Message-Wise Unequal Error Protection Based on Low-Density Parity-Check Codes

We propose a practical message-wise unequal error protection (UEP) scheme using low-density parity-check (LDPC) codes, where one or more special messages are more protected than other ordinary messages. In contrast to the information theoretic cavity coding scheme, which discards the codewords of ordinary messages near those of special messages, the proposed coding scheme performs codeword flipping to separate the codewords of special and ordinary messages without discarding any codewords. To better distinguish the original and flipped codewords, the LDPC codes with all-odd degree check nodes are employed. The decoder performs message type detection and codeword flipping detection based on the unsatisfied check nodes in iterative decoding. We provide performance analysis for both the message type detection and the codeword flipping detection. Moreover, we provide an asymptotic analysis on the detection error exponent to reveal the relationship between the proposed practical coding scheme and the information theoretically optimal cavity coding. Simulation results are provided to show that the proposed practical message-wise UEP schemes offer capacity-approaching protections to both types of messages as if only one type is transmitted.