# QAM Theory- attenuation of signal

#### liam

Joined Apr 22, 2008
11
I'm trying to understand the theory behind QAM, however i cant get my head around how the amplitude of the signal is maintained when it is transmitted across free space.

If a QAM signal at a certain phase and amplitude is broadcast from an antenna, there is going to be attenuation of that signal when it is received at the receivers antenna. Therefore the received signal amplitude will be less, giving a different QAM value??

#### nsaspook

Joined Aug 27, 2009
7,735
I'm trying to understand the theory behind QAM, however i cant get my head around how the amplitude of the signal is maintained when it is transmitted across free space.

If a QAM signal at a certain phase and amplitude is broadcast from an antenna, there is going to be attenuation of that signal when it is received at the receivers antenna. Therefore the received signal amplitude will be less, giving a different QAM value??
The received baseband signal with still have the same relative differences in amplitude (power) from zero 0.0 signal to max 1.0 signal when the 1.0 level is normalized (using a average power over some time period like AGC) to the peak amplitude (power) of the received signal. Using this, you can calculate average energy levels per symbol. QAM is not the best choice for low S/N transmission media like long OTA links.

#### crutschow

Joined Mar 14, 2008
25,660
It's the relative value, not the absolute value, that determines the modulation, and that relative value is maintained even if the signal is attenuated.