Satellite Communications/Carrier Slope


Hello Thomas,

I had a uplink link and when I see the carrier there a slope in the carrier Im using 8PSK modulation but me link can not LOCK  but when I changed to 8QAM  modulation the link LOCK without problem


Im looking forward for your answer
Have a nice day


Good to hear from you again. I hope that all is well apart from your lock problem.

I believe your problem is due to the fact that m-QAM modulation schemes are more efficient the m-PSK modulation schemes.

I have not found details for 8QAM and 8PSK, but have found data for 16QAM and 16PSK.

The reference shows that for a symbol error rate of 0.0001 (1X10^-4), 16QAM requires 19 dB of Es/N0 whereas 16PSK requires 23 dB, a four dB disadvantage.

To summarize…

8PSK is less efficient that 8QAM (probably by about 4 dB).

The roll off observed probably has no connection to your inability to synch.

I hope that this helps.


Thomas Burke, PhD, PMP  

Satellite Communications

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Thomas E. Burke


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Forty years of experience with satellite communications. Have held roles as system engineer on JPL Mariner 9 program and a program manager for a number of classified communication satellite programs. Served as head of TRW / Defense Communications Division / Engineering Development Operation, a 1,400-person organization responsible for all aspects of classified communication satellite design and development.

Ph.D. Chemistry, California Institute of Technology (1969) B.A. magna cum laude Chemistry, University of Minnesota (1962)

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