Abstract:
A broad-band digital satellite communications system for providing broad-band data services. The system comprises a first spacecraft, generally a geo-stationary earth orbit communications device, and at least one controller having broadband communications capability with the first spacecraft. The system also includes at least one second spacecraft, generally a low earth orbit (LEO) communications device. The second spacecraft comprises ; communications capability with the at least one first spacecraft; low data rate communications capability with a land based system, generally a mobile communications service provider; and broadband communications capability with a mobile user subscriber to the mobile communications service provider.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to satellite communications systems and, more particularly, to providing asymmetric data services.  
           [0003]    2. Prior Art  
           [0004]    A number of user applications continue to drive the requirement for high speed data services. Some industry specific examples include remote film editing, medical image transport, financial service data consolidation and backup and Internet communications. As business, government and educational institutions disseminate more information, greater importance is attached to data transfer. In this environment, reliable, high speed data services becomes even more critical. In addition, growth in Internet traffic has caused a strain on the capacity of telephony networks. Network shortcomings include network outages, insufficient access bandwidth, insufficient inter-node bandwidth, and poor spectral efficiency. Providers are required to make significant investments, as well as experience installation delays, to upgrade network infrastructure.  
           [0005]    Corporate LANs/WANs also generate an demand for higher bandwidth. The demand for bandwidth goes up as more and more users are connected. The users, in turn, demand more services and improved network speed. Personal computers are being used to process not only text, but graphics and video as well, all on networks that are increasingly global. High speed networking is also driven by the growth of video distribution, client/server technology, decentralized systems, increased processing power and developments in storage capacity.  
           [0006]    While existing satellite systems offer global service, they do not offer direct connection to the end user at moderate to high data rates. Many of the existing fixed satellite service systems employ wide channel bandwidths and relatively large beam-widths making them more suited to point-to-point trunking service rather than to end user connectivity. The wide area coverage, and constrained flexibility of these systems renders these systems both inefficient and costly to serve many small or isolated users.  
           [0007]    For example U.S. Pat. No. 5,906,337 addresses continuous unbroken links between a geo-stationary and a medium earth orbit satellite with a relationship between orbits that provides continuous contacts. However, the reference does not disclose or suggest exploiting asymmetry data rates for efficient communications.  
           [0008]    U.S. Pat. No. 5,448,623 addresses a single LEO constellation without a coupling to a GEO satellite system and without reference to the asymmetric data flow or directing traffic flow from the ground based upon the position information given by the network control from the satellite data.  
           [0009]    U.S. Pat. No. 5,887,257 is directed to provisions of control signals for channel assignment between various satellites. There is no disclosure or suggestion pertaining to provision of control signals from ground through a first (GEO) to second (LEO) for the purpose of directing a beam (channel) based on a priori position knowledge gained from user of system. Nor does the reference disclose or suggest inter-satellite links between satellites in the two different constellations.  
           [0010]    U.S. Pat. No. 5,890,679 relates only to medium earth orbit satellite constellations of specific configurations. There is no disclosure or suggest pertaining to the provision of broadband data via a geo-stationary earth orbit satellite-to-low earth orbit satellite path that is under ground network operations control.  
           [0011]    U.S. Pat. No. 5,896,558 is directed to all bi-directional links LEO-to-GEO/GEO-to-LEO that involve onboard processor traffic switching. There is no disclosure or suggestion pertaining to asymmetric broadband data provision via one-way link from GEO-to-LEO for broadband data.  
           [0012]    U.S. Pat. No. 5,907,541 addresses a cellular extension satellite systems but not broadband data system and not using GEO+LEO (but only GEO+GEO). No exploitation of data traffic asymmetry is disclosed or suggested.  
           [0013]    U.S. Pat. No. 5,930,254 relates to fast packet switching onboard a satellite without efficiently addressing channel assignments with ground-based data processing and controls distributed to all LEO satellites in view of a GEO.  
           [0014]    U.S. Pat. No. 5,987,233 addresses data caching in a global satellite system but does not disclose or suggest the efficient exploitation of asymmetric data traffic, GEO+LEO, ground based channel assignments with spatial division multiplexing, or anything beyond simply broadcasting and caching of data services.  
           [0015]    U.S. Pat. No. 5,722,042 is directed towards signals that go through high altitude satellites for large terminals and low altitude satellites for small terminals so it is a combination of LEO or GEO but not both, thereby failing to efficiently exploit asymmetric data rates.  
           [0016]    U.S. Pat. No. 4,985,706 is for the simultaneously up-linked signals on a common frequency that do not interfere because pseudo random spreading is used on one up-link to de-couple the power level from the second up-link. There is no suggestion of providing efficient data services via satellite communications.  
