Patent Abstract:
A system and method for tracking a user. The system is adapted for use in a wireless communication system and creates a plurality of beams within a coverage area. A first beam is directed at a user in a first microcell and a number of additional beams illuminate microcells immediately adjacent the first microcell. The system is equipped with a mechanism for detecting movement of the user from the first microcell to one of the immediately adjacent microcells. On the detection of movement of the user, the system redirects the first beam from the first microcell to a second microcell, the second microcell being one of the adjacent microcells. In the illustrative embodiment, the system is implemented in a stratospheric platform based communication system including a hub adapted to communicate with a stratospheric platform. A transceiver and a phased array antenna are disposed on the platform to communicate with the hub and with the user. A second antenna is provided on the platform to communicate with the hub. Beamforming and direction are implemented on the hub and communicated to the platform. The user&#39;s position is detected with a global positioning system receiver, by measuring the strength of a signal received from the user, or by other suitable means. On detection of user movement from the first microcell, the beamforming system redirects the beam to follow the user into a second microcell. Additional beams around the user&#39;s microcell are illuminated to facilitate detection of the users movement.

Full Description:
[0001]    This application is a continuation of now pending application bearing Ser. No. 09/587,758, filed Jun. 6, 2000, for “MCIRO CELL ARCHITECTURE FOR MOBILE USER TRACKING COMMUNICATION SYSTEM”, inventors: Donald C. D. Chang et. al, the entire contents of which are incorporated herein by this reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to communications systems. More specifically, the present invention relates to architectures for data/voice services to mobile users using stratospheric platforms.  
           [0004]    2. Description of the Related Art  
           [0005]    Stratospheric platforms are being considered for data/voice communication applications. Current proposals envision a mounting of transceivers and antennas on aircraft flying at 20-30 kilometers above the earth which will project beams to cell sites located on the ground.  
           [0006]    Copending U.S. patent application Ser. No. 09/588,395, filed Jun. 6, 2000 by D. Chang et al., entitled STRATOSPHERIC PLATFORM BASED MOBILE COMMUNICATIONS ARCHITECTURE, the teachings of which are incorporated herein by reference, addressed the need in the art for a stratospheric platform based communication system offering maximum throughput with the constraints of weight, power and spectrum. In accordance with the teaching of the referenced patent application, communication between users and external networks is facilitated through stratospheric platform and a hub located on the ground. Beamforming is performed at the hub. Signals to and from the user are communicated via directional beams through a phased array antenna on the platform under the directional control of the hub.  
           [0007]    Although this system offers a promising solution to the need in the art for a stratospheric platform based communication system and method for projecting beams of varying cell structure to maximize system capacity within the weight, power and bandwidth constraints thereof and thereby optimize the projection of beams with adequate link margin for billable voice and data transmissions, a further need exists in the art for a system and method for tracking the position or location of a user.  
           [0008]    For certain applications, the ability to track the position or location of the user would allow for more relaxed platform stability requirements and thereby lower the overall costs of the system. One such application is that of the ‘third generation mobil’ communications system. As a stratospheric platform application (SPA) this system would provide high data rate communications to a user enabling simultaneous voice, data and entertainment communication. For a lightweight platform payload, both the transmit power and the antenna aperture can be limited. To close a communication link, a larger aperture is needed for the receive end onboard the platform and a larger aperture and more powerful high power amplifiers are needed for the transmit end as well. These requirements are in direct conflict with the objective of a lightweight payload. With beams tracking the mobile users so that the users are always at the antenna peak directivity or within a contour of 1-dB from the peak, the link would have a 3 to 4 dB advantage over the fixed beams.  
           [0009]    In addition, since beams would be tracking users, there would be no need to form beams where no users are present except for a new acquisition beam that may scan or a big beam that may be used to zoom in. This arrangement may also save beam forming computations depending on the distribution of users.  
           [0010]    In addition, when beams are formed around each user, there may be more opportunities to reuse either the code division multiple access (CDMA) codes or the carrier frequency thereof. This would result in higher system capacity for a limited spectrum.  
