BACKGROUND OF THE INVENTION Field of the Invention The present invention pertains to communication systems, methods and computer program products used in communication systems where components of the system include an elevated platform, such as a satellite, and a ground-based transceiver. More specifically, the present invention is directed to communication systems, methods and computer program products, that use multi-beam antennas in the elevated platform, such as low earth orbiting (LEO) satellite or geostationary (GEO) satellite to communicate on forward and reverse links with a ground-based gateway connected to a terrestrial communication network.

Discussion of the Background Satellites have been used in communication systems since the late 1950's to provide line-of-sight communications between ground-based terminals separated over significant distances. Because the goal of a communication satellite is to relay a signal from one terminal to the next, the satellite is equipped with a receive antenna and a transmit antenna (perhaps integrated into one device) and a power amplifier to boost the signal power before radiating the amplified signal to one of the ground-based terminals. Depending on the specified functions and attributes, the satellite may include on-board processing that manipulates the communication signals before transmission. A simple type of processing is to frequency translate the signal from a"receive frequency"to a"transmit frequency", while more complex processing systems involve decoding the signals before reformatting and broadcasting the signals. For a discussion of components used in satellite communication systems, as well as different types of processing used therein, see Pritchard, W, et al, "Satellite Communication Systems Engineering", Prentice-Hall Inc., 1986, ISBN No. 0-13- 791245-5, and Bhargava, V. et al,"Digital Communication by Satellite", John Wiley and Sons, Inc., 1981, ISBN 0-471-08316-X, the entire contents of both of which being incorporated herein by reference.

One type of processing that is conventionally performed onboard a communications satellite is"beamforming". In beamforming, a signal, or signals, to be broadcast (or received, by implementing a reciprocal procedure) is subdivided into constituent parts, and then the relative phases of the constituent parts are adjusted according to a physical arrangement of antenna elements, and a desired radiation pattern. One type of radiation pattern is a"multi- beam"pattern, where discrete radiation beams may be directed to predetermined locations for communicating with terminales located in the predetermined locations. The relative spacing and gain of the beams is determined by the weighting and phasing algorithm employed by a beamforming device that is co-located with the phased array antenna. Various beamforming techniques and systems are described in Mailloux, R.,"Antenna Array Architecture", Proceedings of the IEEE, Vol. 80, No. 1, pages 163-172, January 1992; Gabriel, W., "Adaptive Processing Array Systems", Proceedings of the IEEE, Vol. 80, No. 1, pages 152- 162, January 1992; and Hwang, Y.,"Satellite Antennas", Proceedings of the IEEE, Vol. 80, No. 1, pages 183-193, January 1992 (hereinafter"Hwang"), the entire contents of each of which being incorporated herein by reference.

U. S. Patent No. 5,422,647 describes a mobile communication satellite payload that uses separate transmit and receive phased array antennas for communicating with ground terminals. The transmit and receive phased array antennas each have a beamforming device used to form multiple beams for covering different locations on Earth. A separate antenna is provided for another radio frequency (RF) link, a two way link, with a ground-based gateway, through which all of the communications signals pass. As seen in Figure 2A, for example, a transmit phased array antenna 42 is integrally formed with a beamformers 50 and power combiners 54. Integrally forming the beamformer with the phased array antenna on the satellite is generally believed to be the superior approach to providing a satellite multi-beam antenna system, as is evident from the discussion at pages 188 and 189 in Hwang, which explains that MMIC technology enables beamformer devices to be placed in the satellite, rather than on the ground in the more advanced communication satellites.

In U. S. Patent No. 5,422,647 the link to/from the gateway 18 employs an antenna 76 that broadcasts a group of channels corresponding with the respective beams, after first being spectrally arranged via a multiplexer and converter. The gateway 18 processes the signals, perhaps connecting them to the public switch telephone network (PSTN). The return link operates in a similar manner, where the burden is on the satellite to covert signals from the group of channels into electrical signals used to feed the respective elements of the transmit phased array antenna 42.

