## 2 Proposed Design  
Figure 1 indicates the simulation setup of the proposed LP modal bidirectional NG-PON system using a GIMMF-FSO link. Figure 1: Proposed architecture of LP[0,1] modal bidirectional 4\(\times\)10 Gbps NG-PON system using two GIMMF-FSO fibers. An ITU-T-standardized NG-PON is realized in the proposed work, which incorporates a set of four downlink and uplink wavelengths at an aggregate data rate of 40 Gbps. For these specified wavelengths, a 0.8 nm channel spacing is used as per the ITU-T standard (G.989 series). In the proposed system, the TIA-492 AAAB standard OM2 MMF is used at a 2.6 dB/km attenuation. The LP[0,1] mode is used to transmit the bidirectional data at distinct operating wavelengths. The proposed LP modal NG-PON-FSO system using a GIMMF link is designed in the OptiSystem v.22 simulation tool under the impact of fiber nonlinearity, link losses, and receiver noise. A spatial optical laser at 1596 to 1598.4 nm is utilized as an input source for modulation in a Mach-Zehnder modulator. Random non-return-to-zero signals are generated at a 10 Gbps data rate for both the downlink and uplink transmission. Four modulated downlink signals, viz., 1596, 1596.8, 1597.6, and 1598.4 nm, passed through a 4:1 multiplexer to launch into a spatial connector followed by a thin lens. The spatial connector (for precisely aligned, rotation = 0\({}^{0}\), distance = 1 \(\upmu\)m, and X/Y shift = 0 \(\upmu\)m) connects signals with transverse mode profiles, which can be rotated and translated. This device ensures the efficient coupling of signals between distinct spatial paths in fiber-FSO links. The thin lens used at the transmitter helps to collect the photons transmitted by laser diodes and then forward them to GIMMF [15]. To minimize the modal crosstalk in a single MMF fiber and for bidirectional transmission, two separate GIMMF cables with FSO links are used for the downlink and uplink transmissions. At the ONT end, a 4:1 demultiplexer is used to distribute the received signals to a distinct spatial PIN photodiode, followed by a low-pass filter and a 3R regenerator. The system performance is evaluated in a BER analyzer. Likewise, for the uplink transmission, four distinct wavelengths, viz., 1524, 1524.8, 1525.6, and 1526.4 nm, are utilized for data transmission from OLT to ONU at an aggregate 40 Gbps data rate. Various generated modes after data transmission through GIMMF are presented in Figure 2. Here, the specified sixteen LP modes are considered as follows: LP[0,1], LP[0,2], LP[4,2], LP[4,1], LP[\(-\)5,2], LP[5,2], LP[3,2], LP[\(-\)3,2], LP[\(-\)5,1], LP[5,1], LP[1,2], LP[\(-\)1,2], LP[\(-\)3,1], LP[3,1], LP[1,1], and LP[\(-\)1,1]. In MDM-based fiber-wireless systems, the LP[0,1] mode is generally preferred over higher-order LP modes, as it offers minimal modal dispersion, low attenuation, and resistance to turbulent environments. The fundamental LP[0,1] mode exhibits lower propagation loss, less turbulence-induced coupling, less diffraction, and a legible beam structure for long transmission distance compared to higher-order modes. It also faces less dispersion, maintains confined intensity distribution, and leads to narrower beam propagation compared to higher-order modes. Better system performance is obtained by using the LP[0,1] mode, followed by LP\({}_{11}\), LP\({}_{21}\), LP\({}_{31}\), and other modes. The system is proposed and investigated in OptiSystem, and its parameter specifications are depicted in Table 1.  
Figure 2: _Cont._