The A* algorithm is an efficient method for decoding short linear block code based on tree search. However, it may suffer very long decoding latency in the worst-case channel condition. In this paper, we propose to apply some restrictions on the Hamming weights of search paths in the tree-search decoding. Simulation results show that with proper selection of path constraints in the A* algorithm, the worst-case decoding complexity (or latency) can be significantly reduced with only slight sacrifice of the bit error rates.

The A* algorithm for decoding linear block codes is known to be an efficient tree-search method for decoding short linear block codes. We propose to employ the early acquired information of some parity bits to speed up the tree search. Analysis for the number of required information bits for obtaining the parity bits is provided. In addition, two versions of the modified A* algorithm which take advantage of the advanced information of parity bits are provided. Simulation results show that there is advantage in reducing the number of search tree edges in each proposed version as compared with two existing A* algorithms which do not consider the early information of parity bits.

Applying interblock memory to binary linear block codes is a method to increase the transmission reliability by increasing the correlation among codewords. In this paper, we consider short block coding with interblock memory. Since short block codes can be efficiently decoded by employing the A* Algorithm, we investigate the performance of short block coding with interblock memory using the A* decoding. In addition, we propose a simple method which can alleviate the effect of error propagation resulting from interblock memory.

We propose a hybrid ARQ (HARQ) scheme for which a rate-1/2 error-correcting code with code length equal to the packet length is used in the HARQ operation, where each packet comprises two subpackets of equal lengths. For the HARQ system with constant packet lengths, the utilization of subpackets enables the flexibility of carrying new message (or data) in the retransmissions. Hence, such a system can gain additional throughput as compared to the system using all-redundancy retransmissions under the conditions of moderate to high signal-to-noise ratios (SNR). In the proposed HARQ scheme, each of the subpackets can be chosen as a data part, or a redundancy part, or a function of two subpackets, where the data part or the redundancy part may belong to different codewords of the rate-1/2 error-correcting code. With a proper design of the function subpacket, we can adaptively control the error-correcting capability used in the HARQ operation. The key idea is that there is possibility of recovering three data subpackets in one retransmission and hence increasing the throughput. The price is that in each retransmission, up to three bits are needed in the feedback channel to indicate which subpacket is successfully recovered.