# DFT-Spread Spectrally Overlapped Hybrid OFDM-Digital Filter Multiple Access IMDD PONs
## 1 Introduction
The most widely adopted and simplest approach is to clip OFDM signals [10; 11]; this approach achieves certain levels of PAPR reduction; however, the clipping still causes significant signal distortion. Another solution is to use multiple signalling and probabilistic techniques, such as pilot-assisted partial transmit sequences (PTSs) and selected mapping (SLM) [12; 13]. However, due to these approaches requiring redundant information to be transported alongside actual data, they have an intrinsic drawback of reducing the useful data rate and increasing the computational complexity. In contrast to the abovementioned PAPR reduction techniques, DFT-spread OFDM is the ultimate technical solution, because it is free from parallel redundant information and has low complexity, since only deterministic DFT and IDFT operations are required in the transceiver [14]. The DFT-spread OFDM technique has already been reported in SSMF IMDD links, including DFT-spread layered/enhanced asymmetrically clipped OFDM systems [15] and probabilistically shaped OFDM-enabled IMDD systems [16]. In addition, the DFT-spread technique also possesses high compatibility for both long-distance [17] and short-reach IMDD transmission systems [18], and shows superior performance in PAPR reduction. Recently, we have applied the DFT-spread technique in the hybrid OFDM-DFMA PONs to further improve the flexibility of the system transmission performance [19]. However, in these PONs, each individual subwavelength spectral region only conveys either a single in-phase (I) or quadrature-phase (Q) channel upstream of the double sideband (DSB) OFDM signal, which halves the spectral efficiency compared with the spectrally overlapped hybrid OFDM-DFMA PONs [6]. By combining the benefits of the DFT-spread technique and the previously reported PON [6], in this paper, we propose DFT-spread spectrally overlapped hybrid OFDM-DFMA IMDD PONs and, via numerical simulations, analyse and optimize their performance characteristics. The simulation results show that when the DFT-spread technique is applied, PAPR reductions of more than 2 dB are attained for a digitally filtered OFDM signal carrying QAM modulated data. More importantly, the proposed PON can achieve a reduction of more than 1 bit in the minimum required DAC/ADC bit resolution, and an improvement of more than 1.4 dB in the upstream power budget. Furthermore, in comparison to the conventional hybrid OFDM-DFMA PON [6], the proposed PON can enhance the aggregate upstream signal transmission rate by factor of up to 10% in a 25 km SSMF IMDD PON transmission system. It is noteworthy that, while the proposed PON still maintains all of the unique advantages associated with the previously reported PONs [6], in the OLT, without utilizing digital MFs, the single FFT operation followed by summing and subtraction operations of the lower sideband (LSB) and upper sideband (USB), and the corresponding DSP-enabled data recovery processes applied in a pipelined approach, can directly demultiplex and demodulate ONU sub-band signals within the same subwavelength spectral region, while the same OLT receiver can also be used to demodulate legacy OFDM signals. The above salient features make the proposed PON a feasible solution for future 5G networks in terms of providing DSP-enabled multichannel aggregation and deaggregation solutions for fronthaul networks [20] to effectively enhance their bandwidth efficiency in comparison with existing common public radio interface (CPRI)-based fronthauls [21].