Digital radio communication apparatus and method of controlling the same

A radio communication apparatus includes: a transmission unit for transmitting a radio wave; and a reception unit for receiving a radio wave and demodulating the same, the reception unit including: an analog filter provided in a first stage of the reception unit; a digital filter provided in a second stage of the reception unit so as to compensate a characteristic of the analog filter by having a filter characteristic of the digital filter varied by varying a tap factor; a test signal generation unit for supplying a test signal to the reception unit; an error state detection unit for detecting an error based on a digital demodulated signal derived from the test signal; and a tap factor setting unit for temporarily setting a tap factor of the digital filter that reduces a level of the error. Supplying of the test signal and detecting of the error are repeated until a target tap factor that minimizes the error is determined, whereupon the target tap factor is set in the digital filter.

BACKGROUND OF THE INVENTION
 Field of the Invention
 The present invention generally relates to digital radio communication
 apparatuses and, more particularly, to a digital mobile radio
 communication apparatus provided with an analog filter and a digital
 filter.
 Recently, with the depletion of radio wave resources foreseen, the
 communication standards stipulate increasingly tight restriction of the
 use of a channel band width. Conventionally, such a restriction has been
 met by improving hardware elements and circuit technology. More
 specifically, the performance of an analog filter is improved for that
 purpose. As the requirements stipulated by the communication standards
 become more strict, it is demanded that a software approach be introduced
 to implement a digital filter or to complement the performance of an
 analog filter.
 FIGS. 1-5 illustrate the technology used in a digital radio communication
 apparatus according to the related art.
 FIG. 1A shows a model of a digital radio transmission system. Referring to
 FIG. 1A, T.sub.b (.omega.) indicates a low-pass filter characteristic of a
 transmission unit, T.sub.r (.omega.) indicates a band-pass filter
 characteristic, F.sub.r (.omega.) indicates a transfer characteristic of a
 transmission path (air), R.sub.r (.omega.) indicates a band-pass filter
 characteristic of a receiver, and R.sub.b (.omega.) indicates a low-pass
 filter characteristic of the receiver. An overall transfer characteristic
 H(.omega.) is given by
 H(.omega.)=T.sub.b (.omega.)T.sub.rb (.omega.)F.sub.rb (.omega.)R.sub.rb
 (.omega.)R.sub.b (.omega.),
 where T.sub.rb (.omega.) indicates an equivalent low-pass filter
 characteristic of T.sub.r (.omega.), F.sub.rb (.omega.) indicates an
 equivalent low-pass filter characteristic of F.sub.r (.omega.) and
 R.sub.rb (.omega.) indicates an equivalent low-pass filter characteristic
 of R.sub.r (.omega.)
 When such a transmission system is to transmit a pulse signal G(.omega.)
 from a signal source, an input waveform for a discrimination circuit is
 given by
EQU r(t)=1/2.pi..intg..sub.-.infin..sup..infin.
 G(.omega.)H(.omega.)e.sup.j.omega.t d.omega. (1)
 FIG. 1B shows an eye pattern of an input waveform for the discrimination
 circuit. Assuming that the signal source transmits a .pi./4-shifted PSK
 modulated signal, there is no intersymbol interference occurring in the
 input waveform of the discrimination circuit if H(.omega.) satisfies the
 Nyquist condition. The eye aperture is open ((a) of FIG. 1B). However, if
 the Nyquist condition fails to be satisfied due to a variation of the
 performance of filter elements that has occurred in the process of
 fabrication, or due to a variation in the operating conditions
 (temperature, power-supply voltage, etc.), intersymbol interference occurs
 so that the eye aperture begins to close ((b) of FIG. 1B).
 FIG. 1C shows a constellation (arrangement of codes) that illustrates the
 above-described relation. Generally, code points on the transmitting side
 ((a) of FIG. 1C) vary (are displaced) in the air as shown in (b) of FIG.
 1C before arriving at the receiving side. If the combination of filters on
 the receiving side satisfies the Nyquist condition, the variation in the
 air settles to a state as shown in (c) of FIG. 1C at a discrimination
 point. That is, the intersymbol distance H at the discrimination point is
 relatively large. However, if there is a deviation in the filter
 characteristic on the reception side, intersymbol interference occurs so
 that it is impossible to properly restore the code points ((d) of FIG.
 1C). That is, the intersymbol distance H at the discrimination point is
 relatively small.
 FIG. 2 shows a relation between a cosine roll-off factor a and the
 constellation in the air. FIG. 2A shows the relation that occurs when
 .alpha.=0.8; FIG. 2B shows the relation that occurs when .alpha.=0.5; and
 FIG. 2C shows the relation that occurs when .alpha.=0.2. The smaller the
 factor .alpha., the smaller the occupied bandwidth so that the more
 preferable it is in terms of efficient use of the bandwidth. Accordingly,
 .alpha. tends to be controlled to maintain it at low level in current
 digital communication systems. However, the constellation in the air
 deviates from that of the point of origination as the level of .alpha. is
 lowered, requiring precise control of the receiver filter in order to
 restore the constellation.
 Conventionally, in order to construct a receiver with a strict requirement
 for selectivity between adjacent channels, a high-performance analog
 filter formed of crystal or ceramic is used.
 FIG. 3A shows a characteristic of attenuation of an analog filter with
 respect to frequency. Generally, in order to obtain a large attenuation, a
 plurality of analog filters are connected in multiple stages so as to
 produce a high performance (large attenuation). Such an approach causes
 the number of required elements to increase, and increases the size and
 cost of the resultant apparatus.
 FIG. 3B shows a group delay characteristic of an analog filter with respect
 to frequency. The delay time of a signal varies with respect to the
 frequency. Therefore, connecting a plurality of analog filters to form
 multiple stages in an attempt to obtain a high-attenuation characteristic
 causes degradation in the group delay characteristic.
 Further, a characteristic of analog elements is subject to a variation that
 occurs in the process of production. The characteristic also varies
 significantly with time and due to a variation in the operating conditions
 (temperature, power-supply voltage, etc.). Thus, it is difficult to
 implement and maintain the precise Nyquist characteristic.
 According to one approach, an analog filter designed to eliminate
 out-of-band noise is used in the first stage, several stages of the
 receiving system are linearized, and the majority of the filter
 performance (the Nyquist characteristic, the attenuation characteristic,
 etc.) is implemented (covered) by the digital filter in a subsequent
 stage.
 FIG. 4 shows a construction of a digital radio communication apparatus
 (portable terminal) according to the related art. The digital radio
 communication apparatus comprises an antenna 1; a transmission/reception
 branching switch 2 (C); a transmitter 3, a frequency synthesizer 4 (SYN),
 a receiver 5, including an RF amplifier (RFA) 6, a first mixer (x) 7, a
 second mixer 9 (x), analog band-pass filters (BPF) 8, 10, 12 formed of
 crystal or ceramic, IF amplifiers (IFA) 11, 13, a quadrature detecting
 unit (QDT) 14 using the QPSK system, an A/D converter (A/D) 15, adaptive
 transversal filters 16, 17 using a digital system, a discriminating
 circuit (DSC) 18, a clock generator (CG) 19, an automatic frequency
 controller (AFC) 20, a voltage controlled oscillator (VCO) 21, and an
 automatic gain controller (AGC) 25.
