## 2 System Overview  
A detailed description of the test setup along with the sensors used is presented in this section. The FSO transceivers were installed on the rooftops of Engineering and Business buildings of Qatar University, which are 600 m apart. The FSO communication link operates at 1 mW fixed optical power (set by the hardware) and at a wavelength of 1550 nm, Figure 1. The FSO units which can work only at wavelength of 1550 nm help to eliminate the effect of absorption phenomenon [1; 3]. The FSO communication link used in the test setup consists of 2 FSO units, 1 transmitter and 1 receiver. The FSO units were periodically calibrated to overcome any loss of transmission due to pointing errors. The unit makes the alignment and collimates the beam automatically. A User Datagram Protocol (UDP) application is running on Field Programmable Gate Arrays (FPGAs) connected to the FSO transceivers. The FPGA development kit NETFPGA-1G-CML (Diglient, Pullman, WA, USA) was used as it is a versatile, low cost network hardware development platform featuring a Xilinx Kintex-7 XC7K325T FPGA (Xilinx, San Jose, CA, USA) and includes four Ethernet interfaces capable of negotiating up to 1 Gbps [14]. The NetFPGA-1G-CML board has 512 MB of 800 MHz Double Data Rate Type 3 (DDR3) that can support high throughput packet buffering. It uses low jitter 200 MHz oscillator that is required to speed up encoding/decoding algorithm.  
One of the FPGA's will act as the server and keep transmitting packets which will be received by the other FPGA which is acting as client. One Gbps capacity was used and was confirmed using the Iperf tool. The transmission of packets occur periodically and the collected packets are compared with the known transmitted packets for computation of PDR. The use of FPGA makes it easier to control the communication and does not occupy larger space. The FPGAs have Ubuntu 9.0 operating system installed in them and the UDP applications are running on the operating system.  
The paper aims at setting an experimental setup for analyzing the FSO performance in Qatar. The metric used for analyzing the performance of FSO is PDR. PDR is defined as the ratio of data packets correctly received by the destination to those generated by the sources. Thus, closer the value of PDR is to 100%, the better is the FSO performance. In general, for wireless optical transmissions, PDR is largely depended on the environment. Mathematically, the PDR is defined as  
\[\text{PDR}=\frac{\text{Number of received packets}}{\text{Number of transmitted packets}}\times 100\% \tag{1}\]  
On the other hand, in order to measure the environmental factors, many sensors were installed at QU near the FSO transceiver as shown in Figure 2, which is also a part of the authors previous work [15]. These sensors measure the temperature, the solar irradiance and the relative humidity.  
Figure 1: Details of the free-space optical (FSO) system installed at Qatar University.  
As shown in Figure 2 the data (Ambient Temperature, Humidity, and Irradiance) are collected from the weather station and correlated with the data collected from the FSO system, i.e., PDR. The sensors used for collecting data from the weather station are stated in detail in Table 1. To investigate the effects of environmental factors on FSO performance, a detailed analysis is done on the collected data in next section. The measured data for each season is also provided.