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In digital telecommunications the data is usually binary, so the number of points in the grid is typically a power of 2 (2, 4, 8, …), corresponding to the number of bits per symbol. The simplest and most commonly used QAM constellations consist of points arranged in a square, i.e. 16-QAM, 64-QAM and 256-QAM (even powers of two).
A diagram with four points, for example, represents a modulation scheme that can separately encode all 4 combinations of two bits: 00, 01, 10, and 11, and so can transmit two bits per symbol. Thus in general a modulation with N {\displaystyle N} constellation points transmits log 2 N {\displaystyle \log _{2}N} bits per symbol.
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The history of modems is the attempt at increasing the bit rate over a fixed bandwidth (and therefore a fixed maximum symbol rate), leading to increasing bits per symbol. For example, ITU-T V.29 specifies 4 bits per symbol, at a symbol rate of 2,400 baud, giving an effective bit rate of 9,600 bits per second.
In a 6 MHz channel, the data rate is at most 36 Mbit/s (for 64-QAM or 8-VSB); the 8-VSB ATSC achieves a data rate of 19.3926 Mbit/s while the 64-QAM J.83b achieves a data rate of 26.970 Mbit/s. While both systems use concatenated trellis/RS coding, the differences in symbol rate and FEC redundancy account for the differences in rate.
Constellation shaping is an energy efficiency enhancement method for digital signal modulation that improves upon amplitude and phase-shift keying (APSK) and conventional quadrature amplitude modulation (QAM) by modifying the continuous uniform distribution of the data symbols to match the channel.
Mapper: The digital bit sequence is mapped into a base band modulated sequence of complex symbols. There are three valid modulation schemes: QPSK, 16-QAM, 64-QAM. Frame adaptation: the complex symbols are grouped in blocks of constant length (1512, 3024, or 6048 symbols per block).
The advantage of APSK over conventional QAM is a lower number of possible amplitude levels and therefore a lower peak-to-average power ratio (PAPR). [2] The resilience of APSK to amplifier and channel non-linearities afforded by its low PAPR have made it especially attractive for satellite communications, including DVB-S2 .