If there is a good margin, higher orders of QAM can be used to gain a faster data rate, but if the link deteriorates, lower orders are used to preserve the noise margin and ensure that a low bit error rate is preserved. As the QAM order increases, so the distance between the different points on the constellation diagram decreases and there is a higher possibility of data errors being introduced. Accordingly there is a balance to be made between the data rate and QAM modulation order, power and the acceptable bit error rate. Whilst further error correction can be introduced to mitigate any deterioration in link quality, this will also decrease the data throughput. However some specific variants of QAM are used in some specific applications and standards.
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In QAM, the constellation points are usually arranged in a square grid with equal vertical and horizontal spacing, although other configurations are possible e. Since in digital telecommunications the data is usually binary , the number of points in the grid is usually a power of 2 2, 4, 8, …. By moving to a higher-order constellation, it is possible to transmit more bits per symbol.
However, if the mean energy of the constellation is to remain the same by way of making a fair comparison , the points must be closer together and are thus more susceptible to noise and other corruption; this results in a higher bit error rate and so higher-order QAM can deliver more data less reliably than lower-order QAM, for constant mean constellation energy. Using higher-order QAM without increasing the bit error rate requires a higher signal-to-noise ratio SNR by increasing signal energy, reducing noise, or both.
If data-rates beyond those offered by 8- PSK are required, it is more usual to move to QAM since it achieves a greater distance between adjacent points in the I-Q plane by distributing the points more evenly. The complicating factor is that the points are no longer all the same amplitude and so the demodulator must now correctly detect both phase and amplitude , rather than just phase. There is a spreading of the spots in the constellation, decreasing the separation between adjacent states, making it difficult for the receiver to decode the signal appropriately.
In other words, there is reduced noise immunity. There are several test parameter measurements which help determine an optimal QAM mode for a specific operating environment. The following three are most significant: [7].
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What is QAM: quadrature amplitude modulation
In QAM, the constellation points are usually arranged in a square grid with equal vertical and horizontal spacing, although other configurations are possible e. Since in digital telecommunications the data is usually binary , the number of points in the grid is usually a power of 2 2, 4, 8, …. By moving to a higher-order constellation, it is possible to transmit more bits per symbol. However, if the mean energy of the constellation is to remain the same by way of making a fair comparison , the points must be closer together and are thus more susceptible to noise and other corruption; this results in a higher bit error rate and so higher-order QAM can deliver more data less reliably than lower-order QAM, for constant mean constellation energy. Using higher-order QAM without increasing the bit error rate requires a higher signal-to-noise ratio SNR by increasing signal energy, reducing noise, or both.
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QAM Formats: 8-QAM, 16-QAM, 32-QAM, 64-QAM, 128-QAM, 256-QAM
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Quadrature amplitude modulation
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