           [0011]    Knowledge of the user location would also allow for fewer CDMA code handoffs. In a fixed-cell-structured system, when a user crosses a boundary of two cells, CDMA code handoff must happen to avoid interference. With a beam following a user scheme, the user would not have to change his CDMA code unless he gets to close to another user who is using the same CDMA code. (A code assignment algorithm is a subject of a copending U.S. patent application Ser. No. 09/735,861 entitled A DYNAMIC CELL CDMA CODE ASSIGNMENT SYSTEM AND METHOD, filed Dec. 12, 2000 by Ying Feria et al.)  
           [0012]    Hence, a need exists in the art for a system and method for tracking the position or location of a user in a stratospheric platform based communication system.  
         SUMMARY OF THE INVENTION  
         [0013]    The need in the art is addressed by the system and method for user tracking of the present invention. The inventive system is adapted for use in a wireless communication system and creates a plurality of beams within a coverage area. A first beam is directed at a user in a first microcell and a number of additional beams illuminate microcells immediately adjacent the first microcell. The system is equipped with a mechanism for detecting movement of the user from the first microcell to one of the immediately adjacent microcells. On the detection of movement of the user, the system redirects the first beam from the first microcell to a second microcell, the second microcell being one of the adjacent microcells.  
           [0014]    In the illustrative embodiment, the system is implemented in a stratospheric platform based communication system including a hub adapted to communicate with a stratospheric platform. A transceiver and a phased array antenna are disposed on the platform to communicate with the hub and with the user. A second antenna is provided on the platform to communicate with the hub. Beamforming and direction are implemented on the hub and communicated to the platform. The user&#39;s position is detected with a global positioning system receiver, by measuring the strength of a signal received from the user, or by other suitable means. On detection of user movement from the first microcell, the beamforming system redirects the beam to follow the user into a second microcell. Additional beams around the user&#39;s microcell are illuminated to facilitate detection of the users movement. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a diagram illustrative of the stratospheric communication system of the present invention with a single stratospheric platform.  
         [0016]    [0016]FIG. 2 is a simplified block diagram of the airborne platform based transceiver system implemented in accordance with the present teachings.  
         [0017]    [0017]FIG. 3 is a simplified block diagram of a hub in accordance with the teachings of the present invention.  
         [0018]    [0018]FIG. 4 is a functional block diagram illustrative of the forward processing of the hub in accordance with the present teachings.  
         [0019]    [0019]FIG. 5 is a functional block diagram illustrative of the forward processing of the platform in accordance with the present teachings.  
         [0020]    [0020]FIG. 6 is a functional block diagram illustrating the processing at the user location in accordance with the teachings of the present invention.  
         [0021]    [0021]FIG. 7 is a functional block diagram illustrating the return path processing performed on the platform in accordance with the teachings of the present invention.  
         [0022]    [0022]FIG. 8 is a functional block diagram illustrating return path processing at the hub in accordance with the teachings of the present invention.  
         [0023]    [0023]FIG. 9 is a diagram which shows how nonuniform cells are created with a fixed platform antenna aperture.  
         [0024]    [0024]FIG. 10 is a set of graphs showing spreading angle as a function of distance from the projected platform location to a user of an elevated beam projection system.  
         [0025]    [0025]FIG. 11 is a block diagram of an illustrative implementation of a code assignment algorithm for use in connection with the communication system depicted in FIG. 1.  
         [0026]    [0026]FIG. 12 is a diagram that illustrates color code assignments based on the number of users in accordance with the method of the present invention.  
         [0027]    [0027]FIG. 13 shows a color 1 code assignment (blue) in accordance with the teachings of the present invention.  
         [0028]    [0028]FIG. 14 shows a color 2 code assignment (pink) in accordance with the teachings of the present invention.  
         [0029]    [0029]FIG. 15 shows a color 3 code assignment (orange) in accordance with the teachings of the present invention.  
         [0030]    [0030]FIG. 16 shows a color 4 code assignment (purple) in accordance with the teachings of the present invention.  
         [0031]    [0031]FIG. 17 depicts an overall code assignment.  
         [0032]    [0032]FIG. 18 is a diagram illustrating a distribution of users sharing a code division multiplexed (CDMA) code in accordance with the teachings of the present invention.  
