Li-Fi: Illuminating the road to higher communication data rates

The radio frequency spectrum is a precious resource and will soon be occupied. It doesn’t take long for Wi-Fi users in urban areas to realize how interference generated by nearby routers can affect the communication performance that network devices can achieve. The first response to this question is to simply add more frequency bands. In addition to the original 2.4GHz frequency band (which still needs to be shared with many other protocols such as Bluetooth), Wi-Fi also adds support for more channels around 5GHz. . However, because there are too many other applications that need to occupy this part of the RF spectrum, the number of frequency bands that Wi-Fi can expand is strictly limited.

Li-Fi: Illuminating the road to higher communication data rates

Author: Mouser Electronics Mark Patrick

The radio frequency spectrum is a precious resource and will soon be occupied. It doesn’t take long for Wi-Fi users in urban areas to realize how interference generated by nearby routers can affect the communication performance that network devices can achieve. The first response to this question is to simply add more frequency bands. In addition to the original 2.4GHz frequency band (which still needs to be shared with many other protocols such as Bluetooth), Wi-Fi also adds support for more channels around 5GHz. . However, because there are too many other applications that need to occupy this part of the RF spectrum, the number of frequency bands that Wi-Fi can expand is strictly limited.

Over time, developers of more advanced Wi-Fi devices have responded to frequency limitations by adopting various techniques to incorporate more data into the core spectrum. These include advanced modulation that sends multiple data bits in each radio symbol. Solutions, and antenna diversity enhancements that can direct transmission to each receiver, and so on. Other solutions have moved Wi-Fi to a frequency range above 10 GHz, which can provide higher bandwidth channels and correspondingly high data rates. But why not further improve the electromagnetic spectrum and use infrared or visible light?

Visible light communication has been deployed to point-to-point backhaul applications to achieve data rates exceeding 100Mbit/s. In application scenarios such as deep canyons, laying cables is simply not feasible. It is also currently studying light-based data transmission to improve the connectivity of systems above the atmosphere and in the ocean. RF scatters quickly in water, so it is difficult to use an extremely low frequency carrier with a corresponding low data rate signal and establish reliable communication on it. According to recent research, although water strongly absorbs the red long-wave end of the visible light frequency, blue-green lasers can be transmitted at a data rate of up to 100 Mbit/s at a distance of tens of meters. For longer-distance applications, NASA has begun to use modulated infrared lasers for ground-to-air communications experiments. The 622Mbits/s channel is switched between different ground stations communicating with orbiting satellites, avoiding the attenuation caused by clouds and fog.

The Li-Fi version of visible light communication is aimed at more practical applications. Despite some adjustments, this technology was developed to take advantage of the LEDs in standard lamps. Many commercial LED luminaires use high-brightness components that produce light at the blue end of the spectrum, and a yellow phosphor coating changes the overall color of the light to white. The role of the phosphor is to reduce the influence of any amplitude modulation imposed on the light source, thereby limiting its bandwidth to around 2MHz. However, if the receiver filters out the yellow component, a data rate of up to 1Gbit/s can be achieved in principle. By making the receiver respond to different components with tunable lamps (usually using a mix of red, green, and blue LEDs), the data rate can be increased to 5Gbits/s or higher. Experiments by the University of Edinburgh team led by Professor Harald Haas (who coined the term Li-Fi) showed that adding laser diodes to the illuminator and making them transmit in parallel can achieve a transmission rate of more than 100Gbit/s.

Li-Fi shares some application attributes with Wi-Fi versions operating in the radio spectrum above 10 GHz. As the frequency of the carrier signal increases, RF communication becomes more directional. Although protocols that use channels above 10 GHz (such as 5G cellular networks) will use reflection to improve reception performance, the communication channel will still be mainly based on line-of-sight transmission.

Because Li-Fi has stronger directionality, it allows the construction of “attocells”, for example, a single user operating under downlight has its own bandwidth. However, Li-Fi is not a pure line-of-sight technology, it has a certain ability to use reflection, so it is no longer necessary to strictly maintain the line-of-sight transmission path. This can be achieved by using coding systems such as Orthogonal Frequency Division Multiplexing (OFDM), which are more complex than the simple binary codes used in early Li-Fi experiments.

