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DB test

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Feb 13th, 2007 at 12:27pm  
Please i need someone to explain how to carry out a Db test on a vsat link.
Thanjks
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Eric Johnston
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Reply #1 - Feb 13th, 2007 at 4:10pm  
If you are referring to measuring the receive signal quality at your VSAT terminal then you need to refer to the manual for the particular modem you are using. Look for instructions on how to read or display the Eb/No (in dB), which is an indication of the receive signal quality.  

Your reception will fail if the receive Eb/No is too low.  The transition from working OK to failed is quite rapid.  Under clear sky conditions you need an operating fade margin of several dB so that your service will continue during moderate rain.   At Ku band, expect failure during very heavy rain - when the fade margin is all used up!  

The threshold value of Eb/No when your service is about to fail varies according to modulation method and coding.   For QPSK 3/4 Turbo FEC the threshold Eb/No for a BER=10E-6 is about 3.8 dB.   Older, less advanced FEC methods like Viterbi and Reed Soloman have higher thresholds.

Another way of measuring receive performance is bit error rate or BER.   Under normal circumstances (clear sky or slight rain) the information error rate is virtually zero and not measurable unless you sit here for ages waiting for an error.  Instead modems have a display that shows the symbol error rate at the receive demodulator input, prior to forward error correction being applied.  At this point there are many errors, like 1 in 1000, even in clear sky, so it is possible to get a meaningful measurement.  Values like 0.001 to 0.00001 are common.   Don't be alarmed at this, remember that the FEC process corrects all these errors to some near zero figure like 10E-12 in your output information stream.

The modem can now compare the input symbol data stream with the output information data stream and work out how many errors are being corrected, and based on its design can also calculate what Eb/No would be needed to give that symbol error rate and thus give an Eb/No display.

Measuring Eb/No directly needs a spectrum analyser.  You look at the carrier hump and determine the height above the noise floor in dB.  This is (Co+No)/No.   At the top of the carrier you are measuring Carrier+Noise, and on either side, Noise only.

To get Eb/No you need to convert your measurement to Co/No and adjust for the FEC ratio and bits/symbol.

Co/No=10log(10^((Co+No)/No)/10-1)

Bits/symbol:  BPSK=1, QPSK=2, 8-PSK=3, 16-QAM=4
FEC = 1/2, 3/4, 7/8 etc

Eb/No = Co/No - 10log(FEC) - 10log(bits/symbol)

If you put this into program try the following check:

Your spectrum analyser measurement (Co+No)/No = 17 dB
Your carrier modulation is 16-QAM
Your FEC rate is 7/8
Your calculated result should come to Eb/No=11.5 dB

Best regards, Eric.
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