         [0033]    [0033]FIG. 19 is a diagram showing an illustrative microcell architecture for use in the mobile user tracking system of the present invention.  
         [0034]    [0034]FIG. 20 is a magnified view of a portion of the diagram of FIG. 19 showing overlap between microcells in accordance with the present teachings. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0035]    Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.  
         [0036]    While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.  
         [0037]    [0037]FIG. 1 is a diagram illustrative of the stratospheric communication system of the present invention with a single stratospheric platform. The inventive system  10  includes a transceiver system  20  mounted on an airborne platform (not shown). In practice, the platform could be an airplane flying in an orbit at 20-30 kilometers (km) above the ground. Those skilled in the art will appreciate that this altitude is 2 to 3 times that of commercial aircraft (i.e., 10 km) and much lower than the altitude of a low earth orbit satellite (1000 km). The transceiver  20  is adapted to communicate with a hub  30  and a plurality of users  40  and  50  located on cells  60  and  70 , respectively, on the earth&#39;s surface. This system is described and claimed in copending application entitled STRATOSPHERIC PLATFORM BASED MOBILE COMMUNICATIONS ARCHITECTURE, Ser. No. 09/588,395, filed Jun. 6, 2000, by D. Chang et al., the teachings of which are already incorporated by reference.  
         [0038]    [0038]FIG. 2 is a simplified block diagram of the airborne platform based transceiver system implemented in accordance with the present teachings. The system  20  includes a feeder antenna  22  adapted to receive a signal from the hub system  30 . The feeder antenna at C, X or other suitable frequency band. The antenna  20  is connected to a radio frequency (RF) downconverter circuit  24 . The downconverter  24  is adapted to downconvert signals received by the antenna. Hence, a C band antenna would be coupled to a C-band RF downconverter  24 . The downconverter  24  outputs a signal at baseband which is demultiplexed by a code division multiplexer  26  into plural separate signals of which  192  are shown in the figure. The multiplexer  26  is bi-directional and serves to multiplex plural signals into a single signal when the system  20  is operating as a receiver. Those skilled in the art will appreciate that the system shown in FIG. 2 is intended for illustration only. Accordingly, the present teachings are not limited to the number of channels or elements shown. Nor is the system limited to the specific circuit configuration shown. Other circuit configurations may be used without departing from the scope of the present teachings.  
         [0039]    The demultiplexed signals feed an RF upconverter  28 . In the illustrative embodiment, the upconverter  28  operates at S-band. The upconverter drives a phased array antenna  29 . As discussed more fully below, the antenna  29  is a single aperture antenna that transmits and receives multiple output beams. The beams are formed and steered by a beamforming network located on the surface in the hub system  30 . Each beam creates a footprint on the surface that provides a cell such as the cells shown at  60  and  70  in FIG. 1.  
         [0040]    As discussed more fully below, the present invention allows the cell size to be non-uniform. That is, at center of coverage, or nadir, the cell can be smaller. As the scan angle increases, the cell sizes increase. The invention allows for a very light weight payload design and full utilization of the resources that a light-weight payload can offer. The present invention forms beams where there are users present with beams of shapes and sizes that are not necessarily uniform. One or more wider beams are formed to provide links supporting lower data rates. These lower data rate links are used to pick up new users trying to get on the system. This allows the coverage area to be greater with limited receiving beams. In addition, by allowing the beam size to be smaller at the center of coverage (nadir of the platform), the code or frequency reuse distance can be reduced, therefore enhancing the total system capacity.  
         [0041]    [0041]FIG. 3 is a simplified block diagram of a hub in accordance with the teachings of the present invention. The hub transceiver system  30  includes an antenna  32  adapted to communicate with the antenna  22  on the airborne platform. The antenna  32  connects to an RF subsystem  34  which provides upconversion and downconversion in a conventional manner. The subsystem  34  receives a baseband signal from a code division multiplexer/demultiplexer  36 . The multiplexer  36  receives inputs from a digital beam former  38  which is fed by conventional multiplexers/demultiplexers, routers, and formatters  39 . The multiplexers/demultiplexers, routers, and formatters  39  are connected to an external network such as the Internet or World Wide Web.  
