# DFT-Spread Spectrally Overlapped Hybrid OFDM-Digital Filter Multiple Access IMDD PONs
## 5 Conclusions  
The novel DFT-spread spectrally overlapped hybrid OFDM-DFMA PON was proposed and numerically simulated for a 25 km IMDD SSMF transmission system. To the best of our knowledge, this is the first work applying the DFT-spread technique to a hybrid OFDM-DFMA PON utilizing spectrally overlapped digital orthogonal filtering, in order to simultaneously reduce PAPRs, optimize CRs, and reduce the minimum required DAC/ADC quantization bits, whilst maintaining the upstream transmission performances and increasing the transceiver complexity. The simulation results showed that, compared to the previously reported PONs, the proposed PON can reduce the PAPR by more than 2 dB, and the optimal CR is reduced by 1 dB. Such a PAPR reduction is independent of the adopted digital filter characteristic, signal modulation format, and ONU sub-band signal spectral location. As a direct result of the PAPR reduction, the proposed PON can reduce the minimum required DAC/ADC resolution bits by more than 1 bit, whilst achieving BERs below the FEC limit. In addition, the proposed PON can improve the upstream power budget by more than 1.4 dB and increase the aggregate upstream signal transmission rate by up to 10%. Figure 7: Magnitude response for excess of bandwidth of \(\alpha=0\) for (**a**) I-phase filter with digital filter lengths 16, 32, 64, and 128; (**b**) Q-phase filter with digital filter lengths 16, 32, 64, and 128. Magnitude response for filter length 64 for (**c**) I-phase filter with excess bandwidth (\(\alpha\)) ranging from 0 to 1, and (**d**) Q-phase filter with excess bandwidth (\(\alpha\)) ranging from 0 to 1. (**e**) Comparison of SNR distribution across all of the subcarriers, using excess bandwidth of \(\alpha=0\) and an optimal value of \(\alpha=0.2\) for both I-phase (CH1) and Q-phase (CH2). It is well known that the speeds of DACs and ADCs play important roles in limiting the maximum achievable signal transmission capacity for DSP-based signal transmission techniques. However, high-speed DACs and ADCs can be very expensive. To achieve a specific signal transmission capacity, the proposed technique has the potential of allowing low-speed DACs/ADCs to be utilized to produce/receive targeted baseband signals, which can then be up-converted to the targeted sub-bands by low-cost electrical components. This could significantly reduce the overall ONU transceiver cost. In addition, the proposed-technique-induced PAPR reductions also considerably relax the requirements of using expensive electrical/optical devices with large linear operation regions and high-resolution DACs/ADCs. As a direct result, the proposed technique may offer a valuable solution capable of reducing the overall network installation cost for implementation in cost-sensitive application scenarios. The experimental demonstrations of point-to-point and multipoint-to-point spectrally overlapped hybrid OFDM-DFMA PON transmissions with/without DFT-spreading are currently being undertaken in our research laboratory, and the corresponding results will be reported elsewhere in due course. Conceptualization, W.J. and J.T. ; data curation, A.S. and J.H. ; formal analysis, A.S.; funding acquisition, J.T. ; methodology, W.J. ; project administration, J.T. ; software, A.S.; supervision, Z.Z., Y.H., R.G. and J.T. ; validation, A.S., Z.Z., R.G. and J.T. ; visualization, A.S. and J.H. ; writing--original draft, A.S.; writing--review and editing, W.J., Z.Z., Y.H., R.G. and J.T. All authors will be informed about each step of manuscript processing including submission, revision, revision reminder, etc. via emails from our system or assigned Assistant Editor. All authors have read and agreed to the published version of the manuscript. This research was funded in part by the DESTINI project, funded by the ERDF under the SMARTExpertise scheme, and in part by the DSP Centre funded by the ERDF through the Welsh Government. Not applicable.