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Enhance Data Rates and Bandwidth with Quadrature Amplitude Modulation

Key Takeaways

  • QAM is a modulation method that combines two amplitude modulated signals into a single channel, doubling the bandwidth of the system. Since two signals are modulated and transmitted to the receiver at the same time, QAM is also called quadrature carrier multiplexing. 

  • The advantages of QAM are higher data rates, high noise immunity, low probability of error values, and increased bandwidth.

  • QAM is used in cable television networks, wireless communication systems, and cellular and digital platforms. 

Person turning on the radio

Radio signals can be amplitude or frequency modulated 

Many people use the radio without knowing the difference between AM radio and FM radio. AM and FM are types of modulation techniques—AM stands for amplitude modulation and FM stands for frequency modulation. While these are the more popular techniques, there are several other types of modulation techniques that can be used, depending on the requirements of a system.

In wireless applications, the idea of transmitting digital information using analog signals while effectively utilizing channel bandwidth has led to the emergence of a new modulation technique called quadrature amplitude modulation (QAM). QAM is a modulation technique used to transmit digital information between two points using analog signals. It allows the transmission of more bits in the same time period, thus, increasing the channel bandwidth. In this article, we will discuss how the QAM method works and its merits.

The Quadrature Amplitude Modulation Method

QAM is the modulation method that combines two amplitude modulated signals into a single channel, doubling the bandwidth of a system. Since the two signals are modulated and transmitted to the receiver at the same time, QAM can also be called quadrature carrier multiplexing. QAM can be utilized in either analog or digital modulation schemes. 

Principles of QAM

In QAM, two carrier signals of the same frequency and a phase shift of 90° are used. The carrier signals can be represented as a sine wave and a cosine wave. 

Input carrier waveforms

QAM carriers have a 90°phase-shift between each other

The two signals are combined at the transmission end using a QAM modulator consisting of the in-phase channel signal (I) and the quadrature channel signal (Q). Output signals are extracted at the receiver end using a QAM demodulator. 

QAM output waveforms

There are two output channel signals in QAM

A QAM modulator and demodulator modulates the in-phase and quadrature components of the modulating signals onto the carrier. The data extracted from the two carrier signals are combined into the original modulating information. 

Modulation

QAM modulator

A QAM modulator is used at the transmission end

The input data signals I and Q are amplitude modulated with sine and cosine carrier waves. The two modulated signals are combined and then processed according to RF requirements. These signals further undergo frequency conversion and amplification to meet the final frequency and amplitude specifications.  

Demodulation

QAM demodulator

A QAM demodulator is used at the receiver end

In the QAM method, demodulation is the reverse of modulation. Signals from the transmitter side enter the demodulator and get split into two signals which are given to separate mixers. The local oscillator supplies demodulation signals of the required frequency, and the signals produced are in-phase and quadrature signals.

The Advantages of QAM

Besides increasing channel bandwidth, there are several other advantages to QAM, including:

  1. The carrier signal can carry a higher number of bits, resulting in a high data rate.

  2. Noise immunity is high, causing less noise interference during data transmission.

  3. The probability of error values is low. 

Applications of the QAM Method

QAM is used in cable television networks to deliver video, audio, and data. The signals are processed and formatted in the hubs and headends of the cable transmission network and are delivered to the customer. At the customer end, the set-top boxes, cable modems, or embedded multimedia terminal adapters convert QAM signals into original video, audio, or data signals. 

In radio communication systems and data delivery applications, QAM is widely used. Most of the high-speed data, video-on-demand, HD television, Internet Protocol cable television, and wireless cellular and digital systems are run by QAM. 

Higher data rates are of great importance in wireless and cellular communication systems. QAM provides this, along with noise immunity, wide bandwidth, and low error values, making it a widely used and respected modulation technique. 

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