With the rapid development of OWC systems, they have gained attention due to their potential for providing high-speed and high-capacity optical communications. The integration of 5G signals with OWC systems (see Fig. 1) offers promising avenues for providing high transmission rates and meeting the growing demand for faster and more reliable wireless connectivity. In this demonstration, a 5G WDM-based bidirectional OWC system with signal remodulation employing cascaded RSOAs to effectively remove the downstream data is practically implemented. For downlink transmission, each of the four optical wavelengths is used to deliver a 9.1-Gbit/s/28-GHz MMW signal using 16-QAM-OFDM modulation. The downstream modulated data on the four optical wavelengths is effectually erased by two RSOAs, and these wavelengths are then reused for upstream optical carriers. The uplink performance is substantially enhanced by utilizing two RSOAs to remove the downstream data. The four upstream optical wavelengths are modulated by an MZM with 9.1-Gbit/s/24-GHz MMW signal using 16-QAM-OFDM modulation. This 5G WDM-based bidirectional OWC system achieves an aggregate transmission rate of 36.4 Gbit/s for both downstream and upstream data. Through 100-m optical wireless link, good BERs and EVMs performance, clear constellation diagrams, and flat electrical spectra are attained for downlink/uplink transmissions.

Figure 2 depicts the configuration of the 5G WDM-based bidirectional OWC systems with cascaded RSOAs. The output of the broadband light source is boosted by an EDFA and efficiently separated into four wavelengths by a 1x4 WDM DEMUX. The four wavelengths of λ₁ (1549.3 nm), λ2 (1550.9 nm), λ3 (1552.5 nm), and λ4 (1554.1 nm) are multiplexed by a 4x1 WDM MUX, and then supplied to an MZM via a polarization controller. The 9.1-Gbit/s/10-GHz 16-QAM-OFDM signal generated by the OFDM transmitter is upconverted to a 9.1-Gbit/s/28-GHz signal using a mixer with an 18 GHz local oscillator signal. The upconverted signal then drives the MZM through a modulator driver. Via two optical circulators (OC1 and OC2), the optical signals are delivered through a 100-m optical wireless link using a fiber collimator with 0.06° divergence angle at the transmitting site and an optical dish antenna with a doublet lens at the receiving site. After circulation by the OC2, the optical signal with four wavelengths is split into two parts utilizing an optical splitter. One part of the optical signal is demultiplexed by a 1x4 WDM DEMUX. The demultiplexed wavelength with downlink OFDM signal is received by a 30-GHz PD, amplified by an LNA, and transmitted to a DSO for downlink performance estimation. Another part of the optical signal is injected into two RSOAs via the OC3 and the OC4 to virtually erase the downstream modulated data and reproduce four pure optical carriers for uplink transmission. 

For uplink transmission, a 9.1-Gbit/s/10-GHz 16-QAM-OFDM signal is upconverted to a 9.1-Gbit/s/24-GHz signal using a mixer with 14 GHz local oscillator signal. Then, the upconverted signal drives an MZM through a modulator driver. After amplification by an EDFA, a VOA optimally controls the optical powers. Through routing by two optical circulators (OC2 and OC1), the optical signal with four wavelengths is transmitted wirelessly through a 100-m optical wireless link using a doublet lens with an optical dish antenna. At the receiving site, a fiber collimator is used to collect the transmitted optical signal. The received optical signal is demultiplexed by a 1x4 WDM DEMUX. The demultiplexed wavelength with uplink OFDM signal is received by a 30-GHz PD and amplified by an LNA. The enhanced electrical signal is next fed into a DSO for uplink performance analysis.

