OOK vs VPPM vs CSK-Difference between OOK, VPPM, CSK modulation

This page compares OOK modulation vs VPPM modulation vs CSK modulation w.r.t. LiFi and mentions difference between OOK, VPPM, CSK modulation types used in LiFi PHY layer. It compares 4-CSK, 8-CSK and 16-CSK modulations. Advantages and disadvantages are also mentioned.

OOK-On Off Keying modulation

In this modulation, data is represented by on and off of the LED. In simple terms, light 'ON state' represents logic '1' and light 'OFF state' represents logic '0'.

OOK modulation technique
Fig-1: OOK Modulation

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

OOK modulation technique under dimming helps to achieve variable data rate and constant range. This is achieved using insertion of compensation time.

VPPM-Variable Pulse Position Modulation

This modulation technique is similar to PPM technique but here pulse width is variable for light dimming control application.

VPPM modulation technique
Fig-2: VPPM Modulation

VPPM modulation technique under dimming helps to achieve variable range and constant data rate. This is achieved by adjusting pulse width. The advantage of this modulation type is that it protects from intra frame flicker. This is due to the fact that pulse amplitude is held constant and dimming is controlled by variable is pulse width.

➨Here logic '0' is mapped using positive pulse at the beginning of period followed with negative pulse.
➨Logic '1' is mapped using negative pulse at the beginning of period followed with positive pulse.

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

Refer PPM Modulation➤ and Manchester code➤ basics.

CSK-Color Shift Keying modulation

The CSK modulation scheme is used to represent information bits in the form of different color wavelengths. Red LED, Green LED and Blue LED 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.

CSK modulation technique
Fig-3: CSK Modulation

Advantages and Disadvantages of CSK

The CSK modulation requires complex design at the transmit and receive end of optical system. This is the only disadvantage of CSK modulation. Inspite of this, the CSK modulation has following advantages due to which it has become very popular.
• Color co-ordinates 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 color of light, amplitude is kept constant and hence total average power of light sources will be kept constant.
• CSK helps achieve variable high bit rate due to higher order modulation support such as 4-CSK, 8-CSK and 16-CSK.

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

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

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