Multiplex radio transmitter and multiplex radio transmission method, multiplex radio receiver and multiplex radio receiving method, and multiplex radio transceiver and multiplex transmission/receiving system

The present invention is directed to a multiplex radio transmission/receiving system. The system includes a plurality of transmission sections provided so as to correspond to a plurality of channels, and a plurality of receiving sections provided so as to correspond to the plurality of channels. Each transmission section includes a modulation section, a first frequency conversion section, a first band-pass filter, a second frequency conversion section, and a second band-pass filter. Each receiving section includes a third band-pass filter, a third frequency conversion section, a fourth band-pass filter, a fourth frequency conversion section, and a demodulation section. By selection of an optimum value for a second intermediate frequency of a transmitter and for a third intermediate frequency of a receiver, a group of transmission radio frequencies (RF) signals and a group of local frequency signals are allocated without overlap. As a result, there can be prepared one type of band-pass filter which has broad band-pass characteristics and which is common among the transmission and receiving sections disposed within the multiplex radio transceiver or among repeaters, thus allowing use of common members.

BACKGROUND OF THE INVENTION
 1) Field of the Invention
 The present invention relates to a multiplex radio transmitter and
 multiplex radio transmission method, a multiplex radio receiver and
 multiplex radio receiving method, and a multiplex radio transceiver and
 multiplex transmission/receiving system suitable for use with a trunk
 multiplex radio communications system.
 2) Description of the Related Art
 In recent years, when large volumes of information including such as video
 data are to be transmitted at high speed, a synchronous multiplex transfer
 mode (Synchronous Transfer Mode) based on a SDH (Synchronous Digital
 Hierarchy) has been widely used as a method of transmitting information
 data. Multiplex radio transceivers are usually located on high ground and
 relay multiplexed trunk signal data at microwave band through use of a
 synchronous multiplex transfer mode
 With regard to frequency bands, the allocation of available channels in
 individual frequency bands is specified in detail in the form of ITU-R
 (International Telecommunications Union Radio Communications Sector)
 recommendations.
 The trunk multiplex radio communications system occupies a frequency band
 called C-band (3.4 GHz to 8.5 GHz), and this C-band is further classified
 into sub-bands: that is, a 4G band, a 5G band, an L6G band, a U6G band, a
 7G band, and an L8G band. As shown in FIGS. 10 through 15, the allocation
 of individual channels within each of the sub-bands is also specified. The
 3.5 G band has been specified only in that the ends thereof have been
 determined as 3.4 GHz and 3.6 GHz, and details of allocation of channels
 within the frequency band are still under discussion and have not yet been
 publicly announced.
 Although a modulation scheme used for a trunk multiplex radio
 communications system within a microwave band has not yet been specified
 by ITU-R recommendations, enterprises adopt a 64 or 128 quadrature
 amplitude modulation scheme. This is because a large volume of data must
 be transmitted within the foregoing frequency bands while fulfilling the
 specifications, such as channel allocations, a per-channel occupied
 bandwidth, and adjacent channel power, and therefore there is adopted a
 QAM scheme having a superior frequency efficiency (a transmission rate per
 1 Hz).
 FIG. 23 shows the principal elements of a transmission section disposed in
 a transmitter of an existing trunk multiplex radio communications system.
 A multiplex radio transmitter 38 shown in FIG. 23 transmits a plurality of
 channel signals having different frequencies while converting them into a
 multiplexed signal and comprises a plurality of transmission sections 31
 corresponding to the individual channels, an antenna 36, and an antenna
 diplexer 37.
 In each transmission section 31, signal data (e.g., STM-1 transmitted over
 an SDH network) received from a synchronous multiplex repeater or a
 transmission-end apparatus are subjected to a baseband processing
 treatment in a baseband processing section (not shown). The thus-processed
 data are modulated and transmitted to a radio circuit. The transmission
 section 31 comprises a modulation section 32, a digital-to-analog
 converter 33, a frequency converter 34, and a band-pass filter 35.
 The modulation section 32 modulates multiplexed trunk signal data and
 comprises a mixer 32a-1, a mixer 32a-2, a 90.degree. phase shifter 32b, a
 local oscillator 32c, and a hybrid section 32d.
 More specifically, I-channel signal data are multiplied by an output signal
 of 70 MHz outputted from the local oscillator 32c. The mixer 32a-2
 multiplies Q-channel signal data by an outputted from the mixer 32a-2
 which is produced by phase-shifting the outputted from the local
 oscillator 32c through 90.degree. by means of the 90.degree. phase shifter
 32b. As a result, the signal data items are modulated into a frequency
 band centered at 70 MHz and are combined into a single waveform by the
 hybrid section 32d. The value of 70 MHz represents an interval between the
 local frequency and the radio frequency (RF).
 The digital-to-analog converter 33 converts a digital signal outputted from
 the modulation section 32 into an analog signal.
 Further, the frequency converter 34 up-converts the thus-modulated signal
 having a 70 MHz band into C-band within the microwave band. The mixer 34a
 multiplies an output from the digital-to-analog converter 33 by a
 microwave carrier which ranges in frequency from about 3 GHz to 8 GHz and
 is outputted from the local oscillator 34b.
 For each channel, the band-pass filter 35 limits a transmission band of a
 modulated signal in a microwave band outputted from the frequency
 converter 34 in such a way as to fulfill specifications defined for an air
 interface.
 The antenna diplexer 37 combines together the band-limited signals
 outputted from the band-pass filter 35.
 The antenna 36 transmits a radio signal outputted from the antenna diplexer
 37.
 Turning to a method for communications between a pair of multiplex radio
 repeaters, there has been employed an FDM (Frequency Division
 Multiplexing) method of enabling simultaneous use of a plurality of
 channels by a transmit-receive channel of each radio receiver being
 allocated a different frequency. FIG. 24 shows an example in which each
 pair of multiplex radio repeaters is allocated a different channel within
 C-band. The frequency band shown in FIG. 24 corresponds to a U6G band
 within the C-band and represents a group of radio frequency (RF) channels
 and a group of local frequencies.
 Channels 1 (6,460 MHz) to channel 8 (6,740 MHz) are allocated to a
 downlink, whereas channel 1' (6,800 MHz) to channel 8' (7,080 MHz) are
 allocated to a corresponding uplink, thus enabling selection of eight
 possible channels.
 There is overlap between the group of radio frequency (RF) channels and the
 group of local frequency channels. To prevent spurious radiation, the
 transmitter must eliminate the local frequency signals so as to prevent
 these signals from being emitted over the group of radio frequency (RF)
 channels, through use of a filter having a narrow bandwidth.
 In contrast, a receiver has a receiving device having the same frequency as
 that transmitted from an opposing repeater. As show in FIG. 25, the
 receiver must separate a channel being used from the eight possible
 channels through use of the band-pass filter having a narrow bandwidth.
 Here, the term "opposing" signifies a repeater with which the current
 repeater is in communication through use of a downlink channel and an
 uplink channel corresponding thereto. The term "opposing" will be used
 herein in the same sense.
 To this end, the band-pass filter 35 provided in the transmission section
 31 is required to have a narrow band as its band-pass characteristics.
 Accordingly, to select the channel being used the band-pass filter of the
 opposing receiver is also required to have a narrow band as its band-pass
 characteristics.
 However, according to the existing technique, a narrow-band filter having
 its center frequency in a microwave band such as that mentioned previously
 is very expensive and bulky.
 Further, since the individual transmission and receiving sections disposed
 in the multiplex radio transceiver transmit data through use of a
 plurality of channels of different frequencies, there is a need for a
 band-pass filter having band-pass characteristics corresponding to each of
 the channels, thus impeding realization of a band-pass filter capable of
 being used in all the channels.
 Moreover, data are transmitted between the repeaters through use of a
 plurality of channels having different frequencies. Even in such a case,
 there is a need for a band-pass filter having band-pass characteristics
 corresponding to each of the channels, thus impeding realization of a
 band-pass filter capable of being used in all the channels.
 SUMMARY OF THE INVENTION
 The present invention has been conceived in view of the foregoing drawbacks
 of the related art, and the object of the present invention is to provide
 a multiplex radio transmitter and a multiplex radio transmission method, a
 multiplex radio receiver and a multiplex radio receiving method, and a
 multiplex transceiver and a multiplex transmission/receiving system,
 wherein a transmitter requires only one type of band-pass filter having
 broad band-pass characteristics; wherein a receiver requires only one type
 of band-pass filter which has the same band-pass characteristics as those
 of the transmitter and which corresponds to frequencies transmitted from
 the opposing transmitter; and wherein band-pass filters having different
 band-pass characteristics for respective channels are replaced with
 identical band-pass filters, thus enabling use of members having common
 characteristics for the transmission sections and receiving sections
 disposed within the multiplex radio transceiver or for repeaters, as well
 as enabling improvement in cost performance.
 To this end, according to one aspect of the present invention, there is
 provided a multiplex radio transmitter which multiplexes a plurality of
 channel signals having different frequencies into another signal and
 transmits the multiplexed signal, the transmitter comprising:
 a plurality of transmission sections provided so as to correspond to
 respective channels, wherein
 the transmission section comprises
 a modulation section which modulates a data signal and outputs the
 modulated signal as a first intermediate frequency signal;
 a first frequency conversion section which converts the frequency of the
 first intermediate frequency signal outputted from the modulation section,
 through use of a first local signal to thus output a second intermediate
 frequency signal which is higher in frequency than the first intermediate
 frequency signal by only the frequency of the first local signal;
 a first filter which eliminates a frequency component of the first local
 signal from the second intermediate frequency signal outputted from the
 first frequency conversion section;
 a second frequency conversion section which converts the frequency of a
 signal outputted from the first filter through use of a second local
 signal to thus output a radio frequency signal which is higher in
 frequency than the second intermediate frequency signal by only the
 frequency of the second local signal; and
 a second filter which has at least a band-pass corresponding to radio
 frequency signals of a plurality of channels to be used; which filters out
 the radio frequency signal outputted from the second frequency conversion
 section; and which eliminates a frequency component of the second local
 signal.
 In the multiplex radio transmitter according to the present invention, a
 baseband signal is temporarily converted into a second intermediate
 frequency, and this second intermediate frequency is selectively set to an
 optimum value. As a result, a group of transmission radio signals and
 local frequency signals are allocated at a microwave band without overlap.
 Consequently, there is required only one type of band-pass filter which is
 capable of permitting collective passage of a group of radio frequency
 (RF) signals and which has broad band-pass characteristics. The
 transmitter is prevented from producing spurious radiation. There is no
 need to use different channels for respective transmission sections
 disposed within the multiplex radio transmitter or for respective
 repeaters, nor is there need to individually prepare band-pass filters
 having different band-pass characteristics, which would otherwise be
 required by the existing multiplex radio transmitter. Further, there can
 be used, for example, a second intermediate frequency which is common to
 all the sub-bands within C-band (3.4 GHz to 8.5 GHz), thus yielding an
 advantage of allowing use of common members in the multiplex radio
 transmitter, as well as allowing inexpensive manufacture of a multiplex
 radio transmitter.
 In a preferred mode, the modulation section can be configured so as to
 output the first intermediate frequency signal by digitally modulating a
 digital data signal and converting the digital data signal into an analog
 data signal.
 As a result, there is yielded an advantage of allowing sending of a signal
 through use of a digital modulation scheme having a superior frequency
 conversion efficiency.
 In a preferred mode, the modulation section can be configured so as to
 output the first intermediate frequency signal by converting a digital
 data signal into an analog data signal and modulating the analog data
 signal in an analog fashion.
 As a result, there is yielded an advantage of allowing the first
 intermediate signal to be freely set.
 In a preferred mode, the second intermediate frequency signal can be formed
 so as to have a frequency whose harmonic component is not superimposed on
 the radio frequency signal.
 As a result, there is yielded an advantage of allowing practical prevention
 of a harmonic component of the image frequency signal group from colliding
 with a group of channels lying in sub-bands within C-band, thus enabling
 an improvement in a signal-to-noise ratio of the receiver in each channel.
 In a preferred mode, the second intermediate frequency signal is formed so
 as to have at least a frequency whose harmonic component of sixth order or
 lower is not superimposed on the radio frequency signal.
 Consequently, of the image frequency signal group, a channel group whose
 harmonic is of 6.sup.th order or lower can be actually prevented from
 colliding with a channel group lying in sub-bands within, e.g., C-band
 (3.4 GHz to 8.5 GHz), resulting in an advantage of improving the
 signal-to-noise ratio of the receiver in each channel.
 In one preferred mode, the second frequency conversion section can be
 configured so as to output the radio frequency signal, by converting the
 frequency of the signal outputted from the first filter through use of the
 second local signal having any of a plurality of frequencies corresponding
 to a plurality of channels.
 As a result, three bands, that is, a radio frequency (RF) signal band, a
 second local signal band, and an image frequency band, can be separated
 from one another without overlap. There is yielded an advantage of
 allowing use of only one type of band-pass filter without use of band-pass
 filters having different band-pass characteristics for transmission
 sections disposed within the multiplex radio transmitter or for repeaters.
 According to another aspect of the present invention, there is provided a
 multiplex radio transmitter which multiplexes a plurality of channel
 signals having different frequencies into another signal and transmits the
 multiplexed signal, the transmitter comprising:
 a plurality of transmission sections provided so as to correspond to
 respective channels, wherein
 the transmission section comprises
 a digital modulation section which digitally modulates a digital data
 signal and converts the digital data signal into an analog data signal, to
 thus output a first intermediate frequency signal;
 a first frequency conversion section which converts the frequency of the
 first intermediate frequency signal outputted from the digital modulation
 section, through use of a first local signal, thus outputting a second
 intermediate frequency signal which is higher in frequency than the first
 intermediate frequency signal by only the frequency of the first local
 signal, wherein the second intermediate frequency signal has at least a
 frequency whose harmonic component of sixth order or lower is not
 superimposed on a radio frequency signal of at least 4 GHz band or higher
 to be used as a transmission signal;
 a first filter which eliminates a frequency component of the first local
 signal from the second intermediate frequency signal outputted from the
 first frequency conversion section;
 a second frequency conversion section which converts the frequency of a
 signal outputted from the first filter, through use of a second local
 signal having any of a plurality of frequencies corresponding to a
 plurality of channels, thus outputting the radio frequency signal which is
 higher in frequency than the second intermediate frequency signal by only
 the frequency of the second local signal; and
 a second filter which has at least a band-pass corresponding to radio
 frequency signals of a plurality of channels to be used; which filters out
 the radio frequency signal outputted from the second frequency conversion
 section; and which eliminates a frequency component of the second local
 signal.
