Time varying convolutional codes revisited

There are several cases where the maximum free distance (MFD) time-invariant convolutional code of a given rate and total encoder memory does not meet the Heller bound or where the encoder that does meet the Heller bound is catastrophic. In order to find codes that meet the Heller bound, it is sometimes necessary to use time-varying convolutional codes. Some periodic time-varying convolutional codes with free distance greater than the best known time-invariant code of the same rate and total encoder memory are presented. These codes also achieve the Heller bound for the given code parameters. In addition, the use of time-varying encoders in order to achieve "spectral thinning" is explored.