Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1997-09-05
2001-09-04
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06285481
ABSTRACT:
BACKGROUND
1. Technical Field
This invention relates to free-space laser communication systems, and more particularly to a free-space atmospheric laser communications error control circuit.
2. Background Information
Free-space atmospheric laser communication systems transmit and receive information by means of a light beam that propagates through the atmosphere. When used for air-to-air or air-to-ground communications, such systems pose a number of challenging problems.
One such problem is that over long distances, or over short distances through a turbulent atmosphere, the beam is absorbed, diffracted and refracted, causing such problems as scintillation in the receiver. What the receiver sees is a “twinkling”, like light from a star. The beam can actually disappear and reappear in a millisecond. The disappearance of the beam is called a “dropout”. Such dropouts can disrupt normal communications 20-30 times a second if not controlled. Dropouts can last from one to ten milliseconds. Another form of disruption is attenuation of the beam due to absorption in haze, mist, fog, snow or other weather phenomenon. During such conditions, it is not uncommon to have the signal go from full-amplitude to nothing every few seconds.
The effect on communications of this “channel property” takes two forms: data can be lost and/or data can be corrupted. This is a problem for interfacing a free-space atmospheric laser communication system with modern network systems, which are designed to operate with a fast, reliable, low-error transport media (the physical layer). Accordingly, it is important to try to minimize or eliminate dropouts and to attain very low bit-error-rates. The present invention addresses the problem.
SUMMARY
The invention includes a control circuit that circumvents the problems of scintillation and other atmosphere-induced degradation of signal propagation in a free-space atmospheric laser communication system by transmitting a “signal strength” data stream between each pair of communicating laser transceivers. The signal strength data stream indicates the signal strength of the sending transceiver as actually received by the remote receiving transceiver. If the sending transceiver receives data from the remote receiving transceiver indicating that the signal strength of the sending transceiver has fallen to or below a selected threshold, or if the sending transceiver cannot detect the signal strength data stream, then the sending transceiver suspends transmission of information packets. The sending transceiver resumes transmission of information packets when the indicated signal strength level returns to an acceptable level. This technique prevents transmitting errors during periods of dropouts or low signal level due to scintillation or other causes. However, at all times, the laser communication system attempts to transmit the signal strength data stream between the transceivers even though information packets are not transmitted.
In another embodiment of the invention, the signal strength data stream is used to regulate gain of the transmitter for each transceiver while the signal strength of such transceiver exceeds the selected threshold.
In another embodiment of the invention, the signal strength data stream is monitored by each transceiver and used to predict when the signal strength being received by a remote receiving transceiver will fall below the selected threshold. This embodiment has the advantage that transmission can be suspended before data dropouts can occur, obviating the need to retransmit lost data.
An advantage of the invention is that it reduces errors by not transmitting data during periods of poor signal transmission, which result in a low signal to noise ratio (SNR); bit error rate is partly determined by the SNR of a channel. Another advantage of the preferred embodiment of the invention is that each transceiver can perform gain control for rapidly gyrating transmission amplitudes by using feedback from a remote transceiver.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
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Fish & Richardson P.C.
Pascal Leslie
Trex Communications Corporation
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