 As a result, the multiplex radio transmitter according to the present
 invention requires only one type of band-pass filter which is capable of
 permitting collective passage of a group of radio frequency (RF) signals
 and which has broad band-pass characteristics, thus preventing the
 transmitter from producing spurious radiation and eliminating a need to
 use different channels for respective transmission sections disposed
 within the multiplex radio transmitter or for repeaters or to individually
 prepare band-pass filters having different band-pass characteristics,
 which would otherwise be required by the existing multiplex radio
 transmitter. Consequently, of the image frequency signal group, a channel
 group whose harmonic is of 6.sup.th order or lower can be actually
 prevented from colliding with a channel group lying in sub-bands within
 C-band (3.4 GHz to 8.5 GHz), resulting in an advantage of improving the
 signal-to-noise ratio of the receiver in each channel.
 In one preferred mode, the frequency of the second intermediate frequency
 signal may be set to a required frequency between 842.00 MHz and 845.02
 MHz, to a frequency of 844 MHz, or to a frequency of 967.1 MHz.
 The frequency can be freely set, and hence there is an advantage of freedom
 in radio circuit design. Further, the use of the foregoing frequency
 results in an advantage of preventing collision between image frequency
 signals of sixth order or lower among the image frequency signal group and
 channel groups provided in sub-bands within C-band (3.4 GHz to 8.5 GHz).
 Still further, there is yielded an advantage of improving the
 signal-to-noise ratio of the receiver in each channel, because in practice
 the influence of the other image frequency signal group is small.
 Still another aspect of the present invention, there is provided a
 multiplex radio transmission method for use with a multiplex radio
 transmitter which multiplexes a plurality of channel signals having
 different frequencies into a signal and transmits the thus multiplexed
 signal, wherein each of a plurality of transmission sections provided so
 as to correspond to channels produces a first intermediate frequency
 signal by modulating a data signal and converts the frequency of the
 modulated data signal, thereby transmitting a radio frequency signal at a
 higher frequency, the method being characterized by the feature that
 when the intermediate frequency signal is converted into the radio
 frequency signal through frequency conversion, the transmission section
 converts the intermediate frequency signal into a second intermediate
 frequency signal having an intermediate frequency between the frequency of
 the intermediate frequency signal and that of the radio frequency signal
 and converts the second intermediate frequency signal into the radio
 frequency signal through frequency conversion.
 As a result, the multiplex radio transmission method according to the
 present invention enables mutual separation of three bands, that is, a
 radio frequency (RF) signal band, a second local signal band, and an image
 frequency band without overlap. Consequently, the transmitter requires
 only one type of band-pass filter which is capable of permitting
 collective passage of a group of radio frequency (RF) signals and which
 has broad band-pass characteristics. The transmitter is prevented from
 producing spurious radiation. There is no need to use different channels
 for respective transmission sections disposed within the multiplex radio
 transmitter or for respective repeaters or to individually prepare
 band-pass filters having different band-pass characteristics, which would
 otherwise be required by the existing multiplex radio transmitter.
 Further, there can be used a second intermediate frequency which is common
 to all the sub-bands within, e.g., C-band (3.4 GHz to 8.5 GHz), thus
 yielding an advantage of allowing use of common members in the multiplex
 radio transmitter, as well as enabling inexpensive manufacture of a
 multiplex radio transmitter.
 According to yet another aspect of the present invention, there is provided
 a multiplex radio receiver including a plurality of receiving sections
 provided so as to correspond to channels for the purpose of receiving, by
 way of a radio propagation path, radio frequency signals transmitted from
 a multiplex radio transmitter disposed so as to oppose the multiplex radio
 receiver, wherein the multiplex radio transmitter has a plurality of
 transmission sections which convert a first intermediate frequency signal
 resulting from modulation of a data signal into a second intermediate
 frequency signal having an intermediate frequency between the frequency of
 the first intermediate frequency signal and that of a radio frequency
 signal; which convert the second intermediate frequency into the radio
 frequency signal; and which are provided so as to correspond to a
 plurality of channel signals having different frequencies, the multiplex
 radio receiver being characterized by the feature that
 the receiving section comprises
 a third filter which has at least a band-pass corresponding to radio
 frequencies of a plurality of channels to be used, which filters out the
 received radio frequency signal, and which eliminates a frequency
 component of an image signal of the radio frequency signal;
 a third frequency conversion section which converts the frequency of a
 signal outputted from the third filter through use of a third local signal
 to thus output a third intermediate frequency signal which is lower in
 frequency than the radio frequency signal by only the frequency of the
 third local signal;
 a fourth frequency conversion section which converts the frequency of a
 signal outputted from the third frequency conversion section through use
 of a fourth local signal to thus output a fourth intermediate frequency
 signal which is lower in frequency than the third intermediate frequency
 signal by only the frequency of the fourth local signal; and
 a demodulation section which demodulates the data signal by demodulation of
 an outputted from the fourth frequency conversion section.
 As a result, the multiplex radio receiver according to the present
 invention enables mutual separation of three bands, that is, a radio
 frequency (RF) signal band, a second local signal band, and an image
 frequency band without overlap. As a result, there is required only one
 type of band-pass filter for use with a receiving radio frequency (RF)
 signal band, thus eliminating a need to filter out a signal for each
 receiving radio frequency (RF) channel. Further, there is no need to
 prepare band-pass filters having different band-pass characteristics for
 respective receiving sections disposed within the multiplex radio receiver
 or for repeaters, thus yielding an advantage of allowing use of members
 and reducing the cost of the multiplex radio receiver.
 In one preferred mode, the transmission section of the multiplex radio
 transmitter is configured so as to produce the first intermediate
 frequency signal by digitally modulating a digital data signal and
 converting the digital data signal into an analog data signal; and the
 demodulation section of the multiplex radio receiver provided so as to
 oppose the transmitter is configured so as to demodulate the data signal
 by converting an outputted from the fourth frequency conversion section
 through analog-to-digital conversion and demodulating the analog data
 signal.
 As a result, bidirectional communication can be established through use of
 the multiplex radio transceiver which digitally modulates and demodulates
 a baseband signal, thereby resulting in an advantage of allowing an
 increase in a transmission rate.
 In one preferred mode, the transmission section of the multiplex radio
 transmitter may be configured so as to produce the first intermediate
 frequency signal by converting a digital data signal into an analog data
 signal and modulating the analog data signal in an analog fashion, and the
 demodulation section of the multiplex radio receiver disposed so as to
 oppose the transmitter may be configured so as to obtain the digital data
 signal by demodulating an analog outputted from the fourth frequency
 conversion section in an analog fashion and converting the demodulated
 analog output through analog-to-digital conversion.
 Accordingly, with the foregoing configuration, bidirectional communication
 can be established through use of a multiplex radio transceiver which
 modulates or demodulates a baseband signal in an analog fashion, thus
 yielding an advantage of allowing the first intermediate frequency signal
 (or the fifth intermediate frequency signal) to be freely set.
 In one preferred mode, the transmission section of the multiplex radio
 transmitter may be configured so as to output the radio frequency signal,
 by converting the frequency of the signal outputted from the first filter
 through use of the second local signal having any of a plurality of
 frequencies corresponding to a plurality of channels, and the third
 frequency conversion section of the multiplex radio receiver disposed so
 as to oppose the transmitter may be configured so as to output the third
 intermediate frequency signal, by converting the frequency of the signal
 outputted from the third filter through use of the third local signal
 having any of a plurality of frequencies corresponding to a plurality of
 channels.
 With such a configuration, the band-pass characteristics of the
 transmission band-pass filter of the transmitter can be made equal to
 those of the receiving band-pass filter. There can be prepared identical
 band-pass filters for the transmission sections of the multiplex radio
 transmitter, for the receiving sections of the multiplex radio receiver,
 or for repeaters, thus yielding an advantage of the ability to use common
 members and to promote cost cutting.
 According to a further aspect of the present invention, there is provided a
 multiplex radio receiving method of receiving radio frequency signals
 transmitted over a radio propagation path from a multiplex radio
 transmitter disposed so as to oppose a multiplex radio receiver, by means
 of a plurality of receiving sections provided so as to correspond to the
 respective channels, wherein the multiplex radio transmitter has a
 plurality of transmission sections which convert a first intermediate
 frequency signal resulting from modulation of a data signal into a second
 intermediate frequency signal having an intermediate frequency between the
 frequency of the first intermediate frequency signal and that of a radio
 frequency signal, which convert the second intermediate frequency into the
 radio frequency signal, and which are provided so as to correspond to a
 plurality of channel signals having different frequencies, the multiplex
 radio receiving method being characterized by the feature that
 the receiving section
 converts the radio frequency signal to a third intermediate frequency
 signal, which is lower in frequency than the radio frequency signal,
 through frequency conversion;
 converts the third intermediate frequency signal to a fourth intermediate
 frequency signal, which is lower in frequency than the third intermediate
 frequency signal, through frequency conversion; and
 demodulates the data signal by demodulation of the fourth intermediate
 frequency signal.
 The multiplex radio receiving method according to the present invention
 enables separation of three bands, that is, a receiving radio frequency
 (RF) signal band, a third local signal band, and an image frequency band,
 from one another without overlap. As a result, there is required only one
 type of band-pass filter for use with a receiving radio frequency (RF)
 signal band, thus eliminating a need to filter out a signal for each
 receiving radio frequency (RF) channel. Further, there can be used the
 third intermediate frequency which is common to all the sub-bands within,
 for example, C-band (3.4 GHz to 8.5 GHz), thus yielding an advantage of
 enabling use of common members in the receiving sections disposed within
 the multiplex radio receiver or in repeaters, as well as allowing
 inexpensive manufacture of a multiplex radio receiver.
 According to a still further aspect of the present invention, there is
 provided a multiplex radio transceiver comprising:
 a multiplex radio transmitter which includes a plurality of transmission
 sections provided so as to correspond to respective channels for the
 purpose of multiplexing a plurality of channel signals having different
 frequencies into another signal and for the purpose of transmitting the
 multiplexed signal, wherein
 the transmission section comprises
 a modulation section which modulates a data signal and outputs the
 modulated data signal as a first intermediate frequency signal;
 a first frequency conversion section which converts the frequency of a
 signal outputted from the modulation section through use of a first local
 signal, thus outputting a second intermediate frequency signal which is
 higher in frequency than the first intermediate frequency signal by only
 the frequency of the first local signal;
 a first filter which eliminates a frequency component of the first local
 signal from the second intermediate frequency signal outputted from the
 first frequency conversion section;
 a second frequency conversion section which converts the frequency of a
 signal outputted from the first filter through use of a second local
 signal, thus outputting a radio frequency signal which is higher in
 frequency than the second intermediate frequency signal by only the
 frequency of the second local signal; and
 a second filter which has at least a band-pass corresponding to radio
 frequency signals of a plurality of channels to be used, which filters out
 the radio frequency signal outputted from the second frequency conversion
 section, and which eliminates a frequency component of the second local
 signal; and
 a multiplex radio receiver which includes a plurality of receiving sections
 provided so as to correspond to the channels to be used by the multiplex
 radio transmitter for the purpose of receiving, by way of a radio
 propagation path, radio frequency signals transmitted from the multiplex
 radio transmitter disposed so as to oppose the multiplex radio receiver,
 wherein the multiplex radio transmitter has the plurality of transmission
 sections which convert a first intermediate frequency signal resulting
 from modulation of a data signal into a second intermediate frequency
 signal having an intermediate frequency between the frequency of the first
 intermediate frequency signal and that of the radio frequency signal
 transmitted from the multiplex radio transmitter, and which convert the
 second intermediate frequency into the radio frequency signal, the
 multiplex radio receiver being characterized by the feature that
 the receiving section comprises
 a fourth filter which has at least a band-pass corresponding to radio
 frequencies of a plurality of channels to be used, which filters out the
 received radio frequency signal, and which eliminates a frequency
 component of an image signal of the radio frequency signal;
 a fifth frequency conversion section which converts the frequency of a
 signal outputted from the fourth filter through use of a fifth local
 signal to thus output a fifth intermediate frequency signal which is lower
 in frequency than the radio frequency signal by only the frequency of the
 fifth local signal;
 a sixth frequency conversion section which converts the frequency of a
 signal outputted from the fifth frequency conversion section through use
 of a sixth local signal to thus output a sixth intermediate frequency
 signal which is lower in frequency than the fifth intermediate frequency
 signal by only the frequency of the sixth local signal; and
 a demodulation section which demodulates the data signal by demodulation of
 an outputted from the sixth frequency conversion section.
 The multiplex radio transceiver according to the present invention enables
 simultaneous and bidirectional transmission and reception of signals. The
 transmitter requires only one type of transmission band-pass filter
 without use of a transmission band-pass filter for each radio frequency
 (RF) channel. Similarly, the receiver requires only a receiving band-pass
 filter having the same band-pass characteristics as those of the
 transmission band-pass filter of the transmitter without use of a
 receiving band-pass filter for each radio frequency (RF) channel.
 Accordingly, there is yielded an advantage of allowing use of common
 members in the transmission sections and receiving sections disposed
 within the multiplex radio transceiver or in repeaters, as well as
 enabling inexpensive manufacture of a multiplex radio transceiver.
