Digital Signal Processing 18 (2008) 189–208 www.elsevier.com/locate/dsp Implementation of a reconfigurable turbo decoder in 3GPP for flat Rayleigh fading Costas Chaikalis Technological Educational Institute (TEI) of Lamia, Department of Electrical Engineering, 3rd km Old National Road, Lamia, Greece Available online 18 April 2007 Abstract Quality of service is a critical consideration in the design of mobile systems, since it allows the user to receive high quality services. Therefore, in 3GPP systems, in order to realise a particular service, the quality of service requirements in terms of perfor- mance and latency, have to be satisfied. Turbo code features include parallel code concatenation, recursive convolutional encoding, nonuniform interleaving and an associated iterative decoding algorithm. Exploiting the quality of service classification according to the priority of latency or performance, possible examples of service scenarios are examined for flat Rayleigh fading channels with emphasis on the turbo decoding algorithm. Particularly, for two operating environments considering SOVA and log-MAP algorithms due to their data-flow similarities, this paper shows that SOVA is clearly optimal for most of the real-time applications, whereas for nonreal time applications with low data rate and small frames log-MAP is preferred. The use of the optimum algorithm in most scenarios results in a more efficient turbo decoder: applications that otherwise would have failed now can be realised.  2007 Elsevier Inc. All rights reserved. Keywords: Mobile communications; Turbo codes; SOVA; Log-MAP; Flat Rayleigh fading 1. Introduction Turbo codes were introduced in [1] and correspond to a powerful channel coding technique, which outperforms all previous known techniques. The performance of turbo codes is just 0.5 dB away from the Shannon limit and they can be applied in any communication system where a significant performance improvement is required or the operating signal-to-noise ratio (SNR) is very low. In fact, low density parity check (LDPC) codes, which were discovered in the early 60’s, can also approach Shannon capacity but this was shown after the discovery of turbo codes. The third generation partnership project (3GPP) belongs to the third generation (3G) of mobile communication systems. Due to its increased efficiency and capability compared to second generation systems, like the successful global system for mobile (GSM), it is expected to dominate the mobile communications market in the next few years. Turbo codes have been adopted as a channel coding scheme in 3GPP systems for data rates higher or equal to 28.8 kbps. They have also been shown to provide impressive coding gains in flat fading channels in the presence of outer block interleaving [2,3]. The soft-input/soft-output (SISO) decoder is the most important element of a turbo decoder. The two main can- didate algorithms to be used in a SISO decoder are the soft output Viterbi algorithm (SOVA) and the log maximum E-mail address: c.chaikalis@ieee.org. 1051-2004/$ – see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.dsp.2007.04.002