Abstract:
An aircraft based communication system ( 10 ) defining a wireless service area ( 20 ) is disclosed. The communication system ( 10 ) includes a communication gateway ( 30 ) connected to a terrestrial based communication network. A first aircraft ( 12 ) is located in proximity to the wireless service area ( 20 ). The first aircraft ( 12 ) communicates with the gateway ( 30 ) and communicates with at least one subscriber ( 24 ) located within the wireless service area ( 20 ). The first aircraft ( 12 ) transmits a first control signal ( 16 ) within the wireless service area ( 20 ). A second aircraft ( 14 ) is located in proximity to the wireless service area ( 20 ). The second aircraft ( 14 ) being operable for communicating with the gateway ( 30 ) and being operable for communicating with the subscriber ( 24 ) located within the wireless service area ( 20 ). The second aircraft ( 14 ) flies in proximity to the first aircraft ( 12 ) for transferring communication sessions ( 56, 58 ) from the first aircraft ( 12 ) to the second aircraft ( 14 ) and maintaining a constant communication link between the subscriber ( 24 ) and the terrestrial based communication network.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to an aircraft based communication system. More particularly, the present invention is directed to a system and technique for implementing plane to plane switchover in an aircraft based wireless communication system.  
           [0003]    2. Discussion  
           [0004]    The increasing need for wireless networks and communication capabilities in outlying and geographically diverse locations has created great demand for wireless systems and wireless service carriers. Many of these new carriers have understandably focused their resources on building as many towers and installing and commissioning as many terrestrial base stations as possible. This strategy increases the coverage area for the communication services of the system and generates revenue for the wireless system operator at a rate limited by the amount of time necessary to construct and commission these base station facilities. However, the build-out rate for base stations is still slow and expensive.  
           [0005]    There exist many geographic regions which do not have wireless communication services implemented through antenna towers and terrestrial base stations. Such areas are often referred to as green field areas. Several options exist for providing wireless communication services to subscribers within these green field areas including but not limited to, installing towers and terrestrial base stations, providing communication service from an aircraft flying overhead, and providing communication service from a satellite constellation.  
           [0006]    Aircraft based wireless communication systems are being seriously considered due to their lower investment in capital equipment, and their reduced lead time for complete implementation. Aircraft based wireless communication systems have a significant advantage over antenna tower based terrestrial systems in that a single aircraft and base station can replace many antenna towers and still cover a large geographic area. Moreover, the aircraft can maintain a line of sight communication link with nearly every subscriber within its communication beam coverage area. However, one disadvantage with an aircraft based wireless communication system is that the aircraft can only provide coverage to the service area for a limited amount of time, as most aircraft must be refueled and the flight crew must be changed.  
           [0007]    In view of the known challenges with terrestrial based wireless communication systems, it is desirable to provide an aircraft based wireless communication system which eliminates the need for additional and costly terrestrial based antenna systems. It is further desirable to provide an aircraft based wireless communication system which is compatible with existing wireless communication system base transceiver stations. To achieve the above goal, it is desirable to implement a system and technique for executing switchover of the real time communication sessions from one servicing aircraft to another replacement aircraft while both aircraft are in flight. Accordingly, it is an advantage of the present invention to minimize any dropped calls or interruption in service experienced by the ground based system subscribers during execution of the plane to plane switchover protocol.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and appended claims, and by referencing the following drawings in which:  
         [0009]    [0009]FIG. 1 is a diagram showing the components of the wireless communication system of the present invention during normal steady state operation;  
         [0010]    [0010]FIG. 2 is a diagram showing the wireless communication system of the present invention during preparation for communication switchover from the servicing aircraft to the replacement aircraft;  
         [0011]    [0011]FIG. 3 is a diagram showing the wireless communication system of the present invention during execution of the communication switchover protocol;  
         [0012]    [0012]FIG. 4 is a diagram showing the wireless communication system of the present invention after completion of the communication switchover protocol;  
         [0013]    [0013]FIG. 5 is a flow diagram showing the steps for executing the communication switchover protocol; and  
         [0014]    [0014]FIG. 6 is a block diagram showing the communication hardware which comprises the payload of each aircraft. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    Turning now to FIG. 1, the aircraft based communication system  10  is shown in accordance with a preferred embodiment of the present invention. Communication system  10  is a cellular based communication system, preferably referred to as a wireless communication system. As shown, wireless communication system  10  includes a primary or servicing aircraft  12  which is outfitted with suitable communication hardware for broadcasting a communication signal  16  into a predetermined geographic area  20 . Preferably, the communication signal  16  is a wireless communication beam pattern comprising a plurality of precisely focused individual beams  22 . The beams  22  may be received by a plurality of wireless communication devices, such as a wireless handset  24 .  
