the main idea of the present invention is to design the rf transceiver module at the remote side and to set an interface unit at the indoor part of the base station and the rrs respectively so as to perform the signal intercommunication between the indoor part of the base station and the rrs and thus improve the flexibility of the base station system . fig3 is a schematic diagram showing a configuration of a base station system according to the present invention . as shown in fig3 , taking td - scdma system as an example , a node b and a rnc are connected through a standard iub interface . the base station includes an indoor part and an outdoor part constituted by a remote rf sub - system ( rrs ). the rss includes a remote interface unit , a rf transceiver unit ( tru ) and an ant . the tru receives an analog if signal from the remote interface unit , converts the signal to an analog rf signal , amplifies the analog rf signal and transmits the amplified analog rf signal through the ant ; or the tru receives and amplifies an analog rf signal from the ant and converts the analog rf signal to an analog if signal , and then the remote interface unit makes processing on the analog if signal and finally transmits the processed interface signal to a local interface unit of the indoor part of the base station system through an optical cable or an if cable . a rnc interface unit ( riu ), for performing an interface function between node b and the rnc ; a central control unit ( ccu ), for performing a main control function of node b and a function of switching signaling and service data between units of node b , wherein data from the rnc is transmitted to the ccu through the riu and data to be transmitted upwardly in node b is transferred to the rnc through the riu ; a digital baseband unit ( bbu ), for performing a processing function of a digital signal at the symbol level and the code chip level at the physical layer and for interchanging a digital baseband signal with the local interface unit ; a gps unit ( gpsu ) and a gps antenna ( gps ant ), for obtaining a gps clock signal through the gps ant , generating a clock signal and provide the generated clock signal to the riu , ccu , bbu in the node b ; and the local interface unit , for converting the digital baseband signal received from the bbu to an interface signal and transmitting the interface signal to the remote interface unit through the optical cable or the if cable , or for converting the interface signal received through the optical cable or the if cable to a digital baseband signal and transmitting the digital baseband signal to the bbu . in order to better understand the objectives , technical solutions and advantages of the present invention , the preferred embodiments will be described in combination with the accompanying drawings . the configuration of the base station described below is the same with that of the conventional base station , which includes an uplink path and a downlink path . fig4 is a schematic diagram showing a configuration of a base station system according to a first embodiment of the present invention . the local interface unit is a local baseband interface unit ( biu - l ). the functions of the local interface unit are : ( 1 ) through the downlink path of the local interface unit , the biu - l receives the digital baseband signal from the bbu , converts the electric signal to a digital baseband optical signal and transmits the optical signal to the remote interface unit , wherein the interface signal is the digital baseband optical signal ; ( 2 ) through the uplink path of the local interface unit , the biu - l receives the interface signal , i . e . a digital baseband optical signal from the remote interface unit , converts the optical signal to a digital baseband electric signal and transmits the electric signal to the bbu . the biu - l may implement its functions by an optical transceiver module with an electric / optical conversion function and an optical / electric conversion function , such as the single mode optical transceiver module pt7320 - 41 or the single mode optical transceiver module pt7320 - 61 . the remote interface unit includes a remote baseband interface unit ( biu - r ) and a digital if unit ( ifu ), wherein the ifu is a digital if module of the conventional rfu . the functions of the remote interface unit are : ( 1 ) through the downlink path of the remote interface unit , the biu - r receives the digital baseband optical signal from the local interface unit , converts the optical signal to the digital baseband signal , i . e ., the digital baseband electric signal , and transmits the electric signal to the ifu and then the ifu performs the baseband signal shaping and filtering , digital up - conversion and d / a conversion on the received digital baseband signal to generates the analog if signal and transmits the analog if signal to the tru ; ( 2 ) through the uplink path of the local interface unit , the ifu receives the analog if signal from the tru , performs the a / d conversion , digital down - conversion , and baseband signal shaping and filtering on the analog if signal to generate the digital baseband signal and transmits the digital baseband signal to the biu - r , and then the biu - r performs converts the electric / optical conversion on the received digital baseband signal to obtain the digital baseband optical signal and transmits the optical signal to the local interface unit . the biu - r may implement its functions by an optical transceiver module with an electric / optical conversion function and an optical / electric conversion function , such as the single mode optical transceiver module pt7320 - 41 or the single mode optical transceiver module pt7320 - 61 . the tru is a rf