QAM (quadrature amplitude modulation)

What is QAM (quadrature amplitude modulation)?

QAM (quadrature amplitude modulation) is a method of combining two amplitude modulation (AM) signals into a single channel. This approach helps double its effective bandwidth. QAM is also used with pulse AM (PAM) in digital systems, like wireless applications.

A QAM modulator works like a translator, helping to translate digital packets into an analog signal to transfer data seamlessly.

QAM is used to achieve high levels of spectrum usage efficiency. This is accomplished by utilizing both the amplitude and phase components to provide a form of modulation. In this scenario, the QAM signal comes with two carriers. Each has the same frequency but differs in phases by 90 degrees, or one-quarter of a cycle, which is the basis for the term quadrature.

One signal is called the I signal, and the other is called the Q signal. Mathematically, one of the signals can be represented with a sine wave and the other by a cosine wave. The two modulated carriers combine at the source for transmission.

At the destination, the carriers separate, and the data is extracted from each. Then, the data is incorporated into the original modulating information.

three kinds of modulation
Examples of three types of modulation

What is the difference between analog and digital QAM?

Some analog transmissions, like AM stereo, use QAM systems. However, QAM comes into its own in data applications. This is because it offers a highly effective form of modulation for data when used in anything from mobile phones to Wi-Fi. QAM is found in most forms of high-speed data transmission.

Analog QAM also enables carriers to transmit multiple analog signals. For example, QAM is used in Phase Alternating Line and National Television Standards Committee systems. In this case, different channels provided by QAM enable the signal to carry components of color or chroma data.

A system known as Compatible QUAM is found in AM stereo radio applications. In this scenario, the different channels enable the required two channels for stereo to be carried by a single carrier. Digital versions of QAM are often called quantized QAM. They are built into most radio communications systems that use data.

For example, radio communications technologies ranging from Long-Term Evolution to Worldwide Interoperability for Microwave Access and Wi-Fi use different types of QAM. As the field evolves, expect to see an increase in QAM systems in radio communication technologies.

What is quadrature amplitude modulation in Wi-Fi?

Since the inception of Wi-Fi 5, networking devices use 256-QAM. Whenever 256 combinations are possible for 8 bits, it's referred to as 256-QAM. When you use a single time period to communicate 10 bits, then it is known as 1024-QAM.

This technology works well in home applications, and the improvements, like 1024-QAM, in Wi-Fi 6 have slightly enhanced bandwidth utilization. This is especially true for high-speed Gigabit Ethernet connections.

Wi-Fi 5 vs. Wi-Fi 6
Wi-Fi 5 vs. Wi-Fi 6 both use QAM.

What are the advantages and disadvantages of using QAM?

The primary benefit of QAM variants is efficient usage of bandwidth. This is because QAM represents more bits per carrier. For example, 256-QAM maps 8 bits per carrier and 16-QAM maps 4 bits per carrier.

Although QAM uses both the amplitude and phase variations to boost the efficiency of radio communication transmissions, there are some significant disadvantages. For example, QAM is more susceptible to noise. This is because the transmission states are close together and a lower level of noise is required to move the signal from one point to another.

Unlike QAM systems, receivers equipped with phase or frequency modulation can limit amplifiers and remove any amplitude noise. This approach improves its noise reliance. Furthermore, amplification of the phase or frequency modulated in a radio transmitter eliminates the need for linear amplifiers.

However, in QAM that contains an amplitude component, linearity must be maintained. Unfortunately, these linear amplifiers fall short when it comes to efficiency, and they consume more power. QAM receivers are also more complex when compared to those of other modulation types and systems. Therefore, they are not the best solution for mobile applications.

What is QAM in cable TV?

QAM systems are popular in the cable television industry.

Multiple system operators (MSOs) and other network operators use QAM to deliver data, video and voice services. QAM systems provide formatting services in hubs where signals are processed and distributed over the cable operator's network. Once at the subscriber's home, cable modems and set-top boxes convert QAM signals back into their original format.

There's been an increasing need for QAM channels among network operators. This explosion in demand is being driven by consumers using high-definition televisions, high-speed data, video conferencing and more. From an overall cost perspective, MSOs need QAM.

This was last updated in August 2021

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