Li-Fi (Light Fidelity) Wireless Communication System

This project implements a Li-Fi communication system using visible light as the transmission medium. An LED transmitter modulates data onto a carrier signal, while a photodiode receiver with analog front-end (AFE) circuitry demodulates and decodes the signal to display received text on an LCD. Demonstrates fundamental optical communication principles using embedded microcontrollers.

Core Concept & Objective

• Goal: Transmit short text messages wirelessly using intensity modulation of an LED, received by a photodiode and decoded by a microcontroller.
• Key Challenge: Convert digital text data into a modulated light signal that can be reliably detected, demodulated, and framed for error-free reception over short distances (1-2 meters).

Technical Design

• Transmitter Side:

  • Microcontroller (Arduino/ESP32) encodes text into frames with start/stop bits and simple checksum.
  • LED Modulation: PWM or direct digital output modulates LED brightness at a carrier frequency (e.g., 1-10 kHz).
  • Framing: Simple protocol with preamble (0xAA), length byte, payload, and CRC/checksum.

• Receiver Side:

  • Photodiode + AFE: Photodiode converts light intensity to current → transimpedance amplifier → bandpass filter → comparator → digital signal.
  • Microcontroller samples the demodulated signal, detects frame sync, extracts payload, validates checksum, and displays on LCD.
  • Signal Processing: Software UART-like decoding with bit timing recovery and frame synchronization.

• Communication Parameters:

  • Range: 1-2 meters (line-of-sight)
  • Data rate: ~100-500 bps (limited by LED/photodiode response and microcontroller sampling)
  • Modulation: On-Off Keying (OOK) with Manchester encoding for DC balance

Project Structure

├── transmitter/ │ ├── tx_main.ino # Text encoding + LED PWM modulation │ └── protocol.h # Framing and checksum functions ├── receiver/ │ ├── rx_main.ino # Photodiode signal processing + LCD display │ ├── afe_circuit.png # Analog front-end schematic │ └── timing_calib.cpp # Bit synchronization algorithm └── docs/ ├── protocol_spec.md # Frame format specification └── performance.md # BER vs distance measurements

Protocol Frame Format

[PREAMBLE: 0xAA x4] [LEN: 1B] [PAYLOAD: 0-32B] [CHECKSUM: 1B] Example: AA AA AA AA 05 "HELLO" 0x2A

Hardware Implementation

Transmitter: Arduino/ESP32 → PWM pin → Current-limiting resistor → High-brightness LED (white/blue) Receiver: Photodiode → Op-amp transimpedance → Bandpass filter → Comparator → MCU interrupt pin MCU → I2C/SPI → 16x2 LCD (HD44780)

Example Operation

1. TX: User enters "HELLO" → framed → modulated onto LED at 2 kHz carrier
2. RX: Photodiode detects blinking → AFE cleans signal → MCU syncs to preamble → extracts "HELLO" → displays on LCD
3. Distance Test: Reliable up to 1.5m in normal room lighting

Key Results

• Reliable range: 1.2-1.8 meters (line-of-sight)
• Bit Error Rate (BER): <1% at 1m distance
• Max payload: 32 characters per frame
• Latency: ~200ms end-to-end for 20-char message

Skills Demonstrated

Optical communication principles • LED modulation (PWM/OOK) • Photodiode analog front-end design • Digital signal processing on MCU • Framing and error detection protocols • Embedded protocol design • Hardware-software integration • Bit timing recovery