BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a radio communication method and a radio communication system between a base station and a mobile station which are applied to a digital cordless telephone system which makes use of a personal handyphone system (PHS) and by which communication by a modem modulation signal such as a facsimile modem signal or a data modem signal or a speech signal is performed by demultiplexing of a PCM (Pulse Code Modulation) signal of 64 kbps and an ADPCM (Adaptive Differential PCM) signal of 32 kbps. 
     2. Description of the Related Art 
     A digital cordless telephone system which makes use of a handyphone system (PHS) is conventionally used. An exemplary one of such digital cordless telephone systems is disclosed, for example, in Japanese Patent Laid-Open Application No. Heisei 5-328425. 
     The cordless telephone system disclosed in the document mentioned above is directed to realization of talking of a high quality by an ADPCM signal of 32 kbps. Further, as a multiplexing system for a speech signal, a circuit having a transmission speed of 64 kbps such as an ISDN (Integrated Services Digital Network) is used. In the multiplexing system, an ADPCM signal of 32 kbps is multiplexed to allow duplex communication by 32 kbps. 
     In this instance, a mutual exchanging apparatus between a μ-law, A-law system of 64 kbps as a digitalization sampling system for a speech signal and an ADPCM system of 32 kbps is provided to allow setting of which one of the systems should be used. By such mutual exchanging processing, if, during communication using the ADPCM system of 32 kbps, a new request for communication to the same communication destination is received, additional communication using the same circuit is possible. 
     However, in the conventional cordless telephone system described above which is directed to realization of talking of a high quality by an ADPCM signal of 32 kbps, when a base station connected to a public network performs data transmission through a radio circuit regarding a modem modulation signal from a facsimile (FAX) modem or a data modem as a speech signal, a PHS mobile station which receives the ADPCM signal of 32 kbps by the transmission converts the ADPCM signal of 32 kbps having a narrow bandwidth once into an analog signal and sends out the analog signal to the facsimile machine or data terminal. 
     Consequently, the data quality is deteriorated considerably and besides the transmission speed is dropped. In this instance, the transmission speed of 9,600 bps is guaranteed for practical use. This also applies to another case wherein a base station is connected to an ISDN network. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a radio communication method and a radio communication system which allow, where they are applied to a digital cordless telephone system or the like which makes use of a handyphone system (PHS) connected to an analog circuit or an ISDN network, data transmission in a wide band regarding a modem modulation signal from a modem of a facsimile machine or a data terminal as a speech signal and allow communication at a high transmission speed without deterioration of the data quality. 
     In order to attain the object described above, according to an aspect of the present invention, there is provided a radio communication method for a radio communication system which includes a base station connected to an analog network and a mobile station connected to the base station by a radio circuit, wherein the base station performs, when an analog signal which is a modem modulation signal such as a facsimile modem signal or a data modem signal is received from the analog network, PCM/CODEC processing of the analog signal, modulates the analog signal into a PCM signal of a high transmission speed, separates the PCM signal into two ADPCM signals of a low transmission speed, allocates the two ADPCM signals of the low transmission speed to two time slots by channel CODEC processing and performs data transmission as two radio communications through the radio circuit, and the mobile station demodulates the data received from the base station into an analog signal by a processing procedure reverse to the processing procedure by the base station and performs communication of the analog signal with a wide bandwidth of the high transmission speed regarding the analog signal as a speech signal. 
     The base station may perform, when a speech signal which is an analog signal is received from the analog network, ADPCM/CODEC processing for the speech signal, allocate the one ADPCM signal to one time slot and perform data transmission through the one radio circuit. 
     According to another aspect of the present invention, there is provided a radio communication method for a radio communication system which includes a base station connected to an ISDN network and a mobile station connected to the base station by a radio circuit, wherein the base station separates a modem modulation signal such as a facsimile modem signal or a data modem signal which is a PCM signal of a high speed sent thereto from the ISDN network into two ADPCM signals of a low transmission speed, allocates the two ADPCM signals of the low transmission speed to two time slots by channel CODEC processing and performs data transmission as two radio communications through the radio circuit, and the mobile station demodulates the data received from the base station into an analog signal by a processing procedure reverse to the processing procedure by the base station and performs communication of the analog signal with a wide bandwidth of the high transmission speed regarding the analog signal as a speech signal. 
