Fig. 1. Concept of clock-synchronized, parallel WDM channel fronthaul structure enabling multifunctional RAN, empowering various applications required in the IMT 2030 usage scenarios [17].

Fig. 2. The principle of EO comb cloning and clock synchronization.

Fig. 3. Experimental setup for the clock-synchronized, self-homodyne fronthaul structure enabled by EO comb cloning. IM: intensity modulator, PM: phase modulator, RF: RF source, AWG: arbitrary waveform generator, IQ Mod.: in-phase and quadrature modulator, PDME: polarization division multiplexing emulator, WSS: wavelength selective switch, VOA: variable optical attenuator, PD: photodetector, OBPF: optical bandpass filter, Coh. Rx.: coherent receiver, VCO: voltage-controlled oscillator, 1/2: 1/2 frequency divider, 1/125: 1/125 frequency divider, PM-EDFA: polarization maintaining erbium-doped fiber amplifier, EDFA: erbium-doped fiber amplifier, LNA: low-noise amplifier.

Fig. 4. Optical spectra of (a) generated carrier comb at Tx (blue line) and cloned comb at Rx (red line) and (b) received signal after 10 km transmission.

Fig. 5. (a) RF spectrum of 25 GHz signal generated by VCO. The inset is the 100 MHz clock signal via subsequent frequency dividing. (b) Single-sideband phase noise PSD of 25 GHz RF source (purple curve), 25 GHz VCO RF output with and without servo loop locking (red and yellow curves), and 100 MHz clock signal after frequency dividing (blue curve).

Fig. 6. (a) Single-sideband PSD of the 100 MHz post-frequency-dividing VCO signal with phase-locked loop with different transmission distances of back-to-back, 4 km, 10 km, and 20 km, marked in red, yellow, blue, and purple, respectively; (b) timing jitter of the 100 MHz clock with different received optical powers of the photodetector.

Fig. 7. The constellations of the 64-QAM signal for Ch. #–1 under three scenarios: without clock synchronization, with digital timing recovery, and with our proposed approach, and Ch. #6. The DSP omission for each recovered signal’s constellation is indicated at the top, highlighting the DSP omission ratio for each case. Note that the carrier phase estimation omission ratio is elaborated in Appendix A.

Fig. 8. (a) BER performance of Ch. #–7 to −1, 3 to 7. All 12 channels used for transmission meet the threshold of 1.0×10−2 for the hard decision FEC with 14% overhead, averaging at BER=7.2×10−3. (b) BER versus received optical power of the signal for Ch. #–1. Minor penalty is observed compared to the ideal electrical back to back and most commonly used DSP-synchronized scenario across different received signal optical powers, showcasing the effectiveness of our approach.

Fig. 9. DSP flows of the traditional coherent transmission and simplified DSP thanks to the clock-synchronized and self-homodyne architecture in our approach.