 CG 19 generates (reproduces) a sampling clock signal SK and a data clock
 signal DK based on the edges of demodulated I/Q signals. AFC 20 detects
 frequency deflection of the IF signal based on the edges of the
 demodulated I/Q signals. An output of AFC 20 is input to DSC 18 and used
 in control of a discriminated phase (phase rotation by .pi./4-shifted QPSK
 and the like). The output of AFC 20 is input to VCO 21 and used to
 maintain the frequency of the IF signal at a regular level.
 Further, the digital radio communication apparatus comprises a TDMA
 synchronization controller 31 for controlling timings according to the
 TDMA system; a codec (CODEC) 32 for converting a sound signal into codes;
 a baseband processor (BBP) 33 of the sound signal; a microphone (MIC) 34;
 a speaker (SPK) 35; a CPU 41 for performing main control (console control
 and call control including location registration, standby, call
 origination, call incoming, and handover) of the apparatus; a main memory
 (MM) 42 embodied by a RAM, a ROM and an EEPROM or the like for storing
 control programs executed by the CPU 42 and associated data; a console
 unit (CSL) 43 operated by a user, including a display unit 44 embodied by
 a liquid crystal or the like for displaying dial numbers and messages, and
 a keyboard (KBD) 45 provided with dial keys; and function keys, and a
 common bus 46 for the CPU 41.
 The CPU 41 controls incoming and outgoing calls via the TDMA
 synchronization controller 31. In a call state, in which a call can
 proceed with respect to a destination terminal, the sound signal from the
 MIC 34 is sampled by the BBP 33 and converted thereby into PCM data. The
 CODEC 32 converts the output of the BBP 33 into code data. The TDMA
 synchronization controller 31 formats the output of the CODEC 32 to
 produce transmitted data TD. The transmitter 3 modulates the transmitted
 data TD into a .pi./4-shifted QPSK signal for transmission via the antenna
 1.
 The wave received by the antenna 1 is amplified by the RFA 6 and converted
 by the mixers 7 and 9 so as to produce a first IF signal and a second IF
 signal, respectively. IFAs 11, 13 and AGC 25 amplify the IF signals to
 have a predetermined level. The IF signals are subject to quadrature
 detection by ODT 14 to produce quadrature detection signals I and Q. The
 detection signals I and Q are subject to A/D conversion by A/D 15. ATFs 16
 and 17 convert the signals I and Q into reproduced signals I and Q having
 minimum errors .epsilon..sub.i and .epsilon..sub.q, respectively, with
 respect to the code points. The reproduced signals I and Q are subject to
 discrimination by DSC 18 so as to produce received data RD. The received
 data RD is input to the TDMA synchronization controller 31 where code data
 of the sound is retrieved. The code data is converted into PCM data by the
 CODEC 32. The PCM data is converted into the sound signal and audibly
 output by SPK 35.
 FIG. 5 shows a construction of an adaptive transversal filter according to
 the related art. The adaptive transversal filter comprises an adaptive
 transversal filter (ATF) 16/17, including a tap factor operator 16A, and a
 FIR (finite impulse response) filter 16B, and consisting of a delay
 circuit (Z.sup.-1) 16a, a multiplier (x) 16b, and an adder (.SIGMA.) 16c;
 and a discrimination unit (DSC) 18, including a discrimination circuit 18a
 for code points, and an error detection unit 18b.
 An output y.sub.j of the FIR filter 16B is given by
 ##EQU1##
 where a tap (weight) factor vector A.sub.j =[a.sub.0j, a.sub.ij, . . . ,
 a.sub.Nj ].sup.T, and an input signal vector x.sub.j =[x.sub.j, x.sub.j-1,
 . . . , x.sub.j-N ].sup.T.
 The discrimination circuit 18a compares the output y.sub.j with a code
 point d.sub.j so as to produce reproduced data RD closest to the code
 point d.sub.j. The error detection unit 18b compares the output y.sub.j
 with the code point d.sub.j so as to produce an error signal
 .epsilon..sub.j =d.sub.j -y.sub.j (=d.sub.j -A.sub.j.sup.T x.sub.j). The
 tap factor operator 16A obtains an optimum tap factor vector A.sub.j+1
 =[a.sub.0j+1, a.sub.1j+1, . . . , a.sub.Nj+1 ].sup.T which causes the
 square of the error .epsilon..sub.j.sup.2 to have a minimum value.
 The optimum tap factor vector A.sub.j+1 is obtained at the next instant
 using the weight vector method of Wiener. However, this method requires
 complex, large-volume operations to be carried out so that real-time
 processing, by a DSP or the like, is impossible when the number of taps N
 is increased. Accordingly, the LMS (least mean square) method is generally
 used to obtain a step-by-step approximation of the optimum tap factor
 vector A.sub.j+1. The LMS method is also called the steepest descent
 method. The tap factor vector A.sub.j+1 for the next instant is given by
EQU A.sub.j+1 =A.sub.j -.mu..gradient..sub.j
 where .mu. indicates a parameter for controlling a convergence
 speed/stability, and .gradient..sub.j indicates an instantaneous gradient.
 The instantaneous gradient .gradient..sub.j is given by
 ##EQU2##
 Accordingly, the following relation holds.
EQU A.sub.j+1 =A.sub.j +2.mu..epsilon..sub.j X.sub.j
 where the parameter .mu. is appropriately set. When .epsilon..sub.j =0,
 A.sub.j+1 =A.sub.j indicates an optimum tap factor vector.
 A combination of the analog filter and the adaptive transversal filter as
 described above can be adapted for variations of the transmission path
 characteristic H(.omega.).
 However, if the adaptive transversal filter is used, it is necessary to
 obtain a next-instant tap factor vector A.sub.j+1 for each symbol
 received, thus imposing a heavy load on the tap factor operator 16A. While
 the number of taps N need to be large in order to obtain a
 high-attenuation characteristic using the digital filter, the processing
 speed of a DSP or the like presents a bottleneck.
 When the LMS method is used, the adaptive process starting with an initial
 vector A.sub.0 is such that, if the level of .mu. is low, the adaptive
 process proceeds with substantially no oscillation so that the optimum
 factor to produce the minimum value of .epsilon..sub.j.sup.2 is obtained
 smoothly. However, the convergence speed is low. If, on the other hand,
 .mu. is high, each of the adaptive steps goes too "far", causing an
 oscillation before arriving at the point that produces the minimum value
 of .epsilon..sub.j.sup.2. In this case, while the convergence speed is
 high, there is a likelihood that divergence may take place. That is, if
 the adaptive transversal filter is used, the receiving system might be
 instable.
 The adaptive transversal filter is designed to minimize an error power
 .epsilon..sub.j.sup.2 with respect to the code point. With the adaptive
 transversal filter, it is impossible to know which of the characteristics
 of the filter of the receiving system (roll-off characteristic,
 attenuation characteristic, group delay characteristic, phase
 characteristic, etc.) is improved. In other words, it is impossible to
 compensate and control a specific characteristic of the filter of the
 receiving system.
