United States Patent [19J
`Honma
`
`1111111111111111111111111111111111111111111111111111111IIIIIIi ml 11111 II
`USOO5557646A
`[IIJ Patent Number:
`[45J Date of Patent:
`
`5,557,646
`Sep. 17, 1996
`
`[54J MULTIPATH ELIMINATING FILTER
`
`[75J
`
`lnvcntor: Souichi Honma, Yokohama, Japan
`
`[73J Assignee: Kabushiki Kaisha Kenwood, Tokyo,
`Japan
`
`(21) Appl. No.: 451,502
`
`[22J Filed:
`
`May 26, 1995
`
`Foreign Application Priority Data
`
`[30J
`Jun. 4, 1994
`[IPI
`Japan .................................... 6-145615
`[51)
`Int. Cl.(; ............................... H03D 1104; H048 7/10;
`H04B lIto
`(52] U.s. CI • .......................... 3751J46; 3751347; 3751350;
`375m2; 4551307
`[581 Field of Search ..................................... 375/350,346.
`375/230, 231,232,233; 364n24.l9, 724.2;
`455152.3. 63, 296, 307, 340, 266, 42
`
`[56}
`
`Refen!oces Cited
`
`U.S. PATENT DOCUMENTS
`
`Primary Emmjl'l~,-Stephen Chin
`Assi,rtant Examiner-Huong Luu
`Attorney, Agel'll. or Finn-Sixbey, Friedman, Leedom &
`Ferguson, P.e.; Gerald J. Ferguson, Jr.; Eric J. Robinson
`
`[57J
`
`ABSTRAcr
`
`First. an adaptive filter 15 provided in a renective wlIve
`detection unit 13 performs an adaptive operation on a digital
`signal inputted to an input tenninal 10 for eliminaling a
`multipalh component from the digital signal. Also, a filler
`update unit 21 updates filter coefficient for a digital filter 18
`comprising an FIR fi lter of lower degree. After the elapse of
`a predetermined time, reflective wave estimation unit 16
`estimates reflection coefficient and delay time of a reflected
`wave contained in the digital signal from a filter coefficient
`at that point of lime, and outputs the estimated values to a
`controller 14. Based on the input estimated values, the
`control!er 14 initializes a delay time of a delay clement of a
`digital filter 23 comprising an IIR filter of first degree of a
`multipalh eliminating adaptive filler 11 and a multiplication
`coefficient of a multiplication element of the same. Then, the
`controller 14 causes the multipath eliminating adaptive filter
`11 10 performance adaptive filter for eliminating a multipath
`component.
`
`811993 Baum et aI .
`5,233,633
`............................ 3751232
`5,268,930 1211993 Sendyk ct al ........................... 375n33
`
`6 Claims, 15 Drawing Sheets
`
`INPUT
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`[6
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`RTL898_1025-0001
`
`Realtek 898 Ex. 1025
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`

`
`u.s. Patent
`
`Sep. 17, 1996
`
`Sheet 1 of 15
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`5,557,646
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`FIG. I
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`RTL898_1025-0002
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`

`
`U.S. Patent
`
`Sep. 17, 1996
`
`Sheet 2 of 15
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`5,557,646
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`u.s. Patent
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`Sep. 17, 1996
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`Sheet 3 of 15
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`u.s. Patent
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`Sep. 17, 1996
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`Sheet 4 of 15
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`5,557,646
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`RTL898_1025-0005
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`

`
`U.S. Patent
`
`Sep. 17, 1996
`
`Sheet 5 of 15
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`5,557,646
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`ELIMINATING
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`PUT
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`12
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`ESTIMATION UNIT
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`DETECTION UNIT
`
`I~
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`CONTROLLER
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`FIG. 6
`
`RTL898_1025-0006
`
`

`
`U.S. Patent
`
`Sep. 17, 1996
`
`Sheet 6 of 15
`
`5,557,646
`
`IIA
`
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`MULTIPATH ELIMINATING
`22 ADAPTIVE FILTER
`23
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`RTL898_1025-0007
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`RTL898_1025-0008
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`RTL898_1025-0009
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`RTL898_1025-0013
`
`

`
`U.S. Patent
`
`Sep. 17, 1996
`
`Sheel13 of 15
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`5,557,646
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`RTL898_1025-0014
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`

`
`U.S. Patent
`
`Sep. 17, 1996
`
`Sheet 14 of 15
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`5,557,646
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`RTL898_1025-0015
`
`

`
`u.s. Patent
`
`Sep. 17, 1996
`
`Sheet 15 of 15
`
`5,557,646
`
`FIR FILTER
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`RTL898_1025-0016
`
`

