Wireless telecommunications system having airborne base station

A wireless telecommunications system with a base station positioned on an airborne platform is provided. The system utilizes a ground-based wireless switching center which communicates via a microwave link with a plurality of airborne base stations positioned above a geographic area being serviced by the telecommunications system. The base stations are supported on an airborne platform having a propulsion system designed to maintain the platform and the base stations in position above the geographic area being serviced. Advantageously, the platform may be positioned in the stratonull which is generally very stable and is located above the troposphere, wherein weather variations occur.

BACKGROUND
 1. Technical Field
 The present disclosure relates generally to telecommunications and more
 particularly to a wireless telecommunications system.
 2. Description of the Related Art
 FIG. 1 depicts a schematic diagram of a portion of a typical wireless
 telecommunications system designated generally as 10. System 10 serves a
 number of wireless terminals 22 and 24 that are situated within a
 geographic area. System 10 comprises wireless switching center 12 that is
 connected to a number of base stations 14.sub.i and that is also coupled
 to local and long distance telephone networks 16. Wireless switching
 center 12 is responsible for, among other things, routing or "switching"
 calls from and to wireless terminals or, alternatively, between a wireless
 terminal and a wireline terminal connected to wireless system 10, via
 local and/or long distance telephone networks 16.
 The geographic area serviced by wireless system 10 is partitioned into a
 number of spatially distinct areas called cells. As depicted in FIG. 1,
 each cell 20.sub.i is schematically represented by a hexagon. In practice,
 however, each cell 20.sub.i usually has an irregular shape that depends,
 for example, on the topography of the terrain serviced by system 10.
 Typically, each cell 20.sub.i contains a corresponding base station
 14.sub.i. Base station 14.sub.i comprises antennas and radios to
 communicate with wireless terminals 22 and 24. Each base station 14.sub.i
 also comprises transmission equipment to communicate with wireless
 switching center 12.
 In designing system 10, engineers allocate a limited number of frequency
 channels to each base station 14.sub.i using well known techniques. Base
 stations 14.sub.i communicate with wireless terminals over these frequency
 channels. Thus, the number of base stations limits the potential capacity
 of system 10 for processing calls to and from wireless terminals.
 As wireless telecommunications has increased in popularity, designers have
 developed techniques for increasing the capacity of these wireless
 telecommunications systems. One common technique is to reduce the size of
 each cell by "splitting" existing cells into multiple cells. Some
 designers have even suggested reducing the size of cells to provide so
 called "microcells" that service very small geographic regions.
 The public has resisted use of microcells for several reasons. First, many
 people fear that electromagnetic radiation from base stations could cause
 health problems. Further, real estate in populated areas is typically
 expensive and accounts for about half the cost of the base station. Thus,
 it is a difficult and expensive proposition to split cells to increase
 capacity of a wireless telecommunications system.
 SUMMARY
 In an exemplary embodiment, a telecommunications system uses airborne
 platforms that are adapted to support wireless telecommunications systems
 components. The airborne platforms are maintained in position relative to
 the ground above a geographic region of the earth to which the wireless
 telecommunications service is to be provided.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 FIGS. 2 and 3 illustrate exemplary embodiments of a wireless
 telecommunications system. It is to be understood that these wireless
 telecommunications systems are intended to encompass any wireless
 communications system utilizing a wireless communication terminal, e.g.,
 cellular, personal communication system (PCS), etc.
 FIG. 2 illustrates wireless telecommunications system 200. Wireless
 telecommunications system 200 includes wireless switching center 212 that
 routes or switches calls to and from wireless terminals. Wireless
 switching center 212 includes a central computer which substantially
 controls operations of communications system 200, including monitoring
 calls, billing, and handoffs.
 In system 200, base stations 214.sub.i are supported on platforms
 226.sub.i. Each platform 226.sub.i has a propulsion system to maintain the
 platform airborne in position relative to the ground above the geographic
 area serviced by system 200. As used herein, the term "airborne" means
 carried by or through the air and includes flight within the atmosphere,
 i.e., flight from the troposphere to the edge of the thermosphere. This is
 in contrast to "spaceborne" which includes flight beyond the atmosphere of
 the earth. Platform 226.sub.i is advantageously positioned at an altitude
 not routinely occupied by military or commercial air traffic. Ideally,
 platform 226.sub.i is located in the stratonull within the stratosphere 15
 to 20 miles above sea level. The stratonull is advantageous because it is
 generally very stable and is located above the troposphere where weather
 phenomena affecting the environment take place. Platform 226.sub.i may
 include a vehicle capable of maintaining a substantially stationary
 position above the earth while supporting at least one base station
 214.sub.i. Such vehicles include lighter-than-air vehicles and high
 altitude aircraft. Specific vehicles are described below.
