At present, WiFi, 3G / 4G cellular networks and related technologies are well applied in various fields. Each of us and the increasing number of Internet of things (IOT) devices are using these technologies, which also leads to the network capacity approaching the limit. Therefore, we need to improve the radio based network capacity to meet the bandwidth requirements, which is challenging.
A key challenge to the growth of access bandwidth based on radio networks is the availability of RF spectrum, which can be divided into two types:
The licensed spectrum needs to be specially granted by government agencies (such as the Federal Communications Commission (FCC)) and used by specific radio applications and specific user groups, usually in specific areas. Due to the huge demand, the cost of RF licensing nationwide may be billions of dollars. People continue to strive to liberate the spectrum from the older and less profitable fields, and transfer it to 5g cellular networks and other fields. However, there are some problems. For example, as spectrum is taken over, billions of televisions or other radio equipment will be phased out. Therefore, licensed spectrum is still scarce and expensive.
In addition to authorized spectrum, another option is unauthorized spectrum. For example, WiFi, Bluetooth, home automation systems, remote control vehicles and other networks usually use a group of bands called instrument, science and medicine (ISM) band. The use of these frequency bands is free to everyone, but there are some detailed regulations. This use is not unique. Anyone can transmit communication on the ISM band. With the increasing number of devices, the available bandwidth of any device may change significantly. There will also be more interference in the ISM frequency band, such as microwave ovens from shared frequency bands, while the frequency bands with higher frequencies (such as above 10GHz) are often not so messy, but their radio propagation characteristics are very poor and can be easily blocked by walls or leaves.
Whether the spectrum used is authorized or not, the maximum capacity of the network depends on frequency, bandwidth, signal-to-noise ratio (SNR), modulation technology, antenna design, protocol and coding mode. Once the maximum capacity is exceeded, the network will slow down, which is why your mobile phone will be very slow in crowded situations. In these cases, too many people send too much data through fixed wireless network capacity. Directional antenna technology such as multiple input multiple output (MIMO) helps to reuse adjacent radio spectrum from different angles, but it also has serious limitations. When the frequency is far beyond the range of FCC or ISM band, what we need to do is to obtain a large amount of bandwidth and effective modulation technology, which drives the emergence of beam communication technology.
Visible light wireless communication (also known as “optical Fidelity Technology”, English name light fidelity (Li fi for short)) is a visible light communication technology for wireless communication. Li FI and related forms of free space optical communication use modulated beams to carry digital data with very high network capacity without using any RF spectrum. A digital message, such as an IP packet, is encoded using standard protocols and light sources for modulation (such as visible light, ultraviolet, infrared, laser or LED, etc.). The emitted light is processed by the optical system, transmitted to the receiver, and then passes through a distance to the optical system receiving light on the remote device. Light is converted into electrical signal through fast photodetector, amplified and demodulated, and then converted into original information for processor on remote equipment. This process is similar for two-way communication. Sometimes different wavelengths of light are used to avoid interference. The capacity of the prototype system has exceeded 100gbps. In 2013, NASA created a distance record for laser communication between the lunar atmosphere and dust environment Explorer (LADEE) spacecraft in lunar orbit and a ground station in New Mexico, with a speed of 622Mbps and a transmission distance of more than 385000 kilometers.
Harald Haas from the University of Edinburgh is a pioneer in the research of Li fi technology and has published a series of excellent Ted speeches and papers on this subject. At the same time, he is also the co-founder of purelifi, a leading commercial product supplier. There are many standards for free space optical communication, among which the most important standard is IEEE 802.15.7. The working principle of these systems is usually to send data streams to free space by modulating a ceiling mounted light source (a small transmitter similar to a WiFi access point (AP) or a specially designed light source) and receive data streams through an optical interface on a remote device. They have been tested for smartphones with Li fi receivers. These systems tend to cover a real room with the same modulated signal of non directional light source. All receiving devices in the room use medium access control (MAC) protocol and encryption technology, so as to retrieve only the part of the data stream they are ready to share.
If we don’t send the same omni-directional optical bit mode to all devices like traditional WiFi, but try to direct a unique beam to each device (a bit like upgraded MIMO), what will be the effect? One method is to place a beam deflector (such as a pair of X-Y scanning galvanometers) in front of the modulation light source and deflect the beam along a path, so that all active devices within its range can be accessed in a fast sequence. The system can buffer the traffic of all endpoints it can receive, set the deflection angle to point to the selected endpoint, and transmit data explosively at the speed of thousands of Mbps until the buffer is exhausted (or the timer is exhausted), and then move the deflector to the next endpoint. It has been proved that beam deflectors used in applications such as laser marking or laser display can do this. The figure below shows a hat sized device integrated with deflectable free space optical network communication technology. If such equipment is hung on the ceiling of the auditorium or installed on the water tower, it can provide fast, safe data bandwidth for thousands of endpoints at the same time. To learn more about this type of network communication technology, please refer to U.S. patent 6650451.
Devices using their own space optical network technology can form overlapping communication areas
What if we want more performance and capacity? We can remove the moving components of the vibrator, which will slow down the whole system and may cause reliability problems, and build multiple transmit / receive beam transceivers into a compact module. Imagine a device that looks like the size of a golf ball. It has hundreds of two-way beams, and each “dimple” will form a beam. When remote devices enter their visual range, a subset of these transceivers is activated. If both fixed bottom station and mobile terminal have these transceivers, a mesh network can be formed. When the transceiver moves, the adjacent beams will switch. This technology will connect a large number of UAVs, IOT devices in factories and other similar applications. U.S. patent 9350448 describes in detail the operation principle of this system and how to use fisheye lens to build a complex optical system of multi beam transceiver.
Li FI and other free space optical technologies bring hope for high-performance networks, which do not use rare and expensive radio spectrum and have higher network capacity. In addition, these technologies are more secure and less susceptible to interference than radio based network technologies.
·Radio network technologies such as WiFi and cellular networks have capacity constraints and are constrained by the available radio spectrum
·From radio technology to optical communication technology such as Li fi to carry information, network capacity, security, anti-interference and system stability can be improved
·Directional networks using deflection or multi beam endpoints can provide better capacity and security
Charles C. Byers (Mr. Byers) is the deputy chief technology officer of the industrial Internet Alliance. He is committed to studying the architecture and implementation of edge computing system, general platform, media processing system and Internet of things. Previously, he was the chief engineer and platform architect of Cisco and a researcher at Alcatel lucent Bell Labs. During his 30 years in the telecommunications network industry, he has made significant contributions in the fields of voice switching, broadband access, network convergence, VoIP, multimedia, video, modular platform, edge computing and the Internet of things. He is also a leader of many standards organizations, including the chief technology officer of the industrial Internet alliance and openfog alliance, as well as the advanced TCA of PICMG Founding member of the advancedmc and MicroTCA subcommittees.
Responsible editor: CT