Adaptive Li-Fi Links for Fluctuating Room Light

ISEF Category: Embedded Systems

Subcategory: Optics  ·  Difficulty: Advanced  ·  Setup: University Lab  ·  Time: Full Year

The Hook

Your room lights can act like a hidden noise source for a light-based internet link. That means your data rate can change even when your LED and photodiode stay the same. If you can sense that noise fast enough, your receiver can pick the best modulation on the fly. That turns a simple demo into a real communication system.

What Is It?

Li-Fi sends data through visible light instead of radio waves. You blink an LED so fast that your eyes only see steady light, while a photodiode turns those flashes back into electrical signals. Think of it like tapping a message in Morse code, but at speeds your eyes cannot follow.

The key idea here is adaptation. OOK, or on-off keying, uses two signal levels. PAM-4 uses four levels, so it can carry more bits per symbol, but it also needs cleaner signals. OFDM, or orthogonal frequency division multiplexing, splits data across many subcarriers, which can help when a channel gets messy. Your receiver MCU estimates ambient-light noise, then chooses the modulation that should work best in that moment.

Why This Is a Good Topic

This topic gives you a clear engineering problem with real numbers to measure. You can test how lighting changes affect signal quality, data rate, and error rate, then compare modulation schemes under the same setup. That makes the project testable, data-heavy, and easy to explain. It also connects to wireless communication, smart lighting, and low-cost sensor networks.

Research Questions

• How does ambient room lighting affect bit error rate in an LED-photodiode Li-Fi link?
• What is the effect of switching from OOK to PAM-4 on throughput at the same received optical power?
• Does adaptive modulation chosen by the receiver MCU reduce packet errors compared with fixed modulation?
• To what extent does flicker from fluorescent or LED room lights change the receiver's noise estimate?
• Which modulation order gives the best throughput under low, medium, and high ambient-light noise?
• How does photodiode biasing affect the signal-to-noise ratio of the received Li-Fi signal?
• What is the effect of adding a simple optical filter on adaptive modulation performance?

Basic Materials

• High-brightness LED or LED module.
• Photodiode with transimpedance amplifier or ready-made photodiode sensor.
• Microcontroller board with ADC and PWM output.
• Breadboard and jumper wires.
• Resistors and capacitors for signal conditioning.
• Stable DC power supply or USB power source.
• Smartphone or lux meter app for approximate light readings.
• Laptop for data logging and analysis.
• Oscilloscope or logic analyzer for checking waveforms.
• Desk lamp and adjustable room lighting sources.

Advanced Materials

• High-speed photodiode with documented bandwidth.
• Low-noise transimpedance amplifier components or evaluation board.
• Microcontroller with fast ADC and DMA support.
• Function generator for transmitter test patterns.
• Oscilloscope with persistence and FFT tools.
• Optical band-pass or neutral-density filters.
• Calibrated lux meter or photometric sensor.
• Motorized or programmable light source for repeatable ambient-light changes.
• Optical bench mounts and alignment hardware.
• Shielded enclosure or dark box for controlled measurements.

Software & Tools

• Python: Logs packets, calculates throughput, and graphs bit error rate versus ambient light.
• NumPy: Handles signal arrays and basic numerical analysis.
• Pandas: Organizes test runs, lighting levels, and modulation choices in tables.
• Matplotlib: Plots SNR, BER, and throughput trends clearly.
• ImageJ: Measures light intensity from photos if you build a simple optical calibration setup.

Experiment Steps

1. Define the channel conditions you want to test, then decide how you will measure ambient-light noise and data quality.
2. Choose a transmitter and receiver architecture that keeps the LED, photodiode, and MCU roles separate and easy to compare.
3. Build a baseline link with one fixed modulation so you can measure a fair starting point.
4. Create a switching rule that maps receiver noise estimates to OOK, PAM-4, or OFDM without changing other variables.
5. Plan controls that keep distance, alignment, and payload size consistent while you change room lighting.
6. Design your analysis so you can compare throughput, bit error rate, and adaptation overhead across lighting conditions.

Common Pitfalls

• Letting the receiver saturate under bright room light, which hides the signal you are trying to measure.
• Comparing modulation schemes with different packet sizes, which makes throughput numbers unfair.
• Changing LED alignment between trials, which mixes optical geometry with modulation effects.
• Ignoring ambient flicker from lamps, which can look like random noise if you do not sample carefully.
• Choosing a switching threshold for modulation that fits one room but fails when the lighting spectrum changes.

What Makes This Competitive

A stronger project goes beyond a simple speed test. You can build a real adaptation rule, then prove it beats fixed modulation under controlled lighting changes. You can also report more than throughput, like BER, latency, and adaptation overhead. If you test multiple light sources or add a noise model that predicts the best modulation, your work starts to look like systems research.

Project Variations

• Test the same adaptive Li-Fi idea with colored ambient light, such as warm and cool LEDs, to see whether spectrum matters as much as brightness.
• Swap the photodiode for a camera sensor or light sensor module and compare how sensor bandwidth changes adaptive performance.
• Keep the hardware fixed, then compare a simple threshold rule with a small ML classifier for choosing OOK, PAM-4, or OFDM.

Learn More

• MIT OpenCourseWare, Signals and Systems: Search MIT OpenCourseWare for lecture notes on sampling, modulation, and noise basics.
• NASA Glenn Research Center, Optics and photonics resources: Search the NASA website for educational pages on light, detectors, and optical communication.
• NIH PubMed: Search PubMed for review articles on visible-light communication and optical wireless communication.
• IEEE Xplore abstracts and author manuscripts: Search for review papers on Li-Fi modulation, BER, and adaptive communication links.
• USGS Spectral data library: Use it to understand how different light sources and filters change spectral content.