Abstract:
A method and apparatus for identifying which one of a plurality of mobile terminals in communication with a ground-based base station, via a transponded satellite, is causing interference with a non-target satellite orbiting in a vicinity of the transponded satellite. The method involves using the base station to sequentially check each of the mobile terminals to identify which one is causing the interference. The check is made by the base station commanding each mobile terminal to modulate the power level of its transmitted signals and then checking with an operator of the interfered with non-target satellite to see if the interference condition has changed. Once the mobile terminal causing the interference condition is identified, the base station can command the mobile terminal to reduce its transmit power accordingly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from provisional application Ser. No. 60/281,357, filed Apr. 4, 2001. 

   FIELD OF THE INVENTION 
   The present invention relates to mobile RF terminals required to conduct bidirectional communications with a base station via a satellite link, and more particularly to a method and apparatus for identifying which one of a plurality of mobile terminals is causing interference with one or more satellites adjacent a target satellite. 
   BACKGROUND OF THE INVENTION 
   With mobile RF terminals located on mobile platforms such as aircraft, cruise ships and other moving platforms, communicating with a ground station via a transponded satellite, there is always the remote possibility, in spite of the safeguards that may be built into the mobile terminal, that the terminal may fail in an unanticipated manner. In such event, there is the possibility that the mobile terminal may cause interference with other satellites orbiting in the geo arc adjacent to the target satellite with which the mobile terminal is communicating. 
   It is also recognized that Fixed Services Satellite (FSS) operators may have difficulty in locating interference from VSAT (Very Small Aperture Terminal) systems that consist of thousands of unsophisticated terminals at remote sites. 
   Therefore, there exists a need for the ground station in communication with a plurality of mobile terminals via a transponded satellite to be able to quickly identify a malfunctioning mobile terminal which is causing interference with non-target satellites and to quickly resolve the interference incident. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a system and method for identifying an interfering mobile terminal from one of a plurality of mobile terminals by analyzing the modulated power level of the signals transmitted by the mobile terminals and determining when the modulated power level of a given mobile terminal varies from an expected modulation level. By detecting this variation, the interfering mobile terminal can be quickly identified and commanded to stop transmitting. 
   The present invention makes use of a base station, preferably a ground station, having a network operations center (NOC) for sending command signals to each mobile terminal. These command signals are relayed to the mobile terminals by a transponded satellite with which each mobile terminal is in communication with. The NOC commands each mobile terminal to vary the power of the signals being transmitted from a transmit antenna located on its associated mobile platform. The mobile terminals could be carried by aircraft, cruise ships or other moving vehicles, but for the purpose of this discussion reference will be made to the mobile platforms as aircraft. 
   The NOC first receives a message from a Fixed Services Satellite (FSS) operator that is experiencing interference from one of the mobile terminals, although the FSS operator will not be able to identify which mobile terminal is causing the interference. The NOC then begins commanding each aircraft, sequentially, to make data rate or transmit power changes (i.e., to modulate) to the signals being transmitted by a transmit antenna of its mobile terminal. The NOC analyzes the received signals and determines if the modulated signals meet the expected power levels, which would indicate that no interference will be caused. If the received signals meet the expected power levels, then the NOC determines that that particular mobile terminal is not the cause of the interference and commands the next mobile platform accessing the transponded satellite to begin transmitting signals in accordance with the commanded data rate modulation scheme. The NOC proceeds to perform the same analysis as described for the first aircraft, and repeats the entire process for each aircraft, one by one, until it determines which mobile terminal is causing the interference. During this process the NOC may also communicate with an operator of the non-target satellite experiencing interference. That operator could inform the NOC as a corresponding increase in the power level is detected at the interfered with satellite. In this manner the NOC can be apprised as soon as the interfering mobile terminal causes the increased level of power to be seen by an interfered with terminal. 
