WO1998035506A2 - A very high data rate communications gateway for ground-space-ground communications - Google Patents

Abstract

A very high data rate communication system for communicating between a plurality of RF ground terminals (10-N) and satellite space station (20). One or more satellites (20) orbiting the earth and first laser communication system (SLC) are carried on said one or more satellites. One or more relay stations (12, 13) is positioned between the satellites and the RF ground stations and at a substantially stationary position above the earth's atmosphere, and at least above the earth's jetstream. An RF communications system (18-N) carried on the one or more relay stations communicate with the RF ground terminals. A second laser communication system (14-1, 14-2, 15-N) on said relay station communicates with the first laser communication system on the one or more satellites. Conversion circuits on the one or more relay stations convert information from the RF communication system on the relay station to information for transmission on the second layer communication system on the relay station. The RF communication system carried by the one or more relay stations includes antennas which form a plurality of narrow spot beams for communicating with the plurality of earth stations.

Description

A VERY HIGH DATA RATE COMMUNICATIONS GATEWAY FOR GROUND-SPACE-GROUND COMMUNICATIONS

REFERENCE TO PROVISIONAL, APPLICATION

The present application is the subject of provisional application Serial No. 60/035,914 filed February 10, 1997 for A VERY HIGH DATA RATE COMMUNICATION SYSTEM FOR COMMUNICATING BETWEEN A PLURALITY OF RF GROUND TERMINALS AND SATELLITE SPACE STATION, which is incorporated herein by reference.

The present invention is directed to a cost-effective very high data rate communication gateway for ground-space- ground communications.

The changes in communications technology are not keeping up with the consumer demand for bandwidth in such areas as multimedia, interactive applications, global media conferences, telecommuting, global interactive designs, and the like. Moreover, the cost of deploying new high data rate infrastructure is a significant barrier to entry for key elements of the global economy in remote locations, rural areas with limited infrastructure, large third world nations with primitive infrastructure but with significant labor forces, and radio frequency bandwidth limitation for all forms of satellite communications. There is, therefore, a global market need to provide global accesses to high volume communications that are of: low-cost to develop, short time to develop, low user cost and equipment, flexible in deployment and to changing technologies and capable of very high data rate full duplex communications .

Limitations to present high data rate communications technology include the following: fiberoptics where data rates are nearly unlimited in the hundreds of gigabits feasible and requires laying fiberoptic cables over-land. Satellite using radio frequency communications is limited to nearly 10% of the carrier frequency per polarization (practical limits are of the order of 1.5 Gbps/polarization/K_u band) and is limited by atmospheric absorption, available antenna real estate with satellite power available, and limited orbital slots and driven by interference issues. As shown in Figure 1, laser communications are nearly unlimited in potential bandwidth but are subject to high atmospheric losses limiting utility and is not cost effective for space-to-ground communications. On the other hand, RF communication bandwidths are limited for satellite communications by frequency allocations (both international and U.S. competition for frequency allocations are strong), orbital locations (limited locations available in geosynchronous orbit), large distances (which drives the need for large power amplifiers and antennas), atmospheric absorption (much less so than later communications), and power weight size constraints (typically costs are $50 to $100 K/lb. Thus, there is a problem of creating a capability that can have the bandwidth of fiberoptics and atmospheric transmission of radio frequency satellite communications . The present invention solves this problem by utilizing the atmospheric penetration capabilities of RF communications and the data rates of laser communication with low power and lightweight.

