Free Distance Bounds for Protograph-Based Regular LDPC Convolutional   Codes
  In this paper asymptotic methods are used to form lower bounds on the free distance to constraint length ratio of several ensembles of regular, asymptotically good, protograph-based LDPC convolutional codes. In particular, we show that the free distance to constraint length ratio of the regular LDPC convolutional codes exceeds that of the minimum distance to block length ratio of the corresponding LDPC block codes. 
David G. M. Mitchell, Ali E. Pusane, Norbert Goertz, Daniel J.   Costello Jr