS applications but will require a much faster switching RF bandwidth of in excess of 40GHz for BFWA applications. Again this component will require the correct biasing circuitry, and for maximum noise performance illuminated with the optimum optical power and the demodulated RF signal amplified by a low noise microwave (LNA) amplifier. Such a photodiode is that of the DSC1 OS by Chipsat and an appropri- ate LNA would be a QLN series amplifier by Quinstar. An alternative is a state of the art optical receiver such as Marconi's 42.5Gbps optical receiver, which consists of a single package incorporation a high-speed photodiode, LNA and appropriate biasing circuitry.

A power amplifier 47 is required to amplify the low output RF signal from the optical receiver 46 to an acceptable level to drive the antenna 49 for free space propa- gation. If efficiency is not a problem then this amplifier should be run in it linear operating region as to reduce the level of distortion introduced by the amplifier to the data carrying RF signal. Alternatively the amplifier can be operated closer to saturation and a predistortion module fitted before this amplifier to linearise the amplifiers input-output characteristics. Alternatively an amplifier linearisation technique such as a feedforward amplifier could be employed. The gain and maximum output power of this amplifier will be determined by the gain of the transmit and receive antennae, the modulation scheme used the propagation distance the weather conditions the RF frequency used the maximum legal radiated transmit power and the RF input level. Automatic gain control may be required on this amplifier to increase/ reduce the radiated RF power in bad/good weather conditions and/or free space path length changes and/or variations in receive antenna gain. An appropriate amplifier for a 28GHz BFWA system may well be something like the Quinstar QLN series

A bandpass filter 48 is required primarily to produce good isolation between transmitted signals and received signals. That is, the transmit antenna will not propagate with any serious power level any signal out side of the transmit band whether they are produced from distortion in previous components or intermodulation/mixing effects. This means that the receiver's front end cannot get saturated by any spurious transmitted signal from the transmit section. Such a filter could be manufactured from waveguide or be printed onto a microwave substrate in such a form as microstrip, stripline, triplate or coplanar waveguide. The input and output to the filter could be waveguide and/or a coaxial microwave connector such as a K connector or V connector or 2.4 mm connector.

The transmit bandpass filter and receive bandpass filter could be replaced by a high isolation diplexer and a single transmit/ receive antenna used.

The antenna 49 is required to allow free space propagation of the data-carrying RF signal. The antenna will also allow gain and directivity to be added to the system this will allow only selected geographical areas to be illuminated with/ receive with adequate power of the data modulated RF signal. The types of antenna that can be used for most types of system, that is 2G, 3G (UMTS), MMDS, BFWA, LMDS propagation is either array antennae or single antennae. Antennae used could be monopoles, dipoles, printed and/or not printed patches, printed and or not printed helixes, conical antennae, spirals (planar and/or conical), horn antennae (ridged and/or not ridged and/or lens type), dish antennae (front fed and/or rear fed), and slot antennae (waveguide and/or printed), for example.

An array antenna could be an array of any size and dimensions of any of the above single antennae.

The uplink system shown in Figure 5 comprises an antenna 51 , which may be the same antenna as in the downlink system or a separate antenna. It is required to re- ceive the free space propagated data carrying RF signal and transform it to a waveguide propagation or coaxial cable propagation RF signal. The antenna will also allow gain and directivity to be added to the system to allow only signals from selected areas illuminated on the ground to be processed. The types of antenna will be the same as for the downlink antenna. A bandpass filter 52 is required primarily to produce good isolation between transmitted signals and received signals. That is any signals received by the antenna that are higher or lower in frequency than the allotted receive bandwidth will experience high attenuation and will not saturate the receivers front end (the LNA). The insertion loss of this filter in the pass band should be as low as possible to increase the sensitivity of the receive system.

