LI-FI: WHAT IT IS AND HOW IT WORKS

Over the last few years, new technologies have been developed to equip existing infrastructures with the communication capabilities with other devices.

PLC (PowerLine Communication) is one popular communication protocol because that enables data transmission via the normal electrical power grid. Although the transmission capability of a PLC system is far lower than that of a cabled or Wi-Fi network, it is possible to remote control the devices in environments where wireless networks are not disturbed and where no other types of network exist.

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In 2011, VLC (Visible light Communication) was created, a new interesting technology in communication protocols that exploits a visible light source to transmit information. The Li-Fi concept was also created: a term which refers to those systems that provide an alternative to existing connectivity systems via a VLC connection between a lighting system and a device. Optical communication is still widely used in many areas, but using illumination as transmitting devices is a true revolution made possible by LEDs. Modern lamps with high performance LEDs – there are home, industrial, automotive lighting systems, etc. – can be adapted for VLC data transmission.

Information transmission

Usually, LED lights are supplied by constant current to control their power. In addition, the light they emit is proportional to the current that runs through the device. In order to transmit information, it is necessary to change the light emitted in a controlled manner, i.e. dynamically adjust the current running through the LEDs. The main objective of lights is to light up a room, so only slight and quick variations in the intensity of the light emitted must be carried out not to alter the visual perception of users. In addition, the functionality of lighting systems must be guaranteed also in case of transmission circuit faults.

Advantages of the technology

In addition to adding functionalities to existing infrastructures, Li-Fi offers connectivity solutions in situations such as for example data transmission in humid environments where Wi-Fi does not work. Li-Fi offers interesting advantages also in terms of safety. For obvious reasons, the information is located near the lighting source. It would therefore be difficult to intercept information in a room under video surveillance because the reception sensor must be in direct visual contact with the source. Li-Fi can be used to have secondary access keys to other networks – for example, it would be possible to supply connectivity in places with Vending Machines or advertising so as to increase the number of users. Li-Fi provides some advantages also in terms of electromagnetic pollution, as electromagnetic radiations in the visible spectrum are not harmful for humans, unlike microwaves, which have a definite impact on our health.

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ROLD’s solution

Transmission circuit

ROLD’s R&D team developed a technology for LiFi communication.

The objective was quite ambitious – the system had to be interfaced with an existing lighting system and the main functionality could not be compromised by any kind of malfunctioning. ROLD’s T-Connection was created with these two characteristics in mind. It makes it possible to connect the transmission circuit to the lighting source directly in parallel. The transmission circuit injects a current proportional to the signal to transmit into the connection node, therefore adjusting the optical power emitted by the LEDs. The transmission module does not need an external power supply, as its supply derives directly from the light’s driver output and is compatible with a voltage between 12 and 32V. The module is equipped with a delayed switching circuit to ensure protection from the inrush current released when the driver is turned on, as it can be quite strong for high-power power supplies. No particular connection is needed, both the driver and the light can be connected using a normal strip connector. It is anyway possible to equip output cables with connectors suitable for the interfacing with existing structures. The frequency of the signal injected into the lamp is adjusted so as to improve the immunity of the transmission to disturbance and enable the correct recovery of incoming data. Adjusting speed, and therefore transmission speed, depends on the transmission bandwidth of the light, therefore it is not possible to know the maximum transmission speed beforehand.

Reception

The reception distance depends on the distance of the transmitted signal, detection capacity and signal/noise ratio of the reception circuit’s acquisition chain. The change in the intensity of lights must not affect the visual perception of users, therefore the signal to be transmitted must be low-entity. Users can control the reception circuit’s acquisition chain. ROLD has created a receiver that can extract weak signals at a distance greater than 6 metres (laboratory tests), so the system can also work in industrial environments. From the signal, the receiver can extract analogue modulating and serial digital signals such as those generated by a RS232, RS485, etc. transmission

www.rold.com

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