## 2 The System's Design, Architecture, and Operating Principles  
The basic system structure, operating description, and design enhancements for this work are presented in this section. The proposed high capacity DWDM-PON system, based on modified cross-seeding architecture, is shown in Figure 1. The Optical Line Terminal (OLT) can be divided into four stages, as illustrated in Figure 1. (A) Stage 1 represents the input light source, which consists of eight CW light sources that have a wavelength spacing of 25 GHz and combine together to be forced into Stage 2. (B) At Stage 2, the optical light sources are utilized to generate subcarriers by using the Double Sideband Optical Carrier Suppression (DSB-OCS) technique. In this work, the DSB-OCS technique is used to increase the system capacity to a record of 8 light sources with 16 subcarriers (i.e., channels) by decreasing the channel spacing to 12.5 GHz. Generally, DSB-OCS has high receiver sensitivity with a small power penalty [24; 25]. At this stage, an Erbium-Doped Fiber Amplifier (EDFA) is used. Each sideband (i.e., subcarrier) acts as a Centralized Light Source (CLS) to be re-modulated for US subcarriers, as shown in Figure 1. (C) The subcarriers are demultiplexed as odd and even subcarriers to ready them for transmission through the two-bidirectional fibers, feed fiber 1 (FF1) and feed fiber 2 (FF2). This part of the operation takes place in Stage 3. In this work, this function is performed with much lower complexity compared to the related studies that utilized the cross-seeding technique [22; 26; 27], but without affecting the performance, as performance will be described later in Section 4. (D) Then, each odd and even subcarrier is modulated by on-off key (OOK) modulation at Stage 4. OOK is the simplest modulation format and relatively inexpensive. Thus, OOK is used to meet the ONU requirements, as mentioned before. The transmitter (Tx) in Figure 2 consists of a Continuous Wave (CW) laser source with a wavelength of (193.025 THz) and an output power of 0 dBm. It is externally modulated at 10 Gb/s with a Non-Return-to-Zero (NRZ) pseudorandom binary sequence in a Mach-Zehnder Modulator (MZM) with a 30 dB extinction ratio. The NRZ format has been widely implemented, mainly due to its signal bandwidth and its relatively easy generation [28]. In addition, stage 4 contains a receiver (Rx) for the US signal, as shown in Figure 2. A p-i-n photodiode/ photodetector (PIN) is used with Low-Pass Filter (LPF) to demodulate the US signal. The PIN specifications are 10 nA dark current and 1 A/W responsivity. The same Tx and Rx will be used again in Stage 7. The odd DS signals are multiplexed to be transmitted through FF1. Similarly, even DS signals are multiplexed to be transmitted through FF2. Figure 1: The proposed high capacity Dense Wavelength Division Multiplexing (DWDM-PON) system based on the modified cross seeding architecture; DeMUX: demultiplexer; Tx: transmitter; Rx: receiver; MUX: multiplexer; DS: downstream path; US: upstream path; FF1: feed fiber 1; FF2: feed fiber 2; RN: remote node; ONUs: optical network units. Stage 5 represents the heart of the cross-seeding technique that eliminates RB. This is done by transmitting the odd DS signals after re-modulation (i.e., Stages 6, 7) as the US through FF2, as indicated in Figure 1. The same procedure will be applied to even DS signals but by passing them through FF1 after re-modulation. Recall that transmission via conventional full-duplex bidirectional techniques suffers from a huge level of RB [28, 29], which forces researchers to find solutions, as indicated in Section 1. Finally, in Stage 6, the RN consists of two multiplexers (MUX) and two demultiplexers (DeMUX). MUX and DeMUX are simpler and lower in cost than Array Waveguide Grating (AWG), which is a complex component consisting of a coupler and fiber grating [30]. Txs and Rxs are used for the demodulation/re-modulation in Stage 7 after the received signal is split by a 20:80 coupler.