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Digital Communications 2 by Didier Le Ruyet, Mylène Pischella

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6Coded Modulations

This chapter presents coding techniques well adapted to high signal-to-noise ratio regimes. We showed in Chapter 3 that when the signal-to-noise ratio is high, large-order quadrature amplitude modulation (QAM) constellations allow us to achieve high spectral efficiencies. Squared QAM constellations, when M = 2m, are regular in the two-dimensional space image2. They are based on the repetition of a reference pattern. This principle can be extended to constellations in higher dimensions than 2. A constellation may be defined as a repetitive infinite grid of points, restricted to a limited area. These repetitive grids are mathematically based on lattices. For instance, squared QAM all follow the same grid, but they are differentiated by their boundary, which gets larger when the size of the constellation increases.

If error-correcting codes are used separately from modulations, they lead to an increase in the required bandwidth, due to the introduced redundancy. Coded modulations combine channel encoding with modulations in order to maintain the same bandwidth, while improving performances. Redundancy is introduced by the use of a larger size constellation than what would be necessary without coding. For instance, let us assume that we want to transmit 2 bits per symbol. Without channel encoding, the symbol period is denoted by T and the bandwidth by B, using quadrature ...

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