Adjusting Transmission Power for QAM

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mdeth87 on 24 Jun 2015
Answered: Drew Davis on 25 Jun 2015
If I transmit the 4-QAM symbol '1.00 + j1.00' but I then wanted to transmit it again at a quarter of the same transmit power. Do I simply divide '1.00 + j1.00' by 4 or do I need to do something different as I am dealing with power?

Answers (1)

Drew Davis
Drew Davis on 25 Jun 2015
If the amplitude of a symbol is Si then the power is proportional to the distance from the origin to the constellation point
power = |Si|^2 / 2
Therefore if you want a signal with quarter power
power / 4 = |Si_new|^2 / 2
|Si_new| = (power / 2)^0.5 = (|Si|^2 / 4)^0.5 = |Si| / 2
Thus the magnitude of the new signal should be half the magnitude of the old signal

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