All signals generated or observed due to human activity (measurement of different physical values, remote control, files, sounds, images, etc.) can be transformed into analog electrical signals. These electrical signals can be processed or transmitted as they are, but in most cases they are digitized beforehand. Once digitized, these signals have different forms regarding the digital coding or error-correcting algorithms used and protocols employed in the transmission systems. To be processed or transmitted, these analog or digital signals can use different mediums.
If the transmission is performed via metallic lines or cables, digital signals directly enter these lines or they modulate more or less complex subcarriers (ADSL, Ethernet).
If transmission occurs with a radiowave, a high-frequency carrier must be available. Due to the congestion of the wireless spectrum, carriers are now principally microwaves, i.e. with frequencies of 1 to 100 GHz (GSM, UMTS, Wi-Fi, Wimax, etc.). Wireless transmission can also be performed by very short pulses having a wide spectrum range, including the microwave spectrum, or these pulses can modulate a carrier in the millimeter wave spectrum. This technique is known as ultra-wideband (UWB) [YAO 09].
If transmission is via fiber optics, digital signals modulate one or several optical carriers (amplitude or soliton modulation), which are themselves transmitted over hundreds of kilometers on optical ...