Multi-node wireless communication system with multiple transponding platforms

ES 2 278 745 T3

DESCRIPTION

Multi-node wireless communication system with multiple transponder platforms.

The present invention relates to a wireless communication system between mobiles and a method of communicating using such a system.

From US 5,423,059 a method is known to improve signal quality in a Simulcast communication system (simulcast in digital and analog). In the simulcast communication system, a communication unit can request enhanced services in its coverage area by transmitting a request for improved signal quality to a central simulcast controller. Upon receiving the request, the central simulcast controller identifies, based on information pertaining to the status of the communication unit, a set of transmitters to improve coverage of the communication unit area. The central simulcast controller determines delays in the enhanced coverage area for each of the transmitters and adjusts the respective delays of each transmitter in such a way as to improve the quality of the transmitted signal for the location of the communications unit.

In particular, the communication unit sends a request for improved signal quality, including its current situation, to the central simulcast controller. Upon receipt of the request, the central simulcast controller distinguishes the status of the requesting unit.

This can be achieved by measuring the arrival time of the request to each transmitter in the system with the help of a local absolute time reference. With this information, the central simulcast controller can effectively triangulate the position of the communication unit.

Once the location of the unit is known, the central simulcast controller determines a set of transmitters required to improve the coverage area and then calculates a new transmission delay requirement for each transmitter. These newly calculated delay requirements are then passed on to the transmitters in such a way that, by changing the delays, destructive interference degrading signal quality is substantially eliminated.

The present invention relates generally to a wireless communication system. More specifically, the present invention relates to a wireless communication system with increased frequency reuse capability, for point-to-point communications.

Current systems for satellite-to-mobile communications, such as Iridium, Globalstar, and ICO, use low-cost user terminals as one of the key features of the system. To maintain the communication link with these common mobile systems, the satellites in the system provide high gain, multi-beam services to subscribers. Low-cost, low-gain portable terminals used by users of these systems transmit and receive signals to and from high-performance satellites that populate most of the atmosphere. Some of these common systems require access to at least two satellites in order to ensure a smooth handover process when the satellites move from one horizon to the other. As a result, the satellite system gains in reliability and availability as the number of satellites increases in the field of view (FOV) of a user. The constellations of satellites offered by these common systems are thus dimensioned to guarantee, at all times, in large coverage areas, the minimum number of satellites within the FOV of a user.

However, all these common satellite-to-mobile communication systems suffer from certain disadvantages. First, they all have limited frequency resources (the term "frequency" is used in this document in a general sense to refer to frequency, time slots, or CDMA code). Any given frequency for a given ground position can only be used by one user at a time. Thus, if a user accesses a satellite using a particular frequency to communicate with its counterpart in the network, other satellites and / or users in the same region cannot use the same frequency source in the same local area. In particular, if a nearby secondary user has a telephone that needs the same frequency source that is being used by the first user, the second user cannot access the system, even through different satellites. This is true regardless of the sophistication of the system, even with systems using multi-beam satellite designs. Even though multiple satellites may be available in a given geographic location, the same frequency spectrum cannot be used by more than one user in a local area. The availability of multiple satellites only serves to increase the availability of the system for the user. However, the total capacity of these mobile communication satellite systems is still limited by their inefficient use of available frequency sources. Thus, the potential growth of these common satellite communication systems is inherently limited.

Furthermore, current communications systems generally only allow mobile-to-hub and hub-to-mobile communications in most constellations of mobile satellites in medium and low Earth orbit. Mobile-to-mobile links require multiple hops between hubs. This means that two or more frequency resources must be allocated by the system to shut down the network.

Clearly, it is desirable to provide a satellite system for mobile-to-mobile communications that alleviates the aforementioned limitations, and makes more efficient use of current sources of the mobile-to-mobile satellite communication system, while also providing much greater opportunities for growth. of the system.

An object of the present invention is to provide a wireless communication system with fewer limitations in frequency reuse for point-to-point communications.

Another object of the present invention is to provide a wireless communication system using individual transponders and mobile terminals that are relatively simple and uncomplicated.

Another object of the present invention is to provide

ES 2 278 745 T3 is a wireless communication system with high reliability by virtue of acceptable degradation.

Still another object of the present invention is to provide a wireless communication system whose complexity is concentrated in central processing concentrator stations.

Yet another object of the present invention is to provide a wireless communication system capable of achieving more precise positioning of satellites and users.

In accordance with the objects of the present invention, a wireless communication system between mobiles is provided. The wireless communication system between mobiles includes a plurality of individual communication transponder platforms. Each of the plurality of individual transponders is in communication with a central processing hub, such that a signal processed by the central processing hub on the forwarding link is radiated with offset time delays, towards the plurality of individual transponders. The radiated signals are then radiated again by the plurality of individual transponders and are coherently received and processed by a mobile user terminal. The return path of the link signal is the opposite of the sending path.