As appreciated by the present inventors, a disadvantage with a conventional satellite based beamforming system like that shown in U. S. Patent 5,422,647, is that placing the infrastructure for forming and processing the beams in space is costly, and reduces system reliability and maintainability. According to conventional practice, the beamforming infrastructure is hosted on the satellite due to the simplicity of directly connecting the beam former network to the phased array antenna elements. However, as presently appreciated, the combined weight, launch load, and reliability constraints associated with arranging the beamforming network in the satellite, gives rise to significant cost inflation for implementing the system. For example, implementing the beamformer in space requires the use of expensive space-grade components to meet reliability considerations. Moreover, at the present time and in the foreseeable future, when satellites are on station, it is economically infeasible to repair or replace failed components. Accordingly, additional expense is often incurred by providing on-board spares that may be brought on-line when a main component fails. However, the preventative measure of adding the redundant parts further drives up the system's cost, size, and launch vehicle launch load.

Another limitation, identified by the present inventors, with conventional satellite- based beam forming networks is that the beam forming operations cannot readily be altered in a convenient fashion, so as to provide an"adaptive"multi-beam communication system.

For example, the system described above in U. S. Patent No. 5,422,647 is representative of conventional systems in which a"hard wired"multibeam structure is established and thus cannot be altered to accommodate changes in market demands or other situations that may benefit from an adaptable antenna system, such as a beam steering system.

As presently appreciated, an improvement on the device and system in U. S. Patent 5,422,647, would include incorporating a programmable processor, or perhaps a pre- programmed processor, application specific integrated circuit (ASIC) or the like, in the beamformer, to allow for the steering or reconfiguration of the number and orientation of the respective beams. However, consistent with the limitations of the conventional approach, the present inventors have recognized that by placing an"intelligent"beamformer in the satellite, further exasperates the satellite cost and reliability issues discussed above.

Traditional multi-beam phased array antennas use a beam-forming network that is an integral part of the antenna. As recognized by the present inventors, there are disadvantages with this approach when a phased array antenna is used in a satellite payload, not the least of which is increased complexity of the satellite payload, and associated cost. Additional problems include the following: fixed beams must be predefined before launch and cannot be changed during the mission life of the satellite and a complex electronically scanned beamformer needs to be incorporated within the satellite payload.

Spaced-based electronics, especially microwave electronics, are significantly more costly than equivalent ground-based systems.

In contrast to conventional techniques, advantages of the proposed invention include: (1) reduced payload complexity and therefore lower cost and project risk (i. e., having a greater investment in a space-based portion of the system, which may be lost if a problem arises with the launch vehicle or deployment of the satellite); (2) a low-cost method of producing one or more scanned beam (s) or simultaneous multiple fixed beams; and (3) an ability for smart antenna functionality such as adaptive beams, anti-jamming nulls, target tracking, etc.

SUMMARY OF THE INVENTION Accordingly, one object of the present invention is to address the above and other- limitations of conventional systems, methods, and computer-program products.

While the discussion in the present section is limited to a summary of the present invention, not all of the attributes of the present invention are included in this summary.

Accordingly, a more complete understanding of all the facets of the present invention will be obtained by reviewing the present document in total.

The present invention is directed to ground-satellite distributed multi-beam communication systems, methods and computer programmed product employed as a component of the system. The phrase"ground-satellite", refers to two components of the overall communication system that cooperate with one another as a single unit to provide the adaptive multi-beam satellite antenna features, albeit with the cost and reliability benefits of ground-based processing. The satellite includes two phased array antennas, transmit and receive, but due to the duality of the antennas, the discussion will focus on only one, usually the transmit phased array antenna. The construction and phasing of the signals used to feed the respective antenna elements is not exclusively produced at the satellite, but rather at a gateway station, located on the ground. By including the beamforming operations on the ground, it is possible to reduce overall system cost, use only one set of parts rather than dual- redundant parts, use normal components rather than space-grade components, reduce payload weight and launch vehicle launch load, and improve system up-time.

Implementing the beamforming operations at the gateway, enables the use of high- performance, upgradably processors that can implement from the ground"smart beamformers"with adaptive beam-steering, and beam shaping attributes.

In exemplary embodiments, the present invention may be employed in Multi-Region Ground Source-Satellite-Ground Network (Reverse Link) and/or Ground Network-Satellite-Multi-Region Ground Reception (Forward Link).

