For several years, it has been examined if the attributes of the wavelengths in C band of the Ultraviolet (UV) spectrum that lie between 200 and 280 nm can be exploited in order to set up short range covert links of low rate in a Non-Line-of-Sight (NLOS) regime. In the present work, it is experimentally investigated and verified that using this band, short range and low rate NLOS links using the same transmitter/receiver pair under different atmospheric conditions without applying extreme power levels can be implemented rather effectively. The transmitter was composed of four Light Emitting Diodes. At the receiving side, an optical filter was followed by a Photo-Multiplier Tube. Initially, we measured the power losses of the channels with and without fog (artificially generated) for ranges up to 20 meters and several transmitters/receiver configurations. Secondly, the performance of Fourth-order Pulse Position Modulation (4-PPM) and Flip Orthogonal Frequency Division Multiplexing (Flip-OFDM) was evaluated for such channels and 10 Kbit/s rate. Applying emissions at 265 nm, NLOS links can operate efficiently especially in harsh environments, as the power losses were lowered when fog appeared. In terms of the modulation formats, 4-PPM performed better in most cases. Better results were obtained for both schemes when the medium became thicker due to the presence of fog. Finally, some initial measurements were realized with a Silicon Carbide PiN photodiode for the same rate but low elevation angles. The performance was exactly the opposite compared to a receiver with inherent gain when the atmosphere thickened.
Short range communications using an optical wireless channel in a non-line-of-sight regime can be attained by exploiting the solar-blind UVC band. Firstly, the power loss and the bandwidth of the diffused wireless channel are examined for several particle and molecular densities of the medium when a receiver with a wide Field-Of-View is considered. Proper transmissions were simulated for two channel cases in order to investigate how the channel affects the signals. The investigated modulation schemes were On-Off-Keying (OOK) and 4-Pulse-Position-Modulation (4-PPM). At the receiving side, estimations with photoelectrons were considered. Compared to OOK, 4-PPM is favored by the detection without threshold and the higher peak optical power. Concerning the influence of the transmission medium, it appears that a sparse medium may limit the performance of both PPM and OOK due to the increased losses and the slight Inter- Symbol-Interference that appears. On the contrary, the estimated channel with broader bandwidth and lower losses for a thicker atmosphere ameliorated the performance of both schemes. Using the previous indication, Code Division Multiple Access (CDMA) transmissions were investigated utilizing the already defined receiver configuration. In CDMA, if the coded signals are transmitted with the same mean and peak power, 4-PPM seems to need more power in order to achieve similar performance to OOK. This confinement of 4-PPM is mitigated in a dense channel. Finally, the linearity of the Power-Current curve of the LEDs at the transmitting side was also taken into account as a factor that may increase the consumption of the sources.