The free distance of fixed convolutional rate 2/4 codes meets the Costello bound
Vladimir Victorovich Chepyzhov, B. J. M. Smeets, Kamil Sh. Zigangirov · IEEE Transactions on Information Theory · 1992
The long standing question whether the free distance of fixed rate convolutional codes is as good as the Costello bound was almost solved by K.S. Zigangirov and J.L. Massey (1987). They proved that this is indeed the case for codes with long branch length and rates 2/c, c>or=5. It is shown that there exist fixed convolutional codes of rate 2/4 whose free distance d/sub free/ meets the Costello bound originally derived for time varying convolutional codes.>