 SUMMARY OF THE INVENTION
 Accordingly, a general object of the present invention is to provide a
 digital radio communication apparatus and a method of controlling the same
 in which the aforementioned problems are eliminated.
 Another and more specific object of the present invention is to provide a
 digital radio communication apparatus and a method of controlling the same
 in which the characteristic of the analog filter is adaptively compensated
 by the digital filter, and in which the compensated characteristic is
 identified and selected.
 Still another object of the present invention is to provide a digital radio
 communication apparatus in which it is possible to control a selected
 characteristic.
 The aforementioned objects can be achieved by a digital radio communication
 apparatus comprising: a transmission unit for transmitting a radio wave;
 and a reception unit for receiving a radio wave and demodulating the same,
 the reception unit comprising: an analog filter provided in a first stage
 of the reception unit; a digital filter provided in a second stage of the
 reception unit so as to compensate a characteristic of the analog filter
 by having a filter characteristic of the digital filter varied by varying
 a tap factor; an RF signal terminal for inputting and outputting a test RF
 signal; a tap factor terminal for inputting and outputting a tap factor of
 the digital filter; and a digital demodulated signal terminal for
 inputting and outputting a digital demodulated signal produced by the
 reception unit.
 According to the controlling method of the present invention, by
 compensating the characteristic of the analog filter using the digital
 filter with the variable filter characteristic, a predetermined (regular)
 characteristic of the receiving system as a whole is obtained in the
 presence of a variation of the characteristic of the analog filter. By
 providing a characteristic controlling terminal (connector or the like),
 it is easy to produce a desired characteristic using an external
 controlling apparatus. Accordingly, the yield of the analog filter is
 improved. In addition to the benefit of ease of adjustment, the benefit of
 significant reduction in cost is provided.
 The aforementioned objects can also be achieved by a method of controlling
 a digital radio communication apparatus, comprising the steps of; a)
 supplying a test RF signal to an RF signal terminal of a reception unit;
 b) detecting an error occurring in a digital demodulated signal derived
 from the test RF signal; c) temporarily setting a tap factor of a digital
 filter that reduces a level of the error; d) repeating steps a)-c) so as
 to determine a target tap factor that minimizes the error, and setting the
 target tap factor in the digital filter.
 According to this aspect of the invention, it is relatively easy to set an
 optimum tap factor that produces a minimum level of error in the reception
 output.
 In further accordance with the invention, the error may be related to one
 of the following: a bit error rate of a digital reproduced signal;
 degradation in an eye pattern of a digital demodulated baseband signal;
 and deviation from code points of the digital demodulated baseband signal.
 According to this aspect of the invention, it is possible to set
 intersymbol interference to a minimum level by controlling a bit error
 rate, degradation in an eye pattern, or deviation from code points.
 The aforementioned objects can also be achieved by a digital radio
 communication apparatus comprising: a transmission unit for transmitting a
 radio wave; and a reception unit for receiving a radio wave and
 demodulating the same, the reception unit comprising: an analog filter
 provided in a first stage of the reception unit; a digital filter provided
 in a second stage of the reception unit so as to compensate a
 characteristic of the analog filter by having a filter characteristic of
 the digital filter varied by varying a tap factor; a test signal
 generation unit for supplying a test signal to the reception unit; an
 error state detection unit for detecting an error based on a digital
 demodulated signal derived from the test signal; and a tap factor setting
 unit for temporarily setting a tap factor of the digital filter that
 reduces a level of the error; wherein supplying of the test signal and
 detecting of the error are repeated until a target tap factor that
 minimizes the error is determined, whereupon the target tap factor is set
 in the digital filter.
 According to this aspect of the invention, by building facilities for
 controlling the reception unit characteristic in the apparatus, setting of
 an optimum tap factor can be performed not only in the process of
 fabrication but also while the apparatus is being used. Accordingly, the
 variation of the characteristic of the analog filter occurring in the
 fabrication process, the variation due to the operating conditions
 (temperature, power-supply voltage and the like), and the variation with
 time can be appropriately compensated. Thus, the characteristic of the
 reception unit can be maintained at optimum levels.
 The transmission unit may subject a data signal originated in the digital
 radio communication apparatus to digital modulation before transmission,
 and a test data signal generated by the test signal generation unit may be
 subjected to digital modulation via the transmission unit and supplied to
 an RF signal terminal of the reception unit.
 Accordingly, the transmission unit within the device can be efficiently
 used, and a similar transfer characteristic T( )=T.sub.b ( )T.sub.r ( ) of
 a transmission unit in a transfer system model can be simulated.
 The aforementioned objects can also be achieved by a digital radio
 communication apparatus comprising: a transmission unit for transmitting a
 radio wave; and a reception unit for receiving a radio wave and
 demodulating the same, the reception unit comprising: an analog filter
 provided in a first stage of the reception unit; a digital filter provided
 in a second stage of the reception unit so as to compensate a
 characteristic of the analog filter by having a filter characteristic of
 the digital filter varied by varying a tap factor; an error state
 detection unit for detecting an error based on a digital demodulated
 signal from the reception unit; and a tap factor setting unit for
 temporarily setting a tap factor of the digital filter that reduces a
 level of the error; wherein setting of the tap factor and detecting of the
 error are repeated until a target tap factor that minimizes the error is
 determined, whereupon the target tap factor is set in the digital filter.
 According to this aspect of the invention, the error state detection unit
 detects a predetermined error state based on a digital demodulated signal
 (signal received in communication) produced in the reception unit. Thus,
 the error state can be monitored on a continuous basis without generating
 a test signal. The reception state of the apparatus can be optimized
 according to the result of monitoring.
 The digital radio communication apparatus may further comprise a call
 controller for controlling incoming calls and outgoing calls, wherein said
 call controller provides facilities of one of the test signal generation
 unit and said error state detection unit.
 Generally, the call controller handles call control signals via the
 transmission unit and the reception unit and is provided with data
 transmission facilities and data reception facilities. By providing the
 call controller with test signal generating facilities so that the call
 controller can generate a test signal using an unoccupied time during
 communication, a test signal generation unit can be omitted. By causing
 the call controller to detect an error state (bit error rate) of the
 received data, the error state detection unit can be omitted.
 In further accordance with the invention, an algorithm for optimizing said
 digital filter by said tap factor setting unit may be based on a principle
 of perturbation.
 The perturbation principle operates such that a tap factor is temporarily
 set on a trial and error basis, the result of setting is evaluated so as
 to control the temporary setting to produce the best evaluation until the
 optimum tap factor is finally determined. Thus, the optimum tap factor is
 arrived at according to a simple process involving determination and
 control.
 In further accordance with the invention, the error may be related to one
 of the following: a bit error rate of a digital reproduced signal;
 degradation in an eye pattern of a digital demodulated baseband signal;
 and deviation from code points of the digital demodulated baseband signal.
 According to this aspect of the invention, it is possible to set
 intersymbol interference to a minimum level by controlling a bit error
 rate, degradation in an eye pattern, or deviation from code points.
