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scpc and tdm/tdma

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ipman
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Sep 10th, 2020 at 3:39pm  
what is scpc and td/tdma system in earth station /vsat satellite system? considering ku band with 48 MHz space segment ,how can we design system of 100Mbps duplex with 100 vsat stations.
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Admin1
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Reply #1 - Sep 11th, 2020 at 9:40am  
Single Channel per Carrier (SCPC) refers to operation of continuous low bit rate carriers, generally in point to point configurations.  The expression 'Single Channel' is somewhat historical since is refers to traffic comprising one analogue audio voice signal input.  Nowadays the 'traffic' over a 10 kbit/s or 64 kbit/s or 128 kbit/s carrier might be digitally encoded voice.

An SCPC carrier might operate 24/7 or might be set up on demand for just of a minute or two when a phone call is made.

Once the bit rate gets up higher the expression 'Time Division Multiplex' (TDM) becomes more applicable as the data stream begins to comprise a sequence of frame blocks or packets containing a variety of end user traffic, voice, data, video etc.

Time Division Multiple Access (TDMA) is when several sites, transmit intermittently on the same frequency, thus sharing the same satellite bandwidth. See my description of how TDMA works.

Regarding your proposed network. Please explain more. Do you plan a hub as well as the 100 VSATs?  What traffic to and from the hub. What traffic to and from each VSAT site.  Star configuration or mesh configuration?. 

A total of 200 Mbit/s through a 48 MHz transponder sounds like a difficult challenge.
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« Last Edit: Sep 11th, 2020 at 11:48am by Admin1 »  
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Reply #2 - Sep 11th, 2020 at 10:02am  
A small VSAT network for internet access purposes, might comprise a teleport hub and 100 remotes. The hub would transmit a 4 Mbit/s carrier, which would be received by all sites, so each site could receive downloads at up to 4 Mbit/s for short periods. The outlink download carrier would be called TDM type and might use a DVB-S type frame structure. Blocks/packets of data would be addressed to particular VSAT sites using IP addresses.
The remote VSATs would only transmit occasionally and would use TDMA. The 100 sites might be grouped in two groups, so that 50 sites operate on one return link frequency and 50 sites on the other return link frequency. The hub would need two TDMA receivers. The burst bit rates of the return links might be 256 kbit/s.

It is hard to see how 100 VSAT site users could afford to pay for 48MHz of satellite bandwidth. Do you have such customers?
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« Last Edit: Sep 11th, 2020 at 11:35am by Admin1 »  
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Reply #3 - Sep 11th, 2020 at 7:20pm  
If you really wanted 100 Mbit/s duplex between just two sites in the same beam coverage I suggest using CDM750 modems with each site transmitting 100 Mbit/s and filling the 48 MHz. With both carriers on top of each other and each site knowing what it is transmitting the interference can by almost cancelled out.  It is called Doubletalk Carrier-in-Carrier.  You will need sufficiently large dishes at both ends.
Check with Comtech that what you propose is feasible.

REFERENCE http://www.comtechefdata.com/files/datasheets/ds-cdm750.pdf
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