 Base station 214.sub.i communicates with wireless switching center 212 via
 microwave link 240.sub.i, operating, for example, in the range of 28 to 38
 GHz. Because of the location of base station 214.sub.i, wireless switching
 center 212 may be located in a rural area R where real estate cost and
 public resistance are minimal. Microwave link 240.sub.i performs the same
 function as land line 15.sub.i does in conventional wireless
 telecommunications system 10. However, positional control information may
 also be communicated to the platform propulsion system via microwave link
 240.sub.i using well known techniques. Communication between base stations
 214.sub.i via microwave link is also envisioned. In a geographic area
 having no long distance telephone network, such as some third world
 countries, inter-base station communication may be used to provide long
 distance telecommunications.
 Each base station 214.sub.i comprises high gain, narrow beam antennas which
 emit narrow beams in assigned ranges, for example, of 824 to 894
 MHz(cellular) or 1900 to 2000 MHz (PCS) to define cells 220.sub.i. The
 narrow beam antennas may be helix array antennas such as those used on
 Navstar global position satellites. Helix array antennas are discussed in
 Antennas, 2d, John D. Kraus, pages 270 and 271, which is incorporated by
 reference. Narrow beams facilitate a more uniform distribution of low
 level RF power reducing radiation levels in populated areas. Because of
 the location of the antennas, typical problems such as multipath fading,
 delay spread, doppler, overload and interference are also greatly reduced.
 FIG. 3 shows an embodiment of the present invention in which the platform
 supporting base station 314 is high altitude balloon 326. High altitude
 balloon 326 may be a balloon, such as disclosed in J. J. Vorchek, U.S.
 Pat. No. 3,746,282 issued Jul. 17, 1973 and J. L. Rand et al., U.S. Pat.
 No. 5,104,059 issued Apr. 14, 1992, both incorporated by reference. These
 high altitude balloons are capable of maintaining altitudes greater than
 120,000 feet above sea level for periods of time exceeding several months
 while supporting a payload.
 High altitude balloon 326 is provided with a streamlined body to facilitate
 horizontal movement while minimizing the effect of winds on balloon 326.
 Balloon 326 includes propulsion system 332 which may be in the form of
 propeller 334 driven by an electric motor. A power supply, such as solar
 cell array 336, is positioned on the balloon envelope to supply energy
 during daytime to drive propulsion system 332 and to charge batteries 338.
 Batteries 338 supply energy to propulsion system 332 during nighttime
 flight. Although balloon 326 is disclosed as having propulsion system 332
 including propeller 334 driven by an electric motor, an appropriate
 propulsion system capable of maintaining balloon 326 in a substantially
 stationary position over the earth may be used, e.g., jet engine, rocket
 engine, ion engine, etc.
 As shown in FIG. 3, an RF signal transmitted by wireless terminal 322 is
 received by base station 314 defining the cell from which the RF signal
 was transmitted. The RF signal is converted to microwave at base station
 314 using modulation and demodulation techniques and base station 314
 communicates via microwave link 340 with wireless switching center 312.
 The RF signal is converted to microwave via block conversion which allows
 a single wireless switching center 312 to handle all of the processing for
 hundreds of base stations. If the signal is directed to another wireless
 terminal, wireless switching center 312 returns the signal along with
 routing data to base station 314 via microwave link 340. Thereafter, the
 signal is routed, according to the routing data, to the appropriate
 wireless terminal. If the signal is directed to a wireline terminal,
 wireless switching center 312 will route the call to the appropriate
 wireline terminal.
 FIG. 4 illustrates wireless telecommunications system 400. Each platform
 426.sub.i supporting base station 414.sub.i in wireless telecommunications
 system 400 comprises an aircraft, preferably configured for long range
 operation, Such as a 747-type aircraft. Several aircraft may be rotated in
 shifts to service a geographic area. Each shift may be extended by
 airborne refueling of aircraft 426.sub.i. Using conventional techniques,
 base station 414.sub.i on each aircraft 426.sub.i executes a handoff to
 base station 414.sub.r on relief aircraft 426.sub.r. Handoffs are
 coordinated by wireless switching center 412 via microwave link 440.sub.i.
 Subsequently, all communication sessions with wireless terminals would be
 handled by relief aircraft 426.sub.r.
 Wireless telecommunications system 400 may provide temporary communications
 within a geographic area in which telecommunications capability has been
 lost. Wireless telecommunications system 400 may also be used to provide
 emergency telecommunications within a geographic area having no
 telecommunications capability. Aircraft 426.sub.i may be flown in a
 substantially circular pattern at any altitude above the geographic area
 being serviced.
 It is to be understood that the above description presents illustrative
 embodiments only. Numerous other arrangements may be devised by one
 skilled in the art without departing from the scope of the invention.