Quadrature Amplitude Modulation (QAM)

Quadrature Amplitude Modulation (QAM)

The quadrature amplitude modulation (QAM) is similar to DSB-SC. The only difference is that QAM sends two message signals over the same spectrum .

Let the two message signals be x₁(t) and x₂(t) .

One of them is sent in phase i.e. by multiplying it with cos ω_ct and the other one x₂(t) is sent in quadrature by multiplying it with sinω_ct .

Finally the two signals are added to obtain the QAM signals as shown in fig.1 .

Fig.1 : Generation of QAM Signal

Hence, the QAM signal is mathematically represented as below :

V_QAM(t) = x₁(t) cos ω_ct + x₂(t) sinω_ct

The coherent detection is used at the receiver to recover the signal x(t) back .

The QAM signal at the receiver input is multiplied by locally generated carrier and the product is passed through a low pass filter .

If we multiply V_QAM(t) by cos ω_ct, then we can recover x₁(t) whereas multiplying V_QAM(t) by sinω_ct can recover  x₂(t) as under :

If we pass this signal through a LPF, then, only the first term will be passed through to the output .

Therefore,

Thus, we recover x₁(t) if  V_QAM(t) multiplied with cosω_c(t) .

Similarly, if sinω_c(t)  is multiplied with V_QAM(t) then, we get,

If this is passed through a LPF, only the second term will be allowed to pass through .

Hence,

Thus, we recover x₂(t), if V_QAM(t) is multiplied with  sinω_ct .

Fig.2 shows the block diagram of QAM receiver. QAM performs like SSB . The QAM system is easy to install .

Fig.2 : QAM Receiver

But, if the locally generated carriers at the receiver have some phase error, then, it will cause a serious problem .