  An orbital-angular-momentum (OAM) mode-group multiplexing (MGM) scheme based
on a graded-index ring-core fiber (GIRCF) is proposed, in which a single-input
two-output (or receive diversity) architecture is designed for each MG channel
and simple digital signal processing (DSP) is utilized to adaptively resist the
mode partition noise resulting from random intra-group mode crosstalk. There is
no need of complex multiple-input multiple-output (MIMO) equalization in this
scheme. Furthermore, the signal-to-noise ratio (SNR) of the received signals
can be improved if a simple maximal ratio combining (MRC) technique is employed
on the receiver side to efficiently take advantage of the diversity gain of
receiver. Intensity-modulated direct-detection (IM-DD) systems transmitting
three OAM mode groups with total 100-Gb/s discrete multi-tone (DMT) signals
over a 1-km GIRCF and two OAM mode groups with total 40-Gb/s DMT signals over
an 18-km GIRCF are experimentally demonstrated, respectively, to confirm the
feasibility of our proposed OAM-MGM scheme.
