Wednesday, May 29, 2019

MIL-STD-188-110 App.C QAM

Recently MIL-STD-188-110 App C  (STANAG 4539) signals were found on a KiwiSDR. This made it possible to complete the corresponding decoder in gr-digitalhf which includes an adaptive channel filter, descrambling, deinterleaving, and convolutional decoding. For the QAM modes descrambling is done by applying an XOR operation on the soft decisions. 

QAM32 constellation obtained after decoding with gr-digitalhf
Using the path metrics of the convolutional decoder it was verified that the whole chain of decoding steps is correct: without puncturing the path metric \(M\) for \(n\) encoded bits at rate \(1/2\) is \(2n\) if there are no errors, so it makes sense to use \(Q=M/2n\) as a measure of data quality. When there is puncturing, e.g., to rate \(3/4\) with puncturing pattern [11, 10], the expected path metric is \(3.5/4\): three out of four bits are transmitted and the probability for the puncture to be correct is \(1/2\). Therefore in this case \(Q\) has to be rescaled by the factor \(4/3.5=8/7\).

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