# Li-Fi

## Summary


Li-Fi, or Light Fidelity, uses light as a medium of data communication. It comes under the umbrella of Optical Wireless Communication (OWC) that includes infrared, visible light and ultraviolet spectrum. Li-Fi uses visible light spectrum in the downlink and infrared spectrum in the uplink. 

Li-Fi promises to achieve data rates in excess of 100Gbps. In 2014, 10Gbps was achieved in a lab environment. A year later, Li-Fi achieved 224Gbps. These rates are many times what current Wi-Fi technology can achieve. For this reason, Li-Fi can be categorized as gigabit communication technology. 

Li-Fi is expected to complement cellular and Wi-Fi technologies, not replace them. While some proprietary Li-Fi products exist in the market, Li-Fi has still some way to go to become standardized and mainstream.

## Discussion

### Why do we need Li-Fi when plenty of wireless technologies exist today?

Among the widely deployed wireless technologies are cellular, Wi-Fi and Bluetooth. These operate in radio frequencies less than 10GHz. This RF spectrum is getting increasingly crowded, and there's a growing demand for more capacity and higher speeds. The visible light spectrum is an appealing alternative. Visible spectrum's capacity is 10,000 times larger. In fact, it's about 670THz of license-free spectrum. 

Cellular can offer higher capacity by making cells smaller but this incurs heavy investment on infrastructure such as base station equipment and transmission towers. LED lighting is becoming common worldwide and this infrastructure can be reused for Li-Fi. This also means that off-the-shelf components such as LEDs can be used to keep costs down. In contrast, millimetre wave technology at 60GHz requires specialized components. 

From the perspective of availability, we're often asked to turn off wireless transmitting devices in aircraft, hospitals and other places where EMI can cause problems. Light is everywhere and there's no problem using it in these places. Line-of-sight and short range characteristics of Li-Fi imply better security and less co-channel interference. 


### What are some use cases of Li-Fi?

Applications that require hundreds of Mbps throughput or microseconds of latency will benefit from Li-Fi. In dense urban areas, Li-Fi indoor attocells can complement cellular infrastructure. Li-Fi can be used for cellular or attocell backhauls. 

Li-Fi can enable Augmented Reality applications. Lighting in a museum can give rich information and interaction around exhibits. At malls, offers can be delivered via Li-Fi. Many household appliances can get connected via Li-Fi, thus enabling the Internet of Things. Li-Fi can also be used for indoor location tracking and navigation. Li-Fi can enable smart wearables. 

Because Li-Fi cannot get through walls, it suits applications that require high security. Only those within the beam of the transmitter can receive data. In environments where radio waves are considered harmful, Li-Fi can be a safe alternative. Radio waves are easily attenuated underwater but this problem doesn't exist for Li-Fi. Vehicle lights can be used for vehicle-to-vehicle, vehicle-to-traffic-light communication and thereby enable autonomous driving technology. 


### What are some important technical details of Li-Fi?

    + Spectrum: Visible spectrum is approximately in the range 400-700nm or 430-750THz. Spectrum above 3THz is unregulated.
    + Bandwidth: LED BW is limited to 20MHz but system BW of 180MHz with 512 subcarriers of DMT waveform has been possible.
    + Area spectral efficiency: At least 0.41 bits/s/Hz/m<sup>2</sup>, which is already 1000x of RF systems.
    + Modulation: Simple methods such as OOK or PWM experience ISI at high data rates. Instead, OFDM or DMT can be used to control light intensity rather than just turning LEDs on and off. There's been significant research to make these methods unipolar. DMO-OFDM is an example.
    + Multiple access: OFDMA is a suitable method and it's already used in IEEE 802.11 and LTE standards. An alternative method would be Wavelength Division Multiple Access (WDMA).
    + Duplexing: Wavelength Division Duplexing (WDD) is the proposed method. Visible spectrum is used for the downlink while IR or RF spectrum is used for the uplink.
    + Topology: Peer-to-peer, star or broadcast.

### Is Li-Fi a proprietary technology or is there a standard for it?

In 2011, IEEE Task Group (TG) 7 released the 802.15.7 standard titled "Short-Range Wireless Optical Communication Using Visible Light". In March 2017, IEEE Task Group 13 started looking into making a new standard 802.15.13 titled "Multi-Gigabit/s Optical Wireless Communications". 

It's been said that IEEE 802.15.7m is focusing on optical camera communications and low rate photodiode communications while IEEE 802.15.13 is for speciality networks for industrial applications. 

The IEEE 802.11 is also involved in standardizing Li-Fi to complement Wi-Fi. This groups suggests that using 802.11 as the base would be a plus for mass deployment and to complement RF spectrum. 

Meanwhile, existing Li-Fi products and their deployments must be considered as proprietary. Any attempt to standardize Li-Fi will likely require support from patent owners.


