LDPC Code-Aided Phase-Ambiguity Resolution for QPSK signals Affected By a Frequency Offset

Low-density parity-check (LDPC) codes provide capacity approaching performances with very long codes in a coherent channel. However, as the block length increases, their performance becomes worse with residual frequency offsets. Residual frequency offsets can be tackled with sub-block decoding and LDPC code-aided phase ambiguity resolution for binary phaseshift keying (BPSK) signals. Sub-block decoding is a practical technique where phase estimation/correction are done over every sub-block in a codeword. In this paper, we extend the concept of LDPC code-aided phase ambiguity resolution to Quadrature phase-shift keying(QPSK) modulated signals through a mapping/demapping technique. Bit-error rate (BER) simulations show that the phase ambiguity algorithm along with sub-block decoding tackle small frequency offsets with a negligible loss. We further show with the help of EXIT charts that LDPC code distributions performing close to capacity and also aiding phase ambiguity resolution can be constructed.