# A Full-Duplex LED-to-LED Visible Light Communication System
## 4 Full-Duplex LED-to-LED VLC  
In previous studies, several modulation schemes have been studied for optical wireless communication. On-off keying (OOK) is the simplest and most conventional modulation method in optical communication, which has the advantages of low complexity and adequate bandwidth achievement. Pulse position modulation (PPM) is suggested in the IEEE 802.15.7 VLC standard for dimming support. PPM is another modulation scheme that can achieve higher power efficiency than OOK. However, PPM requires more complicated systems and bandwidth consumption [16]. Orthogonal frequency division multiplexing (OFDM) is a multi-carrier modulation scheme that shows high frequency efficiency. OFDM also makes systems robust to the low-frequency ambient light from other lighting sources by sending data via high-frequency subcarriers. In order to overcome the disadvantage of the low-data-rate and vulnerability to other lighting sources in LED-to-LED VLC, quadrature amplitude modulation (QAM) and DCO-OFDM are applied in our full-duplex LED-to-LED VLC experiment. Figure 3 shows the experimental setup for the full-duplex LED-to-LED VLC link. A pseudo random binary sequence (PRBS) VLC signal is encoded with a 32-QAM and DCO-OFDM scheme. Hermitian symmetry input is used before the inverse fast Fourier transform (IFFT) process to generate a real and unipolar OFDM signal for an intensity modulation and direct detecting (IM/DD) system [17]. Then, the OFDM signal is directly modulated with an arbitrary function generator (AFG) and sent to a green transmitter LED. The VLC signal emitted by the green transmitter LED is detected by the yellow receiver LED. The received signal is recovered with a fast Fourier transform (FFT), inverse Hermitian symmetry, and 32-QAM de-mapping process. The transmitter and receiver circuits are shown in Figure 2. The DCO-OFDM modulated signal has an amplitude of 0.3 V and an offset of 2.9 V. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Transmitter**} & \multicolumn{4}{c}{**Receiver**} \\ \cline{2-5}  & **Red** & **Yellow** & **Green** & **Blue** \\  & **(628 nm)** & **(587 nm)** & **(520 nm)** & **(465 nm)** \\ \hline Red (628 nm) & 52 μs/2 dB & - & - & - \\ \hline Yellow (587 nm) & 80 μs/1 dB & 52 μs/6 dB & - & - \\ \hline Green (520 nm) & 72 μs/21 dB & 44 μs/22 dB & 96 μs/5 dB & - \\ \hline Blue (465 nm) & 63 μs/10 dB & 64 μs/11 dB & 112 μs/37 dB & 112 μs/12 dB \\ \hline Pc-white (454 nm) & 72 μs/11 dB & 64 μs/13 dB & 112 μs/25 dB & 120 μs/10 dB \\ \hline \hline \end{tabular} The three best performing cases in each experimental result are underlined.  
\end{table} 
Table 2: Rise time and SNR of the LED-to-LED VLC according to the pairs transmitter and receiver LED color sets. Pc, phosphor-converted. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Transmitter**} & \multicolumn{4}{c}{**Receiver**} \\ \cline{2-5}  & **Red** & **Yellow** & **Green** & **Blue** \\  & **(628 nm)** & **(587 nm)** & **(520 nm)** & **(465 nm)** \\ \hline Red (628 nm) & 9.2 kbps & - & - & - \\ \hline Yellow (587 nm) & 5.5 kbps & 16.2 kbps & - & - \\ \hline Green (520 nm) & 34 kbps & 59.1 kbps & 7.9 kbps & - \\ \hline Blue (465 nm) & 19.4 kbps & 20.2 kbps & 38.5 kbps & 12.9 kbps \\ \hline Pc-white (454 nm) & 18.4 kbps & 24.9 kbps & 25.6 kbps & 10.4 kbps \\ \hline \hline \end{tabular} The three best performing cases are underlined.  
\end{table} 
Table 3: Estimated channel capacity of the LED-to-LED VLC according to the pairs of transmitter and receiver LED color sets. The single-sided bandwidth of a subcarrier \(B\) in our full-duplex LED-to-LED VLC system can be calculated as:  
\[B=\frac{1}{2T_{s}} \tag{8}\]  
where \(T_{s}\) is the symbol time duration. The data rate of this system \(D\) is:  
\[D=B\big{(}N_{fft}-2\big{)}\log_{2}M \tag{9}\]  
where \(N_{fft}\) is the size of FFT and \(M\) is the size of constellation. In our experiment, an FFT size of 100 and a constellation size of 32 are used.