The strong directionality of Li-Fi has potential advantages in security applications. Since the signal is largely unaffected by the cone of light under the transmitter, it will not penetrate solid walls at all. For example, some proposed 60GHz Wi-Fi transmission schemes such as IEEE 802.11ax use technologies that make it possible to transmit signals through walls, because the standard working group believes this is essential for overall home adoption. When using Li-Fi, any hacker who wants to intercept the signal must be close to the transmitter and legal receiver. This requirement alone significantly increases the chance of being detected. An application case proposed by the IEEE 802.11bb working group is a desk lamp with Li-Fi function, which can provide a secure wireless connection between the user’s computer and the core network. The uplink channel from the device to the luminaire uses a smaller transmitter that works in the infrared region. This can avoid interference with downlink signals and has the advantage of not distracting device users. In the early stages of technological development, people are worried about whether users will notice the changes in Li-Fi transmitters, whose modulation speed is so high that other effects are not obvious except that the color balance of the total light output may shift. Nevertheless, this is also a factor that lighting designers can compensate.

When installing Li-Fi on ceiling lights, one of its potential shortcomings is co-channel interference. In this case, the light cones converge, so the receiver will not get a clear signal from any transmitter. In addition to being able to utilize the light reflected from walls and other objects used for communication, OFDM-based coding schemes also help to overcome the above-mentioned problems. The IEEE 802.11bb working group has formulated an agreement that can provide at least a data rate of 10Mbits/s, and may increase to a peak of 5Gbits/s, which is ten times the rate of the widely used IEEE 802.11n Wi-Fi based on a 5GHz carrier. The latest version of Wi-Fi and the current more expensive version of IEEE 802.11ac have closed this gap, which can provide 1.73Gbits/s.

One of the promises of Wi-Fi technology is to achieve basically the same peak data rate as Li-Fi. This competition is derived from the IEEE 802.11ax and 802.11ay versions of Wi-Fi that use a carrier frequency of about 60 GHz. These standards improve the problem of too short operating range when trying to build 60GHz Wi-Fi-IEEE 802.11ad for the first time. Some tests have extended the maximum range of IEEE 802.11ay to 300m, making it suitable for office networks. However, its usage mode is different from Li-Fi. One main difference is that a single 802.11ay router can provide services for multiple users, while Li-Fi supporters want to make full use of the concept of attocell, and the backhaul network can be in the same room. Multiple users provide Gbit/s-level transmission services.

Wi-Fi

Li-Fi

Based on wireless technology

Based on photoelectric technology

The signal can penetrate the wall

Sight path required

Vulnerable to potential security attacks

Higher intrinsic safety

May be interfered by other 2.4GHz emission sources

May be subject to co-channel interference

30~40m working distance

Maximum 10m range

3.5Gbps data rate (802.11ax)

Up to 10Gbps data rate

Table 1: Comparison of Wi-Fi and Li-Fi.

Another difference between IEEE 802.11ay and most other protocols is that it can perform other services, which are derived from the algorithms it uses to compensate for obstacles. In terms of potential capabilities, routers can map rooms, detect the presence of people, and even determine gestures. In the Li-Fi environment, these functions are likely to be implemented with the help of a separate camera.

Considering the emergence of new Wi-Fi technology, Li-Fi deployment in traditional homes and offices still requires a lot of effort, but optical-based communication technology has obvious advantages in some environments. For example, on airplanes, the weight of cables used to provide multimedia services to passengers is one of the main obstacles to improving fuel efficiency. By replacing the conventional lighting on each seat with Li-Fi-enabled LEDs, Li-Fi can provide passengers Provide high-speed data transmission. In applications such as hospital operating rooms where RF interference is a major problem, Li-Fi provides a high-bandwidth communication solution. For industrial systems, especially those with a high risk of explosion, Li-Fi may be a safer technology. For example, factories that handle fine powders and volatile chemicals cannot easily use high-frequency RF communications, and data cables require strict protection measures.

Due to Li-Fi’s novel technology, it may be able to achieve more applications in environments where high-speed communication was previously difficult. However, for most cases, if data capacity and convenience are very important considerations, the choice between Li-Fi and Wi-Fi is likely to depend on the requirements of the specific application.

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