An Efficient 4-D 8PSK TCM Decoder Architecture

This paper presents an efficient architecture for a 4-D eight-phase-shift-keying trellis-coded-modulation (TCM) decoder. First, a low-complexity architecture for the transition metric unit is proposed based on substructure sharing. This scheme significantly reduces the required computation without degrading the performance. Then, a new hybrid
T
-algorithm for a Viterbi decoder is developed by applying a
T
-algorithm on both branch metrics (BMs) and path metrics (PMs). TCM encoders usually employ high-rate convolutional codes that yield many more transition paths per state than low-rate codes do. This makes it feasible to purge unnecessary additions by applying the
T
-algorithm on BMs. Applying the
T
-algorithm on BMs instead of PMs allows one to move the ¿search-for-the-optimal¿ operation out of the add-compare-select-unit (ACSU) loop. Hence, the clock speed will not be affected. In addition, by combining the
T
-algorithm on BMs and the
T
-algorithm on PMs, the hybrid
T
-algorithm can reduce the computations required with the conventional
T
-algorithm on PMs by as much as 50%.