There are many good periodically time-varying convolutional codes
P.J. Lee · IEEE Transactions on Information Theory · 1989
It is shown empirically that the number of good periodically time-varying convolutional codes increases exponentially with the period for any set of parameters. Hence, they can be used to enhance the security of cryptosystems without sacrificing error correction capability. It is shown that some periodically time-varying convolutional codes improve the free distance compared with fixed codes.>