         [0042]    The systems depicted in FIGS. 2 and 3 may be implemented in accordance with the teachings of U.S. Pat. No. 5,903,549, issued May 11, 1999 to Von Der Embse and entitled Ground Based Beam Forming Utilizing Synchronized CDMA, the teachings of which are hereby incorporated by reference herein. The number of beams (or simultaneous users) ‘n’ is scaleable at the gateway.  
         [0043]    In accordance with the present teachings, the beam forming circuit  38  generates phasings, weightings and codes for each of a plurality of beams. These beams are multiplexed into a single stream which is transmitted up to the airborne platform  20  via the link provided by the feeders  22  and  32  of FIGS. 2 and 3 respectively. When the stream is received on the airborne platform, it is demultiplexed into separate feeds for the array antenna  29 . The phasing and weighting of the signals provided by the beamforming circuit  38  are effective to generate the plural beams and to steer each beam in a desired direction.  
         [0044]    FIGS.  4 - 8  illustrate the operation of the present invention with respect to forward processing, i.e., from the external network to a user, and return processing, from the user to the external network.  
         [0045]    [0045]FIG. 4 is a functional block diagram illustrative of the forward processing of the hub in accordance with the present teachings. In the illustrative embodiment of the hub processing system  30 , user data is received from one or more Internet Service Providers (ISPs) and directed to circuitry associated with each user by a multiplexer  39 . FIGS.  4 - 8  have been simplified to show circuitry associated with a single user. It will be understood that data directed to each user will be processed in a manner similar to that depicted in FIGS.  4 - 8 .  
         [0046]    Returning to FIG. 4, the multiplexer  39  is adapted to process signals returning to the hub  30  as discussed more fully below. The signal for a single user is selected by the multiplexer  39  and directed to a code division multiple access (CDMA) encoder/decoder  41 . While the encoder/decoder may be implemented in software, in the illustrative embodiment, the encoder/decoder  41  is a digital signal processor which employs a well-known CDMA coding scheme such as an orthogonal (Walsh) code, Gold code and/or Viterbi code to spread the incoming data with a user code. This increases the bandwidth of the incoming signal and allows for a superimposition of signals without interference. The user code is supplied by a dynamic database  43  which performs a lookup of a user code in response to the input of a user ID. The user ID may be supplied by a system controller  45  which performs numerous housekeeping functions in response to input from a system manager interface  47 . For example, the controller  45  is programmed to initialize new users and set up the links therefor. In addition, in accordance with the present teachings, the controller  45  is programmed to recognize conflicts and reallocate codes for certain users as necessary in the manner discussed more fully below.  
         [0047]    A spread user signal is output by the encoder/decoder  41  to a digital beam former  38 . The beam former  38  is a conventional beam forming system which provides element phasing information to direct a beam containing the spread user signal and amplitude information to shape each beam for each user. These beam formed user signals are summed by an adder  37  and input to a CDMA antenna element spreading encoder  36 . In the illustrative embodiment, the element spreading encoder  36  uses orthogonal codes to spread the signal for each element in response to an element code supplied by a register or memory  35 . The user signals might be on the order of  144  kilobits per second bandwidth, spread to 5 megahertz by the encoder  41 , and spread further to 0.5 to 1 gigahertz by the element spreading encoder  36 . The signals from the elements are summed by a second adder  31  and input to the radio frequency stage  34 . In the illustrative embodiment, the RF stage is an RF transceiver which outputs a right-hand circular (RHC) signal in the C-band or X-band range. In practice, a second identical circuit  30 ′ (not shown) would output a left-hand circular (LHC) signal as well. These signals are combined at the antenna  32  and uplinked to the platform  20  depicted in FIG. 1.  
