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How to Improve weak SNR

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bsajidhayat@gmail.com
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Dec 16th, 2020 at 4:56am  
If alignment and RX chain is prefect but still SNR is very low , what is cause and how it can be improve
More over Kindly guide me about basic link budget for a VSAT
which factors are involve and how we can get better link budget.
Regards
SHB
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Reply #1 - Dec 17th, 2020 at 8:30am  
Use a larger, higher gain, receive antenna. e.g. +3dB more gain. Provided that both the uplink C/N and transponder intermodulation C/Im are very much higher, this will give almost 3dB improvement in downlink C/N.  Review the link budget and play around with figures to see what improving the downlink receive gain can do. Increasing the diameter of the dish by 1.4 will double the gain, but note that more skill is needed to assemble a larger dish accurately and point it. Poor skill can easily lose you 3 db in gain.

Be cautious about believe anyone who says alignment is perfect. You may be peaked up pointing on the first sidelobe of the antenna, in which case gain will be 10 to 16 dB down.  Check using crossed strings that the dish rim is flat as distortion can knock several dB off the gain. This can be a problem with heavy LNB/BUC assemblies.  Check that the LNB feed window is clean and that there is no water inside.

Look on spectrum analyser for noise: cross-pol noise due to cross-pol alignment error and also co-pol adjacent satellite interference.

Talk to the satellite network operations centre and consider these options:

Ask about increasing the downlink power.  e.g. + 3dB.  This may not be possible or not permitted in any kind of shared network. May be possible in SCPC system but you will have to pay twice as much or leave half your rented bandwidth unused.

Reduce your information bit rate  to half. This keeps the power constant but increases the power spectral density by 3 dB.

Alter your modulation and coding. If you are trying to do 16-QAM sytem with 0.9 FEC and are complaining about low SNR, then you can get it working again by using say QPSK 7/8FEC but with reduced information bit rate.

Best regards, Eric
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Reply #2 - Dec 18th, 2020 at 5:32am  
Thank you so much Eric

Please tell me about Advantech mini Hub (CCM)

CCM stands for ?

RGDS
Sajid Hayat
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Reply #3 - Dec 18th, 2020 at 9:39am  
CCM means Constant Coding & Modulation.

The outlink carrier is in DVB-S2 format and CCM (Constant Coding & Modulation) is most basic mode of operation. VCM (Variable Coding and Modulation) is an improvement that allows the hub to communicate as best it can to individual or groups of remote sites, given their particular size, location in the beam or rain fade status.

DVB-S2 is a continuous transmission, received by multiple sites.  Although continuous, the stream is made of up blocks of data, destined for different remotes, which can each have their own modulation and coding to suit their circumstances dynamically.

Read more here : https://en.wikipedia.org/wiki/DVB-S2
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« Last Edit: Dec 18th, 2020 at 10:58am by Admin1 »  
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Reply #4 - Dec 20th, 2020 at 6:22pm  
Thank you so much Eric,

Kindly tell me how to calculate the frequency for Both ends of VSAT Terminal If I have 2 M available BW, both terminal are 1.5m KU band, SCPC,
more over I need to know some basic parameters for VSAT link budget, if you can guide me for formulae and the small definition of LB terms , I would be grateful to you. 
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Reply #5 - Dec 20th, 2020 at 6:23pm  
Please tell me the difference of C/N and EBNO
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Reply #6 - Dec 27th, 2020 at 11:22am  
To get you started..

You are trying to make two SCPC links between 1.5m Ku band antennas. You have 2 MHz bandwidth available.

Allocate 1 MHz to each carrier.

Invent a link budget, like this:
https://www.satsig.net/skylinx1.htm

Adjust all the input numbers yourself aiming for a workable uplink and downlink.

If you know the satellite transponder uplink G/T towards your sites then use that data.  You need uplink and downlink beam coverage maps for your satellite (G/T, PFDsat and EIRP). Also the transponder gain setting.

Set the carrier bandwidth to say 800 kHz.
Set the transmit power to something reasonable, say 0.1 to 100 watts. My guess is that about 2 - 3 watts might by about right.  A 10 watt BUC might be a good idea if you want 6 dB of uplink power control for rain.
Is the uplink C/N any good?. If too high or too low then adjust tx power and carrier bandwidth. Use a bigger dish if necessary.
Having made a start with the uplink path you need to find out how much downlink EIRP it will produce. You need the transponder gain setting and the network operations centre's decision on multi-carrier backed off operating point.
A particular input power flux density will produce so much downlink EIRP.  Calculate the downlink EIRP yourself and input that to the link budget. Use a suitable size receive dish.
Check overall C/N is good, considering interference and intermodulation and rain allowance.
Try different modulation methods and FEC coding to increase your customer information rate.
If you really want the maximum capacity use bigger dishes and Comtech CinC modems and transmit two 1.8 MHz carriers on top of each other.

Read all the transponder operating rules and regulations particularly about maximum off-axis and spurious emissions and power spectral density.
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