Color Phase Shift Keying (CPSK)

Color Phase Shift Keying (CPSK) is a modulation technique where information is encoded using both color (wavelength) and phase of a signal, instead of relying only on amplitude or frequency.

It can be thought of as Phase Shift Keying (PSK) with color added as an extra dimension.

Concept Breakdown

• Phase Shift Keying (PSK): Data is encoded by changing the phase of a carrier signal (e.g., 0°, 90°, 180°, 270°).
• Color Dimension: Different colors (wavelengths) are used as additional symbols.

Each transmitted symbol is therefore defined by:

(Phase, Color)

This combined representation allows more bits per symbol to be transmitted.

Why Use CPSK?

• Increases data rate without increasing bandwidth
• Improves spectral efficiency
• Allows better error separation in some systems

Application Areas

CPSK is not common in conventional RF systems, but it is used in:

• Optical communications (fiber optics, free-space optics)
• Visible Light Communication (VLC) / Li-Fi
• Research and experimental communication systems

In optical systems, “color” usually refers to different wavelengths rather than display-style RGB colors, although VLC often uses RGB LEDs directly.

Simple CPSK Example

Assume:

• 4 phases (QPSK → 2 bits)
• 3 colors (Red, Green, Blue → approximately 1.6 bits)

Total bits per symbol:

2 + 1.6 ≈ 3.6 bits per symbol

This achieves higher data throughput at the same symbol rate.

Key Idea

CPSK = PSK + wavelength (color) multiplexing

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Bits per Symbol and Color Count

The number of bits a symbol can represent depends on the number of distinct symbols:

bits per symbol = log₂(number of symbols)

Why 3 Colors ≈ 1.6 Bits

With three distinct colors (R, G, B):

log₂(3) ≈ 1.585 bits

This value is commonly rounded to approximately 1.6 bits.

Fractional Bits in Practice

Although fractional bits cannot be sent directly:

• Over many symbols, the average bits per symbol approaches 1.585
• Block or ternary coding efficiently maps binary data to 3 symbols

Example:

• 3⁸ = 6561 possible states
• log₂(6561) ≈ 12.7 bits
• Average ≈ 1.585 bits per symbol

Intuition

• 2 colors → 1 bit
• 4 colors → 2 bits
• 3 colors sit between them

Why Non-Powers of Two Are Used

• Physical constraints (e.g., RGB LEDs)
• Better energy efficiency
• Reduced channel interference
• Optical systems do not require power-of-two constellations

Colors	Bits per Symbol
2	1.0
3	1.585
4	2.0
8	3.0

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Color Shift Keying (CSK)

Color Shift Keying (CSK) encodes data using different colors (wavelengths) of light. Each symbol corresponds to one color state.

bits per symbol = log₂(M)

where M is the number of colors.

4-CSK

• 4 distinct colors
• Often RGB plus an additional color (White or Amber)

Bits per symbol: log₂(4) = 2

Bits	Color
00	Red
01	Green
10	Blue
11	White

Conceptually similar to QPSK but mapped into color space.

8-CSK

• 8 distinct colors
• Uses combinations of RGB intensities

Bits per symbol: log₂(8) = 3

Bits	Color Mix
000	Red
001	Green
010	Blue
011	Cyan (G+B)
100	Magenta (R+B)
101	Yellow (R+G)
110	Light Blue
111	White

16-CSK

• 16 color symbols in chromaticity space
• Colors differ by precise intensity ratios

Bits per symbol: log₂(16) = 4

(R, G, B) = (0.2, 0.5, 0.3)
(R, G, B) = (0.3, 0.4, 0.3)
...

This scheme is similar to 16-QAM and offers high spectral efficiency, but requires high SNR and careful calibration.

Scheme	Colors	Bits/Symbol	Robustness	Data Rate
4-CSK	4	2	⭐⭐⭐⭐	Low
8-CSK	8	3	⭐⭐⭐	Medium
16-CSK	16	4	⭐⭐	High

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Color and Phase in Practical VLC

Visible light is an electromagnetic wave, meaning it inherently has:

• Amplitude
• Phase
• Wavelength (color)

While physics allows modulation of all three, engineering constraints limit what is practical.

IM/DD Constraint

Most LED-based VLC systems use Intensity Modulation / Direct Detection (IM/DD).

• Only intensity is detected
• Optical phase information is lost

Therefore, classic optical PSK cannot be directly detected in basic VLC systems.

Subcarrier Phase Modulation (Practical)

Phase modulation can still be used by applying PSK or QAM to an electrical RF subcarrier:

1. Generate an RF subcarrier
2. Apply PSK/QAM
3. Use the result to modulate LED intensity
4. Transmit over RGB LEDs

This allows simultaneous use of color and phase in practical VLC.

Coherent Optical VLC (Rare)

True optical phase modulation requires:

• Laser diodes
• Local oscillators
• Coherent detection

While common in fiber optics, this approach is expensive and rare in VLC.

VLC Type	Color	Phase	Practical?
LED IM/DD	Yes	No	Widely used
CSK + RF-PSK	Yes	Yes	Very practical
Optical PSK (Coherent)	Yes	Yes	Rare
RGB + OFDM	Yes	Indirect	Common

Summary

Color and phase can coexist in practical VLC, but phase modulation is applied on a subcarrier rather than directly on the optical wave.