Multi-Region Ground Source-Ground Network (Reverse Link) Ground Network-Multi-Region Ground Reception (Forward Link).

The invention may also be employed in a Mobil Satellite Service (MSS) system to provide a lower-cost satellite payload and increased overall system capacity. The invention could also be employed in a Mobile Terrestrial Service system to provide increased overall system capacity. While the exemplary embodiments focus on a MSS system configuration, the invention is equally applicable for a terrestrial system. Previously, this function has been achieved by using an active phased array antenna and beam-forming all within the satellite payload. This invention addresses a new approach to solving the problem of satellite beam- forming by using ground-based signal processing techniques.

Features of the present invention can be described by an example of a LEO satellite providing Mobile Satellite Service (MSS). Typically, these systems are made of full duplex communications between mobile users and a ground-based communications network such as the Public Switched Telephone Network (PSTN). MSS communications links are made of a Forward Link, where signals from the ground network are routed through the satellite via a Forward Feeder Link and re-txansmitted to the users via a Forward Service Link produced by a multi-beam satellite antenna.

A second communication link, i. e., the Reverse Link that includes the Reverse Service Link, which receives mobile user signals via a multi-beam satellite antenna and routes these signals to the ground network via a Reverse Feeder Link. Multi-beam antennas are used for the Service Links (i. e., Forward and Reverse RF Links to and from users) and allow frequency reuse between geographical regions as a result of spatial isolation. Frequency reuse is a common method of increasing system capacity.

In some systems without onboard processing, each of the frequency bandwidth of each of the beams is usually converted to, or derived from, the Feeder Link band by Frequency Division Multiplexing (FDM) in a third band.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 is a block diagram of a space segment reverse link multi-beam communication system according to the present invention; Figure 2 is a block diagram of a ground station (gateway station) that communicates with a dual polarization feeder link established with the space segment of Figure 1, according to the present invention; Figure 3 is a block diagram of a forward link portion of the ground station according to the present invention; Figure 4 is a block diagram of a forward link portion of the space segment according to the present invention; Figure 5A-5G are graphs comparing a feeder link bandwidth allocation and various inventive approaches for implementing a distributed beamforming system by communicating via the bandwidth restricted feeder link; Figure 6 is a detailed block diagram of portions of the forward link beam forming, beam loader and beam scanner network, as well as the frequency division multiplexing assembly and forward link antenna network according to Figure 3; Figure 7 is a block diagram showing an alternative to the architecture shown in Figure 6 so as to ease bandwidth restrictions on the feeder link; and Figure 8 is a block diagram showing a further alternative to that shown in Figure 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following discussion describes several different embodiments of the present invention and is organized into two main parts. The first part is described in reference to Figures 1,2,3, and 4. The second part describes selected features, functions and obstacles addressed by the selected components of the inventive system as well as the overall system.

To generate the multi beams for the RF Links to and from the users, the satellite payload includes two antenna arrays, one for transmitting signals in the Forward Service Link and a second for receiving signals in the Reverse Service Link. These antenna receive and transmit feed elements are similar to an active phased array antenna, however, they do not have the beam-forming functionality. The following will describe how the beam-forming function is carried out on the ground rather than in the satellite.

The satellite transmit antenna includes radiating elements, usually equally spaced apart within a transmit aperture (although other spacings may be used as well). A radio frequency (RF) power amplifier precedes each radiating feed element. Similarly, the satellite receive antenna has of a number of RF receiving feed elements arranged within an aperture.

Each of the receive elements is followed by a low-noise RF amplifier and filter assembly.

Additionally, the satellite payload has two antenna networks for interfacing with the ground network; one to receive signals from the ground network (Forward Feeder Link) and a second to transmit to the ground network (Reverse Feeder Link).

Reverse Link Description Mobile user signals from a number of geographic regions are present at the receive antenna array. As shown in Figure 1, these signals are received by each of the feed elements, 1, in the satellite receive antenna array and the signal received by each of the array elements is amplified individually by a low-noise amplifier, 3, perhaps after having out of band interference reduced by a bandpass filter 2.. The signal received by each element in the satellite antenna array is now treated as a channel, and hereby called the Element Channel.

