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Basic operation of Sat Modems.. Tx/Rx ?

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Jul 31st, 2014 at 11:34pm  
I will like to know the basic operation of sat modems Tx/Rx state based on different traffic types..Currently working on an energy related project... thanks guys
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« Last Edit: Mar 3rd, 2015 at 7:03pm by Admin1 »  
 
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Reply #1 - Aug 1st, 2014 at 8:15am  
Very small aperture terminals (VSATs) both transmit and receive (TX/RX), to and from a satellite.

Networks generally involve a large teleport hub dish and a large number of small remote VSATs.  In such cases all the VSATs receive a continuous broadcast signal with packets of embedded data intended for all or for individual sites.  VSAT sites transmit short bursts intermittently(e.g <1% of the time), with maybe 100 sites all sharing the same return link satellite bandwidth. 

In the power field (e.g. electricity grid, oil, gas), VSATs may be useful for remote metering or monitoring and control of network equipment. Read more here: http://www.satsig.net/idirect/iDirect-X1-routers.htm
In this case the outdoor version of the modem is enclosed in a waterproof box (model number iDirect X1-OD).

When satellite capacity is shared amongst very many sites, the cost per site is lowered.

A different traffic type is made up of continuous dedicated transmissions, normally called SCPC or Single Channel per Carrier.  This mode is suitable for long term continuous video, music or sound content.  It is simple and reliable and does not rely on a teleport hub.  Links directly between 1.2m or 1.8m diameter VSATs are possible.  The costs per site are much higher (maybe 100 times) because of the continuous dedicated use.

Best regards, Eric.
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« Last Edit: Aug 1st, 2014 at 12:19pm by Admin1 »  
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Reply #2 - Aug 11th, 2014 at 11:35am  
Extending idah's question, I would like to know more about the modulation and coding / error correction methods used on sat links, since I got quite impressed with the worst case scenarios on Tooway and Astra2connect where some basic (degraded data rate) communication is maintained even with 0 to 3dB SNR.

Maybe Eric or someone else would have some links for further reading?
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Eric Johnston
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Reply #3 - Aug 12th, 2014 at 3:34pm  
A system like Tooway uses adaptive coding and modulation (ACM) to keep things going for sites suffering rain.

Considering the downlink to the remote site only, the bit rate in clear sky is 68 Mbit/s. This uses 8PSK modulation and 5/6 rate turbo FEC.
In severe rain the bit rate drops to 28 Mbit/s and the modulation and coding change to QPSK and 1/2 rate turbo.

(Note your data rate for downloads is limited by the routers to a maximum of 20 Mbit/s per second. Remember there are very many customers all sharing the same satellite capacity.  The downlink carrier is made up of a sequence of data packets intended for different sites. Your long term bit rate may be limited to say, 30 kbit/s per month, depending on tariff.)

There are a vast range of FEC methods and ratios and a number of modulation methods, BPSK, QPSK, 8PSK, 16QAM etc.  All of these have their own curve of bit error rate versus C/N. You choose !

This pdf document relates to broadcast satellite TV service:  http://taurus-sales.com/sites/default/files/editor/taurus_adminfiles/TTV%20-%20S...

The general idea is to get the best capacity out of the satellite transponder using both all the bandwidth and all power available simultaneously. If you keep the symbol rate the same the bandwidth stays the same. So just alter the modulation method and FEC during rain to keep the optimum capacity.  You could do the same if you had a variety of receive dish sizes, setting the per packet modulation/coding to the best for each site.  You might also use a different modulation/coding for sites at the beam edge.

Best regards, Eric.
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« Last Edit: Aug 13th, 2014 at 8:37am by Eric Johnston »  
 
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