Bit-to-Symbol Mapping in LDPC Coded Modulation

Various bit-to-symbol mapping schemes are studied in an M-ary modulated system applying low-density parity-check (LDPC) code. By examining different assignment methods of LDPC codeword to two parallel channels with unequal signal-to-noise ratio (SNR), we find out that it is better to assign systematic bits of LDPC codeword to channels with higher SNR, even for regular LDPC code. Therefore, it is more advantageous to map as many systematic bits in LDPC codeword as possible to more significant bits (MSB) of the signal constellation in a LDPC coded M-ary modulation system. The performance gain achieved by this mapping scheme comes at no extra cost in power, rate and complexity.