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TURBO-CODES AND HIGH SPECTRAL EFFICIENCY MODULATION Stkphane Le Goff, Alain Glavieux and Claude Berrou St6phane Le Goff and Claude Berrou, Integrated Circuits for Telecommunications Laboratory Alain Glavieux, Digital Communication Laboratory TELECOM BRETAGNE, FRANCE TELECOM UNIVERSITY BP 832 - 29285 BREST, FRANCE ABSTRACT This paper presents a new coding scheme based on the association of a turbo-code [l] with a bandwith- efficient modulation. It is shown that the new coding scheme provides a substantial coding gain both on Gaussian channels and Rayleigh channels. On a Gaussian channel, it outperforms 64-state trellis-coded modulation (TCM) by 2.5 dB at the bit error rate (BER) of On a Rayleigh fading channel, it outperforms 64-state TCM optimized for that environment. 1 - INTRODUCTION Turbo-codes, newly introduced by Berrou et a1 [l], are binary error-correcting codes built from the parallel concatenation of two recursive systematic convolutional codes and using a feedback decoder. Simulations over a Gaussian channel, using binary modulations (ZPSK or 4-PSK), have shown that turbo-codes possess an error-correcting capability unmatched at the present time. In order to improve the transmission spectral efficiency, it is interesting to combine turbo-codes with a bandwith-efficient modulation. In this paper, we investigate in particular combining a turbo-code of rate R with QAM constellations, over Gaussian and Rayleigh channels. This approach can be considered as an alternative to the conventional Trellis Coded Modulation (TCM). It is generally simpler to implement and moreover results in better performance than TCM, on both Gaussian and Rayleigh channels. 2 - TURBO-CODES PRINCIPLE Turbo-codes result from concatenation of two recursive systematic convolutional (RSC) codes. In Fig. 1, an example of a rate R =1/3 standard turbo-encoder is shown. Two RSC elementary encoders C, and C,, with the same constraint length K =5 and the same polynomial generators (23,35) are used. Both elementary encoder Cl and receive the same information string {dk} but arranged in a different sequence due to the presence of an interleaver I1 and a delay line L1. Given an input bit dk , the encoder outputs at time k are equal to itself since encoding is systematic, and the outputs c: and c: of the elementary encoders and C,. 0-7803-1825-0194 $4.00 0 1994 IEEE I * 4 I , c1 Recursive Systematic Code (23.35) Figure 1 Rate R=l/3 standard turbo-encoder The turbo-decoder is made up of P pipelined identical modules as depicted in Fig. 2 and thus the turbo decoder structure is modular [l]. The pth module input at time k is made up of demodulator outputs (xk )p-I and (Yk )p-l through a delay line and of extrinsic information (zk)p-l provided by the (p-1)th module. Extrinsic information is new information concerning bit but affected by a noise weakly correlated with the noise that perturbs the observations (‘k Ip-1 and ( Yk)p-l.
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This note was uploaded on 01/25/2011 for the course SCE 5441 taught by Professor Lung during the Spring '10 term at Carleton CA.

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