The fast subsampled-updating stabilized fast transversal filter (FSU SFTF) RLS algorithm for adaptive filtering

We present a new, doubly fast algorithm for Recursive Least-Squares (RLS) adaptive filtering that uses displacement structure and subsampled-updating. The Fast Subsampled-Updating Stabilized Fast Transversal Filter (FSU SFTF) algorithm is mathematically equivalent to the classical Fast transversal Filter (FTF) algorithm. The FTF algorithm exploits the shift invariance that is present in the RLS adaptation of a FIR filter. The FTF algorithm is in essence the application of a rotation matrix to a set of filters and in that respect resembles the Levinson algorithm. In the subsampled-updating approach, we accumulate the rotation matrices over some time interval before applying them to the filters. It turns out that the successive rotation matrices themselves can be obtained from a Schur type algorithm which, once properly initialized, does not require inner products. The various convolutions that appear in the algorithm are done using the Fast Fourier Transform (FFT). The resulting algorithm is doubly fast since it exploits FTF and FFTs. The roundoff error propagation in the FSU SFTF algorithm is identical to that in the SFTF algorithm, a numerically stabilized version of the classical FTF algorithm. The roundoff error generation on the other hand seems somewhat smaller. For relatively long filters, the computational complexity of the new algorithm is smaller than that of the well-known LMS algorithm, rendering it especially suitable for applications such as acoustic echo cancellation.