Visible Light Communication

What is VLC?

Visible Light Communication (VLC) uses the visible light spectrum (400–800 THz, wavelengths 380–780 nm) to transmit data. Any LED light source — ceiling lights, traffic signals, display screens, vehicle headlights — can be modulated at high speeds to encode information, turning ordinary illumination into a wireless communication channel. VLC is the foundational technology behind LiFi, but it's broader than LiFi alone.

VLC encompasses any visible light data transmission, including low-data-rate applications like indoor positioning, vehicle-to-vehicle communication, and underwater signaling. The IEEE 802.15.7 standard defines the PHY and MAC layers for short-range optical wireless communication using visible light, supporting data rates from 11.67 kbit/s to 96 Mbit/s.

How VLC Works

The fundamental principle is intensity modulation with direct detection (IM/DD). The transmitter varies the LED's optical intensity at high speed; the receiver photodiode detects intensity variations and converts them to electrical signals. Because LEDs emit incoherent light (no phase coherence), VLC cannot use coherent detection techniques from fiber optics — only the amplitude/intensity of the light carries information.

• Transmitter: An LED light source (ceiling light, traffic light, display screen) is rapidly switched on and off or modulated in intensity. The LED must simultaneously provide illumination and data — the modulation depth is limited to avoid perceptible flicker (<3% variation at >200 Hz)
• Modulation: Data is encoded by varying the LED's intensity. The simplest scheme is OOK (On-Off Keying) — the LED is either on or off. More sophisticated schemes include PPM (Pulse Position Modulation), CSK (Color Shift Keying), and OFDM for higher spectral efficiency
• Channel: Modulated light propagates through the air. The channel includes direct line-of-sight paths and diffuse reflections from walls, ceilings, and surfaces. Diffuse paths provide coverage but suffer multipath distortion
• Receiver: A silicon PIN photodiode or CMOS image sensor detects the light fluctuations and demodulates the data. Image sensors can simultaneously capture data from multiple LED sources (spatial multiplexing)

Modulation Schemes

VLC systems use several modulation techniques, each optimized for different data rate and complexity requirements:

• OOK (On-Off Keying): Simplest — LED on = 1, off = 0. Achieves 1–10 Mbps. Low complexity but flicker risk at low data rates if not carefully managed
• PPM (Pulse Position Modulation): A pulse occupies one of M time slots per symbol. Better power efficiency than OOK but lower spectral efficiency. M-PPM achieves log₂(M)/M bits/symbol
• CSK (Color Shift Keying): Modulates the relative intensities of red, green, and blue LEDs. Maintains constant total luminous flux (no flicker) while encoding data. Defined in IEEE 802.15.7 PHY II. Data rate: 1.25–96 Mbps
• CAP (Carrierless Amplitude Phase): Uses orthogonal filter pairs to create two independent data streams on the same LED. Achieves 50+ Mbps without requiring high-speed ADCs
• DCO-OFDM: Adds DC bias to unipolar OFDM signal. High spectral efficiency but high PAPR. Achieves 100+ Mbps in research systems

IEEE 802.15.7 Standard

IEEE 802.15.7-2018 defines the physical layer (PHY) and medium access control (MAC) for short-range optical wireless communication using visible light. It supports data rates from 11.67 kbit/s to 96 Mbit/s and defines three PHY types:

• PHY I (OOK): Binary OOK with Manchester coding. Data rates: 11.67–266.67 kbit/s. Used for low-rate applications like sensor networks and indoor positioning
• PHY II (CSK/VPPM): Variable PPM and Color Shift Keying. Data rates: 1.25–96 Mbit/s. Used for high-rate data communication. VPPM (Variable Pulse Position Modulation) combines PPM with PWM for dimming control
• PHY III (CSK): Color Shift Keying with spectral efficiency up to 4 bits/s/Hz. Supports simultaneous illumination and communication with flicker constraint
• MAC layer: Supports CSMA/CA channel access, beaconing for synchronization, and association/disassociation procedures. Handles coexistence of multiple VLC networks and dimming control

Applications Beyond LiFi

• Indoor positioning: VLC-based positioning achieves 10–30 cm accuracy for indoor navigation. Each LED transmits a unique ID code; a smartphone camera or photodiode receiver determines position by triangulating received signal strengths. Better than GPS (which doesn't work indoors) and WiFi fingerprinting (1–3m accuracy)
• Vehicle-to-vehicle (V2V): Using car headlights and taillights to communicate speed, braking, and trajectory data. LED headlights modulated at 10–100 kHz can transmit basic safety messages at 1–10 kbps over 50–100m. No spectrum license required
• Smart cities: Street lights as data access points and environmental sensors. LiFi-enabled LED streetlights provide both illumination and free public WiFi. Seoul, San Diego, and Dublin have deployed smart streetlight networks
• Underwater communication: Radio waves don't propagate well underwater (skin depth ~0.2m at 1 GHz), but blue-green light (450–550 nm) penetrates seawater with attenuation of 0.01–0.1 dB/m. Underwater VLC achieves 10 Mbps over 100m in clear water
• Screen-to-device: High-speed data transfer from displays to smartphones via visible light. A digital signage screen can transmit URLs, coupons, or supplementary content to a phone's camera at 10–100 kbps
• Signage: Retail displays that transmit product information to nearby phones. Museum exhibits that trigger audio/video content on visitors' devices
• Aircraft cabin: LED reading lights modulated to provide in-flight internet access without additional RF emissions that could interfere with avionics