A low noise amplifier 53 is required to for several reasons. Firstly the amplifier is required to boost the received signal power level up to an optimum drive level for the optical transmitter (a second stage medium power amplifier may also be required). The second reason for using a low noise amplifier is to increase the sensitivity of the system, that is allow smaller signal levels to be received and still allow the system to function as designed. The input and output to this amplifier could be waveguide and/or coaxial microwave connectors such as K connectors, V connectors or 2.4 mm connectors. Such a low noise amplifier is the QLN from Quinstar.

A medium power amplifier 54 may or may not be required. If the signal ampli- tude received by the receive antenna is very low and the RF power required to drive the optical transmitter is comparatively high then a second stage amplifier will be required firstly to make up the additional gain required to boost the received RF signal an acceptable level for the optical transmitter, and secondly to allow a more commercially available lower gain, lower power low noise amplifier to be used. This amplifier's gain will be dependant on the LNA gain, the receive antenna gain, the received power level and the optimum drive level for the optical transmitter. The RF input and output connectors could be waveguide, K, V, SMA, 2.4mm or any other standard and/or non- standard microwave/mm-wave connector.

A stabilised DC power supply/bias 55 (required if an external optical modulator with no internal DC bias circuitry) is used in order to set the modulation depth and hence determine how linear the RF modulation on to the optical carrier is. An RF bias tee is required to allow superposition of the DC and AC signals on to the same microwave input such a device is the 881 OEF from Inmet Corporation.

The optical transmitter 56 allows the modulation of in the case of BFWA a data modulated 28GHz or 40GHz carrier on to an optical carrier. In the case of UMTS or MMDS this data carrier will be considerably lower in frequency. The optical carrier could be any feasible optical frequency, with the current standard communication wavelengths being between 800nm and 1600nm. The transmitter 56 may be essentially of the same type as in the downlink system described with reference to Figure 4. The power supply unit is a dedicated unit required to drive the laser diode if an external laser diode is used.

The fibre optical cable 45 has been described with reference to Figure 4, while the optical receiver 58 is essentially the same as the receiver 46.

Figures 6 and 7 illustrate an alternative arrangement of downlink and uplink sys- tems. The main difference between this system and the first proposed system is the fact that unmodulated baseband data is sent down the optical fibre and not a data modulated radio frequency modulated on to an optical carrier. In this system the baseband data sent down the optical fibre is modulated at the top end (in the aerostat payload) to whatever specification is required, whether it be BPSK, QPSK, 16QAM, 64QAM or any other digital modulation scheme and on to what ever carrier frequency required for either BFWA, MMDS, LMDS, 2G or 3G standard.

This means that additional hardware will be required on the aerostat. It is envisaged that a microwave/mm-wave synthesised signal generator 61 , a signal modulator 62 of whatever type is required for the standard to be used (BPSK, QPSK, 16QAM, 64QAM, etc..) and operating at what ever carrier frequency is required for the frequency standard (BFWA, MMDS, LMDS, 2G, 3G, etc..) and some possible filtering will all be required.

This system takes some of the tight requirements from the optical transmitter and receiver, as they are no longer required to run at the RF frequencies but at the data rate. For example a BFWA system would have previously required an optical modulator that could switch at 28GHz or 40GHz and now only requires a switching speed of a few hundred MHz.

This system now has the provision to send the data for several channels down the same fibre and use the same transmit/ receive modules but modulate on to different carrier frequencies at the aerostat platform end.

The base station side of the system will require a demodulator to remove the data from the base station carrier, whether, it is BFWA, MMDS, . UMTS, LMDS or any other broadband standard. An optical carrier can then be directly modulated with the baseband data. The additional components required over the components of the original proposed system are a demodulator 63, a microwave/mm-wave synthesised signal generator 64 and a bandpass or low pass filter 65 to remove any out of band.

The uplink system shown in Figure 7 is essentially the same as in the downlink system shown in Figure 6, but reversed. The only major differences are that the modulator and demodulator exchange positions in the system, as do the optical transmitter and receiver.

