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Li-Fi: Wireless Connections Without Radios Charles Byers

Wi-Fi, 3G/4G cellular, and related technologies work pretty well. Everyone—and an increasing number of non-human Internet of Things (IoT) devices—uses them, but, that is just the problem. Networks are approaching the limits of their capacity, and it is challenging to increase radio-based network capacity to meet an exploding demand for bandwidth.

A key challenge to the growth in bandwidth in radio-based access networks is the availability of usable RF spectrum. This problem can be divided two ways:

  • Licensed spectrum
  • Unlicensed spectrum

Licensed spectrum is exclusively granted by a government agency—for example, the United States Federal Communications Commission (FCC)—for use in a specific radio application, by a specific group of users, and often for a specific region or regions of a country. Because of overwhelming demand, choice blocks of nationwide RF spectrum can cost billions of dollars in national spectrum auctions. There are continuous efforts to free up spectrum from older, less profitable uses and move it to things like 5G cellular. However, this has problems too, like rendering billions of TV sets or other radio devices obsolete as their spectrum is taken over. So, the licensed spectrum remains scarce and expensive.

An alternative to the licensed spectrum is the unlicensed spectrum. Networks like Wi-Fi, Bluetooth, home automation systems, remote control vehicles, and others usually use a set of spectral slices called the Instrumentation, Scientific, and Medical (ISM) bands. Use of these bands is free to everyone—subject to some detailed rules—but that use is not exclusive. Anyone can transmit on ISM bands, and as devices come-and-go, the bandwidth available to any one device can change significantly. There also tends to be more interference in ISM bands, for example from microwave ovens that share their spectrum. The bands at higher frequencies—say above about 10GHz—tend to be less cluttered, but their radio propagation characteristics are poor, being easily blocked by walls or foliage.

Regardless of whether the spectrum used is licensed or unlicensed; there is a maximum capacity available that depends upon frequency, bandwidth, Signal-to-Noise Ratio (SNR), modulation techniques, antenna design, protocols, and encoding. Once that capacity is exceeded, the network will slow down. That is why your phone is so slow at crowded events. At these events, too many people are sending too much data on a fixed radio network capacity. Directional antenna techniques like multi-input multi-output (MIMO) help some by reusing spectrum in adjacent areas from different angles, but there are serious limitations. What we need to do is to get lots of bandwidth and efficient modulation techniques at frequencies well beyond the jurisdiction of the FCC or the chaotic ISM bands, and that drives us to light beam communications.

Light Fidelity (Li-Fi) has emerged as a Visible Light Communication (VLC) technology for wireless communication. Li-Fi and related forms of free-space optical communications use modulated light beams to carry digital data at very high network capacities without using any RF spectrum in the process. A digital message, say an IP packet, is encoded with a standard protocol and used to modulate a light source—visible, ultraviolet, infrared, laser, or LED—with a high-speed bit stream. The emitted light is processed by an optical system to direct it towards a receiver. It then crosses through free space to where a portion of the light is received by an optical system at the remote device. The light is converted to an electrical signal by a fast photodetector, amplified, demodulated, and converted back to the original message for use by the remote device’s processor. For bidirectional communication, the process is repeated in the opposite direction—sometimes using a different wavelength of light to avoid interference. Prototype systems have shown capacities over 100Gbps. In 2013, NASA set a distance record for laser communications between the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft in lunar orbit and a ground station in New Mexico at 622Mbps and over a distance of 385000km.

Harald Haas of the University of Edinburgh pioneered much of the work on Li-Fi and developed some excellent TED talks and papers on the subject. He also is the co-founder of a leading provider of commercial products: pureLiFi. There are several standards efforts on free-space optical communication, the most important one being IEEE 802.15.7. These systems typically work by modulating a ceiling-mounted light source—either a little transmitter that is similar to a Wi-Fi Access Point (AP) or a modified light fixture—sending the data streams through free space, and receiving the streams on an optical interface on a remote device. Smartphones including Li-Fi receiver capabilities have been tested. These systems tend to “bathe” an entire room with the same modulated signal from non-directional light sources, and all receiving devices in the room use Medium Access Control (MAC) protocols and encryption to retrieve only the portions of the shared data stream meant for them.

Let’s see if we can do better. What if instead of sending the same omnidirectional optical bit pattern to all devices, as conventional Wi-Fi does, we attempt to direct individual beams of light to each device—sort of like a supped-up optical MIMO? One way to do this is to place a beam deflector—for example, a pair of X-Y scanning galvanometers—in front of the modulated light source, and deflect the beam around a path to visit all the active devices in its range in rapid sequence. This system buffers traffic for all the endpoints it can see, sets the deflection angle to point at the selected endpoint, and bursts data at multi-gigabit speeds until the buffers are exhausted—or a timer expires—before moving the deflector to the next endpoint. It turns out the beam deflectors used in applications like laser marking or laser light shows can be adapted to do this. Figure 1 shows a device about the size of a hatbox equipped with overlapping sectors of this deflected free-space optical networking technology. If such a device was hung from the ceiling of an auditorium or placed on a water tower, thousands of endpoints could be simultaneously served with fast, secure, radio spectrum-free bandwidth. If you want to learn more of the technical details of this sort of network, have a look at my US patent 6,650,451.

Figure 1: Device with overlapping sectors using free-space optical networking technology. (Source: Author)

What if we wanted even more performance and capacity? We can delete the moving parts of the galvanometers, which slow the system down, and may cause reliability concerns, and build multiple transmit/receive beam transceivers into a compact volume. Envision a device similar in appearance to a large golf ball with hundreds of bi-directional beams, one emerging from each dimple. A subset of these transceivers would be activated whenever a remote device entered their angles of view. Both fixed ground stations and movable endpoints would have these transceivers, creating a mesh network. As transceivers move, handoffs would be made to adjacent beams. This technology would be ideal for connecting large swarms of drones, IoT devices in factories, and similar applications. My US patent 9,350,448 has more details on the operation of this sort of system, and how to build the complex optics of the multi-beam transceiver using fish-eye lenses.

Conclusion

In conclusion, Li-Fi, and other free-space optical techniques offer much promise for high-performance networks. They do not use scarce and expensive radio spectrum and offer high capacity. Also, these technologies can be more secure and immune to interference than radio-based network technologies.

Key Points:

  • Radio networking technologies like Wi-Fi and cellular have capacity limits imposed by available radio spectrum.
  • By moving from radio to light-based technologies like Li-Fi to carry the messages, capacity, security, and interference, immunity can be improved.
  • Directional optical networks, using either deflected or multi-beam endpoints, can provide even better capacity and security.


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CHARLES C. BYERS is Associate Chief Technology Officer of the Industrial Internet Consortium, now incorporating OpenFog. He works on the architecture and implementation of edge-fog computing systems, common platforms, media processing systems, and the Internet of Things. Previously, he was a Principal Engineer and Platform Architect with Cisco, and a Bell Labs Fellow at Alcatel-Lucent. During his three decades in the telecommunications networking industry, he has made significant contributions in areas including voice switching, broadband access, converged networks, VoIP, multimedia, video, modular platforms, edge-fog computing and IoT. He has also been a leader in several standards bodies, including serving as CTO for the Industrial Internet Consortium and OpenFog Consortium, and was a founding member of PICMG's AdvancedTCA, AdvancedMC, and MicroTCA subcommittees.

Mr. Byers received his B.S. in Electrical and Computer Engineering and an M.S. in Electrical Engineering from the University of Wisconsin, Madison. In his spare time, he likes travel, cooking, bicycling, and tinkering in his workshop. He holds over 80 US patents.




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