**5.5. Result and analysis**

First, in order to evaluate the all-optical connection FSO system characteristic, the result of the 10 Gbps single wavelength light signal transmission experiment [Arimoto 2007] which is not achievable with conventional FSO system is shown in Figure 19. The bit error rate (BER) is measured by accumulated at 1 minute interval is shown in the figure. The data encoded

attenuation) are measured simultaneously.

**Figure 17.** Photograph of experimental hardware setup

**Figure 18.** Experimental FSO communication system setup

First, in order to evaluate the all-optical connection FSO system characteristic, the result of the 10 Gbps single wavelength light signal transmission experiment [Arimoto 2007] which is not achievable with conventional FSO system is shown in Figure 19. The bit error rate (BER) is measured by accumulated at 1 minute interval is shown in the figure. The data encoded

**5.5. Result and analysis** 

a clock data recovery circuit, and calculation of the bit error is carried out by BERT. The light signal which branched by 3 dB-coupler on the way is used for the monitor of receiving intensity, or other measurement. In a WDM experiment, the wavelength of each signal is made to fit the ITU grid (100 GHz channel spacing), and multi/de-multiplex is performed using a DWDM multi/de-multiplex device. Moreover, a weather monitor device and another optical antenna are installed in the rooftop and weather condition (visibility, precipitation and temperature) and link line condition (scintillation and optical power

optical signal, is amplified by 100 mW booster EDFA shown in Figure 18. The minimum receiving sensitivity of the optical receiver used in this experiment is -37 dBm. Stable transmission with few errors has been achieved over 12 hours. Figure 20 shows the eye patterns measured continuously for 5 minutes at this time. Although the fluctuation of the amplitude direction becomes large when compared with the eye patterns before transmission, it seems that there are very few increases in a time jitter (horizontal axis), and high-speed transmission is possible. In addition, the error sometimes shown in Figure 19 could be burst error produced by the momentary fall of received light intensity of several milliseconds or less, and this influence can hardly be observed on TCP/IP which is commonly used. An experiment using two optical wavelengths and connection to a Giga-bit Ethernet circuit and a 2.5 Gbps test signal was conducted. Optical and electrical signal conversion were performed by the media converter, and the Ethernet circuit performed huge size file transfer of between PCs connected through a switch, and has calculated the throughput from transfer time. In addition, it is connection by 100 Mbps between the switch and PC. Figure 21 shows the measurement result of the throughput of the file transfer performed for every minute and BER accumulated per minute. The minimum receiving sensitivity is -34 dBm which is almost same as the optical receiving module currently used for the media converter and the optical receiver currently used for BER measurement. Although errors sometime appeared in BER measurement, there is almost no change of a throughput almost similar result as in wired network. Since the great portion of error is short-time burst error, the throughput is hardly affected. Moreover, to the experimental result of BER and Ethernet, change considered to be mutual interference among other light signals cannot be found out at all, but has realized stable WDM transmission to it.

**Figure 19.** BER measurement result for 10 Gbps transmission over 1 km distance.

From these results, we demonstrated the next-generation FSO system using all-optical connection technology realized 10 Gbps transmission and also possible to apply WDM transmission which were not able to be realized in the conventional FSO system, and showed that the capability near an optical fiber could be offered. However in presence of strong atmospheric turbulence increased burst errors are observed in the system because of the fall in the received signal power. In order to realize stable communication even under strong atmospheric turbulence more improvement in is needed is required in the tracking system performance. Moreover, it is important to clarify the link design technique about the application distance of atmospheric turbulence and a FSO system.

**Figure 20.** 10 Gbps transmission test eye pattern. (a) Before transmission (b) After 1 km transmission

#### **5.6. Conclusions of the NG-FSO system**

The next-generation optical wireless communication system offered seamless connection of free space and fiber system. The transceiver incorporates a FPM for high-speed beam tracking and control function, therefore, having the capability to mitigate the effects of atmospheric turbulence on the transmitted optical beam. The FSO system performance was verified and error free transmission over an extended period of time was demonstrated. The system performance expressed in terms of BER performance was also evaluated and showed to be consistently above acceptable levels. Stable performance after increasing the system bandwidth using WDM technology was also attained.

**Figure 21.** Throughput and BER characteristics in DWDM transmission
