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Technique for Correcting Symbol Errors in Formatted Modem Transmissions Using Soft Decisions

Technical Description:

This development provides a method for correcting symbol decision errors made by a receiving modem in a formatted modem transmission. Although originally developed specifically for modems which use trellis-coded modulation, this technology is not limited and may be applied to other modulation schemes. Decision errors are usually caused by excessive amounts of noise in a transmission channel. The Viterbi algorithm, an example of dynamic programming, was developed to reduce probability of decision errors by finding the most likely sequence, it is able to find the N most likely sequence in order, where N is some arbitrary value.

Commercial modems have error correction capabilities in some form or another. Trellis coding gain without any bandwidth expansion. Receiving modems determine the most likely sequence that was transmitted by Viterbi decoding. Currently, most modems establish a connection at the highest possible data rate based on channel conditions. If conditions worsen, the modems automatically fall back to a lower rate. On the other hand, if the conditions improve they renegotiate at a higher rate. Adding a more advanced error correction procedure would allow modems to sustain higher data rates for longer periods of time since they would be more tolerable to an increase in channel noise. Additionally, many data link protocols allow for an automatic repeat request (ARC) in the event they received data with errors. Once errors have been detected, the protocol allows the receiving modem to request the erroneous data to be resent by the transmitting side. For this case, this invention would reduce the number of retransmissions allowing higher data throughput.

Commercial Application:

This invention would allow commercial modems to have an enhanced capability of correcting transmission errors at the cost of decoding complexity.

Patent Status:

Issued: United States Patent Number 5,533,033

Released: 1998

Reference Number: Error

If you are interested in exploring this technology further, please call 443-445-7159 or express your interest in writing to the:

National Security Agency
NSA Technology Transfer Program
9800 Savage Road, Suite 6541
Fort George G. Meade, Maryland 20755-6541


Date Posted: Jan 15, 2009 | Last Modified: Jan 15, 2009 | Last Reviewed: Jan 15 2009