Chen Yougan

Comparison of the performance of LDPC codes over different underwater acoustic channels

The fast temporal variations, long multipath delay spreads, and severe frequency-dependent attenuations of underwater acoustic communication channels are extremely complex that impedes underwater acoustic (UWA) data transmission. To alleviate this problem, channel coding is indispensable in UWA communication system to increase the reliability. In this paper, performances of LDPC codes with different parameters over different underwater acoustic communication channels are presented, which is to know how to adjust the encoding and decoding parameters according to different underwater acoustic channels. The results indicate that LDPC codes have a good performance of the estimated BER on the order of 10−4 −10−5 either in positive sound velocity gradient (PSVG) underwater channel or negative sound velocity gradient (NSVG) one, even invariable sound velocity gradient (ISVG) one. In addition, whatever the encoding or decoding parameters is adopted, performance of the LDPC codes over the NSVG underwater channel is the best, compared with PSVG one and ISVG one. Furthermore, in the desired performance of BER of 10−5, the encoding and decoding parameters according to the three different underwater channels specifications are summarized.

A frame synchronization method for underwater acoustic communication on mobile platform

In this paper, we propose a frame synchronization method for underwater acoustic communication on mobile platform. This new method involves transmitting signal based on hyperbolic frequency modulated (HFM) signal as a preamble of the frame, while the receiver uses a correlator which is matched to the transmitted signal. This method can provide us with good signal detection ability which is very important in underwater acoustic (UWA) channels, and can provide Doppler scale estimation for accurate synchronization. It can also provide multiple access capability for UWA communication network. We analyze and simulate the performance of the HFM signal. A contrast is made with the linearly-frequency modulated (LFM) signal compared to the LFM based method. The proposed method works with a robust correlation output of the Doppler Effect and is suited to handle the presence of dense multi-path channels.

RA coding joint adaptive equalization for underwater acoustic communications

Underwater acoustic (UWA) communication channels that vary from stationary with sparse arrivals to rapidly varying and fully reverberant present fading, multipath and refractive properties, greatly impede UWA data transmissions. In order to solve this problem, a receiver with the iterative soft-input/soft-output equalization and decoding that employs Repeat-Accumulate (RA) coding joint adaptive decision feedback equalizer (DFE) is proposed. By using computer simulations, a 5-path UWA channel model is built to establish the UWA communication system model with the proposed receiver structure. The error correction capability of RA codes is discussed and how the parameter affects on the performance of the adaptive DFE with LMS algorithm is analyzed. Simulation results show that the UWA communication system with the proposed channel coding scheme can get about 6dB coding gain comparing with that without channel coding, and the DFE with proper parameters can overcome the effects of the ISI and channel fading, lower the overall bit error ratio (BER). Furthermore, due to the receiver structure which performs in RA decoder with 10 iterations and in adaptive DFE with proper parameters, the reliability of the system can get significant improvement, demonstrating that the iterative soft-input/soft-output equalization and decoding which employs RA coding combined with the adaptive DFE, with algorithmic simplicity, will have good prospect in UWA communications.

Application analysis of QC-LDPC codes in shallow water acoustic channels

The shallow water acoustic (SWA) channel is one of the most challenging channels in wireless digital communications. In this paper, we propose a good performance, low complexity channel coding scheme using quasi-cyclic LDPC (QC-LDPC) codes to improve the communication system reliability for SWA channels. A system simulation model including QC-LDPC codes for the SWA channels is built. With the decoding algorithm of belief propagation (BP), the effects of QC-LDPC codes design parameters, such as girth, circulant permutation matrix size and decoding iterations are discussed. To verify the proposed approach, Quangang Xiaocuo harbor blasting data are used to set up a SWA channel model, and the performances of QC-LDPC codes over the channel model are analyzed. The results show that, with the same parameters, the proposed QC-LDPC channel coding strategy for SWA channels substantially outperforms the conventional (3, 6)-LDPC channel coding strategy, and with less complexity and easy implementation.

High-order modulation based on repeat-accumulate codes for underwater acoustic communications

Several characteristics of underwater acoustic (UWA) communication channels-limited bandwidth, extended multipath and severe fading-impede UWA data transmissions. To solve this problem, we propose to set up a receiver combining repeat-accumulate (RA) coding and high-order modulation, and calculate the channel soft information for RA decoding with high-order modulation. A multipath fading acoustic channel model for the southern Taiwan Strait channel is established based on BELLHOP ray model and its real sound speed profile. The paper also discusses the error correction capability of the different combination of RA coding and high-order modulation. Simulation results show that the system performance may be improved by using error correction coding; however, its data rate will be reduced. While the frequency efficiency grows significantly, the reliability cannot be improved remarkably.