On the Overflow Problem in Finite Precision Turbo Decoding Message Passing

Much attention has been paid recently to the so-called layered decoding of LDPC codes, also known as turbo-decoding message passing (TDMP). The TDMP algorithm for decoding LDPC codes is known to possess some desirable features, such as fast convergence speed, reduced memory requirements and reduced implementation complexity in comparison with standard two-phase message passing algorithm. In this paper we analyze an important issue connected with hardware implementation of TDMP algorithm, namely the finite precision representation of messages influence on the decoding performance. Constrained dynamic range of the finite precision representation of messages entails overflow errors. We present an analysis revealing that in the subsequent decoding iterations, the subtraction of non-overflowed intrinsic message from overflowed extrinsic message is a source of errors that have substantial impact on the decoding results. The analysis is confirmed by simulation results showing significant performance loss. However this performance loss can be almost completely eliminated with a basic modifications in the messages computation algorithm. Effectiveness of the presented modifications is confirmed by simulation results obtained with hardware TDMP decoder implementation that has been developed.