transceiver module of the conventional rfu . since the rf transceiver module of the rfu has the function of amplifying the signal , the pa & amp ; lna unit is omitted in the present invention . the functions of the tru are : ( 1 ) through the downlink path of the tru , the tru receives the analog if signal from the ifu , performs the amplifying , filtering , gain control , conversion from the analog if signal to the analog rf signal on the analog rf signal , performs the amplifying , filtering and gain control on the analog rf signal obtained after the conversion and finally transmits the processed analog rf signal to the ant ; ( 2 ) through the uplink path of the tru , the tru receives the analog rf signal from the ant , performs the amplifying , filtering , gain control , conversion from the analog rf signal to the analog if signal on the analog rf signal , performs the amplifying , filtering and gain control on the analog if signal obtained after the conversion and finally transmits the processed analog if signal to the remote interface unit . the above shows the base station system realized by the digital baseband remote manner according to the present invention . the above solution shows the indoor part and the rrs are divided with the digital baseband signal in this embodiment and the indoor part and the rrs can be connected with the optical cable . when the distance between the indoor part and the rrs is far , the digital baseband remote manner may be used . in addition , the rrs may use different covering modes according to the requirements , such as the single antenna covering , two - antenna covering , four - antenna covering , eight - antenna covering etc . when the digital baseband remote manner is adopted , optical transceivers at different rates can be used , such as electric / optical or optical / electric conversion chips at different rates , to perform the signal conversion . for example , when the single antenna covering mode is adopted , a low rate conversion chip can be used and when a multiple - antenna covering mode is adopted , a high rate conversion chip can be used . the indoor part of the base station can be connected with one or more rrs through one or more interface unit to form a chain , star or mixed connection with the digital optical cable . fig5 is a schematic diagram showing a connection between the indoor part of the base station and the rrs according to the first embodiment of the present invention . suppose there is a bbu processing six carrier frequencies , a tru processing three carrier frequencies and the eight - antenna covering manner is adopted . according to the structure of the base station of the first embodiment of the invention , as shown in fig5 , in the digital baseband remote manner , the base station remote part includes two rrss , and the tru in each rss is the tru that processes three carrier frequencies . the digital baseband unit of the indoor part of the base station includes the bbu that processes six carrier frequencies . the bbu is connected with two biu - ls and the two biu - ls are connected with the biu - rs of the two rrss through the optical cable respectively , and each biu - r is connected with the tru of its rrs . thus , the indoor part of the base station and the two rrss form a star connection , i . e . one base station covers two different areas with two remote rrss and the two rrss cover different areas with the eight - antenna covering manner . fig6 is a schematic diagram showing a configuration of a base station system according to a second embodiment of the present invention . the local interface unit includes an ifu and an iiu - l . the ifu is a digital if module of the conventional rfu . the configuration of the iiu - l includes two parts , i . e ., a downlink path and an uplink path , as shown in fig7 ( a ) and fig7 ( b ) respectively . fig7 ( a ) is a schematic diagram showing modules constituting the downlink path of the local if interface unit and their functions . as shown in fig7 ( a ), the iiu - l downlink path includes the following modules . if the iiu - l is connected with an remote if interface unit ( iiu - r ) of the rrs through an if cable , the iiu - l includes first up - conversion modules , a frequency shift keying ( fsk ) modulating module and a first multiplexing module ; if the iiu - l is connected with the iiu - r of the rrs through an optical cable , the iiu - l includes first up - conversion modules , a fsk modulating module , a first multiplexing module and an electric / optical conversion module . functions of each module are described below . the first up - conversion modules convert the central frequencies of the analog if signals from the ifu of the local interface unit to different central frequencies based on the frequency of the clock signal fclk from the ifu of the local interface unit and transmit the converted analog if signals to the first multiplexing modules . the clock signal fclk is shown in fig6 and is transmitted downwardly to the bbu by the gpsu and then transmitted to the ifu by the bbu ; or may be directly transmitted downwardly to the ifu by the gpsu . the fsk modulating module is for modulating an operation and maintenance ( om ) control signal from the ccu to a certain fixed frequency and outputting the modulated om control signal to the multiplexing module . the om control signal may be used to control the tru to start the rf processing etc . the first multiplexing module is for receiving the fclk signal from the ifu of the local interface unit , the up - converted analog if signals and the om control signal being fsk modulated , and for multiplexing the received signals in frequency , and then for transmitting a multiplexed analog if signal to the iiu - r through the if cable