Department of Architecture, Design, and Media Technology

PhD defence by Quiongxiu Li

On Monday August 30, 2021, Quiongxiu Li will defend her PhD thesis "Privacy-Preserving Distributed Processing Over Networks".

Last modified: 25.08.2021



11:00 – 11:05: Welcome by moderator Jesper Rindom Jensen

11:05 – 11:50: Presentation by Qiongxiu Li

11:50 – 12:30: Lunch & Coffee break

12:30 – 14:30 (latest): Questions

14:30 – 15:00: Assessment

15:30: Announcement from the committee and we will propose a toast for Qiongxiu Li



Assessment committee

  • Professor Antonio G. Marques, King Juan Carlos University, Spain
  • Professor Marc Moonen, KU Leuven, Belgium
  • Professor Petar Popovski, Aalborg University, Denmark 


  • Professor Mads Græsbøll Christensen, Department of Architecture, Design and Media Technology, Aalborg University
  • Professor Richard Heusdens, Netherlands Defence Academy and Delft University of Technology


Department of Architecture, Design and Media Technology Aalborg University


Online session via Zoom.

Online session due to COVID-19 - sign up and questions: If you wish to participate in the defense, please send an email to Kristina Wagner Røjen, and she will invite you to the session.

Be aware that you must be muted during the whole defense, also your camera must be off in order to maintain transmission capacity and prevent technical interruptions.

The defense start exactly at 11.00 am. Please, make sure that you have logged in at least five minutes before that time. The session is open from 10.45 am. You are not allowed to join the online session after the defense has started, neither during the break or the examination. There will be a break between the defense and the opponents’ questions.

You have the possibility to contact the moderator Jesper Rindom Jensen by mail and state your question. Please, also contact Kristina with any further questions regarding the defense.


Privacy has become a primary concern in modern world. Addressing the privacy issue is particularly challenging in the context of distributed processing due to many constraints such as absence of centralized coordination, limited computational resources and inevitable information exchange between different computing units.

This thesis discusses on how to conduct signal processing over a network in a distributed manner without violating privacy. In particular, we focus on practical privacy- preserving solutions that are lightweight in terms of communication and computational cost.

We first investigate existing privacy-preserving approaches which apply well-established cryptographic techniques into distributed processing tools.

Secondly, we propose a new subspace perturbation based method that, instead of applying existing cryptographic techniques, directly exploits the potential of distributed processing tools such as distributed optimization for privacy-preservation. The proposed method is able to alleviate two fundamental limitations in existing approaches: the privacy-accuracy trade-off of differential privacy approaches and expensive communication cost incurred in secret sharing based approaches, respectively.

Thirdly, based on the observation that all the above-discussed algorithms use the idea of inserting noise to mask the private data for privacy-preservation, we propose a new information-theoretical metric that is able to relate and compare all of them in a unified framework.

Fourthly, we observe that there is typically a trade-off between the communication cost and privacy in noise insertion approaches and propose to address this trade-off, by making use of a quantization scheme in a particular way that the accuracy of the algorithm output is not deteriorated.

Finally, continuing with the idea of exploring the potential of existing distributed processing tools for privacy-preservation, we take the first step to investigate the emerging graph signal processing tool and propose a privacy-preserving distributed graph filtering solution using noise insertion. This proposed solution has comparative performance compared with the above proposed subspace perturbation based distributed optimization approaches.