There are two different suboptimal turbo equalizer structures applied in underwater acoustic communications, and the turbo equalizer based on channel estimation (CE-TE) is one of them. And the CE-TE is more complicated with the increased equalizer complexity caused by matrix inverse during iterations, especially for the long time spread of channel and multi-array processing.
Moreover, high rate phase coherent underwater communications have traditionally relied on adaptive equalization methods and spatial diversity to overcome the intersymbol interference (ISI), caused by multipath propagation.
Besides, time reversal combining (TRC) can make use of the spatial diversity and filtering each of the multi-channels by the information of each channel. In addition, TRC makes full use of the energy of the received signal and compress the time spread caused by multipath. However, if the number of the channels is limited, there is still residual ISI after the TRC operation. So these kinds of feature limit the application of TRC.
Motivated by the respective advantages and limitations of the methods of turbo equalizer and TRC, combining CE-TE with TRC can be a very potential technique. Because the TRC compresses the long time spread of the received signals and combines multichannel signal. And then the practical CE-TE eliminates the residual ISI after the TRC operation. So, this kind of scheme can be more efficient and robust.
As a result, a receiver scheme combing time reversal processing with turbo equalization is presented for phase coherent underwater communication. The TRC compresses the ISI induced by multipath propagation in ocean environments, and the CE-TE eliminates residual ISI with a reasonable complexity.
Fig. 1 The Receiver Structure (Image by XU).
The scheme of Receiver is plotted in Fig.1. The channel with intersymbol interference can be equivalent to finite impulse response (FIR) filter.
In the field experiment, proposed scheme can be realized with error-free performance at a 3km range in 6 channels combined. The experimental results provide further evidences that the proposed scheme can be a quite promising option for realizing high-speed and highly reliable data communication in a complex underwater acoustic channel.
This research was supported by Chinese National 863 projects 2002AA401004, 2009AA093301 and 2009AA093601.
The research with the title of "The Union of Time Reversal and Turbo Equalization on Underwater Acoustic Communication" has been included in the OCEANS ’13 MTS/IEEE San Diego (September 23-26) (130501-012).