VLC vs WiFi vs Bluetooth

• Bandwidth density: VLC offers 10–100× higher bandwidth density than WiFi in indoor environments. Each LED cell provides full bandwidth independently, while WiFi bandwidth is shared among all users
• Security: Light is naturally contained within rooms — no wall-penetrating signals. Physical eavesdropping requires line-of-sight access to the room. WiFi signals penetrate walls and can be intercepted from adjacent rooms
• Interference: No electromagnetic interference with existing radio systems. Critical for hospitals, aircraft, and industrial environments with sensitive RF equipment
• Infrastructure: Uses existing LED lighting — no additional spectrum allocation needed. Buildings already have LED ceiling lights; adding VLC is a firmware upgrade
• Limitation: Requires line of sight or reflected light; doesn't work in darkness. Can't replace WiFi for mobile or whole-building coverage

Standards

IEEE 802.15.7-2018 defines the physical layer (PHY) and medium access control (MAC) for short-range optical wireless communication using visible light. It supports data rates from 11.67 kbit/s to 96 Mbit/s and defines three PHY types for different applications. The IEEE 802.11bb standard (2023) extends this to high-rate LiFi for enterprise networking integration with WiFi infrastructure.

LiFi vs VLC: The Distinction

VLC and LiFi are often used interchangeably, but they are not the same thing. VLC is the general term for any visible light data communication, while LiFi is a specific brand name coined by Professor Harald Haas and trademarked by pureLiFi. All LiFi is VLC, but not all VLC is LiFi — the distinction matters for standards, product categorization, and understanding the technology landscape:

• VLC as the umbrella: VLC encompasses everything from low-data-rate indoor positioning (transmitting LED IDs at a few kbps) to vehicle-to-vehicle headlight communication, underwater optical links, and screen-to-device transfers. It uses any visible light source — traffic signals, displays, hand-held flashlights — not necessarily dedicated networking equipment
• LiFi as networking: LiFi specifically refers to bidirectional, high-speed wireless networking using ceiling-mounted LED luminaires as access points. LiFi requires a photodetector on the device for downlink reception and typically an IR LED for uplink transmission. It targets WiFi-replacement speeds (100+ Mbps) with enterprise-grade security and handover between access points
• IEEE standards split: IEEE 802.15.7 defines VLC for low-to-medium rate applications (sensor networks, positioning, signage). IEEE 802.11bb (2023) defines LiFi for high-rate networking within the 802.11 WiFi family. The two standards serve different use cases and do not interoperate
• Hardware differences: A VLC transmitter can be any modulated LED (traffic light, car headlamp). A LiFi access point is a specialized luminaire with an embedded FPGA or ASIC for real-time OFDM modulation, an optical bandpass filter, and a network interface (Ethernet or PoE). LiFi receivers include an avalanche photodiode with TIA (transimpedance amplifier) — far more sensitive than the simple CMOS camera sensor used in many VLC applications

How LED Flickering Encodes Data

The core principle of VLC is that LEDs can switch on and off millions of times per second — far faster than the human eye can perceive. This rapid switching (flickering) encodes binary data. The key engineering challenge is flickering fast enough for high data rates while maintaining constant perceived illumination:

• The flicker fusion threshold: Human vision perceives steady light when flicker frequency exceeds approximately 200 Hz (the flicker fusion threshold). VLC systems operate at modulation frequencies of 1–200 MHz — 1,000 to 1,000,000× above the perceptual threshold. The LED appears perfectly steady to any observer while carrying megabits of data
• Average intensity preservation: The data encoding must maintain the same average light output regardless of the data pattern. If transmitting a long run of 1-bits (LED on), the average brightness increases. Solutions include DC balancing (Manchester encoding ensures equal 0s and 1s), AC-coupled modulation (CSK maintains constant total flux across RGB channels), and preamble/sync patterns that enforce temporal balance
• Modulation depth tradeoff: Deeper modulation (larger intensity swing between 0 and 1) gives better SNR at the receiver but causes more perceptible flicker. The IEEE 802.15.7 standard limits modulation depth to maintain flicker-free illumination. Practical systems use 5–10% modulation depth at 200+ MHz, achieving 10–50 Mbps with OOK. CSK maintains 0% modulation depth by redistributing power between color channels rather than switching on/off
• LED response time: White phosphor LEDs have a modulation bandwidth of only 1–20 MHz due to the slow decay time of the yellow phosphor coating (τ ≈ 100ns). Blue-filtered LEDs (removing the phosphor component) achieve 50–100 MHz bandwidth. RGB LEDs (separate red, green, blue dies) can be independently modulated, enabling wavelength-division multiplexing. Micro-LEDs with smaller junction capacitance achieve 500+ MHz bandwidth in laboratory settings