These and other features of the present invention will become apparent from the following description of the invention, when viewed in accordance with the accompanying drawings and the appended claims.

Figure 1 is a schematic illustration of the geometry of the forwarding link of a mobile satellite communication system, in accordance with the present invention;

Figure 2 is a schematic block diagram illustrating the signal transmission function of a terrestrial telecommunications hub for a wireless communication system, in accordance with a preferred embodiment of the present invention;

Figure 3 is a schematic illustration of the return link geometry of a wireless communication system in accordance with a preferred embodiment of the present invention;

Figure 4 is a schematic block diagram illustrating the signal reception function of a terrestrial telecommunications hub for a wireless communication system in accordance with a preferred embodiment of the present invention;

Figure 5 is a schematic flow diagram illustrating the overall architecture of a multi-transponder wireless communication system, in accordance with a preferred embodiment of the present invention; and Figure 6 is a schematic illustration of a wireless communication system for point-to-point communication, using a variety of different types of transponder nodes, in accordance with a preferred embodiment of the present invention.

Referring now to the figures, the described mobile communication system can be used to break the frequency spectrum limitation discussed above and provides a much more efficient means of reusing the assigned wireless spectrum and mobile satellite multiple times. By removing this limitation of the frequency spectrum in the operation of multiple satellites, the overall capacity of existing wireless communication systems and mobile satellites can be more easily expanded.

Referring now to Figure 1, there is illustrated a satellite communication system 10 between mobiles in accordance with a preferred embodiment of the present invention. In Figure 1, the mobile satellite communication system 10 is depicted in a forward link mode. The mobile satellite communication system 10 includes a terrestrial telecommunications hub 12, a constellation 14 of satellites made up of a plurality of individual satellites 16, and a plurality of portable user terminals 18, such as mobile phones. As described in greater detail below, user terminals 18 can receive signals 20 simultaneously from multiple satellites 16 via wide-beam antennas 22. The terrestrial telecommunications concentrator 12 is in communication with all the satellites 16 of the constellation 14 of satellites, individually and simultaneously. The concentrator 12 also performs user signal pre-processing to compensate for path differentials before sending radiated signals 24 to satellites 16, as described in greater detail below, and similarly performs received signal processing. from satellites.

According to the preferred embodiment, the design of the individual satellites can be significantly simplified over those used in previous mobile systems, since the constellation 14 of satellites behaves as a sparse radiant cluster. It is known that the more satellites 16 that are included in the constellation 14 of satellites, the better the performance obtained from the satellite communication system 10 between mobiles. Simple, small, high-performance satellites are preferred. This is because the behavior of the system 10 is more dependent on the constellation 14 of satellites than on the individual satellites 16.

In a transit mode, illustrated in Figure 1, individual satellites 16 radiate modulated RF energy to a chosen field of view ("FOV"). The system 10 can, however, be operated with reduced capacity and without reconfiguration, even if, for whatever reason, an individual satellite 16 is lost. As a result, the system 10 exhibits acceptable degradation characteristics and provides very high reliability and availability. The greatest complications of the system 10 reside in the ground hubs 12, which locate and track potential users and perform the main beamforming and filtering functions, as described below.

As shown in Figure 2, the processing performed at the terrestrial telecommunications concentrator 12 is diagrammatically illustrated. The concentrator 12 tracks, updates and predicts the differential, time-varying information between various paths between the concentrator 12 and the intended user terminals 18. The accuracy of this information is preferably within a tenth of an RF wavelength. For UHF satellite systems, the required differential path accuracy should be about 10 (ten) centimeters.

ES 2 278 745 T3

For L and S band mobile satellite constellations, the accuracy should be on the order of one (1) centimeter. Unfortunately, the usual or GPS techniques are not able to provide the required accuracy.

In accordance with the present invention, the required accuracy of equivalent path differentials, including all propagation distortion, can be achieved using two-way active calibration and R2N (two-way distance-measuring navigation) techniques. An R2N technique is simply a technique for obtaining positioning information by which the positions of satellites and users are precisely located using multiple calibration sites, and is described in the United States patent application, also pending. , serial number 09 / 209.062, entitled "Method and System for Determining the Position of a Transceiver Unit Incorporating Two-Way Distance Measurement Navigation as a Calibration Reference for GPS," and filed December 10, 1998. Other known techniques may also be used.

The terrestrial communications concentrator 12 has a processing center 26 that processes each signal and is illustrated in transmission mode in Figure 2. The concentrator 12 has the capacity to access the plurality of satellites individually making use of spatial discrimination in the antenna to provide separate signals to different satellites. Alternatively, code identification can also be used to access different satellites independently.

