Use of extrinsic information transfer chart to predict behavior of turbo codes

Abstract

In this paper, we consider turbo codes that are now introduced in various international standards, including the UMTS standard for third generation personal communications and the ETSI DVB-T standard for Terrestrial Digital Video Broadcasting, and review the method of extrinsic information transfer. The convergence properties of the iterative decoding process associated with a given turbo-coding scheme are assessed using the so-called extrinsic information transfer (EXIT) chart analysis technique. This approach provides an opportunity to foresee the bit error rate (BER) of a turbo code system using only the EXIT chart. It is shown that EXIT charts are powerful tools to analyze and optimize the convergence behavior of iterative systems utilizing the turbo principle, i.e., systems exchanging and refining extrinsic information. The idea is to consider the associated SISO stages as information processors that map input a priori LLR's onto output extrinsic LLR's, the information content being obviously assumed to increase from input to output, and introduce them to the design of turbo systems without the reliance on extensive simulation. Compared with existing methods for generating EXIT functions, a suggested approach provides insight into the iterative behavior of linear turbo systems with substantial reduction in numerical complexity.