“They’re still using obsolete old electrons”
Jeff Hecht, City of Light,
Oxford University Press,
Data processing is a part of the telecommunication systems that, in emission, processes the signal before the digital/analog conversion and, in reception, after the analog/digital conversion (see Figures 8.1 and 8.2).
The analog/digital converter performs a scanning or digitizing operation and transforms a continuous signal (analog) in discrete values (binary) according to the Nyquist–Shannon sampling theorem. This theorem states that the sampling frequency of a signal must be equal to or greater than twice the maximum frequency of this signal. For example, in the sound domain, a human ear can capture up to 16 kHz and exceptionally up to 22 kHz. The associated analog/digital converter must sample the audio signal at about 44 kHz to ensure a good reconstruction in the opposite operation (digital/analog conversion). For a standard audio CD, the sampling frequency is equal to 44.1 kHz.
The signal processing includes operations such as filtering, compression, analysis, prediction, modulation, and coding. We discuss in this chapter the modulation and coding parts in a specific wireless optical configuration. The other items (e.g. phase signal use) are not directly related to the specificity of the wireless optical propagation channel. They are already the subject of many relevant books [HAE 03, GLA 96, ZIE 98].