LI-FI (Light Fidelity)

Li-Fi is an advanced light-based communication system capable of transmitting data at high speeds across the visible light, ultraviolet, and infrared spectrums. This wireless technology leverages light waves to facilitate data transfer and positioning between devices. The concept of Li-Fi was first introduced by Professor Harald Haas during a TEDGlobal talk in Edinburgh in 2011.

Li-Fi has the potential to provide high-speed internet connections, even in remote locations. Utilizing common household LED light bulbs for data transmission, Li-Fi can achieve impressive speeds of up to 224 GB per second. In comparison, the fastest Wi-Fi technology, WiGig, which operates in the 60 GHz frequency band, reaches a maximum data rate of 7 GB per second. This makes Li-Fi approximately 100 times faster than current Wi-Fi technologies.

A Brief History

Professor Harald Haas and his team of researchers are pioneers in the field of LIFI. They began their research into VLC technology as early as 2006. Much of their research ventured into the use of light as a medium for the two-way transmission of data, which paved the way for LIFI technology.

Although it wasn’t the exact moment it was invented, the term LIFI was introduced to the world in 2011 during his TED Talk. The talk was entitled "Wireless Data from Every Lightbulb". It was at the talk that he coined the term Light Fidelity (LIFI), a play on the term Wireless Fidelity (WIFI). To demonstrate the capabilities of the new technology, Prof. Haas provided a live demonstration by streaming a large, high-definition video from a standard LED lamp.

Prof. Haas helped start a company that would market LiFi technology in 2012. The company was named pureVLC, but was later renamed pureLiFi, and was an original equipment manufacturer (OEM) that was set to commercialize LiFi products and push for their integration with existing LED-lighting systems.

Through the years, pureLiFi was able to produce a variety of LiFi products designed for easy integration of LiFi into existing devices.

Basic Principle of Li-Fi

Li-Fi is a high-speed, bidirectional networked communication technology that uses light to transmit data. It leverages multiple light bulbs to create a wireless network, enabling mobile communication with impressive data transfer rates.

When an electrical current is applied to an LED light bulb, it emits a stream of photons. LED bulbs, being semiconductor devices, allow the brightness of the emitted light to be modulated at extremely high speeds. This capability enables the transmission of data by varying the light intensity at different rates. A detector then receives the modulated light and interprets the changes in light intensity as data.

Li-Fi operates within the visible light spectrum, a subset of the electromagnetic spectrum, ranging from 400 THz to 790 THz. Each color within this spectrum corresponds to a specific frequency, and the human eye can perceive light in this range, making it an ideal medium for data transmission. As illustrated, the frequency of visible light is significantly higher — potentially 1000 times — than that of radio waves, enabling much faster data transfer rates. Additionally, high modulation rates, minimal interference, and increased bandwidth position Li-Fi as a promising technology for future wireless communication.

Advantages of Li-Fi

Li-Fi offers several advantages over traditional radio frequency-based communication technologies, such as Wi-Fi. Some of the key benefits of Li-Fi include:

1. Wider Bandwidth

• Visible Light Spectrum: LiFi uses the visible light spectrum, which is about 10,000 times larger than the radio frequency spectrum used by WiFi. This wider bandwidth allows for much higher data transfer rates.
• Less Congestion: The radio frequency spectrum is heavily congested due to the numerous devices and applications that rely on it. In contrast, the visible light spectrum is underutilized, reducing interference and allowing for cleaner signals.

2. High Frequency of Visible Light

• Higher Frequencies: Visible light has a much higher frequency compared to radio waves. Higher frequency signals can carry more data per second, leading to faster data transfer rates.
• Shorter Wavelengths: The shorter wavelengths of visible light enable more precise data encoding, further increasing the potential data transfer rates.

3. Advanced Modulation Techniques

• Modulation Speed: LiFi can use advanced modulation techniques, such as orthogonal frequency-division multiplexing (OFDM), to achieve high data rates. These techniques take advantage of the fast switching capabilities of LEDs to encode data at high speeds.
• Multiple Data Streams: LiFi can support multiple data streams by using different colors of light (RGB LEDs), each acting as a separate channel for data transmission. This increases the overall data throughput.

