Reduced-state sequence detection with convolutional codes

Reduced-state sequence detection (RSSD) reduces the state trellis of a channel code by forming the states into classes. States within a class are such that paths into the states lie further than a distance parameter d from each other. An RSSD decoder retains only one survivor per class at each trellis level. The authors apply RSSD to ordinary binary convolutional codes. They first give a class-forming algorithm that finds the greatest reduction. It turns out that no commonly tabulated good code benefits from RSSD. However, RSSD is an effective way to repair weaker codes, such as quick look-in and RCPC codes. Finally, the authors show that RSSD cannot be more efficient than the M-algorithm