Wireless Modem Data Pump
FX909A
Example:
If the byte to be coded is '00101100' then the FEC is derived as follows:
H matrix row:
1
2
3
4
A
B
11101100
00101100
00101100
1
11010011
00101100
00000000
0
10111010
00101100
00101000
0
01110101
00101100
00100100
0
A AND B
Even Parity:
where A is bits 7 - 0 of one row of the H matrix and B is the byte to be coded. The even parity bits
apply to the result of 'A AND B'.
So the word formed will be: '00101100 1000' sent left to right
When the same process is carried out on these 12 bits as above, using all 12 bits of each H matrix
row, the resulting 4 parity bits will be '0000'.
1.5.5.3 Interleaving
The 240 bits of a Mobitex Data Block are interleaved by the modem before transmission to give
protection against noise bursts and short fades. Interleaving is not performed on any bits in the
Mobitex Frame Head.
Considering the 240 bits to be numbered sequentially before interleaving as 0 to 239 ('0' = bit 7 of
byte 0, '11' = bit 0 of FEC for byte 0, ... ,'239' = bit 0 of FEC for byte 19 - see Figure 6), then they will
be transmitted as shown in Figure 13.
The modem performs the inverse operation (de-interleaving) in receive mode on Mobitex Data
Blocks.
ã 1996 Consumer Microcircuits Limited
30
D/909A/4
CMLMICRO [ CML MICROCIRCUITS ]
