A probability diversity combiner for digital h.f. transmission

It is shown that the switched a posteriori probability (SAP) detector is equivalent, for dual diversity with binary transmission, to the familiar maximum a posteriori probability (MAP) detector for processing signals in each diversity branch independently, down to baseband level, and followed by combining the baseband signals in the detector to form a reconstituted signal with the lowest possible error rate. The SAP detector is easy to implement, differing from the conventional switched diversity combiner in switching between branches on a bit-by-bit basis (instead of switching to the highest-SNR branch). The SAP and MAP methods are contrasted in their handling of some samples of impulse noise, and several diversity methods are compared in their handling of Gaussian and non-Gaussian noise. The SAP detector is proposed for use in digital links with modems employing 4-level PSK and differential encoding.