DIGITALLY DEMODULATING A MINIMUM-SHIFT KEYING (MSK) SIGNAL

摘要

A two stage process is applied for recovering the modulating content from the received I-Q waveforms of a MSK modulated signal. In the first stage, at each incoming symbol the I-Q waveform segments of the input belonging to the three most recently received symbols are used in hypothesis testing. A matched filter bank produces ratings for each of the possible three symbol modulating patterns in proportion to the likelihood that the combination in question may have produced the current but by now impaired input segment. While the three symbol window slides symbol-by-symbol over the input the successive hypothesis tests are not independent as each symbol is involved in three consecutive tests. The dependence thus created lays the foundation and provides the branch metrics for applying the Viterbi algorithm for the determination of the modulating symbols in the second stage.