Rapid user acquisition by a ground-based beamformer

A method for rapid acquisition of a specific subscriber by a ground-based beamformer includes the steps of defining a coverage area as an arrangement of a plurality of cells wherein one of the plurality of cells includes a specific subscriber; defining a partition of cell clusters wherein one of the cell clusters includes the one of the plurality of cells that includes the specific subscriber; forming a beam that corresponds to an area of one of the cell clusters; and scanning the beam to the cell cluster that includes the specific subscriber.

BACKGROUND OF THE INVENTION

The present invention relates generally to ground-based beamformers. More specifically, but without limitation thereto, the present invention relates to finding a cell in which a specific user is located within the coverage area of a stratospheric transponder platform linked to a ground-based beamformer.

An antenna array on a stratospheric transponder platform can support multiple subscribers dispersed over a wide coverage area. To reduce weight, power consumption, and cost of the stratospheric transponder platform, a ground-based digital beamformer may be used that performs the beam calculations on the ground from a ground station linked to the antenna elements on the stratospheric transponder platform. In this arrangement, the element excitation signals are passed between the antenna elements on the stratospheric transponder platform and the digital beamformer in the ground station using CDMA techniques, for example.

Solar powered stratospheric transponder platforms have the capability of staying aloft for a long period of time, but their weight and power generation capacity are disadvantageously limited. Ground-based digital beamforming removes the weight and power requirements of the beamformer from the stratospheric platform to the ground station, and also provides an advantageous system architecture known as “spokes and hub.” In this architecture, all system data and communications processing functions are co-located in the ground station. Because system data does not have to be passed back and forth between the stratospheric transponder platforms and the ground station, valuable communications resources are conserved, resulting in faster processing time and system response.

An important function performed by communications systems including ground stations in a spokes and hub architecture is acquiring a specific subscriber, i.e., locating the cell in a platform coverage area in which a specific subscriber is located.

FIG. 1is a diagram illustrating a method100for locating a specific subscriber102in a coverage area104. According to the method100, each cell106within the coverage area104is scanned by stepping a beam108from a beamformer located in a stratospheric transponder platform120sequentially to each cell106until the cell containing the specific subscriber102is located. Although a communications satellite is used to illustrate a transponder platform in this example, other transponder platforms may be used, such as unmanned aircraft and antenna towers

In this example, the coverage area104is 64 km×64 km and is divided into 64 cells106that are each 8 km×8 km. The beam108is stepped in raster scan fashion from cell to cell in each row, and from one row to the next until the specific subscriber102is located, in this example, after stepping the beam10830 times. Assuming a dwell time of T, locating the specific subscriber102requires from T to 64 T, depending on where the cell containing the specific subscriber102is located.

Disadvantageously, the method100requires an average acquisition time of 32T, or more generally, NT/2, where N is the number of cells in the coverage area. Because N may be a large number, processing the many beams slows acquisition time and thus adversely affects the system response.

SUMMARY OF THE INVENTION

The present invention advantageously addresses the needs above as well as other needs by providing a method and apparatus for rapid acquisition of a specific subscriber within a coverage area of a transponder platform.

In one embodiment, the invention may be characterized as a method for rapid acquisition of a specific subscriber that includes the steps of defining a coverage area as an arrangement of a plurality of cells wherein one of the plurality of cells includes a specific subscriber; defining a partition of cell clusters wherein one of the cell clusters includes the one of the plurality of cells that includes the specific subscriber; forming a beam to correspond to an area of one of the cell clusters; and scanning the beam to the cell cluster that includes the specific subscriber.

In another embodiment, the invention may be characterized as a ground-based beamformer for rapid acquisition of a specific subscriber that includes a stratospheric transponder platform having an antenna for one of transmitting and receiving a beam; and a ground station coupled to the stratospheric transponder platform wherein the ground station comprises a beamformer for at least one of zooming the beam to form a beam corresponding to an area of a cell cluster within a partition containing a plurality of cell clusters and scanning the beam to one of the plurality of cell clusters that includes a specific subscriber.

The features and advantages summarized above in addition to other aspects of the present invention will become more apparent from the description, presented in conjunction with the following drawings.

Corresponding reference characters indicate corresponding elements throughout the several views of the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description is presented to disclose the currently known best mode for making and using the present invention. The scope of the invention is defined by the claims.

The method100illustrated inFIG. 1described above for locating the specific subscriber102in the coverage area104may be implemented by a ground-based beamformer in a CDMA communications system gateway hub122. By locating the beamformer at the ground station, the disadvantages of the added weight and power required to include a beamformer in the payload of a stratospheric transponder platform120may be advantageously avoided. Also, the separate beam signals and control signals do not have to be passed back and forth between the ground station and the transponder platform, thus reducing the communications traffic overhead of the system. An example of a ground-based digital beamformer will be described later with reference toFIG. 5.