 The tap factor of said digital filter may be to compensate one or a
 plurality of the following characteristics: a roll-off characteristic of
 said analog filter; an attenuation characteristic of said analog filter; a
 group delay characteristic of said analog filter; and a phase
 characteristic of said analog filter.
 According to this aspect of the invention, unlike the related art wherein
 the adaptive transversal filter is used, the characteristic that needs
 compensating can be selected. Also, it is relatively easy to determine
 whether the compensation takes effect.
 In further accordance with the invention, a plurality of tap factors may be
 stored in a memory.
 A specification, simulation or experiment may be used to determine a
 plurality of typical analog filter characteristics adapted for a variation
 of the characteristic of the analog filter. By retrieving typical
 characteristics, tap factors for compensating the same can be determined
 and stored in a memory. By storing a plurality of such tap factors and by
 sequentially reading out the tap factors in accordance with the adaptive
 control so as to temporarily set the tap factor, optimization control of
 the filter characteristic can be easily performed.
 In further accordance with the invention, a digital signal processor may
 implement facilities of said digital filter and facilities of the memory
 storing the plurality of tap factors.
 By using a DSP, various filter characteristics can be flexibly generated.
 For example, the number of effective taps may be increased or decreased
 appropriately. Since the tap factor of the digital filter of the invention
 is not changed at reception (that is, not changed every time a symbol is
 received), no restriction is imposed on the signal processing time even if
 the number of effective taps is increased.
 A program control of the digital signal processor may implement facilities
 of said error state detection unit and facilities of said tap factor
 setting unit.
 According to this aspect of the invention, the error state (bit error rate,
 eye pattern degradation, deviation from code points) can be detected by
 programmable control without a need for hardware expansion. Also,
 high-quality optimization control of the filter characteristic can be
 easily implemented.
 The aforementioned objects can also be achieved by a digital mobile radio
 communication apparatus for performing communication via a base station of
 a digital mobile communication system, said digital mobile radio
 communication apparatus comprising: a transmission unit for transmitting a
 radio wave; and a reception unit for receiving a radio wave and
 demodulating the same, said reception unit comprising: an analog filter
 provided in a first stage of said reception unit; a digital filter
 provided in a second stage of said reception unit so as to compensate a
 characteristic of said analog filter by having a filter characteristic of
 said digital filter varied by varying a tap factor; and a variable tap
 count control unit for variably controlling the number of effective taps
 of said digital filter depending on conditions that occur in
 communication.
 For example, when an adjacent channel (frequency channel) for a mobile
 station is not being used, the attenuation of the filter may be reduced
 (that is, the number of effective taps may be decreased) so as to reduce
 the operation load on the DSP and decrease the delay in the received and
 reproduced signal. When the adjacent channel is being used, the
 attenuation of the filter may be increased (that is, the number of
 effective taps may be increased) so as to remove jamming from the adjacent
 channel.
 In a type of use in which a response is returned between a base station and
 a mobile station in a short period of time, the number of effective taps
 of the FIR filters 22, 23 is decreased so that the delay in the received
 and reproduced signal is improved to enable quick responses to occur.
 In further accordance with the invention, the variable tap count control
 unit may variably control the number of effective taps of said digital
 filter in accordance with a control signal from the base station.
 According to this aspect of the invention, since the base station (network
 side) keeps track of the channel usage state within a service area, the
 variable tap number facilities of the mobile station can be smoothly
 operated. The base station can control the performance of the mobile
 station in the order of priority. For example, the control of the mobile
 station by the base station may be based on the order of processing speed.
 Alternatively, if the processing speed is not a concern, the control may
 be based on the order of jamming removal capability.
 The variable tap count control unit may temporarily reduce attenuation
 provided by said digital filter with respect to an adjacent channel so as
 to detect a current reception state, and update the number of effective
 taps of said digital filter depending on a result of the detection.
 Since the digital filter can easily modify the attenuation with respect to
 the adjacent channel, it is easy to determine whether the adjacent channel
 is being used. Detection of the reception state is preferably performed in
 an unoccupied time during communication (in the case of TDMA, in
 unoccupied time slots).
 The digital mobile radio communication apparatus may further comprise a
 monitoring control unit for monitoring a usage of a bandwidth for an
 adjacent channel, using unoccupied time in communication, wherein said
 variable tap count control unit may update the number of effective taps of
 said digital filter depending on a result of the monitoring by said
 monitoring control unit.
 The aforementioned objects can also be achieved by a digital mobile radio
 communication apparatus for performing communication via a base station of
 a digital mobile communication system, and for directly communicating with
 another digital mobile radio communication apparatus, in a location
 outside an area served by the base station, said digital mobile radio
 communication apparatus comprising: a transmission unit for subjecting
 data originating in said digital mobile radio communication apparatus to
 digital modulation; and a reception unit for receiving a
 digitally-modulated radio wave and demodulating the same, said
 transmission unit comprising a first digital filter having a filter
 characteristic variable depending on transmission data by varying a tap
 factor, and said reception unit comprising: an analog filter provided in a
 first stage of said reception unit; a second digital filter provided in a
 second stage of said reception unit so as to compensate a characteristic
 of said analog filter by having a filter characteristic of said second
 digital filter varied by varying a tap factor; wherein said digital mobile
 radio communication apparatus comprises a variable roll-off factor control
 unit for variably controlling a roll-off characteristic of said first and
 second digital filters depending on conditions occurring in communication.
 In such a digital mobile communication system, when mobile stations
 communicating with each other leave a service area of the base station,
 the mobile stations begin to communicate with each other with a
 proprietary frequency precision (which is lower than the precision
 provided by the base station). Deviation from the target transmission
 frequency may be such that other channels may receive jamming. According
 to the above aspect of the invention, the roll-off characteristic of the
 first and second digital filters is variably controlled so that jamming
 with respect to other channels is reduced and the communication between
 the mobile station that left the service area can proceed properly.
 The variable roll-off factor control unit may variably control the roll-off
 characteristic of said first and second digital filters in accordance with
 a control signal exchanged between two digital mobile radio communication
 apparatuses communicating with each other, so as to produce a desired
 matching state of the roll-off characteristic.
 By exchanging control signals between the mobile stations communicating
 with each other, jamming with respect to other channels is successfully
 prevented and the roll-off characteristic that occurs between the two
 mobile stations is maintained at a desired matching state.
 The digital mobile radio communication apparatus may further comprise a
 reception level detection unit for detecting a reception level of a
 digital demodulated wave, wherein said variable roll-off factor control
 unit may variably control the roll-off characteristic of said first and
 second digital filters depending on the reception level detected by said
 reception level detection unit.
 For example, when the reception level is relatively high, it means that the
 distance between the two stations is small. In this case, the roll-off
 factor a of the first and second digital filters is reduced so that the
 purpose of reducing the jamming with respect to other channels is best
 served. When the reception level is low, it means that the distance
 between the two stations is large. In this case, the roll-off factor a of
 the first and second digital filters is increased so that the purpose of
 maintaining communication between the two stations is best served.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In the drawings, those components that are identical to each other are
 designated by the same reference numerals.
