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 Satellite Internet forum › General and other topics › Bandwidth calculations for  VSAT Systems
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# Bandwidth calculations for  VSAT Systems

 Ex Member Ex Member Mar 27th, 2013 at 5:29am   Dear EricI am a trainee trying to learn and make career in satcom. Below are some of my questions that will regarding basic concepts of VSAT systems kindly help me in finding the answer.1- I have to calculate the bandwidth required for a 512kbps full duplex circuit. the modem can support QPSK and 8PSK modulations. What are the other parameters I need and how can I calculate bandwidth in MHz required for this SCPC circuit.2- If i have given data rate only how can I calculate symbol rate from this information.Martin Back to top IP Logged
 Eric Johnston Senior Member Offline Personal text from Profile, Options, Top line Posts: 2108 Reply #1 - Mar 27th, 2013 at 9:54am   You start with the information bit rate, 512 kbit/s.Now add on the effect of forward error correction (FEC).  FEC operates with various ratios. Values vary from about 0.5 to 0.95, with the value often alternatively expressed as a fraction, like 1/2, 3/4, 7/8 etc.The transmission bit rate is then, for example 512/(3/4) = 682.67 kbit/s.Next, to find the symbol rate, you need to consider the modulation method.Note:BPSK has one transmission rate bit per symbolQPSK has two transmission rate bits per symbol8-PSK has three transmission rate bits per symbol16-QAM has four transmission rate bits per symbolIf you choose 8-PSK modulation, your symbol rate will be 682.67 / 3 = 227.55 k symbols per second.The amount of transponder bandwidth needed for a 227.55 kbit/s carrier is about 1.2 to 1.4 times the symbol rate. I assume 1.35 below.In the above example 512k 3/4 FEC   8PSKthe bandwidth needed is approx 1.35 x 227.55 = 307 kHz.This is what you pay for. You are assigned a proportional amount of satellite eirp power. If you need more power (due to a too small dish size) then you can pay more.  You will be given proportionally more bandwidth simultaneously as you are basically buying bandwidth plus its fair share of the total transponder power, less the output back off for multicarrier operation.  The 1.35 value is approximate and depends on filter roll off factor and the amount of adjacent carrier interference you can accept.  If you have very small carriers, e.g. 16 ksps, then additional bandwidth may need to be assigned to allow for frequency error and drift.  Your satellite operator will agree with you what bandwidth is needed for your carriers and may insist on a nominal factor of 1.4  If you want to claim for a lesser values make sure you fully understand the output spectrum from your particular make of modem.The least bandwidth (and thus low cost) transponder arrangement is with 16-QAM and an FEC like 7/8 and sharp filtering like 1.2.  Such a carrier requires a very high carrier to noise ratio (C/N) or energy per bit per noise power per Hz (Eb/No) and therefore requires larger earth station dish size, low phase noise and high linearity amplifer (BUC). A less efficient satellite system such as QPSK and FEC 3/4 and 1.4 works well at lower C/N and enables smaller dishes, but costs more due to the larger transponder bandwidth needed.If you have a point to point service with two earth stations then use large dishes to get the transponder cost down. If you can see your own signals you may even be able to put two carriers (the go and return) on top of one another and make further big savings in transponder costs.  If you have many sites save money on the costs of the many dishes and spend more on the satellite transponder lease.Best regards, Eric. Back to top IP Logged