## Information Theory Seminar

### Tuesday, March 27th, 2018

**Time:** 3:00 p.m. **Place:** Jeffery Hall 422

**Speaker:** Silas Fong (University of Toronto)

**Title:** Strong converse theorems for multimessage networks with tight cut-set bound

**Abstract: ** In Shannon’s seminal work that established the maximum coding rate of point-to-point communication, it was shown that communicating reliably over a noisy medium is possible as long as the coding rate is below the capacity, i.e., the Shannon’s limit. Conversely, no reliable communication can be supported for any coding rate above the capacity. For communication engineers, this leaves open the possibility of the following tradeoff between coding rate and error probability: Communicating at a rate above the Shannon’s limit while tolerating a non-zero error probability. In this talk, we focus on various multi-user communication systems where this tradeoff does not exist, i.e., there is a sharp phase transition of the performance of the system (quantified by the error probability) between rates below the Shannon’s limit which can be supported for reliable communication and rates above the Shannon’s limit that must lead to catastrophic failure of communication. In this case, we say that a strong converse exists for the system.

In the first part of my talk, I will briefly discuss the latest development of strong converse results for several common multi-terminal systems including the multiple access channel (MAC), the broadcast channel (BC) and the relay channel. The second part of this talk will cover my recent result which proves a strong converse theorem for any multimessage network with tight cut-set bound. In particular, the result yields the first strong converse theorem for the degraded relay channel. A proof sketch based on the method of types will be presented. The Gaussian version of this result yields the first strong converse theorem for the Gaussian MAC with feedback.

**Biography: **Silas L. Fong is currently a postdoctoral fellow with the Department of Electrical and Computer Engineering at University of Toronto. He received his B.Eng., M.Phil. and Ph.D. degrees in Information Engineering from the Chinese University of Hong Kong in 2005, 2007 and 2011 respectively. He has performed postdoctoral research at City University of Hong Kong from 2011 to 2013, at Cornell University from 2013 to 2014, and at National University of Singapore from 2014 to 2017. His research interests include information theory and its applications to communication networks such as relay networks, wireless networks, and energy-harvesting channels.