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Downstream C/N below limit -7.3, remote stat lost

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Oct 23rd, 2009 at 6:59am  
Hello, my name is Daniel and I have an emergency, there is an station that has been showing intermittence during 4 days, during the night, the communication is lost. I thought this could be weather conditions as there was heavy rain and the Downstream C/N was between 4 and 5, being the low limit equal to 7.3, but this very night, there is no rain and the  Downstream C/N is 4.6 and the Upstream C/N is 6.6, and the communication is lost most of the time, also for latency there is no answer in more or less 80% of the attemps. could this be misalignment?

Please help me with this issue, you can contact me in danielmorganster@hotmail.com or through this forum

Thanks in advance

Daniel
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dot
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Reply #1 - Oct 23rd, 2009 at 8:45am  
Hello Daniboy

The fact that it occurs every night means that it might be related to atmospheric conditions or local interference. It won't cost you anything to check the pointing again as well since someone is going to have to go out to site to check for obvious sources of interference. While you're about that you may as well complete a 1dB compression test and confirm the polarisation is correct too. It may be a good idea to try a waveguide filter before the LNB if it is localised interference.

I assume that the service is C-band and only this site reports this condition?
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TDMAMike
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Reply #2 - Oct 23rd, 2009 at 11:39am  
I agree with dot.  I am thinking Atmospheric or interference. 

What kind of LNB does this unit have? PLL?  DRO? 

What do the temps look like over a 24hr period?  Does the temp change radically from day to night? 

Any chance that he had heavy (heavy)cloud cover and just no rain?  It sounds like the remote is in a fringy area.  What is its downstream SNR during the day (under clear skies)?  It sounds like it may be borderline even under normal conditions (read: operating at 7-8dB SNR).



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Regards, &&&&M
 
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Reply #3 - Oct 25th, 2009 at 5:47am  
Hello and thanks for your reply.

The BUC that I have installed is a 20W Mitec, of which I checked the documentation and as its power is greater that 5 Watts, it is not neccesary to carry on the 1 dB compression tests (correct me if I am wrong).

Also the LNB that is installed is a Norsat 3000 PLL LNB in C Band. the weather is cloudy and rainy, but sometimes its sunny as the day I opened the topic.

The remote is located in Central America, and regarding the typical SNR in a clear day, it has been always between 7 and 8, being the low limit 7.3.

I also had a recommendation that it could be a problem related to the LNB, which could be damaged/degraded or even the LNB cable.

thanks in advance for your cooperation

Daniel
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Eric Johnston
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Reply #4 - Oct 25th, 2009 at 12:14pm  
Maybe water inside the feed system. 

Evaporates in warm daytime, condenses to water drops at night ?  Just a guess..

Best regards, Eric.
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fluffymassive
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Reply #5 - Oct 27th, 2009 at 11:35pm  
i would check:
- alignment
- polarization
- moisture in OMT
- and also perform 1dB compression test.

If all else fails take a spare set of LNB, BUC, and OMT and try it with that.

Cheers,
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Reply #6 - Oct 31st, 2009 at 2:13pm  
Hello, just to mention that the LNB was replaced and that the alarm was cleared, but now there is an alarm that says that power TX power surpases maximum. actual TX power is minus 13dBm (before the change it was at minus 17dBm, and acceptable TX power values are between -35 and -13 dBm) so then the remote is transmitting at maximun power. I tried to adjust the power to -15dBm and the alarm dissappeared for some seconds but then the alarm came on again and the power was again at -13 dBm. How can I stop this automatic behavior that adjust the power back to -13dBm?

Also there is an intermittent alarm that is appearing and clearing intermittently in the Hub side in the Hub Line Card that says that Hub Modem Traffic below CRC.

Please help me with this issue.

Best Regards.

Daniel
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Eric Johnston
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Reply #7 - Oct 31st, 2009 at 3:41pm  
My guess..

The remote dish has become slightly mispointed so the transmit beam is low gain towards the satellite.  The result is a poor signal back into the hub.  The hub tells the remote to increase its power (as it should automatically during heavy rain) and the modem output goes up and hits your limit of -13 dBm, but it is still not good enough and it is trying to go further.  Get the people at the remote to try to peak up the dish pointing more accurately.  Your hub will tell the remote to transmit at lower or higher levels till it gets to the 'sweet spot' for correct hub input signal level.  The stabilised final level, during clear sky, should be significantly less than your -13 dBm limit. The difference is the uplink power control range (for rain fading).

Your limit of -13 dBm is supposed to correspond to the P-1dB compression point of the BUC output.  This is to stop damage to the BUC by overdriving it.  If you start the level quite low, say -25dBm, what happens ?

