Simplicial Convolutional Filters
This work addresses the need for signal processing tools in topological data analysis, particularly for applications like network analysis, but it is incremental as it extends convolutional filters from graphs to simplicial complexes.
The paper tackles the problem of processing signals on simplicial complexes, which generalize graphs to include higher-order structures like edges and faces, by developing simplicial convolutional filters based on Hodge Laplacians. The result is a method that is linear, shift-invariant, equivariant, and computationally efficient, with applications demonstrated in extracting frequency components, denoising edge flows, and analyzing financial and traffic networks.
We study linear filters for processing signals supported on abstract topological spaces modeled as simplicial complexes, which may be interpreted as generalizations of graphs that account for nodes, edges, triangular faces etc. To process such signals, we develop simplicial convolutional filters defined as matrix polynomials of the lower and upper Hodge Laplacians. First, we study the properties of these filters and show that they are linear and shift-invariant, as well as permutation and orientation equivariant. These filters can also be implemented in a distributed fashion with a low computational complexity, as they involve only (multiple rounds of) simplicial shifting between upper and lower adjacent simplices. Second, focusing on edge-flows, we study the frequency responses of these filters and examine how we can use the Hodge-decomposition to delineate gradient, curl and harmonic frequencies. We discuss how these frequencies correspond to the lower- and the upper-adjacent couplings and the kernel of the Hodge Laplacian, respectively, and can be tuned independently by our filter designs. Third, we study different procedures for designing simplicial convolutional filters and discuss their relative advantages. Finally, we corroborate our simplicial filters in several applications: to extract different frequency components of a simplicial signal, to denoise edge flows, and to analyze financial markets and traffic networks.