Performance enhanced HD-MMIM-OFDM based on geometric shaping

A high-dimensional multi-mode index modulation orthogonal frequency division multiplexing (HD-MMIM-OFDM) based on geometric shaping is proposed and demonstrated in cost-effective and power efficient intensity modulation direct detection (IM/DD) systems. Four geometrically shaped three-dimensional (3D-GS) constellation schemes are compared and evaluated in terms of minimum Euclidean distance (MED) and constellation figure of merit (CFM). We select the 3D constellation C3 with the optimal performance as the basis and construct three HD constellation schemes by stacking low-dimensional constellations, namely 4D, 5D, and 6D. To further improve the spectral efficiency (SE) of the system, a multi-mode index modulation (MMIM) based on OFDM is proposed and validated at different fiber lengths. The result shows that compared with traditional 2D-OFDM, the receiver sensitivity of 4D-MMIM-OFDM under the same SE increases by 0.46 dB, and the pseudo-Gray coding scheme further improves the receiver sensitivity by 0.16 dB. When the fiber length increases from 50 km to 80 km, the receiver sensitivity of 6D-MMIM-OFDM increases by 0.97 dB compared to 4D-MMIM-OFDM. It has been demonstrated that as the dimension of the constellation increases, the designed scheme exhibits enhanced tolerance to nonlinearity.