The figure below illustrates how the output power of the BUC varies with input power.  If the modem has output power of -15 dBm and there is a cable loss of 5 dB then the power into the BUC is -20 dBm.
...

Note that overdriving the BUC causes a distorted signal and high error rate.  The hub software may interpret this as low level and increase the BUC drive further and get stuck with poor carrier and overloaded BUC.  Test all sites to determine their P-1dB compression point and then set this as the maximum in the hub config for that site, as the gain of BUCs vary and also cable lengths at each site.

Best regards, Eric.
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TDMAMike
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Reply #8 - Oct 31st, 2009 at 8:47pm  
Daniel,

Are you a Hub Operator or are you on the VSAT side?

Agree with Eric about pointing.  Try to reclaim the additional power the modem took on (when the LNB was swapped) - by repointing that system.  That should bring you back to the -17 plus or minus 2 dB.

The warnings/errors on the line card are expected from a system that is poorly pointed. The system is more than likely so poorly pointed that bursts are arriving LOW on the line card (the system is not within the sweet spot of the UCP)....which is generating the errors.

Get that unit repointed.

M

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Regards, &&&&M
 
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Reply #9 - Nov 11th, 2009 at 6:20pm  
Hello, jut to mention that the antena was repointed and the power returned to a normal level, but the Downstream C/N was only increased to 6.5 oscillating up to 7.5. Also some water was take out from the feeder in the antenna, and a connector in the cable from the Mitec to the feeder was built.

Some CRC errors still arrive to the Hub but just when weather gets bad

it seemed to normalize with the actions taken (thanks to everybody for your cooperation) but then it began to put errors again and regarding the cellular system, we are expeiencing serious issues with the Drops calls, so we are evaluating the IP stats searching for retransmission or lost packets or bottlenecks.

Please give me your opinion about where to look at in this issue

Best Regards

Daniel  PD: How can I include images in this field?
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TDMAMike
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Reply #10 - Nov 11th, 2009 at 6:42pm  
Glad to hear your power has returned to normal/nominal.

Low Downstream C/N could be numerous things.  

1.  The Hub could be under weather (your location being under weather can also cause C/N to drop however, I am sure you are keeping an eye on the weather).
2. Your Hub operator may be at contract power with the satellite provider and it is not enough for a "respectable" C/N (from your operating location).  Where are you located in the beam?  Fringe area, or are you in a favorable spot in the beam?

Do you have any other systems co-located with you, or one in close proximity to you that you can use as a reference terminal/system?  If anything, to compare signal to noise readings?

The CRC errors when under weather are probably due to the fact that your are teetering on or very close to your 1DB compression point (very close to BUC saturation).

Did I read your post correctly that you are backhauling GSM/Cellular?  If so, is that traffic all UDP?
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« Last Edit: Nov 11th, 2009 at 9:23pm by TDMAMike »  

Regards, &&&&M
 
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Reply #11 - Nov 11th, 2009 at 8:06pm  
Do you own your own hub and teleport or are you a VNO? I would be interested to see what your tx line cards are at power wise. Also, if you do operate your own hub, what are the specs? My downstream C/N on C Band is near 13-15 depending on footprint. Though I have a decent sized hub dish (13m) I'm also running at near max power.

So.. details?? Wink
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Reply #12 - Nov 18th, 2009 at 8:20pm  
Hello Guys, the remotes are used to provide cellular backhaul for 7 remote sites, these remotes are located in Puerto Lempira, a remote location in Honduras. The problematic remote station is located eastermost in the country, and we have additional 4 remote units locate at the west at aproximately 40, 58, 65 and 80 miles respectively, and all of these remotes have Downstream C/N of 12 to 13 dB.

At the Hub Side we have and 7.3 meters antenna located in Tegucigalpa and in the hub we have a 12000 iDirect series Chasis, with two line cards. the Hub is transmitting at -3dB and has a 80W Mitec BUC. But I am still trying to get the Downsstream C/N value for the Hub but i have not found this yet, do you know where to find it?.

I will also look for satmex-5 foot print in Honduras, in order to check if this is contributing to the low Downstream C/N value.

Also if anybody has an advanced troubleshhoting manual, please send this to me and it will be highly appreciated. (danielmorgaster@hotmail.com)

any additional coomments or questions just let me know.

Best Regards

Daniel
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Eric Johnston
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Reply #13 - Nov 18th, 2009 at 9:04pm  
On 11 Nov
Quote:
Also some water was take out from the feeder in the antenna

There may be more water in the rings of the conical feed horn, in the LNB transmit reject filter and LNB waveguide. All this will degrade the receive side.

To display pictures here in this forum, put the image file (under 20k bytes file size if possible) on a server somewhere. Then type (img)https://www.exampleserver.com/images/myimage.jpg(/img). Use square brackets not rounded ones.

Best regards, Eric.
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