         [0048]    [0048]FIG. 5 is a functional block diagram illustrative of the forward processing of the platform in accordance with the present teachings. As shown in FIG. 5, the uplinked signal is received by a feeder antenna  22  and fed to LHC and RHC processing circuits  20  and  20 ′ of which only the LHC circuit  20  is shown. Each processing circuit  20  includes an RF transceiver  24  which downconverts the (C-band) uplink feed of one polarization and outputs feeder signals for each antenna element. The element signals are despread by an element decoder  26  in accordance with a code stored in a memory  51  associated with each element. A signal for a given element is upconverted to S-band, in the illustrative embodiment, and combined with the corresponding signal output by the processing circuit  20 ′ for the RHC by a summer  27  which outputs an element downlink signal to a phased array antenna  29 . The phased array antenna  29  forms one or more beams in response to the phasings and weights originally impressed on the element downlink signal by the hub-based beam forming processor  38  of FIG. 4. The beams are thereby directed to an associated user. FIG. 6 is a functional block diagram illustrating the processing at the user location in accordance with the teachings of the present invention. The signal transmitted by the platform is received by an antenna  52  and downconverted into first and second bands by an S-band RF transceiver  54 . One band is selected by the user via an interface  53 . The selected signal is despread by a CDMA decoder  56  which is adapted to decode the signal in accordance with the encoding scheme employed by the encoder  41  of FIG. 4. The decoder  56  outputs a user signal in response to a user code supplied by a memory  55 .  
         [0049]    The return path processing begins with user data being supplied to a summer  64  which combines the user data with location data supplied by a conventional global positioning system (GPS) receiver  62 . The combined signal is spread by a CDMA encoder  58 , which, in the illustrative embodiment, is designed to operate in accordance with the encoding scheme employed by the encoder  41  of FIG. 4. The encoded signal for the selected band is upconverted (to S-band in the illustrative embodiment) and transmitted to the platform  20  via an antenna  68 .  
         [0050]    [0050]FIG. 7 is a functional block diagram illustrating the return path processing performed on the platform in accordance with the teachings of the present invention. The signal from the user is received by the phased array antenna  29  and downconverted by the RF transceiver  28 . The transceiver  28  outputs an element return signal to an encoder  26 . The encoder  26  spreads the signal to avoid interference with the uplink signal and outputs a spread element return signal. The spread element return signal is combined with platform location data supplied by another conventional GPS receiver  55  which is spread by an encoder  56  in response to a code stored in a memory  59 . The spread element return signal and the spread GPS location data are combined with corresponding signals from other elements by a summer  53  and upconverted to C-band before being downlinked to the hub  30  via the antenna  22 .  
         [0051]    [0051]FIG. 8 is a functional block diagram illustrating return path processing at the hub in accordance with the teachings of the present invention. The signal downlinked from the platform is received by the antenna  32  and separated into RHC and LHC downlink feeds. Each feed is downconverted to IF by the C-band RF transceiver  34 . The downconverted signal is despread by the CDMA decoder  36 . The element returns for each user are processed by the beamforming processor  38  in response to the stored phases and weights supplied by the database  47 . The spread user data is then supplied to the CDMA decoder  41  which decodes the data in response to a user code supplied by the database  47 . The decoder  41  outputs user data suitable for transmission over the network connected to the multiplexer  39 .  
         [0052]    In the preferred embodiment, each beam is assigned to a user or a zone. If assigned to a user, the beam is adapted to move with the user to minimize the number of code handovers and to increase antenna directivity in user links. Static beams are formed where no user tracking beams are present for new user detection.  
         [0053]    Conventionally, the beams radiated by the antenna  29  of FIG. 2, would be constrained to provided fixed, uniform footprints or cells on the ground. If the user distribution is uniform, the equal sized cell structure is optimal. However, equal cell size comes at some cost in hardware. To avoid the need for a mechanical tracking system on the antennas, which can be costly and unreliable, the antennas are phased arrays of radiating elements and steered electronically. At a certain altitude from the ground, where a payload carrying platform locates, a same-sized ground projection cells require smaller angular beams as the scan angle increases. To form smaller beams, more antenna array elements will be needed. For a lightweight payload, the number of elements may be limited, thus forming smaller beams at the edge of the coverage may be costly.  
         [0054]    However, in accordance with the present teachings, the beams are formed without regard to a fixed uniform pattern by the digital beam forming network  38 . The system  10  is designed to cover a service area with as many users as possible. If the cell size is uniform on the ground, then the required number of elements in the phased array antenna would be so high that a light-weight payload would not be possible. On the other hand, if some elements are not being utilized to form wider beams at the center of the coverage (or nadir of the platform), the resource (frequency or code) reuse distance would be longer. This would result in fewer users on the system and lower total system capacity.  
