# Testbed for Experimental Characterization of Indoor Visible Light Communication Channels
## 3 Experimental Testbed
### Communication System  
A typical orthogonal frequency-division multiplexing (OFDM) scheme for optical wireless communications was reported in [29], where the input data streams are encoded into quadrature amplitude modulated (QAM) orthogonal subcarriers (a total of \(N_{S}-1\)) for parallel transmission. Note that the number of bits conveyed at a specific subcarrier depends on its SNR. The frequency separation between subcarriers is \(\Delta f=B/N_{S}\), where \(B\) is the modulation bandwidth. The inclusion of a direct current (DC) bias ensures that the OFDM signal is all positive for intensity modulation of the LED. Moreover, to ensure a real-valued time domain signal Hermitian symmetry constraint must be met. This sequence is processed by an inverse fast Fourier transform (IFFT) block to obtain the \(2N_{S}\)-sample  
Figure 3: Electronic board designs: (**a**) Receiver board. (**b**) Emitter board. Figure 2: Mockup design: (**a**) Mockup scheme. (**b**) View of the mockup design. (**c**) Zoom of receiver. (**d**) Zoom of emitters at ceiling. real-valued OFDM signal. Finally, prior to DC-biasing and low-pass filtering a \(N_{E}\)-sample cyclic extension is inserted to the signal at the output of the IFFT block to combat multipath induced intersymbol interference (ISI). At the receiver (Rx), the regenerated OFDM signal is down-sampled to obtain the \((2N_{S}+N_{E})\)-sample sequence which, after removing the cyclic extension, is processed by a FFT block. Following this, just the \(N_{S}-1\) information-bearing symbols are considered throughout the last stages of channel equalization and, finally, QAM demodulation is performed to retrieve the transmitted data bit stream. Due to the slowly time-varying VLC channel characteristics, the channel equalization can be implemented by transmitting training sequences to estimate the channel frequency response prior to the data transmission periods. By transmitting this training symbol an appropriate number \(N_{\text{TS}}\) of times it is possible to estimate the channel response at each specific \(n\)th sub-band as follows [29]:  
\[\hat{H}_{n}=\frac{1}{N_{\text{TS}}X_{n}}\sum_{i=1}^{N_{\text{TS}}}Y_{n,i}, \tag{6}\]  
where \(Y_{n,i}\) is the received symbol at \(n\)th subcarrier during \(i\)th OFDM symbol of the training sequence of \(N_{\text{TS}}\) symbols, and \(X_{n}\) is the training symbol transmitted at that subcarrier, which is the same for all the OFDM symbols at a specific subcarrier. By adopting Saphiro-Rudin sequences [30] for training periods, which enables that certain gain can be applied to the sequence before transmitting due to their low peak-to-average ratio, \(N_{\text{TS}}\) of 20 would be enough to obtain an accurate estimate of \(H_{n}\)[29]. Therefore, \(N_{\text{TS}}\) is set to this value during experimental system evaluation. Adaptive rate algorithms can be applied to enhance the system throughput by tailoring the bits conveyed by each subcarrier to its current available SNR. In this work, a simple and efficient bit-allocation algorithm described in [31] has been adopted, which only performs bit-loading while maintaining a constant transmit power for each subcarrier. Nevertheless, adopting more complex bit- and power-loading algorithms have not shown to offer any noticeable improvement in the throughput in real scenarios [32]. Moreover, the proposed algorithm in this work is concerned with fitting the average probability error to a target, thus enabling additional bit-allocation compared with simple schemes based on just keeping all the subcarriers working below that target bit error rate (BER). Following channel estimation, the received symbols \(r_{n}\) are demodulated to recover the transmitted symbols. The SNR at the \(n\)th sub-band can be estimated by averaging over a series of received symbols at that subcarrier during subsequent OFDM symbols, which is given as:  
\[\gamma_{n}=\Sigma_{i}|\delta_{n,i}|^{2}/\Sigma_{i}|\delta_{n,i}-r_{n,i}|^{2}, \tag{7}\]  
where \(\delta_{n,i}\) are the estimated transmitted symbols.