Abstract
Optical multiple-input multiple-output (MIMO) transmission systems generally employ minimum mean squared error time or frequency domain equalizers. Using an experimental 3-mode dual polarization coherent transmission setup, we show that the convergence time of the MMSE time domain equalizer (TDE) and frequency domain equalizer (FDE) can be reduced by approximately 50% and 30%, respectively. The criterion used to estimate the system convergence time is the time it takes for the MIMO equalizer to reach an average output error which is within a margin of 5% of the average output error after 50,000 symbols. The convergence reduction difference between the TDE and FDE is attributed to the limited maximum step size for stable convergence of the frequency domain equalizer. The adaptive step size requires a small overhead in the form of a lookup table. It is highlighted that the convergence time reduction is achieved without sacrificing optical signal-to-noise ratio performance.
Highlights
Spatial division multiplexing (SDM) enabled by multiple-input multiple-output (MIMO) digital signal processing (DSP) has been established as the method to increase the available capacity in a single fiber [1,2,3]
The criterion used to estimate the system convergence time is the time it takes for the MIMO equalizer to reach an average output error which is within a margin of 5% of the average output error after 50,000 symbols
We denote the system convergence time as the time it takes for the MIMO equalizer to reach an average output error which is within a margin of 5% of the average output error after 50,000 symbols
Summary
Spatial division multiplexing (SDM) enabled by multiple-input multiple-output (MIMO) digital signal processing (DSP) has been established as the method to increase the available capacity in a single fiber [1,2,3]. To unravel the mixed channels at the receiver side, a MIMO weight matrix is used to invert the transmission channel using an adaptive minimum mean squared error (MMSE) algorithm. Two MMSE MIMO equalizers, the time domain equalizer (TDE) and the frequency domain equalizer (FDE), are compared. Both equalizers heuristically adapt a weight matrix to minimize the final output error. A key parameter for the convergence time and final error floor of the MMSE MIMO equalizer is the MIMO adaptation factor, commonly known as the MIMO equalizer’s step size μ. The choice of the step size is critical as making it too small may lead to insufficient channel tracking capabilities, resulting in larger bit error rate or can significantly affect the convergence time
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