4. Spatial Reuse

• Directional Nature: Light is highly directional, allowing for spatial reuse of the same frequencies in different areas without interference. This can significantly increase the overall network capacity.
• Localized Communication: LiFi can provide localized, high-speed communication in specific areas, reducing the load on traditional communication systems and improving overall network performance.

5. Minimal Interference

• Electromagnetic Interference: LiFi is immune to electromagnetic interference from radio frequency devices, making it suitable for environments where such interference is a concern, such as hospitals and aircraft.
• Secure Communication: The line-of-sight nature of LiFi makes it more secure against eavesdropping compared to radio-based communication, which can penetrate walls and be intercepted more easily.

6. Energy Efficiency

• Dual Functionality of LEDs: LiFi uses LEDs, which are already widely used for lighting. The dual functionality of LEDs for both illumination and communication can lead to energy savings and reduced infrastructure costs. Practical Considerations
• Range and Line of Sight: LiFi typically requires a line of sight between the transmitter and receiver, which can be a limitation compared to WiFi. However, this can also be an advantage in terms of security and interference reduction.
• Integration with Existing Infrastructure: While LiFi can complement existing WiFi networks, integrating LiFi into existing infrastructure may require new hardware and adjustments to current systems.

A DIY Model of Li-Fi

Let's build a basic Li-Fi system using Arduino to understand the fundamental principles of Li-Fi technology and how data can be transmitted using light. This project will demonstrate how to transmit data using light and receive it on another device with an Arduino. Later, we'll explore establishing a bi-directional communication system, enabling both devices to transmit and receive data.

Materials Needed:

1. 2 Arduino boards (e.g., Arduino Uno/Nano)
2. Resistors (appropriate values for LEDs and sensors)
3. 2 LED light bulbs
4. 2 light sensors (e.g., photodiodes, LDR)
5. Breadboards and jumper wires
6. USB cables for Arduino
7. A power source for LEDs

Transmitter Circuit

The transmitter circuit consists of an Arduino board connected to an LED light bulb. The Arduino will modulate the light intensity of the LED to transmit data. The LED will emit light pulses that encode the data to be transmitted.

The Circuit Diagram:

1. Connect the LED to the A1 pin of the Arduino board using a resistor to limit the current.
2. Connect the other end of the LED to the ground pin of the Arduino board.
3. Power the Arduino board using a USB cable or an external power source.

Now upload the following code to the Arduino board using the Arduino IDE or any other compatible software.

#define LED_PIN A1
#define PERIOD 15 // in milliseconds // Must be same in both transmitter and receiver
 
char* string = "This is a test transmission";
int string_length;
 
void setup()
{
  pinMode(LED_PIN, OUTPUT);
  string_length = strlen(string);
}
 
void loop()
{
  for(int i = 0; i < string_length; i ++)
  {
    send_byte(string[i]);
  }
  delay(1000);
}
 
void send_byte(char my_byte)
{
  digitalWrite(LED_PIN, LOW);
  delay(PERIOD);
 
  //transmission of bits
  for(int i = 0; i < 8; i++)
  {
    digitalWrite(LED_PIN, (my_byte&(0x01 << i))!=0 );
    delay(PERIOD);
  }
 
  digitalWrite(LED_PIN, HIGH);
  delay(PERIOD);
 
}

Receiver Circuit

The receiver circuit consists of another Arduino board connected to a light sensor (e.g., LDR) that detects the light pulses emitted by the LED. The light sensor will receive the light pulses and decode the data transmitted by the LED.

The Circuit Diagram:

1. Connect one end of the light sensor to the A2 pin and other end to the 5V pin of the Arduino board.
2. Connect a resistor between the light sensor and the ground pin of the Arduino board.
3. Power the Arduino board using a USB cable or an external power source.

