SatSig topic:

CS C/N after ASI Adjacent Channel Interference

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Mar 23^rd, 2015 at 7:27am

I have a question, how do we calculate the CS C/N after ASI(Adjacent Channel Interference), so that we can determine which MODCOD that available for the upstream and downstream?

thanks in advance

Mar 23^rd, 2015 at 7:29pm

Assuming the adjacent carriers are of similar power spectral density.

I would follow the modem manufacturers recommendations regarding carrier spacing and ignore Adjacent carrier interference in the link budget. If someone insists on seeing a figure I suggest something like 28dB.

There are many interferers you can include in the link budget, terrestrial, adjacent beams, adjacent satellites, cross-pol, intermodulation, up and down. Typical values 21 - 30dB. You choose..

You can spend ages in the lab measuring BER versus carrier spacing and writing up learned reports, but I doubt it is worth it. Contact the modem manufacturer. They will have done studies and be able to claim wonderful filter roll-offs and carrier spacings.

What does CS mean ?

Best regards, Eric

Mar 23^rd, 2015 at 7:37pm

It occurs to me on rereading the above that you are referring to adjacent symbol interference. Digital carriers comprise a sequence of symbols. The beginning and end of each symbol slightly interfere with the adjacent symbols. This is a matter for the modem manufacturer. Extreme narrow filtering smears out the symbols and makes ASI worse, using less bandwidth (good) but needs higher C/N for same BER(bad). The modem designers must make a compromise.

Best regards, Eric.

Mar 23^rd, 2015 at 7:50pm

Choose your MODCOD based on overall link budget C/N and the bandwidth you have leased. Theoretically you should choose the one that gives maximum data throughput, but if your system is marginal and subject to rain, has limited or no uplink power margin etc then a more conservative and less efficient MODCOD is advisable, particularly if your customers will complain at the slightest outage.

Mar 24^th, 2015 at 8:37am

Thanks alot Eric for the reply. Well, I have no idea what's CS means. This is one of Intelsat parameters for LBA.

I tried to use the satsig LBA calculator but I am getting big numbers between 60 to 80 dB for C/N, isn't it suppose to be between -1 to 12 dB so we can compare it with C/N of the MODCOD?

May 17^th, 2015 at 9:38am

Eric, CS means Clear Sky

May 17^th, 2015 at 3:45pm

OK, I understand.

Typically you might get 9 dB C/N in clear sky and 6 dB C/N under medium rain fade condition, assuming fixed transmit power.

If we were talking about the return link from a remote site to the hub and your remote had 6 dB of uplink power control range then the hub would detect degradation and tell your remote site to correspondingly temporarily increase its transmit power, e.g by +3 dB from 0.5 watt to 1 watt.

This assumes that your remote site has, say, a 2 watt BUC and that it is normally operating in clear sky with 0.5 watt output (i.e. 6 dB backed off from its rated output power with a single carrier).

An extra benefit may be had from adaptive modulation and coding. For example, under severe rain fade the remote site could switch from transmitting 3/4 QPSK to 1/2 BPSK and also reduce its information rate. The objective being to keep a reliable service going, but at a temporarily reduced bit rate.

If you get improbable C/N ratios like 60 dB to 80 dB in a link budget calculations, check the input data. MHz instead of Hz for carrier bandwidth will give 60 dB error.

Best regards, Eric.

Jun 10^th, 2015 at 9:54am

Thanks alot Eric for the clarification.

Jun 11^th, 2015 at 4:01am

Could you please explain me why bit rate of up link higher than down link (on VSAT site)?

Jun 11^th, 2015 at 6:41am

If your application is VSAT uplinking of outside broadcast TV then a high uplink bit rate is appropriate to the TV quality required. A lower downlink speed will be needed for feedback and control from a hub studio.

Two way video needs the same high bit rate in both directions.

Two way voice needs the same bit rate each way.

Continuous services, like the above are very demanding on satellite capacity, and expensive. Dedicated capacity is required to avoid congestion causing breaks.

Internet browsing via VSAT needs a high download speed and low uplink speed, ratio approx. 5:1 to 10:1. Internet browsing is highly intermittent and may average only 5 kbit/s up and 30kbit/s down, so cost is low as the available capacity of the satellite is time shared by many users. Congestion and packet delays may occur.

Jun 11^th, 2015 at 11:32pm

thank you so much,
could you please show me steps to calculate link budget for a up link and down link?

Jun 12^th, 2015 at 1:23pm

Simple answer: Ask the satellite operator to tell you what you need.

Complex answer: So you get to learn and understand about link budgets :

The first step is to obtain information about the satellite:

For the uplink and downlink beams and transponder of interest:

Downlink beam coverage map marked with EIRPdown (dBW) contours.

Uplink beam coverage map marked with G/Tup(dB/K) and PFDsat (dBW/m^2) contours. Ask and clarify the PFDsat for your particular transponder and your location. The PFDsat contour values can probably be changed on a per transponder basis by altering the transponder gain setting on the satellite.

If you rent just a portion of a transponder, say 2 MHz, you additionally need the Transponder Operating Point. This is a pair of "back-off" figures, e.g. - 4dB input back-off and -1.4 dB output back-off. Ask what is the carrier to intermodulation noise ratio at this operating point.

Find out the location and diameter of the far end hub earth station that will receive your outside broadcast signal.

