Mocomtech CIM-550 - Manual (Page 132)

CiM-550 IP Enabled Satellite Modem Rev. 2
EDMAC CD/CIM550.IOM
106
Adding the extra 5% to the transmitted data rate, the effective Eb/No seen by the user
will degrade by a factor of 10log(1.05), or 0.21 dB (0.07dB in the case of the two BPSK
Turbo rates). The use of an externally synchronized scrambler and descrambler almost
exactly compensates for this degradation. The net effect is that the user will see
effectively identical BER performance whether framing is used or not.
8.1.2 O N THE RECEIVE SIDE
When the demodulator locks to the incoming carrier, it must go through the additional
step of searching for, and locking to the synchronization word. This uniquely identifies
the start of frame, and permits the extraction of the overhead bytes and flag bits at the
correct position within the frame. In addition, the start of frame permits the de-scrambler
to correctly recover the data. The user’s data is extracted, and sent through additional
processing, in the normal manner. The extracted overhead bytes are examined to
determine if they contain valid M&C bytes.