~~Speaker: Ayelet Aharon,
M.Sc. student under the supervision of Prof. Simon Litsyn

Wednesday, March 11, 2015 at 15:30
Room 011, Kitot Bldg., Faculty of Engineering

Adaptive Successive Cancellation List Decoding of Polar Codes

Abstract
Polar codes, invented by Arikan, are the first to achieve the Shannon capacity over discrete memoryless channels (DMC) with feasible implementation complexity of O(n*log(n)), where n is the code's length. Successive Cancelation List (SCL) decoding combined with Cyclic Redundancy Check (CRC) improves polar codes' performances to the extent of being better than those of modern coding techniques. The SCL decoder explores simultaneously L decoding paths which results in running time complexity of O(L*n*log(n)) and space complexity of O(L*n) where L is the list size.

In this work an adaptive version of the SCL decoder, namely ASCL decoder, is proposed. While decoding, the ASCL decoder decides, with the help of predefined thresholds, how many paths to allocate at each decoding step according to the input's noise level and the vulnerability of the currently decoded bit to mistakes. At each decoding step, up to L_max paths can be allocated (if a threshold has isolated more paths the algorithm uses sorting to detect the L_max most probable paths) leading to a space complexity of O(L_max*n). The running time complexity is O(L_average*n*log(n)), where L_average≤L_max is the average list size the decoder uses, which can be monitored by changing the thresholds suitably.

For a given running time complexity, the proposed algorithm's BLER performance is significantly improved in comparison to this of the SCL decoder. Furthermore, for a high enough SNR, the BLER performance of the ASCL decoder with any 2≤L_average≤L_max  is similar to this of the SCL decoder with list size L=L_max. This makes the ASCL decoder a valuable replacement to the SCL decoder.

Unlike the SCL which requires to perform sorting at (almost) each decoding step, the ASCL decoder avoids most of these procedures due to its use of thresholds. By utilizing this property even further, we suggest modified versions of the ASCL decoder which completely avoid sorting at the expense of a relatively small decrease of the BLER performance.

Finally, since the ASCL decoder (as well as any other list decoding procedure), relies on the use of a CRC, we establish a set of guidelines to assist in finding the optimal CRC length for a given SNR value. A wise choice of the CRC can prevent an unnecessary and sometimes significant loss in performance, especially when the decoder's performance has the potential of improving drastically as the SNR grows.

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Speaker: Pavel Kounitsky
M.Sc. student under the supervision of Prof. Anthony J. Weiss and Dr. Yossi Yovel

Wednesday, March 25, 2015  at  15:30
Room 011, Kitot Bldg., Faculty of Engineering

Dynamic Beamforming of Echolocating Bats
Abstract
This work focuses on the engineering aspects of the development of methodologies and analysis tools for multidisciplinary research on ultrasonic bats’ beamforming. 

Echolocating bats perceive their environment acoustically, by emitting ultrasonic pulses and analyzing the received echoes.  The volume of space that is covered by the ultrasound pulse and then is sensed by the bat depends on the emitted beam.  Echolocating bats can rapidly adjust many of their biosonar parameters to optimize sensory acquisition.  The ability to change the ultrasound beam in a functional way is an area of great interest in the fields of zoology, neuroscience and biomimicry.

The research of beamforming poses multiple engineering problems.  This study focuses on mouth emitting bats, that have been hypothesized to change the mouth gape to control the shape of the biosonar beam, and therefore, there are two main challenges: estimation of the mouth gape from a 2D camera image and beam shape restoration from an ultrasonic microphone array.

A neural network approach and a training algorithm have been chosen to estimate the mouth gape, solving both the camera’s angle and zoom variance problems.  Beam restoration involves application of multiple signal processing techniques, such as TDOA analysis, multilateration, flight trajectory Kalman filtering and beam interpolation.

With the developed set of tools, it was possible to perform correlation analysis over the observed features and make several dramatic conclusions about the dynamic beamforming of bats.