Newtec EL170 - Appendix D - Normal and Short Frames and Mixing Frames

Appendix D – Normal and Short
Frames and Mixing Frames
User Manual for EL170/970/470 IP Satellite Modulator/
Demodulator/Modem
version 4.2
197
SHAPING THE FUTURE OF SATELLITE COMMUNICATIONS
APPENDIX D – NORMAL AND SHORT FRAMES AND
MIXING FRAMES
Introduction
This section gives some more information on the limitations when using normal,
short frames and mixing these frames.
Mixing Frames
The LDPC/BCH decoder processes a frame while receiving the next frame. This
processing takes a minimum time and the minimal time is about 4 times longer for
a normal frame as for a short frame. As a normal frame is 4 times longer then a
short one, this is normal behaviour.
Now an issue can arise when mixing short and normal frames. When a short frame
is received, the decoder might be processing a normal frame. So it is clear that the
short frame needs to be long enough so that the decoder has the needed time to
process the normal frame.
Short frames need about 0.3 dB more margin for the same spectral efficiency. But
they are 4 times shorter, so have less latency, which is most visible at low symbol
rates.
Rules for Correct Functioning
This following table gives an overview of the maximum baud rates that can be used
in the use of short and normal frames and mixing normal and short frames. This
with different available ModCods.
• The 32APSK ModCod can never be used for short frames.
• When the maximum symbol rate is higher then 30Mbaud the
following ModCods are not possible in ACM:
- QPSK ¼
- QPSK 1/3
-
QPSK 2/5