           [0017]    U.S. Pat. No. 6,011,951 regards the sharing of frequencies using directional antennas to various satellite constellations. The reference does not address the efficient asymmetric GEO+LEO broadband data service provision through gateway channel SDMA assignment of variously comprised TDMA and CDMA traffic and control signals.  
           [0018]    U.S. Pat. No. 6,002,916 relates to server architecture and does not disclose or suggest an efficient communications system by exploiting asymmetric data traffic with an asymmetric broadband GEO+LEO system. In addition, the referenced systems require onboard processing.  
           [0019]    U.S. Pat. No. 5,995,497 relates to switching of CDMA traffic channels (beams) but does not disclose or suggest efficient communications to ground based stations.  
           [0020]    U.S. Pat. No. 6,016,124 is directed towards base-band digital beam-forming but does not disclose or suggest efficient communications to ground based stations.  
           [0021]    U.S. Pat. No. 5,790,070 regards the steering of beams and methodology to do the same using the satellite as the network node. But the reference does not disclose or suggest an efficient communications system by exploiting asymmetric data traffic with an asymmetric broadband GEO+LEO system.  
           [0022]    The emerging cellular type satellite services serve a very large number of potential subscribers but only at very low data rates. The on-board processing and packet-switched nature of their signal structure severely limits the practical user data rates that can be accommodated within the technology limitations of the processor. Thus, there exists a need for a satellite communications system that serves the demand for high data rate users.  
           [0023]    Moreover, the frequency transmission spectrum is finite resulting in the requirement that the spectrum be used in the most efficient manner. Thus, there exists a need for a satellite communications system that serves the demand for high data rate users and efficiently utilizes the frequency transmission spectrum.  
         SUMMARY OF THE INVENTION  
         [0024]    A broad-band digital satellite communications system for providing data services is provided. The system comprises a first spacecraft, a controller having broadband communications capability with the first spacecraft, and a second spacecraft. The second spacecraft, having a lower orbit than the first spacecraft, comprises communications capability with the at least one first spacecraft, low data rate communications capability with at least one first land based system, and broadband communications capability with at least one second land based system.  
           [0025]    A method for providing asymmetric broadband data services to mobile users is also provided. The method comprises the steps of receiving broadband data at a ground station; transmitting the broadband data on an up-link channel (or channels) from the ground station to a geo-stationary earth orbit satellite. The broadband data received on the up-link channel or channels is combined to form a replica of the broadband data received at the ground station. The replicated broadband data is then retransmitted to a satellite constellation comprised of at least one low earth orbit (LEO) satellite. Each LEO satellite within the constellation has at least one controllable spot beam and each controllable spot beam has at least one down-link communications channel. The method determines if the replicated broadband data exceeds available down-link communications channel capacity associated with the available down-link communication channel(s); and increases the number of available down-link communication channels accordingly if the replicated broadband data exceeds available down-link communication channel capacity. The next method step parses the replicated broadband data on to the number of available down-link communication channels and transmits the replicated broadband data on the number of available down-link communication channels to at least one mobile user.  
           [0026]    A broad-band digital satellite communications system for providing asymmetric broadband data services to mobile users is also provided. The system comprises a geo-synchronous earth orbit (GEO) satellite; a data traffic gateway (DTG) having broadband communications capability with the GEO satellite; a network controller connectable to the at least one DTG; and at least one low earth orbit (LEO) satellite. The LEO satellite comprises communications capability with DTG; communications capability with the at least one GEO satellite; low data rate communications capability with at least one land based system; and broadband communications capability with a mobile user.  
           [0027]    A method for maximizing spectral efficiency in a satellite communications system having a first satellite constellation disposed at a orbit higher than a second satellite constellation is also provided. The first satellite constellation having communications capability with a network controller and the second satellite constellation. The method comprises the steps of allocating the total up-link resources available from the network controller to up-link data to the first satellite constellation. The up-linked data is broadcasted substantially simultaneously from the first satellite constellation to the second satellite constellation. Spot communication beams associated with the second satellite constellation transmit a predetermined fraction of the up-linked data to mobile users.  
           [0028]    A communications system for providing internet data services between a user and an internet is also provided. The system comprises a satellite constellation and a ground station having communications capability with the satellite constellation and the user. The system also comprises a second ground station having communications capability with the satellite constellation and the internet.  
