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SCPC RX ERRORS

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Mwafrika
Ex Member


Jul 22nd, 2008 at 7:34am  
Hi Guys,
I have an iDirect HUB in Kenya on IS-802, Ku band.
I am using a PLL LNB and a 3.8m dish and
A few days ago all the remotes started generating a lot SCPC RX errors.
I have checked and rechecked the RF chan and it is clean.
A frequency scan doesnt reveal an spurious tranmissions either from my dish or the others at the teleport.
What could be causing this and how can I sort this out?
Please help!!!

*****
Nick
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Eric Johnston
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Reply #1 - Jul 22nd, 2008 at 9:29am  
I'm not certain what you mean by SCPC errors.  I assume you are referring to the main outlink carrier from the hub to all the remotes.

Do the errors occur randomly all of the time or do they occur occasionally in bursts around particular times. If you have steady random errors all of the time the cause exists all of the time and should be easier to find.  If the errors occur occasionally at particular times you have some kind on intermittent problem, like intermittent interference or a loose connection.

If you have a continuous problem.  Is the outlink carrier level correct ?  Is the (C+N)/N normal when viewed on a spectrum analyser ?  Is the carrier shape normal ?  Do all sites show low Eb/No all of the time ?.

If the carrier, and the gaps adjacent, look normal then leave the spectrum analyser in max hold mode and see if anything builds up. Watch for little spikes from TDMA bursts that may add up over a while to form an interfering carrier shape.  If the adjacent co-pol carrier is TDMA wait to see if one of the sites on that carrier has a bad spectrum.  If a BUC is overdriven then they will have spectrum spreading and one burst every so often on the adjacent TDMA carrier will spread out and hit yours.

If the carrier shape looks abnormal, with humps or partly filled up guard bands, measure the cross pol spectrum and consider any correlation between the cross-pol spectrum and any humps in the wanted carrier.  Plotting the two spectrums on thin paper and putting one over the other is suggested.  Is so, plan an outage of your outlink carrier and during this outage record and plot the noise floor underneath your wanted carrier slot.  You may detect a cross-pol interferer within your carrier bandwidth.  If you see a TDMA carrier on the cross pol there may be one site amongst many sites on this carrier slot that is badly polarisation aligned and when this site transmits a burst it puts errors into your outlink.

If you do see TDMA type interference tune the spec analyser to the interfering frequency, set the RBW equal to approx the interfering carrier bandwidth and then set zero span and a slow sweep and wait.  Max hold will help record anything. Expect patterns of bursts at the TDMA frame rate.

When did the problem start ?  Tell Intelsat and ask if any new carrier or remote site was being lined up at that time.

If it is an intermittent problem try the spectrum analyser in max hold mode.  Leave it on for many hours waiting to see if it records anything.  You may have to experiment over several days with different settings. If the interferer is brief (under 1 second) make sure you have the sweep rate fast enough.  A 30 second sweep rate may miss a short noise burst. Ultra short bursts like 1mS need 1kHz video bandwidth.

At the hub I recommend that you have a spectrum analyser on all of the time monitoring the outlink and return link spectrum.

Best regards, Eric.
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TDMAMike
Ex Member


Reply #2 - Jul 22nd, 2008 at 12:24pm  
How many remotes?  

How close (in proximity) are they to each other?

Errors constant?

Suggest that you report your findings to Intelsat Carrier Management Center (CMC) and have them take a look at your FWD and return carriers (forward specifically).

My guess is that you have a small (or nearly negligible) carrier under your forward (outbound) carrier causing the errors.  Could be energy from the cross pol or local. That, or something is amiss on the uplink RF chain at the e/s.



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« Last Edit: Jul 22nd, 2008 at 8:16pm by N/A »  
 
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Mwafrika
Ex Member


Reply #3 - Jul 22nd, 2008 at 1:38pm  
I have about 60 remotes distributed all over East and Central Africa.

Nick
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Space
Ex Member


Reply #4 - Jul 27th, 2008 at 2:55pm  
Nick

I am in Kenya also with a similar Hub for internet and p2p africa to africa on idirect.

May be we can meet and exchange ideas.

FYI i had same problem but  i cleaned it up. Whats your size of dish at the hub. What  Kind of SNR do you get on the remotes. What dish size and BUC do you use on the remotes

signal@space.co.ke

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Mwafrika
Ex Member


Reply #5 - Jul 30th, 2008 at 12:58pm  
Hi Space,
When can we sit and we discuss a few stuff?
Would like to know how you sorted out your problem.
Send me a mail on nmuttai@gmail.com

Mwafrika
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dot
Ex Member


Reply #6 - Jul 31st, 2008 at 5:42pm  
Guys. SCPC Rx Errors are reported by iMonitor on a per remote basis. They indicate that something in the transmit path of the hub or receive path at the remote site is at fault.

If it happens at all your sites you need to start looking at the hub. It could be an up-converter, incorrect RCM configuration, carrier configuration or a problem with your HPA.

If it happens to only some remote sites you need to determine what the common item is. Normally this may be the type of lnb in use, local interference, cables or RF component parameters at the hub.

Give me more information and I can help you.
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TDMAMike
Ex Member


Reply #7 - Aug 1st, 2008 at 11:51am  
Just wanted to quickly point out that he said ALL of his nodes were reporting it....and not just one.  Which is why I went the direction of possible energy underneath his operational carriers from the crosspol, or something is amiss in the uplink IF/RF chain at the teleport e/s (HPA, UC, or assoc cabling). 

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Mwafrika
Ex Member


Reply #8 - Aug 4th, 2008 at 9:02am  
Thanks a lot guys. The info has really helped.
Dot, what info exactly can provide you with to help you get a clearer picture?

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dot
Ex Member


Reply #9 - Aug 4th, 2008 at 4:00pm  
Mwafrika, I sent you a PM with my contact details.
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aladin
Ex Member


Reply #10 - Nov 2nd, 2009 at 8:07pm  
hello, i have the same problem in our i monitor,some modems have some doesnt

some modems have around 40000 errors i donno how this may affect internet quality

we always recommend the customers to have RX SNR more than 12 Dbm

i need explanation Please Smiley
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