How does quadrature amplitude modulation work?

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Quadrature amplitude modulation (QAM) is known as a type of method that is used to combine two amplitude modulated signals into one single channel, and in doing so, it creates more effective bandwidth by doubling the amount. It is both a digital and analog modulation scheme and is used extensively in digital telecommunication systems. QAM can be found in pulse amplitude modulation, located in digital systems, for example, in wireless applications. It is also the layout through which cable channels that are digital are encoded and sent through cable television providers. QAM television tuners are compared to the cable counterpart of an advanced television systems committee (ATSC) tuner that is obligated to collect over-the-air broadcasts by digital channels via television stations that are local, not digital. Today, many digital televisions that are cable-ready support both ATSC and QAM standards. Even though QAM utilizes the same six MHz bandwidth that ATSC uses, QAM delivers twice the amount of data, around 38.47 megabits per second – the speed of the transfer of data, because there are no corrections of error. But, the use of QAM necessitates a much clearer path for signals to flow.

Throughout North America, a QAM tuner is a device that is available in certain digital televisions, as well as other devices that create express reception of digital cable channels devoid of the need for a cable box. QAM is most often used in regards to HDTV, high definition television. With digital television, a QAM tuner enables individuals to acquire free reception of digital programs that are not scrambled, sent by local cable providers. What channels remain scrambled by a QAM tuner is dependent on what cable providers declare as an “extra channel” that is not part of the basic cable television package.

In any signal from a QAM, there are two signal carriers present, which are analog message signals, also known as digital bit streams. One signal is known as the I signal, in addition to the second signal, known as the Q signal. These signals, or streams, are conveyed by adjusting the amplitude of these two carrier waves by an amplitude shifting key (ASK) digital modulation scheme, or by using an amplitude modulation (AM) analogue scheme. The two waves, often sinusoids, are differing in their stage by 90 degrees; mathematically, these two signals or waves, are represented with a sine wave, along with a cosine wave. Due to the difference in their phases, these waves are called quadrature components, or quadrature carriers. As these waves are combined, they create a resultant waveform that is a mixture of phase shift keying (PSK), as well as ASK. When the carrier signals reach the destination point, they are then separated and the information from the signals are taken from both and combined to form original modulating information.

By using QAM, there is the possibility to enable the transference of an increased amount of bits per position due to the fact that there exists numerous transfer points. In QAM, a carrier signal is acquired when the phase and amplitude modulation of a carrier signal (the quadrature wave) is summoned and then used for the transfer of data. Because there are a large number of positions of transfer in QAM, it is feasible to transmit a large number of bits for every change in position.

Even though QAM can increase transmission efficiency by using both phase and amplitude variations, there are some considerable drawbacks. First of all, QAM is more vulnerable to noise due to the fact that states are in close proximity to one another, and therefore a decrease in noise is required to relocate a signal to another point. Most receivers can use limiting amplifiers to remove noise. However, this is not possible with QAM. A second drawback is that with QAM, linearity needs to be maintained and there needs to be the use of linear amplifiers which use more noise and are less attractive.

Resource by Will

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