This study proposes a simple scaling factor approach to improve the performance of parallel-concatenated convolutional code (PCCC) and serial concatenated convolutional code (SCCC) systems based on suboptimal soft-input soft-output (SISO) decoders. Fixed and adaptive scaling factors were estimated to mitigate both the optimistic nature of a posteriori information and the correlation between intrinsic and extrinsic information produced by soft-output Viterbi (SOVA) decoders. The scaling factors could be computed off-line to reduce processing time and implementation complexity. The simulation results show a significant improvement in terms of bit-error rate (BER) over additive white Gaussian noise and Rayleigh fading channel. The convergence properties of the suggested iterative scheme are assessed using the extrinsic information transfer (EXIT) chart analysis technique.
 C. Berrou, A. Glavieux and P. Thitimajshima, “Near Shannon Limit Error-Correcting Coding and Decoding: Turbo Codes,” Proceedings of the International Conference on Communications, Geneva, 23-26 May 1993, pp. 10641070.
 J. Hagenauer and P. Robertson, “Iterative (“Turbo”) Decoding of Systematic Convolutional Codes with the MAP And SOVA Algorithm,” Proceedings of the ITG Tagung, Codierung für Quelle, Kanal and Ubertragung, Frankfurt, October 1994, pp. 21-29.
 P. Robertson, E. Villebrun and P. H¨Oher, “A Comparison of Optimal and Sub-Optimal MAP Decoding Algorithms Operating in the Log Domain,” Proceedings of the International Conference on Communications, Seattle, 18-22, June 1995, pp. 1009-1013.
 J. Hagenauer and P. Hoeher, “A Viterbi Algorithm with Soft-Decision Outputs and Its Applications,” Proceedings of IEEE Global Telecommunications Conference, Dallas, 27-30 November 1989, pp. 1680-1686.
 C. Berrou, P. Adde, E. Angui and S. Faudeil, “A Low Complexity Soft-Output Viterbi Decoder Architecture,” Proceedings of IEEE International Conference on Communications, Geneva, Vol. 2, 23-26 May 1993, pp. 737740.
 L. Papke, P. Robertson and E. Villerbrun, “Improved Decoding with the SOVA in a Parallel Concatenated (Turbo-Code) Scheme,” Proceedings of the IEEE International Conference Communications (ICC), Dallas, 23-27 June 1996, pp. 102-106.
 C. H. Wang, W. T. Wang and C. C. Chao, “A Unified Structure of Trellis-Based Soft-Output Decoding Algorithms for Turbo Codes,” IEEE Transactions on Communications, Vol. 52, No. 8, 2004, pp. 1355-1366. doi:10.1109/TCOMM.2004.833025
 G. Colavolpe, G. Ferrari and R. Raheli, “Extrinsic Information in Iterative Decoding: A Unified View,” IEEE Transactions on Communications, Vol. 49, No. 12, 2001, pp. 2088-2094. doi:10.1109/26.974255
 C. X. Huang and A. Ghrayeb, “A Simple Remedy for the Exaggerated Extrinsic Information Produced by SOVA Algorithm,” IEEE Transactions on Wireless Communications, Vol. 5, No. 5, 2006, pp. 996-1002. doi:10.1109/TWC.2006.1633352
 S. Papaharalabos, P. Sweeney, B. G. Evans and P. T. Mathiopoulos, “Improved Performance SOVA Turbo Decoder,” IEE Proceedings—Communications, Vol. 153, No. 5, 2006, pp. 586-590. doi:10.1049/ip-com:20050247
 J. Hagenauer, E. Offer and L. Papke, “Iterative Decoding of Binary Block and Convolutional Codes,” IEEE Transactions on Information Theory, Vol. 42, No. 2, 1996, pp. 429-445. doi:10.1109/18.485714
 L. Lin and R. S. Cheng, “Improvements in SOVA-Based Decoding for Turbo Codes,” Proceedings of the IEEE International Conference on Communications (ICC), Montreal, Vol. 3, 8-12 June 1997, pp. 1473-1478.