OFDM is widely used in 4G mobile networks and is also expected to play a vital role in future 5G and beyond networks. Recently, we have reported new hybrid OFDM-DFMA PONs , where DSP-based digital filtering is employed for individual OFDM signals from various ONUs, and a single FFT operation and following data recovery processes are implemented in the OLT. Hybrid OFDM-DFMA PONs not only support good backward compatibility to OFDM-based 4G networks, but also offers various DSP-enabled SDN solutions of low complexity to meet the stringent requirements of 5G networks. However, due to multiple subcarrier modulation, OFDM suffers from a large peak-to-average power ratio (PAPR), which requires highly linear dynamic ranges of certain properties of modulators and amplifiers. To mitigate this problem, discrete-Fourier-transform (DFT)-spread OFDM is widely considered, which decreases the probability of independently modulated subcarriers being added up coherently by the IFFT .
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