Research On LDPC In Underwater Acoustic Communication Combined With OFDM Technology

It is well known that underwater acoustic (UWA) channel is an extremely complex channel with the characteristics of time-frequency-space variant, strong multipath effect, narrow available bandwidth and high background noise. It is the most difficult channel for wireless communications. Therefore, it is very challenging to accomplish high-speed and reliable communications in such channel.Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier digital modulation technique, which proved to be a good method for high-speed wireless data communications. Applying OFDM to underwater acoustic communications is just at the beginning in our country. OFDM system can eliminate ISI and reduce ICI. However, when the transmitted signals arrive at the receiver through the UWA channel, the signal envelope of each sub carrier may be very different. Some sub carrier may even be severely distorted by deep fading. So it requires the use of channel coding to protect the transmitted data and achieve reliable communications. Many error correction code technologies are applied into the OFDM system, convolution code, R-S code and Turbo code etc.Recently, low-density parity-check (LDPC) codes have attracted much attention particularly in the field of coding theory. LDPC code had discovered by Gallager in 1962. After the great success of Turbo codes, MacKay et al. rediscovered LDPC code and proved its performance is very close to Shannon Limit. Now, LDPC code has become a hot spot of research in the area of error correcting codes and will be applied in many areas. Applying the LDPC code into the OFDM system in the B3G mobile communications also has been studied. This paper tries to study about the performances of LDPC-COFDM used in underwater acoustic communication.In this paper, combined with the characteristic of UWA channel, a COFDM system is firstly set up with MATLAB. And then, the performances as followed have been simulated. The performance contrast of different construction LDPC used in COFDM. Analysis performance of different mapping of MPSK applied into LDPC-COFDM. The performance contrast of the LDPC code applied into OFDM system and other codes applied into OFDM system. And then contrast the influence applied interleaver or not in the LDPC-COFDM system. Finally, LDPC_COFDM systems is set up based on DSP hardware platform and implement underwater acoustic communication. From both simulation results and real test data, COFDM with LDPC codes as its channel coding scheme turns out to be a good choice in underwater acoustic communication.