LiFi (Light Fidelity)

What is LiFi?

LiFi (Light Fidelity) is a wireless communication technology that uses visible light to transmit data. Instead of radio waves like WiFi, LiFi modulates the intensity of LED light at speeds imperceptible to the human eye (millions of times per second), creating a high-speed data connection. The term was coined by Professor Harald Haas of the University of Edinburgh, who demonstrated the concept at TED Global in 2011.

LiFi exploits the vast visible light spectrum (400–800 THz) — approximately 2,600 times larger than the entire radio frequency spectrum allocated for wireless communication. Each LED luminaire becomes an independent access point, creating a dense network of high-capacity cells that can deliver aggregate data rates exceeding 100 Gbps in research demonstrations.

How LiFi Works

A LiFi system consists of an LED transmitter (the "access point"), a photodiode receiver, and a driver circuit that modulates the LED current at high frequency. The modulation is fast enough that the human eye perceives only constant illumination — the LED appears to be simply "on" while simultaneously carrying data at megabit or gigabit rates.

• LED modulation bandwidth: A white phosphor-LED has a modulation bandwidth of only 1–20 MHz due to the slow decay of the yellow phosphor coating. This limits raw data rates to 10–50 Mbps without advanced modulation. Blue-filtered LEDs or RGB LEDs achieve 100+ MHz bandwidth, enabling 100+ Mbps with OOK
• Visible spectrum: Uses the 400–800 THz visible light spectrum (vs WiFi's 2.4/5 GHz radio). The usable optical bandwidth per LED is 10–200 MHz depending on modulation technique
• Downlink: The ceiling-mounted LED bulb transmits data by modulating light intensity. Typical modulation schemes include OOK (On-Off Keying) for simplicity and OFDM for high spectral efficiency
• Uplink: Most implementations use an infrared (IR) LED (850–940nm) on the device to transmit data back to the access point, operating outside the visible spectrum to avoid interference with the downlink
• Receiver: A silicon PIN photodiode or avalanche photodiode on the device converts light intensity variations back to electrical signals. Receiver bandwidth of 50–500 MHz is typical

Modulation Techniques

The choice of modulation scheme determines the achievable data rate, spectral efficiency, and robustness against ambient light interference:

• OOK (On-Off Keying): Simplest scheme — LED is either on (1) or off (0). Achieves 1–10 Mbps with standard white LEDs. Low complexity but poor spectral efficiency (1 bit/s/Hz)
• PPM (Pulse Position Modulation): Information encoded in the position of a pulse within a time slot. Better power efficiency than OOK but lower data rate. Used in low-power LiFi sensors
• CSK (Color Shift Keying): Modulates the relative intensities of red, green, and blue LEDs in an RGB LED. Maintains constant total luminous flux (no flicker) while encoding data. Defined in IEEE 802.15.7
• DCO-OFDM (DC-biased Optical OFDM): Adds a DC bias to the bipolar OFDM signal to make it unipolar (LEDs can only emit positive intensity). Achieves 100+ Mbps. Requires high peak-to-average power ratio (PAPR)
• ACO-OFDM (Asymmetrically Clipped Optical OFDM): Clips negative half of OFDM signal without DC bias. More power-efficient than DCO-OFDM but uses only half the subcarriers. Achieves 50+ Mbps
• Flip-OFDM: Transmits the positive and negative halves of a real-valued OFDM signal in separate slots. Similar to ACO-OFDM but with different framing

IEEE 802.11bb Standard

In 2023, the IEEE published 802.11bb — the first international standard for LiFi as a radio frequency (RF) complement within the 802.11 family. Key features:

• PHY layer: Supports OOK, CSK, and OFDM modulation at data rates from 1 Mbps to over 40 Gbps
• Wavelength: 380–700 nm visible light; optional IR for uplink
• MAC layer: CSMA/CA compatible with 802.11 infrastructure; enables seamless handover between LiFi and WiFi access points
• Integration: LiFi access points connect to existing WiFi/Ethernet infrastructure via standard 802.3 Ethernet
• Handover: Station handover between LiFi APs within 50 ms, enabling mobility