This kind of connection is called a voltage divider. It is used to redistribute the voltage across the light sensor and the resistor. It ususally drops the voltage while maintaianing the frequency of the signal.

Now upload the following code to the Arduino board using the Arduino IDE or any other compatible software.

#define LDR_PIN A2
#define THRESHOLD 100
#define PERIOD 15 // in milliseconds // Must be same in both transmitter and receiver
 
bool previous_state;
bool current_state;
 
void setup()
{
    Serial.begin(9600);
    pinMode(LED_PIN, OUTPUT);
}
 
void loop()
{
  current_state = get_ldr();
  if(!current_state && previous_state)
  {
    print_byte(get_byte());
  }
  previous_state = current_state;
}
 
bool get_ldr()
{
  int voltage = analogRead(LDR_PIN);
  return voltage > THRESHOLD ? true : false;
}
 
char get_byte()
{
  char ret = 0;
  delay(PERIOD*1.5);
  for(int i = 0; i < 8; i++)
  {
    ret = ret | get_ldr() << i;
    delay(PERIOD);
  }
  return ret;
}
void print_byte(char my_byte)
{
  char buff[2];
  sprintf(buff, "%c", my_byte);
  Serial.print(buff);
}

Testing the Li-Fi System

1. Assemble the transmitter and receiver circuits on separate breadboards.
2. Power the Arduino boards using USB cables or external power sources.
3. Upload the transmitter code to one Arduino board and the receiver code to the other Arduino board.
4. Place the LED and light sensor facing each other at a short distance.
5. Open the Serial Monitor on the receiver Arduino IDE to view the transmitted data. Remember to set the baud rate to 9600.
6. The LED should emit light pulses, and the light sensor should detect the pulses and decode the transmitted data.

If everything is set up correctly, you should see the transmitted data displayed on the Serial Monitor of the receiver Arduino IDE. This simple Li-Fi system demonstrates the basic principles of data transmission using light and how it can be implemented using Arduino boards.

Let's understand the procedure:

Usually, while transmitting data, along with the data signal another signal is transmitted simultaneously to indecate the timeings. Its known as the clock signal. In our version, we are using the delay function to indecate the timeings rather than using a clock signal. Although its always recomanded to use a clock signal for better performance.

Here, we've already defined the PERIOD variable in both the transmitter and receiver codes. This variable determines the duration of each bit transmission. But rather than just sending the data, we also need to inform the receiver about the start and end of the data transmission. This is done by sending a start bit and a stop bit. The start bit is sent by turning the LED off for a period of time, and the stop bit is automatically being detected once all the 8 bits of the data are received. So, the receiver knows when the data transmission starts and ends.

After turning the LED off for a period of time, we read our first bit after half of the PERIOD time. This is because the receiver needs to be synchronized with the transmitter. Then the next bit is read after every PERIOD time. This way, we can read all the 8 bits of the data. After that we will turn the LED on for the sake of the next transmission.

Transmitter Code:

• The transmitter code sends a test string "This is a test transmission" character by character.
• The send_byte function sends each character by modulating the LED light intensity (0 for LOW and 1 for HIGH). ASCII encoding is used to represent each character as a byte.
• The PERIOD variable defines the duration of each bit transmission.
• So, as we discussed earlier, the transmitter sends a start bit, followed by 8 data bits, and then a stop bit.
• In line 25, we delay for one PERIOD time after sending start bit.
• Next we start the iteration for sending the data bits. Here, to get each corresponding bit from the whole byte, we use the bitwise AND operator with the byte and a mask. The mask is a byte with only one bit set at the position we want to check. We shift the mask to the left by i bits to get the bit at position i. If the result is not zero, then the bit is 1, otherwise it is 0.
• After sending all the bits, we send the stop bit and delay for one PERIOD time.

