# DFT-Spread Spectrally Overlapped Hybrid OFDM-Digital Filter Multiple Access IMDD PONs
## 1 Introduction  
To effectively cope with the current avalanche of mobile traffic, driven by the unprecedented increase in users' demands for ultrawide bandwidth multimedia and cloud services that have become ubiquitous, fronthauls/backhauls of 5G CRANs capable of converging optical and wireless networks are needed, and require significant changes to network access in order to support the ambitious system requirements [1, 2]. Addressing such technical challenges requires multipronged efforts in different network domains across all layers to not only accommodate the explosive expansion in traffic demand, but also efficiently support dynamic bandwidth provisioning, improved cost-effectiveness, and power efficiency [3]. The emergence of software-defined networks (SDNs) with their extended network-control functionalities leverages the abstraction of different physical resources, and enhances the dynamic reconfigurability, flexibility, scalability, and elasticity of the network [4]. To deliver an SDN-based solution capable of satisfying the abovementioned requirements, a non-incremental solution should be implemented to realize the highly desirable, cost-effective, separately implemented, and independently operated legacy optical and wireless access networks in a converged manner. For cost-sensitive application scenarios such as optical access networks, metropolitan area networks (MANs), and mobile fronthaul/backhaul networks, intensity modulation and direct detection-based passiveoptical networks (IMDD PONs) are considered to be a competitive technical solution due to their excellent cost-effectiveness and power efficiency [5]. To overcome the abovementioned technical challenges, a novel PON technique termed hybrid OFDM-DFMA PON, utilizing spectrally overlapped digital orthogonal filtering, has recently been proposed and extensively investigated [6] where, regardless of the ONU count, matching filter (MF)-free single fast Fourier transform (FFT) operation and the relevant DSP processes are applied in a pipeline approach. In the proposed PON, transceiver-embedded software-reconfigurable digital orthogonal filtering is utilized in each individual ONU, where for upstream transmission two spectrally overlapped digitally filtered orthogonal ONU OFDM sub-band signals occupy the same subwavelength spectral region. In the OLT, for ONU sub-band signal demultiplexing and recovery, a procedure similar to the previously reported hybrid OFDM-DFMA PONs [7] is adopted. Numerical results show that in terms of improving the upstream signal transmission capacity and enhancing the spectral efficiency, the proposed PON outperforms the results previously reported in [7] by a factor of \(\sim\)2. Moreover, in the context of low OLT-DSP complexity, robustness against practical transceiver impairments, enhanced flexibility, and backward compatibility with existing 4G networks, the proposed PON still maintained all the aforementioned salient unique features associated with the hybrid OFDM-DFMA PONs [6]. It is important to mention that the PONs reported in [6] are OFDM-based. The OFDM's multi-subcarrier modulation scheme is well known to produce high PAPRs due to the coherent superposition of orthogonal subcarriers in the time domain [8]. Technically speaking, systems with large PAPRs not only require wide dynamic operating ranges for the transceiver-embedded electrical/optical devices, but also produce high quantization noise for a fixed number of quantization bits, and may force the involved devices to operate in their nonlinear regions, thus introducing nonlinear signal distortion. In addition, a large PAPR may also cause serious nonlinear noise associated with standard single-mode fibre (SSMF) nonlinearities [9]. Therefore, suppressing PAPRs in the PONs reported in [6] is of great importance. Several PAPR reduction techniques have been proposed [10; 11; 12; 13].