Research On LDPC-COFDM System Rejecting Frequency Selective Fading And Narrowband Interference

OFDM, which has good performance in high spectrum efficiency, resistance to multipath fading and simple requirement in channel estimation equalization, has been key technology of broadband wireless access communication system and new generation mobile communication standard. The characteristic of wireless communication channel is very complex. Frequency selective fading (FSF) caused by multipath transmission and narrowband interference (NBI) brought by other systems are main factors on influencing OFDM system communication reliability. The influence is embodied on OFDM signal partial subcarriers, which may result in data transmission errors on corresponding subcarriers. OFDM signal inherent characteristic can be utilized to pointedly reject FSF and NBI influence.A COFDM system based on LDPC coding, which is presented to resolve OFDM system communication reliability problem caused by FSF and NBI in slow wireless channel, is proposed in this dissertation. Firstly, principle of influence to OFDM system caused by FSF and NBI is researched, and error patterns generated by various FSF and NBI are analyzed. Secondly, influenced subcarriers detection method, which is performed by LDPC decoding error pattern acquisition, is provided. While considering the impact of channel additive noise (AWGN) to detection reliability, the subcarrier errors statistical method is proposed to decrease noise influence. Finally, corresponding information bit of influenced subcarriers location is pre-coded when system transmitter is doing LDPC coding. Receiver does LDPC decoding combining transmitter pre-coding, so as to eliminate FSF and NBI influence. The scheme is verified by simulation experiment, and simulation results reveal that receiver can steadily detect subcarriers influenced by FSF and NBI on AWGN condition. The method combining LDPC pre-coding and receiver decoding can significantly decrease FSF and NBI influence, and improve system reliability.LDPC encoder and decoder are the prime components of LDPC-COFDM system. FSF and NBI rejection method in the system does not add overhead to original system, and avoids relatively complex algorithm implementation process, such as channel estimation and interference spectrum detection. In addition, error patterns of OFDM receiver performance due to the impact of FSF and NBI is similar. Therefore, the LDPC-COFDM system and subcarrier detection method based on LDPC encoding and decoding to reject FSF and NBI influence in this dissertation has compatibility to this two channel environment.