     The base station may perform, when the PCM signal of the high transmission speed sent thereto from the ISDN network is a speech signal, PCM/ADPCM processing to convert the speech signal of the received PCM signal into an ADPCM signal of the low transmission speed, allocate the one ADPCM signal to one time slot and perform data transmission through the one radio circuit. 
     A radio communication system according to the present invention may have two forms including a form wherein it comprises a radio base station connected to an analog network and a mobile station connected to the base station by a radio circuit and another form wherein it comprises a radio base station connected to an ISDN network and a mobile station connected to the base station by a radio circuit. Where the radio communication system has the former form, the base station includes a main control section for controlling components of the base station, an analog circuit interface section for receiving an analog signal from the analog network, a signal type discrimination section for discriminating whether the analog signal received by the analog circuit interface section is a modem modulation signal from a facsimile modem or a data modem or a speech signal, a PCM/CODEC section for modulating the analog signal from the analog circuit interface section into a PCM signal of 64 kbps and for conversely demodulating a PCM signal of 64 kbps into an analog signal, an ADPCM/CODEC section for modulating the analog signal from the analog circuit interface section into an ADPCM signal of 32 kbps and for conversely demodulating an ADPCM signal of 32 kbps into an analog signal, a switch section for switching connection of the analog circuit interface section alternatively to the PCM/CODEC section or the ADPCM/CODEC section, a demultiplexing section for demultiplexing a PCM signal of 64 kbps and an ADPCM signal of 32 kbps, a channel CODEC section for performing allocation of communication channels to time slots of the radio circuit and performing framing and deframing of radio frames of a control channel and a communication channel, and a radio interface section for receiving data from the radio circuit and transmitting data through the radio circuit. 
     Meanwhile, the mobile station includes a main control section for controlling components of the mobile station, a radio interface section for receiving data from the base station through the radio circuit and transmitting data through the radio circuit, a channel CODEC section for performing allocation of communication channels to time slots of the radio circuit and performing framing and deframing of radio frames of a control channel and a communication channel, a demultiplexing section for performing demultiplexing of a PCM signal of 64 kbps and an ADPCM signal of 32 kbps, a PCM/CODEC section for demodulating a PCM signal of 64 kbps into an analog signal and for conversely modulating an analog signal into a PCM signal of 64 kbps, an ADPCM/CODEC section for demodulating an ADPCM signal of 32 kbps into an analog signal and for conversely modulating an analog signal into an ADPCM signal of 32 kbps, an analog interface section for performing inputting and outputting processing of an analog signal, a switch section for switching the connection of the analog interface section to the PCM/CODEC section or the ADPCM/CODEC section, and a signal type discrimination section for discriminating whether or not an analog signal received by the analog interface section is a modem modulation signal such as a facsimile modem signal or a data modem signal or a speech signal. 
     On the other hand, where the radio communication system has the second-mentioned form wherein the radio base station is connected to an ISDN network, the base station includes a main control section for controlling components of the base station, a digital circuit interface section for receiving a PCM signal of 64 kbps from the ISDN network, a signal type discrimination section for discriminating whether the digital signal received by the digital circuit interface section is a modem modulation signal such as a facsimile modem signal or a data modem signal or a speech signal, a PCM/ADPCM conversion section for performing conversion between a PCM signal of 64 kbps and an ADPCM signal of 32 kbps, a demultiplexing section for demultiplexing a PCM signal of 64 kbps and an ADPCM signal of 32 kbps, a switch section for switching connection of the digital circuit interface section alternatively to the demultiplexing section or the PCM/ADPCM conversion section, a channel CODEC section for performing allocation of communication channels to time slots of the radio circuit and performing framing and deframing of radio frames of a control channel and a communication channel, and a radio interface section for receiving data from the radio circuit and transmitting data through the radio circuit. 
