## 1 Introduction
### P2 MP Flexible Optical Transceivers  
In the OLT and each ONU, a single pair of the cascaded IFFT/FFT-based P2 MP flexible optical transceivers are deployed to simultaneously establish fast and dynamic multiple ONU-to-OLT and ONU-to-ONU (without involving the OLT) connections. For each ONU's transmitter DSP, the adopted cascaded IFFT-based multi-channel aggregation operations [24] are illustrated in Figure 2a. The operations are capable of dynamically aggregating  
Figure 3: (**a**) Advanced passive remote node architectures for PONs accommodating X ONUs, and (**b**)/(**c**) two practical advanced remote node sub-structure implementations (Sub-structure-1/ Sub-structure-2). (**d**) Conventional remote nodes reported in [15; 16]. Figure 2: Operating principle of the cascaded IFFT/FFT-based multi-channel aggregation/de-aggregation technique embedded in the P2 MP flexible optical transceivers. (**a**) Multi-channel aggregation, (**b**) multi-channel de-aggregation. an arbitrary number of independent channels of adaptively varying bitrates, each of which can be used for either upstream communications or direct inter-ONU communications. To briefly outline the channel aggregation operation, here by taking the \(i\)-th ONU (ONU-\(i\)) as an example, as seen in Figure 2a, to aggregate \(R\) independent channels, the total required IFFT operation count is (\(R\)-1). Assuming the first IFFT operation size is \(2N\), to aggregate the \(r\)-th channel (\(r=\) [2, \(\ldots\), \(R\)]), the required IFFT size for the (\(r\)-1)-th IFFT operation is given by \(2^{r-1}N\). The (\(r\)-1)-th IFFT operation input is a function of the \(r\)-th channel data (\(S_{r}=\left[s_{r}^{1},s_{r}^{2},\ldots,s_{r}^{2^{r-2N}}\right]\)) and the output data of the (\(r\)-2)-th IFFT operation (\(O_{r-2}=\left[o_{r-2}^{1},o_{r-2}^{2},\ldots,o_{r-2}^{2^{r-2N}}\right]\)). Detailed descriptions of this multi-channel aggregation technique can be found in [24]. After cyclic prefix (CP) insertion and parallel-to-serial (P/S) conversion, a complex baseband signal is produced, which consists of multiple independent upstream and direct inter-ONU communication channels. Subsequently, after a digital domain up-sampling operation (M\(\uparrow\)) and an orthogonal digital filtering operation [24], the complex baseband signal is converted to a real-valued radio frequency signal located at a desirable radio frequency spectral region. Following a digital-to-analogue conversion (DAC) process, intensity modulation is performed to produce an optical signal containing the above-described aggregated multiple upstream and direct inter-ONU communication channels produced by ONU-\(i\). In the receivers, the signal detection and demodulation procedures for both the upstream and direct inter-ONU communications are similar, which include direct detection, analogue-to-digital conversion (ADC), serial-to-parallel conversion (S/P), CP removal, FFT operations, and subcarrier identification, as well as ONU signal demodulation [24]. In the receiver DSPs, after CP removal, the FFT operations are performed to separate the signals from different ONUs. For a specific ONU, assuming that the size of the last IFFT operation of the multi-channel aggregation is P and that the digital up-sampling factor is M, the required FFT operation size can be calculated by LFFT = M \(\times\) P [24]. When different ONUs adopt the same FFT sizes, a single FFT operation is sufficient to separate the signals from these ONUs [24]. If different ONUs require different FFT operation sizes, multiple FFT operations, each corresponding to a specific FFT operation size, can be performed [24]. It is worth highlighting that for direct inter-ONU communications, each ONU receiver only performs the FFT operations just for separating the targeted ONUs' signals, which considerably reduces the receiver DSP complexity. After that, the subcarriers of the targeted ONUs are identified and then delivered to the corresponding ONU signal demodulation modules for channel de-aggregations and signal demodulation.