Abstract
When we consider convolutional coded CPFSK over fading channels, the primary design criterion (for high SNR) is to maximize the effective code length (ECL), where ECL is the minimal possible Hamming distance between any two distinct symbol sequences. Define the free branch distance (d free ) as the minimal possible branch distance between any two (unlabeled) parallel paths in the code trellis. That d free achieves the maximum value is a necessary condition for maximizing ECL.