A simple delay-line 4-PPM demodulator with near-optimum performance

Area of Science:

• Optical Communications
• Signal Processing
• Modulation Techniques

Background:

• Digital signal processing is often required for complex modulation formats like 4-pulse position modulation (4-PPM).
• Achieving high sensitivity in optical receivers is crucial for reliable data transmission over long distances.

Purpose of the Study:

• To introduce a simplified 4-PPM demodulator design.
• To demonstrate near-optimal receiver sensitivity without digital signal processing.
• To validate the demodulator's performance with a practical implementation.

Main Methods:

• Utilized analog delay lines and 1-bit comparators for demodulation.
• Employed optical filtering to enhance receiver sensitivity.
• Built and tested a prototype demodulator for the Lunar Laser Communication Demonstration.

Main Results:

• The comparator-based demodulator theoretically achieves sensitivity within 0.23 dB of the optimum 4-ary receiver.
• Measured performance of the prototype was within 1.1 dB of the expected sensitivity.
• The technique shows promise for higher-order and higher-symbol-rate orthogonal modulation formats.

Conclusions:

• A simple, effective 4-PPM demodulator can be realized using analog components.
• The proposed method offers a low-complexity, high-sensitivity solution for optical communication systems.
• This approach is adaptable to more complex modulation schemes, broadening its applicability.