Nonlinear adaptive filtering in kernel spaces

Recently, a family of online kernel-learning algorithms, known as the kernel adaptive filtering (KAF) algorithms, has become an emerging area of research. The KAF algorithms are developed in reproducing kernel Hilbert spaces (RKHS), by using the linear structure of this space to implement well-established linear adaptive algorithms and to obtain nonlinear filters in the original input space. These algorithms include the kernel least mean squares (KLMS), kernel affine projection algorithms (KAPA), kernel recursive least squares (KRLS), and extended kernel recursive least squares (EX-KRLS), etc. When the kernels are radial (such as the Gaussian kernel), they naturally build a growing RBF network, where the weights are directly related to the errors in each sample. The aim of this chapter is to give a brief introduction to kernel adaptive filters. In particular, our focus is on KLMS, the simplest KAF algorithm, which is easy to implement, yet efficient. Several key aspects of the algorithm are discussed, such as self-regularization, sparsification, quantization, and the mean-square convergence. Application examples are also presented, including in particular the adaptive neural decoder for spike trains.