The advantage of moving to the higher order formats is that there are more points within the constellation and therefore it is possible to transmit more bits per symbol. However the points are closer together and they are therefore more susceptible to noise and data errors. more points on the constellation, it is possible to transmit more bits per symbol. 16QAM, 64QAM, etc.īy using higher order modulation formats, i.e. Although data is binary the most common forms of QAM, although not all, are where there constellation can form a square with the number of points equal to a power of 2 i.e. Constellation diagram for a 16QAM signal showing the location of the different pointsĪs shown above, the constellation points are typically arranged in a square grid with equal horizontal and vertical spacing.
The different positions are assigned different values, and in this way a single signal is able to transfer data at a much higher rate. This is known as a constellation diagram. Using QAM there are many different points that can be used, each having defined values of phase and amplitude. Quadrature amplitude modulation, QAM, when used for digital transmission for radio communications applications is able to carry higher data rates than ordinary amplitude modulated schemes and phase modulated schemes.īasic signals exhibit only two positions which allow the transfer of either a 0 or 1. Radio communications systems ranging from cellular technology as in the case of LTE through wireless systems including WiMAX, and Wi-Fi 802.11 use a variety of forms of QAM, and the use of QAM will only increase within the field of radio communications. Here the different channels enable the two channels required for stereo to be carried on the single carrier.ĭigital formats of QAM are often referred to as "Quantised QAM" and they are being increasingly used for data communications often within radio communications systems. In radio applications a system known as C-QUAM is used for AM stereo radio. For example it is used in PAL and NTSC television systems, where the different channels provided by QAM enable it to carry the components of chroma or colour information. The analogue versions of QAM are typically used to allow multiple analogue signals to be carried on a single carrier. Quadrature amplitude modulation, QAM may exist in what may be termed either analogue or digital formats. QAM restores the balance by placing two independent double sideband suppressed carrier signals in the same spectrum as one ordinary double sideband supressed carrier signal. This is very wasteful of the available frequency spectrum. double sideband even with a suppressed carrier occupies twice the bandwidth of the modulating signal. In view of the fact that both amplitude and phase variations are present it may also be considered as a mixture of amplitude and phase modulation.Ī motivation for the use of quadrature amplitude modulation comes from the fact that a straight amplitude modulated signal, i.e. The resultant overall signal consisting of the combination of both I and Q carriers contains of both amplitude and phase variations. Often one signal is called the In-phase or “I” signal, and the other is the quadrature or “Q” signal. As a result of their 90° phase difference they are in quadrature and this gives rise to the name. sine and cosine) are modulated and combined. Quadrature Amplitude Modulation, QAM is a signal in which two carriers shifted in phase by 90 degrees (i.e. Quadrature amplitude modulation concept What is QAM, quadrature amplitude modulation It is able to provide a highly effective form of modulation for data and as such it is used in everything from cellular phones to Wi-Fi and almost every other form of high speed data communications system. QAM, quadrature amplitude modulation has been used for some analogue transmissions including AM stereo transmissions, but it is for data applications where it has come into its own. Quadrature Amplitude Modulation, QAM utilises both amplitude and phase components to provide a form of modulation that is able to provide high levels of spectrum usage efficiency. Modulation formats: Modulation types & techniques Amplitude modulation Frequency modulation Phase modulation Quadrature amplitude modulation, QAM basics QAM theory QAM formats QAM modulators & demodulators Quadrature Amplitude Modulation, QAM Tutorial Includes:
What is QAM: quadrature amplitude modulation QAM: Quadrature Amplitude Modulation combines amplitude & phase changes to give additional capacity & is widely used for data communications.