     Meanwhile, the mobile station is constructed in a similar manner to the mobile station in the radio communication system of the form wherein the base station is connected to the analog circuit. 
     It is to be noted that the base station and the mobile station may be constructed such that they may perform transmission by signal processing reverse to that upon reception. Further, the base station and the mobile station may be constructed so as to serve as a parent machine and a child machine, respectively, of a digital cordless telephone system which makes use of a handyphone system. 
     With the radio communication methods and the radio communication systems described above, where the base station is connected to an analog network, if the analog signal sent thereto from the analog network is a modem modulation signal such as a facsimile modem signal or a data modem signal, the analog signal is modulated into a PCM signal of a high transmission speed (64 kbps). Further, the PCM signal is demultiplexed into two ADPCM signals of a low transmission speed (32 kbps), and the two ADPCM signals of the low transmission speed are allocated to two time slots by channel CODEC processing and data transmission is performed as two radio communications through the radio circuit. 
     Thus, where the radio communication methods and the radio communication systems described above are applied to a digital cordless telephone system which makes use of a handyphone system (PHS) connected to an analog circuit, data transmission with a wide band can be performed regarding a modem modulation signal from a modem of a facsimile machine or a data terminal as a speech signal and communication of a high quality at a high transmission speed can be achieved. 
     On the other hand, where the base station is connected to an ISDN network, a PCM signal of a high transmission speed (64 kbps) in the form of a modulation signal such as a facsimile modem signal or a data modem signal sent from the ISDN network to the base station is separated into two ADPCM signals of a low transmission speed (32 kbps). Then, the two ADPCM signals of the low transmission speed are allocated to two time slots, and data transmission is performed as two radio communications through the radio circuit. 
     Thus, also in this instance, data transmission with a wide band can be performed when an ISDN network is used, and communication of a high quality at a high transmission speed can be achieved. 
     The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference characters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a radio communication system to which the present invention is applied; 
     FIG. 2 is a flow chart illustrating a processing procedure of operation of the radio communication system of FIG. 1 before data transmission of a base station is started; 
     FIG. 3 is a flow chart illustrating a processing procedure of operation of the radio communication system of FIG. 1 before communication and talking of a mobile station is started; and 
     FIG. 4 is a block diagram of another radio communication system to which the present invention is applied where it is connected to an ISDN network. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring first to FIG. 1, there is shown in block diagram a radio communication system to which the present invention is applied. 
     The radio communication system shown in FIG. 1 is constructed as a digital cordless telephone system which makes use of a handyphone system. The radio communication system includes a base station (CS)  10  and a mobile station (PS)  20  which serve as a parent machine and a child machine, respectively. 
     The base station (CS)  10  includes a CS main control section  11  for controlling components of the base station (CS)  10 , and a CS analog circuit interface (I/F) section  12  for receiving an analog signal sent thereto from a public network (analog network). The base station (CS)  10  further includes a CS signal type discrimination section  19  for discriminating whether an analog signal received by the CS analog circuit interface section  12  is a modulation signal from a facsimile modem or a data modem or a speech signal. 
     The base station (CS)  10  further includes a CS-PCM/CODEC section  14  for modulating an analog signal received by the CS analog circuit interface section  12  into a PCM signal of 64 kbps and for conversely demodulating a PCM signal of 64 kbps into an analog signal. 
     The base station (CS)  10  further includes a CS-ADPCM/CODEC section  16  for modulating an analog signal received by the CS analog circuit interface section  12  into an ADPCM signal of 32 kbps and for conversely demodulating an ADPCM signal of 32 kbps into an analog signal. 
     The base station (CS)  10  further includes a CS switch section  13  for connecting the CS analog circuit interface section  12  alternatively to the CS-PCM/CODEC section  14  or the CS-ADPCM/CODEC section  16 . 
