## 1 Introduction
### Dynamic Channel Allocation  
For the proposed current upstream and direct inter-ONU communications, for each ONU, dynamic channel allocation can be made adaptively according to actual user communication requirements. To comprehensively evaluate the dynamic channel allocation-induced impacts on the channel performance, three cases are considered: for Case I (Case III), for each ONU, all the involved four channels are allocated for the upstream (direct inter-ONU) communications. For Case II, for each ONU, CH1/CH2 (CH3/CH4), is used for the direct inter-ONU (upstream) communications. For each considered case, the maximum achievable aggregated overall signal transmission capacity is measured and presented in Figure 7a. Adaptive bit-loading is used for each channel to achieve the maximum aggregated bitrates [24]. The received optical power is fixed at \(-2\) dBm for all the considered cases. It can be found that for the considered three cases, their aggregated overall signal transmission capacity differences are <2.2 Gbit/s, thus indicating that dynamic channel allocation has negligible impacts on aggregated overall signal transmission capacity. It is also interesting to find from the figure that the highest aggregated overall signal transmission capacity is achievable when all the channels are allocated for upstream communications. When more channels are allocated for direct inter-ONU communications, the aggregated overall signal transmission capacity is slightly reduced. To understand the physical mechanisms behind the observed overall signal transmission capacity variations, for each case, the corresponding channel capacities of each ONU are presented in Figure 7b,c. It can be found that for each ONU, in comparison with Case I's CH3/CH4 (Case III's CH1/CH2), Case II's CH3/CH4 (CH1/CH2) can deliver similar transmission capacities. This implies that dynamic channel allocations can only slightly influence the  
Figure 6: Impact of the channel interference effects between ONUs adopting different optical signal modulation techniques. CH1/CH2: direct inter-ONU communication. CH3/CH4: upstream communication. Figure 7: (a) Maximum achievable aggregated network capacities for three different channel allocation cases, and (b)/(c) corresponding channel capacity for ONU1/ONU2. The received optical power: \(-2\) dBm. US: upstream communication. performances of the channels that the channel reallocation operations are activated, and have negligible negative impacts on all other channels. As discussed in Section 3.1, in comparison with the upstream communications, the direct inter-ONU communications suffer from relatively lower chromatic dispersion, while the Rayleigh and Brillouin backscattering effects are, however, more pronounced. Therefore, it is easy to understand that when an upstream channel is reallocated to direct inter-ONU communications, its channel transmission capacity is slightly reduced mainly due to the backscattering effects. The advantages associated with the proposed PONs in terms of overall network throughput, latency, flexibility, scalability, adaptability, and network resource utilization efficiency are more pronounced in comparison with existing PONs with long transmission distances and/or high optical powers. Technically speaking, our previous research [16] indicates that (1) increasing the fiber length (either the distribution fiber length or the feeder fiber length) can enhance the chromatic dispersion effect and the Rayleigh and Brillouin backscattering effects, thus resulting in relatively large signal distortions and lower signal-to-noise ratios (SNRs). Additionally, (2) when the fiber lengths are >25 km and the ONU optical launch powers are <6 dBm, an ONU launch power increase can just lead to a slight increase in the Rayleigh and Brillouin backscattering effects. When the ONU optical launch powers are >6 dBm, a further ONU launch power increase can result in a nonnegligible power increase in the backscattered Brillouin stoke components.