Error detection/correction and fault detection/recovery – Pulse or data error handling – Digital data error correction
Reexamination Certificate
1998-03-05
2001-07-17
Decady, Albert (Department: 2133)
Error detection/correction and fault detection/recovery
Pulse or data error handling
Digital data error correction
C370S389000
Reexamination Certificate
active
06263466
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to data communication systems and, more particularly, to digital satellite data communication systems.
BACKGROUND OF THE INVENTION
In recent years the need for global data networking capability has rapidly expanded. In order to meet this need, broadband satellite communication systems have been proposed as an alternative to land-based communication systems. One type of satellite data communication system is described in a variety of U.S. patents assigned to the assignee of this patent application, including U.S. Pat. Nos. 5,386,953; 5,408,237; 5,527,001; 5,548,294; 5,641,135; 5,642,122, and 5,650,788. These patents and other pending applications assigned to the assignee of this patent application describe a satellite communication system that includes a constellation of low-Earth orbit (LEO) satellites that implement an Earth-fixed cellular beam approach to transmitting data from one location on the Earth's surface to another location. More specifically, each LEO satellite has a communication “footprint” that covers a portion of the Earth's surface as a satellite passes over the Earth. The communication footprint defines the area of the Earth within which ground terminals can communicate with the satellite. Located within each footprint are a large number of cells. During the period of time a cell remains within the borders of a satellite footprint, ground terminals located in the cell transmit data to and receive data from the “servicing” satellite. When a satellite reaches the end of its servicing arc, another satellite in orbit is positioned to “service” the Earth-fixed cells previously covered by the satellite reaching the end of its servicing arc. During servicing, the antennas of ground terminals located in the cells continuously point toward the servicing satellite as it moves in orbit and antennas on the satellite point toward the cells during the time period within which the ground terminals in the cells are allowed to transmit data. Other LEO satellite communication systems employ a satellite-fixed beam approach to transmitting data from one location on the Earth's surface to another location.
Regardless of the nature of the LEO satellite communication system, Earth-fixed cell or satellite-fixed beam, data to be sent from one location on the Earth to another location is transmitted from a ground terminal located within a cell to the satellite serving the cell via an uplink data channel. The data is routed through the constellation of LEO satellites to the satellite serving the cell within which the ground terminal of the designated receiver is located. The latter satellite transmits the data to the receiver ground terminal via a downlink data channel. Thus, the constellation of LEO satellites and the ground terminals form a satellite data communication network wherein each ground terminal and satellite forms a node of the network.
In order for a LEO satellite data communication system to be competitive, it must have a wide bandwidth and be of relatively low cost. Low cost requires that the satellites be light in weight and relatively inexpensive to manufacture. One way of keeping satellite weight and cost low is to minimize the complexity of electronic signal processing hardware, and keep data transmission and reception power requirements low. Unfortunately, low data transmission and reception power conflicts with the need for a highly reliable data communication system because it is relatively easy to lose data contained in low-power signals. One way of improving the reliability of low-power data communication signals that is well known in the satellite communication field is to forward error correction (FEC) code the data to be transmitted. See U.S. Pat. Nos. 5,117,427; 5,446,747; and 5,473,601 for examples of FEC coding of digital data signals.
Typically, data transmissions are broken into digital data “packets” each of which include a header and a payload. The header data packets contain address and control information designed to direct the data packets through the satellite constellation to a desired ground terminal. The payload contains the information being transmitted, which is intended for the satellite or the ground terminal or both. A prior approach of transmitting the data involved either completely or partially FEC decoding and then re-encoding the header and payload data at the satellites. Since at least some payload decoding and re-encoding are required on the satellites with this approach, satellite power and complexity requirements are greater than they would be if no payload decoding and re-encoding occur on the satellites. Furthermore, once the satellites are in orbit, changes to the coding scheme are not possible.
The present invention is directed to a LEO satellite data communication system that uses FEC coding in a novel way to minimize power requirements, minimize complexity of the satellites, and maximize reliability, in part by requiring minimal decoding and re-encoding of the payload data on the satellites.
SUMMARY OF THE INVENTION
In accordance with this invention, a low-Earth orbit (LEO) satellite data communication system is provided. Data to be transmitted from one location on the Earth to another location is assembled into digital data packets each of which includes a header and a payload. The header includes address and other control information, and the payload contains the information to be transmitted. A sending ground terminal first separately encodes the header and payload with a suitable outer forward error correction (FEC) code. If desired, the symbols of the resulting header and payload codewords, derived from a single data packet, may be separately interleaved. The optionally interleaved header and payload codewords are separately inner encoded using a suitable FEC code. The thusly concatenated coded, optionally interleaved header and payload codewords are mixed to produce concatenated coded, optionally interleaved data packets. These concatenated coded, optionally interleaved data packets are transmitted to a receiving satellite via an uplink data communication channel. The receiving satellite first demixes the concatenated coded, optionally interleaved data packets to recover the concatenated coded, optionally interleaved header and payload codewords. Then the concatenated coded, optionally interleaved payload codewords are delayed while the concatenated coded header codewords are first inner decoded to remove the inner FEC code. The symbols of the resulting optionally interleaved, outer encoded header codewords are de-interleaved (if necessary) to regenerate the outer encoded header codewords. The header codewords are then further decoded to remove the outer encoding and recover the header bits of each data packet. The recovered header information is used to determine if the delayed concatenated coded, optionally interleaved payload codewords are intended for the receiving satellite, another satellite or a ground terminal. If for the receiving satellite, the resulting concatenated coded, optionally interleaved receiving satellite payload codewords are inner decoded to remove the inner FEC code. Then the symbols of the resulting interleaved, outer encoded receiving satellite payload codewords are de-interleaved (if necessary) to regenerate the outer encoded receiving satellite payload codewords. Next, the receiving satellite payload codewords are further decoded to remove the outer encoding and recover the receiving satellite payload bits of each data packet. Contrariwise, if the header information indicated that the payload codewords are intended for another satellite or for a ground terminal, the resulting concatenated coded, optionally interleaved payload codewords are applied to a hard decision circuit, which forces the payload codewords to take on +1 or −1 values, which represent logical 0 and 1. The recovered header information is then used to route the data packets through the satellite constellation to either another satellite for which they ar
Griep Karl R.
Hinedi Sami M.
Million Samson
De'cady Albert
Johnson Kindness PLLC
O'Connor Christensen
Teledesic LLC
Ton David
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