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In this paper, we will review our latest developments in improving the spectral efficiency and receiving sensitivity in a direct-detected OFDM transmission system. An RF-tone assisted virtual single sideband (VSSB) OFDM modulation format is proposed to enhance the spectral efficiency in both electrical and optical domains for direct detected OFDM applications. The receiving sensitivity can be improved by using an iterative estimation and cancellation algorithm. Furthermore, the ultimate sensitivity limit using spectrally matched optical filters in direct detection scheme, which can theoretically approach to the performance limit of coherent OFDM systems, will also be discussed. The analytical model, numerical simulations and experimental results of the proposed architectures will be presented.