Low-density parity-check codes for hybrid ARQ system

Abstract

Although Low-density parity-check (LDPC) codes have superb error correcting ability, in order to be a viable solution in practical systems such as a Hybrid Automatic Repeat reQuest (HARQ), besides the performance, there are a few additional issues such as decoding complexity and decoding latency. In this thesis, we focus on the considerations of LDPC codes to be adequate for HARQ system. For the first approach, we propose a $\It{stopping criterion}$ for the decoding failure. Actually, LDPC codes have an inherent stopping criterion, the parity-check constraints (equations). By testing the parity-check constraints, LDPC decoders can detect successful decoding and stop their decoding, which is, however, not possible with Turbo codes. In the first part, we propose a stopping criterion to predict decoding failure of LDPC codes, instead of detecting successful decoding. If the decoder predicts the decoding failure in advance, the receiver can more rapidly response to the transmitter and request for additional parity bits with HARQ system, which reduces overall system latency. The receiver can also save power consumption by avoiding the unnecessary decoder iterations. The proposed stopping criterion makes use of the variations of the number of satisfied parity-check constraints in the Belief-Propagation (BP) decoding which is always tested by the conventional BP decoder to detect successful decoding. Thus, the proposed stopping criterion does not require any additional complexity. Besides the stopping criterion, the counting the number of satisfied parity-check constraints shows behaviors of the BP decoding, which comes, otherwise, from the observations of the changes of log-likelihood ratio (LLR) values in multi-bit resolution with additional complexity. Secondly, we deal with novel HARQ scheme called a $\It{partial chase combining}$, which retransmits partial bits among whole codeword for the retransmission requests. Of existing HARQ scheme, the type-I HARQ sy...