Lowering Error Floors of Irregular LDPC Code on Fast Fading Environment with and without Perfect CSI
Satoshi Gounai, Tomoaki Otsuki Ohtsuki · 2005
Irregular LDPC codes can achieve better error rate performance than regular LDPC codes. However, irregular LDPC codes have higher error floors than regular LDPC codes. The ordered statistical decoding (OSD) algorithm achieves maximum likelihood (ML) decoding. ML decoding is effective in lowering error floors. However, temporal estimates obtained by the OSD algorithm satisfy the parity check equation of the LDPC code. The OSD algorithm cannot lower error floors because the temporal estimates satisfy the LDPC parity check equation. We proposed a concatenated code constructed with an inner irregular LDPC code and an outer cyclic redundancy check (CRC) code. Thanks to CRC, we can detect errors from the codeword estimated by the OSD algorithm. Our proposed LDPC code and decoding can lower error floors in an AWGN channel. However, in wireless access environments, we cannot neglect the effects of the channel. The OSD algorithm needs the ordering of each bit based on reliability. Channel state information (CSI) is used for deciding the reliability of each bit. We evaluate the block error rate (BLER) of the proposed LDPC code decoding in a fast fading channel with and without perfect CSI where 'without perfect CSI' means that only the average of the fading amplitudes is known at the receiver. By simulation, we show that the proposed LDPC code and decoding can lower error floors more than the conventional LDPC code with the OSD algorithm in fast fading channels with and without perfect CSI.