 FIG. 6 shows a principle of the present invention. The digital radio
 communication apparatus according to the invention comprises a
 transmission unit, and a reception unit. The first stage of the reception
 unit includes an analog filter, and the second stage thereof includes a
 digital filter which, with a variable tap factor and a resultant variable
 filter characteristic, compensates the characteristic of the analog
 filter. The apparatus also comprises a terminal for a test RF signal TS, a
 terminal for a digital filter tap factor TC, and a terminal for a digital
 demodulated signal RS from the reception unit.
 FIG. 7 shows a construction of a digital radio communication apparatus
 (mobile terminal) according to a first embodiment of the present
 invention. In the apparatus of FIG. 6, the reception characteristic is
 variably controlled using an external control apparatus.
 Referring to FIG. 7, the apparatus comprises a transmission unit 3,
 including a code converter (DCV) 51 for converting transmitted data TD
 into code data along the quadrature I and Q axes, FIR filters 52, 53 (FIR)
 for implementing a transmission characteristic G(.omega.) shown in FIG.
 1A, a quadrature modulating unit (OMD) 54 operating according to the
 .pi./4-shift QPSK method, and a transmission amplifier (TXA) 55; a
 reception unit 5, including FIR filters (FIR) 22, 23; and an external
 control apparatus 90 for controlling the reception characteristic,
 including a tap factor generation unit (TCG) 91 for generating a tap
 factor for the FIR filters 22, 23, a test signal generation unit (TSG) 92,
 a band-pass filter (BPF) 93, and an error detection unit (ERD) 94 for
 detecting an error in the received signal RD. The other aspects of the
 construction remain the same as the corresponding aspects of FIG. 4.
 The FIR filters 22, 23 of the reception unit 5 have the same construction
 as the FIR filter 16B of FIG. 5. However, the tap factor vector TC is
 supplied by TCG 91 which is external to the apparatus and is latched in a
 non-volatile memory (not shown) within the apparatus. In the following
 description, BPFs 8, 10, 12 are inclusively referred to as analog filters.
 Automatic control (adjustment) of the reception characteristic of the
 apparatus is performed such that connection terminals (indicated by
 circles) of the reception unit 5 are connected to the external control
 apparatus 90 in a fabrication process and in a control operation.
 More specifically, TSG 92 is enabled (EN) by TCG 91 so as to generate a
 simulated RF transmission signal TS for testing the reception
 characteristic of the apparatus. The construction of TSG 92 may be the
 same as the transmission unit 3. However, TSG 92 has a built-in test
 signal generating unit (not shown). The transmission power of the signal
 TS is controlled to be sufficiently low. Preferably, BPF 93 is provided to
 implement the transmission characteristic T(.omega.)=T.sub.b
 (.omega.)T.sub.r (.omega.) of FIG. 1A. The transfer characteristic F.sub.r
 (.omega.) in the air may also be implemented.
 The simulated RF transmission signal TS is input to RFA 6 of the reception
 unit 5. Received data RD corresponding to TS is reproduced and output from
 the reception unit 5. ERD 94 compares the received data RD with the
 transmission test data TP (=TS) so as to detect a bit error rate ER. TCG
 91 repeats transmission of the test data TP and detection of the
 associated bit error rate. For each step of the repetition, the tap factor
 vector TC is adaptively (in a direction of reducing the bit error rate E)
 updated (temporarily set) so as to generate an optimum tap factor vector
 TC that minimizes the bit error rate ER. The tap factor vector TC finally
 determined to be optimum is set in the FIR 22, 23.
 FIGS. 8A-8D show a principle for controlling the reception characteristic
 according to the first embodiment.
 FIG. 8A shows a desired cosine roll-off characteristic R.sub.1 (.omega.)
 for the reception unit 5, occurring when .alpha.=0, .alpha.=0.5, and
 .alpha.=1.0, where .alpha. indicates a roll-off factor. While
 implementation is practically impossible for .alpha.=0, implementation in
 the range .alpha.=0.2-0.8 is possible.
 For example, it is assumed that R.sub.1 (.omega.) for the desired roll-off
 factor of .alpha.=0.5 is implemented. Assuming an analog filter transfer
 characteristic A.sub.1 (.omega.) and a FIR filter transfer characteristic
 F.sub.1 (.omega.) in the receiving system, the relation R.sub.1
 (.omega.)=A.sub.1 (.omega.)F.sub.1 (.omega.) holds.
 However, the transfer characteristic A.sub.1 (.omega.) of the analog filter
 of the first stage has a variation (that is, the transfer characteristic
 may vary from one manufactured apparatus to another, and may also vary
 depending on the conditions). Therefore, some typical variations A.sub.11,
 A.sub.12, . . . may be selected. The transfer characteristic of the FIR
 filter that realizes desired R.sub.1 (.omega.) may be F.sub.11 (.omega.),
 F.sub.12, . . . that satisfy the relation R.sub.l (.omega.)=A.sub.11
 (.omega.)F.sub.11 (.omega.)=A.sub.12 (.omega.)F.sub.12 (.omega.) . . . .
 For other values of .alpha., a similar description may be applied.
 Thus, assuming that A.sub.11 (.omega.) is selected, F.sub.11 (.omega.) for
 compensating the same is known. The tap factor vector TC.sub.11 for
 realizing the known F.sub.11 (.omega.) is determined by the known Fourier
 series method or the like. The Fourier series method operates such that,
 assuming that the target transfer characteristic is F.sub.11 (.omega.) and
 the transfer characteristic to be implemented in the design is D.sub.11
 (.omega.), the tap factor vector TC.sub.11 of D.sub.11 (.omega.) that
 minimizes the error between the characteristics is obtained by using a
 Fourier series. The error .epsilon. and the ith tap factor a.sub.i are
 given by
EQU .epsilon.=.intg..sub.-.pi..sup..pi..vertline.D.sub.l1 (.omega.)-F.sub.l1
 (.omega.).vertline..sup.2 d.omega.
EQU a.sub.i =1/2.pi..intg..sub.-.pi..sup..pi. F.sub.l1 (.omega.)e.sup.j.omega.i
 d.omega. (4)
 Actually, the apparatus is designed using a window function.
 FIG. 8B shows desired attenuation characteristics R.sub.g1
 (.omega.)-R.sub.g3 (.omega.) of the reception unit 5. Assuming that the
 desired attenuation characteristic is R.sub.g2 (.omega.), several typical
 transfer characteristics A.sub.g1 (.omega.), A.sub.g2 (.omega.), . . . may
 be selected in consideration of a variation of the analog filter transfer
 characteristic A.sub.g (.omega.). Accordingly, the FIR filter transfer
 characteristics that realize the desired attenuation characteristic
 R.sub.g2 (.omega.) may be F.sub.g1 (.omega.), F.sub.g2 (.omega.), . . .
 that satisfy R.sub.g2 (.omega.)=A.sub.g1 (.omega.)F.sub.g1
 (.omega.)=A.sub.g2 (.omega.)F.sub.g2 (.omega.). Thus, the tap factor
 vectors TC.sub.g1, TC.sub.g2, . . . corresponding to known F.sub.g1
 (.omega.), F.sub.g2 (.omega.), . . . can be determined. The attenuation
 characteristics A.sub.g1 (.omega.), A.sub.g2 (.omega.), . . . of the
 analog filter may not be flat as indicated in FIG. 8B.
