## 1 Introduction
### Experimental Setups and Parameters  
The experimental setups of the 62.47 Gbit/s@27 km IMDD PON offering simultaneous upstream and direct inter-ONU communications are illustrated in Figure 4, where two ONUs are considered. A dual-channel arbitrary waveform generator (AWG, Keysight M8195A) operating at 32 GSample/s is used to represent these two ONUs. Each ONU produces an optical signal conveying 4 independent channels, of which, 2 high (low) bitrate channels are used for the upstream (inter-ONU) connections. In each ONU's transmitter DSP, the cascaded IFFT-based multi-channel aggregation process presented in Figure 2a is implemented to aggregate 4 independent channels. The IFFT sizes of the required three IFFT operations are 16/32/64. As such, the numbers of data-bearing subcarriers of the aggregated 4 channels are 8/8/16/32. Adaptive bit loading is implemented to maximize the spectral efficiency by adaptively varying each subcarrier modulation format from DBPSK to 64-QAM [24]. Following the cascaded IFFT operations, a CP containing 4 samples is inserted at the beginning of each symbol. After S/P conversion, a 4\(\times\) digital up-sampling operation and a digital orthogonal filtering operation are performed successively [24]. A Hilbert-pair approach [28] is utilized to produce the required two orthogonal digital filter pairs, and the baseband prototype filter has a square-root-raised cosine profile. To effectively reduce imperfect digital filter-induced signal distortions, a digital filter length of 32 and an excess bandwidth factor of 0, which are optimized in [16], are employed in this paper. Subsequently, a signal clipping operation with a clipping ratio of 14 dB and a digital domain time delay operation are \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**ONU COUNT** & & & & & & \\ (\(\mathbf{\Omega}\)) & \(\mathbf{X=4}\) & \(\mathbf{X=8}\) & \(\mathbf{X=16}\) & \(\mathbf{X=32}\) & \(\mathbf{X=64}\) & \(\mathbf{X=128}\) \\ \hline Advanced & 19.7 & 23.15 & 26.43 & 29.73 & 33.9 & 37.05 \\ Remote Nodes & (=2 \(\times\) 6.2 + & (=2 \(\times\) 6.2 + & (=2 \(\times\) 6.2 + & (=2 \(\times\) 6.2 + & (=2 \(\times\) 6.2 + & (=2 \(\times\) 6.2 + & 24.65) \\ (Sub-structure-1) & 7.3) & 10.75) & 14.03) & 17.33) & 21.5) & 24.65) \\ \hline Advanced & 15.3 & 18.75 & 22.03 & 25.33 & 29.5 & 32.65 \\ Remote Nodes & (=2 \(\times\) 4 & (=2 \(\times\) 4 & (=2 \(\times\) 4 & (=2 \(\times\) 4 & (=2 \(\times\) 4 \\ (Sub-structure-2) & + 7.3) & + 10.75) & + 14.03) & + 17.33) & + 21.5) & + 24.65) \\ \hline Conventional & 14.6 & 21.5 & 28.06 & 34.66 & 43 & 49.3 \\ Remote Nodes & (=2 \(\times\) 7.3) & (=2 \(\times\) 10.75) & (=2 \(\times\) 14.03) & (=2 \(\times\) 17.33) & (=2 \(\times\) 21.5) & (=2 \(\times\) 24.65) \\ \hline \hline \end{tabular} Note: In this table, the power losses (in dB) are calculated mainly based on optical coupler insertion losses [26]. The power losses of optical isolators and optical circulators are small (<1 dB) and thus ignored.  
\end{table} 
Table 1: Remote node-induced power losses for inter-ONU signals. implemented successively. The digital domain time delay operation is to adjust the ONU timing for synchronizing the involved two ONUs [24]. To enable the ONUs to achieve the best transmission performances, the AWG- and oscilloscope-embedded pre-compensation functions are used to flatten the PON transmission system frequency responses [24]. The AWG outputs two analogue signals, each having a spectral bandwidth of 8 GHz and an amplitude of 900 mV. Their spectra can be found in Figure 4a,b. Each analogue signal contains 4 independent channels, and their bitrates are given in Table 2. The total bitrates of ONU1 and ONU2 are about 31.29 Gbit/s and 31.17 Gbit/s, respectively, thus resulting in an aggregated bitrate of 62.47 Gbit/s. Following the AWG, two optical intensity modulators (Thorlabs MX35D) are used to perform intensity modulation. Each optical modulator has a 15 kHz linewidth laser source and a 35 GHz Mach-Zehnder modulator (MZM).