Quadrature Amplitude Modulation (QAM)/ QAM Transmitter and QAM Receiver Block Diagram

In this post, we will discuss the Quadrature Amplitude Modulation (QAM). Here we will see, the basic concept of quadrature amplitude modulation and why it is known as quadrature amplitude modulation. The block diagram of QAM transmitter and receiver also has been explained here.

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Introduction of Quadrature Amplitude Modulation (QAM)

Quadrature Amplitude Modulation (QAM) can be used, either with analog signals or digital signals. If the message signal is analog in nature, then we use it with Amplitude Modulation (AM) but when the message signal is in digital form, then Amplitude Shift Keying (ASK) is used.

Using QAM, it is possible to transmit two analog message signals or two digital bit sequences by modulating carrier wave's amplitude using analog modulation technique (AM) or digital modulation technique.

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This modulation scheme is also known as 'Quadrature Carrier Multiplexing'.
Now we will understand, what is the meaning of 'Quadrature' in Quadrature Amplitude Modulation (QAM).
Actually in quadrature amplitude modulation, the two carrier waves are used, that are out of phase by 90 degrees. Therefore this scheme is known as quadrature amplitude modulation, and these carriers are known as Quadrature Carriers.
These modulated waves are summed at the output of the transmitter, therefore the final waveform is the combination of both PSK and ASK in digital case or PM and AM for analogue case.
Now we will understand this concept with the help of block diagrams of QAM Transmitter and QAM receiver.

Block Diagram of QAM Transmitter

In this image, you can see that here we are transmitting two message signals of analog nature x1(t) and x2(t).
Observe that it contains two product modulators. The inputs to the first product modulator is, message signal x1(t) and the carrier wave, while the inputs to the second product modulator are analogue message signal x2(t) and the  carrier wave with a phase shift of 90 degrees. Here we have used a 90 degree phase shifter for phase shifting.

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Mathematical Representation of Multiplexed Signal s(t)

Because we know, when the product modulator is supplied with a message signal and the carrier wave, then we get amplitude modulated wave in the output.
So you can observe here that, quadrature amplitude modulation (QAM) is the combination of both Amplitude Modulation and phase modulation (As phase change of carrier wave takes place here).
This was the case for analog modulation, but if we talk about QAM with digital modulation, then, at the place of analog message signal, we use digital messages. In this case it would be a combination of Amplitude Shift Keying (ASK) and Phase Shift Keying (PSK).
Now see the Output of the QAM transmitter.  Here the outputs of two product modulators are added and the resultant output is the multiplexed signal S(t).
So this QAM scheme enables two modulated signals to occupy the same transmission channel and allows the separation of these two message signals at the receiver output therefore it is also known as 'Bandwidth Conservation Scheme'.

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With this scheme we are simultaneously transmitting two amplitude modulated waves (in case of analog modulation) or two amplitude shift keying signals (in case of digital modulation). Therefore this allows us to use the same transmission channel. And at the receiving end we can easily detect these message signals.
Now it's time to discuss the block diagram of QAM receiver.

Block Diagram of QAM Receiver

As you can see here, block diagram contains 5 components...
Two product modulators, one 90 degree phase shifter, and two low pass filters.
Multiplexed signal S(t), which is output of the QAM transmitter, acts as the input to the QAM receiver.
This multiplexed signal is applied to the inputs of both the product modulators. 
For the First product modulator therefore two inputs are- Multiplexed signal s(t) and the carrier wave while the inputs to the second product modulator are- multiplexed signal s(t) and the carrier wave with 90 degree phase shift.

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Now the outputs of these product modulators are passed through the Low Pass Filters (LPF) as depicted in the image.
So at the outputs of these are low pass filters, we get the message signals.
But one important thing that should be kept in mind here is that, the oscillators used to produce carrier waves at the transmitting and receiving ends must be in coherence i.e. the carriers at the transmitting and receiving ends must be in same phase.
In this way we get back the modulating signals x1(t) and x2(t)(message signals) at the receiving end.

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