Lowering the Error Floor of Turbo Codes With CRC Verification

Abstract

Decoding performance of turbo codes can flatten at moderately high signal-to-noise ratio. This letter proposes a low complexity method for lowering this error floor. This method rests on the observation of the extrinsic information during the iterative decoding process. A set of q most unreliable bits are identified based on their associated extrinsic information. A total of 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</sup> test patterns are then built by inverting the most unreliable bits. The decoded codeword is identified thanks to a cyclic redundancy check detector. This method keeps the turbo coding scheme unchanged as long as an error detection code is serially concatenated with the turbo code. Simulations were performed on a rate-1/3 Long-Term Evolution turbo code and show an improvement of at least one decade in terms of frame error rate in the error floor region. This low complexity method paves the way for further improvements in lowering the error floor of turbo codes.