Abstract:
A cellular radio communication system, a cellular radio communication method and a rake reception method for raising the efficiency of a baseband, and increasing the freedom of the structure of the radio base station. A cellular radio communication system comprises a baseband and first to Nth transmitter-receicvers connected by a transmission line. The baseband is provided with a modulator for producing, by a digital process, modulation signals to be sent from the first to Nth transmitter-receivers, and a demodulator for demodulating digital modulation signals received and analog-to-digital converted by the first to Nth transmitter-receivers. The transmitter-receiver is provided with a digital-to-analog converter for converting a digital modulation signal to a modulation signal to be sent, and an analog-to-digital converter for converting a received modulation signal to a digital modulation signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a cellular radio communication system in which a baseband and a transmitter-receiver are connected via a transmission line, a cellular radio communication method and a rake reception method. 
     DESCRIPTION OF THE PRIOR ART 
     The area where the mobile communications are available is divided into blocks called “cellular” based on the distance that a radio wave can travel from an antenna.  FIG. 1  is a diagram showing three cells in the structure of a conventional cellular radio communication system. In  FIG. 1 , in the case of communicating with a mobile terminal  91  in a cell  31 , a data signal from a switching exchange  25  is converted to a radio signal by a radio base station  21  and sent to the mobile terminal  91  in the cell  31 . On the other hand, a radio signal from the mobile terminal  91  is received by the radio base station  21  and converted to a data signal to be sent to the switching exchange  25 . 
       FIG. 2  shows the structure of a conventional radio base station. A data signal input to a modulator  41  is modulated according to a modulation method such as PSK (Phase Shift Keying), QAM (Quadrature Amplitude Modulation), CDMA (Code Division Multiple Access) and OFDM (Orthogonal Frequency Division Multiplexing), and amplified by a transmitter  43 . Then a radio signal is output from an antenna  17  and reaches to a mobile terminal. A radio signal sent from the mobile terminal is received by the antenna  17 , amplified by a receiver  44  and demodulated to a data signal by a demodulator  42 . 
       FIG. 3  shows the structure of another conventional radio base station. The radio base station comprises a baseband  58  including a modulator  51  and a demodulator  54 , and a transmitter-receiver  59  including a transmitter  55  and a receiver  56 . The baseband  58  and the transmitter-receiver  59  are connected via a transmission line  57 . When the transmission line  57  is an optical cable, an optical signal is transmitted therethrough. When the transmission line  57  is a metallic cable, an electric signal is transmitted therethrough. The electric power for transmission and reception can be effectively used by placing the transmitter-receiver  59  near an antenna to decrease the electric power loss between the transmitter-receiver  59  and the antenna  17 . 
     The Japanese Patent No. 2858514 discloses a typical example of a conventional radio communication system as described above, which realizes the miniaturization of a master station while suppressing the degradation of transmission quality. 
     However, a cellular radio communication system which includes the prior radio base station of the above structure has some problems as follows. 
     First, effective use is not made of the baseband because, in the structure of the radio base station, the baseband and the transmitter-receiver are provided so as to correspond one-to-one with each other. 
     Second, the structure of the radio base station has a small degree of freedom. 
     Third, in a cellular radio communication system using CDMA, RAKE reception is performed by using only multipath components of radio signals received from one antenna of the base station, which limits improvement in the effectiveness of rake reception. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention, in order to overcome the above mentioned problems, to provide a cellular radio communication system, a cellular radio communication method, and a rake reception method for improving the usability of a baseband, the freedom of the structure of a radio base station, and the effectiveness of rake reception. 
     To achieve the object mentioned above, in the first mode of the present invention, a cellular radio communication system comprises one baseband and first to Nth transmitter-receivers connected with each other via a transmission line. The baseband comprises a modulator for producing modulation signals to be transmitted from the first to Nth transmitter-receivers by a digital process and a demodulator for demodulating digital modulation signals, which have been received by the first to Nth transmitter-receivers and undergone analog-to-digital conversion, by a digital process. The transmitter-receiver comprises a digital-to-analog converter for converting a digital modulation signal to a modulation signal to be transmitted and an analog-to-digital converter for converting a received modulation signal to a digital modulation signal. 
     In the second mode of the present invention, a cellular radio communication system comprises first to Mth basebands and first to Nth transmitter-receivers connected with each other via a transmission line. Each of the basebands comprises a modulator for producing a modulation signal to be transmitted by a plurality of transmitter-receivers associated with the baseband by a digital process and a demodulator for demodulating digital modulation signals, which have been received by the plurality of transmitter-receivers associated with the baseband and undergone analog-to-digital conversion, by a digital process. The transmitter-receiver comprises a digital-to-analog converter for converting a digital modulation signal to a modulation signal to transmit and an analog-to-digital converter for converting a received modulation signal to a digital modulation signal. 
     The transmitter-receivers associated with each of the basebands may be changed. 
