University of Hawaii

Department of Electrical Engineering

The Separation Capability of Cosets of Turbo-Like Codes

Date: 2008-11-06
Time: 4:30-5:45 PM
Location: Holmes Hall 389
Speaker: Dr. Mao-Chao Lin

ABSTRACT : Turbo-like codes such as turbo codes, LDPC codes and RA codes are known to have powerful error-correcting capability.   We can obtain cosets of the turbo-like codes by taking pseudo-random sequences as coset leaders. If the number of cosets is not great, these cosets are widely separately as compared to the vectors within each coset. This means that the probability of erroneously detecting a vector in one coset into a vector in another coset is very small.  Hence, the union of these cosets actually forms an unequal-error protection (UEP) code.  UEP codes of similar properties can also be obtained by applying various permutations to code bits or using various interleavers for the turbo-like codes.    In this talk, we will show several applications employing these separation capabilities or UEP capabilities, where multiple candidates are used to represent one message.   The first is the peak-to-average power ratio (PAPR) problem in the OFDM system.   The selective mapping method is a technique of using multiple-candidate representation for alleviating the PAPR problem in the OFDM system.  Based on the separation property, we can construct a turbo coded OFDM using the selective mapping operation without the need of transmitting side information. 

The second is the channel coding for channels with runlength-limited (RLL) constraints. To efficiently combine the runlength-limited (RLL) coding and the channel coding, it has been proposed to directly flip bits which violate the RLL constraint.  Multiple-candidate representation can be used to reduce the number of flipped bits. We can use the cosets of an LDPC code to create the multiple-representation of each message.  The third is the shaping of modulation points in the constellation.  The shaping operation can help reduce the average power.  Using the separation concept, we can obtain shaping gains for turbo coded modulation without sacrificing the overall transmission rate.

Speaker Biography : Mao-Chao Lin received the Bachelor degree and Master degree both in electrical engineering from National Taiwan University in 1977 and 1979 respectively.  He received his Ph.D. degree in electrical engineering from University of Hawaii in 1986.  From 1979 to 1982, he was an assistant scientist of Chung-Shan Institute of Science and Technology at Lung-Tan, Taiwan.  From 2000 to 2003, he was the Chairman of Graduate Institute of Communication Engineering, National Taiwan University.  He is currently a Professor at Department of Electrical Engineering, National Taiwan University.  Since August 2008, he is a Visiting Professor at Department of Electrical Engineering, University of Hawaii, Manoa.  His research interests are in the area of coding theory and its applications.