Advantages Over WiFi

• Speed: Lab demonstrations exceed 100 Gbit/s (using micro-LED arrays); commercial products achieve 150+ Mbit/s per LED
• Security: Light cannot penetrate walls, glass, or opaque barriers — the signal is physically contained within a room. Eavesdropping requires physical presence inside the room
• Interference: No electromagnetic interference with sensitive equipment (hospitals, aircraft cockpits, industrial control). No conflict with existing RF services
• Density: Each light bulb is an independent access point — a typical office ceiling with 50 lights creates 50 independent cells, each delivering full bandwidth. No shared-medium contention
• Efficiency: Uses existing LED lighting infrastructure — the lighting and communication functions are served simultaneously. No additional spectrum allocation or licensing required
• Directionality: Narrow beam angles reduce interference between adjacent cells and improve spatial reuse

Limitations

• Line of sight: Requires direct or reflected light path to the receiver. Direct path provides highest data rate; reflected paths suffer multipath dispersion and reduced SNR
• Sunlight interference: Bright ambient light (direct sunlight >100 klux) saturates the photodiode receiver, reducing SNR. Mitigation: optical bandpass filters and differential detection
• Uplink: Most implementations still use RF (WiFi or IR) for the return path, limiting full LiFi symmetry
• Range: Typically limited to a single room per access point (3–5 meters vertical distance). Effective range depends on LED power and receiver sensitivity
• Shadowing: Objects blocking the direct optical path cause signal loss. Multi-LED arrays and diffuse reflections provide some robustness
• Standardization: IEEE 802.11bb is very new (2023); ecosystem maturity lags WiFi by a decade

Real-World Deployments

• Hospitals: Oledcomm deployed LiFi in French hospitals (Lille, Paris) where WiFi RF interference with medical equipment (infusion pumps, patient monitors, MRI) is a concern. LiFi provides data connectivity without RF emissions in patient areas
• Schools: LiFi classrooms in Paris provide high-speed connectivity to students without electromagnetic interference. Each desk LED doubles as a learning lamp and data access point
• Offices: PureLiFi's Light Antenna products provide enterprise-grade indoor wireless. A single ceiling luminaire serves as both illumination and a multi-gigabit access point
• Aircraft: LiFi in-seat entertainment systems reduce wiring weight (each WiFi access point adds ~0.5 kg of cabling). Air France has trialed LiFi for passenger connectivity
• Industrial: Factory floors with heavy electromagnetic interference from motors and welding equipment benefit from LiFi immunity to RF noise

How LiFi Works: The Hardware Chain

A complete LiFi link requires careful engineering at both transmitter and receiver. The hardware chain converts digital data to modulated light and back:

• LED driver circuit: The transmitter contains a high-speed current driver that modulates the LED bias current at the required frequency. A Class-AB amplifier topology provides linear modulation around the DC bias point (typically 350mA for a 1W white LED). The driver must maintain linearity to within 1% for OFDM — nonlinear distortion causes intermodulation products that corrupt subcarriers. A bias tee combines the DC bias (from a constant-current source) with the AC data signal (from the DAC/FPGA). The DAC sample rate must be 2× the modulation bandwidth (Nyquist): a 100 MHz LiFi link requires a 200 MSPS DAC, typically implemented in an FPGA
• Photodetector at receiver: The receiving device uses a silicon PIN photodiode (Responsivity: 0.4–0.6 A/W at 450nm) or an avalanche photodiode (APD, gain 10–100×) to convert incoming photons to electrical current. The photodiode is connected to a transimpedance amplifier (TIA) that converts the photocurrent (nanoamps to microamps) to a voltage signal. The TIA bandwidth must match the LiFi modulation bandwidth — a 100 Mbps link requires a TIA with at least 70 MHz bandwidth. A transimpedance gain of 10–50 kΩ is typical, balancing sensitivity against noise. An optical bandpass filter (400–550 nm for blue-filtered systems) rejects ambient light and sunlight, improving SNR by 10–20 dB
• Modulation/demodulation chain: At the transmitter: digital data → LDPC/Turbo encoder → QAM mapper → IFFT (OFDM) → DAC → LED driver → modulated light. At the receiver: photodiode → TIA → ADC → FFT (demodulation) → QAM demapper → LDPC/Turbo decoder → digital data. The entire chain is implemented in real-time on an FPGA or ASIC. Latency is typically <1 ms for the modulation/demodulation path, comparable to WiFi
• Why visible light provides massive bandwidth: The visible light spectrum spans 400–800 THz (380–780 nm wavelength). This is approximately 400,000 THz of raw optical bandwidth — compared to the entire RF spectrum allocated for wireless (roughly 100 GHz total). Even though each LED only modulates a tiny fraction of this bandwidth (10–200 MHz), the spatial reuse is extraordinary: each of the 50+ LED cells in an office provides independent bandwidth, giving an aggregate capacity that scales with the number of lights rather than competing for shared spectrum