 According to a further aspect of the present invention, there is provided a
 multiplex radio transceiver comprising:
 a multiplex radio transmitter which includes a plurality of transmission
 sections provided so as to correspond to respective channels for the
 purpose of multiplexing a plurality of channel signals having different
 frequencies into another signal and for the purpose of transmitting the
 multiplexed signal, wherein
 the transmission section comprises
 a modulation section which modulates a data signal and outputs the
 modulated data signal as a first intermediate frequency signal;
 a first frequency conversion section which converts the frequency of a
 signal outputted from the modulation section through use of a first local
 signal, thus outputting a second intermediate frequency signal which is
 higher in frequency than the first intermediate frequency signal by only
 the frequency of the first local signal;
 a first filter which eliminates a frequency component of the first local
 signal from the second intermediate frequency signal outputted from the
 first frequency conversion section;
 a second frequency conversion section which converts the frequency of a
 signal outputted from the first filter through use of a second local
 signal, thus outputting a radio frequency signal which is higher in
 frequency than the second intermediate frequency signal by only the
 frequency of the second local signal; and
 a second filter which has at least a band-pass corresponding to radio
 frequency signals of a plurality of channels to be used, which filters out
 the radio frequency signal outputted from the second frequency conversion
 section, and which eliminates a frequency component of the second local
 signal; and
 a multiplex radio receiver which includes a plurality of receiving sections
 provided so as to correspond to the respective channels for the purpose of
 receiving, by way of a radio propagation path, radio frequency signals
 transmitted from the plurality of transmission sections of the multiplex
 radio transmitter, the multiplex radio receiver being characterized by the
 feature that
 the receiving section comprises
 a third filter which has at least a band-pass corresponding to radio
 frequencies of a plurality of channels to be used, which filters out the
 received radio frequency signal, and which eliminates a frequency
 component of an image signal of the radio frequency signal;
 a third frequency conversion section which converts the frequency of a
 signal outputted from the third filter through use of a third local signal
 to thus output a third intermediate frequency signal which is lower in
 frequency than the radio frequency signal by only the frequency of the
 third local signal;
 a fourth frequency conversion section which converts the frequency of a
 signal outputted from the third frequency conversion section through use
 of a fourth local signal to thus output a fourth intermediate frequency
 signal which is lower in frequency than the third intermediate frequency
 signal by only the frequency of the fourth local signal; and
 a demodulation section which demodulates the data signal by demodulation of
 an outputted from the fourth frequency conversion section.
 The multiplex radio transmission/receiving system according to the present
 invention enables simultaneous and bidirectional transmission and
 reception of signals. The transmitter requires only one type of
 transmission band-pass filter without use of a transmission band-pass
 filter for each radio frequency (RF) channel. Similarly, the receiver
 requires only a receiving band-pass filter having the same band-pass
 characteristics as those of the transmission band-pass filter of the
 transmitter without use of a receiving band-pass filter for each radio
 frequency (RF) channel. Accordingly, there is yielded an advantage of
 enabling common use of the members in the transmission sections and
 receiving sections disposed within the multiplex radio transceiver or in
 repeaters, as well as enabling inexpensive manufacture of a multiplex
 radio transceiver and promoting cost cutting. Further, the harmonic of the
 second intermediate frequency does not collide with signals at sub-bands
 within, for example, C-band, and there is yielded an advantage of
 improving a signal-to-noise ratio of the receiver, as well as preventing
 superimposition of resultant spurious radiation of an image signal on a
 main signal.
 BRIEF DESCRIPTION OF THE DRAWINGS
 FIG. 1 is a schematic diagram showing the configuration of a trunk
 multiplex radio transmission/receiving system using a digital modulation
 scheme according to a first embodiment of the present invention;
 FIG. 2 is a block diagram showing a digital modulation section according to
 the first embodiment of the present invention;
 FIG. 3(a) shows a spectrum of an outputted from a digital modulation
 section provided at the transmission end according to the first
 embodiment;
 FIG. 3(b) shows the band-pass characteristics of a first band-pass filter
 provided at the transmission end or those of a fourth band-pass filter
 provided at the receiving end according to the first embodiment;
 FIG. 4 shows frequency allocation of a radio frequency (RF) signal group
 and a local signal group and the band-pass characteristics of a second
 band-pass filter disposed at the transmission end or those of a third
 band-pass filter disposed at the receiving end according to the first
 embodiment;
 FIG. 5 is a block diagram showing a digital demodulation section according
 to the first embodiment;
 FIG. 6 is a diagrammatic representation showing a frequency conversion
 section which produces an image frequency according to the first
 embodiment;
 FIG. 7 is a plot visually showing image frequencies according to the first
 embodiment by mapping a straight line (f/F2)=m.multidot.(F1/F2)+n;
 FIG. 8 is an enlarged view showing the principal element shown in FIG. 7
 according to the first embodiment and showing collision between
 transmission radio frequency (RF) signals and image frequencies;
 FIG. 9 is a plot showing straight lines so as to prevent collision between
 harmonics of P1 and sub-bands within C-band according to the first
 embodiment;
 FIG. 10 shows a table showing numbers of (uplink/downlink) channels within
 4G band, correspondence between frequencies, and other specifications
 according to general ITU-R recommendations;
 FIG. 11 shows a table showing numbers of (uplink/downlink) channels within
 5G band, correspondence between frequencies, and other specifications
 according to general ITU-R recommendations;
 FIG. 12 shows a table showing numbers of (uplink/downlink) channels within
 L6G band, correspondence between frequencies, and other specifications
 according to general ITU-R recommendations;
 FIG. 13 shows a table showing numbers of (uplink/downlink) channels within
 U6G band, correspondence between frequencies, and other specifications
 according to general ITU-R recommendations;
 FIG. 14 shows a table showing numbers of (uplink/downlink) channels within
 7G band, correspondence between frequencies, and other specifications
 according to general ITU-R recommendations;
 FIG. 15 shows a table showing numbers of (uplink/downlink) channels within
 L8G band, correspondence between frequencies, and other specifications
 according to general ITU-R recommendations;
 FIG. 16 is a schematic diagram showing the entire configuration of a trunk
 multiplex transmission/receiving system which employs a digital
 modulation/demodulation scheme according to a modification of the first
 embodiment;
 FIG. 17 is a schematic diagram showing the entire configuration of a trunk
 multiplex transmission/receiving system which employs an analog modulation
 scheme according to a second embodiment;
 FIG. 18 is a block diagram showing an analog modulation section according
 to the second embodiment;
 FIG. 19 is a block diagram showing an analog demodulation section according
 to the second embodiment;
 FIG. 20(a) shows a spectrum of an outputted from the analog modulation
 section disposed at the transmission send according to the second
 embodiment;
 FIG. 20(b) shows the band-pass characteristics of a first band-pass filter
 disposed at the transmission end or those of a fourth band-pass filter
 disposed at the receiving end according to the second embodiment;
 FIG. 21 shows frequency allocation of a radio frequency (RF) signal group
 and a local signal group and the band-pass characteristics of a second
 band-pass filter disposed at the transmission end or those of a third
 band-pass filter disposed at the receiving end according to the second
 embodiment;
 FIG. 22 is a schematic diagram showing a trunk multiplex
 transmission/receiving system using an analog modulation/demodulation
 scheme according to a modification of the second embodiment;
 FIG. 23 is a block diagram showing the principal elements of a transmission
 section of a multiplex radio transmitter;
 FIG. 24 shows allocation of a radio frequency (RF) signal group and a local
 signal group of a multiplex transmitter; and
 FIG. 25 shows a group of radio frequency (RF) channels of a multiplex radio
 transmitter and a band-pass filter provided for each channel or a group of
 radio frequency (RF) channels of a multiplex receiver and a band-pass
 filter disposed for each channel.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 (A) Description of a First Embodiment
 Preferred embodiments of the present invention will now be described
 hereinbelow by reference to the accompanying drawings.
 FIG. 1 is a circuit diagram showing the configuration of a trunk multiplex
 radio transmission/receiving system using a digital modulation scheme
 according to a first embodiment of the present invention. A trunk
 multiplex radio transmission/receiving system 22 using a digital
 modulation scheme and shown in FIG. 1 relays multiplexed trunk signal data
 (STM-1) between synchronous multiplex repeaters or between a synchronous
 multiplex repeater and a transmission-end apparatus. The trunk multiplex
 radio transmission/receiving system 22 comprises a multiplex radio
 transmitter (hereinafter often referred to simply as a "transmitter") 20a
 which transmits a plurality of channel signals having different
 frequencies while converting them into a multiplexed signal; a multiplex
 radio receiver (hereinafter often referred to simply as a "receiver") 20b
 which receives a plurality of multiplexed channel signals having different
 frequencies; and a radio propagation path 29.
 Here, the transmitter 20a digitally modulates multiplexed trunk signal data
 (STM-1) received from a synchronous multiplex repeater or a
 transmission-end apparatus and transmits the thus-modulated data to the
 radio propagation path 29. The transmitter 20a comprises a plurality of
 radio transmission sections 10a provided so as to correspond to respective
 channels; an antenna 8a; and an antenna diplexer 9a.
 A frame format of the trunk multiplex signal data (STM-1) comprises an SOH
 (Section Over Head) which will serve as a header and multiplexed data.
 The SOH consists of additional bits for establishing a layer 2 link between
 synchronous multiplex repeaters or between a synchronous multiplex
 repeater and a transmission-end apparatus, thus embodying features such as
 an error correction capability, a client service information capability, a
 resending control capability, or the like.
 A transmission rate of STM-1 is 155.52 Mbps and corresponds to 2,016
 channels of a telephone line, provided that the transmission rate per
 channel is 64kbps. A modulation rate corresponds to a value resulting from
 dividing a transmission rate of 155.52 Mbps by loge (a multivalued
 modulation number). More specifically, in a case where a 64 QAM scheme is
 used, there is obtained a modulation rate of 155.52 (Mbps)/6(bits)=about
 26 Mbps. In a case where a 128 QAM scheme is used, there is obtained a
 modulation rate of 155.52 (Mbps)/7(bits)=about 22 Mbps.
 As set forth, the modulation rate is univocally determined by means of a
 STM-1 transmission frame format and by a multivalued digital modulation
 number.
 Each radio transmission section 10a subjects the multiplexed trunk STM-1
 data to a required baseband processing treatment in a baseband processing
 section and digitally modulates the thus-processed data. The transmission
 section 10a comprises a digital modulation section 2a, a first frequency
 converter 3a, a first band-pass filter 4a, a second frequency converter
 5a, a second band-pass filter 6a, and a high power amplifier 7a.
 The digital modulation section 2a digitally modulates the STM-1 data
 processed by the baseband processing section. As shown in FIG. 2, the
 digital modulation section 2a comprises a hybrid section 11a, a mapping
 section 12a, a roll-off filter 13a-1, a roll-off filter 13a-2, a
 quadrature modulation section 14a, and a digital-to-analog converter 18a.
 The hybrid section 11a converts the STM-1 data into parallel data through
 serial-to-parallel conversion.
 The mapping section 12a maps data bits of the I- and Q-channels into a
 constellation (or an arrangement of signal points) in groups data
 bits--which are equal in number to a multivalued modulation number--and
 encodes the amplitude and phase of the transmitted signal.
 The roll-off filter 13a-1 processes the I-channel signal through waveform
 shaping at a rate which is four times the modulation rate, in order to
 prevent intersymbol interference at the receiving end. Similarly, the
 roll-off filter 13a-2 processes the Q-channel signal at a rate which is
 four times the modulation speed.
 The quadrature modulation section 14a performs a baseband modulation
 operation by multiplying the oversampled I-channel signal--which is an
 outputted from the roll-off filter 13a-1--and the oversampled Q-channel
 signal-which is an outputted from the roll-off filter 13a-2--by a clock
 signal which is four times as fast as the modulation rate. The quadrature
 modulation section 14a comprises a mixer 15a-1, a mixer 15a-2, a
 quadrature carrier signal generator 16a, and an adder 17a.
 The mixer 15a-1 multiplies the I-channel signal--which is oversampled at a
 rate four times the modulation rate and is outputted from the roll-off
 filter 13a-1--by a quadrature carrier signal which is as fast as the
 I-channel signal.
 More specifically, as is obvious from boxes provided in FIG. 2, the
 I-channel data signal--whose amplitude is sampled at a rate four times the
 modulation rate f.sub.Q --is multiplied by
 cos[2.multidot..pi..multidot.(4.multidot.f.sub.Q).multidot.t] which is
 produced by the quadrature carrier signal generator 16a and assumes three
 values 1,0,-1,0,1 . . . , whereby the data are converted into a data
 string of I,0,-I,0,I,0,-I,0,I,0,-I, . . . .
 Similarly, the Q-channel data signal--whose amplitude is sampled at a rate
 four times the modulation rate f.sub.Q --is multiplied by
 sin[2.multidot..pi..multidot.(4.multidot.f.sub.Q).multidot.t] which is
 produced by the quadrature carrier signal generator 16a and assumes three
 values 0,1,0,-1,0 . . . , whereby the data are converted into a data
 string of 0,Q,0,-Q,0,Q,0, -Q,0,Q,0, . . . .
 The adder 17a adds the I-channel data outputted from the mixer 15a-1 to the
 Q-channel data outputted from the mixer 15a-2, thus outputting a data
 string of I,Q,-I,-Q,I,Q,-I,-Q,I,Q,-I, . . . because of only a phase
 difference of (.pi./2) between cos and sin.
 In short, in view of an output of the modulation section, the I-channel
 data and the Q-channel data appear to be selectively output.
 More specifically, the quadrature carrier signal generator 16a is a counter
 provided within an LSI (Large Scale Integration) circuit and produces the
 carrier signal by dividing the fastest clock signal. Accordingly, an
 arbitrary frequency cannot be selected.
 The digital-to-analog converter 18a converts digital data outputted from
 the quadrature modulation section 14a into analog data and produces an
 output spectrum such as that shown in FIG. 3 (a). The spectrum comes about
 while being centered on the modulation rate f.sub.M.
