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Lattice Semiconductor
Convolutional Encoder User’s Guide
Punctured Codes and Depuncturing
After convolutional encoding, some of the encoded symbols can be selectively removed before transmission. This
process, called “puncturing”, is a data compression method used to reduce the number of bits transmitted. Figure 3
shows an example of puncturing.
Figure 3. Puncturing Process
After convolutional coding
i 0
i 1
Input data
i 2 i 3 i 4
i 5
i 6
a 0 a 1 a 2 a 3 a 4 a 5 a 6
b 0 b 1 b 2 b 3 b 4 b 5 b 6
Puncture pattern
Puncture pattern
superimposed
Final punctured output
1
1
0
1
1
0
a 0 a 1 a 2 a 3 a 4 a 5 a 6
b 0 b 1 b 2 b 3 b 4 b 5 b 6
a 0 b 0 b 1 a 2 a 3 b 3 b 4 a 5
If puncturing is employed in the encoder, the decoder will have to “depuncture” the data before decoding. Depunc-
turing is done by inserting NULL symbols for the punctured symbols. NULL symbols are equidistant from either ‘0’
or ‘1’.
Convolutional Encoder Core Description
Internal Architecture
Figure 4 shows the modules of the Convolutional Encoder and their interconnectivity. A brief description of the
modules follows.
Figure 4. Convolutional Encoder Internal Architecture
din
clk
Encoder
Puncture
Unit
dout
reset_b
dout_valid
din_valid
Control Unit
pd_start
rfi
Encoder
This module takes input data and performs convolutional encoding. The encoder uses generator polynomials con-
?gured by the user. When punctured encoding is enabled, the encoder performs 1/2 rate encoding irrespective of
the encoder rate. The puncture unit will use the 1/2 rate code to generate the appropriate user-programmed rate.
Puncture Unit
This unit performs data puncturing, as previously explained. The input is a two channel data stream and the output
is always a one channel output. The unit is capable of performing puncturing of any block size and any rate.
Control Unit
The control unit generates the handshake signals dout_valid, rfi and pd_start using din_valid and the
status of the decoder. It also generates various control signals required by the encoder and puncture unit.
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