## 4 Evaluation of the Logarithmic Transimpedance Circuit
### Experimental Evaluation in Long Range Outdoor Conditions  
The purpose of the next set of experiments is to demonstrate that the proposed logarithmic transimpedance solution is compatible to long range automotive applications and with outdoor conditions. The spectral analysis of the optical interferences during the tests is showed in Figure 5b. For these tests, the standard compliant LED traffic light is used as a VLC emitter (see Table 6). The VLC emitter is configured to broadcast data modulated using standard On-Off-Keying (OOK) modulation and Manchester coding [5]. Concerning the modulations frequencies, these tests are performed at 11 kHz. Nevertheless, modulation frequencies up to 100 kHz can be used as well. Yet, as these tests are meant to evaluate the resilience to noise, the evaluation for higher frequencies has not been addressed. In order to perform such tests, the signal provided by the logarithmic transimpedance light sensing solution is further processed using standard signal processing techniques (see Figure 1). Thus, the signal is filtered, amplified and finally converted to digital data. Then, the digital signal is introduced into a 180 MHz microcontroller and the BER is determined (in real time) by comparing the received bits with the original message. As the aim of this article is to present the benefits of the logarithmic transimpedance solution, full details regarding the signal processing are not within purposes of this article. Nevertheless, these signal processing techniques follow the general structure of a VLC sensor. These tests are performed in outdoor conditions for a communication distance of about 50 m in variate lighting conditions (see Figure 7). At this distance, the irradiance of the received VLC signal further decreases, whereas the influence of the background light becomes more stringent (i.e., lower SNR). In this case, we approximated the irradiance of the received data signal to be around 0.076 \(\upmu\)W/cm\({}^{2}\) (i.e., the approximation is made based on the optical irradiance measured at 1 m). For these tests, the VLC receiver's Field of View (FOV) is set at 30\({}^{\circ}\) (i.e., \(\pm\)15\({}^{\circ}\)). For such distances, the selected FOV is compatible to mobile applications as the solid angle between a traffic light and an on-vehicle VLC receiver is smaller than 3.5\({}^{\circ}\). The configuration used during the outdoor testing can be summarized as follows:  
* LED-based traffic light VLC emitter (see parameters in Table 6);
* PIN photodiode logarithmic transimpedance circuit based VLC emitter (see architecture in Figure 1 and parameters in Table 3);
* Receiver FOV: 7,5\({}^{\circ}\)/15\({}^{\circ}\)/30\({}^{\circ}\);
* Off the shelf IR-reject optical filter;
* Modulation type: OOK;
* Coding type: Manchester;
* Optical clock: 11 kHz;
* Asynchronous transmission;
* Receiver distance: 50 m;
* Conditions: outdoor uncontrolled conditions; tests performed in different moments of the day in order to evaluate the impact of the sun for various optical power levels and relative orientations with respect to the VLC emitter;
* Measured parameters: real time BER determination;  
The experimental setup for these tests is shown in Figure 7a, whereas the summary of the experimental evaluation is provided in Table 7. The results confirm that the usage of the logarithmic circuit enables the VLC sensor to provide reliable communications in outdoor conditions, achieving BER lower than 10\({}^{-6}\). For subsequent tests, the logarithmic transimpedance based VLC receiver is evaluated in outdoor sunny conditions. For these tests, the VLC receiver is orientated toward the traffic light VLC emitter and the communication parameters are evaluated while the sun is traveling the sky. As these experiments are performed during a different moment of the day, from morning to sun fall, the VLC receiver is exposed to different sunlight powers and different incidence angles.