As shown in Figure 2, assuming there are "H" users, signals from user 1 to user H, generally identified by reference number 28, are input to processing center 26. The positions of the various users (1 to H), designated by reference numeral 30, are generally determined by the circuitry from the various user signals 28. The various user signals 28 for users 1 through H are then combined for transmission to the different satellites 16, as indicated generally by reference number 32. In this case, the signal is sent to N satellites . The combined signals are then amplified, filtered, upconverted, and then further amplified, as generally indicated by reference numeral 36. These signals are then delivered to a multibeam antenna 38 where beamforming processing is performed so that the signals can be transmitted to each of the N satellites by radiating signals 24. Beamforming processing can be carried out in the baseband or low IF frequency band, by digital or analog means. For a signal with a small bandwidth (less than a few MHz), execution by digital means can offer economic advantages. The processed signals 24, radiated from the ground hub 12 to each satellite, are amplified, filtered, and then re-radiated by each of the multiple satellites 16 to simultaneously reach designated user positions. Consequently, the signals radiated from the multiple satellites will be coherently received by a simple handheld terminal wide beam antenna 22.

Equivalently, the effect of the spatial processing carried out by the processing center 26 is to focus the signal intensity on the user from multiple satellites 16, which act as separate, scattered parts of a large active reflector. Thus, ground processing will incorporate different time delays into signals 24 that are radiated through various pathways (ie, transponders). The time delays will be introduced into the signals 24 as if the satellites were located on an ellipsoidal surface, the two foci of which were located exactly at the positions of the concentrator 12 and the designated user 18, respectively. In constellations in low and medium Earth orbit, users 18 and hub 12 will always be in the near field of the sparse cluster.

In the receive mode, illustrated in Figure 3, individual satellites 16 pick up RF signals from the same FOV. Figure 3 illustrates the geometry of the return link for receiving signals sent from the user terminals 18 to the terrestrial telecommunications concentrator 12. As shown in Figure 3, two groups of links are involved: the links (signals) between the users 18 and the satellites 16, generally indicated by reference numeral 40; and those between the satellites 16 and the concentrator 12, generally indicated with the reference number 42. To achieve the best performance, the user antennas 22 are able to illuminate, preferably, all the satellites 16 involved. This will lead to a restriction on the variation of the gain of the user antenna 22 with respect to the group.

As in the case of the forward link geometry, the satellites 16 will amplify the signals 40 received from the users 18 and will re-radiate the signals 42 back to the hub 12. The hub 12 can receive the signals 42 independently, but at the same time. time, from satellites 16; and will sum the signals 42 from different satellites 16 coherently in the post processor 44, as illustrated in Figure 4.

The signal flows and the block diagram illustrated in Figure 4 show the reception function of the post processor 44 and the hub 12. The signal flows are the opposite of the corresponding ones in Figure 2. Therefore, it will not be repeated. in detail the reception process. However, links 42, from satellites 16 to hub 12 are received at multibeam, beamforming antenna 34 and then transmitted to receiver and downstream converters 46 before the signals are separated. The signals are separated depending on the user from which they are received, as generally indicated by reference number 48, and then sent to the specific user, 1 to H, as generally indicated by reference number 50. It should be understood that both the receiving and transmitting functions are necessary parts for the calibration of the link path and the location of the user's position.

The technique of the present invention has been shown to significantly reduce mean sidelobe levels. This has been determined to be due to three factors. In the first place, the proposed architecture is not a periodic grouping but a sparse grouping with random separations, lacking diffraction lobes. Although the average level of

ES 2 278 745 T3 the side lobes at a single frequency is relatively high, the level decreases with increasing bandwidth. Second, the large sparsely populated cluster formed by the satellites constitutes a large extended aperture. Thus, all land users are in the near field of the extended aperture and the wavefronts received by all users are spherical rather than flat. Consequently, the effects of scattering are much more pronounced than they would be in the far field. The dispersion grows very rapidly when scanning with a probe outside the main beam and very effectively blurs the power distribution in a finite signal bandwidth. Third, the communication system is preferably designed with a large frequency bandwidth. The information signal will therefore be spread over this entire bandwidth via CDMA or via short duration waveforms for TDMA schemes.

Figure 5 diagrammatically illustrates the operation of the invention, which enables increased reuse of the precious frequency spectrum by multiple satellites. Among the advantages offered by this system is the absence of any limitation on the reuse of frequencies by additional satellites for point-to-point communications. Instead, the capacity of this system is only limited by the total RF power of the satellites. Furthermore, the preferred embodiment allows the use of simple and inexpensive satellite designs, since the more satellites included in the constellation, the better the performance of the entire system. The system also provides high reliability by virtue of acceptable degradation, as well as the concentration of complicated treatments in the concentrators.

