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In this paper, we propose an algorithm to determine the -dimensional optimum signal constellation design (DD-OSCD), in minimum mean-square error sense, for channel capacity achieving source distribution. We then describe a -dimensional coded modulation scheme, employing orbital angular momentum modes, two polarization states, and two in-phase/quadrature coordinates, suitable for use in future few-mode fiber (FMF) applications. Monte Carlo simulations indicate that the proposed low-density parity-check (LDPC)-coded polarization-division-multiplexed (PDM) quadrature-multiplexed (QM) DD-OSCD significantly outperforms conventional PDM-QAM (by 4.32 dB for 64-ary 3-D constellation at a BER of ). In addition, the DD-OSCD outperforms sphere packing constellations (by 0.33 dB for 8-ary 3-D constellation at a BER of ).