SIMULATIVE INVESTIGATION OF 16X5 GB/S & 16X10 GB/S WDM-FSO TRANSMISSION SYSTEM UNDER DIFFERENT ATMOSPHERIC CONDITIONS

In Free Space Optic (FSO) system free space requires as a communication media. Atmospheric attenuation due to communication media can degrade a system performance. To overcome this losses BER or Q-factor of a system are improved by using different type of photodetector at receiver side & using optical receiver at link.. This paper describe the design and simulation of 16-channel WDM based Free Space Optical communication link under various Atmospheric conditions at different bit rates (5Gbps/10Gbps) using APD & PIN photo-detector & the performance of system using APD & PIN photo-detector is compared. The NRZ modulation format & semiconductor optical amplifier (SOA) is used in this design. In this simulation it observed that APD photo-detector at bit rate 5Gbps/10Gbpsgive better performance at higher attenuation.


INTRODUCTION
Free Space Optical Communication system is widely used now a day's due to its high Bit Rate, huge bandwidth and minimum bit error. FSO system provide very efficient communication. In FSO communication system electrical signal first converted into light form for this purpose Laser source is used which include LED's and Laser diodes. On the other hand at receiver side optical-detector is used to covert light signal back into electrical form. [1] The photodetector is an essential element of any optical communication system which connected at receiver side as shown in " Fig.1 P-region c.
Intrinsic region First of all electron-hole pairs are generated in depletion region by photo absorption. These generate more pair of electron and hole through impact ionization which swept out depletion region very fast i.e the transit time is very less. [2,3] APD photo-detector-An Avalanche photo-detector (APD) is a semiconductor device which exploits the photoelectric effect to convert light to electricity. APD photo-detector provide a built-in first stage of gain. APD was invented in 1960's. APD structural configuration is same as of PIN photodiode. When high reverse bias voltage is apply to APD it show an internal current gain effect. This internal current gain effect is due to impact ionization. [4] In APD higher gain is possible due to internal multiplication. Where in PIN photo-detector current at output is proportional to light incident. So gain in case of PIN photo-detector is low compared to APD photo-detector.

SYSTEM DESIGN
The FSO system is sub divided into three parts : Transmitter, FSO channel, Receiver. The Transmitter section has four subsystem: Pseudo Random Bit Stream to generate the sequence of bit. NRZ Modulator to encode the signal. CW array laser for transmission of light signal and forth is Mach-Zehnder modulator to modulate the data. [5] The basic transmitter section is shown in " Fig.2." System I. In system I 16x5Gbps system is design using APD & PIN photo-detector. Firstly 16x5Gbps system using APD photo-detector is investigated for 4 different atmospheric conditions. Then 16x5Gbps system is design using PIN photo-detector. [7] System II. In this system 16x10Gbps system is design using APD & PIN photo-detector.16x10Gbps system is design firstly using APD photo-detector. Investigate it for very clear, clear, light haze, heavy haze atmospheric attenuation. Secondly design 16x10Gbps system using PIN photodetector and comparing it with 16x10Gbps system using APD photo-detector. [8] The simulation parameter are shown in Table .I.

RESULT AND DISCISSION
The 16-channel WDM based Free Space Optical communication link has been analyzed at 5Gbps/10Gbps in term of Bit Error Rate and Q-factor. The Eye diagram for 16X5 WDM-FSO system using APD & PIN photo-detector are shown in " Fig.4" and "Fig.5" respectively. [9] The parameters Q-factor and BER are mention in Table. II.   Similar as 16X5Gbps a 16X10Gbps system is design using APD & PIN photo-detector. 16X10Gbps system using APD photo-detector give better result as compare to 16X10 using PIN photo-detector. The Eye diagram foe 16X10Gbps WDM-FSO system using APD& PIN photo-detector are shown in " Fig.8" and "Fig.9". The parameters Bit Error Rate, Link Range and Quality factor are mention in Table   . (a)   Fig.11. BER v/s Link Range for 16x10Gbps WDM-FSO system (a)using APD photo-detector (b) using PIN photo-detector

CONCLUSION
In this paper, 16-channel WDM based Free Space Optical transmission system is design and investigate at 5Gbps/10Gbps with APD & PIN photo-detector. The BER and Q-factor values for four atmospheric conditions are evaluated. The maximum link range for 16x5Gbps system using APD photo-detector is 453 km achieved for PIN photo-detector it is up to 388 km. In 16x10Gbps system using APD photo-detector maximum link range 370 km and using PIN photo-detector it is up to 590 km. So it is conclude that 16x5Gbps system using APD photo-detector having best performance at higher attenuation. For lower attenuation 16x10Gbps system using PIN photo-detector is best.