## Quadrature Amplitude Modulation (QAM)QAM is similar to Double Sideband Modulation Suppressed Carrier (DSBSC), generating two sidebands symmetric to the carrier frequency. It sends QAM has applications in various fields, including in today's technology. The example of QAM in analog modulation is Quadrature refers to the phase shift of The upper wave is the 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 ## QAM ModulatorUnlike 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. m cosω Product = m The second message signal is sent in Quadrature. The message signal is multiplied by sinωct. m sinω Product = m 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 - ω The product of both the signals is further added. It is given by: M 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: ## QAM DemodulatorDemodulation 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: M The carrier signal can be cosω - If M
_{Q}is multiplied with cosω_{c}t, we get the first signal m_{1}(t) back. Thus, it helps to recover the first message signal. M_{Q}x cosω_{c}t = (m_{1}(t)cosω_{c}t + m_{2}(t)sinω_{c}t) x cosω_{c}t M_{Q}cosω_{c}t = m_{1}(t)cos^{2}ω_{c}t + m_{2}(t)sinω_{c}tcosω_{c}t M_{Q}cosω_{c}t = 1/2 [(1 + cosω_{c}t) m_{1}(t) + m_{2}(t)sin2ω_{c}t] M_{Q}cosω_{c}t = 1/2 [m_{1}(t) + cosω_{c}t m_{1}(t) + m_{2}(t)sin2ω_{c}t] - If M
_{Q}is multiplied with sinω_{c}t, we get the signal m_{2}(t) back. Thus, it helps to recover the second message signal. M_{Q}x sinω_{c}t = (m_{1}(t)cosω_{c}t + m_{2}(t)sinω_{c}t) x sinω_{c}t M_{Q}sinω_{c}t = m_{1}(t)cosω_{c}t sinω_{c}t + m_{2}(t)sin^{2}ω_{c}t M_{Q}cosω_{c}t = 1/2 [m_{1}(t) sin2ω_{c}t + m_{2}(t) - m_{2}(t)cos2ω_{c}t]
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 m 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. ## Advantages of QAMThe advantages of the Quadrature Amplitude Modulation are as follows: **Increased bandwidth** QAM has a bandwidth greater than DSBSC because it can transmit two signals on the same spectrum.**Low noise interference** QAM has low noise interference because the two signals are close together and requires a low noise to transmit the signal from one point to another.
## DisadvantagesThe disadvantages of Quadrature Amplitude Modulation are as follows: **Complex Receiver** The receiver of QAM is complex because it needs two carrier signals of opposite phases to recover two message signals.**Consumes more power** The requirement of additional amplifiers and filters increases its power consumption.
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