## 5 Discussion  
In the section of experimental results above, it is clearly shown that the quality of the transmitted RF signal with the optical-electronic joint compensation is improved significantly. Although some works demonstrate that the timing suppression method with a single optical phase compensator can achieve low-noise frequency transfer, this technique cannot solve the problem of a limited compensation range, as the peak-to-peak timing fluctuation of a long fiber link in long-term scale is very large. At the same time, some works have also reported fiber-based frequency transfer that only use electronic compensation, but it is difficult for the electronic compensation technology to provide high-resolution timing jitter compensation. In order to allow the authors to fully understand the progress of the fiber-based RF transfer techniques, we demonstrate the results of some fiber frequency transfer experiments in recent years, as shown in Table 1. From the table, we can find that when the frequency of the transferred signal is higher than 10 GHz and the fiber length exceeds 50 km, it is difficult to have a technology of fiber-based transfer that can provide timing jitter at the level of a hundred of femtoseconds.  
In this paper, we combined an optical fiber stretcher and an electronic phase shifter to build an optical-electronic joint compensator. With the advantages of the large range and high resolution of the joint phase shifter, a long-term and ultra-low jitter frequency transfer over long fiber link was achieved. By contrasting the experimental results of timing fluctuation in a couple of hours with and without phase compensator, it proved that our optical-electronic joint phase compensation method validly suppresses the ambient perturbation introduced timing fluctuation in a long time. In addition, the instability of our frequency transfer is superior to a widely used H-master clock, which indicates that our proposed frequency transfer method can be used to disseminate atomic clocks over long-distance fiber-optic links in long-time scale.  
By comparing the commonly used frequency transfer techniques based on microwave communication systems such as GPS and two-way satellite time and frequency transfer (TWSTFT) methods, the fiber-based frequency transfer using phase-compensation has the advantages in cost, accuracy and reliability. First, the power of the laser transceiver components used in the optical frequency transfer system is less than 100 mw, and the power of the electric components and the servo controller is also in the level of watt. The typical microwave communication method requires high-power electronic components. Secondly, the optical frequency transfer can reach the precision of picosecond and femtosecond, while the precision of the microwave communication method can only reach the order of nanosecond. Finally, the radio channel used by microwave communication is far less reliable and safer than the fiber transmission link. Therefore, the fiber-based frequency  
Figure 5: Frequency instability results for the fiber-based RF transfer. (i) Free-running link without phase compensation. (ii) With optical–electronic joint phase compensation. (iii) The measurement floor for the short link. (iv) For H-master clock.  
transfer with phase compensation has become an important and mature approach in the high-precision time-frequency area.