Earth-fixed cell beam management for satellite communication system

• US 5,408,237 A
• Filed: 07/08/1993
• Issued: 04/18/1995
• Est. Priority Date: 11/08/1991
• Status: Expired due to Term

• Current Assignee: Wengen Wireless LLC (Intellectual Ventures LLC)
• Original Assignee: Teledesic, LLC (Michael J. Larson, MBA)
• Inventors: Sturza, Mark A., Patterson, David P.
• Primary Examiner(s): Issing, Gregory C.
• Application Number:
  US08/088,714
• Time in Patent Office: 649 Days
• Field of Search: 342/354, 342/357, 342/368, 455/12.1, 455/13.3, 379/58
• US Class Current: 342/354
• CPC Class Codes: H01Q 1/288 Satellite antennas
• CPC Class Codes: H01Q 15/02 Refracting or diffracting devices, e.g. lens, prism
• CPC Class Codes: H01Q 25/008 lens fed multibeam arrays
• CPC Class Codes: H01Q 3/24 varying the orientation by switching energy from one active radiating element to another, e.g. for beam switching
• CPC Class Codes: H04B 7/1851 Systems using a satellite or space-based relay H04B7/18508, H04B7/18521 take precedence; providing specific services H04B7/18523 - H04B7/18576
• CPC Class Codes: H04B 7/18532 Arrangements for managing transmission, i.e. for transporting data or a signalling message
• CPC Class Codes: H04B 7/18541 for handover of resources
• CPC Class Codes: H04B 7/2041 Spot beam multiple access

Abstract

Earth-fixed cell beam management methods which may be employed to allocate beams generated by a constellation of low Earth orbit satellites (12) flying in orbits below geosynchronous altitudes are disclosed. These beams (19) are electronically steered so that they illuminate "Earth-fixed cells" (26) as opposed to "satellite-fixed cells." In a system that employs satellite-fixed cells, the "footprint" of the beams propagated by a spacecraft defines the zone on the ground called a "cell" which is illuminated by the spacecraft. This satellite-fixed cell moves constantly as the spacecraft moves around the globe. In sharp contrast, an "Earth-fixed cell" (26) is a stationary region mapped onto the surface of the Earth (E) that has permanent fixed boundaries, just like a city or a state. Although the rapidly moving satellites (12) still shine their beams over the ground in rapidly moving footprints (16), the locations of the footprints at any given time do not determine the location of the unchanging Earth-fixed cells (26). The great advantage provided by using cells having boundaries that are fixed to an Earth-fixed grid (20) is realized when a subscriber being served by one satellite must switch to another beam in the same satellite or to a second satellite because the first is moving out of range below the local horizon. With satellite-fixed cells, this "hand-off" involves the assignment to the terminal of a new communication channel within the new beam or new satellite. This assignment process takes time and consumes processing capacity at both the terminal and the satellite. It is also subject to blocking, call interruption, and call dropping if there is not an idle communication channel in the next serving beam or satellite. The Earth-fixed cell method avoids these problems by allocating communication channels (frequency, code, and/or time slot) on an Earth-fixed cell basis rather than on a satellite-fixed cell basis. Regardless of which satellite/beam is currently serving a particular cell, the terminal maintains the same channel assignment, thus ameliorating the "hand-off" problem.

Claims

1. A method for allocating a plurality of beams (19) transmitted from and received at positions in Earth orbit for communicating with a plurality of portable (P), mobile (M) and fixed (F) terminals and gateways (G) comprising the steps of:

forming a plurality of footprints (16) using a plurality of electronically steered, antenna elements (13a, 13b and 13c) which generate said plurality of beams (19);

said plurality of antenna elements (13a, 13b and 13c) being carried onboard a plurality of satellites (12) flying in orbits (11) below geosynchronous altitude;

one of said plurality of satellites (12) including a first serving satellite (12a) and a second satellite (12b) positioned next to said first serving satellite (12a);

said plurality of footprints (16) illuminating portions of an Earth-fixed grid (20) with said plurality of beams (19);

said plurality of beams (19) being capable of conveying a plurality of packets (Pk);

said Earth-fixed grid (20) defining a plurality of Earth-fixed supercells (24);

said plurality of Earth-fixed supercells (24) including a plurality of Earth-fixed cells (26);

said plurality of Earth-fixed cells (26) including a target Earth-fixed cell (26t);

tracking a target Earth-fixed cell (26t) by using a sequence of individual antenna elements (13a, 13b, 13c) aboard said first serving satellite (12a) so that said plurality of beams (19) are maintained over said target Earth-fixed cell (26t) as long as one of said plurality of individual antenna elements (13a, 13b, 13c) aboard said first serving satellite (12a) is capable of serving said target Earth-fixed cell (26t);

measuring the distance from said first serving satellite (12a) which is currently serving said target Earth-fixed cell (26t) to the centroid of said target Earth-fixed cell (26t);

measuring the distance from said second satellite (12b) which is the next closest satellite to said target Earth-fixed cell (26t) compared to said first serving satellite (12a) to the centroid of said target Earth-fixed cell (26t); and

switching responsibility for providing said plurality of beams (19) to said target Earth-fixed cell (26t)

from said first serving satellite (12a) to said second next satellite (12b)

when the distance from said second next satellite (12b) to the centroid of said target Earth-fixed cell (26t) is less than

the distance from said first serving satellite (12a) to the centroid of said target Earth-fixed cell (26t).

2. An apparatus for communicating among plurality of portable (P), mobile (M) and fixed (F) terminals and gateways (G) comprising:

a plurality of satellites (12) operating in low Earth orbit;

each of said plurality of satellites (12) having a plurality of electronically steered antennas (13);

said plurality of electronically steered antennas (13) being capable of generating a plurality of beams (19);

said plurality of satellites (12) flying in orbits (11) below geosynchronous altitude;

said plurality of electronically steered antennas (13) being capable of forming a plurality of footprints (16); and

an Earth-fixed grid (20) mapped over the surface of the Earth (E);

said plurality of footprints (16) illuminating portions of said Earth-fixed grid (20) with said plurality of beams (19);

said plurality of beams (19) being capable of conveying a plurality of packets (Pk);

said Earth-fixed grid (20) defining a plurality of Earth-fixed supercells (24);

said plurality of Earth-fixed supercells (24) including a plurality of Earth-fixed cells (26);

said Earth-fixed grid (20), said Earth-fixed supercells (24) and said Earth-fixed cells (26) being used to allocate said plurality of beams (19) to provide communications services to said plurality of portable (P), mobile (M) and fixed (F) terminals and gateways (G) .