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A novel optical signal modulation concept of adaptively modulated optical orthogonal frequency division multiplexing (AMOOFDM) is proposed, and a comprehensive theoretical model of AMOOFDM modems is developed. Numerical simulations of the transmission performance of the AMOOFDM signals are undertaken in unamplified multimode fiber (MMF)-based links using directly modulated distributed feedback (DFB) lasers (DMLs). It is shown that 28 Gb/s over 300 m and 10 Gb/s over 900 m transmission of intensity modulation and direct detection (IMDD) AMOOFDM signals at 1550 nm is feasible in DML-based links using MMFs with 3-dB effective bandwidths of 200 MHz·km. Apart from a higher signal capacity, AMOOFDM also has a greater spectral efficiency and is less susceptible to different launching conditions, modal dispersion, and fiber types, compared with all existing schemes. In addition, a large noise margin of about 15 dB is also observed. The bits of resolution of analog-to-digital converters (ADCs) and the cyclic prefix of AMOOFDM symbols are the main factors limiting the maximum achievable performance, on which the influence of DMLs is, however, negligible under the optimum operating condition.