A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) Direct Detection <s> Simulations show orthogonal frequency division multiplexing (OFDM) with optical single sideband modulation can adaptively compensate for dispersion in 4000-km 32×10Gbps WDM SMF links with 40% spectral efficiency. OFDM requires no reverse feedback path so can compensate rapid plant variations. <s> BIB001 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) Direct Detection <s> Based on a comprehensive theoretical model of a recently proposed novel technique known as adaptively modulated optical orthogonal frequency-division multiplexing (AMOOFDM), investigations are undertaken into the impact of an analog-to-digital converter involved in the AMOOFDM modem on the transmission performance of AMOOFDM signals in unamplified intensity-modulation and direct-detection (IMDD) multimode-fiber (MMF)-based links. It is found that signal quantization and clipping effects are significant in determining the maximum achievable transmission performance of the AMOOFDM modem. A minimum quantization bit value of ten and optimum clipping ratio of 13 dB are identified, based on which, the transmission performance is maximized. It is shown that 40-Gb/s-over-220-m and 32-Gb/s-over-300-m IMDD-AMOOFDM signal transmission at 1550 nm with loss margins of about 15 dB is feasible in the installed worst case 62.5-mum MMF links having 3-dB effective bandwidths as small as 150 MHz middot km. Meanwhile, excellent performance, robustness to fiber types, and variation in launch conditions and signal bit rates is observed. In addition, discussions are presented of the potential of 100-Gb/s AMOOFDM signal transmission over installed MMF links <s> BIB002 </s> A Survey on OFDM-Based Elastic Core Optical Networking <s> 1) Direct Detection <s> A combination of orthogonal frequency-division multiplexing (OFDM) and compatible single-sideband modulation (CompSSB) using a standard direct-detection scheme is suggested to overcome chromatic dispersion without explicit compensation. Since the proposed type of SSB modulation does not require a spectral gap between optical carrier and subcarriers, it is highly spectrally efficient and the complexity in the analogue part is reduced compared to known direct-detection schemes for OFDM. <s> BIB003
Direct-detection optical OFDM is realized by sending the optical carrier along with the OFDM baseband so that direct detection with a single photodiode can be used at the receiver to convert the optical field back into the electrical domain. DDO-OFDM can be classified into two categories according to how the optical OFDM signal is generated: (1) (2) non-linearly-mapped DDO-OFDM, where the optical OFDM spectrum does not display a replica of baseband OFDM, but aims to obtain a linear mapping between baseband OFDM and optical intensity BIB003 . The advantage of direct-detection optical OFDM is its relatively-simple implementation and low cost. Therefore, DDO-OFDM has a broader range of applications, such as long-haul transmission BIB001 [53], multi-mode fiber, and shortreach single-mode fiber transmission BIB002  . However, DDO-OFDM is less bandwidth efficient, and it has lower OSNR sensitivity compared to CO-OFDM.