Optimized DC-free turbo coding scheme under the map decoding algorithm

A useful tool in the design of reliable digital communication systems is channel coding. Turbo codes have been shown to yield an outstanding coding gain close to theoretical limits in the additive white Gaussian noise (AWGN) channel. In this paper, we applied Wadayamapsilas schemes of the direct current DC-free codes in a cascaded structure with turbo codes under the Maximum a Posteriori (MAP) decoding algorithm. The work done by Deng and Herro [1] impose relatively strong restrictions on the decomposition of the codeword. Wadayama removed such restrictions to treat any decomposition of the codeword. This is the main difference between the two approaches and it is why we used Wadayamapsilas. The proposed scheme achieves the DC-free coding and error-correcting capability simultaneously. A given sequence becomes DC-free if and only if the absolute Running Digital Sum (RDS) value of the sequence is bounded by a constant for any time instant. The RDS control encoder generates a sequence giving the turbo coded sequence a bounded RDS value which results in a high error correcting capability. The high performances in terms of Bit Error Rate (BER) which is very close to the Shannon limits of this simple cascaded structure allows us to expect a new results with other capacity approaching EC block codes, such as low density parity check codes.

1. Introduction

ERROR-CONTROL (EC) codes and constrained sequence (CS) codes are channel codes that are used to improve the performance of digital communication systems. EC codes enable detection and/or correction of errors when the coded sequence is corrupted due to an imperfect channel. CS codes make it possible for the channel-input sequences to avoid some patterns that are susceptible to corruption in practical systems. CS codes include runlength-limited (RLL) codes and DC-free codes.