Crosstalk mitigation using adaptive time reversal

Cancellation of dolphin sonar clicks in a communication signal based on adaptive time reversal processing

In long-range underwater communication, conventional time reversal processing (CTRP) is used to mitigate the distortion caused by multipath and temporal spreading. However, signals produced by marine animals can contaminate communication signals. Impulsive signals, such as dolphin sonar clicks, have wide bandwidths and short pulse durations, making it difficult to isolate the communication signal. This letter proposes a method to cancel these sounds by estimating the Green's function of marine animal clicks and applying adaptive time reversal processing (ATRP). The effectiveness of the click nulling was verified by comparing the performances of CTRP and ATRP with seagoing experimental data.

Iterative channel estimation-based soft successive interference cancellation for multiuser underwater acoustic communications

This paper presents a single-carrier multiuser receiver in underwater acoustic communications with strong multiple access interference (MAI) combining passive time-reversal (PTR) and direct-adaptation-based turbo equalization. The receiver works in an iterative block by block manner. In the first iteration, when the symbols of all users are detected by successive interference cancellation (SIC) in the block, the channel impulse response (CIR) of each user is then updated by post-SIC signal to achieve more accurate MAI reconstruction and PTR combination for the next block. In the following iterations, the a priori mean of symbol in the current block is available and used to further improve the accuracy of reconstructed MAI and the updated CIR. The proposed receiver is demonstrated using experimental data collected at Songhua Lake, China, in 2019. The results show that single-carrier nine-user communication data with quadrature phase-shift keying can be successfully recovered, with an average data rate of 1.67 kbps for each user.

Equivalence of adaptive time reversal and least squares for cross talk mitigation

A recent paper [H. C. Song et al., J. Acoust. Soc. Am. 127, EL19-EL22 (2010)] demonstrated adaptive time reversal (ATR) using at-sea experimental data which significantly suppressed the cross talk among users (or transmitters) over conventional time reversal. In this letter, the ATR approach is shown essentially equivalent to the least squares solution to an over-determined system.

Experimental demonstration of multiuser communication in deep water using time reversal

Multiuser communication is demonstrated using experimental data (450-550 Hz) collected in deep water, south of Japan. The multiple users are spatially distributed either in depth or range while a 114-m long, 20-element vertical array (i.e., base station) is deployed to around the sound channel axis (~1000 m). First, signals received separately from ranges of 150 km and 180 km at various depths are combined asynchronously to generate multiuser communication sequences for subsequent processing, achieving an aggregate data rate of 300 bits/s for up to three users. Adaptive time reversal is employed to separate collided packets at the base station, followed by a single channel decision feedback equalizer. Then it is demonstrated that two users separated by 3 km in range at ~1000 m depth can transmit information simultaneously to the base station at ~500 km range with an aggregate data rate of 200 bits/s.

Time reversal multiple-input/multiple-output acoustic communication enhanced by parallel interference cancellation

Multiple-input/multiple-output (MIMO) techniques can lead to significant improvements of underwater acoustic communication capabilities. In this paper, receivers based on time reversal processing are developed for high frequency underwater MIMO channels. Time reversal followed by a single channel decision feedback equalizer, aided by frequent channel updates, is used to compensate for the time-varying inter-symbol interference. A parallel interference cancellation method is incorporated to suppress the co-channel interference in the MIMO system. The receiver performance is demonstrated by a 2008 shallow water experiment in Kauai, Hawaii. In the experiment, high frequency MIMO signals centered at 16 kHz were transmitted every hour during a 35 h period from an 8-element source array to a wide aperture 16-element vertical receiving array at 4 km range. The interference cancellation method is shown to generate significant performance enhancement, on average 2-4 dB in the output signal-to-noise ratio per data stream, throughout the 35 h MIMO transmissions. Further, communication performance and achieved data rates exhibit significant changes over the 35 h period as a result of stratification of the water column.

High-rate multiuser communications in shallow water

Passive multiuser communications in shallow water previously was demonstrated in the 3-4 kHz band using a time reversal approach. This paper extends those experimental results in three respects. First, a larger bandwidth at higher frequency (11-19 kHz) is employed allowing for the use of various symbol rates (or bandwidths). Second, two different shaping pulses are examined: a raised cosine filter and LFM (linear frequency modulation) chirp. Third, the adaptive time reversal approach with spatial nulling is applied to suppress the crosstalk among users. It is shown that the use of a larger bandwidth is beneficial along with the time reversal receiver which can handle significant intersymbol interference with minimal computational complexity. In addition, adding each user degrades the performance by about 4 dB for the benefit of linear increase in data rate. It is demonstrated that an aggregate data rate of 60 kbits/s can be achieved with a 7.5 kHz bandwidth (a spectral efficiency of 8 bits/s Hz) by three users distributed over 4.2-m depth at a 2.2 km range in shallow water using 16-QAM (quadrature amplitude modulation).