Adaptive Nonlinear Equalizer for Full-Duplex Underwater Acoustic Systems

Abstract

To enable full-duplex (FD) operation in underwater acoustic systems, the strong selfinterference (SI) from the near-end transmitter should be efficiently cancelled. Digital cancellation is considered as the main approach for the SI cancellation. It is believed that the key challenge in achieving a high level of cancellation is to overcome the nonlinear distortion in the transmit and receive chains. The majority of the nonlinearity is introduced by the power amplifier (PA), which can be dealt with by using the PA output as the reference signal for digital cancellation. Further nonlinearity comes mainly from the hydrophone pre-amplifier. For applications working in half-duplex mode, the pre-amplifier can normally be assumed to be linear due to the low received signal level. For FD operations, the strong SI might result in a nonlinear response of the pre-amplifier, and this nonlinearity should be equalized to achieve a high cancellation performance. In this paper, we propose a technique for equalizing the nonlinearity in the preamplifier. This is done using a basis expansion model of the nonlinear equalizer response. More specifically, the Legendre polynomials are used as the basis functions. The expansion coefficients are updated to reduce the mean squared error or a cost function derived based on the power spectrum of the received signal. The performance of the equalizer is evaluated using the experimental data with artificial nonlinear distortion and with real nonlinearity introduced by the hydrophone pre-amplifier. Both sets of results demonstrate that nonlinear distortions can be effectively equalized using the proposed adaptive equalization technique.

Full Duplex Physical and MAC Layer-Based Underwater Wireless Communication Systems and Protocols: Opportunities, Challenges, and Future Directions

“…• ANE. A novel ANE-based digital cancellation scheme is proposed in [60], in which the PA's output signal serves as a guide for digital cancellation to resolve non-linear distortion in the transmitter and receiver chains. The equalization has been performed by using the basis expansion model (BEM), which can model the inverse of the preamplifier response.…”

“…Owing to the fact that the SDFE-based TRDC [22] system employs a hybrid self-interference cancellation scheme, while HSIC [41] employs analog interference cancellation to suppress the interference, and A-SIC [25] employs the NLMS algorithm to suppress the SI signal for the increase in spectral efficiency. The FSK based FD-UWAC systems [29], RLS-adaptive [25] and ANE [60] adopts the FD-UWAC channel, and significantly enhancement the spectral efficiency. However, the FSK based UWAC system in [29] has shown high spectral efficiency compared to other digital schemes such as RLS-adaptive [25] and ANE [60].…”

“…The FSK based FD-UWAC systems [29], RLS-adaptive [25] and ANE [60] adopts the FD-UWAC channel, and significantly enhancement the spectral efficiency. However, the FSK based UWAC system in [29] has shown high spectral efficiency compared to other digital schemes such as RLS-adaptive [25] and ANE [60]. The FSK-based UWAC system [29] sends data at a rate of 21 bits per second over a bandwidth of 60 kHz, which improves spectral efficiency.…”

“…TRDC [22] transmits the signal up to a range of 10 m, while the transmission ranges of ML-SC-IBFD [23] and A-IBFD [24] achieve an effective transmission range of 7.2 m. Apart from that, the HSIC [41] system only transmits signals up to 0.5 m, while the A-SIC [25] system provides no detail about the transmission range. The transmission range of BPSKbased FD-UWAC systems including RLS-adaptive [17,25] and ANE [60] has been tested for low transmission range. However, in [29], the FSK-based FD-UWAC system was evaluated for a longer transmission range of 5 m to 1000 m while maintaining significant energy efficiency and battery life.…”

Adaptive Filtering for Full-Duplex UWA Systems With Time-Varying Self-Interference Channel

“…The signal distortions in the SI channel and their influence onto the SI cancellation performance have not been yet well understood. Such issues have been partly addressed separately or in some combination as the PA nonlinearity [8], [9], the nonlinearity in the preamplifier of the hydrophone [20], passband to baseband conversion [15], modelling of the near-end surface reflections [14], modelling of the self-loop interference through the mounting system [21], acoustic isolation of the projector and hydrophone [22], etc. In this paper, we propose an improved technique for dealing with the fast time-variability of the SI channel in digital SI cancellers.…”