FIG. 2is a diagram illustrating another method200for locating a specific subscriber in a coverage area by performing both zooming and scanning of a beam. The beam may be formed by a beamformer on the transponder platform or by a ground-based beamformer using the CDMA communications system in the example ofFIG. 1. The coverage area104is defined by the arrangement of a plurality of cells106. In this example, each of the cells106has the same area. An initial 8×8 cell cluster201is defined that, in this example, includes the entire coverage area. A beam202is zoomed to form a beam that corresponds to the area of the initial cell cluster201. A locating signal is received from the specific subscriber102that confirms that the specific subscriber102is in one of the cells106included in the initial cell cluster201. The initial cell cluster201is then partitioned into four new 4×4 cell clusters204,206,208, and210. The beam202is zoomed to become the smaller beam212that corresponds to the area of one of the new 4×4 cell clusters204,206,208, and210and is scanned or stepped in raster fashion to each new cell cluster successively until a locating signal is received from the specific subscriber102. The cell cluster containing the specific subscriber102, i.e., cell cluster208, is then partitioned into four new 2×2 cell clusters224,226,228, and230. The beam212is zoomed to become the smaller beam214that corresponds to the area of one of the new 2×2 cell clusters224,226,228, and230and is scanned in raster fashion to each new cell cluster successively until a locating signal is received from the specific subscriber102in cell cluster230. The cell cluster230containing subscriber102is then partitioned into four new 1×1 cell clusters234,236,238, and240. The beam214is zoomed to become the smaller beam216that corresponds to the area of one of the new 1×1 cell clusters234,236,238, and240and is scanned in raster fashion to each new cell cluster until a locating signal is received from subscriber102in cell cluster240. Because the new 1×1 cell clusters234,236,238, and240now contain only one cell each, the specific subscriber102has now been located or acquired.

Only 10 beam steps were required in this example compared to 30 steps in the example ofFIG. 1, thus reducing the total dwell time by 67 percent. On average, a dwell time of only 8.5T (based on a minimum of four and a maximum of 13 beam steps) is required for the approach used in this example compared to 32T in the approach ofFIG. 1, nearly four times faster.

Another advantage of the method200ofFIG. 2is that it may be implemented with existing ground-based digital beamformers simply by adding software using standard beamforming functions and a beam zooming method, such as the one described below. As an example, the ground-based beamformer in a CDMA communications system gateway hub122inFIG. 1can perform the zooming and scanning of the beam214.

FIGS. 3A–3Care beam plots illustrating a method300for zooming a beam that may be used with the embodiment illustrated inFIG. 2. InFIG. 3A, individual beams302,304,306, and308each have a width angle θ that corresponds to the area of one of the plurality of cells106inFIG. 2. InFIG. 3B, the beams302and304are added to form a zoomed beam310, and the beams306and308are added to form a zoomed beam312. Each of the zoomed beams310and312has twice the width angle of the individual beam302, thus corresponding to a cell cluster size of 2×2, or four cells.

InFIG. 3C, the zoomed beams310and312are added to form a zoomed beam320that has four times the width angle of the individual beam302, thus corresponding to a cell cluster size 4×4, or 16 cells. Other beam combinations may be used according to this method to zoom beams to correspond to the areas of various cell cluster sizes.

FIG. 4is a diagram illustrating the method illustrated inFIG. 2further enhanced by a traffic model. A further advantage of beam zooming is realized by adapting to a real-time communications traffic pattern that may have a diurnal cycle and differ from zone to zone within the coverage area. Traffic models may be updated, stored, and accessed in the traffic model by a ground-based beamformer to further reduce the acquisition time. The “spokes and hub” architecture is well suited to the use of traffic models because cell size and search strategy based on traffic conditions and models may be updated at the gateway hub without passing information back and forth between the gateway hub and the transponder platform, thus conserving valuable communications resources.

Special events may have a great influence on the traffic model. For example, an initial 2×2 cell cluster402that is smaller than the coverage area104may be defined for a known high traffic area stored in the traffic model that contains only four 1×1 cell clusters416,418,420, and422. The beam202is zoomed directly to the beam414that corresponds to the area of the cell cluster402, and a locating signal is received from the specific subscriber102that confirms that the specific subscriber102is in one of the cells106included in the initial cell cluster402. The initial cell cluster402is partitioned into four new 1×1 cell clusters416,418,420, and422, and the beam414is zoomed to the beam415that corresponds to the area of one of the four new 1×1 cell clusters416,418,420, and422. The beam415is scanned sequentially until the specific subscriber102is acquired in the cell cluster420.

