## 2 System Model  
The proposed multi-user full-duplex VLC system for smart homes is shown in Figure 1. For a particular smart home scenario, there are two types of traffic that mainly need to be taken into consideration: traffic from the user data and from smart devices. In order to account for these two types of traffic and ensure transmission occurs without any interruption, a well-defined mechanism needs to be implemented. The system can best be understood if it is broken into two sections: uplink and downlink. In this section, a simple overview of the two sections is given, followed by a more detailed description in Section 3.  
Figure 1: Proposed multi-user full-duplex visible light communication (VLC) system.  
Since red-green-blue (RGB) LEDs are used in the proposed communication system, three primary channels are available for use. Red and green lights are chosen over blue light as the latter has a shorter wavelength, which means it travels a comparatively shorter distance before being scattered. The proposed system is divided into downlink and uplink sections, the majority of data used by the users and devices flows in the downlink section. To ensure that the system is robust, red and green light are used for user data and device data respectively. Unmodulated blue light is used for downlink transmission. The intensity of the three lights is controlled so that the output light is white, making it easy for the human eye to adjust to the environment such that people can continue their daily tasks without interruption. Receivers present in the downlink section are equipped with red or green optical filters so that the user data and the device data are separated. To ensure simplicity in the system design, the ACO-OFDM scheme is used to modulate the signal on the downlink channel. A brief description of ACO-OFDM is given in Section 3.  
A phosphorus LED, which is a blue LED coated with a phosphorus layer to emit white light, is used in the uplink section. Blue is chosen over red or green to avoid interference between uplink and downlink transmission. Receivers present in the uplink section are equipped with blue optical filters to receive incoming data. An OOK scheme is chosen as the uplink modulation scheme as it simplifies the system design, ensures robust performance and helps uplink devices like smartphones, which have limited power resources, to extend their battery life and efficiently utilize their computational power.  
The channel and LED deployment designed for use in smart homes is given in [28]. Results obtained in [29] show that circular LED deployment provides best average received optical power distribution (ROPD) on the receiving plane. It performs better than traditional rectangular- and array-based deployment of LEDs on the ceiling. Additionally, the number of LEDs required in this configuration is lower than that required by traditional deployment techniques, thus reducing the power consumption and deployment cost. Moreover, it utilizes particle swarm optimization (PSO) to create room specific deployment to ensure that every room obtains the minimum luminosity required as per the IEEE 802.15.7 VLC Standard [30]. Consequently, it has been chosen as part of our system design, as shown in Figure 1.