Contents I. Introduction
The demand of bandwidth to serve increasing services such as video on mobile terminals is growing every year, according to Edholm's law of bandwidth [1]. As nowadays almost all radio bands are allocated, bandwidth-limited, and almost saturated, new methods for wireless data transmissions are currently investigated. The physical layer for wireless links will thus be strongly developed within the next years as the traffic in the wireless regime is supposed to become larger than that of the wired networks lines. Optical wireless communications (OWC) in near-infrared have been shown to be capable of reaching free-space gigabit per second (Gbps) capacities [2], [3], and, recently, a record of capacity has been reported by using four light beams with orbital angular momentum multiplexing [4]. However, as the wavelength is very small (around m), fog effects and scattering by dust of particles introduce high fading effects [2]. OWC systems thus require very good channel qualities, and an outdoor use in real weather conditions may be difficult to achieve tens of Gbps at long range without specific techniques as adaptative optics. THz-based systems requires an additional optical-to-THz converter, but the wavelength increase will provide enhanced robustness to channel propagation [5]. THz communications are very promising as it opens a huge space for new services using the last available wavelengths in the electromagnetic spectrum (frequencies are still free in the sub-millimeter region, i.e., above 300 GHz). The THz frequency range is very attractive as it provides more data bandwidths in accordance with the increase of the carrier frequencies. Carrier frequencies in the 200–300-GHz atmospheric window enable data links, and several demonstrations have been reported in the last years. Future data-rates handled by THz carriers are expected to become comparable to fiber-optic guided ones, ensuring a high-speed continuity from backbone networks to end-users.