Your objective will be to determine what uplink dish and power you need, for your choice of bit rate.

Having obtained all the above you can then start playing with link budgets.
Start here: https://www.satsig.net/link-budgets/paksat-vsat-return-link.htm

Note that outside broadcast TV tends to involve a small antenna and a moderately high power transmitter. Such arrangements (antenna size and power spectral density) must be approved by the satellite operator as there is risk of unacceptable interference into nearby satellites. Always transmit at low power first under verbal supervision from the NOC. Beware the RF radiation hazard from high power microwave transmitters.

Jun 13^th, 2015 at 12:00am

thank you so much,
I am planning to buy a Satcom on the move (VSAT on ship) but I don't know why I can't transmit high bit rate at VSAT on ship? maybe it bigger bandwith so it interference to another satellite?

Jun 13^th, 2015 at 10:26am

If someone says you can't transmit your wanted bit rate then ask for an explanation.

Inmarsat provide world wide beam coverages, suitable for maritime use.
Read: https://www.inmarsat.com/wp-content/uploads/2013/10/Inmarsat_FleetBroadband_Best...
Offering 256 kbit/s streaming data (up or down) with 55cm radome antenna and 3 axis dynamic tracking.
Higher quality video may be recorded on the ship and then later played back and transmitted at lower speed.

If you intend to operate in a limited sea area where there is good beam coverage from a national or regional satellite operator then you may find a better deal.

Jun 13^th, 2015 at 11:29am

Thanks
I want to transmit 6Mbps video , live news from ship , But I see in datasheet satellite modem and satcom on the move equipment that is difficult, no equipment can't transmit that bit rate

Jun 17^th, 2015 at 4:15am

Hello,
Please explain me what is off axis, I found this in some document but I am not sure what is off axis . for example off axis affect to EIRP spectral density ....

Jun 17^th, 2015 at 11:00am

An antenna has an on-axis transmit gain that varies with its size.
The EIRP it sends towards the intended satellite is the power into the feed (in dBW) plus the antenna on-axis gain (in dBi).

Antennas have off-axis gain (in dBi) also, which reduces as the angle away from the main beam pointing direction increases. The manufacturer will have measured the off-axis sideobe pattern in order to get approval for use.

Example:

When a new satellite is proposed the owner needs to coordinate with nearby satellite operators to agree acceptable interference levels into and from nearby satellites. These agreements typically specify a maximum power spectral density that is acceptable, expressed, for example, as dBW/4kHz, dBW/40kHz, dBW/200kHz etc.

To find out if your system will be acceptable determine the power (dBW) into your feed and the bandwidth (Hz) of your transmit signal. Determine your antenna off-axis gain towards the adjacent satellite (e.g. transmit gain at 3.6 deg off axis.) Add together and adjust to express the result in terms of 4, 40 or 200 kHz or whatever.

If it does not pass consider using a broader carrier (e.g. BPSK and 1/2 rate FEC) so your signal is wider and has a lower power spectral density. You might also consider using a larger transmit dish so you don't need such high power into the feed, for the same EIRP to the satellite. Choosing a satellite with small spot uplink beams will help.

Jun 17^th, 2015 at 5:53pm

so that means power density of sidelobe? if diameter large will have low sidelobe will better then smaller dish?

Jun 17^th, 2015 at 9:04pm

To achieve a particular EIRP towards the intended satellite, a larger antenna will need less power at the feed, since the main beam gain is larger.

All quality antennas for transmit VSAT use have the same off-axis gain envelope, approx. 29 - 25log(theta) dBi. This is regardless of size.
example: at 3.7 deg off axis the gain is 29-25log(3.7) = 14.8 dBi.

This off-axis gain envelope formula starts at the side of the main beam and continues out to -10 dBi, which applies round the back. Refer to your manufacturer for specified off-axis gain envelopes for your antenna, or measure it.

Jun 19^th, 2015 at 9:50am

Thanks,
Is highier frequency will smaller beam width (boresight) ?

Jun 19^th, 2015 at 1:21pm

If the antenna size is unchanged, an increase in frequency makes the main beam beamwidth narrower and the main beam gain, on axis, increases.

See: https://www.satsig.net/pointing/antenna-beamwidth-calculator.htm

'boresight' is the word referring to the direction of the main beam.

Jun 19^th, 2015 at 2:19pm

thanks,
Could you tell me how to choose an antenna?

Jun 19^th, 2015 at 4:32pm

Find out from the satellite operator what specifications an antenna in their network must meet.

Explain if it is intended for mobile use. Describe the operational area you want it to work within. They may not permit mobile operation at all or may have special requirements.

Ask them about link budgets, bit rates, required dish sizes, powers, modulation, coding, polarisation isolation, sidelobes, tracking systems etc. Tell them about both ends of the proposed link.

Before ordering anything check that it will be acceptable to the satellite operator. Some operators will have "type-approved" systems. Anything unusual may be required to undergo extensive testing prove it does not cause unacceptable interference to other users of the same satellite or to users of adjacent satellites, prior to being given permission to go into operational service.

For something very complicated, that is at significant risk of not working properly, such as adverse environment maritime or vehicle mobile, I would suggest you ask to see an already installed working system and talk to the customer about their experience and the skills their staff need to keep it working.

Jun 22^nd, 2015 at 9:16am

thank you so much,
i see many modem use spectral spread, what is benefit of technology