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

To enable full-duplex (FD) in underwater acoustic (UWA) systems, a high level of self-interference cancellation (SIC) is required. This can be achieved by using a combination of SIC methods, including digital SIC. For digital SIC, adaptive filters are used. In time-invariant channels, the SI can be effectively cancelled by classical recursive least-square (RLS) adaptive filters, such as the sliding-window RLS (SRLS) or exponential-window RLS, but their SIC performance degrades in time-varying channels, e.g., in channels with a moving sea surface. Their performance can be improved by delaying the filter inputs. This delay, however, makes the mean squared error (MSE) unsuitable for measuring the SIC performance. In this article, we propose a new evaluation metric, the SIC factor (SICF), which gives better indication of the SIC performance compared to MSE. The SICF can be used to evaluate the performance of digital SIC techniques without the need of implementing a full FD system. A new SRLS adaptive filter based on parabolic approximation of the channel variation in time, named SRLS-P, is also proposed. The SIC performance of the SRLS-P adaptive filter and classical RLS algorithms (with and without the delay) is evaluated by simulation and in lake experiments. The results show that the SRLS-P adaptive filter can significantly improve the SIC performance, compared to the classical RLS adaptive filters.