LED Flickering for Data Encoding:

  Human perception:    ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓  (steady light)
  Actual modulation:   █░░░█░░░░░█░░░█░░░░░█  (20 MHz OOK)
  Data encoded:        1  0  0  1  0  0  0  1  0  0  0  0  1

  At 20 MHz, each bit = 50ns — 4,000× faster than eye can detect

Modulation Techniques for VLC

The modulation scheme determines the achievable data rate, power efficiency, and flicker characteristics of a VLC system. Each technique balances these tradeoffs differently:

• OOK (On-Off Keying): The simplest approach — LED fully on for 1, fully off for 0. Achievable data rate depends on LED bandwidth: 1–10 Mbps with standard white LEDs, 50–100 Mbps with blue-filtered or RGB LEDs. Manchester encoding (NRZ-to-RZ conversion) ensures DC balance by guaranteeing a transition every bit period. The disadvantage is that the LED turns fully off during 0-bits, reducing average illumination. Dimming-compatible variants (VPPM) adjust the pulse width to maintain target brightness while encoding data
• CSK (Color Shift Keying): Defined in IEEE 802.15.7 PHY II, CSK modulates three color channels (R, G, B) simultaneously while keeping their sum constant. A constellation diagram shows the available color states arranged in a circle — each point represents a specific R/G/B intensity combination. Moving between constellation points changes the color slightly but the total brightness never changes, eliminating flicker entirely. PHY II supports data rates from 1.25 to 96 Mbps using CSK or VPPM modulation. The receiver must have separate photodetectors with color filters for each channel, or a single detector with sequential RGB LED modulation
• OFDM (Orthogonal Frequency Division Multiplexing): The highest-performance VLC modulation. Divides the LED bandwidth into many narrow subcarriers (typically 64–1024), each modulated independently with QAM (Quadrature Amplitude Modulation). Because LEDs emit positive-only intensity, the bipolar OFDM signal must be made unipolar through DC biasing (DCO-OFDM) or asymmetric clipping (ACO-OFDM). DCO-OFDM achieves 100+ Mbps but requires high peak-to-average power ratio (PAPR), pushing the LED into nonlinear regions. ACO-OFDM clips the negative half of the signal, using only odd subcarriers — sacrificing half the bandwidth for better power efficiency

Practical LiFi Deployments

While VLC is a broad research field, LiFi is the commercialized, networking-focused application. Current deployments demonstrate real-world performance characteristics:

• The pureLiFi Light Antenna: PureLiFi's enterprise product is a ceiling-mounted luminaire that combines LED illumination with LiFi networking. It contains an FPGA for real-time OFDM signal generation, a blue-filtered white LED array for downlink (up to 43 Mbps per access point), and an IR photodiode array for device uplink. Each Light Antenna covers a 5–7 meter diameter cell with PoE (Power over Ethernet) connectivity. The system integrates with standard Ethernet switches and appears as a regular 802.11bb access point to connected devices
• Data rates in practice: Laboratory demonstrations have achieved 1 Gbps using RGB LEDs with wavelength-division multiplexing and 10+ Gbps using micro-LED arrays. In commercial deployments, realistic throughput is 15–50 Mbps per access point — comparable to 802.11n WiFi but with the advantage of dedicated bandwidth per cell. An office with 20 LiFi access points provides 20 × 43 Mbps = 860 Mbps aggregate throughput, with each user getting full bandwidth in their cell
• Requirements for deployment: LiFi requires (1) line-of-sight or strong reflected path to the LED luminaire, (2) an LED bulb with an embedded modulator driver and optical bandpass filter (standard LEDs cannot be retrofitted), (3) a photodetector on the receiving device (either built-in or a USB dongle), and (4) PoE-capable Ethernet infrastructure to power and connect each LiFi access point. The LED luminaire must be within 2–4 meters of the receiver for reliable high-speed connectivity
• Where LiFi is deployed today: Hospitals (Oledcomm in French hospitals — eliminates RF interference with medical equipment), offices (pureLiFi installations in UK and French companies), factories (where heavy machinery creates RF noise that degrades WiFi), and research labs. LiFi is not yet cost-competitive with WiFi for general consumer use — a LiFi access point costs $200–500 vs $50–100 for a WiFi 6E access point