         [0055]    Consequently, the system  10  of the present invention is designed with a dynamic cell structure utilizing all the resources available on a light-weight payload to form beams as small as possible and reuse the frequency or CDMA code as often as possible to enhance the total system capacity. That is, the system  10  allows the cell sizes to be non-uniform. At center of coverage, or nadir, the cell can be smaller. As the scan angle increases, so does the cell size. In addition, the cell shape is not restricted to be perfectly circular. The cell shape may be elongated as the scan angle increases. This is depicted in FIG. 9 below.  
         [0056]    [0056]FIG. 9 is a diagram which shows how nonuniform cells are created with a fixed platform antenna aperture. The scenario illustrated in this figure assumes a nominally circular antenna array situated at ‘A’ and oriented normal to the local vertical. The antenna is at an altitude ‘h’ above the ground point ‘O’, which is at the center of the coverage area. Now consider the beam formed by this antenna having maximum directivity in the direction of a user located at point ‘C’. Assuming that differential spreading of the antenna gain across the beam width can be neglected, the contour of constant antenna gain on the ground plane, denoted ‘BDEF’ in the figure, will be elliptical in form. Furthermore, the long-side spreading (∠CAD and ∠CAB) will be equal, as will the short-side spreading angles (∠CAE and ∠CAF). The distances CB, CD, CE, and CF can be easily calculated as  
         CB   h     =       tan        (       ∠      OAC     +     [           long-side               spreading-angle           ]       )       -     tan                   ∠        (   OAC   )                     CD   h     =       tan   (     ∠      OAC     )     -     tan                   (       ∠      OAC     -     [           long-side               spreading-angle           ]       )                   CE   h     =       CF   h     =       1     cos   (     ∠      OAC     )            tan        (     [           short-side               spreading-angle           ]     )                                 
 
         [0057]    Assuming the antenna aperture is constant, the spreading angles are then a function of the distance between a user and the projected platform location on the ground (OC). The larger the OC is, the larger the spreading angles are, as shown in FIG. 10.  
         [0058]    [0058]FIG. 10 is a set of graphs showing spreading angle as a function of distance from the projected platform location to a user of an elevated beam projection system. Note that the short-side spreading angle may be different from the long-side spreading angle.  
         [0059]    In practice, optimal utilization of system resources calls for multiple (e.g. 200) beams to be generated. As mentioned above, in the preferred embodiment, each beam would track a user if a user were present. To maximize system capacity, the frequencies are reused by assigning codes to each beam.  
         [0060]    In the illustrative implementation, a group of 64 codes is divided into 4 subgroups. Each subgroup of codes is referred to as one color of codes and has 16 individual codes. In the illustrative embodiment, there are four colors of codes. The assignment of one color of codes is independent of the other colors of codes. The same color of codes can be reused outside a criterion. In the illustrative embodiment, a 20 decibel (dB) sidelobe contour criterion is employed. In accordance with this criterion, a beam collision will be detected when the user moves into another cell and receives a signal therein at a level of 20 dB down from maximum or higher. The 20 dB sidelobe contours can be of different sizes and shapes throughout a coverage area.  
         [0061]    [0061]FIG. 11 is a block diagram of an illustrative implementation of a code assignment algorithm for use in connection with the communication system depicted in FIG. 1.  
         [0062]    [0062]FIG. 12 is a diagram that illustrates color code assignments based on the number of users in accordance with the method of the present invention. The method  100  includes the step of assigning codes with as many colors as possible ( 110 ). Next, the code colors are sorted with the number of users in descending order (step  120 ). Hence, as shown in FIG. 12, purple  122  might be used for 5 users, orange  124  might be used for 10 users, pink  126  might be used for 25 users and blue  128  might be used for 30 users.  
         [0063]    Returning to FIG. 11, at step  130 , if a new user enters a cell, the system checks to determine if the new user can be assigned to the first code color using the 20 dB sidelobe contour criterion. If not, at step  140 , the system  10  tries the next color. At step  150 , the system endeavors to find a code in the code color and at step  160 , the code is assigned to the user.  
