**5. General conclusion of this chapter**

FSO system can spread as a reliable solution for high bandwidth and short distance. There are some factors which must be taken into consideration during the design of FSO system as controllable and uncontrollable factors. Controllable factors include wavelength, transmission range, beam divergence, loss occurred between transmitter and receiver and detector sensitivity. Uncontrollable factors include visibility, rainfall rate, raindrop radius, atmospheric attenuation and scintillation.

Atmospheric attenuation may be absorption or scattering. Absorption lines at the visible and IR wavelengths are narrow and separated. So, we can ignore absorption effect at the wavelength identified as atmospheric windows. Wavelength at FSO system must be eye safe and able to transmit a sufficient power during the bad weather condition. Mie scattering represents the main affects on FSO systems. The main cause of Mie scattering is fog and hazy. Attenuation caused by fog in Yemen is so important for Taiz as the low visibility range can less than 0.05 km during the extensive fog according to the data taken from metrology authority. Transmission in this city may be cut off, so the distance between the transmitter and receiver must be reduced. However, Sana'a and Aden cities the weather is clear during the whole year in comparison with Taiz city. Rayleigh scattering we can ignore it at the visible and infrared wavelength as its effect on the ultraviolet wavelengths is huge. This scattering occurs when the molecules size is less than the wave length of the laser beam.

Non-selective scattering independent on wavelength and occurs when the molecules size is bigger than wavelength and it occurs due to the rainfall. Generally, FSO system is so adequate in Yemeni environment according to the previous results. The performance of wavelength 1550 nm is better at the bad weather conditions in comparison with wavelengths 850 nm and 780 nm. Furthermore, the wavelength 1550 nm allows a high power may reach to over 50 times in comparison with the wavelengths 850 nm & 780 nm.

By analyzing results obtained at chapter four, we conclude that we are able to improve the performance of transmission of FSO system at the bad weather conditions by using the wavelength 1550 nm and short distance between transmitter and receiver.
