OOK vs VPPM vs CSK Modulation: A Detailed Comparison

In the world of wireless communication, modulation techniques play a crucial role in transmitting information efficiently and accurately. Among the popular methods, On-Off Keying (OOK), Variable Pulse Position Modulation (VPPM), and Color Shift Keying (CSK) are widely used in various applications, especially in Visible Light Communication (VLC) systems.

Understanding the differences between these modulation schemes is essential to determine the best choice for a specific application. This article dives into the key features, advantages, and drawbacks of OOK, VPPM, and CSK modulation, highlighting how each performs in terms of efficiency, data rate, and complexity.

We will also compare 4-CSK, 8-CSK, and 16-CSK modulations.

OOK Modulation: On-Off Keying

In this modulation technique, data is represented by the “on” and “off” states of the LED. In simple terms, a light “ON” state represents logic ‘1’, and a light “OFF” state represents logic ‘0’.

Fig-1: OOK Modulation

In the LiFi standard, OOK uses Manchester codes to represent digital information in the form of 1s and 0s. Here, encoding is done using the edge transition concept, where a low-to-high transition represents logic ‘1’, and a high-to-low transition represents logic ‘0’.

OOK modulation under dimming helps achieve a variable data rate and constant range. This is achieved by inserting compensation time.

VPPM: Variable Pulse Position Modulation

This modulation technique is similar to PPM (Pulse Position Modulation), but here the pulse width is variable for light dimming control applications.

Fig-2: VPPM Modulation

VPPM modulation under dimming helps achieve variable range and constant data rate. This is achieved by adjusting the pulse width.

The advantage of this modulation type is that it protects against intra-frame flicker. This is because the pulse amplitude is held constant, and dimming is controlled by the variable pulse width.

• Here, logic ‘0’ is mapped using a positive pulse at the beginning of the period, followed by a negative pulse.
• Logic ‘1’ is mapped using a negative pulse at the beginning of the period, followed by a positive pulse.

For VPPM modulation to be efficient, the duration of the period containing the pulses should be long enough so that pulse positions can be distinguished.

CSK Modulation: Color Shift Keying

This modulation scheme represents information bits using different color wavelengths. Red, Green, and Blue LEDs are used at the transmit end to produce different colors of different wavelengths to code the information bits.

The diagram maps various wavelengths marked in ‘blue’ with its color counterparts.

Fig-3: CSK Modulation

Advantages and Disadvantages of CSK Modulation

CSK modulation requires a complex design at both the transmit and receive ends of the optical system. This is its primary disadvantage.

Despite this, CSK modulation has the following advantages, which have contributed to its popularity:

• Color coordinates are used to represent information, and the same is represented in the form of binary codes for simplicity in programming.
• As information is represented by the color of light, the amplitude is kept constant, and hence the total average power of the light sources will be kept constant.
• CSK helps achieve a variable high bit rate due to higher-order modulation support, such as 4-CSK, 8-CSK, and 16-CSK.

Difference between 4-CSK, 8-CSK, and 16-CSK Modulation Types

• Here, the raw bit rate of the modulation scheme equals the bits per CSK symbol time base optical clock rate.
• In 4-CSK, 2 bits are assigned per symbol, and there are a total of four symbols from S0 to S3.
• In 8-CSK, 3 bits are assigned per symbol, and there are a total of eight symbols from S0 to S7.
• In 16-CSK, 4 bits are assigned per symbol, and there are a total of sixteen symbols from S0 to S15.

Conclusion

Each modulation technique—OOK, VPPM, and CSK—offers unique advantages depending on the application requirements. OOK is simple and power-efficient but limited in terms of data rate. VPPM offers greater flexibility with pulse position modulation, while CSK stands out for its ability to transmit data using multiple color channels, making it suitable for high-speed applications like VLC. Understanding these differences allows engineers and system designers to select the most suitable modulation scheme for their communication needs.