or converting the multiplexed analog signal to an optical signal by the electric / optical conversion module and then transmitting the optical signal to the iiu - r . fig7 ( b ) is a schematic diagram showing modules constituting the uplink path of the local if interface unit shown in fig6 and their functions . as shown in fig7 ( b ), the iiu - l uplink path includes the following modules . if the iiu - l is connected with the iiu - r through an if cable , the iiu - l includes a first demultiplexing module and first down - conversion modules ; if the iiu - l is connected with the iiu - r through an optical cable , the iiu - l includes an optical / electric conversion module , a first demultiplexing module and first down - conversion modules . functions of each module are described below . the first demultiplexing module is for receiving the analog if signal from the second multiplexing module of the iiu - r , demultiplexing the analog if signal in frequency and outputting the demultiplexed analog if signals to the first down - conversion modules . the first down - conversion modules convert the central frequencies of the analog if signals at different central frequencies from the first demultiplexing module to central frequencies of the analog if signals outputted from the ifu based on the frequency of the clock signal fclk from the ifu of the local interface unit and transmits the converted analog if signals to the ifu of the local interface unit . the ifus in fig7 ( a ) and in fig7 ( b ) refer to the same ifu . conversion or mixing , fsk modulation and multiplexing / demultiplexing in frequency are well - known technologies so will not be described in detail here . what will be emphasized here is an implementation of the base station system of the present invention . the remote interface unit is a remote if interface unit ( iiu - r ). the iiu - r also includes two parts , i . e ., the downlink path and the uplink path . the structures of the two parts are the same as that of the uplink path and the downlink path of the iiu - l shown in fig7 ( b ) and fig7 ( a ). the differences are that in the iiu - r , the fsk modulating module is replaced by a fsk demodulating module which is in the downlink path for demodulating the om control signal being fsk modulated from the iiu - l and recovers the om control signal . the fsk demodulation is well - known technology and will not be described in detail here . in addition , in the remote interface unit , the frequency base fclk signals from the up - conversion modules and the down - conversion modules are the fclk signals of the analog if signals demultplexed by the demultiplexing module of the remote interface unit . if the iiu - l is connected with the iiu - r through an if cable , the iiu - r includes a second demultiplexing module , second down - conversion modules and a fsk demodulating module ; if the iiu - l is connected with the iiu - r through an optical cable , the iiu - r includes an optical / electric conversion module , a second demultiplexing module , second down - conversion modules and a fsk demodulating module . functions of each module are described below . the second demultiplexing module receives the analog if signal from the first multiplexing module of the iiu - r , demultiplexes the signal and outputs the demultiplexed analog if signals to the second down - conversion modules , the demultiplexed fclk signal to the tru , the second up - conversion modules and the second down - conversion modules , the demultiplexed om control signal being fsk modulated to the fsk demodulating module . the second down - conversion modules convert the analog if signals from the second demultiplexing module based on the frequency of the fclk signal from the second demultiplexing module , convert the central frequencies of the analog if signals at different central frequencies from the second demultiplexing module to the original central frequencies and outputs the converted analog if signals to the tru . the fsk demodulating module demodulates the modulated om control signal from the second demultiplexing module to the original om control signal and outputs the demodulated signal to the tru . if the iiu - l is connected with the iiu - r through an if cable , the iiu - r includes second up - conversion modules and a second multiplexing module ; if the iiu - l is connected with the iiu - r through an optical cable , the iiu - r includes second up - conversion modules , a second multiplexing module and an electric / optical conversion module . functions of each module are described below . the second up - conversion modules convert the central frequencies of the analog if signals from the tru to different central frequencies based on the frequency of the fclk signal from the second demultiplexing module and output the converted analog if signals to the second multiplexing module . the second multiplexing module receives the converted analog if signals from the second up - conversion modules , multiplexes the received signals in frequency and outputs the multiplexed analog if signals to the first demultiplexing module of the iiu - l . if the indoor part of the base station is connected with the rss through an if cable , the multiplexing module of the iiu - l is connected with the demultiplexing module of the iiu - r through the if cable and the demultiplexing module of the iiu - l is connected with the multiplexing module of the iiu - r through the if cable and at this time , the interface signals transmitted between the iiu - l of the local interface unit of the indoor part of the base station and the iiu - r of the rrs are the analog if electric signals . if the indoor part of the base station is connected with the rss through an optical cable , the electric / optical conversion module of the iiu - l is connected with optical / electric conversion module of the iiu - r through the optical cable and the optical / electric conversion module of the iiu - l is connected with electric / optical conversion module of the iiu - r through the optical cable and at this time , the interface signals transmitted between the iiu - l of the local interface unit of the indoor part of the base station and the iiu - r of the rrs are the analog if optical signals . the tru is a rf transceiver module of a conventional rfu . the functions of the tru have been described in the first embodiment and will not be described in detail here . its functions are also to perform the conversion between an analog if signal and an analog rf signal . the differences are that in the current embodiment , through the downlink path , the tru receives the analog if signals outputted from the down - conversion modules of the iiu - r , converts the analog if signals to the analog rf signals and transmits to the ant ; through the uplink path , the tru receives the analog rf signals from the ant , converts the analog rf signals to the analog if signals and transmits to the up - conversion modules of the iiu - r . in addition , as shown in fig7 ( a ) and fig7 ( b ), there are n up / down - conversion modules , wherein the number n is decided by the number m of antennas in the antenna covering mode adopted by the base station in practice . if the base station adopts an eight - antenna covering mode , i . e . m = 8 , n = m , which means there are eight up / down - conversion modules . the above is the base station system realized by the if remote manner according to the present invention . the above solution shows the indoor part and the rrs are divided with the analog if signal in this embodiment and the indoor part and the rrs can be connected with the optical cable or the if cable . when the distance between the indoor part and the rrs is near , the if remote manner may be used . similarly , in the if remote manner , the rrs may use different covering modes according to the requirements , such as the single antenna covering , two - antenna covering , four - antenna covering , eight - antenna covering etc . when the if remote manner is adopted , optical transceivers at different rates can be used , such as electric / optical or optical / electric conversion chips at different rates , to perform the signal conversion . the indoor part of the base station can be connected with one or more rrs through one or more interface unit to form a chain , star or mixed connection with the optical cable or the if cable . fig8 is a schematic diagram showing a connection between the indoor part of the base station and the rrs according to a second embodiment of the present invention . suppose each remote tru processes three carrier frequencies , a four - antenna covering manner is adopted and each bbu performs the baseband processing for the three carrier frequencies . according to the structure of the base station of the second embodiment of the invention , as shown in fig8 , in the if remote manner , if the ifu processes three carrier frequencies with the eight - antenna covering , each rss of the base station remote part includes one tru which processes three carrier frequencies and the digital baseband unit of the indoor part of the base station includes two bbus each processes three carrier frequencies . the two bbus are connected with the ifu and the ifu is connected with the remote iiu - r through one iiu - l by an if cable or an optical cable . the two rrss are connected with each other through an if cable or an optical cable . thus , the indoor part of the base station and the two rrss form a star connection , i . e . one base station covers two different areas with two remote rrss and the two rrss cover different areas with the four - antenna covering manner . fig9 is a schematic diagram showing a configuration of a base station system according to the third embodiment of the present invention . the local interface unit includes three units that are the biu - l , ifu and iiu - l and the two remote interface units includes three units that are the biu - r , ifu and iiu - r respectively . the indoor part further includes the riu , ccu , bbu 1 , bbu 2 and gpsu . it should be noted here that in the current embodiment , the biu - l and the ifu of the indoor part may correspond to one bbu respectively , as shown in fig9 , i . e . the configuration and function of bbu 1 and bbu 2 are the same , and may also share one bbu . the rrs further includes the tru . the structure of the current embodiment shows in practice , this kind of base station system may be more flexible . one base station structure can support two remote manners so that the user may flexibly choose the remote manner according to the distance between the indoor part of the base station and the rrs . for example , when the distance is short , the if remote manner may be used because the if cable is cheap . and the user may flexibly choose the remote manner according to the price of the fiber module . for example , if the analog fiber is cheap , the if remote manner may be used and if the digital fiber is cheap , the digital baseband remote manner may be used . specifically , if the base station of the invention supports both the covering in a building and the covering of a large remote area , a rrs adopting the if remote manner may be used to realize the covering in the building and another rrs adopting the digital baseband remote manner may be used to realized the covering in a large remote area . the above descriptions are only preferred embodiments of the present invention and will not limit the scope sought for protection in the claims .