## 1 Introduction
### Advanced Passive Remote Nodes  
For PONs accommodating a large number of ONUs, the advanced passive remote node architecture without using optical filters, illustrated in Figure 3a, can be used to considerably reduce the inter-ONU signal power losses. As seen in the figure, assuming the ONU count is X, this advanced remote node just consists of a 1-by-X optical coupler (OC-1), an X-by-X optical coupler (OC-2), and X sub-structures. For each sub-architecture, its Port-1 (P1) is connected to an ONU, its P2 is connected to a port of OC-1, and its P3 and P4 are connected to OC-2. Using different optical components, as presented, respectively, in Figure 3b,c, the sub-structures can be realized using two diffident approaches, all of which are capable of delivering the same functionalities. As seen in Figure 3a, each ONU's inter-ONU and upstream signal is first transmitted to its corresponding sub-structure in the remote node and then split into multiple copies (the number of copies depends on optical couplers used in this sub-structure). One copy is delivered to the OLT through the sub-structure's P2 and OC-1. One copy is transmitted to OC-2 through P3, which is then split and broadcasted to all ONUs by OC-2 through all sub-structures' P4 ports. The optical isolators and circulators used in the advanced remote nodes' sub-structure-1 and sub-structure-2, as shown in Figure 3b,c, allow only one direction transmission of optical signals from each ONU to be redirected to all other ONUs, thus reducing unwanted optical interferences between different signals produced by the OCs. For downstream signal transmissions, each downstream signal is broadcasted to all ONUs through OC-1 and sub-structures-embedded optical couplers. _Upstream/downstream signal power losses_: In comparison with standard PONs employing a 1-by-X optical coupler at the remote nodes, the PONs equipped with the sub-structure-1 (sub-structure-2)-based advanced remote nodes lead to an extra \(\sim\)6.2 dB (\(\sim\)4 dB) power loss for both the upstream signals and the downstream signals due to the use of 1-by-3 (1-by-2) optical couplers in the sub-structures. Such estimations are made based on the maximum insertion losses of commercial 1-by-3/1-by-2 optical couplers of \(\sim\)6.2 dB/4 dB. For example, for 128 ONUs, a 128-port optical coupler in the standard PON remote node can result in a power loss of \(\sim\)24.65 dB [26], and the upstream/downstream signal power losses caused by the sub-structure-1/sub-structure-2-based advanced remote nodes are thus \(\sim\)30.85 dB (6.2 dB + 24.65 dB)/\(\sim\)28.65 dB (4 dB + 24.65 dB). Such power losses would be acceptable to PONs with E2-class power budgets of 35 dB [26]. _Inter-ONU signal power losses_: For the proposed techniques with two advanced remote node sub-structures being implemented, the resulting inter-ONU signal power losses are estimated and presented in Table 1. For comparisons, the inter-ONU signal power losses of the conventional concurrent direct inter-ONU and upstream communication techniques reported in [15; 16] are also calculated and shown in Table 1. The conventional techniques use X-by-3 optical couplers at the remote nodes, as presented in Figure 3d, and the inter-ONU signals need to pass the coupler twice. It can be found that in comparison with the conventional techniques, the proposed techniques can considerably reduce the inter-ONU signal power losses when the ONU count is \(\geq\)16. For 128-ONUs, \(>\)12.2 dB (\(>\)16.6) reductions in the inter-ONU signal power losses can be obtained if the remote nodes use sub-structure-1 (sub-structure-2). In addition, for 128-ONUs, the sub-structure-1/sub-structure-2-based advanced remote nodes can result in \(\sim\)37.05 dB/32.65 dB inter-ONU signal power losses, respectively. These power losses would be acceptable to PONs with E2-class power budgets [26] because inter-ONU communications have significantly shorter transmission distances in comparison with upstream communications.