Dish Size for DVB-S / DVB-S2 / DVB-S2X

esto

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Currently I have a 80cm dish, and generally it does fine, but when it rains I notice some HD channels start breaking up. DVB-S uses QPSK, and DVB-S2 doubles the modulation and uses 8PSK. So I understand it's more sensitive to rain, obstacles, etc. and needs a higher receive level than standard DVB-S signals.

Is there any kind of reference chart available that shows the sensitivity differences, and/or dish size recommendation differences? eg. "If your RSL is 48dBw you need a 60cm dish for DVB-S signals, or 80cm dish for DVB-S2 signals."

Also, when 4k comes to satellite (DVB-S2X), modulation rates will have to increase again, I suppose to 16PSK (altho Wikipedia mentions only 64PSK minimum?) How realistic is this going to be in terms of dish size? Eg. right now I need an 80cm dish for DVB-S, if I want DVB-S2 I need to upgrade to 100cm, and with DVB-S2X I will need, what a 1.5m dish I guess? I don't see DVB-S2X being too realistic for anyone who doesn't have a 100% clear view of the sky and living in a place where it never rains.
 

Terryl

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I don't see the type of RF modulation compression dropping the RF signals from the transponders by that much.(if any at all)

I would go to satbeams.com and select the satellite and it's beam your looking for, then zoom into your location, and then click on your location, this will give you the estimated dish size needed for that satellite and transponder.

If you have weather fade problems then a bigger dish would be recommended, if 80 cm is shown then go for a 1 meter or larger.
 

esto

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Thanks, yes I check SATbeams, but they just give dish size recommendations based on the beam, not the specific transponder (which may have different modulation rates).

The RSL (Recieve Signal Level) is fixed between the sat and the dish, but for higher order modulation (eg. 16-PSK, 64-PSK, etc.) you will need a stronger signal to decode it in poor weather compared to lower order modulation like QPSK.

In LOS microwaves, they have Adaptive Modulation, meaning when the weather is good, it uses the highest modulation available (256-QAM in this case)to have maximum bandwidth, but when weather gets bad, then it drops to lower modulation (4-QAM in this case), because it can be decoded easier, but then the bandwidth drops. See attached pic:


AMR_0.png


The same basic principal applies to satellites as well. Right now DVB-S uses QPSK (4-PSK). DVB-S2 uses 8-PSK, so it requires a stronger signal to be decoded when bad weather or obsticals are present, compared to DVB-S which uses 4-PSK. If DVB-S2X will use something like 16 or 64 QAM, then alot of people are going to need to upgrade dish size to decode this modulation, because of it's sensitivity.
 

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This has been discussed when DVB-S2/8PSK first came into play as the requirement for a better signal quality became necessary. This knocked out a lot of "fringe reception" signals after a few channels changed modulation.

I am sure there is a complex mathematical formula to calculate the required signal level for a specific modulation and FEC rate. FEC is important here as the lower the FEC, the more signal you need. FEC rate of 1/2 needs 2.7dB C/N to lock in DVB-S QPSK modulation, whereas with 2/3 you would need 4.4dB and with 3/4 you would need 5.5dB C/N to lock with the same modulation. Some HD channels on Sky had FEC of 8/9 - this allowed them to squeeze higher bit rates into audio and video, as less of the bandwidth would be taken up by error correction. FEC of 1/2 means that 50% of packets are used for error correction, whereas FEC 8/9 means that only 11% of packets is used for error correction and the rest for audio/video.

If and when Sky decide to pump out UHD with new modulations, I am sure that they will somehow ensure that the millions dishes won't need to be upgraded to larger ones by playing with FEC and transponder strength. I believe each TP's power can be adjusted from the uplink site - could be wrong though.

Discuss.
 

esto

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If and when Sky decide to pump out UHD with new modulations, I am sure that they will somehow ensure that the millions dishes won't need to be upgraded to larger ones by playing with FEC and transponder strength. I believe each TP's power can be adjusted from the uplink site - could be wrong though.
They could increase power on the DVB-S2X transponders, but then they would have to lower the power somewhere else on the satellite, in order to respect the satellites power balance. Or else launch newer satellites capable of more power overall.
http://en.wikipedia.org/wiki/Power_budget

I don't know what kind of FEC commercial satellites use, but in general, they are all pretty similar, and unless some great new magical code is developed, there isn't much they can do there. Higher modulation is more susceptible to errors, that's just physics.

As an alternative to increasing modulation, they could increase the channel bandwidth and keep current modulations to get similar thruput needed to acheive UHD TV.
 

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the signal from dvbs2x will be more increce of the dvbs2 ?
 
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