As is long known, for radio frequency communications in satellites, the key problems are large antennas as required for narrow beams. RF antennas must be large to obtain narrow beams, and the RF spectrum is busy with lots of use and interferences, and there is limited bandwidths due to the frequencies used. A summary of the pros and cons for satellite communication applications of lasers is, for the pro side, they (1) have very narrow beam, (2) high data rates are feasible (5 GBPS ) , and (3) lower power in free space. The problems with laser communications may be summarized as, (1) they are blocked by clouds and rain, (2) can encounter severe distortions during wind or other turbulence and (3) it is a maturing technology. For radio frequencies, practical radio frequency satellite communications are below 50 GHz due to atmospheric losses. It is a mature technology which penetrates wind, clouds, rain, and has bandwidths up to 5 GHz. The problems with RF satellite communications are: (1) it is difficult to get narrow beams with smaller antennas, (2) requires relatively high power in free space, and (3) the narrow bandwidths require large apertures . There have been significant technology developments in high altitude long-dwell stratospheric lighter-than-air vehicles and aircraft and very high-speed optical communications . (asynchronous

distribution of data from large bandwidth communications systems . Low-cost microwave receivers with cellular and global SATCOM systems available are in development. Thus, related communications technologies which the present invention can utilize include all communication technologies presently employed over radio frequencies and laser frequencies that can be used for communications between space and ground. Geostationary platform technologies that are incorporated in this invention include long-dwell high-altitude aircraft and lighter-than- air vehicles and propulsion systems that permit long-dwell at high altitudes (10-30 miles) for long periods of time. Present communications systems that are known to exist communicate directly from space to ground and directly through the atmosphere. This invention significantly reduces the limitations of direct space-to-ground or ground-to-space through the implementation of an intermediate sub-orbital high altitude data relay system.

The basic objective of this invention is to provide a cost effective application of high data rate communications that are feasible for the space environment with high data rate communications capabilities that are feasible and available within the earth's atmosphere.

As shown above, high data rate communications between space and ground terminals is limited by two factors: atmospheric limitations on coherence bandwidth and frequency use limitations due to international treaties and U.S. government regulations. These limitations restrict present useable bandwidth to a few gigabits in the radio frequency spectrum. Research and development activities in the use of laser communications have the potential for several hundred gigabits of communication but are fundamentally limited by atmospheric turbulence, clouds, and rainfall under extreme stability requirements on both the satellite and ground terminals. As shown above, the feasibility of using lasers to communicate between ground terminals and orbiting spacecraft has been complicated by atmospheric turbulence and absorption due to cloud cover. Thus , while above atmospheric communication by laser has been demonstrated and emerging technology in laser devices enables very high data rate communications in the order of tens of gigabits, the present invention takes advantage of the above-the- atmosphere use of laser communications and the small, low power and lightweight (as compared to radio frequency application), high data rate. These advantages are coupled with the advantages of radio frequency applications which enables through the atmosphere communications (transmission through clouds, turbulence and precipitation) .

The coupling of these two technologies, according to this invention, through the use of an intermediate earth gateway/relay terminal preferably located in altitude 10 -

25 miles above the ground, permits very high data rate connectivity between space and ground over the entire earth without the present-day constraints of atmospheric losses, frequency constraints and interference problems. By placing the gateway terminal relatively close to the ground

(10 - 30 miles) the above-discussed problems on the radio frequency system are minimized due to short path lengths while the laser communication implementation problems of atmospheric absorption are reduced or eliminated. Thus, the present invention provides a unique opportunity to combine the advantages of both laser and radio frequency communications and minimize the disadvantages of each technology. This implementation has several high-value civilian and defense related applications, including: 1. The intermediate gateway concept with its low cost, high data rate connectivity between any point on the earth. Through the use of a gateway relay, satellite communication between any two earth locations and very high data rate communications are feasible.

2. The application of the invention to global telecommunications service is also possible by the use of multiple low data rate downlinks from the gateway form.

The advantages of the invention include the provision of satellite communications with the unique flexibility and the operation of service locations independent of local weather or geographic considerations since the gateway system operates above the atmosphere. This reduces a need for expensive terrestrial communications infrastructure to provide services to other terrestrial communication hubs or gateway or directly to end customer drop point. The invention also provides significant opportunities to provide high bandwidth services to more locations that are remote to measure communications trunks such as Alaska,

Hawaii and ocean island nations. Finally, the invention has the potential to provide global competition to communications providers that have monopolistic controls of local communications markets due to high cost of infrastructure. This should create the opportunity to reduce global communications cost and increase connectivity and services to the world population.

