High data rates, robustness, low energy consumption, data security, electromagnetic compatibility (EMC) and network capability: the requirements for exchanging data are enormous in the field of manufacturing and process automation, where industrial systems are becoming increasingly complex and more and more sensors, machines and control and regulation units are communicating with each other. With these requirements, the demand for a replacement for today's predominant wired fieldbus systems is growing. This is because wireless systems promise greater reliability and safety than wear-prone and expensive special cables or slip rings, particularly in the case of moving or moving system parts such as gripper arms or lifting equipment. They also enable faster installation and offer greater flexibility. Last but not least, they are always required when laying a signal cable from the sensors or actuators to the control unit is completely impossible or very time-consuming, thus achieving significant cost savings in assembly and maintenance.

The state of the art

The state of the art in industrial systems today is wired transmission protocols ranging from a few kbit/s to 12.5 Gbit/s. Representative examples are Ethernet 10/100/1000Base-T, Ethercat, Profibus or Profinet. Higher data rates are achieved, for example, with 10GBaseT or PCIe with copper cables or 100GbE via fiber optics. In comparison, there are various radio-based technologies which, however, must be critically scrutinized in terms of net data rate and reliability. These include, for example, IEEE 802.11n or 802.11ac with theoretical gross data rates of 600 Mbit/s and 1300 Mbit/s respectively. If a network of this type is operated in a certain area with obstacles, EMC interference and several subscribers, the net data rate may be reduced due to the temporary interference of the transmission channel. Real-time communication is almost impossible with these wireless technologies and is always associated with limitations.

In contrast to radio solutions, optical wireless communication - also known as Li-Fi technology - works with a physically limited bandwidth in the THz range. Unlike radio-based communication, there is no government regulation of the spectrum worldwide. A practical limitation of the data rate is given by available optoelectronic components, which are used for modulation and demodulation. Unlike radio technologies, wireless communication based on light is much less sensitive to interference. In addition to the high net data rates of up to 12.5 Gbit/s, Li-Fi systems ensure stable operation in EMC-affected areas. They can operate at different wavelengths - mainly in the visible and near infrared range.

There are currently Li-Fi systems with various proprietary approaches. In addition, various organizations are working on standardization. The IEEE standard 802.15.7, for example, offers data rates from a few kbit/s up to 100 Mbit/s. Higher data rates are currently under discussion. The Infrared Data Association (IrDA) has published well-known standards (SIR, MIR, FIR, VFIR) with up to 16 Mbit/s as well as newer versions (Giga-IR) with up to 1 Gbit/s. Versions for data rates of 5 and 10 Gbit/s are currently being worked on.

Line of sight - disadvantage and advantage at the same time

The biggest disadvantage and advantage of Li-Fi technology is the need for a line of sight. Multi-path propagation, as is known from infrared remote control and can be used for simple handling, is theoretically conceivable with higher data rates. However, with the technologies available today, this is not feasible in a competitive market environment. This is why Li-Fi focuses primarily on point-to-point and point-to-multipoint solutions. In this respect, cables, connectors and radio links with line of sight can be replaced by Li-Fi solutions with high data rates, real-time capability and interference immunity. In contrast, classic wireless networks offer the greatest possible mobility at comparatively low data rates, whereas real-time capability is usually not an issue.

How Li-Fi works

In general, a Li-Fi HotSpot data link consists of a permanently installed optical transmitter/receiver module that represents an access point for a defined spatial application area. A second optical module can be placed in this area and establish a data connection. The distance between the two modules and the spot size of the emitted light are important for Li-Fi HotSpots. In a first approximation, the spot size changes linearly with the distance. The distance can be increased until the minimum sensitivity of the receiver is reached. Alternatively, the beam angle can be reduced to further increase the distance. A major advantage of Li-Fi is that several data links can be set up in parallel using the room multiplex method without interference between the individual connections. And each connection can operate at the maximum available data rate. There is also the option of using time-division multiplexing to establish point-to-multipoint connections.

In addition to stationary installation, applications in which two Li-Fi modules move in relation to each other on a rail system are also conceivable. If, for example, a moving camera is installed for process control, the video data can be transmitted wirelessly in real time during movement. A solution with cables would have disadvantages in terms of robustness and durability. Although special cable chains could compensate for this disadvantage, the speed of the moving modules would then be limited.