           [0029]    A method for providing internet data services between at least one user and an internet is also provided. The method comprises the steps of transmitting data from the at least one user to a satellite constellation and assigning the data to at least one communications channel within the satellite constellation. The method also comprises the steps of steering at least one satellite spot communication beam associated with the communication channel(s) to illuminate the geographical position of the internet and parallel transmitting data to the internet. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein:  
         [0031]    [0031]FIG. 1 is a pictorial schematic of data flow from a land-based system to a mobile user incorporating features of the present invention;  
         [0032]    [0032]FIG. 2 is a pictorial schematic of data flow from a land-based system to a mobile user incorporating overlapping beam features of the present invention shown in FIG. 1; and  
         [0033]    [0033]FIG. 3 is a method flow chart for using the systems shown in FIGS.  1  or  2  to transmit data from a ground station to a mobile user;  
         [0034]    [0034]FIG. 4 is an alternate method flow chart for using the systems shown in FIGS.  1  or  2  to transmit data from a ground station to a mobile user;  
         [0035]    [0035]FIG. 5 is a pictorial diagram of a communications system incorporating features of the present invention; and  
         [0036]    [0036]FIG. 6 is a method flow chart for using the system shown in FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0037]    Referring to FIG. 1, there is shown a pictorial representation of a system  10  for providing data services to mobile users and incorporating features of the present invention. Advantageously this system exploits the natural asymmetry between low rate requests for data and the high rate data requested. First, through the use of the user return links that issue data requests, such as a session on the internet where large downloads are requested, and then through the application of user position information already present within the system via low rate communications links  9  between the user  16  and the LEO satellite  12 . Furthermore, the gateway operations and control center (GOCC)  13  controls low data rate, such as cellular service operations, and provides data services such as broadband data services. User position information is derived from the low data rate service to make appropriate data channel and beam assignments and to initialize and update the beam pointing algorithms that control the phased-array antennas  3  onboard the LEO satellites  12 . The Data Traffic Gateway (DTG)  14  is provided to centralize the interface to the service provider backbone and provide up-link communications with the geo-stationary earth orbit (GEO) satellite. Addition of the DTG  14  links it to the GOCC and the service provider gateways. It is appreciated that providing appropriate LEO satellites  12  data channel and beam assignments from the GOCC  13  to the DTG  14  to the GEO  11  satellite and then to the LEO satellite  12  constellation obviates the requirement for line-of-sight LEO satellite control centers.  
         [0038]    Referring to FIG. 3 there is shown a method flow chart for using the system shown in FIGS.  1  or  2  to transmit data from a ground station to a mobile user. Data is transmitted from a geo-stationary satellite (GEO) (FIG. 1, item  11 ) and received  47  on a low earth orbit (LEO) satellite constellation (FIG. 1, items  12 ), where a satellite constellation may be one or more LEO satellites. A geographical position of a user requesting the data is determined  49  and the data is transmitted  50  from the LEO (FIG. 1, items  12 ) constellation to the user (FIG. 1, item  16 ).  
         [0039]    Referring now to FIG. 1, digital signals are transmitted from the DTG  14  to a geo-stationary (GEO)  11  satellite within multiple Fixed Satellite space-to-Earth frequency bandwidth allocations on multi-band up-link channels  5 . These signals are combined onto a signal that is transmitted from the GEO  11  satellite to a satellite constellation of low earth orbit (LEO) satellites  12 . A subset of the satellites  12  in the LEO constellation are simultaneously illuminated by the GEO satellite  11 . Each LEO satellite  12  transmits multiple, independently directed spot beams  18 , each providing a predetermined fraction of the digital signal traffic received from the GEO-LEO inter-satellite link  19 . The fraction of the digital signal may be determined by channel allocation requirements, spot beam overlap, and/or channel transmission environment. Each LEO phased-array antenna  3  independently points separate beams  18  to user terminals  16  tracked by the position information available from user voice/data low rate links  9  between the service provider gateway  15  and the LEO satellite  12 . Broadband data requests are processed through the interactive voice/data low rate links  9  that comprise a LEO constellation based cellular telephone system. Broadband data are transmitted to users from the data traffic gateway  14  through the GEO spacecraft  11  and then to the user terminal  15  through one or more of the LEO satellites  12  within view of the user terminal  16 . Thus, the system maintains a low data rate return link from each user through the conventional cellular telephone extension function and distributes high rate data as might be requested from an internet service provider through the spot beams. LEO satellite traffic and beam assignments and their associated tracking trajectories and traffic hand-offs are managed through the central Gateway Operations and Control Center (GOCC)  13  and the local service provider gateways  15 . The digital up-links to the GEO satellite are also managed from the GOCC  13 . Multiple GEO satellites and data traffic gateways are used to achieve global broadband coverage.  
