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On turbo encodedBicm

Turbo modulation codée avec entrelacement de bits

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Abstract

The superior performance of the binary turbo codes has stimulated vigorous efforts in generating bandwidth efficient modulation schemes adhering to these codes. Several approaches for the integration of turbo-coding and modulation have emerged in recent years but none seem to dominate. In the bit interleaved coded modulation (Bicm) scheme is used to achieve high bandwidth and power efficiency, while separating coding and modulation. As is now well known, theBicm scheme achieves capacity remarkably close to the constellation channel capacity. The turbo-Bicm scheme enjoys high coding diversity (well suited for fading channels), high flexibility as well as design and implementation simplicity, while maintaining good power efficiency. The system comprises one standard turbo code, an interleaver, a mapper and a modulator at the transmitter, corresponding to a demodulator, a de-interleaver and a turbo decoder at the receiver. A modified system, which improves on performance by incorporating the demodulation in the iterative decoding procedure, is investigated, and some performance gain is demonstrated, especially for low rate codes. Information theoretic arguments for the somewhat minor potential improvement in performance are detailed. The preferred mapper and interleaver for this system are considered. Extending previous works, for higher level modulations, we analyze a system including a convolutional code, an interleaver, a differential encoder (De), a mapper and a modulator at the transmitter. As for theBpsk modulation, the serial concatenation of a convolutional code withDe outperforms the single convolutional code. The serial concatenation withDe approach is analyzed also for a turbo code, where it is found to fail in achieving performance improvement. Several structures for the serial concatenation withDe are examined. These results are substantiated through the ‘spectral thinning’ phenomena of the weight distribution of the convolutional and turbocodes.

Résumé

Les bonnes performances des turbo codes binaires ont suscité de gros efforts de recherche pour associer ces codes aux modulations à grande efficacité spectrale. Plusieurs approches ont été proposées durant ces dernieres années, mais aucune ne semble s’imposer. Dans cet article le principe de la modulation codée avec entrelacement de bits (Bicm) est utilisé pour atteindre une efficacité spectrale élevée et de forts rendements en puissance, tout en séparant le codage et la modulation. Le systèmeBicm, c’est maintenant bien connu, permet d’atteindre une bonne efficacité spectrale. Le système turbo-Bicm apporte une importante diversité de codage (bien adaptée aux canaux à évanouissement), une grande flexibilité aussi bien en ce qui concerne la simplicité de conception et d’implantation que du rendement de puissance. Le systéme comprend, du coté émetteur, un turbo code standard, un entrelaceur, un codeur binaire à symbole et un modulateur. Le récepteur est constitué d’un démodulateur, d’un désentrelaceur et d’un turbo-décodeur. Un systeme modifié permettant d’améliorer les performances en intégrant la démodulation dans le processus de décodage itératif, est décrit. L’amélioration des performances est démontrée, en particulier dans le cas des codes à faible rendement. Des propositions théoriques conduisant à des améliorations mineures potentielles des performances sont détaillées. Développant de précédents travaux sur les modulations de haul niveau, un système incluant à l’émission un code convolutif, un entrelaceur, un codeur différentiel, un codeur binaire à symbole et un modulateur est analysé. Pour la modulationMdp2, la concaténation série d’un code convolutif et d’un codeur différentiel conduit à de meilleures performances qu’un simple code convolutif. La concaténation série basée sur un codage différentiel est analysée égalemenl pour un turbo code. Dans ce cas, il n’y a pas d’amélioration des performances. Plusieurs structures pour la concatenation série avec un codeur différentiel sont étudiées. Les résultats sont justifiés par les phénomènes de « rétrécissement spectral » de la distribution des poids dans les codes convolutifs et les turbo codes.

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  1. Department of Electrical Engineering, Technion—Israel Institution of Technology, 32000, Haifa, Israel

    Ilan Abramovici & Shlomo Shamai

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  1. Ilan Abramovici
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  2. Shlomo Shamai
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Abramovici, I., Shamai, S. On turbo encodedBicm . Ann. Télécommun. 54, 225–234 (1999). https://doi.org/10.1007/BF02998584

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