# Testbed for Experimental Characterization of Indoor Visible Light Communication Channels
## 5 Experimental Results  
Figure 12 illustrates different views of the mockup manufactured to experimentally evaluate VLC systems. In the general view of the testbed shown in Figure 12a, we can see the instruments controlled by a laptop: two arbitrary waveform generators Tektronix AFG31152 which are used to send the OFDM signals towards the four lamps and an oscilloscope Agilent MSO-X3052A which acquires the signal detected at optical receiver. Several tests have been developed to assess the suitability of this design. In this preliminary testbed evaluation, just one of the four channels of the \(2\times 2\) array of photodiodes Hamamatsu S5107 (the physical area of each photodetecting element is 0.25 cm\({}^{2}\)) is acquired, configuring a multiple-input single-output (MISO) system. Use of the four channels, which are already operational, will be required in our future work which will be focused on studying MIMO schemes. The emitters are four standard red-green-blue (RGB) COTS LEDs not optimized for communication, thus the transmission bandwidth was established to 10 MHz. We consider that extensive commercial deployment of VLC systems will be carry out on standard LED luminaries, where transmission bandwidths will be limited to tens of MHz, therefore our design is in accordance with practical implementations similarly to other experimental studies [8; 17; 32]. Figure 11: Simulated throughput in bits per symbol (BPS) at the receiver plane of the mockup by using the adaptive OFDM algorithm described in Section 3.2 (with \(P_{e}=3.8\times 10^{-3}\)) for two situations: (**a**) Receiver pointing upward (\(\text{th}_{\text{max}}=6.1\) BPS, \(\text{th}_{\text{mean}}=5.8\) BPS, \(\text{th}_{\text{min}}=5.1\) BPS). (**b**) Receiver oriented as defined by Equation (14) (\(\text{th}_{\text{max}}=6.1\) BPS, \(\text{th}_{\text{mean}}=5.9\) BPS, \(\text{th}_{\text{min}}=5.3\) BPS). Table 3 presents the parameters set for the adaptive OFDM communication system during experiments as described in Section 3.2. Figure 13 shows some interesting results obtained when the receiver is located at the center of the mockup floor and the four lamps are transmitting the same information. Operating under the forward error correction (FEC) 7% BER limit of \(3.8\times 10^{-3}\), the achieved throughput was 4.8 BPS (45.4 Mbps for the transmission bandwidth of \(B_{T}=10\) MHz) with a measured BER of \(1.5\times 10^{-3}\). The achieved throughout is significantly inferior to the estimation depicted in Figure 11 for an ideal theoretical VLC channel, where many practical impairments are not modeled, for example, imperfect channel estimation, slight timing synchronization, sampling and acquisition errors, non-linear effects and clipping, and so forth at electronic devices. This reinforces the idea that experimental evaluation is crucial for a better characterization of VLC systems. Despite all the practical impairments, we can see how the measured BER is below the target, which demonstrates that the communication algorithm perfectly adapts to the actual channel situation during transmission. Moreover, with this testbed we are not interested in obtaining optimal data rates but evaluating throughput variations when the receiver situation inside the room changes. Figure 12: Views of the manufactured mockup: (**a**) General view of the experimental testbed. (**b**) Graphical user interface developed in Python. (**c**) Zoom of optical receiver. (**d**) Zoom of emitters at ceiling.