# Agenda

## PhD Thesis Defence

- Monday, 28 January 2019
- 12:00-14:30
- Aula Senaatszaal

### Graph-Time Signal Processing Filtering and Sampling Strategies

**Elvin Isufi**

The necessity to process signals living in non-Euclidean domains, such as signals defined on the top of a graph, has led to the extension of signal processing techniques to the graph setting. Among different approaches, graph signal processing distinguishes itself by providing a Fourier analysis of these signals. Analogously to the Fourier transform for time and image signals, the graph Fourier transform decomposes the graph signals decomposes in terms of the harmonics provided by the underlying topology. For instance, a graph signal characterized by a slow variation between adjacent nodes has a low frequency content.

Along with the graph Fourier transform, graph filters are the key tool to alter the graph frequency content of a graph signal. This thesis focuses on graph filters that are performed distributively in the node domainâ€“that is, each node needs to exchange information only within its neighbor to perform a given filtering operation. Similarly to the classical filters, we propose ways to design and implement distributed finite impulse response and infinite impulse response graph filters.

One of the key contributions of this thesis is to bring the temporal dimension to graph signal processing and build upon a graph-time signal processing framework. This is done in different ways. First, we analyze the effects that the temporal variations on the graph signal and graph topology have on the filtering output. Second, we introduce the notion of joint graph-time filtering. Third, we presentpr a statistical analysis of the distributed graph filtering when the graph signal and the graph topology change randomly in time. Finally, we extend the sampling framework from the reconstruction of graph signals to the observation and tracking of time-varying graph processes.

We characterize the behavior of the distributed autoregressivemoving average (ARMA) graph filters when the graph signal and the graph topology are time-varying. The latter analysis is exploited in two ways: i ) to quantify the limitations of graph filters in a dynamic environment, such as a moving sensors processing a time-varying signal in a sensor network; and i i ) to provide ways for filtering with low computation and communication complexity time-varying graph signals.

We develop the notion of distributed graph-time filtering, which is an operation that jointly processes the graph frequencies of a time-varying graph signal on one hand and its temporal frequencies on the other hand. We propose distributed finite impulse response and infinite impulse response recursions to implement a two-dimensional graphtime filtering operation. Finally, we propose design strategies to find the filter coefficients that approximate a desired two-dimensional frequency response.

We extend the analysis of graph filters to a stochastic environment, i.e., when the graph topology and the graph signal change randomly over time. By characterizing the first and second order moments of the filter output, we quantify the impact of the graph signal and the graph topology randomness into the distributed filtering operation. The latter allows us to develop the notion of graph filtering in the mean, which is also used to ease the computational burden of classical graph filters.

Finally, we propose a sampling framework for time-varying graph signals. Particularly, when the graph signal changes over time following a state-space model, we extend the graph signal sampling theory to the tasks of observing and tracking the time-varying graph signal froma few relevant nodes. The latter theory considers the graph signal sampling as a particular case and shows that tools from sparse sensing and sensor selection can be used for sampling.

Additional information ...### Agenda

- Thu, 24 Jan 2019
- 10:00
- Aula Senaatszaal

### PhD Thesis Defence

#### Pan Liu

- Thu, 24 Jan 2019
- 14:30
- Aula Senaatszaal

### PhD Thesis Defence

#### Jeroen van Gemert

#### Efficient computational methods in Magnetic Resonance Imaging

How to design dielectric pads that can be used to increase image quality inMagnetic Resonance Imaging, and how to accelerate image reconstruction times using a preconditioner.

- Thu, 24 Jan 2019
- 09:30
- Lipkenszaal (HB 01.150)

### MSc SS Thesis Presentation

#### Bastian Generowicz

#### Improving Ultrafast Doppler Imaging using Subspace Tracking

fast tracking to distinguish blood motion from clutter signals caused by slow moving tissue.

- Mon, 28 Jan 2019
- 12:00
- Aula Senaatszaal

### PhD Thesis Defence

#### Elvin Isufi

#### Graph-Time Signal Processing Filtering and Sampling Strategies

Design of graph filters where nodes exchange information only with their neighbors to perform a given filter operation: how can distributed FIR and IIR graph filters be made?

- Thu, 7 Mar 2019
- 13:30
- HB 17.150

### Signal Processing Seminar

#### Andreas Spanias

#### Sensor and Machine Learning at The Arizona State University

Overview of research projects at the ASU SenSIP Center