LiFi Hardware Chain:

  TRANSMITTER (Ceiling LED Luminaire):
  Ethernet ──► FPGA ──► DAC ──► LED Driver ──► White LED
               │                    │
               │  OFDM modulation   │  350mA DC bias
               │  LDPC encoding     │  + AC signal
               └────────────────────┘

  RECEIVER (Device):
  Photodiode ──► TIA ──► ADC ──► FPGA ──► Ethernet
      │            │               │
   400-550nm    10-50kΩ        OFDM demod
   bandpass     transimpedance  LDPC decode
   filter       amplifier

LiFi Standards

LiFi standardization is evolving across two distinct standards bodies, reflecting the technology's dual nature as both a lighting function and a wireless network:

• IEEE 802.15.7 — Visible Light Communication: The original VLC standard, published in 2011 and revised in 2018. Defines three PHY types: PHY I (OOK, 11.67–266.67 kbit/s), PHY II (VPPM/CSK, 1.25–96 Mbit/s), and PHY III (CSSK, up to 96 Mbit/s). The MAC layer supports CSMA/CA channel access, beacon-based synchronization, and dimming control. 802.15.7 targets low-to-medium rate applications: sensor networks, indoor positioning, and signage. It does not support the high data rates or handover mechanisms required for enterprise LiFi networking
• IEEE 802.11bb — Light Fidelity: Published in 2023 as an amendment to the 802.11 WiFi family. Defines LiFi as a complementary PHY layer alongside WiFi (802.11a/b/g/n/ac/ax). Key specifications: modulation OOK, CSK, and OFDM at data rates from 1 Mbps to 40 Gbps, wavelength 380–700 nm (visible) with optional IR uplink, CSMA/CA MAC compatible with existing WiFi infrastructure, and inter-AP handover within 50 ms. 802.11bb LiFi access points connect to standard Ethernet switches and appear as regular 802.11 infrastructure to clients. This enables seamless LiFi-WiFi roaming — a device moves from WiFi coverage into a LiFi cell and back without dropping the connection
• How LiFi coexists with fluorescent lighting: Fluorescent lights produce light through mercury vapor discharge, which emits UV radiation that excites a phosphor coating. The discharge inherently flickers at twice the AC mains frequency (100 Hz or 120 Hz depending on region). This flickering produces optical noise at 100–120 Hz and its harmonics (up to ~10 kHz). LiFi modulation operates at MHz frequencies — orders of magnitude above fluorescent flicker harmonics. An optical bandpass filter on the LiFi receiver rejects the broadband fluorescent emission outside the LiFi wavelength band. Additionally, LiFi uses differential detection or DC-blocking filters to remove the slowly-varying fluorescent ambient component. CFL (compact fluorescent lamp) and LED lights present different challenges: CFLs produce broadband visible light with 120 Hz flicker, while white phosphor LEDs produce a smooth spectrum with minimal flicker. LiFi systems are designed to reject both types of ambient noise
• Coexistence with sunlight: Direct sunlight produces approximately 100,000 lux of broadband visible illumination — far brighter than any LiFi LED. Without filtering, sunlight saturates the photodiode receiver and destroys the LiFi signal. Solutions include: narrow optical bandpass filters (10–50 nm bandwidth centered on the LiFi LED wavelength), differential photodetector pairs that cancel common-mode ambient light, and automatic gain control (AGC) that adjusts the receiver gain based on ambient light level. In practice, LiFi works reliably indoors where ambient light is 100–1,000 lux. Direct sunlight exposure (>50,000 lux) through a window degrades performance but does not eliminate it when filtering is used