The preferred embodiment creates a demand for a large number of low cost satellites and also makes use of R2N techniques to achieve satellite and user location. The greater the number of users using this system, the greater the precision with which satellite and user positions can be determined. However, even more important than the actual positions of the users and the satellites, are the lengths of the paths traveled by the signals. Therefore, periodic calibration techniques applied directly to these path lengths can be much simpler and more economically effective. The system also benefits from the large percentage of bandwidths available with CDMA and TDMA systems.

As shown in Figure 5, the present invention is divided into three segments: a hub segment 52 containing the terrestrial telecommunications hub 12, a space segment 54 containing a plurality of individual satellites 16, and a segment 56 of users having a plurality of user terminals 18. The hub segment also has a processing center 26 that pre-processes the transmission signals and a post processor 44 to post-process the received signals.

User terminals 18 simultaneously receive and transmit signals from / to multiple satellites 16 through broadband antennas. User terminals 18 may lack the ability to separately access individual satellites in space segment 14. The concentrator 12 pre-processes the signals destined for each local user as they transmit and post-processes the signals supplied to each local user as they receive them to compensate for path differentials. These corrections are calculated separately and are applied to the signals transmitted to each satellite 16 of the space segment 54 or to those received from each of them. While the invention has heretofore been described in relation to a plurality of satellites 16, it should be understood that a variety of other transponder nodes may be used in place of or in combination with one or more of the satellites 16.

Referring again to Figure 6, an illustrative wireless communication system 100 is shown in accordance with the preferred embodiment. The illustrative wireless communication system 100 includes a central hub 102, a transmitter tower 104, a satellite transponder 106, a high-altitude platform 108, an antenna 110, and an intended user 112. The intended user 112 may be a stationary user. or a mobile user.

The central hub 102 is in direct communication with the antenna 110 in order to process the signals transmitted and received by the antenna 110, in accordance with the techniques described above. In accordance with this illustrative embodiment, the central hub 102 also communicates with the transmitter tower 104 via line 114, in order to perform the processing of signals transmitted and received by the transmitter tower 104. Line 114 may be a terrestrial cable or it may represent wireless communication between central hub 102 and transmitter tower 104. Also, although only a single antenna 110 and a single tower 104 are illustrated, it will be understood that multiple antennas and multiple towers. In addition, tower 104 can be part of a regional or national tower-based cellular network for inter-fixed or mobile-to-mobile communications.

As shown, in the example, the central hub 102 processes signals 116 that are transmitted from the antenna 110 to a platform 108 located at high altitude. On the forward link, signal 116 is then transmitted from platform 108 at high altitude to intended user 112, as generally represented by signal 118. On the return link, the intended user sends a signal 118 to platform 108 at high altitude, which then sends a signal 116 to antenna 110, which has been processed by central hub 102. In addition, central hub 102 it also handles signals 120 that are transmitted by antenna 110 to a satellite transponder 106. Satellite transponder 106 then transmits signal 122 to intended user 112. On the return link, the intended user 112 sends a signal 122 to the satellite transponder 106 which then sends the signal 120 to the antenna 110 for processing by the central hub 102.

In addition, the central hub 102 sends a signal 114 to the transmitter tower 104 which, in turn, communicates with the intended user 112 through a signal 124. On the return link, the intended user 112 communicates with the tower 104 to through a signal 124, said tower then communicating with the central concentrator 102 through a line 114 for signal processing. Although only described

In ES 2 278 745 T3 be a single transponder node of each type (ie satellite, high altitude platform or tower), a specific system can use any combination of such transponder nodes. It should be understood that, according to the described system, any high-altitude platform system can be used, such as manned or unmanned aircraft, balloons or airplanes. Furthermore, in accordance with the described system, any space-based system that includes one or more spacecraft for point-to-point communications can be used.

The illustrative system 100 described can greatly improve frequency reuse efficiency for point-to-point communications, such as in multiple telephony and bidirectional Internet protocol. This improved capability is the result of the fact that the information for the intended receiver (user) 112 will arrive from all of the in-phase transponders. Thanks to this configuration, information for unanticipated users will generally arrive out of phase. For unanticipated users, out-of-phase signals will appear as noise. The behavior of the proposed invention will depend on the spatial separations between the various transponder nodes, as well as on the remote users. The communication bandwidth between transponders and users will also affect behavior. The effect of these on the disclosed system 100 can be determined in a variety of known ways.

Thus, the present system 100 may be comprised of a plurality of transponder nodes that are only part of a pure tower-based system, a high-altitude platform system, such as a stratospheric platform, or a communications satellite system. Alternatively, the system may consist of a plurality of transponders that are selected from all or some of the aforementioned types of transponder nodes.

The invention having now been fully described, it will be apparent to those of ordinary skill in the art that many changes and modifications can be made therein without departing from the scope of the invention as set forth herein.