“…Unfortunately, these benefits are accompanied by high-cost installations, including narrow-linewidth laser sources at the transmitters, local oscillators, 90-degree optical hybrids, and extra signal processes accounting for the phase and frequency offset estimations at the receivers. On the other hand, DD-OFDM can adopt a low-cost DFB laser with MHz-level linewidth [6], eliminates the local oscillators and optical hybrids, and needs not estimate the phase and frequency offsets, therefore making the DD-OFDM quite convenient to be implemented. Besides, a direct-detected PDM-OFDM has been demonstrated with the self-polarization diversity technique for doubling spectral efficiency (SE) and enhancing PMD tolerance [7].…”