 FIG. 8C shows a desired group delay characteristic R.sub.d1 (.omega.) of
 the reception unit 5. Assuming that the group delay characteristic of the
 analog filter is A.sub.d1 (.omega.), the group delay characteristic of the
 FIR filter for realizing the group delay characteristic is F.sub.d1
 (.omega.).
 FIG. 8D shows a desired linear phase characteristic Re.sub.1 (.omega.) of
 the reception unit 5. Assuming that the analog filter phase characteristic
 is A.theta..sub.1 (.omega.), the phase characteristic of the FIR filter to
 realize the linear phase characteristic R.theta..sub.1 (.omega.) is
 F.theta..sub.1 (.omega.).
 Generally, the desired correction characteristic F(.omega.) of the
 reception unit 5 is a synthesis of one or two of the characteristics
 F.sub.1 (.omega.)-F.theta.(.omega.). For example, given that the
 correction characteristic of the roll-off characteristic is F.sub.11
 (.omega.) and the correction characteristic of the attenuation
 characteristic is F.sub.g2 (.omega.), the synthesized characteristic is
 obtained such that F.sub.1g (.omega.)=F.sub.11 (.omega.)F.sub.g2
 (.omega.). Thus, the correction characteristic adapted for actual
 variations of the analog filter is obtained. The tap factor generating
 unit 91 adaptively (that is, according to the principle of perturbation)
 selects a desired characteristic from among known characteristics F.sub.1
 (.omega.)-F.theta.(.omega.), F.sub.1g (.omega.) in accordance with the bit
 error rate ER so as to obtain the tap factor vector TC that realizes the
 selected characteristic by computation. Alternatively, the tap factor
 generating unit 91 may read out the tap factor vector TC from a ROM or the
 like.
 The tap factor vector TC obtained by computation or stored in the ROM or
 the like may be tap factor vectors TC.sub.11, TC.sub.12 that directly
 correspond to F.sub.11 (.omega.), F.sub.12 (.omega.). Alternatively, it
 may be a transitional tap factor vector (difference between vectors) for
 transition from F.sub.11 (.omega.) to F.sub.12 (.omega.).
 FIG. 9 is a flowchart for controlling the reception characteristic
 according to the first embodiment.
 When the control apparatus 90 is started, the flow as shown in FIG. 9 is
 started. In S1, an error flag ERF and a bit error rate descent detection
 flag DWNF indicating a decrease in the bit error rate ER are reset. In S2,
 a default tap factor vector TC (for example, .alpha.=0.5) is set in the
 FIR filters 22, 23. In S3, a burst of the test signal TS is transmitted.
 In S4, the bit error rate ER of the received data RD is detected and
 latched. In S5, the characteristic of the FIR filters 22, 23 is enhanced
 by a small increment (for example, .alpha.=0.6 is set). In S6, the test
 signal TS is transmitted again. In S7, the bit error rate ER of the
 received data RD is detected and latched. In S8, the previous bit error
 rate and the current bit error rate are compared with each other so as to
 determine whether the bit error rate ER has increased or decreased. In the
 case of a decrease (including the case where the rate remains unchanged),
 it is determined that the characteristic is improving. In S15, the bit
 error rate descent detection flag DWNF is set, and the flow returns to S5.
 If it is determined that the bit error rate ER has increased, it is
 determined that worsening of the characteristic takes place. In S9, the
 characteristic of the FIR filters 22, 23 is dropped by a small decrement
 (for example, .alpha.=0.5 is set). In S10, the test signal TS is
 transmitted. In S11, the bit error rate ER for the received data RD is
 detected and latched. In S12, a determination is made as to whether the
 bit error rate has decreased. In the case of a decrease (including the
 case where the rate remain unchanged), it is determined that the
 characteristic is improving. In S16, DWNF is set, thus returning to S9.
 If it is determined in S12 that the bit error rate is increasing, a
 determination is made in S13 as to whether DWNF=1. If DWNF=1, it is
 determined that worsening of the characteristic takes place after an
 improvement, so that, in S14, the characteristic of the FIR filters 22, 23
 is returned to the immediately preceding setting, thus terminating the
 process. If DWNF=0, it is determined that worsening of the characteristic
 takes place without an improvement being detected previously. Thus, in
 S17, the error flag ERF is set and the process is terminated. In this
 case, the initial conditions may be changed, for example, so that the
 above process is repeated.
 While the description above relates to the control of the roll-off
 characteristic F.sub.1 (.omega.), similar descriptions may also be applied
 to the control of the attenuation characteristic F.sub.g (.omega.), the
 group delay characteristic F.sub.d (.omega.), the phase characteristic
 F.theta.(.omega.) and the synthesized characteristic F.sub.1g (.omega.).
 The adaptive control as described above is only one example. Variations may
 also be made. For example, a first point where a small-decrement drop (or
 a small-increment enhancement) of the characteristics of the FIR filter
 22, 23 produces worsening of the characteristic may be determined.
 Subsequently, a second point where a small-increment enhancement (or a
 small-decrement drop) of the characteristics of the FIR filter 22, 23
 produces worsening of the characteristic may be determined. The
 characteristic that occurs at the midpoint may be selected as the final
 setting.
 According to the first embodiment, the deviation of the characteristic of
 the analog filter is adaptively compensated. The first embodiment provides
 ease of a controlling procedure and improvement in the yield. Additional
 benefits are that the size of the analog filter is reduced, the cost
 thereof is reduced, and the performance of the apparatus is improved
 thanks to optimization of the filter characteristic as a whole.
 Unlike the related art which uses an adaptive transversal filter, the
 invention provides selective compensation and control of one or two of the
 characteristics because various characteristics (roll-off characteristic,
 attenuation characteristic and the like) constructing the reception
 characteristic R(.omega.) are clearly differentiated from each other.
 Further, the first embodiment provides a more stable operation of the FIR
 filters.
 FIG. 10 shows a construction of a digital radio communication apparatus
 according to a second embodiment of the present invention. The external
 control apparatus 90 as described above is built into the digital radio
 communication apparatus.
 Referring to FIG. 10, the digital radio communication apparatus comprises
 an RF switch (RFS) 26, an attenuator (ATT) 27, a parameter memory (PM) 28
 embodied by a non-volatile memory such as a ROM or an EEPROM, and an
 address register (ADR) 29.
 RFS 26 switches an output from the TXA 55 to either the
 transmission/reception branching switch 2 or the attenuator 27. ATT 27
 attenuates the output level of the TXA 55 to produce a received wave of
 the RFA 6. By operating the TXA 55 directly so as to reduce the output
 level thereof, ATT 27 can be omitted.
 PM 28 stores plural predetermined sets of tap factor vectors TC for
 implementing the cosine roll-off characteristic F.sub.1 (.omega.), the
 attenuation characteristic F.sub.g (.omega.), the group delay
 characteristic F.sub.d (.omega.), the phase characteristic
 F.theta.(.omega.) and the synthesized characteristic F.sub.1g (.omega.),
 which characteristics are set in the FIR filters 22, 23 in order to
 compensate the characteristic of the analog filter. Each of the tap factor
 vectors TC is selected and read out according to the contents of ADR 29.
