QAM Theory and Formulas

- the basic theory and relevant formulas or equations behind QAM quadrature amplitude modulation give additional insight into its operation..

The basic QAM theory aims to express the operation of QAM, quadrature amplitude modulation using some mathematical formulae.

Fortunately it is possible to express some of the basic QAM theory in terms of relatively simple equations that provide some insight into what is actually happening within the QAM signal.

QAM theory basics

Quadrature amplitude theory states that both amplitude and phase change within a QAM signal.

The basic way in which a QAM signal can be generated is to generate two signals that are 90° out of phase with each other and then sum them. This will generate a signal that is the sum of both waves, which has a certain amplitude resulting from the sum of both signals and a phase which again is dependent upon the sum of the signals.

If the amplitude of one of the signals is adjusted then this affects both the phase and amplitude of the overall signal, the phase tending towards that of the signal with the higher amplitude content.

As there are two RF signals that can be modulated, these are referred to as the I - In-phase and Q - Quadrature signals.

The I and Q signals can be represented by the equations below:

I = A cos(Ψ)   and   Q = A sin(Ψ)

It can be seen that the I and Q components are represented as cosine and sine. This is because the two signals are 90° out of phase with one another.

Using the two equations it is possible to express the signal as:.

cos(α + β) = cos(α)cos(β) - sin(α)sin(β)

Using the expression A cos(2πft + Ψ) for the carrier signal.

A cos(2πft + Ψ) = I cos(2?ft) - Q sin(2πft)

Where f is the carrier frequency.

This expression shows the resulting waveform is a periodic signal for which the phase can be adjusted by changing the amplitude either or both I and Q. This can also result in an amplitude change as well.

Accordingly it is possible to digitally modulate a carrier signal by adjusting the amplitude of the two mixed signals.

By Ian Poole


<< Previous   |   Next >>


Share this page


Want more like this? Register for our newsletter





Tick-Tock: What Your Engineers Could be Spending Time Doing if They Weren’t Stuck Designing a Display? Markku Riihonen | 4D Systems
Tick-Tock: What Your Engineers Could be Spending Time Doing if They Weren’t Stuck Designing a Display?
As soon as any design project is embarked upon, the clock starts ticking. The length of time needed to develop a system can impinge heavily on its commercial success. Windows of opportunity could be missed if it takes too long to complete, with products from rival companies gaining greater market share.









Radio-Electronics.com is operated and owned by Adrio Communications Ltd and edited by Ian Poole. All information is © Adrio Communications Ltd and may not be copied except for individual personal use. This includes copying material in whatever form into website pages. While every effort is made to ensure the accuracy of the information on Radio-Electronics.com, no liability is accepted for any consequences of using it. This site uses cookies. By using this site, these terms including the use of cookies are accepted. More explanation can be found in our Privacy Policy