SERVER, DATA CACHING METHOD, AND COMMUNICATION SYSTEM CLAIM OF PRIORITY The present application claims priority from Chinese patent 5 application No. 201310101545.7 filed on March 28, 2013, the content of which is hereby incorporated by reference into this application. BACKGROUND OF THE INVENTION This invention relates to a server, a data caching method, and a 10 communication system, and more particularly, to a server, a data caching method, and a communication system, for sending encoded data to a plurality of transmission nodes in case where a user terminal is a mobile terminal. Hitherto, in a wireless network, each base station caches a 15 plurality of files. When a user terminal requests one file, if the file has been cached by a base station (also referred to as local base station) belonging to a scope in which the user terminal is located, the local base station directly provides a service to the user terminal. On the other hand, if the file has not been cached by the local base station, the local 20 base station acquires the file data from a network and sends the acquired file data to the user terminal (refer to US 200510 102300 A l ) . In US 2005/0102300 A l , the base station is required to cache the entire content of the file, causing the consumption of the storage area of the base station. 25 Moreover, in EP 2202888 A l , there is disclosed "Raptor code", which is a typical implementation of fountain codes, which generate approximate infinite encoded packets from limited original packets, achieving a low encoding and decoding complexity. However, EP 2202888 A1 only proposes the application of fountain codes in data transmission in order to improve the efficiency and reliability of the data transmission, and an improvement of the storage efficiency by utilizing the fountain code to encode and store the data is not taken into consideration. Further, in WO 20061 12 1900 A2, there are disclosed a multiple source wireless communication system and method. In the multiple source wireless communication system and method, in a cellular network, when a user terminal can receive communications from a plurality of base stations, Reed-Solomon coding (RS coding) or rateless encoding 10 such as Tornado encoding or Raptor encoding is employed to encode a source data block including M packets, N packets of different subsets are sent from each of the base stations, the packets are received from the plurality of base stations at the receiver, and the source data block can be derived based on K (K=M+A) out of the N packets, where A< M, M>K, and N encoded symbols are stored in the storage module 102. Each encoded symbol is marked by a unique sequence number. Because the Raptor code is a system code, that is to say, the first K encoded symbols are the original symbols. When the 20 user terminal collects K+o (K) encoded symbols, the user terminal can decode the downloaded encoded symbols and acquire the original file, where 020. For example, when it is assumed that one user terminal requests a file with a size of 8 MB, the file is divided into K (K=8,192) original 25 symbols, in which the size of each symbol is L ( L = l kB), and further encoded into N (N=6,553,600) encoded symbols by Raptor code. After encoding, the control module 101 of the server 100 sends N encoded symbols stored in the storage module 102 to the plurality of connected base stations for caching through the encoded symbol transmission interface (i. e., the communication module 104). A method of sending N encoded symbols to the plurality of connected base stations for caching by the control module 101 is described below. It is assumed that there are i base stations, each of 5 the base stations can cache an arbitrary number of encoded symbols, and the i-th base station caches Ci encoded symbols. In order to optimize the cache efficiency, the statistics of the mobile behavior of the user terminal and/or the states of the base stations are performed, and the numbers of encoded symbols cached by 10 different transrnission nodes are pertinently designed. Concretely, the number of the encoded symbols sent to each of the base stations is determined by the control module 101 based on a combination of the state of the base station in the network and the characteristics of the file. 15 The characteristics of the file mainly include a timeout time Ttimeout for downloading the file, The timeout time Ttimeouits dependent on the type of the content downloaded by the user terminal. The content providing server assigns this parameter while sending the content. The state of the base station includes the coverage area of the base 20 station, the average stay time of the user terminal in the base station, the number of the user terminals in the base station, the traffic load sf the base station, the handover frequency (including the frequency of handover incoming and the frequency of handover outgoing) of the user terminal in the base station, and the average channel state of the user terminal in the 25 base station. The average reference values of the stay time Tstay of the user terminal in the macro base station and the average number Urn,,, of the user terminals of the macro base station are counted by the operator, and are assigned in the cache module. The average number UreIayo f the user terminals in the relay stations and the average number Umicroo f the user terminals in the micro base stations are respectively counted by each base station and are acquired from the cache module of the corresponding micro base stations or relay stations. On the design of the cache strategy, when other conditions of base 5 stations are the same, the