Features and Range of the FSO by Use of the OFDM and QAM Modulation in Different Atmospheric Conditions

摘要

The FSO (Free Space Optics) communication uses the visible or infrared light for transmission. As well as cable optics FSO also uses laser for the data transmission, but the data flow is not transmitted in the fiber but in the air. This technology does not require expensive fiber optic cables and or ensure the licensing zone as it is in the case of radio networks. As well as in the cable transmissions are different modulations used. Nowadays, the most used modulations are QAM and OFDM. OFDM belongs to the border group of modulations with more carrier waves, where the information's are transmitted via subcarrier waves with lower data flow and baud rate. It is mainly used in broadband wire and wireless communications. OFDM provides very high signal resistance against the interference, chromatic dispersion (CD) and polarization mode dispersion (PMD)[1]. Modulation method involves the use of several hundreds to thousands of subcarrier waves. QAM is a composite modulation, which uses for symbol creation the combination of ASK (Amplitude Shift Keying) and PSK (Phase Shift Keying). Each state is represented by a specific value of the amplitude and phase. It's actually multistate modulation, which is able to transmit n bits by m symbols. That means that more bits are transferred at one moment. This paper deals with the modulations used in FSO. Most used modulation in FSO is OOK (On-Off Keying) , but modulations OFDM and QAM are (modulation ) ways of the future. The main task was to determine how much is the reach of modulations changing with the changes of density (visibility) of fog and the set transceiver power. As software environment for simulations has been used OptiSystem program. For the simulation of the atmosphere the FSO component has been used. In this component were simulated attenuations, which are responding to varying densities of fog . At different intensities of fog were changing the received power.