Shashi Kiran Chilappagari Michael Chertkov Mikhail G. Stepanov Bane V. Vasic Instanton-based Techniques for Analysis and Reduction of Error Floors of LDPC Codes http://arxiv.org/abs/0903.1624 2009 CoRR abs/0903.1624 db/journals/corr/corr0903.html#abs-0903-1624 informal publication