Code Rates and Puncturing
Convolutional encoders add additional information to the encoded bit stream. This is always at a fixed integer value.
A rate 1/2 encoder produces 2 output bits for every input bit
Since this represents a massive transmission overhead it is also possible to “puncture” the code (or simply remove bits) before transmission to save bandwidth.
The puncturing scheme below is 2/3 rate puncturing and is based on rate 1/2 coding
Puncturing schemes have a pattern which determines which bits are removed
0 1 0 1 0 0 0 0 0 1 0 1 0 1 0 1 1 0 1 0 1 0
1 0 0 1 1 0 1 0 0 0 1 1 1 0 1 0 1 0 1 1 1 0
Decoder replaces punctured bit locations with “0” before decoding
- Requires external synchronisation to indicate punctured positions
Rates 2/3, 3/4, 4/5, 5/6, 6/7 7/8 supported
Removed Information bits marked in red