The downlink/uplink BERs under different received MMW powers over a 100-m optical wireless link are exhibited in Fig. 3(a). For downlink OFDM signal transmission, a 3.8x10⁻³ (FEC limit) BER is attained at received MMW powers of -26.9 (λ₁, 1549.3 nm) and -27 (λ₂, 1550.9 nm) dBm. For uplink OFDM signal transmission (two RSOAs), a 3.8´10⁻³ BER is acquired at received MMW powers of -27.2 (λ₁) and -27.3 (λ₂) dBm. To have more correlation with the number of RSOA and uplink BER performance, we remove one RSOA to evaluate the uplink BERs over 100-m optical wireless link. For uplink OFDM signal transmission (one RSOA), a 3.8x10⁻³ BER is acquired at received MMW powers of -23.9 (λ₁) and -24.1 (λ₂) dBm. At 3.8x10⁻³ BER, power penalty improvements of 3.3 dB (λ₁) and 3.2 dB (λ₂) are observed when using two RSOAs. Furthermore, Fig. 3(b) shows the EVMs at wavelengths of λ₁ and λ₂ (downlink/uplink) and at different received MMW powers. Through a 100-m optical wireless link, the EVMs of downlink/uplink 16-QAM-OFDM signals remain below the 12.5% 3GPP limit when the received MMW powers are higher than -28.9 (λ₁), -29.1 (λ₂), -29.3 (λ₁, two RSOAs, uplink), and -29.4 (λ₂, two RSOAs, uplink) dBm, respectively. To verify the relationship between the number of RSOA and uplink EVM performance, we change two RSOAs to one RSOA to evaluate the EVMs. For uplink signal using 16-QAM-OFDM modulation (one RSOA), a 12.5% EVM is obtained at received MMW powers of -25.5 (λ₁) and -25.7 (λ₂) dBm. At 12.5% EVM, power penalty degradations of 3.8 dB (λ₁) and 3.7 dB (λ₂) exist when using one RSOA. When using one RSOA, the downstream modulated data is incompletely erased, leading to interference to degrade the uplink EVM performance. As for the constellation diagrams, Figs. 3(c) and 3(d) show the connected constellation diagrams of 9.1-Gbit/s/28-GHz and 9.1-Gbit/s/24-GHz 16-QAM-OFDM signals at wavelength of λ₁ (downlink/uplink), over 100-m optical wireless link and at -26.9-dBm received MMW power. Clearly, each downlink/uplink 16-QAM-OFDM signal has a distinct constellation diagram with BER of 3.8x10⁻³ (Fig. 3(c)) and 2.6x10⁻³ (Fig. 3 (d)). 

To clarify the improvement attained by employing two RSOAs, comparisons for constellation diagrams with scenarios involving no RSOA, one RSOA, and two RSOAs are presented. Figs. 4(a), 4(b), and 4(c) show the related constellation diagrams of 9.1-Gbit/s/24-GHz 16-QAM-OFDM signal at wavelength of λ₁ (uplink) through 100-m optical wireless link and at -27.2 dBm received MMW power, in the scenarios with no RSOA, one RSOA, and two RSOAs. In the scenario with no RSOA, the constellation diagram shows a blurred pattern with 4.7x10⁻¹ BER (Fig. 4(a)). In the scenario with one RSOA, the constellation diagram presents a somewhat blurred pattern with 5.4x10⁻³ BER (Fig. 4(b)). In the scenario with two RSOAs, however, a clear and distinct constellation diagram with 2.6x10⁻³ BER (Fig. 4(c)) is attained. In the scenario with no RSOA, the downstream modulated data is not erased and this can lead to the simultaneous modulation of downstream and upstream data on the same optical carrier. This situation brings on strong interference that degrades uplink performance and results in a blurred constellation diagram. In the scenario with one RSOA, the downstream modulated data is incompletely suppressed and this can lead to partial downstream data and all upstream data modulating the same optical carrier. This situation can cause interference, leading to reduced uplink performance and somewhat blurred constellation diagrams. As for the scenario with two RSOAs, the downstream modulated data is virtually eliminated, ensuring that only upstream modulated data exists on the optical carrier. This situation will not cause interference from the downstream modulated data.