base station in which the average stay time of the user terminal is longer caches a larger number of encoded symbols, the base station with a larger number of user terminals caches a larger number of encoded symbols, the base station with higher traffic load caches a smaller number of encoded symbols, the base station with poor 10 channel state caches a smaller number of encoded symbols, and the base station with frequent handover caches a smaller number of encoded symbols. Several application modes in which the number of the encoded symbols sent to each base station is determined by the control module 15 10 1 based on a combination of the state of the base station in the network and the characteristics of the file are described below. Application Mode 1 In Application Mode 1, the control module 101 allocates cached data to the plurality of transmission nodes in a homogeneous cellular network. In the homogeneous cellular network, each of the transrriission nodes in the cellular network has the same size and is located in the regular hexagonal cellular network. In this case, each of the transmission nodes has the same coverage area, and thus has the same number of the sent encoded symbols. The encoded symbols to be sent to each of the transmission nodes for caching can be determined by the timeout time, the mobile coefficient (obtained in advance by statistics) of the user terminal, and the size of the file together. In the actual system, the average stay time Tstay of the user terminal in one transmission node may be counted, and in this case, the number of the encoded symbols to be sent to each of the transmission nodes for caching is K* Tsta,/ Ttimeout. Application Mode 2 In Application Mode 2, the control module 101 allocates the cached data to the plurality of transmission nodes in a heterogeneous cellular network. 10 In the heterogeneous cellular network, each of the transmission nodes in the cellular network does not have exactly the same size, and hence the number of the encoded symbols to be sent to each of the transmission nodes for caching is required to be determined by taking into further consideration the parameters of the transmission node itself, 15 i.e. the average number of the user terminals in the transmission node etc. For example, the number of the encoded symbols to be sent to each of the transmission nodes for caching is proportional to the average number of the user terminals in the base station. 20 When it is assumed that the average number of the user terminals in the macro base station is Urn,,, and the average number of the user terminals in one relay station is Urelayt, he number of the encoded syn~buls to be allocated to the relay station is K*TstayT/ timeout*Um~ay/Umacro. 25 Application Mode 3 In a normal cellular network, some transmission nodes are under the condition of high load, while other transmission nodes are under the condition of low load. Therefore, the number of the encoded symbols to be sent to each of the transmission nodes for caching is determined in view of the amount of the traffic load. For a low load transmission node, the transmission rate can be increased to send more encoded symbols in a shorter time. For a high load transmission nodes with limited transmission capacity, 5 the number of encoded symbols for caching needs to be reduced. For example, the number of the encoded symbols to be sent to each of the transmission nodes for caching has a negative correlation to the traffic load of each of the transmission nodes. The high load transmission nodes caches fewer encoded symbols, while the low load 10 transmission nodes caches more encoded symbols. Application Mode 4 Other conditions are the same in the cells (i.e., the coverage areas of the base stations). For some files related to the geographic locations, 15 the probability of the user terminal requesting the file is directly related to the location of the user terminal. When it is assumed that the probability of the user terminal in each of the cells requesting the file is P1, P2, P3 ..., respectively, the ratio of cached contents in the corresponding base stations is approximately PI: P2: P3 . . . 20 These are just four special cases, and for specific design, the determination is required to be made by taking into comprehensive consideration the average stay time of the user terminal in the base station, the number of the user terminals in the base station, the traffic load of the base station, the handover frequency of the user terminal in 25 the base stations, the average channel state of the user terminal in the base station, and the timeout time of downloading the file. According to the embodiment of this invention, the file is cached after being encoded, which can improve the cache efficiency of wireless communication nodes several times. For the cell including more relay stations, this scheme can effectively reduce the downlink energy consumption. Moreover, when the number of the symbols to be cached by each of the base stations is determined, by taking into consideration the 5 factors such as the average stay time of the user terminal in the base station, the number of the user terminals in the base station, the traffic load of the base station, the handover frequency of the user terminal in the base stations, the average channel state of the user terminal in the base station, and the timeout time of downloading the file, the 10 appropriate amount of cached data can be allocated to each of the base stations, the cache area in each of the base stations can be properly utilized, the cache efficiency can be improved, the utilization of the cached data in the base station can be improved, and the download efficiency of the user terminal can be improved. 