Satellite links
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Satellite links
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A satellite link comprises two parts, the uplink and the downlink.
First, consider the uplink. The earth station transmits a signal. This signal comes from the transmitter which may be a solid state power amplifier (SSPA) or travelling wave tube amplifier (TWTA). Most commonly VSAT terminals have solid state power amplifiers mounted at the dish and as close to the feed as possible to minimise waveguide attenuation losses. These dish mounted units are often block up converters (BUC) or Transmit Receive Integrated Assembly (TRIA) which change the frequency of the signals from L band (in the cross site inter facility link (IFL) cable) to the microwave frequency for transmission (C band, Ku or Ka band). BUCs have a rated output power, such as 2 watts for single carrier operation or 0.5 watts for multi-carrier operation. For ease of calculation the 2 watts power needs to be converted to dBW by doing 10 x log (power in watts), so a 2 watt BUC has a single carrier output power capability of +3 dBW (2 watt) or, for multi-carrier operation -6 dBW (0.25 watt) output power per carrier for each of two equal power carriers.
The output power of the BUC is fed to the dish which concentrates the power in the direction of the satellite rather than allowing the power to be radiated evenly in all directions. This characteristic of the antenna is called gain, measured in dBi, which means gain relative to an isotropic, omni-directional antenna.
The combination of BUC power and satellite dish gain produces equivalent isotropic radiated power (EIRP), so for example. 2 watt BUC power + 40 dBi antenna gain produces 43 dBW EIRP.
The transmit EIRP of the earth station may be achieved by having a variety of sizes of BUC power and dish size. A large dish with low power BUC can produce the same EIRP as a small dish with high power BUC. There are limiting considerations to this. Small dishes may cause unacceptable interference to adjacent satellites. To minimise cost, choose a larger dish plus lower power BUC and take account of the cost of the electricity used.
Find the distance to the satellite as this will give you the spreading loss in the up satellite link. Distances between approx 35860 km (sub satellite point) to approx 41756 km (edge of visibility) and are applicable for geostationary satellites.
The satellite receive beam will have a G/T value for the direction from your earth station. Review the uplink beam coverage map and determine the satellite receive G/T in the direction from your site. Values like -8 to +10 dBK are typical. Broad, earth coverage global beams have the lowest G/T; their beam width is approx 17.5 deg, which is what the earth looks like from a geostationary orbit position. Spot beams (say 1 deg diameter) have the highest uplink G/T.
Now learn this link budget equation:
Go to the link budget calculator and play with some numbers. The EIRP you can transmit can be varied by changing the BUC power and dish size and so, as a result, the uplink C/N will vary. You obviously need a decent uplink C/N (say more than 10 or 20 dB) but once it is adequate how do you decide what correct EIRP is needed?. Note how the link budget calculator tells you what is the uplink power flux density that you are producing at the satellite. Write this figure down.
You need to consider the required required power flux density into the satellite.
If you were transmitting a single large 36 MHz satellite TV carrier and aiming to saturate the transponder you would need to produce the PFDsat for the transponder. The satellite up-link beam pattern will have contours specifying both G/T and PFDsat. Read off the PFDsat for your site and this will tell you the PFD that you need to produce for single carrier, full transponder operation. You can ask the satellite operator to adjust the satellite transponder gain, and thus PFDsat, by setting attenuator switches on the satellite. This will allow you to trade off earth station costs, convenience and quality. Higher gain might be attractive if your uplink were a mobile TV uplink truck or if you were having problems producing enough uplink power. The penalty is lower uplink C/N and greater susceptibility to uplink interference.
The satellite operator will normally have several nominal transponder gain set settings. e.g. low gain for multi-carrier operation amongst large dishes, medium gain for single carrier operation and high gain for multi-carrier VSAT return links.
If you were transmitting a small carrier into a multi-carrier operation transponder you need to do the following calculation as a starting point. Note that the satellite will have a PFDsat and input back off specified (e.g. 6 dB input back off for multi-carrier operation). Note your carrier bandwidth and the transponder bandwidth. I am assuming here that you want your fair share of the satellite power, proportional to the bandwidth. This is a good starting point but you may prefer to have your fair share of the power (and pay the normal amount) or have more power (and pay more) depending on your dish sizes. As a rule it will be better to always spend more on larger dishes and reduce your space segment costs.
Now adjust your uplink EIRP till you get the required PFD at the satellite. Check that the uplink C/N is still reasonable.
The downlink EIRP from the satellite is either:
or
Consider the downlink receive earth station. This will have a diameter size, receive frequency and system noise temperature. Put these together and you will get the receive earth station G/T. The equation for G/T is: Earth station G/T = Gain - 10 log (system noise temperature)
Now use the link budget equation for satellite links:
Earth station intermodulation noise: If you are operating a
multi-carrier BUC put in say 30 dB interference
Uplink interference from other earth stations pointed to nearby satellite:
If you are a low power spectral density uplink put 25 dB, otherwise 30 dB.
Uplink interference from multiple beams on same satellite: In any, put 30
dB.
Uplink cross polar interference: Put in 30 dB, if you can't trust the
installers and NOC staff, put in 25 dB.
Transponder intermodulation: If multi-carrier the put in 21 dB
Down-link interference from other nearby satellite: If you are a low power
spectral density uplink put 25 dB, otherwise 30 dB.
Down-link interference from multiple beams on same satellite: In any, put
30 dB.
Down-link cross polar interference: Put in 30 dB, if you can't trust the
installers, put in 25 dB.
Finally add them all together to obtain the total link budget C/N
| Pre-calculated satellite link budget with
useful default parameters inserted (may be amended and recalculated)
SCPC / DAMA 8kbit/s mesh
network satellite link budget - Skylinx
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Other useful links:
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► Page created 30 Sept 2005, amended 13 May 2008 Eric Johnston
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