The ensemble performance of parallel and serial concatenated turbocodes is considered, where the ensemble is generated by a uniform choiceof the interleaver and of the component codes taken from the set of timevarying recursive systematic convolutional codes. Following thederivation of the ensemble weight enumeration functions of randomparallel and serial concatenated codes, improved upper bounds on the bitand message error probabilities of these ensembles of codes are derivedand the influence of the interleaver length N and the memory length ofthe component codes m are investigated. The improved bounding techniqueproposed here, which is based on the tangential-sphere bound, iscompared to the conventional union bound and to a recent alternativebounding technique by Duman and Salehi (1997) which incorporatesmodified Gallager bounds. The advantage of the derived bounds isdemonstrated, especially in the region above the cutoff rate, includingthus a portion of the rate region where the performance of turbo codesis most appealing
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