The radio spectrum is a precious resource that is quickly being used up. It won’t be long before Wi-Fi users in urban areas realize how interference from nearby routers can affect the communication performance that network devices can achieve. The first response to this question was to simply add more bands, Wi-Fi added support for more channels around 5GHz in addition to the original 2.4GHz band (which still needs to be shared with many other protocols like Bluetooth) . However, because there are so many other applications that need to occupy this part of the RF spectrum, the number of frequency bands that Wi-Fi can extend is severely limited.
Over time, developers of more advanced Wi-Fi devices have responded to frequency constraints by employing various techniques to bring more data into the core spectrum, including advanced modulation that sends multiple bits of data in each radio symbol solutions, and antenna diversity enhancements that can direct transmissions to individual receivers, among others. Other schemes have moved Wi-Fi to the frequency range above 10GHz, which can provide higher bandwidth channels and correspondingly high data rates. But why not push the electromagnetic spectrum even further and take advantage of infrared or visible light?
Visible light communication has been deployed in point-to-point backhaul applications to achieve data rates in excess of 100Mbit/s, while in application scenarios such as deep canyons, laying cables is simply not feasible. Light-based data transmission is also being investigated to improve the connectivity of systems above the atmosphere and in the ocean. RF scatters very quickly in water, making it difficult to take very low frequency carriers with correspondingly low data rate signals and establish reliable communications over them. According to recent research, blue-green lasers can transmit at data rates of up to 100Mbit/s over distances of tens of meters, although water also strongly absorbs the red long-wave end of visible light frequencies. For longer-range applications, NASA has already begun experiments with ground-to-air communications using modulated infrared lasers. The 622Mbits/s channel is switched between different ground stations communicating with orbiting satellites, avoiding attenuation due to clouds and fog.
The Li-Fi version of visible light communication is aimed at more practical applications. Despite some tweaks, this technology was developed to take advantage of LEDs in standard luminaires. Many commercial LED luminaires use high-brightness components that produce light at the blue end of the spectrum, with a yellow phosphor coating changing the overall color of the light to white. The role of the phosphor is to reduce the effect 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, data rates up to 1 Gbit/s can in principle be achieved. Data rates can be increased to 5Gbits/s or higher by making the receiver respond to different components with tunable luminaires (usually using a mix of red, green and blue LEDs). Experiments by a team at the University of Edinburgh led by Professor Harald Haas (who coined the term Li-Fi) have shown that adding laser diodes to illuminators and making them transmit in parallel can achieve transmission rates in excess of 100Gbit/s.
Li-Fi shares some application properties with versions of Wi-Fi that operate in the radio spectrum above 10GHz. RF communications become more directional as the carrier signal frequency increases. Although protocols using channels above 10GHz, such as 5G cellular networks, will use reflections to improve reception performance, the communication channel will still be primarily based on line-of-sight transmission.
Since Li-Fi is more directional, it allows the construction of “attocells”, e.g. a single user operating under a downlight with its own bandwidth. However, Li-Fi is not a pure line-of-sight technology, it has a certain ability to take advantage of reflections, so that 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 employed 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 below the emitter, it won’t penetrate solid walls at all. Some of the proposed 60GHz Wi-Fi transmission schemes, such as IEEE 802.11ax, employ technology that makes it possible to transmit signals through walls, as the standards working group believes is critical for overall home adoption. When using Li-Fi, any hacker who wants to intercept the signal must be close to the transmitter and legitimate receiver, and this requirement alone significantly increases the chances of detection. One use case proposed by the IEEE 802.11bb working group is a Li-Fi-enabled desk lamp that provides a secure wireless connection between a user’s computer and the core network. The uplink channel from the device to the luminaire uses a smaller transmitter operating in the infrared region. This avoids interference with downlink signals and has the benefit of not distracting the user of the device. In the early stages of technology development, there were concerns about whether users would notice changes in Li-Fi transmitters whose modulation speeds were so high that other than a possible shift in the color balance of the total light output, other effects were not noticeable. Still, it’s a factor that luminaire designers can compensate for.
One of the potential drawbacks of installing Li-Fi to ceiling lights is co-channel interference. In this case, the light cones intersect, so the receiver doesn’t get a clear signal from either transmitter. OFDM-based coding schemes help overcome the above-mentioned problems, in addition to their ability to utilize light reflected from walls and other objects used for communication. The IEEE 802.11bb working group has developed a protocol that can provide data rates of at least 10Mbits/s and possibly a peak of 5Gbits/s, which is ten times the rate of the widely adopted IEEE 802.11n Wi-Fi based on the 5GHz carrier. The latest version of Wi-Fi, as well as the current, more expensive version of IEEE 802.11ac, has closed the gap, offering 1.73Gbits/s.
One of the promises of Wi-Fi technology is to achieve peak data rates that are basically consistent with Li-Fi, and this competition stems from the IEEE 802.11ax and 802.11ay versions of Wi-Fi that use carrier frequencies around 60GHz. These standards improve upon the short range encountered in the first attempt to build 60GHz Wi-Fi-IEEE 802.11ad. Some tests have extended the maximum range of IEEE 802.11ay to 300m, making it suitable for office networks. However, its usage patterns are different from Li-Fi, with one major difference being that a single 802.11ay router can serve multiple users, while Li-Fi proponents want to take advantage of the concept of attocells, where the backhaul network can serve multiple users in the same room. Multiple users provide Gbit/s-level transmission services.
Wi-Fi Li-Fi
Based on wireless technology Based on optoelectronic technology
Signal can penetrate walls Requires line-of-sight path
Vulnerable to potential security attacks Higher intrinsic security
May be interfered by other 2.4GHz sources May be interfered by co-channel
30～40m action distance, maximum 10m action distance
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 that stem 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 a Li-Fi environment, these functions are likely to be implemented with the help of a separate camera.
Considering the emergence of the new Wi-Fi technology, Li-Fi deployment in traditional homes and offices still needs a lot of effort, but light-based communication technology has obvious advantages in some environments. In airplanes, for example, the weight of cables used to provide multimedia services to passengers is one of the main obstacles to improving fuel efficiency. By replacing the regular lights on each seat with Li-Fi-enabled LEDs, Li-Fi can provide passengers with Provides high-speed data transmission. In applications such as hospital operating rooms where RF interference is a significant issue, 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, plants dealing with fines and volatile chemicals cannot readily adopt high-frequency RF communications, and data cables require strict protection.
Thanks to Li-Fi’s novel technology, it may enable 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.