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Downlink satellite EIRP vs (C+N)/N at rx site

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Ex Member

Feb 12th, 2014 at 6:17pm  

I need some help in my considerations Smiley

What is more important for me for my RX stations ? EIRP from satellite or real (C+N)/N measured on spectrum analyzer.

Example - i have two satellites to choose from with different EIRP at my territory:

first - 56 dBW
second - 50 dBW

i have installed RX antennas - 1.8 m
first and second satellite operator can give me 15 db (C+N)N at rx sites
Price is the same for slot.

So from my point of view which offer better ? or are the same?

I think there is a different only for sat operator - with EIRP 50dBW  more power must be concentrate at my carrier so some bandwith from transponder will be don't used (as a power for me ?)

Best Regards
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USN - Retired
Ex Member

Reply #1 - Feb 13th, 2014 at 3:29pm  
Can't answer without first knowing the source of your EIRP numbers. Are you referring to EIRP as radiated to you on the satellite downlink? or the uplink EIRP as reported by the distant earth station?

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Eric Johnston
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Reply #2 - Feb 13th, 2014 at 5:13pm  
It's possible that 50 or 56 dBW EIRP refer to a transponder total downlink power at saturation with single carrier, like most big satellite tv carriers.

If you have a small carrier, say 1 MHz wide then the transponder will be operating multi carrier and backed off to make inter-modulation acceptable. If the back off is 3 dB then total transponder operating power is 47 or 53 dBW.

Your carrier will normally be given its fair share of the 36 MHz transponder bandwidth e.g 47 - 10 log (1/36).   If you pay for more power (and waste bandwidth) you can have more to get whatever downlink C/N you want. Other people will get less than fair  share. Ideally use your fair share of both power and bandwidth simultaneously by adjusting modulation type and FEC coding.

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Oasis Networks
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Reply #3 - Feb 14th, 2014 at 7:32am  
I would like to approach it from a different perspective, more practical and less theoretical.

First question to ask, is what is the purpose of this link? Is it a VSAT, or FTA (you are only receiving) or broadcast (you are only transmitting)? Reason for asking, is that besides the EIRP, you should also consider what is happening on the other side of the link. For example, if you would like to use the link for VSAT or any type of two-ways communication, you should consider the other side dish size and G/T. Needless to say other practical parameters, such as helpdesk availability, electricity and equipment redundancy etc.

Another point to consider, in case that you would like to broadcast and other to receive your transmission, is the elevation angle at the receiving sites. Always better to go for satellite with higher elevation, this way you can assure you will have easier installation at the remote sites.

Hope it helps,
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Ex Member

Reply #4 - Feb 17th, 2014 at 6:15pm  

First - thank you for your support - i still learn Smiley

@USN - Retired   EIRP = radiated to me satellite downlik

@Eric Johnston - it will be tv carrier - with with this example parameters:

DVB-S2 SR 3000, 8PSK FEC 2/3 ROLL-OFF 0,2

so this transponder will be operate in multicarrier mode
and yes 56 and 50 dBW refer to full transponder

@Oasis Networks - transmission will be point to multipoint.
I have about 100 RX stations with installed 1,8 m RX antennas - this transmission will be only for this stations.

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Oasis Networks
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Reply #5 - Feb 17th, 2014 at 7:56pm  
Hi again,

If you will just broadcast from your side a DTH transmission, why would you worry too much about the satellite EIRP? As you will transmit only. Or you refer to the EIRP at your customers?
Without calculating, I believe both satellites will be good, if the receive will be done with a 1.8m dish and you are transmitting just a single channel. Sounds like you have enough margin there.

Another point worth to be considered - you can transmit your broadcast as a feed to a teleport, and then they will mux it into one of the bouquet they operate over the area. This is a very practical solution as well, especially if you need to playout the channel but don't have the equipment to do it; or in case you would like to use a popular satellite which ran out of free capacity.

Kind Regards,
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Reply #6 - Feb 17th, 2014 at 8:37pm  
Get hold of the downlink beam coverage map and note the EIRP contours. Note that sites will receive different power levels depending on location.  Sites near the beam peak will have values about 3 or 4 dB higher than at the beam edge contour.

If you get a contract that says you will be given a (C+N)/N =15 dB at all your sites, that is excellent.

In service, it is not realistic to go measuring the value at all of your sites so make sure it is agreed and that a specific reference site be used for carrier initial line up and for in service verification. Both you and they should have physical access to measure the (C+N)/N using a spectrum analyser.   The reference site is unlikely to be at beam edge so the measurement value at the reference site should be a few dB higher than 15 dB, according to its position in the beam.  You need to meet 15 dB for your least well located sites, nearest beam edge.

Regarding choice of satellite it is obvious that it will be easier for the operator of the higher power (56 dBW) satellite to meet your requirements. Note the points raised by Oasis above. Higher beam elevation angles are good.  Ask about lifetime of the satellite ? Is it about to go into inclined orbit ?  Does the transponder still have an on-board spare ?  Is the transponder pre-emptible (special cheap tariff), which means it is a risk of being used as a spare and taken to replace someone else's transponder.

Are the frequencies being offered the same?  Consider if the exact frequencies offered are subject to interference from terrestrial WiMax where your sites will be located.

Is there a rather close adjacent satellite that causes interference into small 1.8m dishes at the particular frequency?

Do the satellites have linear or circular polarisation?.  In the tropics C band linear tends to suffer cross-pol interference.  Circular is better in the tropics.

In the future might you transition to another satellite and have to repoint all your dishes?

Are you dealing with the satellite owner ?  Be wary of sub-leasing capacity from someone who has already bought a long term lease and is possibly in financial trouble. ALWAYS talk to the satellite owner.
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