     The base station (CS)  10  further includes a CS demultiplexing section  15  for demultiplexing a PCM signal of 64 kbps into two signals equivalent to ADPCM signals of 32 kbps, and a CS channel CODEC section  17  for performing allocation of communication channels to time slots in the radio circuit and performing framing and deframing of radio frames of a control channel and a communication channel. The control channel and communication channel are used to control the transmission and send and receive data. The framing and deframing of radio frames enables communications to be broken into packets and transmitted with error checking algorithms. 
     The base station (CS)  10  further includes a CS radio interface section  18  for receiving data sent thereto through the radio circuit by means of a CS antenna Ant and performing transmission of data through the radio circuit (CS antenna Ant). 
     Meanwhile, the mobile station (PS)  20  of the communication system includes a PS main control section  21  for controlling components of the mobile station (PS)  20 , a PS radio interface (I/F) section  28  for receiving data sent thereto from the base station (CS)  10  through the radio circuit by means of the PS antenna Ant and performing data transmission through the radio circuit (PS antenna Ant), and a PS channel CODEC section  27  for performing allocation of communication channels to time slots in the radio circuit and performing framing and deframing of radio frames of a control channel and a communication channel. The mobile station (PS)  20  further includes a PS demultiplexing section  25  for performing demultiplexing of a PCM signal of 64 kbps and two signals equivalent to ADPCM signals of 32 kbps. 
     The mobile station (PS)  20  further includes a PS-PCM/CODEC section  24  for demodulating a PCM signal of 64 kbps into an analog signal and for conversely modulating an analog signal into a PCM signal of 64 kbps, and a PS-ADPCM/CODEC section  26  for demodulating an ADPCM signal of 32 kbps into an analog signal and for conversely modulating an analog signal into an ADPCM signal of 32 kbps. The mobile station (PS)  20  further includes a PS analog interface (I/F) section  22  for performing processing of input and output signals each in the form of an analog signal. 
     The mobile station (PS)  20  further includes a PS switch section  23  for switching the connection of the PS analog interface section  22  to the PS-PCM/CODEC section  24  or the PS-ADPCM/CODEC section  26 , and a PS signal type discrimination section  29  for discriminating whether or not an analog signal received by the PS analog interface section  22  is a modem modulation signal from a facsimile modem or a data modem or a speech signal. 
     Operation of the radio communication system is described in detail below with reference to FIGS. 1,  2  and  3 . FIG. 2 illustrates in flow chart a processing procedure of operation of the radio communication system before data transmission of the base station (CS) is started, and FIG. 3 illustrates in flow chart a processing procedure of operation of the radio communication system before communication and talking of the mobile station (PS) is started. 
     First, in an initial state, the CS switch section  13  of the base station (CS)  10  is in a switched state wherein it connects the CS analog circuit interface section  12  to the CS-ADPCM/CODEC section  16  (on between a-c). Meanwhile, it is assumed that the PS switch section  23  of the mobile station (PS)  20  is connected to the PS-ADPCM/CODEC section  26  side (on between d-f). 
     If the base station (CS)  10  receives an analog signal sent thereto from a public network (analog network) at the CS analog circuit interface section  12  thereof (step S 11 ), then the CS signal type discrimination section  19  discriminates whether or not the received analog signal is a modem modulation signal from a facsimile modem or a data modem or a speech signal (step S 12 ). For the discrimination, for example, a method of detecting an answer tone of 2,100 Hz or, particularly from a facsimile modem, a CNG signal is available. 
     If the analog signal is a modem modulation signal from a facsimile modem or a data modem, then the CS switch section  13  disconnects the CS analog circuit interface section  12  from the CS-ADPCM/CODEC section  16  (off between a-c) and now connects the CS analog circuit interface section  12  to the CS-PCM/CODEC section  14  (on between a-b) under the control of the CS main control section  11  (step S 13 ). 