         [0064]    Illustrative code assignments are shown in FIGS.  13 - 17 .  
         [0065]    [0065]FIG. 13 shows a color 1 code assignment (blue) in accordance with the teachings of the present invention.  
         [0066]    [0066]FIG. 14 shows a color 2 code assignment (pink) in accordance with the teachings of the present invention.  
         [0067]    [0067]FIG. 15 shows a color 3 code assignment (orange) in accordance with the teachings of the present invention.  
         [0068]    [0068]FIG. 16 shows a color 4 code assignment (purple) in accordance with the teachings of the present invention.  
         [0069]    [0069]FIG. 17 depicts an overall code assignment. As shown in FIG. 13, each cell  70  is created by a beam directed to a respective user  50 . Note that although the beams overlap, no two beams overlap a user. This is depicted in FIG. 18.  
         [0070]    [0070]FIG. 18 is a diagram illustrating a distribution of users sharing a code division multiplexed (CDMA) code in accordance with the teachings of the present invention. At anytime, a user would not be located inside the contour of another user using the same code. In FIG. 18, the stars are the users. The oval shaped contours are the isolation forbidden zone. That is, any other user using the same code cannot locate inside the forbidden zone. As in the figure, for each oval contour, there is only one user locate in the center of the contour. FIG. 18 shows the code sharing to one of the CDMA codes. For each different code, a similar figure can be drawn.  
         [0071]    Returning to FIG. 8, the beamforming processor  38  utilizes the GPS location data in the return signal with respect to the coordinates of the platform and the coordinates of the user to generate beam direction control signals for each user. In accordance with the present teachings, each coverage area or cell, is subdivided into a plurality of microcells. This is depicted in FIG. 19.  
         [0072]    [0072]FIG. 19 is a diagram showing an illustrative microcell architecture for use in the mobile user tracking system of the present invention. As shown in FIG. 19, the architecture is implemented within a cell 200 as a plurality of microcells #1-61 with precomputed beam weights. In the illustrative embodiment, the microcells correspond to &lt;1 dB antenna directivity roll-off from the peak. The present teachings are not limited to the size and shape of the microcells shown. A variety of shapes and sizes may be used to create uniform or nonuniform patterns without departing from the scope of the present teachings.  
         [0073]    In accordance with the present teachings, for a user located at microcell #7 a finite number of beams is formed around the user (e.g. at microcells #1, 2, 6, 7, 8, 13 and 14). So long as the user remains in microcell #7, the beam direction is unchanged. However, if the user moves to microcell #8, then the signal received from microcell #8 will be greater than the signal received from microcell #7. At this point, narrow beams are shifted from microcells #1, 2, 6, 7, 8, 13 and 14 to microcells #2, 3, 7, 8, 9, 14 and 15. In short, in accordance with an illustrative embodiment of the present teachings, a narrow beam is directed to the cell at which the user is located based on strongest return signal, GPS location or other information and to an area surrounding the user defined as microcells in the preferred embodiment. The system continues to track the user as the user travels from microcell #7 to microcell #59 as depicted in FIG. 19.  
         [0074]    For mobile users, the beam weights would have to be updated on some basis. For example, assuming a user is moving at a speed under 120 km/hr, and a beam size (with a 4-dB rolloff) of 8 km, the beam weight update rate of approximately once every minute might be adequate. As the user moves along a boundary between microcells, he may be registered with one cell, then another, then back to the original cell. This ‘ping-pong’ effect may be mitigated by overlapping the cells as depicted in FIG. 20.  
         [0075]    [0075]FIG. 20 is a magnified view of a portion of the diagram of FIG. 19 showing overlap between microcells in accordance with the present teachings. With the overlap between adjacent cells, a user will not be assigned to a new microcell immediately when he enters a new microcell&#39;s boundary. On the contrary, a new microcell assignment will occur when the user leaves the area of the old cell completely.  
         [0076]    Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. The present teachings allow for a very light weight payload with full utilization of the resources that a light-weight payload can offer. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.  
         [0077]    It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.  
         [0078]    Accordingly,

Technology Classification (CPC): 7