The receive frequency bandwidth of each Element Channel is filtered with channel filter 4 and phased coherently converted with mixer 4 to a Feeder Link frequency band. In this example, the Element Channels of all the receive elements are Frequency Division Multiplexed into the Feeder Link frequency band using combiner 6, and switch 7. Other methods can be employed, including demodulation of the mobile user signal digitally multiplexed and remodulated onto the Feeder Link. The Feeder Link frequency band containing all of the Element Channels is converted to the transmit frequency with mixer and filters 8,9 and then amplified by a High-Power Amplifier assembly 10 and transmitted to a ground station on earth using at least a single polarization. A signal generation assembly 11 generates the appropriate local oscillator frequencies for frequency conversion operations.

Figure 1 illustrates a functional block diagram of the space segment describer herein.

Figure 2 illustrates a functional block diagram of the ground station segment describer herein. A ground station receives the Feeder Link band with an antenna assembly 13, amplifies the respective signals with low noise amplifiers 14 converts the respective signals to an intermediate band frequency with mixers 15 and multiplexes and filters the signals with respective multiplexers 16 and filters 17. The signals are then phase coherently converted with a mixer 19 and filter 20 to a common frequency band. These signals within the individual Element Channels correspond to the signals received by each of the satellite receive antenna array elements. These signals from the Element Channels feed into a ground- based beam-forming network 21. Signal source 22 produces the references signals used to produce local oscillators signals and other reference signals.

This beam-forming network is used on the ground to form the multiple satellite uplink beams or scanning uplink beams (Beam Channels) that cover specific regions on earth. This beam-forming function can be performed at any convenient frequency or at baseband. The beamformer can be optimized for specific requirements of the geographic region being served.

Forward Link Description Figure 3 illustrates a functional block diagram of the Forward Link ground segment described herein. The Forward Link (Ground Network to mobile user) works in a similar way to that discussed above for the ground segment receiver link. Groups of signals, destined for mobile users within a common geographical region, are assigned to a corresponding satellite downlink coverage beam. The signals arrive from various wired and wireless links through a gateway interface network 30 and ground station interface assembly 31. These groupings of signals, or Beam Channels, are passed through a beamformer assembly 32 on the ground. Like the Reverse Link, these downlink beams are adapted to provide the best coverage for the geographical region being served. Beam-forming can be achieved at any convenient frequency or at baseband (prior to modulation).

The outputs of the beamformer network are the Element Channels. These are phase coherently converted and multiplexed on to the Forward Feeder Link frequency band. This example uses a Frequency Division Multiplexing Assembly 33 for these Element Channels to multiplex onto the Feeder Link frequency band; other multiplexing and signal processing methods can be employed as well. This composite signal is then amplified by a High-Power Amplifier and forward link antenna network 34 in the earth station and transmitted to the satellite. A signal generator 35 is used to produce reference and local oscillator signals.

Figure 4 illustrates a functional block diagram of the Forward link space segment described herein. The satellite receives the Feeder Link signal through an antenna 40 and conveys the signal in two paths, as shown to respective power dividers 43. The power dividers divide the signals into N/ (2k) signals, which are in turn phase coherently downconverted through down converters 45 that convert the individual Element Channels to a common Forward Service Link frequency band. These individual Element Channels are then divided by k, amplified, filtered and radiated by the individual radiating elements in the satellite array transmit antenna, which is made up of"divided by"k"transmit feed"circuits 47. Multiple or scanning satellite downlink beams are formed by the spatial recombination of the Beam Channels as generated by the beamformer on the ground. A power optimization system 50 includes a processor that, depending on the mode of operation of the space segment, will adjust the power amplifier gain control, which affects power consumption, as well as a low noise amplifier gain control. A coherent oscillator assembly 53 provides local oscillator signals and reference signals for the space segment.

Coverage Versatility The multi-beam antenna system and method allows optimization for specific requirements of a geographic region. For example, regions that have a very high user-density may use a plurality of smaller beams to increase the amount of service link frequency reuse and the overall system capacity of the very high user-density regions. Small, high-density regions, such as a city or metropolitan area, can have a dedicated beam. Beams may be scanned to compensate for the motion of the satellite by tracking a specific geographic region. Tracking a specific geographic region minimizes beam-to-beam handoff and provides an optimum RF Link.

