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SLM-7650 Satellite Modem
Theory of Operation
4.3.4.5
16-QAM
16-phase Quaternary Amplitude Modulation The modulator
generates a constellation with 16 discrete phase
angle/amplitude states for 16-QAM. There is a large
reduction of the symbol rate when using 16-QAM. The
symbol rate for 3/4 FEC is 1/3 of the data rate. This severe
reduction in symbol rate requires more power on the satellite
transponder. Reed Solomon encoding is absolutely
necessary.
Phase noise and non-linear links will have a detrimental effect on 16-QAM.
4.3.5
Bit Error Rate
The Bit Error Rate of any digital circuit is the number of bit errors, divided by the bit
rate, divided by the total time of the test.
Example: What is the bit error rate when 2 errors occurred in 10 minutes with a data rate
of 256 kHz?
A communications link that is free of errors is always desired, but most links will operate
with a certain number of uncorrectable errors over a period of time. Forward Error
Correction is necessary to reduce the errors to an acceptable level.
The BER performance curves are an important reference for modem performance. The
slope of the BER curve is an indication of the speed at which the BER performance can
degrade under link impaired conditions. There is a point that the modems will not
operate any more. This point is referred to as the BER Threshold point. Once the BER
Threshold point is reached then the modem will lose lock. The performance of a modem
at or near threshold is dependent on the type of FEC that is in operation. The Turbo
Product Codec will have the least pronounced threshold effect. The modem will stay
locked at very low EbNo values where the bit error correction is as low as 1 x 10
is, of course, unusable at that point, but the error correction will regain substantial error
correction for very small increases in signal input.
4.3.6
Symbol Rate
Symbol rate is the rate that the modulated output signal changes from one phase or
amplitude state to another. It is represented by the actual bandwidth of the transmitted
carrier at the 3 dB down points from the top of the spectral waveform. It is useful for
determining the frequency allocation of the satellite transponder. It is also used to
calculate the frequency separation for each carrier on the satellite.
BE/Rate/time
2/256k/(10x60) = 1.3 x 10
4–11
MN/SLM7650.IOM
-8
Revision 4
-2
. Data
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