     In the third mode of the present invention, a cellular radio communication method applied to a cellular radio communication system comprising one baseband and first to Nth transmitter-receivers connected with each other via a transmission line, comprising the steps of: producing, at the baseband, a modulation signal to be sent by each of the first to Nth transmitter-receivers by a digital process; converting, at the transmitter-receiver, a digital modulation signal to a modulation signal to be sent and outputting the signal from an antenna; converting, at the transmitter-receiver, a modulation signal received by the antenna to a digital modulation signal; and demodulating, at the baseband, a digital modulation signal, which has been received and undergone analog-to-digital conversion at each of the first to Nth transmitter-receivers, by a digital process. 
     In the fourth mode of the present invention, a cellular radio communication method applied to a cellular radio communication system comprising first to Mth basebands and first to Nth transmitter-receivers connected with each other via a transmission line, comprising the steps of: modulating, at the baseband, signals to produce modulation signals to be sent by a plurality of transmitter-receivers associated with the baseband by a digital process; converting, at the transmitter-receiver, a digital modulation signal to a modulation signal to be sent and outputting the signal from an antenna; converting, at the transmitter-receiver, a modulation signal received by the antenna to a digital modulation signal; and demodulating, at the baseband, digital modulation signals, which have been received and undergone analog-to-digital conversion at the plurality of transmitter-receivers associated with the baseband, by a digital process. 
     In the step of modulating or demodulating, the transmitter-receivers associated with each of the basebands may be changed. 
     Multiple multipath components may be produced from CDMA signals received by the transmitter-receivers in multiple cells to select and synthesize high-level components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and further objects and novel features of the invention will be more fully understood from the following detailed description when the same is read in connection with the accompanying drawings in which: 
         FIG. 1  is a diagram showing the structure of a conventional cellular radio communication system; 
         FIG. 2  is a diagram showing the structure of a conventional radio base station; 
         FIG. 3  is a diagram showing the structure of another conventional radio base station; 
         FIG. 4  is a block diagram showing the structure of a base station according to the first embodiment of the present invention; 
         FIG. 5  is a block diagram showing the structure of a base station according to the second embodiment of the present invention; 
         FIG. 6  is a block diagram showing the structure of a baseband in the case of communication using CDMA according to the third embodiment of the present invention; and 
         FIG. 7  is a diagram showing the structure of a rake reception system in which rake reception is performed using a received signal from transmitter-receivers in a plurality of cells of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 4  is a block diagram showing the structure of a base station according to the first embodiment of the present invention. Referring to  FIG. 4 , each of N pieces of base stations is comprised of a baseband  71  and first to Nth transmitter-receivers connected via a transmission line  68 . The baseband  71  comprises a modulator  61 , a demodulator  62  and a transmission-reception interface  63 . 
     Each transmitter-receivers is comprised of a digital-to-analog converter  64 , an analog-to-digital converter  65 , a transmitter  66 , a receiver  67  and a transmission-reception interface  63 . One baseband carries out baseband operations for the first to Nth transmitter-receivers. 
     The baseband  71  performs digital processing including CPU software processing by using a digital signal. That is, the baseband  71  performs time-sharing processing or parallel processing by one or more digital processing circuits. 
     The modulator  61  performs modulation by a modulation method such as PSK (Phase-Shift Keying), QAM (Quadrature Amplitude Modulation), CDMA (Code Division Multiple Access), OFDM (Orthogonal Frequency Division Multiplexing) or multiplex modulation by a plurality of modulation methods. The modulator  61  produces modulation signals to be sent by the first to Nth transmitter-receivers by the time-sharing processing or the parallel processing. 
     The demodulator  62  performs demodulation by a demodulation method such as PSK, QAM, CDMA, OFDM or multiplex demodulation by a plurality of demodulation methods. The demodulator  62  demodulates modulation signals received from the first to Nth transmitter-receivers by the time-sharing processing or the parallel processing. 
     The baseband  71  and the first to Nth transmitter-receivers are connected by the transmission line  68  and transmit and receive digital signals through the transmission-reception interfaces  63 . Examples of the transmission line(s) include N pieces of transmission lines connecting the baseband  71  and the first to Nth transmitter-receivers, one virtual transmission line time-shared by the baseband  71  and the first to Nth transmitter-receivers for digital signal transmission such as the Internet using TCP/IP protocol, or an optical transmission line in which digital signals to/from the baseband  71  and the first to Nth transmitter-receivers are time-division multiplexed or wavelength multiplexed. 
     Referring to  FIG. 4 , a description will be given of the operation of the base station according to the first embodiment. An input data signal is modulated at the modulation processor  61  of the baseband  71 . The modulator  61  produces a modulation signal to be sent to the first to Nth transmitter-receivers through the time-sharing processing or the parallel processing. A modulated digital signal is transmitted to the transmission-reception interface  63  in the baseband  71 . The transmission-reception interface  63  transmits the digital signal to the transmission-reception interface  63  of the respective first to Nth transmitter-receivers via the transmission line  68 . 
     The transmission-reception interface  63  of each first to Nth transmitter-receiver transmits the received digital signal to the digital-to-analog converter  64 . The digital-to-analog converter  64  outputs a modulated analog signal. When the frequency of the modulated analog signal is the same as that of a frequency band used for radio communication, the modulated signal is amplified by the transmitter  66  and output from an antenna  17 . When the frequency of the modulated signal is lower than that of a frequency band used for radio communication, the signal is converted to the frequency of the frequency band, amplified by the transmitter  66  and output from the antenna  17 . 
