Implementation of a Two-Step SOVA Decoder with a Fixed Scaling Factor

Taek-Won KWON  Jun-Rim CHOI  

Publication
IEICE TRANSACTIONS on Communications   Vol.E86-B   No.6   pp.1893-1900
Publication Date: 2003/06/01
Online ISSN: 
DOI: 
Print ISSN: 0916-8516
Type of Manuscript: PAPER
Category: Wireless Communication Technology
Keyword: 
Viterbi,  SOVA,  soft output Viterbi algorithm,  turbo decoder,  scaling factor,  frame size,  

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Summary: 
Two implementation schemes for a two-step SOVA (Soft Output Viterbi Algorithm) decoder are proposed and verified in a chip. One uses the combination of trace back (TB) logic to find the survivor state and double trace back logic to find the weighting factor of a two-step SOVA. The other is that the reliability values are divided by a scaling factor in order to compensate for the distortion brought by overestimating those values in SOVA. We introduced a fixed scaling factor of 0.25 or 0.33 for a rate 1/3 and designed an 8-state Turbo decoder with a 256-bit frame size to lower the reliability values. The implemented architecture of the two-step SOVA decoder allows important savings in area and high-speed processing compared with the one-step SOVA decoder using register exchange (RE) or trace-back (TB) method. The chip is fabricated using 0.65 µm gate array at Samsung Electronics and it shows higher SNR performance by 2 dB at the BER 10-4 than that of a two-step SOVA decoder without a scaling factor.