Research On The Stopping Criteria For High Speed Turbo Decoders

Turbo code, a channel coding technique with excellent performance, is adopted in3GPP LTE and LTE-Advanced system. In the standard of LTE-A system, a peaktransmission rate as high as1Gbit/s downlink and500Mbit/s uplink is required. In order tomeet this requirement, we need to improve traditional turbo decoders, design and verifynew decoding algorithms and structures which can meet the high transmission raterequirement of future wireless telecommunication system.Stopping strategy for turbo iterative decoding is a key technique to improve decodingspeed. It saves the average iteration number significantly at the cost of acceptabledegradation in BER(Bit Error Rate) performance, thus making an improvement in iterativedecoding speed. In this paper, we did a research and made a modification on stoppingcriteria of turbo decoders. The S3criterion is selected to be modified since simulationresults for existing stopping criteria show that it outperforms other criteria at high data rateapplication scenario. A new stopping criterion based on S3is proposed in this paper,which uses the statistics value of the soft values from SISO decoders as a metric to judgewhether the iterative decoding process should be terminated. Furthermore, this paper alsoproposed a novel criterion based on cosine similarity. In cosine similarity algorithm, thecosine similarity of extrinsic value sequences from both SISO decoders is monitored andused to determine whether to terminate the iterative decoding process. Simulation resultsshow that both of the newly proposed algorithms can reduce average iteration numbersignificantly when compared with existed stopping criteria, thus improving the decodingspeed of turbo decoders. The S3-modefied criterion is suitable for high bit signal-to-noiseenvironment, and cosine similarity criterion fits low bit signal-to-noise environment. Bothalgorithms proposed by this paper can be applied to high speed turbo decoders accordingto the implementation environment to meet the increasing date rate requirement of moderndigital communication systems.