     A radio signal received from the antenna  17  of each first to Nth transmitter-receiver is amplified by the receiver  67 , and then converted to a digital signal at the analog-to-digital converter  65 . Incidentally, to moderate the processing speed at the baseband  71 , the radio signal may be converted to a digital signal at the analog-to-digital converter  65  after changing the frequency of the radio signal to a lower frequency. The digital signal is sent from the transmission-reception interface  63  in the transmitter-receiver to the transmission-reception interface  63  in the baseband  71  via the transmission line  68 , then sent to the demodulator  62 . At the demodulator  62 , the digital signal sent from each of the first to Nth transmitter-receivers is demodulated by the time-sharing processing or the parallel processing to output a data signal. 
       FIG. 5  is a block diagram showing a base station according to the second embodiment of the present invention. In  FIG. 5 , first to Mth basebands and first to Nth transmitter-receivers are connected through transmission-reception interfaces  63  by a transmission line  68 . Examples of the transmission line  68  include a transmission line time-shared by the first to Mth basebands and the first to Nth transmitter-receivers for digital signal transmission such as the Internet using TCP/IP protocol, and an optical transmission line in which digital signals to/from the first to Mth basebands and the first to Nth transmitter-receivers are time-division multiplexed or wavelength multiplexed. 
     Referring to  FIG. 5 , a description will be given of the operation of the base station according to the second embodiment. The basebands and the transmitter-receivers are connected via the transmission line  68 . Each baseband takes charge of a modulation-demodulation processing for the plural transmitter-receivers as in the first embodiment. For example, the first baseband  71  is in charge of the first transmitter-receiver  72  and the third transmitter-receiver, while the Mth baseband  74  is in charge of the second transmitter-receiver, the fourth transmitter-receiver and the Nth transmitter-receiver  73 . The association between each baseband and transmitter-receivers changes dynamically. Depending on the number of mobile terminals in a cell and the use condition of the cell, a load of modulation-demodulation processing on the baseband which controls a plurality of cells changes. When a load of processing on a particular baseband increases and the baseband is overloaded, a part of the load is transferred to another baseband which has a small load. In case of a failure in the baseband, its load is transferred to another baseband or to other basebands. 
       FIG. 6  is a block diagram showing the detailed structure of a baseband using CDMA radio signals according to the third embodiment of the present invention. A modulator  87  comprises the first modulator  81  which performs modulation such as PSK, QAM, and OFDM, and a diffuser  82 . A demodulator  88  comprises a back diffuser  85 , a first demodulator  84  which performs demodulation and a rake receiver  86 . 
     Referring to  FIG. 6 , a description will be made of the operation of the baseband  75  using CDMA radio signals. The modulator  87  performs modulation such as PSK, QAM, and OFDM at the first modulator  81 , and then performs diffusion at the diffuser  82  to produce a CDMA signal. The CDMA signal is sent to a transmitter-receiver by the transmission-reception interface  63  via the transmission line  68 . 
     On the other hand, the CDMA signal received by the transmitter-receiver is sent to the demodulator  88  by the transmission-reception interface  63  of the baseband  75  via the transmission line  68 . The demodulator  88  performs back diffusion at the back diffuser  85 , and then performs demodulation such as PSK, QAM, and OFDM at the first demodulator  84 . 
     The rake receiver  86  performs rake reception of CDMA signals not only from a transmitter-receiver in one cell but also from transmitter-receivers in a plurality of cells. As in the case where a mobile terminal is moving between cells, when a mobile terminal is in the coverage area of a plurality of cells, the rake receiver  86  performs rake reception of CDMA signals from the transmitter-receivers in the plurality of cells. For that reason, as shown in  FIG. 7 , one baseband is in charge of a plural adjacent cells to enlarge the cell area for which the baseband can perform rake reception. 
     In  FIG. 7 , when a mobile terminal  91  is within two cells  35  and  36 , a transmitter-receiver  25  and a transmitter-receiver  26  digital-analog converts CDMA signals received from the mobile terminal  91 , and then transmits the signals to the baseband  75 . The rake receiver  86  in the baseband  75  extracts multipath components  101  and  102  of the same data from each CDMA signal. From the multipath components  101  and  102 , the high-level components ( 111 ,  112 ,  113 ) are extracted and synthesized. The synthesized signal is demodulated in the first demodulator  84 , and a data signal is output. 
     As set forth hereinabove, in accordance with the present invention, a plurality of radio base stations each comprise one baseband and a plurality of transmitter-receivers. Thus, the number of the baseband can be reduced. 
     Moreover, a plurality of basebands and a plurality of transmitter-receivers are connected with each other via one or more transmission lines. Thereby, the association between each baseband and the transmitter-receivers can be changed freely according to the situation. 
     Further, high-level multipath components received from a plurality of antennas are extracted and synthesized for rake reception, which can be performed without using handover even when a mobile terminal is moving between cells. 
     While preferred embodiments of the invention have been described using specific terms, the description has been for illustrative purpose only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.