15 In the embodiment of this invention, the types of the fountain codes used are not limited. As for the fountain codes, while some fountain codes are random encoding, which can generate an arbitrary number of symbols, the fountain code like Raptor code, which needs to ensure the decoding quality, has an upper limit of the number of encoded 20 symbols but the value of the upper limit is huge. Therefore, in general, calculating and allocating the encoded symbols for caching in such a manner as described above iil any one of Application Modes 1 to 4 causes no problem. However, in some extreme cases, when the number of the base stations is huge, even a state in which each base station caches one 25 encoded symbol on average cannot be realized, and at this moment, such a mode as illustrated in FIG. 6, in which encoded symbols are reused among base stations, can be adopted. In FIG. 6, each hexagonal box represents a base station, the number labeled in each hexagonal box represents the encoded symbol cached by the base station itself, and the base stations labeled with the same number reuse the encoded symbol among those base stations. For example, in FIG. 6, four base stations labeled with the number " 1" reuse the encoded symbol among those four base stations. The communication system of this invention is described below. The communication system of this invention is realized mainly in two processes: a process in which the server sends the encoded symbols to the base station and then the base station caches the encoded symbols; and a process in which the user terminal downloads the symbols. 10 Firstly, with reference to FIG. 7, a signaling exchange process in which the communication system of the embodiment of this invention as illustrated in FIG. 1 sends the encoded symbols to the base stations is described. As illustrated in FIG. 7, firstly, the server sends a request of 15 inquiring the state of the base station to each of base stations 1, 2 and 3 via the backbone network. Next, each of base stations 1, 2 and 3 returns the state of the base station itself to the server after receiving the request of inquiring the state of the base station sent from the server. 20 Next, after the server receives the state information of the base station from each of base stations 1, 2 and 3, the base station information management module 105 of the server employs the state information of the base station received to update the state of the base station. 25 Next, the server calculates the number of the encoded symbols based on the updated state of the base station. Calculating the number of the encoded symbols here is performed in such a manner as described above in any one of Application Modes 1 to 4. Specifically, the appropriate numbers of symbols to be cached by each of base stations 1, 2 and 3 are calculated based on the state information of each of base stations 1, 2 and 3. Then, the server generates encoded symbols. Next, the server sends the corresponding number of the encoded 5 symbols to each of base stations 1, 2 and 3 based on the calculated number of the encoded symbols to be cached by each of the base stations. Base stations 1, 2 and 3 cache the encoded symbols in their caches respectively after receiving the encoded symbols sent from the server, and each of base stations 1, 2 and 3 generate a file information 10 list and a cached symbol list based on the encoded symbols received. With this, the first main process of the communication system of the embodiment of this invention ends. The other main process of the communication system of this invention, i.e. the download process of the user terminal, is described below. 15 The download process of the user terminal is summarized as follows. 1) Starting the download When the user terminal initiates a download request, the user terminal initiates a query request to the closest base station, and the 20 base station reports back the serial numbers (need to be defined) of the content cached in the base station. The user terminal records the serial numbers of received symbols on local, and hence the user terminal determines whether there are the serial numbers to be downloaded in the base station based on the serial numbers of symbols recorded on local 25 and the serial numbers of cached content in the base station reported back by the base station, and further accordingly determines whether the base station is to be connected or be regarded as an excluded base station. When it is determined that there are serial numbers to be downloaded in the base station, the user terminal starts the request for each symbol to the base station one by one. 2) Handover When the cached content of the currently connected base station is completely downloaded, or when the user terminal moves out of the service 5 area of the original base station, the user terminal needs a handover. Similar to the current handover process, the user terminal notifies the original connected base station that the user terminal aborts the download, and starts the request to a new base station. As in the case of starting the download, the base station reports back the serial numbers of the content 10 cached in the base station, and each of the user terminals initiates the download requests for the corresponding data. 3) Pausing the download If the content which is not downloaded by the user terminal cannot be found in the neighboring base stations that can be connected and the 15 time does not exceed the timeout time Ttimeoutht,e download is paused until a new base station that can be connected is found. 