   The above-described method can be used to check a single mobile terminal for interference within a time span of about 5–10 seconds. A transponded satellite accommodating 20–30 aircraft can be checked typically in less than 5 minutes. Once the interfering mobile terminal is identified, it can be commanded by the NOC to shut down or to reduce its data transmission rate, thus effectively reducing the power level of its transmitted signals. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, in which: 
       FIG. 1  is an exemplary system for enabling communications between a ground-based component and a plurality of mobile platforms; 
       FIG. 2  is a simplified block diagram of a mobile terminal located on each mobile platform; and 
       FIG. 3  is a graph showing the signals transmitted from a mobile platform modulated in accordance with an exemplary modulation scheme commanded by a ground-based component, from which the ground-based component is able to determine if the signals are causing interference with an FSS satellite adjacent a target satellite with which the mobile platform is communicating. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , there is shown a system  10  for implementing the method of the present invention. The system  10  provides data content to and from a plurality of mobile platforms  12   a – 12   f  in one or more distinct coverage regions  14   a  and  14   b.  The system  10  generally comprises a ground segment  16 , a plurality of satellites  18   a – 18   f  forming a space segment  17 , and a mobile terminal  20  disposed on each mobile platform  12 . The mobile platforms  12  could comprise aircraft, cruise ships or any other moving vehicle. Thus, the illustration of the mobile platforms  12  as aircraft in the figures herein, and the reference to the mobile platforms as aircraft throughout the following description should not be construed as limiting the applicability of the system  10  to only aircraft. 
   The space segment  17  may include any number of satellites  18  in each coverage region  14   a  and  14   b  needed to provide coverage for each region. Satellites  18   a ,  18   b ,  18   d  and  18   e  are preferably Ku or Ka-band satellites. Satellites  18   c  and  18   f  are Broadcast Satellite Services (BSS) satellites. Each of the satellites  18  are further located in a geostationary orbit (GSO) or a non-geostationary orbit (NGSO). Examples of possible NGSO orbits that could be used with this invention include low Earth orbit (LEO), medium Earth orbit (MEO) and highly elliptical orbit (HEO). Each of the satellites  18  includes at least one radio frequency (RF) transponder, and more preferably a plurality of RF transponders. For example satellite  18   a  is illustrated having four transponders  18   a   1 – 18   a   4 . It will be appreciated that each other satellite  18  illustrated could have a greater or lesser plurality of RF transponders as required to handle the anticipated number of aircraft  12  operating in the coverage area. The transponders provide “bent-pipe” communications between the aircraft  12  and the ground segment  16 . The frequency bands used for these communication links could comprise any radio frequency band from approximately 10 MHz to 100 GHz. The transponders preferably comprise Ku-band transponders in the frequency band designated by the Federal Communications Commission (FCC) and the International Telecommunications Union (ITU) for fixed satellite services FSS or BSS satellites. Also, different types of transponders may be employed (i.e., each satellite  18  need not include a plurality of identical types of transponders) and each transponder may operate at a different frequency. Each of the transponders  18   a   1 – 18   a   4  further include wide geographic coverage, high effective isotropic radiated power (EIRP) and high gain/noise temperature (G/T). 
   With further reference to  FIG. 1 , the ground segment  16  includes a ground station  22  in bidirectional communication with a content center  24  and a network operations center (NOC)  26 . A second ground station  22   a  located in the second coverage area  14   b  may be used if more than one distinct coverage area is required for the service. In this instance, ground station  22   a  would also be in bidirectional communication with the NOC  26  via a terrestrial ground link or any other suitable means for establishing a communication link with the NOC  26 . The ground station  22   a  would also be in bidirectional communication with a content center  24   a.  For the purpose of discussion, the system  10  will be described with respect to the operations occurring in coverage region  14   a.  However, it will be understood that identical operations relative to the satellites  18   d – 18   f  occur in coverage region  14   b.  It will also be understood that the system  10  may be scaled to any number of coverage regions  14  in the manner just described. 
   The ground station  22  comprises an antenna and associated antenna control electronics needed for transmitting data content to the satellites  18   a  and  18   b.  The antenna of the ground station  22  may also be used to receive data content transponded by the transponders  18   a   1 – 18   a   4  originating from each mobile terminal  20  of each aircraft  12  within the coverage region  14   a.  The ground station  22  may be located anywhere within the coverage region  14   a.  Similarly, ground station  22   a,  if incorporated, can be located anywhere within the second coverage area  14   b.    