In regions where there is a low-density of users or users are spread over a large geographic region, fewer broad beams may be used to provide coverage. A less complex beam-forming network lowers ground station cost of servicing regions of low usage (i. e. low revenue) while maintaining user service.

With the beam-forming function performed on the ground and within the ground station and not in the satellite payload, beam-forming requirements can be re-optimized for different geographical regions. Coverage needs will change over the lifetime of the system; ground-based beam-forming allows coverage requirements to be changed with time without affecting the design of the satellite. In addition, interference rejection or interference mitigation functions can be incorporated into the uplink beamformer on the ground in real time.

Figure 6 is a block diagram showing a related ground-based segment 100 that communicates through an RF link 150 to the satellite 200. The system shown in Figure 6 is generally the same as what a composite diagram of Figures 3 and 4 would show, although omits of the"in-line"processing components included in Figures 3 and 4 for simplification purposes. As shown, a RF feeder link 150 interconnects the ground-based station (gateway) 100 to the satellite 200. The link from the satellite 200 to the ground station 100 is similar in operation, and thus not further described. The ground-based station includes a beamformer network (BFN) 160 that performs the function of the"Forward Link Beam Forming, Beam Loader, and Beam Scanner Network"of Figure 3. The BFN includes has a central processing unit (CPU) 162 that is connected by way of a bus to a controllable phase delay bank 164.

The phase delay bank 164 controllably adjusts respective phases of channel elements under processor control, according to a predetermined beamforming algorithm being executed by the CPU 162. ROM 166 holds the predetermined beamforming algorithm therein, in the form of computer readable instructions. Alternatively, an external control device 14 provides updated beamforming programs to either the CPU 162 or the RAM 167 for execution by the CPU 162. If the ROM 166 is implemented as field programable memory, the ROM 146 may also receive computer controlled program updates, or manual updates.

The I/O device 165 corresponds with the"Ground Station Interface Assembly"of Figure 3 and serves as the interface with the public switched telephone networks, proprietary terrestrial networks (such as cellular networks) and the external control unit 14. The I/O device also connects to the bus internal to the BFN 160.

N element channels are output to the"Frequency division multiplexing Assembly"of Figure 3, represented generally by the controllable oscillator 170, mixers, and combiners 192 and 182. The controllable oscillator 170 and respective mixers for the 96 different channel elements align the respective channel elements in two overlapping frequency bands, as will be discussed with respect to Figures 5A and SB, although maintained separate as left and right and circularly polarized composite signals 180,190. Polarization diversity, a feature of the present invention, is used to conserve the bandwidth allocated to the feeder link 150. For example, if the feeder link 150 is restricted to 160 MHZ, both the LHCP signal 180 and RHCP signal 190 can occupy the 160 MHZ band without interfering with one another.

The respective left and right hand circular polarized signals (180,190) are provided to an orthogonal mode transducer 199 that combines the different channels before transmitting the same by a transmit antenna (phased array or reflector type, for example, 198). The transmitted energy is then received at the satellite by an antenna 201 and then separated by another orthogonal mode transducer 203. The aggregation of left hand element channels 1-48 are passed to a 1: 48 power divider 205, although distributed power dividers may be used as well as shown in Figure 4. Respective outputs are then down converted with respective local oscillator signals and amplified by a solid state power amplifier 210 before being fed to one of N antenna elements 212. In the present embodiment, the number of antenna elements, and corresponding element channels is 96, although other numbers of element channels and phased array elements may be used as well.

A characteristic feature of the configuration shown in Figure 6 is that all the beam forming processing is done at the ground station 100, and may be reconfigured by using the CPU 160 to control the relative phase delays implemented in the controllable phase delay circuit 164. Additionally, when a lesser number of beams is desired, the CPU 160 is able to adaptively reconfigure the system from the ground, either by implementing a new program in the CPU 162, or reconfiguring the program by way of the external control device 14.