 The first frequency converter 3a converts a first IF signal (a first
 intermediate frequency signal) outputted from the digital modulation
 section 2a into a second IF signal (a second intermediate frequency
 signal) which is higher in frequency than the first IF signal by only the
 frequency of a first local signal of the first frequency converter. In the
 first frequency converter 3a, a mixer 3a-1 mixes a signal outputted from a
 first local signal oscillator 3a-2 into the first IF signal outputted from
 the digital modulation section 2a.
 The way the frequency value of the second IF signal is determined will be
 described later.
 The first band-pass filter 4a eliminates a frequency component of the first
 local signal outputted from the first frequency converter 3a, thus
 permitting passage of only the second IF signal. FIG. 3(b) shows the
 band-pass characteristics of the first band-pass filter 4a. As shown in
 this drawing, the first band-pass filter 4a permits passage of only the
 frequency of 844 MHz, which is the frequency of the second IF signal (the
 way the frequency of the second IF signal is determined will be described
 later) but prevents the passage of the first local signal whose frequency
 (818 MHz) is lower than that of the second IF signal by about 26 MHz.
 The second frequency converter 5a outputs a radio frequency (RF) signal by
 up-converting an outputted from the first band-pass filter 4a through use
 of a second local signal capable of oscillating eight kinds of
 frequencies. A mixer 5a-1 mixes a second local signal outputted from a
 second local signal oscillator 5a-2 with the second IF signal outputted
 from the first band-pass filter 4a.
 For example, FIG. 4 shows allocation of a group of frequencies of the
 second local signal and radio frequencies (RF), through use of the U6G
 band as a sub-band. As shown in the drawing, the group of frequencies of
 the second local signal and the group of radio frequency (RF) signals do
 not overlap one another and are separated in frequency from one another.
 Further, the second band-pass filter 6a has at least a band-pass width
 corresponding to radio frequency signals of a plurality of channels to be
 used and filters the radio frequency (RF) signal outputted from the second
 frequency converter 5a, thus eliminating a frequency component of the
 second local signal.
 The band-pass characteristics of the second band-pass filter 6a shown in
 FIG. 4 are such that the center frequency lies in the microwave band, and
 the second band-pass filter has a band-pass width of 340 MHz.
 Specifically, the pass-band width corresponds to the sum of the radio
 frequency (RF) group of 280 MHz and the bandwidth guard space of 30 MHz
 provided on either side thereof. As a result, individual channels of the
 group of radio frequency (RF) signals can be collectively passed through
 use of only one type of second band-pass filter having broad band-pass
 characteristics, without need for a second band-pass filter having narrow
 band-pass characteristics for each radio frequency (RF) signal channel.
 A signal-to-noise ratio of the receiver can be improved, so long as the
 value of the second IF frequency is selected in such a way as to prevent
 collision between the image frequency obtained by multiplication of the
 second IF frequency by the local signal and a signal in another sub-band
 within C-band.
 The high power amplifier 7a amplifies a radio frequency (RF) signal
 outputted from the second band-pass filter 6a with high efficiency and low
 distortion.
 The antenna 8a is connected to each of the radio transmission sections 10a
 via the antenna diplexer 9a and transfers to the radio propagation path 29
 the digitally-modulated radio frequency (RF) signals outputted from the
 individual radio transmission sections 10a. Specifically, the antenna 8a
 is configured as an antenna to be shared among the radio transmission
 sections 10a.
 The antenna diplexer 9a connects the radio frequency (RF) signals outputted
 from the individual radio transmission sections 10a to the shared antenna
 8a.
 In contrast, the receiver 20b digitally demodulates the radio frequency
 (RF) signal, which has passed through the radio propagation path 29, and
 sends the STM-1 signal to another synchronous multiplex repeater or
 another transmission-end apparatus. To this end, the receiver 20b shown in
 FIG. 1 comprises an antenna 8b, an antenna diplexer 9b, a plurality of
 radio receiving sections 10b provided so as to correspond to the
 individual channels, and a baseband processing section (not shown).
 The antenna 8b is connected to each of the radio receiving sections 10b via
 the antenna diplexer 9b and receives a radio frequency (RF) signal. More
 specifically, the antenna 8b is constituted as an antenna to be shared
 among the radio receiving sections 10b.
 Each radio receiving section 10b digitally demodulates the radio frequency
 (RF) signal received by the antenna 8b and outputs the STM-1 signal after
 having subjected the RF signal to a baseband processing treatment. The
 radio receiving section 10b comprises a low-noise amplifier 7b, a third
 band-pass filter 6b, a third frequency converter 5b, a fourth band-pass
 filter 4b, a fourth frequency converter 3b, and a digital demodulator 2b.
 The low-noise amplifier 7b amplifies the radio frequency (RF) signal
 received by the antenna 8b with little noise.
 The third band-pass filter 6b has at least a band-pass width corresponding
 to radio frequency signals of a plurality of channels to be used. The
 third band-pass filter 6b eliminates a frequency component of the image
 signal from the radio frequency (RF) signal outputted from the low-noise
 amplifier 7b, thus selecting a desired signal. As a result, individual
 channels of radio frequency (RF) signals can be collectively passed
 through use of only one type of band-pass filter without need for a
 band-pass filter for each radio frequency (RF) signal channel. The third
 band-pass filter 6b has band-pass characteristics such as those shown in
 FIG. 4. The third IF signal used in the receiver has a frequency of 844
 MHz; the reason for this will be described later.
 The third frequency converter 5b converts the frequency of the radio
 frequency (RF) signal outputted from the third band-pass filter 6b,
 through use of a third local signal, thereby outputting a third IF signal
 whose frequency is lower than that of the radio frequency (RF) signal by
 the frequency of the third local signal. A mixer 5b-1 mixes a third local
 signal outputted from a third local signal oscillator 5b-2 with the radio
 frequency (RF) signal outputted from the third band-pass filter 6b.
 The fourth band-pass filter 4b eliminates an image signal from the
 outputted from the third frequency converter 5b, thus permitting passage
 of only the third IF signal.
 As shown in FIG. 3(b), the band-pass characteristics of the fourth
 band-pass filter 4b permit passage of only a frequency of 844 MHz, which
 corresponds to the third IF signal, but prevent passage of the first local
 signal (818 MHz) whose frequency is lower than that of the third IF signal
 by about 26 MHz. The way a value of the second IF signal is determined
 will be described later.
 The fourth frequency converter 3b down-converts the third IF signal
 outputted from the fourth band-pass filter 4b into a fourth IF signal
 whose frequency is lower than that of the third IF signal. A mixer 3b-1
 mixes a fourth local signal outputted from a fourth local signal
 oscillator 3b-2 with the radio frequency (RF) signal outputted from the
 fourth filter 4b.
 The digital demodulation section 2b digitally demodulates the outputted
 from the fourth frequency converter 3b and as shown in FIG. 5 comprises an
 analog-to-digital converter 18b, a quadrature detection section 14b, an
 integrator 13b-1, an integrator 13b-2, and a reverse mapping section 12b.
 The analog-to-digital converter 18b converts the fourth IF signal received
 from the fourth frequency converter 3b through analog-to-digital
 conversion.
 The quadrature detection section 14b demodulates a digital signal outputted
 from the analog-to-digital converter 18b and comprises a hybrid section
 17b which converts an outputted from the analog-to-digital converter 18b
 through serial-to-analog conversion; a quadrature carrier signal generator
 16b which produces quadrature carrier signals
 cos[2.multidot..pi..multidot.(4.multidot.v).multidot.t] and
 sin[2.multidot..pi..multidot.(4.multidot.v).multidot.t] (v: modulation
 rate); a mixer 15b-1 which reproduces a bit string having amplitude values
 by multiplication of I-channel data by the quadrature carrier signal; and
 a mixer 15b-2 which reproduces a bit string having amplitude values by
 multiplication of Q-channel data by the quadrature carrier signal.
 The integrator 13b-1 extracts the bit position of demodulated data
 regarding the modulation rate "v" from an I-channel bit string which is
 outputted from the quadrature detection section 14b and is sampled at high
 speed. Similarly, the integrator 13b-2 extracts the bit position of
 demodulated data regarding the modulation rate "v" from a Q-channel bit
 string which is outputted from the quadrature detection section 14b and is
 sampled at high speed.
 The reverse mapping section 12b decodes the amplitude and phase of the
 transmitted signal from the demodulated data bits regarding the modulation
 speed outputted from the integrators 13b-1 and 13b-2, thereby extracting
 demodulated data in an actual signal space in groups of data bits which
 are equal in number to the multivalued modulation number.
 The demodulated data are subjected to a required baseband processing
 treatment in the baseband processing section and are sent as STM-1 data to
 another synchronous multiplex repeater or a transmission-end apparatus.
 Full duplex communication is established between the two opposing repeaters
 over the radio propagation path 29 through use of required channels. Of
 these channels, one type of channel is used for a backup purpose. A master
 repeater transmits a radio signal at, e.g., a downlink transmission
 frequency X which is one of seven types of channels. A slave repeater
 which opposes the master repeater receives the thus-transmitted signal
 having the transmission frequency X. Simultaneously, the slave repeater
 transmits a signal at an uplink transmission frequency X', and the master
 repeater which opposes the slave repeater receives the signal having the
 transmission frequency X'.
 For example, frequencies in a U6G band within C-band are allocated in a
 manner such as that shown in FIG. 13; namely, the uplink channel is
 allocated channel 1' (a frequency of 6800.0 MHz) to channel 8' (a
 frequency of 7080.0 MHz); and the downlink channel is allocated channel 1
 (a frequency of 6460.0 MHz) to channel 8 (a frequency of 6740.0 MHz).
 Since the transmitter and the receiver each select one channel from the
 eight types of channels, they become symmetrical to each other in terms of
 band-pass characteristics.
 For example, in a case where a frequency of 844 MHz is selected as the
 second and third IF frequencies, three bands, i.e., the radio frequency
 (RF) signal, band the local signal band, and the image frequency band, can
 be allocated as shown in FIG. 4 in such a way as not to overlap one
 another. As a result, the local signal band is separated from the group of
 radio frequency (RF) signals, and the transmitter requires only one type
 of band-pass filter capable of permitting collective passage of the
 plurality of radio frequency (RF) signals. Members having common
 specifications can be used for the individual radio transmission sections
 disposed within the multiplex radio transmitter or for repeaters.
 Therefore, there is no need for individual band-pass filters having
 different band-pass characteristics, which would otherwise be provided for
 individual radio transmission sections or repeaters.
 So long as the second IF signal is selected so as to prevent collision
 between the image frequency obtained by multiplication of the second IF
 frequency by the local signal and a signal in another sub-band within
 C-band, the transmitter will be prevented from producing spurious
 radiation, and the signal-to-noise ratio of the receiver can be improved.
 Similarly, the receiver does not require in its front-end stage filters
 having band-pass characteristics corresponding to the channels used for
 individual radio receiving sections or repeaters. The receiver requires
 only one type of band-pass filter, thus enabling use of members having
 common specifications for the individual radio receiving sections provided
 within the multiplex radio receiver or for repeaters.
 The intermediate frequency determined in consideration of such a special
 value will be referred to as an "optimum second IF" at the sending end and
 as an "optimum third IF" at the receiving end.
 A method of selecting the optimum second and third IF frequencies will now
 be described in detail.
 The optimum second IF signal F1 (or the optimum third IF signal) is
 determined in consideration of the following:
 (1) The radio frequency (RF) signal band and the local signal band must be
 sufficiently separated from each other in order to prevent overlap among
 the radio frequency (RF) signal band, the local signal band, and the image
 frequency band and to separate them from one another. This is because
 small difference between the bands may cause the local signal band to
 mixedly exist within the radio frequency (RF) signal band.
 (2) The lower limit of the optimum second or third IF frequency is taken
 into consideration.
 (3) The image frequency [a theoretically infinite number of frequency
 groups produced from a harmonic of the local signal and a harmonic of the
 radio frequency (RF) signal] must be prevented from falling within the
 radio frequency (RF) signal band. In short, the radio frequency (RF)
 signal is protected from the influence of the image frequency.
 (4) The second IF frequency is selected not only so as to become effective
 within one transmit/receive sub-band within C-band (ranging in frequency
 from 3.4 GHz to 3.5 GHz) but also so as to become useful as a shared
 second IF among all the other sub-bands.
 The foregoing items (1) through (4) will now be examined.
 With regard to item (1), a frequency difference on the order of several
 tens of mega-hertz is evidently insufficient, and hence the frequency
 difference is set to about several hundreds of mega-hertz.
 Next, item (2) is examined. Since the second IF frequency at the sending
 end and the third IF frequency at the receiving end are equal to each
 other, attention is focused on only the second IF frequency at the sending
 end.
 The lower limit of the optimum second IF frequency described in item (2) is
 determined by the bandwidth of the sub-band. As shown in FIG. 4, when the
 transmission radio frequency (RF) signal is extracted for each band
 through use of the common band-pass filter, in each frequency band the
 band-pass of the filter must be wider than the sum of the bandwidth of
 channels 1 through 8 and a band-width guard space widths provided at each
 end of the band, in order to prevent the upper limit of the local signal
 band from overlapping with the lower end of the radio frequency (RF)
 signal band. Of the individual sub-bands, the sub-band having the widest
 bandwidth is U6G band (680 MHz). Assuming that the optimum second IF
 frequency is F1, we have
EQU F1&gt;680 (1).
 Item (3) will now be described by reference to FIGS. 6 and 7.
 FIG. 6 schematically shows a frequency converter which produces an image
 frequency. The frequency converter shown in FIG. 6 multiplies the
 fundamental harmonic P1 of the second intermediate frequency by the
 fundamental harmonic P2 of the carrier frequency. Here, the harmonics of
 P1 and P2 are also multiplied together, thereby resulting in unwanted
 image frequency components. An image frequency f.sub.i is expressed by the
 sum of an integral multiple of P1 and an integral multiple of P2.