         [0040]    Referring now to FIG. 4 there is shown a method flow chart for using the systems shown in FIGS.  1  or  2  to transmit data from a ground station to a mobile user. FIGS. 1 and 4 and the following numerical example illustrate features of the present invention. Select for this example frequency division multiplexed (FDM) signals with a center band spacing of 57.14 MHz, guard bands between channels of 7.14 MHz and no guard bands at the edges of the up-link and down-link bands. Identify 4.8 GHz of up-link bandwidth for the Earth-to-GEO data link that uses dual-polarization techniques to transmit 84 channels, nominally 50 MHz each, within the 27.5-to-29.9 GHz Region 2 FCC allocation for Earth-to-space communications. Data is received  31  at the data traffic gateway  14  and evaluated  32  for required up-link channel capacity. If available channel capacity is exceeded the data is parsed  34  on to multiple up-link channels before the date is substantially parallel up-linked  33  to the GEO satellite  11 . Receivers on board the GEO satellite  11  translate  35  the FDM signals into the 59-64 GHz band allocated for inter-satellite communications. All 84 channels are substantially simultaneously broadcasted  36  on a single polarization into an earth coverage beam that includes, for example, the 1414 km altitude of the GLOBALSTAR satellites. Receivers on each of the LEO satellites  12  translate the FDM channels from the inter-satellite link into the 400 MHz band of 19.7-to-20.1 GHz that is allocated for mobile satellite space-to-earth communications. The next step determines  38  if the data exceeds available down-link channel capacity. This step may be predetermined at the GOCC  13  or the GEO  11  from known channel capacity and data down-link requirements. In addition, the channel down-link capacity may be dynamically determined  38  by one or more of the LEO satellites. If the channel capacity is exceeded more channels are added  40 . For example, there are at least,  12  beams on a GLOBALSTAR LEO satellite that can be independently formed within the active phased-array antenna. With general regard to communications beam forming reference can be had to “Digital Beamforming in Wireless Communications”, by John Litva and Titus KwokYeung Lo, ISBN 0-89006-712-0, the disclosure of which is incorporated by reference in its entirety.  
         [0041]    Each beam may contain up to 7 channels from the inter-satellite link. Each beam is independently pointed to a user within the spot beam coverage. In addition, since a LEO satellite within the LEO constellation has a view of +/−54 degrees to all earth terminals with at least a 10 degree elevation angle view of the satellite, many more than 12 spot beams with coverage areas of about 2 degrees may be simultaneously pointed to deliver high rate data to multiple locations without generating significant interference between beams. Thus, if the total channel capacity of an individual satellite within the LEO constellation is exceeded then channels from a satellite with overlapping beam coverage are allocated to carry a fraction of the data signal (FIG. 2, item  18 ). The next step  42  steers the beams associated with the allocated channels to illuminate the user. The last step spreads  37  the data onto the allocated channels to be transmitted  39  to the user.  
         [0042]    It is readily appreciated from this example that space division multiplexing (SDM) using a multi-beam phased-array antenna can provide many times frequency reuse. In this manner, all allocated up-link and down-link bandwidth with high data rate signals through the translation of the multi-band FDM traffic onto a single optical carrier signal for the GEO-to-LEO inter-satellite link are advantageously utilized. Multi-spot-beam phased-array antennas customized for each allocated down-link band may then be used to fold the many up-link channels into the many down-link beams, by utilizing a beams set for each of the various down-link bands.  
         [0043]    It is readily appreciated that the efficiency of spectral resource allocation comes from the mapping of subsets or fractions of the up-link spectrum onto spot beams; effectively using some or all of a particular bandwidth allocated for space-to-Earth communications.  
         [0044]    Referring now to FIG. 5 there is shown a pictorial diagram of another communications system incorporating features of the present invention. Referring also to FIG. 6 there is shown a method flow chart for using the system shown in FIG. 5. Terminal  51  transmit data  62  to a satellite constellation  55  through modem  52  and ground station  53 . The data is received  63  on the satellite constellation  55 , where the satellite constellation may comprise one or more satellites. The geographical position of an internet  59  is determined  64  and the data is transmitted  65  to the internet through ground station  54 , modem bank  56 , and routers  58 .  
         [0045]    It is readily appreciated that features of the present invention allow rurally located users, or other users where internet connection is not economically feasible, to have access to internet services. In addition, it is also readily appreciated that the internet access is not limited to narrow-band data services but includes broadband services as well.  
         [0046]    Lastly, it should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.