Real-World Performance

Laboratory LiFi speeds of 1+ Gbps are impressive, but real-world performance depends heavily on environmental conditions. Understanding these factors is critical for practical deployment planning:

• Attenuation through materials: Visible light cannot pass through opaque materials. Clear glass transmits 80–90% of visible light, making LiFi viable through office windows and glass partitions. Tinted glass (common in modern offices) transmits 30–60% depending on tint density. Polycarbonate and acrylic panels transmit 70–90%. Drywall, wood, and metal are completely opaque to LiFi. Fog and smoke scatter visible light, reducing range by 10–30 dB/m in dense fog. Rain attenuates at approximately 3 dB/km — negligible for indoor LiFi but relevant for outdoor VLC links
• How furniture blocks LiFi: Office furniture, human bodies, and equipment create shadow zones where the direct optical path from the ceiling LED to the device is blocked. A person standing between the LED and receiver can attenuate the signal by 20–40 dB. Desks, partitions, and shelving similarly create dead zones. Mitigation strategies include: multi-LED arrays (3–4 LEDs per luminaire covering overlapping sectors), diffuse ceiling reflections (white surfaces reflect 70–85% of visible light), and handoff to adjacent LiFi cells when the primary cell is shadowed. In practice, a well-designed LiFi deployment uses overlapping cells with 1.5–2× coverage margin
• Cell handoff problem: When a user moves between LiFi access points, the device must hand off from one cell to another without dropping the connection. Unlike WiFi (where cells overlap substantially and RF propagation is gradual), LiFi cells have sharp boundaries — the device is either in the LED cone or it isn't. The handoff must happen within 50 ms (802.11bb specification) to avoid TCP connection drops. This requires: pre-scanning of adjacent cells (the device's photodetector monitors signal strength from neighboring LEDs), predictive handoff algorithms that initiate handoff before the current cell signal degrades below threshold, and coordinated MAC-level signaling between adjacent LiFi APs. In research prototypes, handoff latency of 5–20 ms has been achieved. In commercial deployments, the handoff is managed by the same controller that handles WiFi roaming, ensuring consistent behavior
• Mobility impact on throughput: Stationary devices achieve the highest LiFi data rates because the direct optical path provides maximum SNR. Walking while connected reduces throughput due to: rapid SNR fluctuations as the device moves through interference nulls, handoff interruptions when crossing cell boundaries, and angular dependence of LED emission (intensity drops as the cosine of the angle from the LED normal). A user walking at 1.5 m/s through an office with 3-meter cell spacing experiences handoffs every ~2 seconds, with each handoff causing a 10–50 ms throughput interruption. The aggregate throughput during mobility is typically 30–50% of the stationary peak rate
• Mixed LiFi-WiFi performance: In practice, LiFi is most effective as a complement to WiFi rather than a replacement. The optimal strategy uses LiFi for high-bandwidth stationary applications (desk work, video conferencing) where the device is within the LED cell, and WiFi fallback for mobility, roaming, and areas without LiFi coverage. 802.11bb enables this by defining a unified MAC layer — the device sees LiFi and WiFi as a single network with different PHY layers. Automatic band steering directs devices to LiFi when signal quality is sufficient, and falls back to WiFi when LiFi conditions degrade

LiFi Performance vs Environment:

  Condition                    Throughput     SNR
  ──────────────────────────   ────────────   ──────
  Stationary, direct path      43 Mbps        35 dB
  Stationary, 45° angle        30 Mbps        28 dB
  Walking (1.5 m/s)            15-25 Mbps     20-30 dB
  Through clear glass           35 Mbps        30 dB
  Through tinted glass          15 Mbps        18 dB
  Shadowed (person blocking)   0 Mbps (handoff)
  Direct sunlight (>50 klux)   5-10 Mbps      10-15 dB
  Dense fog                    2 Mbps         8 dB