Based on the traffic model, there is a high probability that the position of the specific subscriber102is in the high traffic area defined by the cell cluster402, which is much smaller than the entire coverage area104. The reduced initial cluster size advantageously reduces the size of the initial cell cluster and consequently the total dwell time even further than the method illustrated inFIG. 2. Alternatively, a sequential search using mixed cell cluster sizes may be used if only one acquisition code address for subscriber102(e.g., CDMA systems) is used. If two acquisition code addresses are used for subscriber102, the high traffic area may be allocated a dedicated acquisition code and two searches may be conducted in parallel using two beams. One beam would be used to perform the search described with reference toFIG. 2, and the other beam would be used to perform the search described with reference toFIG. 4. If the position of subscriber102is successfully predicted by the traffic model, the latter search would be terminated before the former, thus reducing the acquisition time by the fastest method.

The traffic model allows the acquisition of the specific subscriber102to be adapted to real-time traffic conditions stored in user position files residing in the gateway station or hub122. In communications systems having a beamformer in the transponder platform, the traffic model information would have to be transmitted back and forth between the gateway and the transponder platform, which would actually increase the time delay for acquiring the specific subscriber102. In a ground based beamforming system, however, the traffic model may advantageously be implemented in the ground station without the delay of relaying the traffic model information to the transponder platform.

FIG. 5is a block diagram of an exemplary ground-based beamformer500that may be used to implement the methods illustrated inFIGS. 2 and 4. The ground-based beamformer500includes a ground station52and a stratospheric transponder platform54.

Shown in the ground station52are a data processor506that interfaces in this example with communications traffic504to and from internet service providers502, a traffic model module508, a digital beamformer509, beam signals (1-N)510, element signals (1-M)512, a CDMA multiplexer/demultiplexer514, CDMA signals515, A C- or X-band RF subsystem516, and a feeder link518.

The data processor506performs multiplexing, demultiplexing, routing, and formatting of beam signals510according to well-known techniques. The data processor506is coupled to the digital beamformer509and includes the functions of beam zooming and scanning beams in raster sequence as described above and illustrated inFIG. 2.

The traffic model module508coupled to the data processor506may be included for storing and updating traffic models as described above to further improve subscriber acquisition time as illustrated inFIG. 4. The traffic model module508includes the user position files and real-time traffic conditions as described forFIG. 4.

The beam signals510are received as input to the digital beamformer509when transmitting to the subscribers534or generated as output from the digital beamformer509to the data processor506when receiving signals from the subscribers534. The digital beamformer509receives as inputs or generates as outputs element signals512corresponding to the beam signals510. The digital beamformer509may be implemented using well-known techniques. A code division multiple access (CDMA) mux/demux514multiplexes/demultiplexes the element signals512as described above to/from a C- or X-band RF subsystem516according to well-known techniques. The C- or X-band RF subsystem516inputs/outputs CDMA signals515and transmits/receives C- or X-band signals517to/from a feeder link518that links the element signals512between the ground station52and the stratospheric transponder platform54.

The stratospheric transponder platform54includes a feeder link522, a C- or X-band RF subsystem524, and a CDMA mux/demux526that may be implemented according to well known techniques as described above. An S-band RF subsystem530amplifies element signals (1-M)528for transmitting/receiving by an antenna array532to/from the subscribers534on the beams108. The operation of antenna array532is assumed to be reversible between transmit and receive modes, thus the beamforming method of this example applies both to transmitting and receiving signals. The stratospheric transponder platform54may be, for example, a communications satellite, an unmanned aircraft, or an antenna tower.

For the user acquisition function, the stratospheric transponder platform54and the gateway hub52only receive the locating signal and do not transmit any signals to the subscriber534being acquired. Thus, inFIG. 5, the locating signal originates from the subscriber534being acquired and is relayed by the stratospheric transponder platform54to the gateway hub52. When receiving the locating signal from the subscriber534being acquired, the inputs to the digital beamformer are the element signals512and the outputs are the beams510. One or two of the beams may be used for user acquisition. The zooming and scanning functions may be performed mathematically in the digital beamformer in two steps: (1) multiplying the M element signals by an appropriate set of M weights represented as complex numbers, and (2) summing the M weighted signals to determine the output beam signal. The sets of complex weights may be computed using well known techniques.

FIG. 6is a flow chart600of the method illustrated inFIG. 2for rapid user acquisition for stratospheric transponder platforms by beams formed by the ground-based beamformer ofFIG. 5. Step602defines a coverage area as an arrangement of cells such as that shown inFIG. 2. Step604defines an initial cell cluster that includes the cell containing the specific subscriber. The initial cluster may be selected as the entire coverage area or a portion of the coverage area determined by a traffic model as explained above. Step606zooms the beam to form a beam that corresponds to an area of one of the cell clusters. Step608scans the beam to the cell cluster that includes the specific subscriber. Step610partitions the cell cluster that includes the specific subscriber to define a new partition of cell clusters. Step612repeats steps606,608, and610until the partition contains only one cell, i.e., the cell containing the specific subscriber.

Other modifications, variations, and arrangements of the present invention may be made in accordance with the above teachings other than as specifically described to practice the invention within the spirit and scope defined by the following claims.