Reference is made to the text by Stevens G. Lambert and William L. Casey entitled_. LASER COMMUNICATIONS IN SPACE, copyright 1995 by Artch House, Inc., Section 12.1.2.1 entitled "High Data Rate Low Earth Orbit (LEO) to GEO Crosslinks". There, reference is made to use of lasers for crosslinks and portions of the up/downlinks which suggests the combination of RF and laser communications technology. "The approach is to cross-link with optic communications and up/downlink with laser systems to subsonic aircraft flying above/around the clouds. These wide body types of aircraft are not new acquisitions but rather are already in the inventory. " The present invention is a significant improvement in that the present invention is predicated on the use of airships operating at low speeds so that there is no turbulence created which would corrupt the laser link. In other words, the present invention does not include airplanes or rockets.

Moreover, the altitude of the airship is critical in the invention because atmospheric turbulence corrupts laser communications. The fact that the airship is operating at an altitude above the jet stream, is practically stationary, and is above most of the atmosphere makes the laser portion of the invention viable. The combination of low density atmosphere and relatively low wind conditions reduces most of the distortion effects on the atmospheric channel. This combination of conditions not only allows the link to be closed but enables high data rate communications, data rates in excess of 4 GBPS , to be feasible. The invention would generally not work on high speed aircraft or air vehicles operating at altitudes in or below the jet stream. In addition, high altitude aircraft operating at high speeds create high levels of turbulence that would corrupt the laser link making the viability difficult and likely not feasible. Thus, this invention does not extend to airplanes or rockets because those platforms are not suitable for the practice of the present invention .

The altitude of the air vehicle and substantial payload capability enable the use of RF communications to move high data rates without the need for large antennas or high power microwave devices.

As discussed above in connection with the Lambert et al text, it has been suggested to postulate the use of subsonic aircraft using lasers to satellites and RF from aircraft to the ground. This idea sounds similar on the surface of the present invention but neglects to recognize that maintaining a reliable laser link from an aircraft would be difficult i.ξ_rini iupϋUiafa-La due to the atmospheric induced scintillation which would vary from about 05 milliseconds causing instantaneous path fades in excess of 60 db. The depth of fades would require the laser link powers to be in the multi- 100' s of watts for saαs± r? applications and outages most of the time. The present 16, '1 invention takes into account the impacts of atmospheric turbulence on the laser communications. It may be possible to relay lasers from the aircraft up to a satellite as the turbulence effects would be lower on the SATCOM side of the link. However, it is unlikely that the system would perform reliably due to beam tilt and scintillation. The present invention is in the sweet spot of the atmosphere and air platforms taking advantage of the altitude and relative stability and fixed position to permit laser communications without heroic atmospheric compensation systems which not only are big and expensive, but have never been demonstrated as being feasible for communications. To date there is no identified technology

CDiVri UiO ^ that has proven to permit transmission of laser communications between space and aircraft or the ground. This invention utilizes readily available technology in a unique fashion. The RF portion of the system is also unique in that it is coupled with the laser system to allow full transmission of high data rates to the ground from space without requiring large amounts of keep-out zones for interference rejection requirements.

