## 3 Proposed Multi-User Full-Duplex Visible Light Communication System
### Downlink Section
#### 3.1.1. Asymmetrically-Clipped Optical Orthogonal Frequency-Division Multiplexing  
VLC uses intensity modulation with direct detection (IM/DD) and the signal generated by the OFDM modulator is complex and bipolar. However, the main requirement of a VLC system is that it is given only real and positive values from the optical modulator as input. As a result, it is necessary to make the OFDM signal unipolar and real. The ACO-OFDM modulator inherently performs this task.  
To ensure that the output of ACO-OFDM is real and unipolar, the following procedure is followed [24]. There are N sub-carriers, out of which only odd ones are used to send data. At the transmitter, the source bits are first modulated using a Gray-coded M-ary quadrature amplitude modulation (QAM) mapping block, where M is the modulation order, which produces a complex bit stream. Subsequently, in order to maintain the real OFDM symbols in the time domain, the encoded data are passed through a Hermitian symmetry block. The data are then passed on to an inverse fast Fourier transform (IFFT) block and a clipper. A cyclic prefix (CP) is later added to avoid inter-symbol interference (ISI), which then drives the LED. The addition of a CP has a negligible impact on the SNR or the spectral efficiency of the VLC system. To reduce the peak-to-average-power ratio (PAPR), clipping is performed to combat two types of distortions, namely, the out-of-band distortion and the in-band distortion. The former is reduced by filtering and the latter is reduced by the addition of the CP. In ACO-OFDM, only odd sub-carriers are used for signal transmission [32]. The signal is subsequently converted from analog to digital using an analogue-to-digital converter (ADC) and later using optical intensity modulation (IM), data is sent over the optical wireless channel.  
At the receiver end, using direct detection (DD), the signal is received by a PD. This will convert the signal from optical to electrical domain. The estimated signal is then passed to the ACO-OFDM  
Figure 2: Block diagram of the downlink section. S/P: serial to parallel; IFFT: inverse fast Fourier transform; P/S: parallel to serial; CP: cyclic prefix; DAC: digital-to-analogue converter; LPF: low pass filter; ACO-OFDM: asymmetrically clipped optical orthogonal frequency-division multiplexing; and ADC: analogue-to-digital converter.  
demodulator block. After demodulation, the signal goes through the de-mapping block and the original data stream is recovered.  
The received signal at the receiver in the time domain is represented as follows:  
\[y(t)=\ h(t)x_{aco}(t)+\ n(t) \tag{1}\]  
where channel \(h\) can be either line-of-sight (LOS) or non-line-of-sight (NLOS). The main property of \(h\) is that it is always real and positive. The received signal in the frequency domain can be expressed as:  
\[Y\ =HX_{aco}\ +N \tag{2}\]  
where \(X_{aco}\) is the ACO-OFDM signal, \(H\) is the optical wireless communication (OWC) channel response and \(N\) is the noise, which for simplicity, is assumed to be additive white Gaussian noise (AWGN) with mean equal to zero and variance \(\sigma^{2}\). Perfect synchronization between transmitter and receiver is assumed.  
The expression of LOS optical channel and NLOS optical channel is given by Equations (3) and (4) respectively. The total diffused channel response can be computed by Equation (6).  
\[H_{los}(0)=\left\{\begin{aligned} &\frac{A_{r}(m_{1}+1)}{2\pi d^{2}}\cos^{ \text{  
At the receiver side after receiving the signal through the blue optical-filtered PD, a trans-impedance amplifier is used to amplify the electrical signal. This electrical signal is then passed through a post-equalization block so that the signal level is amplified and the original data is received. The performance of the system is analyzed in Section 6.