## 1 Introduction
Apart from the aforementioned requirements to remote node modifications, IMDD point-to-multipoint (P2 MP) flexible optical transceivers [15; 16; 24], which allow multiple low-speed transmitters to concurrently communicate with multiple high-speed receivers, are also envisaged to be used for delivering the desirable concurrent direct inter-ONU and upstream communications by dynamically and flexibly aggregating/de-aggregating multiple inter-ONU and upstream channels. The utilization of such transceivers is beneficial for reducing the overall network transceiver count [16] and further improving network scalability and upgradability [24]. For previously reported IMDD P2 MP optical transceivers, multiple channels are aggregated/de-aggregated in digital signal processing (DSP) by using digital orthogonal filtering (DOF) [15; 16], cascaded inverse fast Fourier transform (IFFT)/fast Fourier transform (FFT) operations [24], and a single-IFFT/FFT operation (i.e., different channels use different subcarrier groups) [25]. In comparison, DOF-based channel aggregation/de-aggregation possesses a relatively large transceiver DSP complexity for aggregating/de-aggregating a large number of channels. On the other hand, a single-IFFT/FFT operation-based alternative requires extra DSP processes or frequency guard bands (i.e., null subcarriers) for reducing interferences between signals from different ONUs. The previously reported cascaded IFFT/FFT operations for channel aggregation/de-aggregation are preferable, where DOF is also employed for reducing interferences, as the operation possesses several advantages in terms of low transceiver DSP complexity, high spectral efficiency, and additional physical layer network security (because multiple channels are aggregated in two dimensions, i.e., the time domain and the frequency domain) [24]. To effectively deliver the desirable inter-ONU communications in future PON-based mobile access networks with a large number of ONUs, in this paper, a novel concurrent direct inter-ONU and upstream communication network is proposed and experimentally demonstrated. An advanced passive remote node architecture is proposed, which, depending on the employed optical components, can be implemented using two approaches. In comparison with an X-by-3 optical coupler-based conventional remote node [15; 16], these two remote node implementation approaches can reduce the inter-ONU signal power losses by \(>\)12.2 dB and \(>\)16.6 dB, respectively. In addition, the cascaded IFFT/FFT-based IMDD P2 MP flexible transceivers are also used in all of the ONUs and the OLT. The proposed network is experimentally evaluated in a 27 km, \(>\)62.47 Gbit/s upstream IMDD PON. The experimental results show that the network has three salient features, including (1) concurrently providing ultra-dense, fast, and dynamic ONU-to-ONU and upstream (ONU-to-OLT) connections each adaptively tailored according to specific requirements of an inter-ONU/upstream communications; (2) excellent tolerance to various transmission system impairments including chromatic dispersion, the Rayleigh and Brillouin backscattering effects, and the channel interference effects; and (3) supporting dynamic channel allocations without considerably compromising the channel performances. P2 MP Flexible Optical Transceiver and Advanced Remote Node-Enabled Concurrent Direct Inter-ONU and Upstream Communications in IMDD PONs  
The schematic diagram of the proposed concurrent direct inter-ONU and upstream communication IMDD PONs is illustrated in Figure 1. The involved transceiver multi-channel aggregation/de-aggregation processes and advanced remote node architectures are also presented in Figures 2 and 3, respectively. In discussing the proposed network, special attention is given to upstream communications and direct inter-ONU communications only. Figure 1: Schematic diagram of the proposed concurrent direct inter-ONU and upstream communication techniques in IMDD PONs.