         [0016]    With reference to FIG. 2, the communication system  10  also includes a secondary or replacement aircraft  14  which is also capable of broadcasting a communication signal  18  into the predetermined geographic region  20 . The communication signal  18  is also preferably a wireless communication beam pattern comprising a plurality of precisely focused individual beams  22 , which may also be received by one or more wireless handsets  24 . The primary communication signal  16  is shown as a beam pattern of solid circles  26 , and the secondary communication signal  18  is shown as a pattern of dotted line circles  28 .  
         [0017]    Referring now to FIGS. 1 and 2, the ground based communication system  30  represents the existing communication hardware associated with most ground based wireless communication systems. As shown, communication system  30  includes a base transceiver station (BTS)  32  having a main receiver  34 , a diversity receiver  36 , and a transmitter  38 . The BTS  32  functions as a communication gateway between the aircraft  12 ,  14  and a terrestrial based communication network, such as a public telephone and/or data network. A high speed data converter  40  provides a high speed data link to the aircraft  12 ,  14 . Therefore, high speed data converter receives signals from the transmitter  38  and provides converted signals to the main receiver  34  and diversity receiver  36  of the BTS  32 . A variety of data converters can be used with the communication system of the present invention. While a C-band converter is preferred, other converters such as K-band and S-band converters can also be employed.  
         [0018]    The C-band converter  40  is also connected via a bi-directional communication path  42  to a primary antenna  44 . The C-band converter  40  is also connected via a bidirectional communication link  46  to a secondary antenna  48 . While two separate antennas  44 ,  48  are shown, it is within the scope of the present invention to use a single multiplexed antenna for communicating with the aircraft  12 ,  14 . The ground based communication system  30  is preferably an existing 800 MHz or 1900 MHz wireless communication system operating under either the Is-95 or Is-136 wireless communication standard. While one of the above mentioned wireless systems is preferred, the plane to plane switchover protocol of the present invention can be used with many other types of wireless communication systems and standards.  
         [0019]    Turning briefly to FIG. 6, the payload  80  of the servicing aircraft  12  is shown in block diagram form. It should be understood that replacement aircraft  14  also includes a similar payload  80 . As shown, the communication hardware forming payload  80  includes a signal repeater  82  which further includes signal control and processing hardware. The signal repeater  82  communicates bidirectionally with a C-band signal generator  84  which is connected to a suitable C-band antenna  86 . The signal repeater  82  also communicates bidirectionally with a wireless beam signal generator  88  which is connected to a suitable wireless beam antenna array  90 . As will be appreciated the C-band antenna  86  communicates with the antennas  44 ,  48  connected to the ground based C-band converter  40 , and the wireless beam antenna array  90  is particularly suited for broadcasting the wireless communication beam patterns  16 ,  18 . Because the C-band antenna  86  and the wireless beam antenna array  90  are mounted externally on each aircraft  12 ,  14 , they are preferably covered by an aerodynamic cowling  92 .  
         [0020]    With reference to FIG. 1, the steady state operation of communication system  10  will be described in greater detail. During the steady state operation, wireless communication service is provided by a single aircraft such as servicing aircraft  12 . It is preferred that the aircraft  12 ,  14  fly over the wireless service area at an altitude of about 30,000 to 50,000 feet. However, one skilled in the art will readily appreciate that the aircraft  12 ,  14  can fly at a variety of altitudes depending upon the system implementation and aircraft capabilities. During the steady state operation of the wireless communication system, one of the two C-band ground antennas  44 ,  48  are used for the feeder link  56  between the ground based communication system  30  and the servicing aircraft  12 . As shown in FIG. 1, the primary antenna  44  is being used. Also shown is that the replacement aircraft  14  is in transit to the predetermined geographic area  20  from an airport  50 . During steady state operation, the BTS  32  is receiving the repeated signal from the servicing aircraft  12  on the main receiver path  34 . A wireless service subscriber  24  within geographic area  20  perceives a single RF communication path. Accordingly, an advantage of the present invention is that an existing wireless service subscriber  24  can communicate with the aircraft  12 ,  14  without any modifications to their wireless telephone or data device. Additionally, the communication hardware carried by the aircraft  12 ,  14  can communicate with existing terrestrial based telephone and data networks.  
         [0021]    Turning now to FIG. 2, the preparation for executing a plane to plane communication switchover protocol is shown. During switchover preparation, the replacement aircraft  14  moves into position along a flight path in proximity to the primary servicing aircraft  12 . The replacement aircraft  14  replicates the communication signal  16  transmitted by servicing aircraft  12  as wireless communication beam pattern  18 . This communication signal  18  is broadcasted from the replacement aircraft  14  toward the ground at low power, and is broadcasted in such a way that the wireless communication beam pattern  18  substantially overlaps the existing wireless communication beam pattern  16 . The overlap between the communication beam patterns  16 ,  18  is preferably greater than 80%. For example, if the overlap becomes less than 80%, an significant number of subscribers will be required to handoff to a different beam. A goal of the present invention is to minimize the number of handoffs during execution of the plane to plane switchover.  