 CPU 41 uses unoccupied time in communication to switch RFS 26 to the
 attenuator 27 side. CPU 41 also transmits a test signal via the TDMA
 synchronization controller 31 and the transmission unit 3. The test signal
 is not output to the antenna 1 but is input to RFA 6 via RFS 26 and ATT
 27. The reception unit 5 demodulates the test signal so as to produce the
 received data RD. The received data RD is input to CPU 41 via the TDMA
 synchronization controller 31. By comparing the received data RD with the
 known test data TP, the bit error rate ER is determined. CPU 41 repeats
 transmission of the test signal and determination of the bit error rate
 ER. For each step of the repetition, the read address of PM 28 is
 adaptively changed so as to obtain, in a final stage, an optimum tap
 factor vector TC that minimizes the bit error rate ER. The optimum tap
 factor vector TC obtained in the final stage is set in the FIR filters 22,
 23. Thereafter, RFS 26 is switched to the antenna 1 side. With this,
 communication is enabled.
 By having the built-in control facilities, the digital radio communication
 apparatus according to the second embodiment can not only compensate a
 variation that occurs in the process of fabrication but can also adapt for
 various communization hazards during an operation. The feature of the
 second embodiment may also be useful for an operation not prompted by
 detection of communication hazards. For example, a that produces
 R(.omega.) may be controlled to be small or the attenuation characteristic
 may be enhanced as a precaution to prevent jamming from an adjacent
 channel. The characteristics may be modified in a variety of ways in
 accordance with the communication conditions.
 FIG. 11 shows how an error rate is determined according to a variation of
 the second embodiment. For example, the bit error rate ER may be
 determined on the basis of the data received in communication.
 More specifically, FIG. 11 shows a frame format of a transmitted and
 received frame in the TDMA (PDC) system. One upstream frame (20 ms)
 comprises three channels (time slots T1-T3). One superframe (720 ms)
 comprises 36 frames. The signal for one channel comprises guard bits R, G
 for burst transmission, a preamble P, communication data TCH, a
 synchronization word SW, a still flag SF indicating a content of TCH, a
 known color code CC which varies depending on frequency, and arbitrary
 control data SACCH. The numerals below the reference symbols indicate
 number of bits.
 The bit error rate ER of variable data like the communication data TCH
 cannot be determined without a specific error checking system being
 introduced between a transmitter and a receiver. However, the preamble P,
 the synchronization word SW and the color code CC are known on the
 reception side as well as on the transmission side. Therefore, this
 information can be used to determine the bit error rate ER.
 CPU 41 retrieves known bit information from the received data RD received
 in a wait state or in communication, so as to determine the bit error rate
 ER. When the determined bit error rate ER exceeds a predetermined
 threshold level, CPU 41 changes the characteristic of the FIR filters 22,
 23 appropriately.
 FIG. 12 shows a construction of a digital radio communication apparatus
 according to a third embodiment of the present invention. The facilities
 of the FIR filters 22, 23, and the facilities for setting an optimum tap
 factor vector for the FIR filters 22, 23 are implemented by program means.
 The facilities of the FIR filters 22, 23 are implemented by an operation
 described by the equation (2). The facilities for setting an optimum tap
 factor vector for the FIR filters 22, 23 are implemented by a process
 similar to the process of FIG. 9. According to the third embodiment, the
 facilities are split so that DSP 47 and CPU 41 perform respective
 facilities. For example, DSP 47 controls PM 28 so as to read tap factor
 vectors TC therefrom. CPU 41 is provided with facilities for transmitting
 a test signal, determining the bit error rate ER based on the received
 data RD, and adaptively controlling the FIR filters 22, 23 based on the
 bit error rate ER. Alternatively, as described with reference to FIG. 11,
 CPU 41 may be provided with facilities for determining the bit error rate
 ER based on the received data RD received in communication, and adaptively
 controlling the FIR filters 22, 23 based on the bit error rate ER.
 In further accordance with the third embodiment, DSP 47 implements the
 operations of the FIR filters 22, 23. Distortion in the eye can be
 detected based on the output signals I, Q of the FIR filters 22, 23. Since
 the distortion of the eye is directly related to degradation in the
 reception, the FIR filters 22, 23 can also be adaptively controlled based
 on the distortion.
 FIGS. 13A and 13B show a method of detecting degradation in the reception
 according to a variation of the third embodiment. In the illustrated
 method, degradation in the reception (error state) is detected based on
 the eye distortion.
 FIG. 13A shows how the eye distortion is detected based on an aperture
 ratio 1A. Referring to FIG. 13A, IRF indicates a reference level which
 differentiates between upper quadrants and lower quadrants. (1)-(6)
 indicate sampling data occurring at the discrimination points. R2
 indicates a register for detecting a minimum value of the input data I in
 the upper quadrants. R3 indicates a register for detecting a maximum value
 in the lower quadrants. The sampling data (1)-(6) may occur in any
 sequence. While FIG. 13A shows an eye pattern resulting from a variation
 of amplitude, an eye pattern resulting from a variation of phase also
 appears as a variation of amplitude.
 At the outset of the detection period, R2 is preset to a maximum level of
 the upper quadrants, and R3 is preset to a minimum level of the lower
 quadrants. The subsequent input data I is compared with the reference
 level I.sub.RF. When the data is in the upper quadrants, R2 is updated so
 as to latch the data lower in level than the previous data. When the data
 is in the lower quadrants, R3 is updated so as to latch the data higher in
 level than the previous data. Accordingly, after a predetermined period of
 time has elapsed, R2 latches the minimum value (3) in the upper quadrants,
 and R3 latches the maximum value (4) in the lower quadrants. The eye
 aperture ratio 1A is obtained such that IA=.vertline.(3)-(4).vertline..
 The same description also applies to the Q axis.
 FIG. 13B shows measurement of the eye distortion based on a variation
 (variance .delta.I.sup.1, .delta.I.sub.2) of the eye amplitude. Referring
 to FIG. 13B, R1 indicates a register for detecting a maximum value of the
 input data I in the upper quadrants, and R4 indicates a register for
 detecting a minimum value in the lower quadrants. The operations of R2 and
 R3 are similar to the operations of R1 and R4, respectively. A description
 will now be given of the operations of R1 and R4.
 At the outset of the detection period, R1 is preset to a minimum level of
 the upper quadrants, and R4 is preset to a maximum level of the lower
 quadrants. The subsequent input data I is compared with the reference
 level I.sub.RF. When the data is in the upper quadrants, R1 is updated so
 as to latch the data higher in level than the previous data. When the data
 is in the lower quadrants, R4 is updated so as to latch the data lower in
 level than the previous data. Accordingly, after a predetermined period of
 time has elapsed, R1 latches the maximum value (1) in the upper quadrants,
 and R4 latches the minimum value (6) in the lower quadrants. The eye
 variation .delta.I.sub.1 in the upper quadrants is obtained such that
 .delta.I.sub.1 =.vertline.(1)-(3).vertline., and the eye variation
 .delta.I.sub.2 in the lower quadrants is obtained such that .delta.I.sub.2
 =.vertline.(4)-(6).vertline.. An average of .delta.I.sub.1 and
 .delta.I.sub.2 is obtained and is designated as a variation in the eye
 amplitude. The same description also applies to the Q axis.