EQU f.sub.i =m.multidot.P1+n.multidot.P2 (2)
 where "m" and "n" represent integers. For example, f.sub.i =P1+P2
 represents a signal obtained when m=1 and n=1. This signal is at the upper
 side of the carrier and is not eliminated by the band-pass filter: namely,
 the signal corresponds to the transmission radio frequency (RF) signal. In
 contrast, f.sub.i =-P1+P2 represents a signal obtained when m=-1 and n=1.
 This signal is at the lower side of the carrier and is eliminated by the
 band-pass filter.
 Equation (2) is changed into
EQU f.sub.i /P2=m.multidot.(P1/P2)+n (3)
 When equation (3) is plotted while taking (P1/P2) as the horizontal axis
 and (f.sub.i /P2) as the vertical axis, there are obtained a group of
 straight lines having slopes "m" and intercepts "n" on the vertical axis.
 FIG. 7 shows segments of exemplary straight lines having intercepts n=1,
 n=-2, n=-3, n=4, and n=5.
 A group of straight lines having n=1 are defined by f.sub.i =P1+P2 (m=k,
 n=1: k is an integer) which represents a group of straight lines having an
 intercept of one and slopes ranging from +.infin. to -.infin..
 Groups of straight lines having intercepts n=-2, n=-3, n=4, and n=5 are
 represented respectively by
 f.sub.i =P1+P2 (m=k, n=-2: k is an integer),
 f.sub.i =P1+P2 (m=k, n=-3: k is an integer),
 f.sub.i =P1+P2 (m=k, n=4: k is an integer), and
 f.sub.i =P1+P2 (m=k, n=5: k is an integer).
 These expressions represent groups of straight lines having changed
 intercepts and slopes ranging from +.infin. to -.infin..
 Here (P1/P2) represents a ratio of P2 to P1 and assumes a value between 0
 and 1. The ratio physically represents the upper limit of P1. More
 specifically, carrier P2 assumes a large value ranging in frequency from
 3.4 GHz to 8.3 GHz, and carrier P1 representing a signal frequency assumes
 a value which does not exceed the value of P2. Only a hatched region
 provided in FIG. 7; namely, the extent to which (P1/P2) assumes 0 through
 1, requires attention. FIG. 8 is an enlarged view of the hatched region.
 The vertical axis (f.sub.i /P2) represents the magnitude of the image
 frequency fi normalized with respect to P2, i.e., a value which expresses
 extension or contraction of the scale of the vertical axis.
 In FIG. 8, the straight line designated by P1+P2 corresponds to the
 transmission radio frequency (RF) signal, and hence the conditions
 provided in item (3) must be taken into consideration. Specifically, the
 radio frequency (RF) signal is protected from the influence of the image
 frequency.
 The thick line (P1+P2) shown in FIG. 8 represents a radio frequency (RF)
 sent to a radio channel. Two straight lines crossing each other on the
 line (P1+P2) are merely image frequencies which come into collision with
 the line (P1+P2) at that frequency.
 The range of the horizontal axis (P1/P2) must be limited to a range which
 can be of practical use. This is because as mentioned above (P1/P2)
 represents a ratio of P2 to P1 and cannot assume a value close to 1.
 The optimum second intermediate frequency F1 is determined, and a possible
 range of the carrier frequency F2 is examined, whereby a range of (F1/F2)
 is plotted on the coordinate system comprising the (P1/P2) axis and the
 (f.sub.i /P2) axis.
 First, in view of the foregoing items (1) and (2), the optimum second
 intermediate frequency F1 must be lower than 3.4 GHz and greater than at
 least 680 MHz which is the maximum value of the radio frequency (RF)
 signal bandwidth.
 Further, in view of item (4), the optimum second intermediate frequency F1
 must be considerably low so that it can be shared among sub-bands within
 C-band. For example, in a case where the transmission radio frequency (RF)
 signal band is the 4G band and where the second optimum intermediate
 frequency F1 is set to a large value such as 3 GHz, the carrier F2 is
 allocated to a low value such as 620 MHz to 860 MHz.
 Further, in consideration of the fact that many existing band-pass filters
 have their center frequencies around 800 MHz, the optimum second
 intermediate frequency F1 is set to 800 MHz.
 The optimum second intermediate frequency F1 is not limited to 800 MHz but
 can be set to an arbitrary value within the foregoing range. Hence,
 (F1/F2) also assumes a range which is substantially the same as that of
 the second intermediate frequency.
 At this time, the third intermediate frequency F2 assumes a value within a
 range in the vicinity of C-band [e.g., a range (2.6 GHz to 7.5 GHz) lower
 than C-band by 800 MHz], and hence (F1/F2) assumes a value within a range
 from 800/7500 to 800/2600 (=0.11 to 0.31).
 In consideration of item (3), attention is paid to the order of the
 harmonic within a range of m=-6-+6 and n=-6 to +6.
 In this way, the range of the horizontal axis (P1/P2) falls within a range
 from 0.11 to 0.31, and therefore the types of image frequencies falling
 into the range are limited to the following six points A, B, C, D, E, and
 F: namely,
EQU (A) 2F2-2F1 (5)
EQU =3F2-5F1 (6)
EQU =4F1 (7)
EQU (B) 3F2-6F1 (8)
EQU (C) 2F2-3F1 (9)
EQU =5F1 (10)
EQU (D) 2F2-4F1 (11)
EQU =6F1 (12)
EQU (E) 2F2-5F1 (13)
EQU =7F1 (14)
EQU (F) 2F2-6F1 (15)
EQU =8F1 (16)
 where the symbol "=" represents an identical point. Of these frequencies
 (A), (B), (C), (D), (E), and (F), there are selected frequencies wherein
 at least one equation is represented by a term of 5.sub.th order or lower:
 namely, (A), (C), (D), and (E). Since frequencies including a harmonic
 term of 6 order or higher, such as (B) and (F), cause great attenuation,
 they are omitted. Point (D) receives very small influence from 6F1 but
 chiefly receives influence from 2F2 to 4F1. The same applies to frequency
 (E). More specifically, it is essential only that frequencies 4F1, 5F1,
 6F1, and 7F1, which greatly affect F1+F2, be omitted from existing
 sub-bands within C-band.
 As can be seen from item (4), it is essential only that the optimum second
 intermediate frequency F1 is determined so as to prevent the harmonic of
 the optimum second intermediate frequency F1, i.e. m.multidot.F1, from
 coming into collision with existing sub-bands within C-band.
 FIG. 9 shows overlaps between the harmonic f=m.multidot.F1 of the second
 intermediate frequency F1 and the individual sub-bands within C-band. The
 horizontal axis represents frequency, and the vertical axis represents
 orders of harmonic components of the second IF. FIG. 9 shows seven types
 of sub-bands within C-band: namely, 3.5G band (3.5 G), 4 G band (4GL,
 4GU), 5 G band (5GL, 5GU), L6G band (L6GL, L6GU), U6G band (U6GL, U6GU), 7
 G band (7GL, 7GU), and L8G band (L8GL, L8GU). Six straight lines (G), (H),
 (I), (J), (K), (L), and (M) are plotted on the coordinate system. From the
 drawing, it is understood which one of the sub-bands overlaps with the
 m-order harmonic of the optimum second IF F1.
 For example, a straight line (H) shown in the drawing is drawn freehand so
 as to satisfy the following conditions.
 A harmonic of fourth order does not come into contact with the lower end of
 3.5 G band (ranging in frequency from 3400 MHz to 3600 MHz).
 A harmonic of fifth order passes through a range from channel 7 to channel
 1' in 4 G band (i.e., 3860 MHz to 3940 MHz) where no channels are
 allocated.
 A harmonic of sixth order passes through a range from channel 7 to channel
 1' in 5 G band (i.e., 4670 MHz to 4730 MHz) where no channels are
 allocated.
 A harmonic of seventh order does not come into contact with the lower end
 of L6G band (channel 1=5945.20 MHz).
 There will be described in detail conditions under which the six straight
 lines (G) to (M) do not come into contact with the individual sub-bands.
 FIGS. 10 through 15 show the allocation of channels within the individual
 sub-bands 4 G band, 5 G band, L6G band, U6G band, 7 G band, and L8G band
 defined by ITU-R recommendations. These channel allocations will be
 referred to one by one. The band-width guard space provided on each end of
 the frequency allocation has a medium or small capacity of
 .+-.10M.times.2=.+-.20 MHz or a large capacity of .+-.15M.times.2=.+-.30
 MHz.
 Straight line (G) is determined in such a way that a harmonic of
 5.multidot.F1 passes through a gap between the upper end of 3.5 G band and
 the lower end of 4 G band.
 3600+20&lt;5.multidot.F1&lt;3620-30
 .thrfore.No solution
 Straight line (H) is determined in such a way that a harmonic of
 4.multidot.F1 does not come into contact with the lower end of 3.5 G band;
 such that a harmonic of 5.multidot.F1 passes through a gap in 4 G band;
 and such that a harmonic of 6.multidot.F1 passes through a gap in 5 G
 band.
 4.multidot.F1&lt;3400-20
 .thrfore.F1&lt;845
 3860+30&lt;5.multidot.F1&lt;3940-30
 .thrfore.778&lt;F1&lt;782
 4670+30&lt;6.multidot.F1&lt;4730-30
 .thrfore.783&lt;F1&lt;783
 .thrfore.No solution under these three conditions
 Straight line (I) is determined in such away that a harmonic of
 4.multidot.F1 does not come into contact with the lower end of 3.5 G band;
 such that a harmonic of 5.multidot.F1 does not come into contact with the
 upper end of 4 G band; such that a harmonic of 6.multidot.F1 does not come
 into contact with the upper end of 5 G band; and such that a harmonic of
 7.multidot.F1 does not come into contact with the lower end of L6G band.
EQU 4.multidot.F1&lt;3400-20
EQU .thrfore.F1&lt;845
 4180+30&lt;5.multidot.F1
EQU .thrfore.F1&gt;842.00
EQU 4970+30&lt;6.multidot.F1
EQU .thrfore.F1&gt;833.33
EQU 7.multidot.F1&lt;5945.20-30
EQU .thrfore.F1&lt;845.02
EQU 842.00&lt;F1&lt;845.02 (17)
 Straight line (J) is determined in such a way that a harmonic of
 4.multidot.F1 does not come into contact with the upper end of 3.5 G band;
 such that a harmonic of 7.multidot.F1 does not come into contact with the
 upper end of L6G; and such that a harmonic of 7.multidot.F1 does not come
 into contact with the lower end of U6G band.
 3600+20&lt;4.multidot.F1
 .thrfore.F1&gt;905
 6404.79+30 &lt;7.multidot.F1&lt;6460.0-30
 .thrfore.919.3&lt;F1&lt;918.6
 .thrfore.No solution under these two conditions
 Straight line (K) is determined in such a way that a harmonic of
 7.multidot.F1 passes through a gap in U6G band; and such that a harmonic
 of 8.multidot.F1 does not come into contact with the lower end of L8G
 band.
 6740+30 &lt;7.multidot.F1&lt;6800-30
 .thrfore.967.14&lt;F1&lt;967.14
 8.multidot.F1&lt;7747.70-30
 .thrfore.F1&lt;964.7
 .thrfore.No solution under these two conditions.
 However, F1=967.14 MHz can be used for approximation up to the harmonic of
 seventh order (18).
 Straight line (L) is determined in such a way that a harmonic of
 8.multidot.F1 does not come into contact with the upper end of L8G band.
 8266.57+30&lt;8.multidot.F1
 .thrfore.F1&gt;1037
 .thrfore.Provided that F1 corresponds to the minimum frequency of 3400 MHz
 within C-band, (F1/F2) is determined as 1038/(3400-1038)=0.44 which is out
 of the defined rage of 0.11 to 0.31 shown in FIG. 6, and therefore F1&gt;1037
 is omitted.
 Straight line (M) is determined in such a way that a harmonic of
 6.multidot.F1 does not come into contact with the lower end of 3.5 G band.
EQU 6.multidot.F1&lt;3400
EQU .thrfore.F1&lt;566 (19)
 As a result, the frequencies defined in (17), (18), and (19) can be adopted
 as an intermediate frequency.
 From Equation 1 resulting from (1), the optimum second IF is adopted in
 consideration of (17) and (18). Specifically,
EQU 842.00&lt;F1&lt;845.02 (20)
EQU F1=967.1 MHz (possible for a harmonic of up to seventh order) (21).
 Similarly, the optimum third IF used at the receiving end also adopts the
 frequencies defined in (20) and (21).
 As mentioned above, according to the first embodiment, a transmission
 signal is subjected to the first frequency conversion treatment, and the
 thus-converted signal is further subjected to a frequency conversion
 treatment performed at the transmission end, thus producing a second IF.
 The transmission signal is subjected to a third frequency conversion
 treatment performed at the receiving end, and the thus-converted signal is
 further subjected to a fourth frequency conversion treatment. As a result,
 an arbitrary second or third intermediate frequency can be set, and image
 frequencies can be prevented from falling into the group of radio
 frequencies (RFs).
 So long as the second IF frequency F1 and the third IF frequency F2 assumes
 the frequency defined in (20) or (21), in practice harmonics of the second
 IF frequency F1, i.e., 4.multidot.F1, 5.multidot.F1, 6.multidot.F1, and
 7.multidot.F1, do not affect radio signals of existing sub-bands within
 C-band, and therefore the signal-to-noise ratio of the receiver can be
 improved.
 Further, so long as the second IF F1 and the third IF F2 assume the
 frequency defined in (20) or (21), three bands, i.e., the transmission
 radio frequency (RF) signal band, the local frequency channel band, and
 the image frequency band, are separated from one another without overlap.
 Further, the receiver can receive a signal while these three bands are
 separated from one another.
 As a result, the transmitter requires only one type of band-pass filter
 having a pass-band for permitting passage of a plurality of radio
 frequencies (RF), thus eliminating the need for a band-pass filter for
 each radio frequency (RF) channel.