     Since the CS switch section  13  connects the CS analog circuit interface section  12  to the CS-PCM/CODEC section  14  in this manner, a path is formed for the CS-PCM/CODEC section  14 , and the CS-PCM/CODEC section  14  thus modulates the received modem modulation signal into a PCM signal of 64 kbps (step S 14 ). 
     The CS demultiplexing section  15  demultiplexes the modulated PCM signal of 64 kbps into two signals equivalent to ADPCM signals of 32 kbps (step S 15 ). The CS channel CODEC section  17  allocates the two signals equivalent to ADPCM signals of 32 kbps to two time slots and transmits them as two radio communications from the CS radio interface section  18  through the radio circuit (CS antenna Ant) (step S 16 ). 
     When the mobile station (PS)  20  receives the data sent thereto through the radio circuit by the PS antenna Ant and the PS radio interface section  28  (step S 31  in FIG.  3 ), the PS signal type discrimination section  29  discriminates whether the received data are conversion data of a modem modulation signal which has been allocated to two time slots and transmitted separately as such or conversion data of a speech signal which has been allocated to one time slot and transmitted as such (step S 32 ). 
     If the discrimination reveals that the received data are conversion data of a modem modulation signal allocated to two time slots and transmitted separately as such, then the PS switch section  23  disconnects the PS analog interface section  22  from the PS-ADPCM/CODEC section  26  (off between d-f) and now connects the PS analog interface section  22  to the PS-PCM/CODEC section  24  (on between d-e) under the control of the PS main control section  21  (step S 33 ). 
     For the discrimination, for example, a method wherein, when two call termination requests arrive at the mobile station (PS)  20  from the base station (CS)  10  during communication control, the received data are discriminated to be conversion data of a modem modulation signal is available. 
     The two signals allocated to two time slots and equivalent to ADPCM signals of 32 kbps are separated by the PS channel CODEC section  27  and forwarded to the PS demultiplexing section  25  (step S 34 ). The PS demultiplexing section  25  multiplexes the two signals equivalent to ADPCM signals of 32 kbps and demodulates the multiplexed signal into a PCM signal of 64 kbps (step S 35 ). The PS-PCM/CODEC section  24  demodulates the received PCM signal of 64 kbps into an analog signal (step S 36 ). 
     Through the PS analog interface section  22  for which a path has been formed by selection of the PS-PCM/CODEC section  24  by the PS switch section  23  under the control of the PS main control section  21 , the demodulated modem modulation signal is transmitted to the facsimile machine, data terminal or the like thereby to start communication (step S 37 ). 
     On the other hand, if the analog signal received by the CS analog circuit interface section  12  of the base station (CS)  10  is a speech signal (step S 12  in FIG.  3 ), whether or not the analog signal is a speech signal is discriminated by the CS signal type discrimination section  19 . The CS switch section  13  forms a path immediately without cutting the connection between the CS analog circuit interface section  12  and the CS-ADPCM/CODEC section  16  (on between a-c) (step S 17  in FIG.  3 ). The CS-ADPCM/CODEC section  16  for which a path has been formed in this manner modulates the received speech signal into an ADPCM signal of 32 kbps (step S 18 ). 
     Further, the CS channel CODEC section  17  allocates the modulated ADPCM signal to one time slot and transmits it as one radio communication through the radio circuit via the CS radio interface section  18  and the CS antenna Ant (step S 19 ). The mobile station (PS)  20  receives the data sent thereto through the radio circuit using the PS antenna Ant and the PS radio interface section  28 . Then, when the data are conversion data of a speech signal allocated to one time slot, the PS signal type discrimination section  29  discriminates that the data are a speech signal. 
     Here, the PS switch section  23  forms a path immediately without cutting the connection between the PS analog interface section  22  and the PS-ADPCM/CODEC section  26  (on between d-f) under the control of the PS main control section  21  (step S 38  of FIG.  3 ). 
