# Investigation of OFDM-Based HS-PON Using Front-End LiFiSystem for 5G Networks
## 2 Proposed Model  
Figure 1 depicts the conceptual diagram of the PON/LiFi model for a 5G network. Signals transmitted from the core network's OLT are distributed to four optical network units (ONUs) via a single-mode fiber (SMF) in an optical distribution network (ODN). The PON integrates a line-of-sight (LOS) indoor optical wireless channel at the front end to provide a high-quality signal to each ONU. An LED in a room serves users' equipment such as computers, phones, and other smart devices in indoor LiFi scenarios. The optical wireless network in the visible spectrum constitutes a blue LED (450 nm or 666.2 THz; bandwidth = 1 GHz). The LED receives data from the PON via an SMF and stores it. The light flashes at an extremely high speed that is invisible to the human eye. At the receiver, a photodetector (PD) reads all scintillations to extract the data [19]. It employs OFDM modulation with four downlink (DN) frequencies (187.5, 187.6, 187.7, and 187.8 THz) and four uplink (UP) frequencies (195.3, 195.4, 195.5, and 195.6 THz), following the ITU-T channel spacing of 100 GHz. Due to its higher signal to noise ratio (SNR) and better receiver sensitivity, 16-QAM modulation is used for OFDM modulation. In order to support high-speed and long-haul transmissions, coherent detection is used to take into account fiber impairments and LiFi channel noise [20]. For 5G-based hybrid wired-wireless transmission, Figure 2 shows a systematic block diagram of a full-duplex HS-PON/LiFi model using 16-QAM-OFDM modulation over the backend SMF link and the frontend LiFi channel link. Figure 1: Conceptual diagram of the proposed PON/LiFi model. OptiSystem software was used to design and analyze this model. Simulation parameters include 32,768 samples and 50\(\times\)109 symbols/sec. There are four transmitters and receivers used for both DN and UP transmission signals, as shown in the figure. DN transmitters (TX) use continuous-wave laser diodes with a \(-6\) dBm CW output power and 0.15  
Figure 2: Block diagram of proposed OFDM-based bidirectional HS-PON using LiFi model. MHz linewidth. Downlink transmitters generate a random bit sequence at a high bit rate of 50 Gbps that is fed into a 16-QAM sequence generator that encodes four bits per symbol. A 16-QAM signal is modulated using an OFDM modulator with 1024 fast Fourier transform points, 512 subcarriers, and 64 cyclic prefix points. Two low-pass cosine filters (with a 0.2 roll off factor) are used to filter the in-phase (I) and quadrature (Q) signals generated by the OFDM modulator, followed by two electrical gain components (gain = \(-\)0.008). Mach-Zehnder modulators (MZMs) then process the signal. In order to combine the generated modulated signals at a specific frequency, a 2:1 power combiner is incorporated. Thus, four HS-PON/OFDM downlink signals are multiplexed via a 4:1 ideal Mux at four downlink frequencies between 187.5 and 187.8 THz. SMFs are used to transmit signals considering fiber attenuation, nonlinearities, and dispersion. Pre- and post-amplification over the SMF link was achieved with two optical amplifiers with gains of 13 dB. Using a 2:1 bidirectional splitter, downlink signals can be directed to a specific LiFi user at the downlink receiver. In order to split the downlink signals into two pairs, 50:50 couplers are used. Coherent orthogonal reception at ONU is enabled by this setup, as well as a local oscillator and 900-phase shift device. The signal is converted from the optical domain into electrical domain using two pairs of PIN PDs, followed by two electrical subtractors, followed by two electrical amplifiers. The QAM sequence detector and OFDM demodulator are then used to demodulate the signal. The QAM sequence detector and OFDM demodulator use the same parameters as those in the modulation process at the OLT to reconstruct transmitted signals. To determine the binary sequences as well as the mapped electrical signals, a QAM sequence detector is used [20; 21].