QAM is similar to Double Sideband Modulation Suppressed Carrier (DSBSC), generating two sidebands symmetric to the carrier frequency. It sends two message signals over the same spectrum.
QAM has applications in various fields, including in today's technology. The example of QAM in analog modulation is PM (Phase Modulation), where the signal's amplitude is constant, and the phase varies. Another example is PSK (Phase Shift Keying) in digital modulation, where also the phase of the signal varies. But, QAM using Shift keying has greater applications in digital communication to transmit a high number of bits over the same spectrum.
Quadrature refers to the phase shift of 90 degrees. It means four and is derived from 360 degrees/4 = 90 degrees. The components used in the modulation are cosine waves and sine waves, which differ by a phase shift of 90 degrees.
These two waves are shown below:
The upper wave is the sine wave, and the lower is the cosine wave.
QAM is a family of analog modulation as well as digital modulation. It can be used to vary the amplitudes in analog modulation and two digital bitstreams using ASK (Amplitude Shift Keying) in digital modulation. Here, we will discuss QAM based on the analog modulation.
Unlike DSB, Quadrature Amplitude Modulation sends two message signals over the same spectrum. The first message signal is sent by multiplying it with the carrier signal.
m1(t) = The first message signal
cosωct = The first carrier signal
Product = m1(t)cosωct
The second message signal is sent in Quadrature. The message signal is multiplied by sinωct.
m2(t) = The first message signal
sinωct = Quadrature carrier signal
Product = m2(t)sinωct
Quad means four. The Quadrature carrier signal means a phase shift of 90 degrees. It means that both the carrier signals differ by a phase shift of 90 degrees.
cos (90 - ωct) = sinωct or sin (90 - ωct) = cosωct
The product of both the signals is further added. It is given by:
MQ (t) = m1(t)cosωct + m2(t)sinωct
The process of multiplication of the message signal and the carrier signal is known as mixing or modulation.
The block diagram of a simple analog Quadrature Amplitude Modulation is shown below:
Demodulation is the reverse process of modulation. It recovers the original signal from the product to make it available to the receiver. Here, the demodulation is based on the coherent detection method. The carrier is multiplied with the output of the modulator and then applied to the low pass filter. QAM does not have carrier with the signal like DSBSC transmission. Hence, the carrier is selected, which is coherent with the original baseband signal to prevent further phase shifts and distortion.
It is given by:
MQ x (carrier signal) = (m1(t)cosωct + m2(t)sinωct) x (carrier signal)
The carrier signal can be cosωct or its Quadrature sinωct. Let's discuss both the possibilities of the carrier signal.
The function of a low pass filter is to allow a certain band of frequencies to pass through it. In QAM, a low pass filter brings out the message signals m1(t) and m2(t) by suppressing the components around the frequency 2ωc.
The block diagram of the QAM demodulator is shown below:
QAM can give the performance like Single Sideband (SSB) transmission if there is no phase error. It is because a phase error can result in co-channel interference. In practical communication, carrier synchronization is maintained by sending the carrier in between messages. It prevents interference in the channel.
The advantages of the Quadrature Amplitude Modulation are as follows:
The disadvantages of Quadrature Amplitude Modulation are as follows:
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