# Free Space Optic Receiver with Strongly Overlapped Photodetectors' Field of View
## 4 FSO Receiver Measurements
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Active Photodetector Count** & **1** & **2** & **3** & **6** \\ \hline Measured mean \(BER\) value [/] & 4.25 \(\times\)\(10^{-5}\) & 2.49 \(\times\)\(10^{-5}\) & 1.66 \(\times\)\(10^{-9}\) & / \\ (short optical route) & & & & \\ Corresponding \(SNR\) value [dB] & 17.91 & 18.19 & 21.45 & / \\ (short optical route) & / & / & 4.79 \(\times\)\(10^{-5}\) & 2.72 \(\times\)\(10^{-8}\) \\ Measured mean \(BER\) value [/] & / & / & 17.85 & 20.72 \\ (long optical route) & & & & \\ Calculated \(SNR\) value [dB] & 14.31 & 14.59 & 17.85 & 20.72 \\ related to the long optical route & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Mean \(BER\) and \(SNR\) values differentiated by the number of actively operating photodetectors. a simpler \(x^{7}+x+1\) (n = 7) pseudorandom sequence or with the only one word (16 bits) repeated over and over again. However, the measurement was still unstable and synchronization was frequently lost. Even if the synchronization succeeded, the measured values would not be accurate due to the short data string and the presence of harmonic components in the spectrum of such testing signal. For the above reasons, two optical routes had to be used for measurement. The short optical route from the transmitter to the receiver was created via the only main reflective surface: the room's ceiling (Figure 13a). The transmitter was deflected to provide reflection just above the receiver. The plaster's natural scattering ability was beneficial. The distance between the transmitter and the receiver on the floor was 1.5 m. Room height was 3 m and the floorplan dimensions were 6 m \(\times\) 3.5 m. The transmitter and receiver were positioned so that no line of sight existed between them. The long optical route was established via numerous reflections from the ceiling and walls. This time, the transmitter had no deflections, as shown in Figure 13b. The distance between the transmitter and the receiver on the floor was doubled to 3 m. The room height was again 3 m, and similarly, the floorplan dimensions were 6 \(\times\) 3.5 m. No line of sight was permitted between the transmitter and the receiver. Under these conditions, signal coverage was almost uniform. The last row in Table 3 is related to the long optical route \(SNR\) values and shows the calculated \(SNR\) values for the two actively operating photodetectors via the short optical route,  
Figure 13: Plan of (**a**) the short and (**b**) the long optical route prepared for receiver testing. i.e., \(17.85-(21.45-18.19)=14.59\) dB, and similarly for only one actively operating photodetector, i.e., \(17.85-(21.45-17.91)=14.31\) dB. If a link could have been established for only one actively operating photodetector via the long optical route, the \(BER\) value would have been very high, achieving \(4.63\times 10^{-3}\). The \(SNR\) difference between only one and all six actively operating photodetectors was 6.41 dB, which is close to the theoretical value of 4.8 dB calculated above. As a reminder, the theoretical value of 4.8 dB was calculated as the \(SNR\) difference between only one and all six operating photodetectors with uniform signal coverage. In other words, this calculated difference covered only photodetector inherent noise suppression. The better result of 6.41 dB appears to be an inaccuracy arising from the unequal short and long optical routes. However, useful signal and noise processing differs, and no tool exists to precisely handle noise. Adding signals from the photodetectors means decreasing instead of increasing peak-to-peak total noise values. The measured result of 6.41 dB encourages more investigation with perhaps the same or more photodetector receivers. The active operation of only one or two photodetectors does not differ much in the \(BER\) or \(SNR\) parameters. Compared to more photodetectors, the situation with two actively operating photodetectors was exceptional due to the presence of error bursts.