# A 40 Mb/s VLC System Reusing an Existing Large LED Panel in an Indoor Office Environment
## 2 Experimental Setup
### MultiCAP for the Downlink Visible Light Communication  
It is well known that LEDs, especially those that are used for illumination, suffer from low 3-dB bandwidth (up to a few MHz) and decaying frequency responses [38], as shown in Figure 5b. Unlike radio frequency (RF), wireless technologies where multi-path induced fading is highly critical, multi-path fading is not a major concern and it poses limited impact on the overall system frequency response in indoor VLC systems. One main reason is that most practical indoor VLC systems are static line-of-sight (LOS) channels. The other reason is that the coherence bandwidth of a tpicyal indoor non-LOS (NLOS) channel is much higher than the LED's 3-dB bandwidth, thus making the LED capable of approximately representing the frequency response of the entire VLC system. Recent experimental results have shown that a maximum root mean-square (RMS) delay spread of 14.2 ns was measured for a 3 m NLOS link in a worst practical indoor scenario [39; 40]. The coherence bandwidth can be estimated to be in the order of several tens of MHz, which is still much higher than the LED panel's bandwidth of 1.4 MHz that was used in this work. A multi-carrier version of carrierless amplitude and phase (MultiCAP) modulation with bit-loading is adopted to overcome the unideal channel response [41]. When compared with OFDM, MultiCAP has a lower peak-to-average power ratio (PAPR), because typical MultiCAP implementations tend to use a smaller number of carriers. In addition, the modulation and demodulation of MultiCAP can be implemented with reduced complexity, due to the absence of fast Fourier transform (FFT) and inverse FFT operations. The parameters and algorithms for the modulation and demodulation are described, as follows. Suppose that a full band of \(B_{\text{mod}}\) is divided evenly into \(N\) sub-bands. Each sub-band is transmitting a band-pass CAP signal, which is similar to quadrature amplitude modulation (QAM). For example, the \(n\)th sub-band is centred at the carrier frequency \(f_{n}=\frac{2n-1}{2N}B_{\text{mod}}\), where \(n=1,\cdots,N\). Because the band is divided evenly, the baseband symbol rate \(R_{s}\) in each sub-band is the same and is equal to \(\frac{B_{\text{mod}}}{N(1+\beta)}\) when using the square-root raised cosine (SRRC) filter with a roll-off factor of \(\beta\) for pulse shaping. The MultiCAP filter bank consists of \(N\) pairs of pulse shaping filters, as given by:  
\[g_{\text{1n}}(t) =g(t)\cos 2\pi f_{n}t \tag{1}\] \[g_{\text{Qn}}(t) =g(t)\sin 2\pi f_{n}t \tag{2}\]  
where  
\[g(t)=\frac{\sin[\pi(1-\beta)t/T_{s}]+4\beta t/T_{s}\cos[\pi(1+\beta)t/T_{s}]} {\pi t/T_{s}[1-(4\beta t/T_{s})^{2}]} \tag{3}\]  
is the SRRC pulse and \(T_{s}=1/R_{s}\) is the baseband symbol period. The synthesis of pulse shaping filters in (1) with built-in carriers is the major difference between CAP and QAM. Because of this feature, CAP does not require extra free-running oscillators and mixers to shift the spectrum to the target frequency range, which is beneficial when designing low-cost systems. Block 6 of Figure 1a illustrates the schematic diagram for the implementation of modulation and demodulation in MultiCAP. At the transmitter, for the \(n\)th sub-band, a random symbol sequence \(D_{n}\) is split into two branches, i.e., an in-phase (I) sequence \(D_{\text{1n}}\) and a quadrature (Q) sequence \(D_{\text{Qn}}\), after the QAM-\(2^{k_{n}}\) constellation mapping. Here, \(k_{n}\) denotes the QAM constellation order (in bits) that is used in the \(n\)th sub-band. Subsequently, the baseband symbol sequences \(D_{\text{1n}}\) and \(D_{\text{Qn}}\) are up-sampled and filtered by the \(n\)th in-phase and quadrature filters of \(g_{\text{1n}}(t)\) and \(g_{\text{Qn}}(t)\), respectively. Finally, the \(n\)th CAP signal is generated by subtracting the I path signal from the Q path signal, which is given by:  
\[s_{n}(t)=D_{\text{1n}}*g_{\text{1n}}(t)-D_{\text{Qn}}*g_{\text{Qn}}(t) \tag{4}\]where \(*\) is the time-domain convolution.