 Similarly, the receiver also requires only one type of band-pass filer
 having a pass-band corresponding to the channel of a signal transmitted
 from an opposing transmitter, thus eliminating the need for a band-pass
 filter for each radio frequency (RF) channel.
 Band-pass filters having different filtering characteristics have been
 required for individual radio transmission sections disposed within a
 multiplex radio transmitter, for individual receiving sections disposed
 within a multiplex radio receiver, and for repeaters. In contrast,
 according to the first embodiment, even if a multiplex communications
 system is allocated several channels, a band-pass filter having common
 specifications can be used for the individual radio transmission sections
 disposed within another multiplex radio transmitter, for individual radio
 receiving sections disposed within another multiplex radio receiver, or
 for repeaters, thus promoting common use of members. In this connection,
 the cost of a multiplex radio transmitter can be reduced.
 (A1) Modification of the First Embodiment
 In the first embodiment, a repeater disposed at the transmission end is not
 equipped with a radio receiving unit, and a repeater disposed at the
 receiving end is not equipped with a radio transmission unit. However,
 each of these repeaters may be configured so as to have both a radio
 transmission unit and a radio receiving unit.
 In such a case, as shown in FIG. 16, a digital modulation type trunk
 multiplex transmission/receiving system 23, in which each of transmission
 and receiving stations has a radio transceiver unit, relays multiplexed
 trunk signal data (STM-1) between synchronous multiplex repeaters or
 between a synchronous multiplex repeater and a transmission-end apparatus.
 The trunk multiplex transmission/receiving system 23 comprises a multiplex
 radio transceiver 21a, a multiplex radio transceiver 21b, and a radio
 propagation path 29.
 Each of the multiplex radio transceivers 21a, 21b has the plurality of
 radio transmission sections 10a corresponding to a plurality of channels
 and the plurality of radio receiving sections 10b corresponding to the
 plurality of channels. The multiplex radio transceivers 21a and 21b are
 disposed so as to oppose each other. Further, the trunk multiplex
 transmission/receiving system 23 comprises eight frequency converters.
 In the multiplex radio transceiver 21a, each radio transmission section 10a
 digitally modulates multiplexed trunk signal data (STM-1) received from
 another synchronous multiplex repeater or a transmission-end apparatus,
 through use of a plurality of channels having different frequencies. The
 thus-modulated signals are transmitted at C-band to a radio channel.
 Simultaneously, radio signals, which have been digitally modulated by
 individual radio transmission sections 10a of the counterpart multiplex
 radio transceiver 21b through use of other frequencies within the
 identical band, are received by radio receiving sections 10b of the
 multiplex radio transceiver 21a. The thus-received signals are digitally
 demodulated and are subjected to a baseband processing treatment, whereby
 STM-1 data are resent to another synchronous repeater or transmission-end
 apparatus. The multiplex radio transceiver 21a comprises the plurality of
 radio transmission sections 10a, the plurality of radio receiving sections
 10b, the antenna 8a, the antenna diplexer 9a, and the baseband processing
 section (not shown).
 Each of the individual radio transmission sections 10a of the multiplex
 radio transceiver 21a receives multiplexed trunk signal data (STM-1) sent
 from another synchronous multiplex repeater or a transmission-end
 apparatus. The thus-received signal data are digitally modulated by the
 digital modulation section 2a by way of the baseband processing section,
 whereby a first IF signal is output. The first frequency converter 3a
 up-converts the first IF signal to a second IF signal which is filtered
 out by the first band-pass filter 4a. The thus-filtered signal is
 up-converted to a radio frequency (RF) signal by means of a second
 frequency converter 5a. The thus-converted signal is then sent through an
 antenna at C-band to the radio channel by way of the second band-pass
 filter 6a, the high power amplifier 7a, and the antenna diplexer
 By way of the antenna 8a, the radio receiving sections 10b of the multiplex
 radio transceiver 21a receives the respective radio signals which have
 frequencies differing from the transmission frequencies of the multiplex
 radio transceiver 21a and which are transmitted at the identical band from
 the individual radio transmission sections 10a of the multiplex radio
 transmitter 21b disposed so as to oppose the multiplex radio transceiver
 21a. The thus-received signals are passed through the seventh band-pass
 filter 6b by way of the antenna diplexer 9a and the low-noise amplifier
 7b, thus filtering out image signals included in the signals. An output
 from the band-pass filter 6b is down-converted to a seventh IF signal,
 through use of the seventh frequency converter 5b. An output from the
 seventh frequency converter 5b is further down-converted to an eighth IF
 signal by means of the eighth frequency converter 3b, and an outputted
 from the eight frequency converter 3b is digitally demodulated by means of
 the digital demodulation section 2b, whereby the STM-1 data having been
 subjected to a baseband processing treatment are transmitted to another
 synchronous multiplex repeater or a transmission-end apparatus.
 Similarly, by way of the individual radio receiving sections 10b,the
 multiplex radio transceiver 21b disposed so as to oppose the multiplex
 radio transceiver 21a receives the radio signals which are transmitted
 from the multiplex radio transceiver 21a after having been digitally
 modulated by individual radio transmission sections 10a and which have
 frequencies differing from the transmission frequencies of the multiplex
 radio transceiver 21a in the identical band. The thus-received radio
 signals are digitally demodulated and retransmitted to another synchronous
 multiplex repeater and a transmission-end apparatus. Concurrently,
 multiplexed trunk signal data (STM-1) transmitted from another synchronous
 multiplex repeater or a transmission-end apparatus are subjected to a
 baseband processing treatment. The signal data are then digitally
 demodulated by means of the individual radio transmission sections 10a,
 through use of a plurality of channels having different frequencies. The
 thus-modulated signals are transmitted to the radio channel at C-band. As
 is the case with the multiplex radio transceiver 21a, the multiplex radio
 transceiver 21 comprises the plurality of radio transmission sections 10a,
 the plurality of radio receiving sections 10b, the antenna 8a, the antenna
 diplexer 9a, and the baseband processing section (not shown).
 The individual radio receiving sections 10b of the multiplex radio
 transceiver 21b are identical to the individual radio receiving sections
 10b of the multiplex radio transceiver 21a. More specifically, by way of
 the antenna 8b, the individual radio receiving sections 10b receive radio
 signals transmitted from the individual radio transmission sections 10a of
 the opposing multiplex radio transceiver 21. The thus-received signals are
 passed through the third band-pass filter 6b by way of the antenna
 diplexer 9b and the low-noise amplifier 7b, thereby an image signal from
 the signals. The thus-removed image signal is down-converted into the
 third IF signal through use of the third frequency converter 5b, and the
 down-converted signal is further passed through the fourth band-pass
 filter 4b. An output from the fourth band-pass filter 4b is down-converted
 again through use of the fourth frequency converter 3b. The thus-converted
 signal is digitally demodulated by means of the digital demodulation
 section 2b and is subjected to a baseband processing treatment, thereby
 producing STM-1 data. The STM-1 data having been subjected to a baseband
 processing treatment are transmitted to another synchronous multiplex
 repeater or a transmission-end apparatus.
 Further, the individual radio transmitting sections 10a of the multiplex
 radio transceiver 21b are identical to the individual radio transmitting
 sections 10a of the multiplex radio transceiver 21a. More specifically,
 the multiplexed trunk signal data (STM-1) transmitted from another
 synchronous multiplex repeater or a transmission-end apparatus are
 subjected to a baseband processing treatment. The signal data are then
 digitally modulated by means of the digital modulation section 2a, through
 use of a plurality of channels having different frequencies. A fifth IF
 signal outputted from the digital modulation section 2a is up-converted to
 a sixth IF signal by means of a fifth frequency converter 3a, and the
 thus-converted signal is filtered out by means of a fifth band-pass filter
 4a. The signal is further up-converted to a radio frequency (RF) signal by
 means of a sixth frequency converter 5a. An outputted from the sixth
 frequency converter 5a is transmitted to the radio channel at C-band from
 the antenna 8b by way of the sixth band-pass filter 6a, the high power
 amplifier 7a, and the antenna diplexer 9b.
 The transmission frequency employed by the individual radio transmission
 sections 10a of the multiplex radio transceiver 21 is different from that
 employed by the individual radio transmission sections 10b of the
 multiplex radio transceiver 21b. As in the case of the first embodiment,
 the frequencies are selected so as not to affect radio signals of existing
 sub-bands within C-band.
 The radio diplexer 9a and the antenna diplexer 9b transmit a radio
 frequency (RF) signal outputted from the respective radio transmission
 sections 10a to the individual radio receiving section 10b without leakage
 and separate the received radio frequency (RF) signal so that the
 individual radio receiving sections 10b can receive the radio frequency
 signals without leakage.
 The antenna 8a is connected to the respective radio transmission sections
 10a by way of the antenna diplexer 9a. The radio frequency (RF)--which is
 outputted from each radio transmission section 10a and is digitally
 modulated--is sent to the radio channel 29 from the antenna 8a. The
 antenna 8a is also connected to each radio receiving section 10b, thus
 receiving the radio frequency (RF) signal.
 In short, the antenna 8a serves as a transmission antenna shared among the
 radio transmission sections 10a and as a receiving antenna shared among
 the radio receiving sections 10b. Further, the antenna 8b is identical
 with the antenna 8a.
 In FIG. 16, members which are assigned the same reference numerals as those
 used in the first embodiment are identical with or identical in function
 with those described in the first embodiment, and hence their further
 descriptions will be omitted here.
 In the present embodiment, there is employed a digital
 modulation/demodulation scheme in which a baseband signal is digitally
 modulated and demodulated. Accordingly, multiplexed trunk signal data
 (STM-1) can be bidirectionally sent and received.
 The multiplex radio transceiver 21a acquires multiplexed trunk signal data
 (STM-1) and digitally modulates the signal data through a baseband
 processing treatment, by means of each radio transmission section 10a. The
 digitally modulated signal is up-converted to the second IF signal by
 means of the first frequency converter. The thus-converted signal is
 further up-converted to a radio frequency (RF) signal by means of the
 second frequency converter. The radio frequency signal is then
 transmitted. The radio receiving section 10b of the multiplex radio
 transceiver 21b receives the thus-transmitted signal. The signal is
 down-converted to the third IF signal by means of the third frequency
 converter, and the third IF signal is further down-converted to the fourth
 IF signal by means of the fourth frequency converter. The fourth IF signal
 is digitally demodulated and is subjected to a baseband processing
 treatment. The signal is then transmitted to another synchronous multiplex
 repeater or a transmission-end apparatus.
 The multiplex radio transceiver 21b acquires multiplexed trunk signal data
 (STM-1) and digitally modulates the signal data through a baseband
 processing treatment by means of each radio transmission section 10a. The
 digitally modulated signal is up-converted to the sixth IF signal, by
 means of the fifth frequency converter. The thus-converted signal is
 further up-converted to a radio frequency (RF) signal by means of the
 sixth frequency converter. The radio frequency signal is then transmitted.
 The radio receiving section 10b of the multiplex radio transceiver 21a
 receives the thus-transmitted signal. The signal is down-converted to the
 seventh IF signal by means of the seventh frequency converter, and the
 seventh IF signal is further down-converted to the eighth IF signal by
 means of the eighth frequency converter. The eighth IF signal is digitally
 demodulated and subjected to a baseband processing treatment. Multiplexed
 trunk signal data (STM-1) are extracted from the thus-processed signal and
 transmitted to another synchronous multiplex repeater or a
 transmission-end apparatus.
 The bidirectional transmission of data mentioned above enables significant
 improvement in the efficiency of transmission of multiplexed trunk signal
 data.
 Without regard to the frequency of a primary modulation output, an
 arbitrary second or third intermediate frequency can be set.
 Further, so long as the second or third intermediate frequency is set to
 the value selected in the first embodiment, harmonics of the intermediate
 frequency F1, i.e., 4.multidot.F1, 5.multidot.F1, 6.multidot.F1, and
 7.multidot.F1, do not intercept existing sub-bands within C-band.
 Further, so long as the second and third intermediate frequencies are set
 to the value selected in the first embodiment, three bands, i.e., the
 transmission radio frequency (RF) signal band, the local frequency channel
 band, and the image frequency band, are separated from one another without
 overlap. Accordingly, in practice, the radio frequency (Rf) signal can be
 prevented from colliding with the image frequency.
 As a result, the two multiplex radio transceivers which are disposed so as
 to oppose each other require only one type of transmission band-pass
 filter having a pass-band for permitting passage of a plurality of radio
 frequency (RF) signals, thus eliminating a need to filter out a
 transmission signal for each radio frequency (RF) channel.
 Further, the two multiplex radio transceivers require only one type of
 receiving band-pass filter having a pass-band for permitting passage of a
 plurality of radio frequency (RF) signals, thus eliminating a need to
 filter out a received signal for each radio frequency (RF) channel.
 In short, band-pass filters having different filtering characteristics have
 been required for individual radio transmission sections disposed within a
 multiplex radio transmitter, for individual receiving sections disposed
 within a multiplex radio receiver, and for repeaters. In contrast,
 according to the modification of the first embodiment, even if a multiplex
 communications system is allocated several channels, a band-pass filter
 having common characteristics can be used for the individual radio
 transmission sections disposed within another multiplex radio transmitter,
 for individual radio receiving sections disposed within another multiplex
 radio receiver, or for repeaters, thus promoting common use of members. In
 this connection, the cost of a multiplex radio transmitter can be reduced.
 (B) Description of a Second Embodiment
 In the first embodiment and its modification set forth, the transmission
 section of the radio repeater digitally modulates multiplexed trunk signal
 data (STM-1), and the digitally-modulated signal is up-converted in two
 steps and then sent to the radio propagation path 29. In contrast, the
 receiving section of the opposing radio repeater down-converts a received
 radio frequency (RF) signal in two steps. The thus-converted signal is
 digitally demodulated and is subjected to a baseband processing treatment,
 whereby multiplexed trunk signal data (STM-1) are obtained.