The system illustrated in Figures 6 and 7 can be simplified by removing the demodulator and signal generator from the base station in the downlink system and supplying the optical transmitter with base-band data straight from the base station, with a corresponding simplification of the base station in the uplink system. In another alternative configuration, illustrated by Figures 8 and 9, which are diagrammatically similar to Figures 6 and 7 and in which like reference numerals identify components which are the same as in Figures 6 and 7, the downlink system involves the use of a down-converted data modulated radio frequency signal modulated on to an optical carrier and sent down the optical fibre. In this system the radio on fibre signal sent down the optical fibre can be up-converted at the top end (in the aerostat payload) to whatever carrier frequency is required, for example BFWA, MMDS, LMDS, 2G or 3G standard, or it can simply be up-converted to the original carrier frequency before down-conversion. This means that additional hardware will be required on the aerostat. It is envisaged that a microwave/mm-wave synthesised signal generator and an up-converting mixer 83 and a down-converting 82 mixer will be required operating at what ever carrier frequency is required for the frequency standard (BFWA, MMDS, LMDS, 2G, 3G, etc.). Some possible filtering may also be required. This system takes some of the tight requirements from the optical transmitter and receiver, as they are no longer required to run at the previously high RF frequencies but at lower intermediate frequencies. For example a BFWA system would have previously required an optical modulator that could switch at 28GHz or 40GHz and now could require a switching speed as low as a few hundred MHz. This system now has the provision to send several different intermediate frequencies for several different channels down the same fibre and use the same transmit/receive modules but up-convert to the same or different carrier frequencies at the aerostat platform end.

The base station side of the system will require a down-converting mixer and RF synthesiser to produce a lower intermediate frequency (IF) than the base station carrier frequency, whether it is BFWA, MMDS, UMTS, LMDS or any other broadband standard. An optical carrier can then be directly modulated with the IF. The additional components required over the components of the original proposed system are several mixers, a microwave/mm-wave synthesised signal generator and several bandpass or low pass filters to remove any problematic out of band signals. The disadvantage of this system is its increased complexity.

The uplink system is very much the same as the downlink but in reverse. The only major differences are that the down-converting mixer 82 and up-converting mixer 83 exchange positions in the system, as do the optical transmitter and optical receiver.

Claims

1. A communication system, comprising a base station connected to a communications network and arranged to provide a radio frequency communications signal to radio transmitting means carried by an aerostat tethered above the base sta- tion, characterised in that the radio frequency communications signal is conveyed from the base station to the transmitting means by an optical fibre communications link carried by the tether.

2. A communications system according to Claim 1 , wherein the optical fibre communications link comprises an optical cable which is separate from the tether but supported thereby.

3. A communications system according to Claim 1 , wherein the optical fibre communications link comprises an optical cable forming at least a part of the tether.

4. A communications system according to Claim 1, 2 or 3, comprising electrical power-generating means carried by the aerostat.

5. A communications system according to Claim 4, wherein the power- generating means comprise wind turbines.

6. A communications system according to Claim 4 or 5, wherein the power-generating means comprise solar panels.

7. A communications system according to Claim 1 , 2 or 3, wherein the tether carries an electrical power cable.

8. A communications system according to any preceding claim, wherein the aerostat carries downwardly-directed antennae for transmitting to and/or receiving radio signals from ground-based equipment.

9. A communications system according to any preceding claim, wherein the aerostat carries at least one upwardly-directed antenna for establishing communications with satellite-based communications equipment.

10. A communications system according to Claim 8 or 9, wherein the aerostat comprises a gas envelope and the antennae are mounted within the gas envelope.

1 1. A communications system according to Claim 8 or 9, wherein the downwardly-directed antennae are carried on a platform supported beneath the aerostat.

12. A communications system according to Claim 9, 10 or 1 1 , wherein the or each upwardly-directed antenna is mounted on an upper surface of the aerostat.

1 3. A communications system according to any of Claims 12 to 1 6, wherein the downwardly-directed antennae comprise a steerable antenna capable of directing a radio signal to a selected location on the ground.

