# Free Space Optic Receiver with Strongly Overlapped Photodetectors' Field of View
## 5 Conclusions  
An FSO multiple photodetector receiver, using equal signal combining, was described in this paper. Instead of using additional optical elements, the receiver included six photodetectors providing omnidirectional receiving capability. We used the near infrared optical spectrum. Due to high background light levels, the receiver employed ordinary PIN photodiodes. The photodiodes were fixed to truncated pyramid walls; the walls and base were at an angle of 45\({}^{\circ}\). The receiver was designed for interior use with multipath light signal propagation. The FOVs of the photodiodes were highly overlapped and covered one hemisphere. We discussed the separation of optical and electrical noise and demonstrated that highly overlapped FOVs eliminate electrical (PDAs) noise and are advantageous in environments with reflections or low optical noise. In other words, highly overlapped FOVs provide parallel information processing and additive noise suppression. To preserve this advantage, falling rays must be over a minimum angle of 32\({}^{\circ}\) with respect to the receiver's base. The procedure used to derive the minimum angle was described and generalized for different photodiode holders. We continued with PIN photodiode selection and follow-up amplifier design. Feedback amplifier topology appeared best for the application. For the final PDAs, PIN photodiodes BP104F and DiFET operational amplifiers OPA627 BP were selected. With respect to slower power LEDs, the optical receiver was designed for a 2 Mbit\(\cdot\)s\({}^{-1}\) data stream.  
The FSO multiple photodetector receiver noise parameters were indirectly measured by evaluating the bit error rate (\(BER\)) parameters. The calculations between the \(BER\) and \(SNR\) parameters were performed with the _erfc_ function in Excel. Measurements were differentiated by the number of actively operating photodetectors. All measurements confirmed our theoretical assumptions. The operation of all six photodetectors in a multipath light signal propagation environment provided an exceptional communication radius over 6 m at a low \(BER\) value of 2.72 \(\times\) 10\({}^{-8}\) with FOVs highly overlapped. By contrast, the operation of only three photodetectors failed, with a high \(BER\) value of 4.79 \(\times\) 10\({}^{-5}\) for the same configuration and FOVs only slightly overlapped.  
[PERSON] proposed the sensor system idea and designed the sensor. [PERSON] and [PERSON] edited the manuscript, also tested and validated the experiments and performed its final edits.  
This article was supported by the Ministry of Education of the Czech Republic (Projects No. SP2019/67, SP2019/80, and SP2019/85). This research was partially supported by the Ministry of Education, Youth and Sports of the Czech Republic through Grant Project no. CZ.1.07/2.3.00/20.0217 within the framework of the Operation Programme Education for Competitiveness financed by the European Structural Funds and from the state budget of the Czech Republic. This article was also supported by the Ministry of the Interior of the Czech Republic within the projects Nos. VI20152020008 and VI2 VS/444, as well as the Ministry of Industry and Trade of the Czech Republic within the project Nos. FV10396 and FV20581. It was also supported by the Grand Agency of the Czech Republic (project # 15-215475) and the European Regional Development Fund in the Research Centre of Advanced Mechatronic Systems project, project number CZ.02.1.01/0.0/0.0/16_019/0000867 within the Operational Programme Research, Development and Education.  
The authors declare no conflict of interest.  
Figure 14: VGA amplification (one of four stages) vs. control voltage \(V_{diff}\) dependency.