         [0022]    As part of the switchover preparation protocol, both ground antennas  44 ,  48  are used for transmitting and receiving the feeder link communication signals  56 ,  58  to/from the aircraft  12 ,  14 . The aircraft  12 ,  14  function as signal repeaters for communicating between the BTS  32  and the predetermined geographic region  20 . The BTS  32  receives a strong signal on the main receiver  34  via the primary antenna  44 , and a weaker diffracted signal on the diversity receiver  36  via the secondary antenna  48 . During switchover preparation, a wireless subscriber  24  within the geographic region  20  perceives a multipath reception condition because the communication signal  16  and the communication signal  18  are transmitted within a common frequency band. For wireless handsets, such as a CDMA handset, the RAKE receiver internal to the handset functions to resolve the multipath signals. As is known in the art, the RAKE receiver of a CDMA wireless handset has the ability to separate out and receive multipath signals (two signals on the same frequency).  
         [0023]    Referring to FIG. 3, the switchover execution procedure is shown. During switchover execution, the replacement aircraft  14  increases the power of its control signal  18  up to normal amplitude levels, and the servicing aircraft  12  reduces the power level of its control signal  16  until the control signal can no longer be detected. The power level of the control signal  18  can be increased to its normal operating power level before the power level of the control signal  16  is attenuated. Alternatively, the power level of the control signal  18  can be increased while simultaneously attenuating the power level of the control signal  16 . In this context the term control signal refers collectively to voice traffic and control signal information. The power level changes of the control signals can be performed by the hardware carried in each aircraft  12 ,  14 . Alternatively, the power level changes of the control signals can be performed by the ground based C-band converter  40 .  
         [0024]    At this point, both C-band ground antennas  44 ,  48  are used for the feeder link, broadcasted to/received from the wireless subscribers  24  within the geographic area  20 . It is preferred that the subscriber links from each aircraft  12 ,  14  to the wireless subscribers  24  are broadcasted at the same frequency, such that they appear to be identical to the subscriber for creating or simulating a multipath condition such that they can be simultaneously received by a single handset receiver. Also at this point, both aircraft are receiving and repeating signals from the wireless subscribers  24  back to the ground antennas  44 ,  48 . Further, both receivers  34 ,  36  of BTS  32  are receiving the multipath signals from the aircraft  12 ,  14 . As the power of the control signal from the servicing aircraft  12  fades and the power of the control signal from the replacement aircraft  14  increases, the BTS  32  switches reception from the main receiver  34  to the diversity path receiver  36 , and the communication sessions are effectively transferred or switched over from the servicing aircraft  12  to the replacement aircraft  14 . At this point, there is minimal frequency and timing shift of the control signals received by the BTS  24 . Also from the wireless subscriber&#39;s view, there is minimal frequency and timing shift of the control signals. More particularly, this means that the plane to plane switchover is transparent to the handset  24 , and a wireless subscriber within the predetermined geographic area  20  is unable to perceive the switching from the main receiver  34  to the diversity path receiver  36 .  
         [0025]    Turning now to FIG. 4, completion of the plane to plane communication switchover protocol is shown. After the switchover protocol from the servicing aircraft  12  to the replacement aircraft  14  is complete, the replacement aircraft  14  is then considered the primary servicing and communicating aircraft. At this point, operation once again has the appearance of normal steady state operation. The BTS  32  is receiving the control channel  56  on the secondary antenna  48 . After the switchover protocol is complete the BTS  32  can continue receiving signals on the diversity path receiver  36 , or as shown, the C-band converter  40  may switch signal reception from the diversity receiver  36  to the main receiver  34 . This communication link from the aircraft  14  to the antenna  48  is shown as a solid line  56 , and the communication link from the antenna  48  through to the main receiver  34  of the BTS  32  is shown as a dashed line  54 . Additionally, as shown responsibility for carrying the feeder link  56  has switched from the primary antenna  44  to the secondary antenna  48 .  
         [0026]    While the present invention has been described within the context of a two aircraft system, one skilled in the art will readily appreciate that a single aircraft  12  can be used for providing communication service to subscribers  24  within the service area  20 . The aircraft  12  is intended to include any type of aircraft capable of maintaining coverage over the service area  20 , including but not limited to an unmanned airplane with in-flight refueling capabilities, a dirigible type airship, and a balloon. Additionally, the switchover protocol of the present invention may be made between two different types of aircraft.  
