Design of a Modified Bi-Directional SOVA

In this paper, a directional bi-directional Soft-Output Viterbi algorithm (SOVA) based Turbo decoder architecture is proposed. By performing an extra pass through the decoder trellis, effectively bi-directional decoding, decoder performance can be improved. The bi-directional decoding results in a 60% increase in operations over the conventional SOVA decoder, while incurring no significant increase in architecture size. The paper also proposes the use of extrinsic information in selecting the SOVA algorithm, resulting in improved decoding performance. The use of the extrinsic information as a reliability measure does not introduce any inherent latency and requires a negligible increase in hardware. Following both architecture proposals, the paper discusses practical implementation using finite precise arithmetic. Simulation results show that by combining both proposed techniques, an extra coding gain of 0.4dB can be obtained over the conventional SOVA decoder in an Additive White Gaussian Noise (AWGN) channel. Key–Words: Information rate, adaptive modulation, adaptive coding, fading channels, concatenated coding