4) Finishing the download The user terminal starts an attempt at decoding after having collected K encoding packets, and when the attempt is successful, 20 decoding is made in order to finish the download without sending a new symbol request. On the other hand, when the attempt is not successful, a new attempt of decoding is taken again after the download of the next symbol is finished. 5) Timeout handling 25 A timeout time Ttimeouist set based on the extent of the delay tolerance of the downloaded content. When the download is still not finished over the timeout time, the download is not paused, and when the content which is not downloaded by the user terminal cannot be found in the neighboring base stations that can be connected, an original symbol request for completing the content is sent to the content source on the Internet directly via the backbone network of the base station. With reference to FIG. 8 and FIG. 9, the download process of the user terminal is described below in detail. FIG. 8 is a sequence diagram of one example in which the user terminal device in the communication system of the embodiment of this invention executes a download once without timeout. It is assumed that the number K of the original symbols equals to 300, and through the sending process as illustrated in FIG. 8, base station 1 10 caches the symbols 1 to 100, base station 2 caches the symbols 10 1 to 200, and base station 3 caches the symbols 201 to 300. As illustrated in FIG. 8, initially the user terminal is located in the coverage area of base station 1 and always is in a moving state. The user terminal requests the relevant information of the file 1 (the size S of the file, 15 the number K of the original symbols, the number N of the encoded symbols, and the size L of the encoded symbols) from base station 1. Base station 1 requests the file information from the server, and reports the file information back to the user terminal after receiving the file information from the server. Next, the user terminal continues to request the cached symbol list in 20 the base station from base station 1, and base station 1 returns the cached symbol list stored in base station 1 to the user terminal (symbols 1 to 100 in this example). Next, the user terminal requests each symbol from base station 1. After the 59th symbol is completely received, the user terminal moves 25 out of the coverage area of base station 1 and moves into the coverage areas of base station 2 and base station 3. It is assumed here that the user terminal communicates with the base station with stronger signal (here assumed as base station 2). The user terminal requests the cached symbol list of base station 2 and requests the cached symbols of base station 2 one by one. After all cached symbols (101 to 200) of base station 2 are completely downloaded, the user terminal adds base station 2 into the base station exclusion list. Next, the user terminal is still located in the coverage area of base 5 station 3, and hence a handover to base station 3 for communication is made. Firstly, the cached symbol list of base station 3 is requested, and then the cached symbols of base station 3 are requested one by one. After the cached symbol 241 of base station 3 is completely downloaded, the user terminal decodes the received symbols, generates the 10 source file, stops the download, and completes the download. FIG. 9 is a signaling interaction diagram of one example in which the user terminal device in the communication system of the embodiment of this invention executes a timeout download once. As illustrated in FIG. 9, the download process is similar to the 15 example of FIG. 8, but the difference is that the user terminal moves into the coverage area of base station 2 after leaving the coverage area of base station 1. After all cached symbols of base station 2 are completely downloaded by the user terminal, when all base stations are in the base station exclusion list, then the user terminal enters a wait state. When the timeout timer 20 indicates timeout, the user terminal requests the original symbols from the server via base station 2 to finish the download. FIGS. 10A to 10C are flowcharts illustrating the download action of the user terminal device in the communication system of this invention. As illustrated in FIG. 10A, firstly, the user terminal sends the file 25 information requests to all base stations based on an ID of the requested file (Step S101). Next, a determination is made as to whether the user terminal receives the signals from the base stations which are not in the excluded base station list (Step S102). When the determination is that the user terminal receives the signals from the base stations which are not in the base station exclusion list (being "Yes" in Step S102), the user terminal is connected to the base station with the strongest received signal of the base stations whose signals are received (Step S103). When the determination is 5 that the user terminal does not receive the signals from the base stations which are not in the excluded base station list (being "No" in Step S102), the procedure goes to Step S 110. In Step S104, the user terminal determines whether the cached symbol list of the connected base station is received. When the 10 determination is that the cached symbol list is received ("Yes" in Step S104), the user terminal further determines whether there is a symbol not being received in the cached symbol list of the connected base station (Step S105). When the determination is that the cached symbol list is not received ("No" in Step S 104), the procedure goes to Step S 11 1. 