Tomoharu YOKOTA     Kazuhiko ASAKA     Hideki MATSUMOTO
Tsuyoshi YAMAGUCHI     Chiharu KASAI

Introduction     Configuration of study system Weather dependency of the Link Quality of the Free Space Optical Transmission System Simulation of optical propagation characteristics Conclusion

4 Simulation of optical propagation characteristics

The simulation software of this study uses FASCODE model as the basic atmosphere structure and HITRAN database parameter as the standard air component.

(1) Theoretical formula and simulation of optical propagation loss in the air

At first the optical propagation simulation software was compared with the estimation formula which includes the visibility and is frequently used for the estimation of the Free Space Optical Transmission System and is suitable for the intuitive understand of optical attenuation. The estimation formulae are the popular formula (1) and (2).

	(dB/km)	formula (1)
	(dB/km)	formula (2)

L : propagation loss (dB/km) V : visibility (km) λ : wave length (nm)
q : size distribution of scattering particles,  0.585V ^1/3 (V ＜ 6km)

Formula (2) defines the attenuation coefficient caused by scattering particles similarly with formula (1), but includes the wave length dependency and the size distribution of particles.

Figure 4-1 shows the relation between the atmospheric attenuation and the visibility obtained by formula (1), (2) and the simulation based on the wave length 845nm, and shows also the simulation of Fog (VIS = 0.2km, 0.5km), Rural (VIS = 5km) and Urban (VIS = 5km) as the default setting of propagation environment. The result of the simulation closely resembled with that of formulae. There is a small difference caused by the influence of scattering particles (dust, fog and rain drop) except the visibility. But the optical propagation loss can be sufficiently estimated by the simulation used the visibility as parameter.

(2) Weather and result of simulation

The influence of the rain was simulated setting the rain rate as the main parameter. The rain appears frequently and affects the Free Space Optical Transmission System.

In simulation software, the estimation of the transmission against the rain rate is calculated using Marshall-Palmer formula. Marshall-Palmer formula is as follows.

t = exp (-0.365 R0.63 ×s)

formula (3)

t : transmission R : rain rate (mm/hr) s : path length (km)

At first when the visibility is fixed (VIS = 23km) and the rain rate is deviated, the result almost coincides with formula (3) and the simulation of the attenuation caused by rain rate is correct. But when the visibility is short (10km, 1km), the difference between them gets larger. This means that the attenuation caused by scatter and absorption from other element is larger than that caused by simple rain. In other word the element that reduces the visibility is the influence of fog or haze except rain and the influence of fog or haze appears more strongly than rain.

Conclusively the visibility data is more effective than the rain rate to estimate the optical attenuation at the actual field in rain.

Figure 4-1 Relation between Atmospheric Attenuation and Visibility

5 Conclusion

The following items are verified on the weather dependency of the transmission quality.

�@	The weather condition of the visibility less than 1km, in such a condition the Free Space Optical Transmission System is affected, appears at the time of rain and the percentage of it is very rare, only 0.32%, as the result of the measurement since October, 2001 to March, 2002.
�A	The data of the optical propagation loss is almost equal to the value from the estimation formula in the worst case and the most part of it is less than the formula. Accordingly there is no problem to estimate the propagation loss and the link quality of the Free Space Optical Transmission System using the estimation formula.
�B	The link-down did not happen and the worst bit error rate was 4.3×10 -8 . The link quality is very high for wireless system.

The following items are verified as the result of the simulation of the optical propagation loss.

�@	The result of the simulation setting the visibility as parameter, coincides with the value from the estimation formula. It is possible for the simulation to estimate the optical propagation loss using the visibility.
�A	As the result of the simulation including the visibility and the rain rate, the simulation is correct if the visibility is long. But the difference gets larger if the visibility is short. The simulation using the visibility including the weather condition is desirable and the simulation using only the rain rate is not desirable.

The deviation of the link quality of the Free Space Optical Transmission System caused by the weather condition was verified. That is, the Free Space Optical Transmission System has the strong relation with the weather condition, especially the visibility, and it is possible to estimate the link quality of the Free Space Optical Transmission System using the visibility. The visibility is possible to be measured by the visibility meter and obtained from the weather bureau of that area. In this study it was verified that the link design of Shinagawa area is possible using the data from the Meteorological Agency in Ohtemachi, Tokyo and the similar estimation in another area is possible using the data from the weather bureau.

The Free Space Optical Transmission System is the system of high speed, broad band and high flexibility, and is easy to be setup and highly reliable. It is effective for the construction of the access network of the broad band internet.

Back