### How does Li-Fi stack up against Wi-Fi?

Wi-Fi is based on RF, which relies on electric fields to carry information. Li-Fi relies on optical intensity. RF is complex-valued and bipolar while Li-Fi is real-values and unipolar (non-negative). 

Li-Fi has the advantage over Wi-Fi in terms of capacity: 10,000 times more capacity. Visible light spectrum is being used for lighting but not for communication yet. Hence this spectrum is free of interference. Wi-Fi IEEE 802.11ac has a theoretical maximum throughput of about 7Gbps but practical values are at best 200Mbps. 

LED lighting infrastructure is already installed in many places and can be reused for Li-Fi communications. The cost of LEDs is also dropping. Moreover, solar panels can be used as Li-Fi receivers for data sent from laser stations or lamp posts. Hence Li-Fi has a better proposition in terms of cost and efficiency. Li-Fi can also be used in places such as aircraft and hospitals where EMI due to RF is usually a concern. For location-aware services, Li-Fi may complement iBeacon. 

Li-Fi can be seen as complementing Wi-Fi. Reasearch is ongoing regarding hybrid Li-Fi/Wi-Fi networks. 


### What are the common criticisms of Li-Fi?

Li-Fi systems need to emit and capture light, implying that they can't be used at night. IR spectrum could be used at night but at the expense of throughput. In bright sunlight, Li-Fi systems may not work that well due to light interference. Li-Fi has limited range and requires line-of-sight. However, the use of OFDM along with M-QAM can enable non-LOS communications. Concerns about lower LED energy efficiency when used for Li-Fi have been dispelled. 

While Harald Haas has claimed that existing lighting infrastructure can be used, the reality is that LEDs come in many varieties and there will be compatibility issues with Li-Fi adapters. Moreover, electrical wiring will need to be converted to network wiring for the transmission of data. Consumer devices will need to be replaced to handle Li-Fi signals. 

For immediate adoption, a temporary issue is that Li-Fi is not a mature technology and the standards are not yet in place. When Li-Fi last mile is achieving hundreds of gigabits per second, backhaul infrastructure also needs to scale equivalently. However, backhauls reaching a few terabits per second have been reported. 


### What are essential components of Li-Fi?

Li-Fi uses LEDs as transmitters and photodetectors/photodiodes as receivers. The former can be phosphor-coated LEDs or RGB LEDs. For receivers, photodiodes can be p-intrinsic-n (PIN) photodiodes or avalanche photodiodes (APD). Off-the-shelf components can be used for these but they need to be driven by signal processing firmware to make them communicating devices. Recent advancements include Resonant Cavity LEDs and MicroLEDs that achieve higher data rates. LEDs have a trade-off between output power and optical efficiency. To solve this, Laser Diodes (LD) have been proposed to target more than 100Gbps data rates. 

## Milestones

Aug  
2003

Japanese researchers report the use of white LEDs to transmit information at 400Mbps while also serving the primary purpose of indoor lighting. 

Nov  
2003

The Visible Light Communications Consortium (VLCC) is established, comprising of major companies in Japan. The aim is to provide communication capability through LED lightings. 

2006

OFDM is first used for VLC since the high crest factor that's usually a problem for radio communications is not an issue for VLC when intensity modulation and direct detection is used. 

2009

Scottish Enterprise funds a project named "D-Light". Project work begins in January 2010 at the University of Edinburgh. Project objectives mention the use of off-the-shelf LEDs to achieve 100Mbps under normal lighting, BER of 10E-4, 1-4m range and real-time signal processing. Thus, the project has a strong practical rather than academic focus. 

Jul  
2011

At TEDGlobal event, Harald Haas coins the term *Li-Fi*, which stands for *Light Fidelity*. He explains four limitations with radio communications: capacity, efficiency, availability, security. He claims that Li-Fi overcomes these limitations. A few months later, D-Light project member Gordon Povey announces that project goals have been met with a measured data rate of 102.5Mbps. 

Oct  
2011

The Li-Fi Consortium is launched to promote Optical Wireless Communications (OWC). While Visible Light Communications (VLC) considers only the visible spectrum, OWC includes infrared as well. 

2012

Italian researchers achieve 1Gbps data rate using Discrete Multitone (DMT) modulation. 

Feb  
2015

Researchers at the University of Oxford report Li-Fi speed of 224Gbps by aggregating six channels with each channel achieving 37.4Gbps. 

Sep  
2015

Harald Haas demonstrates the use of solar cells as Li-Fi receivers while at the same time converting light energy to electrical energy. 

Mar  
2017

Company PureLiFi claims to deliver 45Mbps with its Li-Fi products. 

Jul  
2017

IEEE 802.11 Light Communication(LC) Topic Interest Group (TIG) publishes a report on OWC. The report points out why 802.11 may be better than alternatives that other IEEE groups have proposed. This is an input to IEEE 802.11 Working Group (WG) for potential standardization of OWC.