Multiplex communications – Communication over free space – Repeater
Reexamination Certificate
1999-06-17
2004-04-20
Tran, Cong Van (Department: 2683)
Multiplex communications
Communication over free space
Repeater
C370S318000, C455S003020, C455S013200
Reexamination Certificate
active
06724737
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of satellite communication systems, and, in particular, to systems and methods of controlling the bit error rate of transmissions from a terminal to a satellite in order to compensate for rain fade and the like.
2. Description of Related Art
Typical satellite communications systems used to connect remote terminals to other such terminals such as cellular phone systems, or to connect broadcast control center to individual terminals such as used in TV satellite broadcasting, or to control earth observation satellites, all require means to compensate for radio link rain fade and the like. There have been numerous systems for compensating for changes in transmission conditions between a remote terminal and the satellite. An early example can be found in NASA's ACTS System. In the ACTS System rain fade is estimated at the ground station by monitoring three separate beacons on board the satellite and forward error correction (FEC) rate is adjusted accordingly. However, such a system is only suitable for experimental satellite communication systems, for it takes up to much of the satellites available assets and is not adapted for use with remote terminals. U.S. Pat. No. 5,511,079 “Apparatus And Method For Controlling Forward Error Correction Encoding In A very Small Aperture Terminal” by D Dillon a system and process that controls the FEC of a VSAT earth station in order to compensate for signal loss between the VAST and hub station due to rain fade and the like. In detail, the hub earth station is adapted to measure the echo signal produced by the satellite's transponder in response to a transmission therefrom. The hub station is further capable of generating a signal that is transmitted to the VSAT earth stations that is indicative of the propagation conditions at the hub. The VSAT earth station is capable of receiving this signal and in response thereto determines the proper FEC in accordance with the propagation conditions both locally at the VSAT and remotely at the hub. In other words, the VSAT slows or speeds up the data transmission rate depending upon propagation conditions. However, such a system is designed for use with a VSAT that does not have means to measure the quality of a received or transmitted signal. Furthermore, it is not usable where the satellite incorporates on board processing and the uplink and downlink signals are decoupled.
U.S. Pat. No. 5,721,742 “Terminal, Network, And Communication System” by I. Okajima, et al. discloses a communication system where the data for transmission is FEC encoded based on the type of message and not on the propagation conditions at any point along the link. The decision to apply an FEC is made solely in the VSAT based on a predetermined message type (length) criteria. Information is included in the sent message to allow the receiving terminal to correctly decode the message. Here no attempt is made to compensate for atmospheric conditions, such as rain fade.
U.S. Pat. No. 4,047,151 “Satellite Communications System And Apparatus” by P. Baran relates to communication systems where the data for transmission is encoded in a spread spectrum scheme that allows gradual consumption of resources in response to propagation conditions. However, no mechanism for obtaining propagation conditions is provided.
U.S. Pat. No. 4,261,054 “Real-time Adaptive Power Control In Satellite Communications Systems” by H. Scharia-Nielsen discloses a transponded communication systems wherein means are provided to adjust the sending VSATs uplink power based on another VSAT's reported received detected decode errors. This adaptive power control (not adaptive FEC) is based on the summation of the propagation conditions between the sender and the monitoring VSAT (one or both of which may be in rain) and may not be representative of the sender-to-receiver VSAT path. It does not allow the VSAT to be its own link quality monitor and allowing power and/or FEC adaptation based solely on its VSAT-to-satellite propagation conditions.
U.S. Pat. No. 5,335,249 “Method And Apparatus For Spread Spectrum Communications” by T. E. Krueger, et al. discloses a communications system that includes means to select between using narrow band or spread spectrum for a link between a sender and receiver, and to optimize the spread spectrum parameters to meet the link reliability required. The method establishes a link and selects the “best” parameters. When the link degrades below the reliability threshold the process is reset to begin again. However, this invention does not use a dynamic adaptive coding/power process in response to changing link propagation conditions.
U.S. Pat. No. 5,603,096 “Reverse Link, Closed Loop Power Control In A code Division Multiple Access System” by K. S. Gilhousen, et al. discloses a communications systems where the receiver measures the sender's power and compares it to a stored expected value. The receiver then instructs the sender to increase/decrease its transmitter level to stay within a given received power level window. This invention does use received power measurements but does not provide for the sender to determine its own power needed in response to local link propagation conditions.
U.S. Pat. No. 4,309,764 “Technique For Increasing the Rain Margin Of A Satellite Communication System” by A. Acampora applies to transponded communications systems where a VSAT uses an unspecified mechanism to adjust its uplink power to overcome rain fades and notifies the Hub to notify the sender to use additional return path resources to overcome a downlink fade. The Hub controls the assignment of resources. In this system use of additional assets to counter rain fades is applied to both VSAT-to-satellite links, even if only one link is experiencing a rain fade.
U.S. Pat. No. 5,768,684 “Method And Apparatus For Bi-Directional Power Control in A Digital Communication System” by J. E. Grubb, et al. discloses a processing satellite system where the receiving nodes (including the satellite) measure the received power level from a sender using an estimated bit error rate. This received power level is then sent back to the sender such that the sender can adjust its transmitting power level to match the link propagation conditions. The sender does not perform its own uplink and downlink power level measurements and does not include adaptive coding to increase the overall link margin dynamic range.
U.S. Pat. No. 5,699,365 “Apparatus And Method For Adaptive Forward Error Correction In Data Communications” by J. T. Klayman, et al. discloses a communications system where the receiver monitors the FEC error results from messages sent to it. When the errors reach a threshold, the receiver informs the sender to use a more/less robust FEC. However, the same FEC is employed from the sender to the receiver, and this wastes band width on portions of the link not needing a robust FEC.
Therefore, what is needed is an active closed loop adaptive coding system between the satellite and the VSAT without using any Hub or network control center (NCC) measurement data in the process. Furthermore, it is also important to permit the adjusting of both the VSAT's uplink and downlink FEC rates and uplink power to meet propagation conditions local to the VSAT-to-satellite link only, thus allowing higher throughput in both directions without using increased satellite resources. The NCC's local propagation conditions should remain immaterial and only independently operated on its own Hub-to-satellite link.
Thus, it is a primary object of the invention and method of providing a terminal to satellite communication system that can compensate for changes in propagation conditions therebetween.
It is another primary object of the invention to provide a system and method of providing an adaptive coding system for insuring error free communications between a satellite and remote terminal when propagation conditions therebetween deteriorate.
It is a further object of the invention t
Boyden Albert K.
Lee Chester P.
Lockheed Martin Global Telecommunications, INC
Tran Cong Van
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