   The content center  24  is in communication with a variety of external data content providers and controls the transmission of video and data information received by it to the ground station  22 . Preferably, the content center  24  is in contact with an Internet service provider (ISP)  30 , a video content source  32  and a public switched telephone network (PSTN)  34 . Optionally, the content center  24  can also communicate with one or more virtual private networks (VPNs)  36 . The ISP  30  provides Internet access to each of the occupants of each aircraft  12 . The video content source  32  provides live television programming, for example, Cable News Network® (CNN) and ESPN®. The NOC  26  performs traditional network management, user authentication, accounting, customer service and billing tasks. The content center  24   a  associated with the ground station  22   a  in the second coverage region  14   b  would also preferably be in communication with an ISP  38 , a video content provider  40 , a PSTN  42 , and optionally a VPN  44 . An optional air telephone system  28  may also be included as an alternative to the satellite return link. 
   Referring now to  FIG. 2 , the mobile terminal  20  disposed on each aircraft  12  will be described in greater detail. Each mobile terminal  20  includes a data content management system in the form of a router/server  50  (hereinafter “server”) which is in communication with a communications subsystem  52 , a control unit and display system  54 , and a distribution system in the form of a local area network (LAN)  56 . Optionally, the server  50  can also be configured for operation in connection with a National Air Telephone System (NATS)  58 , a crew information services system  60  and/or an in-flight entertainment system (IFE)  62 . 
   The communications subsystem  52  includes a transmitter subsystem  64  and a receiver subsystem  66 . The transmitter subsystem  64  includes an encoder  68 , a modulator  70  and an Up-converter  72  for encoding, modulating and up-converting data content signals from the server  50  to a transmit antenna  74 . The receiver subsystem  66  includes a decoder  76 , a demodulator  78  and a down-converter  80  for decoding, demodulating and down-converting signals received by the receive antenna  82  into base band video and audio signals, as well as data signals. While only one receiver subsystem  66  is shown, it will be appreciated that preferably a plurality of receiver subsystems  66  will typically be included to enable simultaneous reception of RF signals from a plurality of RF transponders. If a plurality of receiver subsystems  66  are shown, then a corresponding plurality of components  76 – 80  will also be required. 
   The signals received by the receiver subsystem  66  are then input to the server  50 . A system controller  84  is used to control all subsystems of the mobile system  20 . The system controller  84 , in particular, provides signals to an antenna controller  86  which is used to electronically steer the receive antenna  82  to maintain the receive antenna pointed at a particular one of the satellites  18 , which will hereinafter be referred to as the “target” satellite. The transmit antenna  74  is slaved to the receive antenna  82  such that it also tracks the target satellite  18 . It will be appreciated that some types of mobile antennas may transmit and receive from the same aperture. In this case the transmit antenna  74  and the receive antenna  82  are combined into a single antenna. 
   With further reference to  FIG. 2 , the local area network (LAN)  56  is used to interface the server  50  to a plurality of access stations  88  associated with each seat location on board the aircraft  12   a.  Each access station  88  can be used to interface the server  50  directly with a user&#39;s laptop computer, personal digital assistant (PDA) or other personal computing device of the user. The access stations  88  could also each comprise a seat back mounted computer/display. The LAN  56  enables bidirectional communication of data between the user&#39;s computing device and the server  50  such that each user is able to request a desired channel of television programming, access a desired website, access his/her email, or perform a wide variety of other tasks independently of the other users on board the aircraft  12 . 
   The receive and transmit antennas  82  and  74 , respectively, may comprise any form of steerable antenna. In one preferred form, these antennas comprise electronically scanned, phased array antennas. Phased array antennas are especially well suited for aviation applications where aerodynamic drag is important considerations. One particular form of electronically scanned, phased array antenna suitable for use with the present invention is disclosed in U.S. Pat. No. 5,886,671, assigned to The Boeing Co. 
   Referring further to  FIG. 1 , in operation of the system  10 , the data content is preferably formatted into Internet protocol (IP) packets before being transmitted by either the ground station  22 , or from the transmit antenna  74  of each mobile terminal  20 . For the purpose of discussion, a transmission of data content in the form of IP packets from the ground station  22  will be referred to as a “forward link” transmission. IP packet multiplexing is also preferably employed such that data content can be provided simultaneously to each of the aircraft  12  operating within the coverage region  14   a  using unicast, multicast and broadcast transmissions. 
