# Visible Light Communication: A System Perspective--Overview and Challenges
## 4 Hybrid VLC
### LED-to-LED System  
A typical VLC system uses LED as transmitter and photodetector as a receiver. Most of the research work has experimented with one-way VLC system in the downlink direction. To take full advantage of the large unregulated bandwidth of VLC, ideally, a bidirectional VLC system should be used for downlink and uplink communication. To build a two-way VLC-VLC system, an LED transmitter and photodetector would be required at both ends for downlink and uplink transmission. However, because of user device sensitivity to orientation changes and aesthetically not pleasing, it has not attracted much research for indoor wireless communication. [PERSON] et al. [85; 86] have developed a LED-to-LED VLC system based on the concept of [PERSON] et al. [87]. The VLC system is based on a single LED, which can be used both for transmission and reception. The discharging characteristic of LED was used for receiving the incoming light, which in turn translates to information. The LED capacitance changes with the intensity of receiving light. A threshold was set based on the experiments to determine the discharging for the different intensity of light to infer the incoming symbols of ON and OFF. In another reported work, smart phone was used to relay the VLC to a toy car [88]. The obvious drawback is that LED is less sensitive to light. A high-intensity light would be needed which will increase the cost and decrease the communication range. Also, a single LED for transmission and reception means that it would require multiplexing. Furthermore, fine tuning would be required to keep the LED illuminated without transmitting any useful information. This would decrease the maximum achievable data rate. [PERSON] et al. [58] have proposed a full fibre integrated LED-to-LED VLC system. A fiber optic link is used to connect all VAP's to address the backbone bottleneck. A hybrid access protocol is designed using frequency division multiplexing for the uplink and downlink in fiber transmission, and time division multiplexing for bidirectional VLC transmission. Total system throughput of 8 Gb/s was achieved for eight VAP; each offered a 500 Mbps downlink and uplink transmission. The same network architecture can be extended to support upto 100 Gb/s wireless access system in the future. [PERSON] et al. [89] have proposed a system which uses LED enabled devices to communicate on a two-way VLC channel without the use of an exclusive photodetector, while simultaneously providing illumination. Manchester coding with on/off keying (OOK) modulation is employed to achieve the best tradeoff between illumination and data rates. A prototype is developed to achieve data rates in the order of kbps at a distance of ten of centimetres. [PERSON] et al. have used 682 nm visible-vertical-cavity surface-emitting laser (VCSEL) for the bidirectional VLC system [90]. The downstream of the proposed system is based on orthogonal frequency division multiplexing (OFDM) and quadrature amplitude modulation (QAM) modulation. To further enhance bandwidth efficiency bit-loading and power-loading algorithms are used. Results show that a single VCSEL bidirectional-VLC system can achieve 10.5 Gbits/s OFDM downstream traffic and 2 Mbits/s upstream. [PERSON] et al. have investigated the VLC channel at the application level under different settings and scenario for low cost embedded VLC devices [91]. It investigates the effect of platform parameters such as transmitter and receiver position, the nature of receiver (whether the receiver is PD or LED-based), and the protocol level parameters such as symbol rate and frame payload size. The experimental setup involves adopting OpenVLC 1.0 with BeagleBone board and running a Debian Linux. The parameters investigated are throughput and packet reception ratio. Experiments show that channel conditions change for different scenarios and parameters need to be tuned for different scenarios such as day condition, mobility etc. The usage of a simple PD could reduce the impact of ambient light on the overall throughput.