DESCRIPTION OF THE DRAWINGS The above and other objects , advantages and features of the invention will become more apparent when considered with the following specification and accompanying drawings wherein:

Figure 1A is a diagrammatic illustration showing how clouds and moisture absorb laser beam energy resulting in loss of communications, Figure IB illustrates how wind and turbulence diffract and distort and corrupt laser beams making it difficult for communications to be successfully accomplished, and Figure 1C illustrates how for radio frequency communications RF antennas must be large to achieve narrow beams ;

Figure 2 illustrates in a diagrammatic fashion the overall system concept of an earth gateway incorporating the invention;

Figure 3 is a diagrammatic illustration of the application of the invention to a particular system;

Figure 4 is a schematic diagram illustrating the earth's components of the gateway; Figure 5 is a schematic diagram of the air vehicle details;

Figure 6 is a block diagram of the high altitude communications payload incorporating the invention;

Figure 7 is a block diagram of the communications equipment at the earth's terminal;

Figure 8 is a block diagram of the RF communications terminal on the high altitude platform;

Figure 9 is a block diagram of the RF communications terminal on a high altitude platform; Figure 10A is a schematic illustration of fixed service aspects of the invention;

Figure 10B is a schematic illustration of mixed applications service concepts incorporated in the invention; and Figure IOC illustrates mobile services which can be utilized in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION

Referring now to Figure 2, a plurality of RF ground terminals 10-1, 10-2...10-N and 11-1, 11-2, 11-3...11-N communicate over radio frequency links (typically less than 50 GHz) with a respective lighter-than-air craft or airships 12 and 13 (only two being indicated). The lighter-than-air aircrafts 12 and 13 have a critical altitude so as to avoid atmospheric turbulence which corrupts laser communication. The airships 12 and 13 are operating at least above the Jetstream and in the range of 10 - 40 nautical miles above the earth's surface and are practically stationary and above most of the atmosphere to accommodate operation of the laser portion of the invention. The combination of low density atmosphere and relatively low wind conditions reduces most of the distortion present on the atmospheric channel. The lasers 14-1, 14-2 on airship 12 and 15-1, 15-2 on airship 13 are coupled via coupling circuits 16, 17 to the radio frequency receivers and transmitters. RF transceivers 18-1, 18- 2...18-N and 19-1, 19-2...19-N are coupled via data routing circuits PR (Figure 6) to the respective laser transceivers 14 and 15, respectively, and. vice versa. Laser transceivers and telescopes 14 and 15 are oriented to communicate with orbiting satellites 20 which are provided with laser communications SLC. As indicated in Figure 3, system parameters for a given system are illustrated. Note that for a given frequency of 48 GHz, a modulation scheme of lδPSKjfull duplex is used. The data rate is about 6 Gbps per terminal per polarization. Polarization diversity is about 3.0 GHz per channel with total capacities being the total data rate of 60 Gbps with 12 space laser links at 5 Gbps. For the ground stations, the frequency is likewise at about 48 GHz and a typical antenna size is about six feet with the transmitter power of about 1 watt.

As shown in Figure 4, the earth-based components consist essentially of the ground terminals 10, the RF links from the ground terminals to the RF antennas on the lighter-than-air vehicle 12 with the data routing function being performed by a control unit PR to couple (route) data to the laser transceivers 14 for communicating with the laser transceivers SLC on the satellites 20. Note that the preferred altitude is 10 to 25 nautical miles (but can extend to 40 nautical miles) and that in the preferred embodiment, narrow spot beams are transmitted from the lighter-than-air vehicle 12 to the ground terminals 10 and 11 so as to minimize adjacent channel interference. The airlift mechanism or lighter-than-air aircraft 12 is illustrated as having an attitude and position control. The antennas on the lighter-than-air craft have narrow beams (preferably less than 1°) with the radio frequency terminals typically operating below 100 GHz (the number of terminals being unlimited except by the platform space). The antenna beamwidths are selected to minimize adjacent link interference. The links are full duplex, simplex or half-duplex per application.

In a preferred embodiment, the vehicle is a lighter- than-air vehicle that can hover at altitudes where atmospheric losses for laser or EHF permits high reliability communications . Typical altitudes are greater than 10 nautical miles but at least greater than the jet stream. Periods for the lighter-than-air craft on station for days to years and the platform stability meet communications terminal requirements.