     The ADPCM signal of 32 kbps allocated to one time slot is inputted to the PS-ADPCM/CODEC section  26 , by which it is subsequently demodulated into an analog signal (step S 40 ). The thus demodulated analog signal is transmitted as a speech signal to the telephone terminal through the PS-ADPCM/CODEC section  26  and the PS switch section  23  and through the PS analog interface section  22  for which a path has been formed thereby to perform talking (step S 41 ). 
     On the other hand, when transmission from the radio communication system is to be performed, the mobile station (PS)  20  discriminates, by the PS signal type discrimination section  29  thereof, whether an analog signal received by the PS analog interface section  22  is a modem modulation signal of a facsimile modem or a data modem or a speech signal and switches the PS switch section  23  in accordance with the discrimination. Thereafter, a procedure reverse to that for the termination is performed thereby to perform talking. 
     Referring now to FIG. 4, there is shown in block diagram another radio communication system to which the present invention is applied where it is connected to an ISDN network. 
     The radio communication system is basically similar to the radio communication system described hereinabove with reference to FIG. 1 except that the base station (CS)  10  is connected to an ISDN network and includes a CS digital circuit interface section  31  for receiving a PCM signal of 64 kbps sent thereto from the ISDN network and a CS-PCM/ADPCM conversion section  32  for performing conversion between a PCM signal of  64  kbps and an ADPCM signal of 32 kbps in place of the CS analog circuit interface section  12 , CS-PCM/CODEC section  14  and CS-ADPCM/CODEC section  16  of the base station (CS)  10  of the radio communication system of FIG.  1 . 
     In operation, in the base station (CS)  10 , when a PCM signal of 64 kbps sent thereto from the ISDN network is a modem modulation signal from a facsimile modem or a data modem, the CS signal type discrimination section  19  discriminates that the PCM signal of 64 kbps is a modem modulation signal. In response to the discrimination, the CS switch section  13  connects the CS digital circuit interface section  31 , which has received the PCM signal of 64 kbps, to the CS demultiplexing section  15  (on between a-b) and forwards the received PCM signal of 64 kbps as it is to the CS demultiplexing section  15 . Thereafter, communication with the facsimile machine or data terminal is started in accordance with a processing procedure similar to that in the radio communication system of FIG.  1 . 
     On the other hand, when the PCM signal of 64 kbps sent thereto from the ISDN network is a speech signal, in the base station (CS)  10 , the CS signal type discrimination section  19  discriminates that the PCM signal of 64 kbps is a speech signal, and the CS switch section  13  connects the CS digital circuit interface section  31 , which has received the PCM signal of  64  kbps, not to the CS demultiplexing section  15  but to the CS-PCM/ADPCM conversion section  32  (on between a-c). 
     Then, the CS-PCM/ADPCM conversion section  32  converts the received PCM signal of 64 kbps into an ADPCM signal of 32 kbps. The thus converted ADPCM signal of 32 kbps is transmitted to the CS channel CODEC section  17 . Thereafter, talking with the telephone terminal is started in accordance with a processing procedure similar to that in the radio communication system of FIG.  1 . 
     It is to be noted that, in transmission, communication and talking via the ISDN network is performed in accordance with a processing procedure reverse to the processing procedure for termination. 
     Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the invention as set forth herein.
The invention provides a radio communication method and a radio communication system which allow data transmission in a wide band regarding a modem modulation signal as a speech signal and allow communication at a high transmission speed without deterioration of the data quality. When a base station receives an analog signal (facsimile or data modem modulation signal) from a public network, a CS switch section connects an analog circuit interface section to a CS-PCM/CODEC section. The CS-PCM/CODEC section modulates the modem modulation signal into a PCM signal of 64 kbps, which is then demultiplexed into two signals equivalent to ADPCM signals of 32 kbps by a CS demultiplexing section. Thereafter, a CS channel CODEC section allocates the two signals equivalent to ADPCM signals of 32 kbps to two time slots and transmits the data through two radio circuits. A base station performs demodulation of received data into an analog signal by a processing procedure reverse to that by the base station. Communication is performed with a wide bandwidth of 64 kbps regarding a modem modulation signal as a speech signal.