Scaling Factor at receiver

Dear All,

I am wondering if we need to re scale our modulations symbols at receiver. Let me put this way, suppose we have 64QAM symbols at transmitter side ready for transmission. We scale them by 1/sqrt(42) in order to make their average energy equal to 1. Do we need to multiply with sqrt(42) at receiver before demodulating them?

Waiting

Talha

Answer 1

[Walter Roberson]

Consider the case of transmitting by radio with an omnidirectional antenna. The signal energy would dissipate over the surface of the every-expanding sphere of radius c*t where c is the speed of light. The surface area is 4/3 * Pi * r^2, so for a given size of receiver, the received signal strength is going to reduce by 1/4 as the distance doubles.

Now, taking in to account that the received amplitude falls with the square of the distance, do you think it is going to be important whether you multiply the received amplitude by a linear constant such as sqrt(42) ?

QAM reception works by detecting constellation position. Constellation position detection is not going to be affected by linear multiplication of the received magnitude.

Answer 2

[Talha Rahman]

So it means no scaling at the receiver.... doesn't matter what modulation i am employing at transmitting ...receiver doesn't bother

Answer 3

[Talha Rahman]

Hi Mr. Walter,

i think you were wrong about not scaling the constellation at receiver. In fact, when i didn't i had my error rate quite high above 0.2 whereas i applied scaling factor at receiver and i got my error rate almost vanished after some iteration.

Best Regards

Talha