Free-Ride Feedback and Superposition Retransmission Over LDPC Coded Links

In this paper, we propose a new transmission scheme for the scenario where two nodes attempt to exchange messages and the conventional low-density parity-check (LDPC) codes are implemented for error correction. In the proposed scheme, the ACK/NACK feedback information is transmitted along with the payload data by free-ride codes, while the re-transmitted codewords are superimposed (XORed) on the current codewords, both of which cost neither extra bandwidth nor transmission power. Firstly, we present a syndrome channel model and derive its capacity (referred to as accessible capacity) with a lower bound, implying that the reliable transmission of extra bits (feedback information) is possible. Then, the performance of the extra bits is analyzed by the dependency testing (DT) bound for the syndrome channels. Moreover, motivated by the DT bound, we present a low-complexity DT-like decoder for the free-ride codes. For the superposition retransmission, we present a practical implementation, where those unsuccessfully decoded codewords are sparsely interleaved and superimposed onto the current codewords. In addition, the presented transmission scheme is combined with the conventional hybrid automatic repeat request (HARQ) protocol, resulting in a throughput-enhanced conjunction HARQ scheme. Numerical results show that the word error rate (WER) of the LDPC codes can be significantly reduced by using the presented transmission scheme, but without any extra bandwidth or transmission power. They also show that the presented conjunction HARQ schemes can achieve a throughput improvement up to 80% over fading channels in comparison with the original 5G HARQ scheme.