         [0027]    With reference to FIG. 5, the relative timing of the events forming the plane to plane switchover protocol of the present invention are shown. At block  60 , the replacement aircraft  14  arrives on station ready to initiate wireless service. At block  62  the BTS  32  transmits the same forward link signals  56 ,  58  (including control signal information) to both aircraft  12 ,  14  (respectively). The information to the replacement aircraft  14  is attenuated by the C-band converter  40 . At block  64 , both aircraft  12 ,  14  retransmit the forward link signals (or subscriber link signals) into the geographic area  20 . Likewise, both aircraft  12 ,  14  receive the uplink signals from a wireless subscriber and retransmit these signals to the BTS  32 . At block  66 , the wireless handset  24  receives signals from both aircraft  12 ,  14  and uses it&#39;s multipath resolver (RAKE receiver) to process the signals and resolve the multipath condition. The handset transmits normally, and does not require any modification.  
         [0028]    At block  68 , the BTS  32  receives signals from both aircraft  12 ,  14  and uses the BTS diversity inputs  34 ,  36  to process the signals. At block  70  the C-band converter  40  slowly changes the relative power of the transmitted signals, reducing the amplitude of the signal from the servicing aircraft  12  and increasing the amplitude of the signal from the replacement aircraft  14 . Alternatively, changing of the relative power can be handled by the signal control and processing hardware  82  carried by each aircraft  12 ,  14 . Continuing with block  72 , the handset modifies its multipath resolver as the relative signal levels change. If the control channel (forward link signal) from a different beam transmitted by the replacement aircraft  14  becomes stronger than the control channel from the servicing aircraft  12 , a beam to beam handoff is requested.  
         [0029]    At block  74 , the ground based C-band converter  40  changes the relative amplitude slow enough to grant the handoff request. Where there is a large difference between the two aircraft signal levels, the converter  40  stops transmitting to the original servicing aircraft  12 . Finally at block  76 , the plane to plane switchover process is complete. The original servicing aircraft  12  stops transmitting the uplink signal to the BTS  32  and also stops transmitting the forward link signal to the geographic area  20 .  
         [0030]    The present invention uses diversity inputs on a standard BTS to handle two communication inputs from two separate aircraft. The communication switchover protocol associated with the present invention allows for slowly changing relative powers to the BTS, thus forcing transparent switchovers from the old beam pattern or beam structure to the new beam pattern or beam structure. The communication protocol of the present invention also allows transmission of both signals to the aircraft on different polarizations.  
         [0031]    The present invention allows a service provider to rapidly cover a large geographical area without having to deploy multiple base stations across that area. This allows early entry into new markets and easy system expansion after initial deployment. This can be done without any changes to the wireless infrastructure or to the subscriber handset. The present invention provides coverage over a wide geographical area by linking the wireless infrastructure on the ground with a high altitude repeater that is carried on an aircraft. As part of the present invention, a technique is described for minimizing the dropped call impact of a plane to plane switchover in an aircraft based wireless communication system. As set forth above, the technique involves simultaneously transmitting control channels from both planes while changing the relative power (amplitude) levels of the control signals.  
         [0032]    The wide-area coverage provided by the present invention saves capital expenses by locating the infrastructure equipment of the wireless communication system in a single center. The service provider does not have to buy any land, buildings, or towers to install hundreds of base transceiver stations (BTSs) to cover the area. Instead, the wireless operator simply installs a mobile switching office and a reduced amount of BTS equipment needed for terrestrial deployment.  
         [0033]    The central location of all the wireless infrastructure equipment in the present invention allows the wireless operator to operate and maintain the system with a smaller crew. Moreover, the wireless operator saves all the capital expenses (trucks, test equipment, etc.) and all the recurring costs (personnel to visit the sites) that would be needed to operate and maintain geographically dispersed facilities.  
         [0034]    The wireless communication system of the present invention complements a terrestrial coverage by filling up any coverage holes left in the terrestrial deployment and by extending the coverage to suburban and rural areas where site deployment is not economical. In a new system, the present invention provides immediate coverage of the entire service area, compared to the slow rollout of sites that is typical in terrestrial deployments.  
         [0035]    The present invention can also provide services for other digital wireless technologies such as Personal Communication Service (PCS) 1900, IS-95 Code Division Multiple Access (CDMA), IS-136 Time Division Multiple Access (TDMA), Global System for Mobiles (GSM) and Integrated Digital Enhanced Network (iDEN). In addition, the present invention can be used for third generation wireless systems such as Universal Mobile Telecommunications System (UMTS) or CDMA-2000 as well as broadband data systems such as Local Multipoint Distribution Systems (LMDS).  
         [0036]    The foregoing discussion discloses and describes exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.