 In contrast, an analog modulation/demodulation scheme may be alternatively
 be used.
 FIG. 17 is a circuit diagram showing the configuration of a trunk multiplex
 radio transmission/receiving system using an analog modulation scheme
 according to a second embodiment of the present invention. A trunk
 multiplex radio transmission/receiving system 24 using an analog
 modulation scheme relays multiplexed trunk signal data (STM-1) between
 synchronous multiplex repeaters or between a synchronous multiplex
 repeater and a transmission-end apparatus. The trunk multiplex radio
 transmission/receiving system 24 comprises a multiplex radio transmitter
 51a which transmits a plurality of channel signals having different
 frequencies while converting them into a multiplexed signal; a multiplex
 radio receiver 51b which receives a plurality of multiplexed channel
 signals having different frequencies; and a radio propagation path 29.
 Here, the multiplex radio transmitter 51a modulates multiplexed trunk
 signal data (STM-1) received from another synchronous multiplex repeater
 or a transmission-end apparatus in an analog fashion and transmits the
 thus-modulated data to the radio propagation path 29. The multiplex radio
 transmitter 51a comprises a plurality of radio transmission sections 50a
 provided so as to correspond to respective channels; an antenna 48a; an
 antenna diplexer 49a; and a baseband processing section (not shown).
 Each radio transmission section 50a subjects the multiplexed trunk signal
 data (STM-1) to a baseband processing treatment and modulates the
 thus-processed data in an analog fashion. The transmission section 50a
 comprises an analog modulation section 42a, a first frequency converter
 43a, a first band-pass filter 44a, a second frequency converter 45a, a
 second band-pass filter 46a, and a high power amplifier 47a.
 The analog modulation section 42a modulates the STM-1 data, which have been
 subjected to a baseband processing treatment, in an analog fashion and
 sends the thus-modulated signal to the radio channel. As shown in FIG. 18,
 the analog modulation section 42a comprises a hybrid section 60a, a
 mapping section 61a, a roll-off filter 62a-1, a roll-off filter 62a-2, a
 mixer 64a-1, a mixer 64a-2, a local oscillator 65a, a 90-degree phase
 shifter 66a, and a hybrid section 67a.
 The hybrid section 60a converts the STM-1 data into parallel data through
 serial-to-parallel conversion.
 The mapping section 61a maps data bits of the I- and Q-channels into a
 constellation (or an arrangement of signal points) in groups of data
 bits-which are equal in number to a multivalued modulation number-at a
 modulation rate "w".
 The roll-off filter 62a-1 processes the I-channel data at a rate
 (4.multidot.W) which is four times the modulation rate "w", in order to
 prevent intersymbol interference from arising at the receiving end.
 Similarly, the roll-off filter 62a-2 processes the Q-channel signal at a
 rate (4.multidot.W) which is four times the modulation speed "w".
 The digital-to-analog converter 63a-1 converts a digital outputted from the
 roll-off filter 62a-1 into an analog value. Similarly, the
 digital-to-analog converter 63a-2 converts a digital outputted from the
 roll-off filter 62a-2 into an analog value.
 The mixer 64a-1 mixes an I-channel analog signal outputted from the
 digital-to-analog converter 63a-1 with an analog cosine signal outputted
 from the local oscillator 65a. Similarly, the mixer 64a-2 mixes a
 Q-channel analog signal outputted from the digital-to-analog converter
 63a-2 with an analog sine signal outputted from the 90-degree phase
 shifter 66a. The oscillation frequency of the local oscillator 65a is 70
 MHz, as specified by ITU-R. In contrast with digital modulation, analog
 modulation theoretically enables selection of an arbitrary value as the
 oscillation frequency.
 The hybrid section 67a merges the I-channel data outputted from the mixer
 64a-1 and the Q-channel data outputted from the mixer 64a-2 into one
 signal. FIG. 20(a) shows an output spectrum of the thus-merged signal
 which appears in the vicinity of the oscillation frequency f.sub.L.
 The first frequency converter 43a up-converts a first IF signal outputted
 from the analog modulation section 42a into a second IF signal which is
 higher in frequency than the first IF signal by only the frequency of a
 first local signal of the first frequency converter. In the first
 frequency converter 43a, a mixer 43a-1 mixes the first local signal
 outputted from a first local signal oscillator 43a-2 into the first IF
 signal outputted from the analog modulation section 42a.
 The first band-pass filter 44a eliminates a frequency component of the
 first local signal outputted from the first frequency converter 43a, thus
 permitting passage of only the second IF signal. FIG. 20(b) shows the
 band-pass characteristics of the first band-pass filter 44a. As shown in
 this drawing, the first band-pass filter 44a permits passage of only the
 frequency of 844 MHz, which is the frequency of the second IF signal, but
 prevents the passage of the first local signal whose frequency (818 MHz)
 is lower than that of the second IF signal by about 26 MHz.
 The second frequency converter 45a outputs a radio frequency (RF) signal by
 up-converting an outputted from the first band-pass filter 44a through use
 of a second local signal having eight kinds of frequencies. A mixer 45a-1
 mixes a second local signal outputted from a second local signal
 oscillator 45a-2 with the second IF signal outputted from the first
 band-pass filter 44a.
 For example, FIG. 21 shows allocation of a group of frequencies of the
 second local signal and radio frequencies (RF), through use of the U6G
 band as a sub-band and through use of an up-link. As shown in the drawing,
 the group of radio frequency (RF) signals and the group of frequencies of
 the local signal are not superimposed on one another but are separated in
 frequency from one another.
 Further, the second band-pass filter 46a has at least a band-pass width
 corresponding to radio frequency signals of a plurality of channels to be
 used and filters the radio frequency (RF) signal outputted from the second
 frequency converter 45a, thus eliminating a frequency component of the
 second local signal.
 The band-pass characteristics of the second band-pass filter 46a shown in
 FIG. 21 are such that the center frequency lies in the microwave band, and
 the second band-pass filter has a band-pass width of 340 MHz.
 Specifically, the pass-band width corresponds to the sum of the radio
 frequency (RF) group of 280 MHz and the bandwidth guard space of 30 MHz
 provided on either side thereof. As a result, individual channels of the
 group of radio frequency (RF) signals can be collectively passed through
 use of only one type of second band-pass filter having broad band-pass
 characteristics, without need for a second band-pass filter having
 band-pass characteristics for each radio frequency (RF) signal channel.
 The high power amplifier 47a amplifies a radio frequency (RF) signal
 outputted from the second band-pass filter 46a with high efficiency and
 low distortion.
 The antenna 48a is connected to each of the radio transmission sections 50a
 via the antenna diplexer 49a and sends the analog-modulated radio
 frequency (RF) signals outputted from the high power amplifier 47a.
 In contrast, the multiplex radio receiver 51b demodulates in an analog
 fashion the radio frequency (RF) signal, which has passed through the
 radio propagation path 29, and outputs STM-1 data after having subjected
 the demodulated signal to a baseband processing treatment. To this end,
 the receiver 51b shown in FIG. 17 comprises an antenna 48b, a plurality of
 radio receiving sections 50b provided so as to correspond to the
 individual channels, and an antenna diplexer 49b.
 The antenna 48b is connected to each of the radio receiving sections 50b
 via the antenna diplexer 49b and receives a radio frequency (RF) signal.
 More specifically, the antenna 48b is constituted as an antenna to be
 shared among the radio receiving sections 50b.
 Each radio receiving section 50b digitally demodulates the radio frequency
 (RF) signal received by the antenna 48b and outputs STM-1 data after
 having subjected the RF signal to a baseband processing treatment. The
 radio receiving section 50b comprises a low-noise amplifier 47b, a third
 band-pass filter 46b, a third frequency converter 45b, a fourth band-pass
 filter 44b, a fourth frequency converter 43b, and an analog demodulator
 42b.
 The low-noise amplifier 47b amplifies the radio frequency (RF) signal
 received by the antenna 48b with little noise.
 The third band-pass filter 46b has at least a band-pass width corresponding
 to radio frequency signals of a plurality of channels to be used. The
 third band-pass filter 46b eliminates a frequency component of the image
 signal from the radio frequency (RF) signal outputted from the low-noise
 amplifier 47b. The third band-pass filter 46b has band-pass
 characteristics such as those shown in FIG. 21. The third IF signal used
 in the receiver has a frequency of 844 MHz.
 The third frequency converter 45b down-converts the frequency of the radio
 frequency (RF) signal outputted from the third band-pass filter 46b and a
 third local signal outputted from the third local oscillator 45b-2,
 through use of the mixer 45b-1, thereby outputting a third IF signal whose
 frequency is lower than that of the radio frequency (RF) signal by the
 frequency of the third local signal.
 The fourth band-pass filter 44b eliminates an image signal from the
 outputted from the third frequency converter 45b, thus permitting passage
 of only the third IF signal. As shown in FIG. 20(b), the fourth band-pass
 filter 44b permit passage of only a frequency of 844 MHz, which
 corresponds to the third IF signal, but prevents passage of the first
 local signal (818 MHz), whose frequency is lower than that of the third IF
 signal by about 26 MHz.
 The fourth frequency converter 43b down-converts the third IF signal
 outputted from the fourth band-pass filter 44b into a fourth IF signal
 whose frequency is lower than that of the third IF signal. A mixer 43b-1
 mixes a fourth local signal outputted from a fourth local signal
 oscillator 43b-2 with the third IF signal outputted from the fourth filter
 44b.
 The digital demodulation section 42b demodulates in an analog fashion the
 outputted from the fourth frequency converter 43b and comprises, as shown
 in FIG. 19, a hybrid section 60b, a local oscillator 65b, a 90-degree
 phase shifter 66b, a mixer 64b-1, a mixer 64b-2, an analog-to-digital
 converter 63b-1, an analog-to-digital converter 63b-2, an integrator
 62b-1, an integrator 62b-2, and a reverse mapping section 61b.
 The hybrid section 60b separates an outputted from the fourth frequency
 converter section 43b.
 The local oscillator 65b is made up of an analog oscillator of 70 MHz, and
 the 90-degree phase shifter 66b shifts the phase of a signal outputted
 from the local oscillator 65b by 90 degrees.
 The mixer 64b-1 multiplies the I-channel data outputted from the hybrid
 section 60b by an outputted from the local oscillator 65b, thereby causing
 the I-channel data to fall within a baseband. Similarly, the mixer 64b-2
 multiplies the Q-channel data outputted from the hybrid section 60b by an
 outputted from the 90-degree phase shifter 66b, thus causing the Q-channel
 data to fall within the baseband.
 Through analog-to-digital conversion, the analog-to-digital converter 63b-1
 converts the fourth IF signal received from the mixer 64b-1. Similarly,
 through analog-to-digital conversion, the analog-to-digital converter
 63b-2 converts the fourth IF signal received from the mixer 64b-2.
 The integrator 62b-1 extracts the bit position of demodulated data
 regarding the modulation rate "v" from a bit string formed by sampling an
 outputted from the analog-to-digital converter 63b-1 at high speed.
 Similarly, the integrator 63b-2 extracts the bit position of demodulated
 data regarding the modulation rate "v" from a bit string formed by
 sampling an outputted from the analog-to-digital converter 63b-2 at high
 speed.
 The reverse mapping section 61b decodes the amplitude and phase of bits
 which have an amplitude and are outputted from the integrators 62b-1 and
 62b-2, thereby reproducing original data in groups of data bits which are
 equal in number to the multivalued modulation number.
 Full duplex communication is established between the two opposing repeaters
 over the radio propagation path 29 through use of a required number of
 channels. Of these channels, one type of channel is used for a backup
 purpose. A master repeater transmits a radio signal at, e.g., a downlink
 transmission frequency X which is one of seven types of channels. A slave
 repeater which opposes the master repeater receives the thus-transmitted
 signal having the transmission frequency X. Simultaneously, the slave
 repeater transmits a signal at an uplink transmission frequency X', and
 the master repeater which opposes the slave repeater receives the signal
 having the transmission frequency X'.
 Specifically, the transmitter and the receiver each select one channel from
 the eight types of channels, and they become symmetrical to each other in
 terms of band-pass characteristics.
 In this way, in a case where a frequency of 844 MHz is selected as the
 second and third IF frequencies, three bands, i.e., the radio frequency
 (RF) signal band, the local signal band, and the image frequency band, can
 be allocated as shown in FIG. 21 in such a way as not to overlap one
 another. As a result, the local signal band is separated from the group of
 radio frequency (RF) signals, and hence the transmitter requires only one
 type of band-pass filter capable of permitting collective passage of the
 plurality of radio frequency (RF) signals. There can be used common
 members in the radio transmission sections disposed within the multiplex
 radio transmitter or in repeaters. Therefore, there is no need for
 individual band-pass filters having different band-pass characteristics,
 which would otherwise be provided for individual radio transmission
 sections or repeaters.
 Similarly, the receiver does not require in its front-end stage filters
 having band-pass characteristics corresponding to the channels used for
 individual radio receiving sections or repeaters. The receiver requires
 only one type of band-pass filter, thus enabling use of common members for
 individual radio receiving sections in the multiplex radio receiver or for
 repeaters.
 As mentioned above, according to the second embodiment, after having
 performed the first frequency conversion through analog modulation, the
 transmitter converts, e.g., the second IF signal, into an optimum second
 IF signal of 844 MHz obtained in the first embodiment. Further, after
 having performed the fourth frequency conversion by converting the third
 IF signal into an optimum third IF signal of 844 MHz obtained in the first
 embodiment, the receiver demodulates the frequency-converted signal in an
 analog fashion, thus enabling setting of the second or third intermediate
 frequency to an arbitrary value. In practice, an image frequency is
 prevented from falling within the radio frequency (RF) signal.
 Harmonics of the second and third intermediate frequencies F1, i.e.,
 4.multidot.F1, 5.multidot.F1, 6.multidot.F1, and 7.multidot.F1, do not
 affect radio signals of existing sub-bands within C-band.
