Fig. 1. Schematic diagram of an optical fiber-connected radio access network fronthaul. In 5G fronthaul, CUs or DUs serve as the signal control and processing units of the distributed network, while AAUs handle the wireless radio signal up/down-conversion and emission.

Fig. 2. Designed OFDM frame structure with multiple LFM pilots. By leaving one or more subcarriers empty at a certain interval among communication subcarriers in OFDM modulation, the LFM pilots are located in unused frequency slots. The inserted pilot sequence can facilitate both communication signal demodulation and vibration sensing.

Fig. 3. Fronthaul OFDM communication DSP flow for the transmitter and receiver sides. (a) The QAM-modulated OFDM symbols are generated and projected, with a cyclic prefix added to each symbol. A synchronization header is included at the beginning of each frame, followed by several training symbols to enable initial channel estimation. LFM pilots are inserted at specific intervals of subcarriers in the frequency domain. Before transmitting the signal to the fiber and wireless channels, calibration is performed using S-parameter measurements. (b) Owing to the heterodyne A-RoF architecture adopted, distortions from both fiber and wireless channels are jointly processed. LFM pilots assist in both digital down-conversion to the baseband and channel estimation. After their information is extracted, the LFM pilots can be removed.

Fig. 4. Using LFM pilots for frequency offset estimation (FOE) and channel estimation (CE). Stage 1: the received OFDM spectrum with the LFM pilots. Stage 2: a chirp is inversely multiplied to convert the lower sideband LFM pilots into discrete multi-tone frequencies. Stage 3: after a digital filter isolates the lower sideband pilots, another chirp is applied to transform the upper sideband LFM pilots into discrete multi-tone frequencies, which are then filtered out. Stage 4: the multiple LFM pilots are separated from the OFDM payloads. The LFM part is sent to the FOE and CE module, and the payloads are sent to conventional DSP.

Fig. 5. Operating theory of the LFM/frequency-modulated continuous-wave (FMCW) pulse-compression DAS.

Fig. 6. Matched filters are used to obtain ϕ-OTDR traces from different LFM pilots. In the frequency domain, LFM pilots are inserted at specific intervals of subcarriers within the OFDM spectrum, separated from the subcarriers carrying payloads. At the sensing receiver, matched filters are applied to each LFM pilot, reconstructing the RBS spectrum at each point along the fiber. If strain occurs at a specific point, the demodulated RBS spectrum shifts accordingly.

Fig. 7. The two demodulation methods of DAS for recovering vibration are frequency demodulation and phase demodulation. In frequency demodulation, the DSP compares the RBS spectra at different times, while in phase demodulation, the DSP compares the differential phase at different times.

Fig. 8. MMW optical-wireless fronthaul experimental setup. MSG, microwave signal generator; MZM, Mach–Zehnder modulator; WSS, wavelength selective switch; PM-OC, polarization maintained optical coupler; IQM, IQ modulator; EDFA, erbium-doped fiber amplifier; EA, electrical amplifier; Cir, circulator; PZT, piezoelectric transducer; VOA, variable optical attenuator; PD, photodiode; RTO, real-time oscilloscope; ICR, integrated coherent receiver.

Fig. 9. (a) Baseband frequency spectrum of the transmitted OFDM symbols, consisting of 896 subcarriers and a bandwidth of 1.8 GHz. (b) PAPR of the entire OFDM signal, including both LFM pilots and OFDM payloads. The impact of different initial phase distributions on PAPR is analyzed.

Fig. 10. (a) Relationship between communication performance and CSPR is evaluated for 64-QAM-OFDM modulation, a transmission distance of 10 km, a signal-to-pilot ratio (STPR) of 6 dB, and a fiber launch power of 7 dBm. The BER remains below the 15% soft-decision forward error correction (SD-FEC) threshold when the CSPR is less than 13 dB. (b) Additionally, the communication performance is measured for different modulation formats and fiber input powers at CSPR = 10 dB. The modulation formats include QPSK, PCS-16-QAM, 16-QAM, PCS-64-QAM, and 64-QAM, while the launch power ranges from 5 dBm to 13 dBm.

Fig. 11. (a) Communication performance is evaluated for different STPRs and pilot intervals, with CSPR = 10 dB and LP = 10 dBm. (b) Relationship between the BER of all subcarriers and the sweeping bandwidth of the LFM pilots.

Fig. 12. (a) RBS spectrum after matched filtering along the 10-km fiber. Different colors represent varying RBS intensities across different frequencies and distances. (b) Time-distance waterfall plot near the end of the 10 km fiber, where a 60 m PZT is located.

Fig. 13. (a) Applied PZT voltages and their corresponding frequency shifts in the RBS spectrum. The fitted curve demonstrates high linearity. (b) The RBS spectrum at the vibration point shifts proportionally to the vibration intensity, while the sensing resolution is determined by the equivalent pulse width.

Fig. 14. (a) Recovered chirp-wave vibration with a frequency sweep from 20 Hz to 100 Hz. (b) Comparison of phase demodulation (blue line) and frequency demodulation (red line) for recovering the same sine-wave vibration under a 2V_pp voltage applied to the PZT.

Fig. 15. (a) 50-Hz vibration is sampled over a duration of 2 s, and its normalized single-sideband power spectral density is calculated to determine the sensing sensitivity at the fiber end. (b) The relationship between the sensing sensitivity and the STPR shows that sensing sensitivity decreases as STPR increases.

Fig. 16. MMW radio signal generation and reception setup. (a) The OFDM symbol beats with a single-frequency light after being received by the photodiode. The photonics-assisted MMW generation eliminates the need for an up-conversion process in the electrical domain. (b) A 64-element MMW phased array serves as the transmitter (Tx) antenna, while a 16-element MMW phased array serves as the receiver (Rx) antenna. The transmission data from the Tx are fed by the photodiode, and the received radio signal from the Rx is sent to the RTO. A reflective surface is used to construct a fixed-length wireless channel. The experimental MMW wireless transmission operates in the Ka band, ranging from 27.2 GHz to 29 GHz.