One obstacle of moving the beamforming operations to the gateway, is if the phased array has a large number of elements, the respective element channels, when aggregated, place fairly high bandwidth demands on the feeder link 150. Moreover, suppose each of the beams formed by the satellite's phased array antenna transmits all MHZ signal, then each of the 96 antenna elements will expect to be fed by an 11 MHZ signal. This issue is further explored below in the discussion regarding Figures 5A-5G.

Suppose the feeder link 150 has a bandwidth of 160 MHZ, as shown in Figure 5G.

The 24 beams, each having a signal bandwidth of 11 MHZ, could fit within the 160 MHZ allocation, assuming one-half of the signals from the beams are allocated to the LHCP channel and the other half are allocated to the RHCP channel. However, according to the present invention, because the BFN 160 is provided on the ground station, separate signal paths are needed for the respective antenna elements (96 in the present example). Various forms of signal compression are employed to package all of the information and energy contained in the 96 element channels into the 160 MHZ bandwidth allocated to the feeder link. Figures 5C and 5D represent these cases where the relative bandwidths occupied by each channel element is approximately 25% of the channels shown in Figures 5A and 5B.

The respective spacings in Figures 5A-5G should not be construed as having been drawn to scale.

In one embodiment, the BFN 160 includes a digitizer that digitizes the respective channel elements. The data is then compressed in bandwidth by various M-ary modulation schemes or other multi-bit per channel symbol modulation schemes, such as 64 QAM. An inverse operation is performed at the satellite 200 to demodulate the signals. Advanced modulation techniques are a possibility because the communications channel is bandwidth limited, not power limited. Other modulation schemes may be used as well, such as partially overlapping the main lobes of separate FDM signals, and suppressing the self-interference at the satellite imposed by the overlapping modulation. Once again the present description is focusing on the forward link, although the discussion is also applicable to the reverse link.

Aspects of one type of overlapping modulation technique and interference suppression technique using a multi-channel correlation receiver is found in U. S. patent no. 4,293,945, issued to Atia, A., et al., the entire contents of which is incorporated herein by reference.

Once digitized, the redundancy in the respective element channels may further be reduced using recognized data compression techniques. While the advanced modulation techniques, overlapping and self-interference suppression techniques and data compression techniques have been discussed separately, combinations of each of the options, as well as the alternative embodiments discussed below, are features of the present invention.

Another alternative to reducing the overload on the feeder link bandwidth restriction is to reduce the number of elements in the satellite's phased array antenna system, thereby reducing the number of element channels. Figures 5E and 5F, show this later alternative, where only 48 element channels are supported.

Figures 7 and 8 describe alternatives that further easy the bandwidth restriction of the feeder link 150 by distributing selected power divider functions between the gateway 100 and satellite 200. In both cases, an initial stage of beamforming is performed at the gateway 100, but four (for example) of the element channels are combined into respective aggregate channels, where the bandwidth of the of the aggregate channel is about 25% of the sum of four separate element channels. In the case of Figure 7, each of the aggregate channels is passed to the satellite 200 and then down converted with the appropriate mixer, as shown. A subsequent 1: 4 power division and phase weighting operation is performed in the signal processor 205. The four outputs from each signal processor 205 is then amplified and fed to an element 212 of the phased array antenna. Figure 8 is similar to Figure 7, except the aggregate channels are amplified with the solid state amplifier before being input to the signal processor 219.

An attribute of the present system is that linear components are used in the signal processing chain such that the linearity of the respective signal components is preserved, thus enabling the beamforming operations to be conveniently distributed between the satellite and ground station.

With regard to the computer-based mechanisms and processes discussed in the present description, these mechanisms and processes may be conveniently implemented using a conventional general purpose microprocessor programmed according to the teachings in the present specification, as will be appreciated to those skilled in the relevant arts. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will also be apparent to those skilled in the relevant arts.

An attribute and embodiment of the present invention thus also includes a computer- program product that may be hosted on a storage medium and include instructions that can be used to program a computer to perform a process in accordance with the present invention.

This storage medium can include, but is not limited to, any type of disk including floppy disks, optical disks, CD-ROMs, magneto-optical disks, ROMs, RAMs, EPROMs, EEPROMs, flash memory, magnetic or optical cards, or any type of media suitable for storing electronic instructions.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.