 Further, three bands, i.e., the transmission radio frequency (RF) signal
 band, the local frequency channel band, and the image frequency band, are
 separated from one another without overlap. Similarly, these three bands
 are separated from one another even in the receiver.
 As a result, the multiplex radio transmitter requires only one type of
 band-pass filter having a pass-band for permitting passage of a plurality
 of radio frequencies (RF), thus eliminating a need to filter a signal for
 each radio frequency (RF) channel as well as a need to prepare band-pass
 filters having different band-pass characteristics for respective
 transmission sections or for repeaters.
 Similarly, the multiplex radio receiver requires only one type of band-pass
 filter having a pass-band for permitting passage of a plurality of radio
 frequencies (RF), thus eliminating a need to filter a signal for each
 radio frequency (RF) channel as well as to prepare band-pass filters
 having different band-pass characteristics for respective receiving
 sections or for repeaters.
 In short, band-pass filters having different filtering characteristics have
 been required for individual radio transmission sections disposed within a
 multiplex radio transmitter, for individual receiving sections disposed
 within a multiplex radio receiver, and for repeaters. In contrast,
 according to the second embodiment, even if a multiplex communications
 system is allocated several channels, a band-pass filter having common
 specifications can be used for the individual radio transmission sections
 disposed within another multiplex radio transmitter, for individual radio
 receiving sections disposed within another multiplex radio receiver, or
 for repeaters, thus promoting common use of members. In this connection,
 the cost of a multiplex radio transmitter can be reduced.
 (B1) Modification of the Second Embodiment
 In the second embodiment, a repeater disposed at the transmission end is
 not equipped with a radio receiving unit, and a repeater disposed at the
 receiving end is not equipped with a radio transmission unit. However,
 each of these repeaters may be configured so as to have both a radio
 transmission unit and a radio receiving unit.
 In such a case, the transmitter and receiver each have a configuration such
 as that shown in FIG. 22, wherein a digital baseband modulation section
 shown in FIG. 16 is changed to an analog baseband modulation section. An
 analog type trunk multiplex radio transmission/receiving system 25, in
 which the transmitter and the receiver each have a radio transceiver,
 relays multiplexed trunk signal data (STM-1) between synchronous multiplex
 repeaters or between a synchronous multiplex repeater and a
 transmission-end apparatus. The trunk multiplex transmission/receiving
 system 25 comprises a multiplex radio transceiver 71a, a multiplex radio
 transceiver 71b, and a radio propagation path 29.
 Each of the multiplex radio transceivers 71a, 71b has the plurality of
 radio transmission sections 50a corresponding to a plurality of channels
 and the plurality of radio receiving sections 50b corresponding to the
 plurality of channels. Further, the trunk multiplex transmission/receiving
 system 25 comprises eight frequency converters.
 In the multiplex radio transceiver 71a, each radio transmission section 50a
 subjects to a baseband processing treatment multiplexed trunk signal data
 (STM-1) received from another synchronous multiplex repeater or a
 transmission-end apparatus and modulates the thus-processed signal in an
 analog fashion, through use of a plurality of channels having different
 frequencies. The thus-modulated signals are transmitted to a radio channel
 such as a C-band channel. Simultaneously, radio signals, which have been
 modulated in an analog fashion by individual radio transmission sections
 50a of the counterpart multiplex radio transceiver 71b through use of
 other frequencies within the same band, are received by radio receiving
 sections 50b of the multiplex radio transceiver. The thus-received signals
 are demodulated in an analog fashion and are subjected to a baseband
 processing treatment, whereby STM-1 data are resent to another synchronous
 repeater or transmission-end apparatus. The multiplex radio transceiver
 71a comprises the plurality of radio transmission sections 50a, the
 plurality of radio receiving sections 50b, the antenna 48a, and the
 antenna diplexer 49a.
 Each of the individual radio transmission sections 50a of the multiplex
 radio transceiver 71a receives multiplexed trunk signal data (STM-1) sent
 from another synchronous multiplex repeater or a transmission-end
 apparatus. The thus-received signal data are subjected to a baseband
 processing treatment and are modulated to an analog signal by means of the
 analog modulation section 42a. The first frequency converter 43a
 up-converts the first IF signal outputted from the analog modulation
 section 42a to a second IF signal which is filtered out by the first
 band-pass filter 44a. The thus-filtered signal is up-converted to a radio
 frequency (RF) signal by means of a second frequency converter 45a. The
 thus-converted signal is then sent to a radio channel from the antenna 48a
 by way of the second band-pass filter 46a, the high power amplifier 47a,
 and the antenna diplexer 49a.
 By way of the antenna 48a, the radio receiving sections 50b of the
 multiplex radio transceiver 71a receives the respective radio signals
 which have frequencies differing from the transmission frequencies of the
 multiplex radio transceiver 71a and which are transmitted at the identical
 band from the individual radio transmission sections 50a of the multiplex
 radio transceiver 71b disposed so as to oppose the multiplex radio
 transceiver 71a. The thus-received signals are passed through the seventh
 band-pass filter 46b by way of the antenna diplexer 49a and the low-noise
 amplifier 47b, thus filtering out image signals included in the signals.
 An outputted from the band-pass filter 46b is down-converted to the
 seventh IF signal, through use of the seventh frequency converter 45b. An
 outputted from the seventh frequency converter 45b is further
 down-converted to the eighth IF signal by means of the eighth frequency
 converter 44b, and an outputted from the eight frequency converter 43b is
 demodulated in an analog fashion by means of the analog demodulation
 section 42b. The thus-demodulated signal is subjected to a baseband
 processing treatment, whereby the STM-1 data are transmitted to another
 synchronous multiplex repeater or a transmission-end apparatus.
 Similarly, in the multiplex radio transceiver 71b disposed so as to oppose
 the multiplex radio transceiver 71a, each radio transmission section 50b
 receives a radio signal, which is modulated in an analog fashion and is
 sent by means of the radio transmission section 50a of the multiplex radio
 transceiver 71a, and demodulates the received signal in an analog fashion.
 The thus-demodulated signal is resent to another synchronous multiplex
 repeater or a transmission-end apparatus. Simultaneously, multiplexed
 trunk signal data (STM-1) transmitted from another synchronous multiplex
 repeater or a transmission-send apparatus are subjected to a baseband
 processing treatment, and the thus-processed data are demodulated in an
 analog fashion by means of the respective radio receiving sections 50a,
 through use of a plurality of channels having different frequencies. The
 thus-demodulated data are sent to a radio channel. The multiplex radio
 transceiver 71b comprises the plurality of radio transmission sections
 50a, the plurality of radio receiving sections 50b, an antenna 48b, and an
 antenna diplexer 49b in the same manner as does the multiplex radio
 transceiver 71a.
 By way of the antenna 48b, as in the case of the radio receiving section
 50b of the multiplex radio transceiver 71a, the radio receiving sections
 50b of the multiplex radio transceiver 71b receives the respective radio
 signals which have frequencies differing from the transmission frequencies
 of the multiplex radio transceiver 71b and which are transmitted from the
 individual radio transmission sections 50a of the multiplex radio
 transceiver 71a disposed so as to oppose the multiplex radio transceiver
 71b. The thus-received signals are passed through the third band-pass
 filter 46b by way of the antenna diplexer 49b and the low-noise amplifier
 47b, thus filtering out image signals included in the signals. An
 outputted from the band-pass filter 46b is down-converted to the third IF
 signal, through use of the third frequency converter 45b. An outputted
 from the third frequency converter 45b is passed through the fourth
 band-pass filter 44b, thus down-converting an outputted from the band-pass
 filter 44b to the fourth IF signal. An outputted from the fourth frequency
 converter 44b is demodulated in an analog fashion by means of the analog
 demodulation section 42b. The thus-demodulated signal is subjected to a
 baseband processing treatment, whereby the STM-1 data are transmitted to
 another synchronous multiplex repeater or a transmission-end apparatus.
 As in the case of the radio transmission section 50a of the multiplex radio
 transceiver 71b, each of the individual radio transmission sections 50a of
 the multiplex radio transceiver 71b receives multiplexed trunk signal data
 (STM-1) sent from another synchronous multiplex repeater or a
 transmission-end apparatus. The thus-received signal data are subjected to
 a baseband processing treatment and are modulated to an analog signal by
 means of the analog modulation section 42a. The fifth frequency converter
 43a up-converts the fifth IF signal outputted from the analog modulation
 section 42a to the sixth IF signal. The sixth IF signal is filtered by
 means of the fifth band-pass filter 44a and is up-converted to a radio
 frequency (RF) signal by means of the sixth frequency converter 45a. The
 thus-converted signal is then sent to a radio channel at C-band from the
 antenna 48b by way of the sixth band-pass filter 46a, the high power
 amplifier 47a, and the antenna diplexer 49b.
 In FIG. 22, the members assigned the reference numerals, which are the same
 as those used in the first embodiment, the modification of the first
 embodiment, and the second embodiment, designate analogous members or
 members having similar functions, and hence their further explanations
 will be omitted here.
 The second local signal used in the radio transmission section is different
 in frequency from the sixth local signal used in the radio transmission
 section of the opposing multiplex radio transceiver. As in the case of the
 first embodiment, the frequencies are selected so as not to intercept
 other sub-bands within C-band. The third local signal used in the radio
 receiving section is different in frequency from the seventh local signal
 used in the radio transmission section of the opposing multiplex radio
 transceiver. The frequencies are selected so as not to intercept other
 sub-bands within C-band, as in the case of the first embodiment.
 As mentioned above, according to the present modification, there is
 employed an analog modulation/demodulation scheme in which a baseband
 signal is modulated and demodulated in an analog fashion. Accordingly,
 multiplexed trunk signal data (STM-1) can be bidirectionally sent and
 received.
 The multiplex radio transceiver 71a acquires multiplexed trunk signal data
 (STM-1) and modulates the signal data in an analog fashion, through a
 baseband processing treatment, by means of each radio transmission section
 50a. The modulated signal is up-converted to the second IF signal by means
 of the first frequency converter. The thus-converted signal is further
 up-converted to a radio frequency (RF) signal by means of the second
 frequency converter. The radio frequency signal is transmitted. The radio
 receiving section 50b of the opposing multiplex radio transceiver 71b
 receives the thus-transmitted signal. The signal is down-converted to the
 third IF signal by means of the third frequency converter, and the third
 IF signal is further down-converted to the fourth IF signal by means of
 the fourth frequency converter. The fourth IF signal is demodulated in an
 analog fashion and is subjected to a baseband processing treatment. The
 signal is then transmitted to another synchronous multiplex repeater or a
 transmission-end apparatus.
 The multiplex radio transceiver 71b acquires multiplexed trunk signal data
 (STM-1) and modulates the signal data in an analog fashion through a
 baseband processing treatment by means of each radio transmission section
 50a. The modulated signal is up-converted to the sixth IF signal by means
 of the fifth frequency converter. The thus-converted signal is further
 up-converted to a radio frequency (RF) signal by means of the sixth
 frequency converter. The radio frequency signal is transmitted. The radio
 receiving section 50b of the multiplex radio transceiver 71a receives the
 thus-transmitted signal. The signal is down-converted to the seventh IF
 signal by means of the seventh frequency converter, and the seventh IF
 signal is further down-converted to the eighth IF signal by means of the
 eighth frequency converter. The eighth IF signal is demodulated in an
 analog fashion and is subjected to a baseband processing treatment.
 Multiplexed trunk signal data (STM-1) are extracted from the
 thus-processed signal and are transmitted to another synchronous multiplex
 repeater or a transmission-end apparatus.
 The bidirectional transmission of data mentioned above enables the
 efficiency of transmission of multiplexed trunk signal data to be
 significantly improved.
 An arbitrary second or third intermediate frequency can be set, and an
 image frequency does not fall within the group of radio frequency (RF)
 signals.
 Further, so long as the second or third intermediate frequency is set to
 the value selected in the first embodiment, harmonics of the intermediate
 frequency F1, i.e., 4.multidot.F1, 5.multidot.F1, 6.multidot.F1, and
 7.multidot.F1, do not intercept existing sub-bands within C-band.
 Further, so long as the second and third intermediate frequencies are set
 to the value selected in the first embodiment, three bands, i.e., the
 transmission radio frequency (RF) signal band, the local frequency channel
 band, and the image frequency band, are separated from one another without
 overlap.
 As a result, the two multiplex radio transceivers which are disposed so as
 to oppose each other require only one type of transmission band-pass
 filter having a pass-band for permitting passage of a plurality of radio
 frequency (RF) signals, thus eliminating a need to filter out a
 transmission signal for each radio frequency (RF) channel.
 Further, a receiving device of the radio transceiver requires only one type
 of receiving band-pass filter having a pass-band for permitting passage of
 a plurality of radio frequency (RF) signals, thus eliminating a need to
 filter out a received signal for each radio frequency (RF) channel.
 In short, band-pass filters having different filtering characteristics have
 been required for individual radio transmission sections disposed within a
 multiplex radio transmitter, for individual receiving sections disposed
 within a multiplex radio receiver, and for repeaters. In contrast,
 according to the modification of the second embodiment, even if a
 multiplex communications system is allocated several channels, a band-pass
 filter having common characteristics can be used for the individual radio
 transmission sections disposed within another multiplex radio transmitter,
 for individual radio receiving sections disposed within another multiplex
 radio receiver, or for repeaters, thus promoting common use of members. In
 this connection, the cost of a multiplex radio transmitter can be reduced.
 (C) Others
 In the first embodiment, the modification of the first embodiment, the
 second embodiment, and the modification of the second embodiment, the
 optimum second intermediate frequency is not limited to a frequency of 844
 MHz. It goes without saying that the frequencies defined in (20), (21) can
 also be used as the optimum second intermediate frequency. Other values
 which are referred to when the frequency is determined are not limited to
 the foregoing examples. Further, a band to be used is not limited to
 C-band.
 Needless to say, the present invention is susceptible of various
 modifications within the scope of the gist of the present invention.