15 When the determination of Step S105 is that there exists a symbol not being received ("Yes" in Step S105), the procedure goes to Step S106. On the other hand, when the determination of Step S105 is that all symbols have been received ("No" in Step S105) does not exist, the procedure goes to Step S112 and the current connected base station is added into the base 20 station exclusion list. In Step S106, the user terminal requests one cached symbol not being received. In Step S107, the user terminal determines whether the download is successful. When the determination is that the download is unsuccessful 25 ("No" in Step S107), the procedure returns to Step S102. On the other hand, when the determination is that the download is successful ("Yes" in Step S 107), the procedure goes to Step S 108, in which a determination is made as to whether the number of the collected symbols is more than the number of the original symbols. When the determination is that the number of the collected symbols is not more than the number of the original symbols ("No" in Step S108), the procedure returns to Step S105. On the other hand, when the determination is that the number of the collected symbols is more than the number of the original symbols ("Yes" in Step S108), decoding is 5 made and a determination is made as to whether decoding is successful (Step Slog), and when the determination is that decoding is unsuccessful ("No" in Step Slog), the procedure returns to Step S105. On the other hand, when the determination is that decoding is successful ("Yes" in Step Slog), the process ends. 10 In addition, in Step S110, a determination is made as to whether it exceeds the timeout time. When the determination is that it exceeds the timeout time ("Yes" in Step S1 lo), a determination is made as to whether the signal of the base station can be received (Step S113), and when the determination is that the signal of the base station cannot be received, Step 15 S113 is repeated. On the other hand, when the determination is that the signal of the base station can be received ("Yes" in Step S113), a request for the symbol not being received with the smallest serial number is sent to the server via the base station (Step S114), decoding is made, and a determination is made as to whether decoding is successful (Step S115). 20 When the determination is that decoding is unsuccessful (being "No" in Step S1151, the procedure returns to Step S114. On the other hand, when the determination is that decoding is successful ("Yes" in Step S 115), the process ends. When the determination is that it does not exceed the timeout time ("No" in Step S 1 lo), the procedure returns to Step S 102. 25 In the above, the processes of caching and downloading the data of the communication system of this invention have been described. According to the communication system of the embodiment of this invention, the file is cached after being encoded, which can improve the cache efficiency of the wireless communication nodes several times. For the cellular including more relay stations, this scheme can effectively reduce the downlink energy consumption. Moreover, according to the communication system of the embodiment of this invention, the appropriate amount of the cached data 5 can be allocated to each of the base stations, the cache area in each of the base stations can be properly utilized, the cache efficiency can be improved, and the utilization of the cached data in the base station can be improved. This invention is not limited to the above-described embodiments but 10 includes various modifications. The above-described embodiments are explained in details for better understanding of this invention and are not limited to those including all the configurations described above. A part of the configuration of one embodiment may be replaced with that of another embodiment; the configuration of one embodiment may be incorporated to 15 the configuration of another embodiment. A part of the configuration of each embodiment may be added, deleted, or replaced by that of a different configuration. For example, the case in which there is only one user terminal in the communication system has been described, but it should be 20 understood that there may be more than one user terminal in the communication system. The above-described configurations, functions, processing modules, and processing means, for all or a part of them, may be implemented by hardware: for example, by designing an integrated circuit. The 25 above-described configurations and functions may be implemented by software, which means that a processor interprets and executes programs providing the functions. The information of programs, tables, and files to implement the functions may be stored in a storage device such as a memory, a hard disk drive, or an SSD (a Solid State Drive), or a storage medium such as an IC card, or an SD card. The drawings shows control lines and information lines as considered necessary for explanation but do not show all control lines or information 5 lines in the products. It can be considered that almost of all components are actually interconnected. WHAT IS CLAIMED IS: 1. A server comprising: a storage module for storing original files and characteristics of the original files; an encoding module for encoding the original files stored in the storage module to generate encoded symbols; an information acquisition module for communicating with at least one transmission node and acquiring state information from the at least one transmission node; 10 a control module for calculating a number of the encoded symbols to be sent to the at least one transmission node for caching based on the characteristics of the original files stored in the storage module and the state information of the at least one transmission node acquired by the information acquisition module; and 15 a communication module for sending the encoded symbols to the at least one transmission node based on the number of the encoded symbols calculated by the control module. 