USE OF FORWARD ERROR CORRECTION IN A GROUND-TO-AIR-TO-GROUND TELEMETRY AND CONTROL LINKS
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AbstractA forward error correcting (FEC) codec/interleaving system has been developed to improve the signal to noise ratio of a full duplex channel composed of a low rate command and control uplink and a high rate telemetry downlink. The system includes two MIL spec airborne 20 square inch cards and a ruggedized modified LINKABIT standard LV7017HS Convolutional Encoder Viterbi Decoder Ground Unit. The coding parameters used are code rate 1/2, constraint length K=7, 8 levels of soft decision and data rates of up to 20 Kbps for the command link and up to 10 Mbps in the telemetry downlink. Since Viterbi Decoders work best on memoryless Gaussian Noise channels, interleaving is used to randomize the effects of fading, jamming or other bursty types of noise making them appear like white Guassian additive noise to the decoder. A coherent binary PSK modem will ideally provide 10^-5 bit error rate (BER) with Eb/No = 9.6 dB (bit energy per noise ratio). The use of the FEC codecs in the sort decision mode achieves the same BER with 5 dB lower Eb/No, meaning either an increase of the communication range with the same RF equipment or substantial decrease of RF power or antenna size for the same communication range. The same system could be used in a hard decision mode and still provide a gain of more than 3 dB at BER of 10^-5. The coding gain increases as the required BER decreases. The described system could be easily adapted to any command and control link or telemetry link at the above mentioned data rates.
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