   The IP data content packets received by each of the transponders  18   a   1 – 18   a   4  are then transponded by the transponders to each aircraft  12  operating within the coverage region  14   a.  While multiple satellites  18  are illustrated over coverage region  14   a,  it will be appreciated that at the present time, a single satellite is capable of providing coverage to an area encompassing the entire continental United States. Thus, depending upon the geographic size of the coverage region and the mobile platform traffic anticipated within the region, it is possible that only a single satellite incorporating a single transponder may be needed to provide coverage for the entire region. Other distinct coverage regions besides the continental United States include Europe, South/Central America, East Asia, Middle East, North Atlantic, etc. It is anticipated that in service regions larger than the continental United States, that a plurality of satellites  18  each incorporating one or more transponders may be required to provide complete coverage of the region. 
   The receive antenna  82  and transmit antenna  74  are each preferably disposed on the top of the fuselage of their associated aircraft  12 . The receive antenna  74  of each aircraft receives the entire RF transmission of encoded RF signals representing the IP data content packets from at least one of the transponders  18   a   1 – 18   a   4 . The receive antenna  82  receives horizontally polarized (HP) and vertically polarized (VP) signals which are input to at least one of the receivers  66 . If more than one receiver  66  is incorporated, then one will be designated for use with a particular transponder  18   a   1 – 18   a   4  carried by the target satellite  18  to which it is pointed. The receiver  66  decodes, demodulates and down-converts the encoded RF signals to produce video and audio signals, as well as data signals, that are input to the server  50 . The server  50  operates to filter off and discard any data content not intended for users on the aircraft  12  and then forwards the remaining data content via the LAN  56  to the appropriate access stations  88 . In this manner, each user receives only that portion of the programming or other information previously requested by the user. Accordingly, each user is free to request and receive desired channels of programming, access email, access the Internet and perform other data transfer operations independently of all other users on the aircraft  12   a.    
   Referring further to  FIG. 1 , a transmission of data content from the aircraft  12   a  to the ground station  22  will be described. This transmission is termed a “return link” transmission. The antenna controller  86  causes the transmit antenna  74  to maintain the antenna beam thereof pointed at the target satellite  18   a . The channels used for communication from each mobile terminal  20  back to the ground station  22  represent point-to-point links that are individually assigned and dynamically managed by the NOC  26  of the ground segment  16 . For the system  10  to accommodate several hundred or more aircraft  12 , multiple aircraft will need to be assigned to each transponder carried by a given satellite  18 . The preferred multiple access methods for the return link are code division multiple access (CDMA), frequency divisional multiple access (FDMA), time division multiple access (TDMA) or combinations thereof. Thus, multiple mobile terminals  20  may be assigned to a single transponder  18   a   1 – 18   a   4 . Where a greater number of aircraft  12  incorporating a mobile terminal  20  are operated within the coverage region  14   a , then the number of transponders required increases accordingly. 
   The receive antenna  82  may implement a closed-loop tracking system for pointing the antenna beam and for adjusting the polarization of the antennas based on receive signal amplitude. The transmit antenna  74  is slaved to the point direction and polarization of the receive antenna  82 . An alternative implementation could use an open-loop tracking method with the pointing direction and polarization determined by knowledge of the aircraft&#39;s  12  position and attitude using an on-board inertial reference unit (IRU) and knowledge of the location of the satellites  18 . 
   Encoded RF signals are transmitted from the transmit antenna  74  of the mobile terminal  20  of a given aircraft  12  to an assigned one of the transponders  18   a   1 – 18   a   4 , and transponded by the designated transponder to the ground station  22 . The ground station  22  communicates with the content center  24  to determine and provide the appropriate data being requested by the user (e.g., content from the world wide web, email or information from the user&#39;s VPN). 
   An additional concern that must be taken into account with the system  10  is the potential for interference that may result from the small aperture size of the receive antenna  82 . The aperture size of the receive antenna  82  is typically smaller than conventional “very small aperture terminal” (VSAT) antennas. Accordingly, the beam from the receive antenna  82  may encompass adjacent satellites along the geosynchronous arc. This can result in interference from satellites other than the target satellite being received by a particular mobile terminal  20 . To overcome this potential problem, the system  10  preferably uses a lower than normal forward link data rate that overcomes the interference from adjacent satellites. For example, the system  10  operates at a preferred forward link data rate of at least about 5 Mbps per transponder, using a typical FSS Ku-band transponder (e.g., Telstar-6) and an antenna having an active aperture of about 17 inches by 24 inches (43.18 cm by 60.96 cm). For comparison purposes, a typical Ku-band transponder usually operates at a data rate of approximately 30 Mbps using conventional VSAT antennas. 
