Different modulation techniques may be compared theoretically on the basis of their Nyquist bandwidth and the normalized carrier-to-noise ratio. The real carrier-to-noise ratio (allowing for all imperfections) needs to be considered in order to define real systems.
Detailed information on this subject is given in Appendix 1.
3.2 Modulation methods
A suitable modulation method is selected by taking into account the system requirements. For instance, if spectrum efficiency is not a major issue and/or high interference tolerance is important, a simple modulation method should be used. The features of simple modulation methods are:
– easy implementation in all frequency bands,
– robustness against propagation effects,
– high tolerance against all kinds of interferences,
– high system gain characteristics.
On the other hand, a multi-state modulation method improves spectral efficiency on a route. Typical applications for multi-state modulation methods are high capacity trunk, junction and access networks.
Careful design of the multi-state constellation for QAM schemes might achieve some system gain against non‑linear distortion while retaining a fairly simple implementation.
Consideration of the carrier-to-noise requirement for the same BER when changing from 16-state to 512‑state modulation, for example, shows the need for significant increases in peak power, average power and peak-to-average power ratio. This places more stringent requirements on the high power amplifier, and will in many cases require the use of linearization measures, such as pre-distortion.
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