# Investigation of OFDM-Based HS-PON Using Front-End LiFiSystem for 5G Networks
## 3 Results and Discussions
As the SMF length increases, the received power decreases for all transmitted signals. In the proposed model, noise is primarily caused by heat generated in electronic signal processing, leading to  
Figure 5: (**a**) Bit sequence at 50 Gbps; (**b**) 16-QAM constellation diagram at Tx; (**c**) timing diagram at Tx; LP cosine filter output for (**d**) I-channel, (**e**) Q-channel, and OFDM modulation signal at Tx; emission spectra of modulation outputs (**f**) single channel transmitted signal at 187.7 THz (**g**) before SMF and (**h**) after 50 km SMF; demodulated output for (**i**) I channel and (**j**) Q channel; (**k**) 16-QAM constellation diagram at Rx; (**l**) electrical spectrum at QAM sequence decoder; (**m**) timing diagram at Rx; (**n**) optical spectrum of blue LED; (**o**) optical spectrum after 10 m LiFi range; (**p**) eye pattern in BER analyzer. Figure 6: Optical spectra of (**a**) DN and (**b**) UP transmission. performance degradation. We consider both shot noise and thermal noise during optical signal conversion at the PD [29]. Over a 10-100 km range, both DN and UP transmission signals are able to receive 0 dBm to \(-17\) dBm of power, as shown in Figure 7a,b. In addition, 187.7 THz (in DN) and 195.5 THz (in UP) signals both decline sharply compared to other signals. Since the wavelengths of high-frequency signals (187.8 THz and 195.6 THz) are shorter than those of lower frequencies, they can travel farther without being absorbed or scattered by the atmosphere. Moreover, the lower-frequency signals travel less than the higher-frequency signals do over long distances due to the fact that the low-frequency signals experience fewer cycles than do higher-frequency signals. The 187.7 THz and 195.5 THz signals transmitted also suffer from higher inter-channel interference (via frequency offset) and inter-symbol interference (via multi-path channels). The optical power received for 187.5, 187.6, 187.7, and 187.8 THz signals is \(-16\), \(-17\), \(-17\), and \(-16.9\) dBm, for BER limits of 10-3 over 100 km fiber. As well, at a 100 km SMF range, 195.3, 195.4, 195.5, and 195.6 THz signals have \(-16\), \(-17\), \(-17\), and \(-16.5\) dBm power sensitivities. By increasing the transmitted power, the signal quality can be improved. Figure 7a,b show constellation diagrams for a 16-QAM modulation format on 30 and 80 km fiber at 187.7 THz in DN and 195.5 THz in UP. The BER is clearly below the limit over long-range transmissions for the constellations. Thus, the hybrid fiber-LiFi link enables transmission over long distances at aggregate data rates of 200 Gbps without requiring any digital processing units. Table 2 illustrates the optical spectra of hybrid HS-PON/OFDM using the LiFi system for different fiber lengths and a fixed LiFi range of 10 m at an input power of \(-6\) dBm and a 50 Gbps channel rate. The system utilizes four HS-PON downlink channels (187.5-187.8 THz) separated by a 100 GHz channel spacing, while the LiFi system uses one blue LED (450 nm or 666.2 THz). With increasing SMF ranges from 10 to 100 km, the power in the main tones decreases, but increases in the side lobes. It is possible to obtain a maximum optical power of \(-30\) dBm in a 10 km SMF range with an LED signal power of \(-62\) dBm. In contrast, for SMF and LiFi channels at 100 km, a maximum optical power of \(-45\) and \(-70\) dBm, respectively, isobtained. In addition, the side lobes are between \(-60\) dBm and \(-70\) dBm over a 10-100 km fiber range. Owing to the essence of fiber impairments like linear and  
Figure 7: SMF range versus received optical power performance of different frequency signals in (**a**) DN and (**b**) UP directions; insets: corresponding constellation diagrams. Compared to the recent multiple channels of OFDM-based PONs in Refs.