   Using a standard digital video broadcast (DVB) waveform, the forward link signal typically occupies less than 8 MHz out of a total transponder width of 27 MHz. However, concentrating the transponder power in less than the full transponder bandwidth could create a regulatory concern. FCC regulations presently regulate the maximum effective isotropic radiated power (EIRP) spectral density from a transponder to prevent interference between closely spaced satellites. Accordingly, in one preferred embodiment of the system, spread spectrum modulation techniques are employed in modulator  70  to “spread” the forward link signal over the transponder bandwidth using well known signal spreading techniques. This reduces the spectral density of the transponded signal, thus eliminating the possibility of interference between two or more mobile terminals  20 . 
   It is also equally important that the transmit antenna  74  meets regulatory requirements that prevent interference to satellites adjacent to the target satellite  18 . The transmit antennas used in most mobile applications also tend to be smaller than conventional VSAT antennas (typically reflector antennas that are 1 meter in diameter). Mobile transmit antennas used for aeronautical applications should have low aerodynamic drag, be lightweight, have low power consumption and be of relatively small size. For all these reasons, the antenna aperture of the transmit antenna  74  is preferably smaller than a conventional VSAT antenna. VSAT antennas are sized to create an antenna beam that is narrow enough to illuminate a single FSS satellite along the geosynchronous arc. This is important because FSS satellites are spaced at 2° intervals along the geosynchronous arc. The smaller than normal antenna aperture of the transmit antenna  74  used with the present invention, in some instances, may create an antenna beam that is wide enough to irradiate satellites that are adjacent to the target satellite along the geosynchronous arc, which could create an interference problem. The chance of this potential problem arising is greatly reduced by employing spread spectrum modulation techniques on the return link transmissions as well. The transmitted signal from the transmit antenna  74  is spread in frequency to produce an interfering signal at the adjacent satellite that is below the threshold EIRP spectral density at which the signal would interfere. It will be appreciated, however, that spread spectrum modulation techniques may not be required if the angular spacing between satellites within a given coverage region is such that interference will not be a problem. 
   The present invention relates to a system and method for quickly detecting which one of a plurality of mobile RF terminals  20  is causing interference with an FSS satellite  18  orbiting in the vicinity of a transponded target satellite, such as satellite  18   a , with which the mobile terminals  20  are communicating. 
   Referring to  FIG. 3 , a graph  100  of a modulated signal transmitted from a first one of the mobile terminals  20  is shown. The NOC  26  initiates the method of the present invention by commanding the first mobile terminal  20  to transmit signals at a changing data rate (i.e., power level). In this example, the data rate changes from 160 kbps to 16 kbps, and the duty cycle of the signal is 50%. 
   The NOC  26  then checks with the operator of the interfered with FSS satellite to determine if the interfering condition is being modulated in the same manner (i.e., has increased and/or decreased) as the modulated signal transmitted from the first of the mobile terminals  20 . If the interfering condition is not affected, then the NOC  26  determines that the first mobile terminal  20  is not causing the interference. The NOC  26  then commands a second one of the mobile terminals  20  accessing the target satellite  18   a  to transmit signals in accordance with the modulation profile of the signal level shown in graph  100 . If the NOC  26  determines that this mobile terminal  20  is also not interfering, it will continue to test each mobile terminal  20 , in sequential fashion, until it determines which mobile terminal is causing the interference with the satellite. Once it determines which mobile terminal  20  is causing the interference, it either commands that mobile terminal  20  to reduce its transmit power level to a suitable level that will not cause interference, or to stop transmitting entirely. 
   The NOC  26  may also monitor the target satellite  18   a  and/or the interfered with FSS satellite. If the NOC  26  can detect the interfering signal, the NOC can directly determine whether the interfering signal is affected by the power modulation. In this manner, the NOC  26  can determine which mobile terminal  20  is causing the interference without continuous communication with the operator of the interfered with FSS satellite. 
   Using the above-described process, the mobile terminal  20  of each aircraft  12  can be checked by the NOC  26  within a time span of about 5–10 seconds. An entire transponded satellite accommodating up to about 30 aircraft can be checked in less than about 5 minutes. 
   Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. It will also be appreciated that the variations of the preferred embodiments in specific embodiments herein could readily be implemented in other ones of the embodiments. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.