MIMO equalization with adaptive step size for few-mode fiber transmission systems.

Area of Science:

• Optical Communications
• Signal Processing
• Telecommunications Engineering

Background:

• Optical multiple-input multiple-output (MIMO) systems are crucial for high-capacity data transmission.
• Minimum mean squared error (MMSE) equalizers, in time and frequency domains, are standard for mitigating signal impairments in optical MIMO.
• Faster convergence of these equalizers is essential for efficient system operation and adaptability.

Purpose of the Study:

• To investigate methods for reducing the convergence time of MMSE equalizers in optical MIMO systems.
• To experimentally evaluate the convergence performance of time domain equalizers (TDE) and frequency domain equalizers (FDE).
• To analyze the factors influencing convergence speed differences between TDE and FDE.

Main Methods:

• An experimental 3-mode dual polarization coherent transmission setup was utilized.
• Convergence time was measured as the duration for the equalizer to achieve an output error within 5% of its final state after 50,000 symbols.
• Adaptive step size strategies were explored, requiring minimal overhead via a lookup table.

Main Results:

• The convergence time of the MMSE TDE was reduced by approximately 50%.
• The convergence time of the MMSE FDE was reduced by approximately 30%.
• The observed difference in convergence reduction was linked to the FDE's limited maximum step size for stable convergence.

Conclusions:

• Significant reductions in equalizer convergence time are achievable in optical MIMO systems.
• Adaptive step size techniques offer a viable method for accelerating TDE and FDE convergence.
• These convergence improvements were attained without any degradation in optical signal-to-noise ratio (OSNR) performance.