## 6 Moving Cell Based on C-RAN
### Moving Cell  
In the typical handover process, the train needs to firstly disconnect with the old BS, and then establish a new connection with the next BS. This kind of handoff process will inevitably increase the handoff delay, which can not meet the extremely frequent handoff caused by the ultra-high speed of vactrains. In the concept of moving cell, the handover will not occur due to no cells crossing. Figure 6: C-RAN system architecture for vactrain [42]. The primary reason is that the BS can move simultaneously with the UEs, that is, moving cell. Under this assumption, the communication link between BS and UE can be considered always static regardless of the speed due to no handoffs. At present, based on the above ideas, some physically moving base station schemes have been studied and proposed [46; 47]. However, in fact, these physical moving BS solutions can not be put into practice in HSR systems. [PERSON] and [PERSON] proposed a moving frequency scheme based on optical handoff, which can reduce the handoff delay to 5 ns to 1 ms [48]. The low handover delay can meet the handover requirements of vactrains, so we adopt the moving frequency scheme in the C-RAN architecture. The concept of moving frequency can be implemented in C-RAN architecture, which is illustrated in Figure 6. Specifically, the cloud processing center can deploy the next RAU and activates the corresponding leaky waveguide that the train will access through the optical transmission network according to the exact train location information [49]. The ring network can get all the RAUs that are within the range of the cloud processing center connected with the same optical fiber. In addition, using Wavelength Division Multiplexing (WDM), every RAU can be assigned to a specific wavelength. Further, the fixed Optical Add Drop Multiplexer (OADM) is deployed in each RAU, thus the fixed wavelength is terminated in each RAU. The idea is then to put the desired frequency for a certain RAU on the right wavelength, which can be done through the use of some optical switches in combination with a WDM laser. Figure 7 shows a basic optical switching structure, through which the optical switches can be realized. The WDM laser will generate a beam of light that contains the desired wavelengths. The entire beam is then sent to the optical switch, where the right wavelengths are passed to the different RF modulators (each generating the same RF). The mux will multiplex all the modulated wavelengths, and then transmitted through the optical fiber to the right RAU. The RAU equipped with the OADM dropping the corresponding wavelength will transmit the information on RF through leaky waveguide. In other words, the traditional handover process is replaced by the optical switching process, which could be as low as 5 ns to 1 ms. Based on this, the overlapping length of leaky waveguides should be at least 0.3 m with the train speed of 1000 km/h and the optical signal switching process delay of 1 ms. The basic principle of moving frequency with the consideration of single frequency RF signal is shown in Figure 8.  
Figure 8: Basic principle to implement moving frequency concept. Figure 7: Basic architecture in the cloud processing center to implement moving cell [42]. \(T_{1}\): The train operates within the leaky wave radiation coverage of \(RAU_{i-1}\). In the cloud processing center, the RF signal \(f_{i}\) is modulated to the optical signal with the wavelength \(\lambda_{i-1}\). Then, the optical signal will be transferred through the optic fiber to the \(RAU_{i-1}\). The \(RAU_{i-1}\) will transfer the information on RF signal to the leaky waveguide, which is responsible for the train-ground wireless communication. \(T_{2}\): The train reaches the overlapping radiation coverage area of \(RAU_{i-1}\) and \(RAU_{i}\). The specific position for optical switching can be predicted by fast handoff algorithm [50]. In the switching process, the RF signal \(f_{i}\) will be modulated to the optical signal with the wavelength \(\lambda_{i}\).