Receiver Code:

• The receiver code reads the light sensor value and detects the light pulses emitted by the LED.
• The get_ldr function reads the analog value from the light sensor and returns a boolean value based on a predefined threshold.
• In every iteration of the loop, the receiver checks for a falling edge (transition from HIGH to LOW) in the light sensor value. This indicates the start of a new byte transmission.
• Once the falling edge is detected, the receiver reads the 8 data bits and reconstructs the byte by calling the get_byte function.
• The get_byte function reads each bit by calling the get_ldr function and reconstructs the byte by shifting the bits to the left.
• The reconstructed byte is then printed to the Serial Monitor using the print_byte function.

This simple Li-Fi system demonstrates the basic principles of data transmission using light and how it can be implemented using Arduino boards. You can experiment with different data transmission rates, distances, and light sources to explore the capabilities of Li-Fi technology further.

Bi-Directional LI-FI System

To establish a bi-directional communication system, you can modify the transmitter and receiver codes to enable both devices to transmit and receive data. By incorporating two-way communication, you can create a more interactive Li-Fi system that allows devices to exchange data in real-time.

Circuit Diagram:

The Code for Bi-Directional Communication:

This code is for both the transmitter and receiver. You can use the same code for both devices.

#define LED_PIN A1
#define LDR_PIN A2
 
#define THRESHOLD 100
#define PERIOD 15
 
// receive
bool previous_state;
bool current_state;
 
// transmit
int string_length;
 
void setup () {
  Serial.begin(9600);
  pinMode(LED_PIN, OUTPUT);
  digitalWrite(LED_PIN, HIGH);
}
 
void loop () {
 
  // Transmit
  if (Serial.available()) {
    String string = Serial.readString();
    int string_length = string.length();
 
    long int t1 = millis();
    for(int i = 0; i < string_length; i ++)
    {
      send_byte(string[i]);
    }
    long int t2 = millis();
    Serial.print("\n[SENT] => " + string);
    Serial.print("[TIME TAKEN BY LAST OPERATION] => "); Serial.print(t2-t1); Serial.print(" ms\n\n");
 
    delay(1000);
  }
 
  // Receive
  current_state = get_ldr();
  if(!current_state && previous_state)
  {
    print_byte(get_byte());
  }
  previous_state = current_state;
}
 
// Receive
bool get_ldr()
{
  int voltage = analogRead(LDR_PIN);
  return voltage > THRESHOLD ? true : false;
}
char get_byte()
{
  char ret = 0;
  delay(PERIOD*1.5);
  for(int i = 0; i < 8; i++)
  {
    ret = ret | get_ldr() << i;
    delay(PERIOD);
  }
  return ret;
}
void print_byte(char my_byte)
{
  char buff[2];
  sprintf(buff, "%c", my_byte);
  Serial.print(buff);
}
 
// Transmit
void send_byte(char my_byte)
{
  digitalWrite(LED_PIN, LOW);
  delay(PERIOD);
 
  //transmission of bits
  for(int i = 0; i < 8; i++)
  {
    digitalWrite(LED_PIN, (my_byte&(0x01 << i))!=0 );
    delay(PERIOD);
  }
 
  digitalWrite(LED_PIN, HIGH);
  delay(PERIOD);
}

This code combines the transmitter and receiver functionalities into a single program. The device can now send and receive data using the same Arduino board. The Serial object is used to read and write data to the Serial Monitor. When the device receives data from the Serial Monitor, it transmits the data using the send_byte function. When the device detects light pulses from the LED, it receives the data and prints it to the Serial Monitor using the print_byte function.

This is a Half Duplex mode of communication. In this mode, the devices can either send or receive data at a time, but not both simultaneously. To achieve full-duplex communication, where devices can send and receive data simultaneously, you can implement a more complex system with multiple LEDs and light sensors.

Conclusion

Li-Fi is a promising wireless communication technology that offers high-speed data transmission using visible light. By leveraging the visible light spectrum, Li-Fi can achieve data rates that are significantly faster than traditional radio frequency-based technologies. The advantages of Li-Fi, such as wider bandwidth, high frequency of visible light, advanced modulation techniques, spatial reuse, minimal interference, and energy efficiency, make it an attractive alternative for future wireless communication systems.