 While the arrangement is used to obtain the magnitude of the maximum
 variation of the eye, statistical variance of the eye may be obtained
 instead. Detection of the eye distortion may be conducted only with
 respect to the I axis and the Q axis. Both the eye aperture ratio and the
 eye variation may be used to determine the eye distortion. The
 above-described facilities for detecting the eye distortion can be
 implemented by hardware of a maximum value detection circuit and a minimum
 value detection circuit. The constructions of FIGS. 7 and 10 can detect
 the eye distortion as an error state.
 Detection of the reception error state may be performed by measuring a
 variance .epsilon. from code points of the digital demodulated signals I
 and Q. The tap factor vector that minimizes .epsilon..sup.2 is similar to
 the optimum tap factor vector in the adaptive transversal filter. The
 optimum tap factor vectors for a variety of analog filters are obtained so
 that they are adapted to FIR filters 22, 23 according to the adaptive
 control as described with reference to FIG. 9.
 FIG. 14 shows a digital radio communication system according to a fourth
 embodiment of the present invention, wherein the number of effective taps
 of the reception filter is varied depending on the local conditions
 occurring during communication. Referring to FIG. 14, the digital radio
 communication system comprises a base station (BS) 70, and a mobile
 station (MS) 60 having the construction as shown in FIG. 12. The number of
 effective taps of the FIR filters 22, 23 implemented by DSP 47 is
 configured to be variable.
 For example, BS 70 transmits a control signal for controlling the reception
 attenuation characteristic depending on the conditions surrounding MS 60
 during communication. CPU 41 receiving the control signal variably
 controls the number of effective taps of the FIR filters 22, 23.
 More specifically, when an adjacent channel (frequency channel) to that for
 MS 60 is not being used, BS70 reduces the attenuation of the filter (that
 is, decreases the number of effective taps) so as to reduce the operation
 load on the DSP and decrease the delay in the received and reproduced
 signal. When the adjacent channel is being used, the attenuation of the
 filter is increased (that is, the number of effective taps is increased)
 so as to remove jamming from the adjacent channel.
 In a type of communication in which a response is returned to be from MS 60
 to BS 70 in a short period of time, the number of effective taps of the
 FIR filters 22, 23 is decreased so that the delay in the received and
 reproduced signal is improved to enable quick responses to occur.
 The base station can control the performance of the mobile station in the
 order of priority. For example, the control of the mobile station by the
 base station may be based on the order of processing speed. Alternatively,
 if the processing speed is not a concern, the control may be based on the
 order of jamming removal capability.
 CPU 41 uses local time slots or unoccupied time (unoccupied time slots)
 occurring in communication so as to temporarily reduce the attenuation
 provided by the FIR filters 22, 23 with respect to the adjacent channel,
 and to detect a current reception error state. When the adjacent channel
 (frequency channel) is not being used (causing no reception error), CPU 41
 decreases the attenuation provided by the FIR filters 22, 23 so as to
 decrease the load on DSP 47 and reduce the delay in the received and
 reproduced signal RD. When the adjacent channel is being used (causing a
 reception error), CPU 41 increases the attenuation provided by the FIR
 filters 22, 23 so as to prevent jamming from the adjacent channels.
 In this case, BPF 8, 10, 12 have a relatively large bandwidth. Further, BPF
 12 may be implemented by an FIR filter 12 so that the attenuation with
 respect to the adjacent channels is temporarily reduced.
 CPU 41 uses unoccupied time occurring in communication so as to monitor the
 usage of the adjacent channel (frequency channel). When the adjacent
 channel is not being used (no carrier is detected or received data cannot
 be reproduced), CPU 41 decreases the attenuation provided by the FIR
 filters 22, 23 so as to decrease the load on DSP 47 and reduce the delay
 in the received and reproduced signal RD. When the adjacent channel is
 being used (a carrier is detected, or received data can be reproduced),
 CPU 41 increases the attenuation provided by the FIR filters 22, 23 so as
 to prevent jamming from the adjacent channels.
 FIG. 15 shows a digital radio communication system according to a fifth
 embodiment of the present invention, wherein the roll-off factor and the
 number of effective taps of the transmission filter and the reception
 filter are configured to be variable.
 FIR filters 52, 53 in the transmission unit 3 are implemented by DSP 48.
 The roll-off factor and the number of effective taps are configured to be
 variable. The FIR filters 22, 23 of the reception unit 5 have the same
 construction.
 In some types of digital mobile communication systems, when mobile stations
 MSA, MSB communicating with each other leave a service area of BS 70, MSA,
 MSB begin to communicate with each other with a proprietary frequency
 precision (which is lower than the precision provided by the base
 station). Deviation from the target transmission frequency of MSA, MSB may
 be such that other channels may receive jamming. According to the fifth
 embodiment, MSA, MSB exchange a control signal for variably controlling
 the roll-off characteristics (and, in addition to that, the attenuation
 characteristics, if necessary) of the FIR filters 52, 53, 22, 23, so as to
 attain a desired matching state (having an overlap with the
 transmission/reception bandwidth). In this way, jamming with respect to
 other channels is reduced and the communication between the mobile
 stations can proceed properly.
 FIG. 16 shows a construction of a digital radio communication system
 according to a sixth embodiment of the present invention, wherein the
 roll-off factor and the number of effective taps of the transmission
 filter and the reception filter are variably controlled depending on the
 reception level (RSSI).
 Referring to FIG. 16, the system comprises an IF amplifier (IFA) 36, a
 reception level detecting unit (RSSID) 37, and an A/D converter (A/D) 38.
 For example, it is assumed that MSA, MSB communicate with each other
 outside an area served by BS 70. When the reception level is high, it
 means that the distance between the two stations is small. In this case,
 the roll-off factor .alpha. of the FIR filters 52, 53, 22, 23 is decreased
 (if necessary, the attenuation characteristic may be enhanced) so as to
 prevent jamming with respect to other channels. When the reception level
 is low, it means that the two stations are removed from each other by a
 great distance. In this case, the roll-off factor .alpha. is increased (if
 necessary, the attenuation is reduced) so that the two stations can
 continue to communicate properly.
 While it is assumed in the foregoing embodiments that an FIR filter is used
 as a digital filter, the present invention is equally applicable to an
 apparatus in which an IIR (infinite impulse response) filter is used. The
 transfer function of an IIR filter (standard format according to z
 conversion) is given by
 ##EQU3##
 Moreover, while the foregoing embodiments are assumed to be applied to a
 portable terminal operated on the TDMA system, the present invention is
 equally applicable to a radio communication apparatus operated on the CDMA
 system.
 Application of the present invention is not limited to portable terminals
 but may be extended to various types of digital radio communication
 apparatuses (a base station, a radio repeater station, an earth station
 and a substation).
 The present invention is not limited to the above-described embodiments,
 and variations and modifications may be made without departing from the
 scope of the present invention.