# Testbed for Experimental Characterization of Indoor Visible Light Communication Channels
## 5 Experimental Results
\begin{table}
\begin{tabular}{l r} \hline \hline
**Parameter** & **Value** \\ \hline Total number of subcarriers (\(N_{S}\)): & 64 \\ Number of information subcarriers (\(N_{SI}\)): & 63 \\ Maximum number of bits per subcarrier (\(b_{\max}\)): & 8 (256-QAM) \\ Target bit error rate (\(P_{e}\)): & \(3.8\times 10^{-3}\) \\ Modulation bandwidth (\(B_{T}\)): & 10 MHz \\ Cyclic prefix extension (\(N_{E}\)): & 4 \\ OFDM symbol period (\(T_{S}\)): & 6.6 \(\upmu\)s \\ Number of training sequences (\(N_{\text{TS}}\)): & 20 \\ Number of random-data symbols (\(N_{\text{symbols}}\)): & 2500 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters for Adaptive OFDM System. Figure 13: Experimental results obtained at the center of the mockup receiver plane when the four lamps transmit the same information: (**a**) Timing metric during a transmission. (**b**) Average of measured SNR. (**c**) Symbol constellations at receiver. Figure 13a illustrates the timing metric during the evaluation of a received OFDM signal. The inset shows a transition between a random-data transmission and the start of a new training period. The narrow impulses shown in blue indicates the start of a new frame composed of \(N_{\text{TS}}=20\) training sequences and \(N_{\text{symbols}}\gg N_{\text{TS}}\) OFDM sequences conveying the random data, which will be used for synchronization at the receiver end. The Shapiro-Rudin training sequences can be easily identified in the figure due to its characteristic shape. All the other impulses following the first impulse will be ignored by the synchronizing module. However, during the transmission of random-data OFDM symbols there are no impulses with sufficient height that will cross the dedicated threshold level. This timing method allows the receiver to synchronize with the incoming signal so as to proceed to perform channel estimation and data demodulation. There is no need for another different channel to carry out synchronization between emitted signals and receiver, thus a real optical communication is emulated. Additionally, Figure 13b presents the average of the measured SNR as compared with a theoretical fourth-order low-pass response. Finally, Figure 13c depicts the symbol constellations obtained at receiver end for this transmission. Figure 14 shows the experimental throughput and SNR measured when just one lamp is used to transmit data. In all the experiments, the receiver was located defining a \(7\times 7\) matrix from an initial point \(x=7.5\) cm, \(y=7.5\) cm, with shift movements between neighboring positions of \(7.5\) cm in x- or y-direction. In addition, when not specified, the receiver was oriented vertically pointing upward during the test. Moreover, this one-lamp experiment was performed without walls, thus direct LOS links were evaluated. We can observe how the system throughput and SNR are degraded rapidly when the receiver moves beyond \(30\) cm (the half of the mockup width) from the lamp location (this can be seen more easily in the throughput map of Figure 17a). Therefore, the coverage radius, where the data rate is above the half of its maximum, is approximately the half of the communication room width. We can also see how there is a zone in the opposite area of the room to the lamp position where the communication was no possible (insufficient SNR was obtained and null data rate was observed). This prompts the idea that independent VLC links without almost interference between them could be established at opposite areas of the room but no more than two of these links could operate simultaneously because interference when receiver only moves in either x- or y-direction from the lamp is evident as the communication, although seriously degraded, is still possible. Therefore, a coordinated cooperative scheme between lamp transmissions would be required [12].