14. A communications system according to Claim 13, wherein the steerable antenna comprises a steerable horn antenna.

1 5. A communications system according to Claim 1 3, wherein the steerable antenna comprises an antenna array and control means are provided for controlling the signals applied to separate elements of the array such that the resultant radio beam transmitted from the array can be selectively directed towards a chosen location on the ground.

16. A communications system according to Claim 15, wherein the control means is arranged to control signals supplied to the antenna array in such a manner that the effective width of the resultant radio beam on the ground can be selectively varied.

17. A communications system according to Claim 1 5 or 16, wherein the control means form part of the base station.

18. A cellular communications network, comprising a base station connected thereto, an aerostat provided with a communications link to the base station, and cellular network transmitting and receiving antennae mounted on the aerostat and connected to the base station, and through the base station to the remainder of the net- work, via the communications link.

19. A cellular communications network according to Claim 18, comprising a plurality of antennae mounted on the aerostat and arranged to transmit signals to different areas on the ground constituting different cells within the network.

20. A cellular communications network according to Claim 19, wherein the antennae comprise an antenna array, and control means are provided for controlling the signals applied to separate elements of the array such that the resultant radio beam transmitted from the array can be selectively directed towards a chosen location on the ground.

21. A cellular communications network according to Claim 20, wherein the control means is arranged to control signals supplied to the antenna array in such a manner that the effective width of the resultant radio beam on the ground can be selectively varied.

22. A cellular communications network according to Claim 21 , wherein the width of the radio beams directed towards different cell areas on the ground is con- trolled so as to vary the relative sizes of the cells according to signal traffic conditions within the cells.

23. A cellular communications network according to Claim 20, 21 or 22, wherein the control means form part of the base station.

24. A cellular communications network according to any of Claims 1 9 to 23, wherein two or more antennae can be arranged to transmit to the same cell area on the ground, whereby the capacity of a cell can be selectively varied according to signal traffic conditions within the cell.

25. A method of controlling a cellular communications network, comprising providing a plurality of cellular network transmitting and receiving antennae on an aero- stat positioned above and in communication with a ground base station connected to said network, and controlling the antennae and the communications therewith in such a manner as to create and manage a plurality of different network cells on the ground beneath the aerostat

26. A method according to Claim 25, comprising dynamically varying the area of each cell according to the relative volumes of signal traffic generated within the cells.

27. A method according to Claim 25, comprising dynamically varying the capacity of each cell by allocating a greater or smaller number of antennae thereto according to the volume of signal traffic generated within the cell.

28. A method according to Claim 25, 26 or 27, comprising using an antenna array on the aerostat and controlling the elements of the array to vary the size and/or capacity of each cell.

29. A method according to Claim 28, comprising controlling the elements of the array so as to vary the direction and/or beam width of radio signals transmitted therefrom.

30. A method according to any of Claims 25 to 29, comprising controlling the switching of signals to the antennae from the base station.

31. A communications system comprising a communications satellite in earth orbit, and a plurality of earth stations for transmitting communications signals to and receiving signals from the satellite whereby any of the earth stations may transmit communications signals to any of the other stations, wherein at least one of the earth stations comprises a base station in communication with an aerostat maintained at a predetermined location relative to the base station, the aerostat carrying at least one antenna for transmitting signals to and receiving signals from the satellite and being maintained at an altitude above the earth's surface such that radio signal attenuation between the satellite and the aerostat is substantially smaller than that between the satellite and the ground at the same location.

32. A communications system according to Claim 31, wherein the aerostat is maintained at an altitude above the earth's surface such that signals between the aerostat and the satellite are unaffected by weather.

33. A communications system according to Claim 31 or 32, wherein the aerostat is maintained at the predetermined location and altitude by means of a tether.

34. A communications system according to Claim 33, wherein the commu- nication between the base station and the aerostat is by way of a cable carried by the tether.

35. A communication system according to Claim 36, wherein the cable is an optical fibre cable.