Effect of Mode Coupling on Signal Processing Complexity in Mode-Division Multiplexing

Mode-division multiplexing systems employ multi-input multi-output (MIMO) equalization to compensate for chromatic dispersion (CD), modal dispersion (MD) and modal crosstalk. The computational complexity of MIMO equalization depends on the number of modes and on the group delay (GD) spread arising from CD and MD. Assuming the strong-coupling regime, in which the total system length far exceeds the correlation length of modal fields, we quantify the GD spread arising from MD, showing that it can be reduced significantly by mode coupling. We evaluate the computational complexity of various MIMO single-carrier equalizers, considering separate or combined equalization of CD and MD, in the time or frequency domain. We present numerical examples for the optimally designed graded-index depressed-cladding fibers supporting D=6, 12, 20 or 30 modes in two polarizations. Assuming a 2000-km system length, a 1-km correlation length, and a combined CD+MD frequency-domain equalizer, the complexity (in complex multiplications per two-dimensional symbol) is a factor 1.4, 1.7, 2.2, 2.8 times higher for D=6, 12, 20, 30 than for polarization-multiplexed systems in standard single-mode fiber (D=2).