Convolutional Code - Recursive and Non-recursive Codes

Recursive and Non-recursive Codes

The encoder on the picture above is a non-recursive encoder. Here's an example of a recursive one:

One can see that the input being encoded is included in the output sequence too (look at the output 2). Such codes are referred to as systematic; otherwise the code is called non-systematic.

Recursive codes are almost always systematic and, conversely, non-recursive codes are non-systematic. It isn't a strict requirement, but a common practice.