# Free Space Optic Receiver with Strongly Overlapped Photodetectors' Field of View
## 4 FSO Receiver Measurements  
The FSO multiple photodetector receiver noise parameters were measured indirectly by evaluating the bit error rate (\(BER\)) parameters. The measurement block diagram is depicted in Figure 10. A BA400 (SyntheSys Research, Menlo Park, CA, USA, 1992) tester was used to create the \(BER\) evaluation core. The level translators matched the \(BER\) tester ECL and codec PE TTL interfaces. As stated above, the PE codec employs the DP83910A device. Because of different asymmetrical and symmetrical port architectures, a cabling translator was added. The optical transmitter equipped with power LED VSMY99445 is shown in Figure 11a. The assembled FSO multiple photodetector receiver described here is shown in Figure 11b. Other equipment used for \(BER\) measurement is shown in Figure 12.  
All measurements were performed at night with a stable diffusion LED illumination of 400 lx. The \(BER\) measurements operated over a 2 Mbit\(\cdot\)s\({}^{-1}\) data stream and x\({}^{23}\) + x\({}^{5}\) + 1 (n = 23) pseudorandom sequence. The calculations between the \(BER\) and the \(SNR\) parameters were provided by the \"erfc\" Excel function according to the following definition [33; 34]:  
\[BER=\frac{1}{2}\text{erfc}\left(\frac{\sqrt{SNR}}{2\sqrt{2}}\right)\ [/]. \tag{34}\]  
The \(BER\) measurement of the FSO multiple photodetector receiver was differentiated by the number of the actively operating photodetectors. As the occurrence of errors and the measured \(BER\) values are random, a higher number of measurements was required. The volume of the transferred data was always at least 10 times higher than the expected \(BER\) reciprocal value. Table 3 shows the measured data. Figure 11: (a) Optical transmitter equipped with power LED VSMY99445 (Vishay, Malvern, PA, USA, 2015); (b) assembled FSO receiver. Figure 10: Bit error rate (\(BER\)) evaluation block diagram. It follows from the \(BER\) measurements that the change in noise parameters from only one to all six actively operating photodetectors was too large. The first short optical route allowed \(BER\) measurements of a lower number of actively operating photodetectors, whereas, for a higher number of actively operating photodetectors, the \(BER\) measurements were zero. However, the second-long optical route permitted \(BER\) measurements with a higher number of actively operating photodetectors, whereas for a lower number of actively operating photodetectors, it was difficult to establish a link. Notably, our goal was to focus on only one optical route. The experiences were as follows:  
When using short optical route, the \(BER\) measurements were possible for one, two and three actively operating photodetectors. However, the \(BER\) measurement for all six actively operating photodetectors was zero for time in units of hours. As can be calculated from the last row of Table 3, the \(BER\) difference between only one and all six actively operating photodetectors is over five orders. Since the estimated \(BER\) value for all six actively operating photodetectors and short route would be between \(10^{-10}\) and \(10^{-11}\), the time needed for measurements would be huge, reaching around 140 h (almost one week). The total measurement time would have to be extended at least to one month because several measurements are required. The long optical route was successfully tested for only three or six actively operating photodetectors. For lower numbers of actively operating photodetectors, there was problem with resynchronization at the receiver part of the BER tester Synthesys BA400. The results improved using  
Figure 12: Synthesys BA400 BERT (SyntheSys Research, Menlo Park, CA, USA, 1992), PE codec, level and cabling translators, and Krohn-Hite 3202 band-pass filter (Krohn-Hite, Brockton, MA, USA, 1972) during PE spectrum evaluation.