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Errors on my CDM 600 Modem

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Ex Member
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Feb 2nd, 2012 at 8:04am  
Hello Guys,

I just finished an installation on a jackup rig in Nigeria, after the installation i got an Eb/No rate of 9db and the link communicates well. but i noticed that when the Eb/No drops to about 5.3 db it begins to have some errors and the link will become unusable at that point. Please any one with an idea of what the problem could be, should advice.
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Eric Johnston
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Reply #1 - Feb 2nd, 2012 at 11:37am  
The Eb/No of 9 dB should be stable and not drop down unless there is heavy rain.

You are probably using a modulation method and FEC code rate that works well at 9 dB and fails at 5.3 dB.

The question to address is why does the Eb/No drop down to 5.3 dB ?.

Is it due to movement of the antenna causing the beam to become slightly mispointed ?  Is the antenna loose and swinging in the wind ?  Push gently on the antenna in all directions while watching the Eb/No. It should be stable. Did the rig move slightly ?

Is it due to a poor connector on a cable ?  Waggle the cables while watching the Eb/No.  There should be no change.

I presume the Eb/No measurement you are referring to is measured at the rig.  It would be interesting to ask what Eb/No is being recorded at the far end teleport ?  A failure in either direction of transmission will cause a loss if IP connectivity.

Are there any other sites receiving the same signal from the satellite ? Do they see the same changes ? Is their service affected similarly ?

Is the change sudden ?  Did the antenna move ?  Did a cable move ?  Did anything happen at the same time ? Did the receive frequency drift out of the demod capture range ?

Is the change gradual ?  Satellite movement ?  If the 9 dB is not the peaked up value and in fact somewhere on the side of the beam then slight satellite movement which should cause negligible change will cause significant regular daily changes (e.g. 9 dB down to 5.3 dB and back up again) as the satellite moves up and down the side of the beam.  Peak up.  Is the change temperature related ? Read out the tuning error ?  Is the change weather related ? Was it raining heavily at the site or at the land end teleport when the signal dropped ?

Just some ideas ...

If it is an emergency and you have a point to point service and you want to keep some kind of service going temporarily, despite the problem, then drastically reduce the information bit rate and alter the modulation/FEC rate to keep the carrier bandwidth constant.  Try QPSK 21/44 Turbo which should work perfectly at Eb/No= 5.3 dB. Get help from the teleport end. They may be able to control your modem remotely (when the signal is good!) but there is a risk they could lose control so it is a good idea to have someone on the rig to manually intervene if necessary.  Make sure the remote is configured to go to transmit inhibit if it loses the signal from the hub.

Best regards, Eric.
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Reply #2 - Feb 2nd, 2012 at 5:57pm  
Thanks for you Reply to my challenge. Am presently working out changing my Modulation type I will also look at other suggested options. Thanks a-lot,  really appreciate it.
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Eric Johnston
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Reply #3 - Feb 2nd, 2012 at 7:15pm  
I note that, for a BER=10E-8 :

8-PSK 3/4  Turbo needs Eb/No=9.8 dB (includes 3 dB margin)  Symbol rate = 0.44 x bit rate.

QPSK 3/4 Turbo rate needs Eb/No=7.4 dB (includes 3 dB margin)  Symbol rate = 0.67 x bit rate.

Remember to keep the symbol rate and bandwidth unchanged by reducing the bit rate, otherwise the carrier bandwidth will get wider and you will interfere with adjacent carriers.  
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