Payload Features

The laser links between the air vehicle and space relay satellites and a narrow beam below RF links between air vehicle and ground. This means that small antennas can provide narrow beams and use separation between users or other vehicle limits to solve radio interference problems. The short ranges between the earth and air vehicle limit the need for high power transmitters . Most communications are achievable with less than 1 watt power. Figure 6 illustrates the block diagram showing the high altitude communications payload. There, each antenna 30, 31 can be coupled to one or more RF transceivers 32-1, 32-2...32-N. The RF transceivers are coupled via data routing and function as units PR and laser transceivers 14-1, 14-2 and their associated laser telescopes LT. Figure 7 illustrates one example of the communication block diagram at the earth's terminal. While a base antenna is illustrated, the antennas can be arrays, dishes or horns . Note that the polarization can be linear or circular or a combination. The antennas 35 are coupled by diplexer 36 to receiver/demodulator circuitry 37-1 and demultipliers 38 to a user or data source 39. Data from the user or data source 39 is processed through multiplexer modulator 40 in RF transmitter 41 to diplexer 36. Figure 9 illustrates the RF communications terminal on the high altitude platform or lighter-than-air ship. Note that the signals from the RF system are coupled to an optical transmitter 50 and optical switch 51 to the laser telescope 14, and signals from the laser telescope are coupled via the optical switch 51 to receiver 53 which couples the signals to the RF system for transmission to earth. In this disclosure, the laser is not the only link option to the satellite communication. This link can also use extra high frequency EHF links depending on cost, data rate and frequency of allocation or any other frequency band which atmospheric and climatic conditions affect adversely. The system as disclosed provides for fixed high data rate service with multiple distributions possible. It provides a massive data switch in the sky and can support various commercial and military operations with robust global information handling capabilities. Figure 10A illustrates fixed service concepts including the fixed ground terminals. Figure 10B illustrates mixed applications wherein some of the RF ground terminals are fixed ground terminals, others can be mobile services and others can be airborne services. Figure IOC illustrates the system as applied where all of the ground terminals are mobile.

The altitude of the airship and the speed that it is traveling is critical. The altitude must be sufficient that the airship is at least above the Jetstream and is practically stationary and is above most of the atmosphere where disturbances in the air produce turbulence which adversely affects laser communications. By positioning the airship at an altitude such that the low density of the atmosphere and relatively low wind conditions, most distortion affects in the atmospheric channel are substantially reduced or eliminated. This combination of conditions not only allows the link to be closed but enables high data rate communications: data rates in excess of 4 GBPS are entirely feasible. The invention excludes high-speed aircraft or air vehicles operating at altitudes in or below the Jetstream.

While preferred embodiments of the invention have been illustrated and described, it will be appreciated that various other embodiments , adaptations and modifications of the invention will be greatly apparent to those skilled in the art.

Claims

WHAT IS CLAIMED IS:

1. A very high data rate communication system for communicating between a plurality of RF ground terminals and satellite space station comprising: one or more satellites orbiting the earth and first laser communication means carried on said one or more satellites , one or more relay stations positioned between said one or more satellites and said RF ground stations and at a substantially stationary position above the earth's atmosphere, and at least above the earth's Jetstream, an RF communication means carried on said one or more relay stations for communicating with said RF ground terminals , a second laser communication means on said relay station for communicating with said first laser communication means on said one or more satellites, and conversion means on said one or more relay stations for converting information from said RF communication means on said relay station to information for transmission on said second laser communication means on said relay station.

2. The communication system defined in Claim 1 wherein said RF communication system carried by said one or more relay stations includes means to form a plurality of narrow spot beams for communicating with said plurality of earth stations .

3. The communication system defined in Claim 1 wherein said one or more relay stations includes an airship at said substantially stationary position and from about 10 to about 25 miles above the earth's surface.

4. The communication system defined in Claim 2 wherein said one or more relay stations includes one or more airships at substantially stationary positions and from about 10 to about 25 miles above the earth's surface.