NSF Feedback

Adam Williamson

I am a Ph.D. student in the UCLA Department of Electrical Engineering, working in the Communication Systems Laboratory with Professor Rick Wesel.

Much of my research has focused on the benefits of feedback in communication systems. In this feedback work, I'm investigating how incremental redundancy enables transmission at rates close to capacity with short blocklengths (i.e., in several hundreds of bits rather than several thousand that have traditionally been required for fixed-length block codes). This is important because it reduces the number of symbols required to transmit short messages. I've demonstrated several constructions for deterministic coding schemes that surpass the information-theoretic lower bounds on rate at short blocklengths, both for the decision feedback and information feedback settings.


For blocklengths under one hundred bits, it's especially important to account for the latency cost of the stopping rule used with variable-length codes, since transmitting additional parity bits for error detection (e.g., in the form of a CRC) imposes rate loss. One alternative to explicit error-detection that I've explored is the Reliability-Output Viterbi Algorithm (ROVA), which determines when to terminate transmission based on the word-error probability of the decoded convolutional code sequence. Due to the rate loss of terminated convolutional codes at short blocklengths, I created the ROVA for tail-biting convolutional codes.

For more details about my feedback work, see this Feedback Overview, as well as the following paper and slides from ISIT 2012:
  • A. R. Williamson, T.-Y. Chen, and R. D. Wesel, "A Rate-Compatible Sphere-Packing Analysis of Feedback Coding with Limited Retransmissions". IEEE Int. Symp. Inf. Theory (ISIT), Cambridge, MA, USA, July 1 - 6, 2012.
    Available at http://arxiv.org/abs/1202.1458. [Slides]

In addition, I'm interested in applying information-theoretic concepts and communication system design techniques to a variety of other problems, such as coding for storage. I've explored system-level approaches to use mutual information to optimize the lifetime of flash memory devices and have investigated techniques to mitigate cell-to-cell interference in flash memory.

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Contact Information:
adamroyce - ucla - edu