## 3 Experimental Setup  
In order to verify the performance of this scheme, we built an experiment of actual multiple-access time and frequency transfer over fiber and free-space links based on a femtosecond OFC, as shown in Figure 3. The experimental setup was designed with a master site, A, and two slave sites, B and C. All sites were placed at the same local end, and a rubidium atomic clock provided the reference source. At the far end, we designed a flat mirror to realize the light reflection between the master and the slave sites to form an optical link. The optical comb source of site A is an erbium-doped fiber femtosecond laser with a repetition frequency of 100 MHz, with a center wavelength of 1550 nm, power of \(\sim\)60 mW, and pulse width of \(\sim\)200 fs [30]. After the optical signal passes through an optical coupler (OC), one of the low-power signals is extracted through PD1, power splitter (PS), and BFP to obtain a repetition frequency 100 MHz signal. This signal is phase-detected with the 100 MHz signal synthesized by the rubidium atomic clock (FE-5680A, FEI Communications, Mitchel Field, NY, USA) in a mixer. After the phase error is generated, it is fed back to the laser through a proportional integral (PI) controller to form a phase-locked loop to achieve the stabilization of the OFC. The high-power signal output by OC enters the 3.9 km long optical fiber through a circulator, and then another OC outputs two optical signals of equal power, which are sent to free space through two collimators (CMs) and two telescopes. The two spatial lights reach sites B and C, respectively, after being reflected by the remote mirror. The distance of the entire free-space link is about 100 m. At sites B and C, the pulse optical signal transmitted by site A is collected by a CM and sent to PD3 and PD4 through a circulator, and the electrical signal containing repetition frequency and 2 nd harmonic frequency is output. At site B, the signal output by PD3 passes through a BFP and an amplifier (AMP) to obtain a 100 MHz signal; at site C, the signal output by PD4 passes through a BFP and AMP to obtain a 200 MHz signal. At this point, the process of transmitting RF signals from the master site to each slave site is realized. The light sources of sites B and C are two CW lasers with a wavelength of 1550 nm, output power of \(\sim\)20 mW, and linewidth of \(\sim\)3 MHz. The time and frequency source of site B is a 100 MHz signal synthesized by the rubidium atomic clock. The 100 MHz electrical signal is modulated onto the light through an amplitude modulation EOM and then sent to the free-space link through a circulator and a CM. The time and frequency source of site C is a 200 MHz signal synthesized by the rubidium atomic clock. The 200 MHz electrical signal is also modulated onto the light through another amplitude modulation EOM and then sent to the free-space link through a circulator and a CM. As shown in Figure 3, the two optical signals arrive at site A along the same optical path mentioned above, are collected by two CMs, combined at the OC, passed through the 4 km optical fiber, and converted into an electrical signal by the circulator and PD2. It should be pointed out that the electrical signal contains both 100 MHz and 200 MHz RF signals sent simultaneously by sites B and C. It is divided into two electrical signals through a PS and then passed through two BPFs, respectively. Finally, two single-frequency 100 MHz and 200 MHz signals are extracted. In order to obtain the time-frequency difference of site A, the electrical signal output by PD1 passes through a PS and two BPFs to obtain two reference signals (with frequencies of 100 MHz and 200 MHz, respectively). The phase difference between the reference 100 MHz signal and the 100 MHz signal received from site B is measured using a digital phase detector (DPD) to obtain the phase difference value \(\varphi_{\rm AB}\).