2. The server according to claim 1, wherein: the characteristics of the original files include a timeout time for downloading a file; and the state information includes a coverage area of a transmission node, an average stay time of a user terminal in a transmission node, a number of the user terminals in a transmission node, an amount of a traffic load of a transmission node, a handover frequency of a user terminal in the transmission nodes, and an average channel state between a user terminal and a transmission node. 3. The server according to claim 2, wherein, when calculating the 30 number of the encoded symbols to be sent to each of the at least one transmission node for caching, the number of the encoded symbols to be sent to the each of the at least one transmission node for caching has a positive correlation to each of the coverage area of the transmission node, 5 the average stay time of the user terminal in the transmission node, the number of the user terminals in the transmission node, and the average channel state between the user terminal and the transmission node, and has a negative correlation to each of the amount of the traffic load of the transmission node and the handover frequency of the user terminal in the 10 transmission nodes. 4. A data caching method for sending cached data to at least one transmission node coupled to the server, the data caching method including steps of: 15 encoding original files stored in the server to generate encoded symbols; communicating with the at least one transmission node and acquiring state information from the at least one transmission node; calculating a number of the encoded symbols to be sent to the at 20 least one transmission node for caching based on characteristics of the original files stored in the server and the acquired state information of the at least one transmission node; and sending the encoded symbols to the at least one transmission node based on the calculated number of the encoded symbols. 5. A communication system comprising a server, at least one transmission node, and at least one user terminal, the server and the at least one transmission node being coupled by wired connection, the at least one transmission node and the at least one user terminal being coupled by wireless connection, the at least one transmission node being configured to receive and cache encoded symbols sent from the server, the at least one user terminal being configured to request the encoded symbols cached by the at least one transmission node from the at least one transmission node, 10 the server including: a storage module for storing original files and characteristics of the original files; an encoding module for encoding the original files stored in the storage module to generate encoded symbols; 15 an information acquisition module for communicating with the at least one transmission node and acquiring state information from the at least one transmission node; a control module for calculating a number of the encoded symbols to be sent to the at least one transmission node for caching based on the 20 characteristics of the original files stored in the storage module and the state information of the at least one transmission node acquired by the information acquisition module; and a communication module for sending the encoded symbols to the at least one transmission node based on the number of the encoded symbols 25 calculated by the control module. 6. The communication system according to claim 5, wherein the at least one user terminal requests the encoded symbols cached in the at least one transmission node from the at least one transmission node while moving. 7. The communication system according to claim 5, wherein: the characteristics of the original files include a timeout time for 5 downloading a file; and the state information includes a coverage area of a transmission node, an average stay time of a user terminal in a transmission node, a number of the user terminals in a transmission node, an amount of a traffic load of a transmission node, a handover frequency of a user terminal in the 10 transmission nodes, and an average channel state between a user terminal and a transmission node. 8. The communication system according to claim 5, wherein, when calculating the number of the encoded symbols to be sent to each of the at 15 least one transmission node for caching, the number of the encoded symbols to be sent to the each of the at least one transmission node for caching has a positive correlation to each of a coverage area of a transmission node, an average stay time of a user terminal in a transmission node, a number of the user terminals in a transmission node, and an average channel state 20 between a user terminal and a transmission node, and has a negative correlation to each of an amount of a traffic load of a transmission node and a handover frequency of a user terminal in the transmission nodes. 9. A server, substantially as herein described with reference to 25 accompanying drawings and examples. 10. A data caching method, substantially as herein described with reference to accompanying drawings and examples. 1 1. A communication system, substantially as herein described with reference to accompanying drawings and examples.