Radio spectrum is a precious resource and it quickly gets filled up. It did not take long for users of Wi-Fi in urban areas to understand how interference from nearby routers would affect the communications performance they could achieve from their own network equipment.

One of the first responses to this problem was to simply add more frequency bands. In addition to the original 2,4 GHz band, which still needs to be shared with many other protocols (including Bluetooth), Wi-Fi added support for additional channels around 5 GHz. However, the number of frequency bands into which Wi-Fi can expand is severely limited – because there are too many other applications that need ­access to their own portions of the RF spectrum.

Over time, developers of more advanced Wi-Fi equipment have countered the frequency restriction issue by employing a variety of techniques to push more data into the core spectrum. These range from advanced modulation schemes that transmit multiple data bits in each radio symbol to antenna-diversity enhancements that make it possible to steer transmissions towards individual receivers.

Other proposals have moved Wi-Fi into the 10 GHz-plus range. This can provide wider-bandwidth channels and commensurately high data rates. But, why not go further up the electromagnetic spectrum and make use of infrared or visible light instead?

Visible light communication has already been deployed for point-to-point backhaul applications in order to achieve data rates in excess of 100 Mbps where it is impractical to deploy cables, such as across deep canyons. Light-based transmissions are also being investigated for their ability to improve the connectivity of systems both above the atmosphere and below the waves.

RF scatters quickly in water, making it hard to establish reliable communications above signals that employ extremely low-frequency carriers and which have commensurately low data rates. Although water also strongly absorbs the red end of the visible light frequencies, blue-green lasers can carry transmissions at data rates of up to a hundred Mbps over several tens of metres, according to recent research studies.

Aiming at much longer distance applications, NASA has begun trials of ground-to-space communications using a modulated infrared laser. The 622 Mbps channel avoids the attenuation caused by clouds by switching between different ground stations that cooperate to communicate with an orbiting satellite.

The Li-Fi version of visible light communications is targeted at more down-to-earth applications. It was developed to take advantage of the LEDs that go into standard light fittings, albeit with some adjustments.

Many commercial LED light fittings use a high-brightness element that produces light at the blue end of the spectrum. A coating of yellow phosphor shifts the overall colour of the light to white. The phosphor’s action slows down the effect of any amplitude modulation imposed on the source light, limiting its bandwidth to around 2 MHz. However, if a receiver filters out the yellow component, it is possible to achieve data rates as high as 1 Gbps, in principle.

Having receivers that respond to the different components of a light fitting with tuneable colour, which normally use a mixture of red, green and blue LEDs, it is possible to push the data rate to 5 Gbps or more. Experiments by a University of Edinburgh team led by Professor Harald Haas (who coined the term Li-Fi), have shown that adding laser diodes to the luminaries and having them transmit in parallel could achieve a transmission rate of more than 100 Gbps.

Li-Fi shares some usage attributes with versions of Wi-Fi that operate in the 10 GHz-plus part of the radio spectrum. As the frequency of the carrier signal increases, RF communications become more directional. Although protocols that make use of 10 GHz-plus channels, such as 5G cellular, will take advantage of reflections to improve reception performance, communications channels will predominantly be based on line-of-sight transmission.

As Li-Fi has even greater directivity, it allows the construction of ‘attocells’ where a single user operating under a downlight, for example, has the bandwidth to themselves. However, Li-Fi is not purely a light-of-sight technology. It has some ability to employ reflections, which thereby avoids the need to maintain strictly line-of-sight transmission paths. This is enabled by the use of coding systems, such as orthogonal frequency-division multiplexing (OFDM), that are more complex than the simple binary codes employed in early Li-Fi experiments.

The directivity of Li-Fi is provides a potential advantage in terms of security. As well as being largely restricted to a cone of light underneath the transmitter, signals do not penetrate solid walls at all. Some proposed 60 GHz Wi-Fi transmission schemes, such as IEEE 802.11ax, utilise techniques that make it possible to send signals through walls as this is seen by the standard’s working group as important for overall usability in homes.

With Li-Fi, any hacker who wants to intercept the signal needs to be close to both the transmitter and the legitimate receiver. That factor alone clearly increases the chance of detection.

A use case proposed by the IEEE 802.11bb working group is that of a Li-Fi-enabled desk lamp which provides a secure wireless connection between the user’s computer and the core network. The uplink channel from device to light fitting makes use of a smaller emitter operating in the infrared region. This avoids interference with the downstream signal and also has the benefit of not distracting the device’s user.

In the early stages of the technology’s evolution, there was some concern over whether users would notice flicker from Li-Fi-enabled transmitters. The modulation speed is so high though that the effect is unnoticeable other than a possible shift in the colour balance of the overall light output. However, this is a factor for which luminaire designers can compensate.

One potential drawback with Li-Fi when fitted to ceiling lights is that of co-channel interference. Here the light cones intersect, so a receiver will not get a clear signal from either transmitter. OFDM-based coding schemes help overcome this issue in addition to making light reflected off walls and other objects usable for communication.

The IEEE 802.11bb working group has proposed a protocol that at minimum provides a data rate of 10 Mbps rising to a peak of 5 Gbps, which is 10 times faster than the widely implemented IEEE 802.11n form of Wi-Fi, based on a 5 GHz carrier. The more recent and currently much more expensive IEEE 802.11ac version of Wi-Fi narrows this gap. It can deliver 1,73 Gbps.

Wi-Fi promises to match Li-Fi’s peak data rates. This competition will come from the IEEE 802.11ax and 802.11ay versions of Wi-Fi that employ carrier frequencies around 60 GHz. These standards improve on the short range suffered by the first attempt to build a 60 GHz Wi-Fi - IEEE 802.11ad.

Some tests have pushed the maximum range of IEEE 802.11ay to 300 m, making it suitable for office networks. However, its usage model is different to Li-Fi. One key difference is that a single router is expected to serve multiple users whereas Li-Fi proponents expect to make the most of the attocell concept, with backhaul networks providing the ability to serve Gbps sessions to multiple users within the same room.

Another difference between IEEE 802.11ay and most other protocols is that it can perform additional services that arise from the algorithms used to compensate for obstructions. Potentially, routers can map rooms, detect the presence of people and can even determine gestures. In a Li-Fi environment, these functions would more likely be implemented with the help of separate cameras.

Although Li-Fi will need to slug it out with the newer forms of Wi-Fi in conventional home and office deployments, light-based communication has some clear-cut advantages in certain environments. In aircraft, for example, the weight of cables used to deliver multimedia services to passengers is a major obstacle to building more fuel-efficient vehicles. Li-Fi makes it possible to deliver high data rates to individuals simply by replacing the conventional lights over each seat with a suitable Li-Fi-enabled LED.

Li-Fi provides a solution for high-bandwidth communications where interference from RF is problematic, such as in the operating theatres of hospitals. It is potentially a much safer technology for industrial systems, particularly those where there is a high risk of explosion. For example, plants that handle fine powders and volatile chemicals cannot easily employ high-frequency RF communications and stringent safeguards are needed for electrical data cables.

Thanks to its novel approach, Li-Fi will probably find use cases in environments where it has previously been difficult to implement high-speed communication. For most situations though, where considerations of data capacity and convenience are uppermost, the choice between either Li-Fi or Wi-Fi is likely to come down to the specific requirements of the application.

For more information contact TRX Electronics, authorised Mouser Independent Representative in South Africa, +27 12 997 0509, info@trxe.com, www.trxe.com