## 3 Results
### Proof-of-Concept Demonstration
#### 3.2.2 Uplink Communication Performance  
For uplink communication, firstly, an optical camera monitors multiple smart-farming units with a full region-of-interest (full-RoI) using full pixels. A faster communication link can be constructed by tracking a specific unit by limiting the FoV but increasing the frame rate (tracked -RoI) using partial pixels. The frame rate from the camera is 34.8 fps and 849.6 fps for the full-RoI (1440 \(\times\) 1080 pixels) case and tracked-RoI (80 \(\times\) 4 pixels) case, respectively.  
Figure 8: (**a**) BER using 2-PAM (without equalizer) at distance of 0 cm, 25 cm, 50 cm and 85 cm; (**b**) BER using 2-PAM (with equalizer) at distance of 0 cm, 25 cm, 50 cm and 85 cm.  
Figure 9: (**a**) Eye-diagram of the received 2-PAM (OOK) signal of 20 Mb/s for the downlink communication at the distance of 25 cm; (**b**) that with DFE.  
Figure 10a,b shows the measured signal-to-noise ratio (SNR) of the full-RoI case and the tracked-RoI case at four different distances. The result of the full-RoI case shows that at the distance of 50 cm, the highest SNR of 23.5 dB is achieved. The SNR gradually decreases as the distance deviates from 50 cm. The SNR variation is in accordance with the light intensity captured by the camera, as shown in Figure 7. Figure 10b shows the SNR variation for the tracked RoI case. Overall, there is a smaller variation in the SNR distribution across the distances in this case. The distance of 25 cm shows the highest SNR of 16.3 dB. This is because, in the tracked-RoI case, the camera tracks and samples the brightest point with a faster frame rate.  
Unlike the downlink test, it is difficult to change symbol rates under the limited frame rates of the optical camera. Hence, in this investigation, the depth of modulation level is changed. Figure 11a shows the BER performance at different distances in the case of full-RoI. 2-PAM, 4-PAM, and 8-PAM schemes are tested. The symbol rate of 17 Hz is applied. Actual data rates are 17 b/s, 34 b/s, and 51 b/s for 2-PAM, 4-PAM and 8-PAM, respectively. Although 8-PAM can provide the highest data rate, the BER performance is poor due to insufficient SNR. In 4-PAM, reliable communication is possible at 50 cm where the SNR is the highest. For 2-PAM, BERs are less than \(10^{-3}\) at all distances except for the case of 0 cm, as the SNR at this point is the worst due to the blockage.  
Figure 11b shows the BER for the tracked-RoI cases. The symbol rate of 420 Hz is applied. Actual data rates are 420 b/s, 840 b/s, and 1260 b/s for 2-PAM, 4-PAM and 8-PAM, respectively. 2-PAM results show less than \(10^{-3}\) at all distances (even at 0 cm). It means as long as there is a small portion of the reflected light to track and sample from the smart-farming unit, this method can reliably construct the uplink communication link. Next, to push the data rate further, the effectiveness of DFE on this optical camera-based receiver is tested. When the DFE is applied to 4-PAM results at 25 cm and 50 cm with relatively high SNRs in the case of tracked-RoI, a successful recovery with BER less than \(10^{-3}\) is achieved. Figure 12a,b show the eye diagrams of the received 840 s b/s 4-PAM  
Figure 10: (**a**) SNR for full ROI using 2-PAM at the distance of 0 cm, 25 cm, 50 cm and 85 cm; (**b**) SNR for tracked ROI using 2-PAM at the distance of 0 cm, 25 cm, 50 cm and 85 cm.  
signal at 25 cm, before and after applying the DFE, showing a wider eye-opening in the equalized case.