Furuno FELCOM 12, Service Manual

Page: 38 / 118

2-7
Convolutional Encoding
The end of scrambled bit stream data of 120 bits is affixed with 6 bits of continuous
flash bits and sent to the half-rate convolutional encoder which consists of a 7 bits
shift register (constraint length-7) and two polynomials. 126 bits of signalling packet
input are encoder to 252 symbols. The first state of the shift register is all zeroes.
X0 X1 X2 X3 X4 X5 X6
G1=X0+X2+X3+X5+X6
Start state
: 0 0 0 0 0 0
Coded symbols
(To interleaver)
exclusive OR
Formatted
(Scrambled) data
G2=X0+X1+X2+X3+X6
G2
G1
+
+
(Half-rate: Output of 2 bits
with input of 1 bit)
exclusive OR
Figure 2-7 Encoding
Unique Word
The 252 symbols from the encorder is affixed with an unique word (uw) to be used for
bit synchronization at the recorder. The uw is 64 bits in length on the SES signalling
channel and 64
×
2 bits on the NCS/CES TDM channel.
Interleaving and
Permuting Encoder
UW symbol
BPSK modulator
or
interleaver
(added)
Figure 2-8 UW
Interleaving and Permuting Encoder
The figure below shows the interleave matrix, consisting of 64 rows by 162 columns.
Symbol data comes in column by column. Note that columns 0 and 1 are filled with
unique words.
0 1 2 ....................................161
0
1
2
63
62
...............
Column
Row
1 2 3 ... 64 ... 128 65 10240 ... 10177 ..................
UW 64bits UW 64bits
1
2
3
64
65
66
67
128
10177
10177
10177
10240
10239
63 127
Bit stream
Figure 2-9 Interleave Matrix
Interleave Matrix