`
`1
`MULTIPATH ELIMINATING FlLTER
`BACKGROUND OF THE INVEl\'TION
`
`2
`determined by a steepest gradient of F. Hence, update filter
`coefficient c'" as follows, for use at next time (n+l),
`
`5,557,646
`
`where a is a fixed convergence parameter.
`In the example of AO, 17, an operator 2 squares an
`absolute value ofY ... and a subtracter 3 subtracts a reference
`amplitude value of 1 from the absolute value squared to
`obtain f", A filter update unit 4 performs an expected value
`10 arithmetic and an update calculation on a fi lter coefficient.
`Thus, an updated filter coefficient is sct for thc FIR filter 1.
`FIG, 18 shows another example of a multipath eliminat(cid:173)
`ing adaptive filter, The filter of FIG. 18 comprises an f1R
`filter having a nonzero filter coefficient only at points
`
`I' corresponding to integer multiples of a delay time of a
`
`reflected wave. Furthennore. a mulliplier for level adjust(cid:173)
`ment is provided on the subsequent stage side of the FIR
`filler for normalizing an amplitude of a direct wave to 1. It
`is described in literature 3 below as an improved version of
`20 an invention disclosed in literature 2.
`Literature 2: Japanese Palent Application Laid-open No,
`14052711987
`Literature 3: Japanese Patent Application Laid-open No.
`6262811991
`When one reflected wave is involved and when a reflec(cid:173)
`tion coefficient at nonnalization by a direct wave is taken as
`r and a delay time of the reflected wave as t, transfer function
`HMP of multipath is represented by
`
`1. Field of Ihe Invention
`The present invention relates to a multipath eliminating S
`filter, and particularly to a muitipath eliminating filler which
`eliminates a multi path component arising at reception of an
`FM modulated signal or a phase modulaled signal using an
`adaptive filter having variable filter characteristics.
`2 Description of Prior Art
`When an FM modulated signal or a phase modulated
`signal is to be received and demodulated, it is known that
`multipath transmission (multi path), where an undesirable
`rellcctcd wave component cause{! by a building, mountain or
`the like is superimposed on a direct wave component, takes
`place, causing a deterioration in quality of reception such as
`an increase in distonion of a demodulated signal and the
`like. For a stationary receiver, this problem can be coped
`with by sharpening directivity of an antenna for tuning in to
`a direct wave. This measure, however, is not applicable to a
`mobile receiver. Thus, for a mobile receiver, it is proposed
`that an adaptive filter is used as a method of eliminating
`multipath distortion. This method utilizes a property thai. an
`amplitude (envelope) of an FM modulated signal is fixed. A 25
`digital filter is inserted at an intermediate frequency stage
`preceding an amplifier limiter, thereby varying filter char(cid:173)
`acteristics so thaI an output amplitude of the digital filter is
`fixed .
`FIG. 17 shows an example of a conventional multipath 30
`eliminating adaptive filter. The filler ofAO. 17 comprises an
`FIR filter having sufficient degree as described in literature
`I below, for example.
`Literature 1: J. R. Treichler, B. G. Agee: "A New
`Approach to Multipalh Correction of constant Modulus 33
`Signals", TEEEE Trans. vol. ASSP· 3I , No. 2, pp 4.5~71
`(1983)
`In FIG. 17, a digital signal of an NO converted intcnne(cid:173)
`diate frequency signal is inpulled to input tcnninallN. With
`a value at time n of the input digital signal taken as x"' 40
`degree of an FlR (Finite Impulse Response) filter 1 as N,
`coefficient of the FJR filter 1 as c", (lc::O to N), and a value
`at time n of an output digital signailo be outputted 10 output
`tcnninal OlIT as Y", Y" is expressed by
`
`Transfer function HEQ to be realized by the multipath
`eliminating adaptive filler is an inverse function of H.wl' as
`represented by
`
`11(1 + r(")
`
`l - rr +,lrl' - ,..1r :l< +
`
`.,. + (-I"h'''+ ., .
`
`Thus,. HHQ is realized at an F1R filter 5 by selecting an
`appropnate L.
`In the example of FIG. 18, after squaring an absolute
`value of output y .. from the FIR filter 5 at an operator 6, the
`absolute value squared is multiplied by a variable gain
`coefficient g at a level adjusting multiplier 7 for nonnaJizing
`4j an amplitude of a direct wave 10 L
`In lhis case, evaluating fu nction F in an adaptive algo-(cid:173)
`rithm is expressed by
`
`Its matrix representation is
`
`so
`
`where CT=[Co, C" c3 •. •• , C;,]' xT=[x", x .... " x"_:.t •.. , ,
`x ......... l. and the superscrip( .. " represents a transposed
`matrix,
`With a reference amplitude value taken as I, error E" is
`expressed by
`
`A subtracter 8 subtracts 1 from an output of the multiplier
`7 to obtain error E~ and outputs it to a tiller update unit 9.
`Updalc expressions for updating r, t, and g by a method
`" of steepest gradient are
`
`In an adaptive
`expressed by
`
`algorithm, evaluating function F is 60
`
`where E [*) indicates an expected value arithmetic.
`Removal of multipath distortion is synonymous with
`minimization of F. Filter coefficient c~ for minimizing F is
`
`RTL898_1025-0017
`
`where aI- a) arc fixed convergence parameters.
`The filter update unit 9 perfonns an expected value
`arithmetic and an update calculation on r, t, and g. Thus,
`M updated filler ebaracteristics r and t arc set for the FIR fi lter
`5, and an updated gain coefficient g is set for the multiplier
`7.
`
`

`
`5,557,646
`
`3
`If a unit delay time (here, a sampling period for a digital
`signal) u of a delay clement of the FIR fi lter 5 is fixed and
`is far smaller than t, fi lter coefficient ck is obtained from r
`and t by the following expressions:
`
`,
`
`The obtained Cot is set for the FIR filter 5 as an update fil ter
`coefficient. P9vlluJ (v is a variable assuming an integer 10
`greater than 0), and a maximum integer not exceeding vtlu.
`
`4
`calculates filler characteristics and an amount of level
`adjustment for minimizing an error detected by the error
`detection means and which updales fi lter cbaracteristics for
`the digital filter and an amount of level adjustment for the
`level adjuslment means; and reflective wave estimation
`means for estimating characteristics of a reflected wave from
`filter characteristics of the digital filter of the adaptive filter
`at a point of time when an error in the adaptive filter
`becomcs small LO a certain extent.
`A further aspect of the prescnt invention is characterized
`by comprising: level detection means fordettcting a level of
`the digital Signal to be inputted to said mul tipath eliminating
`adaptive filter; and level adjustment amount initialization
`means for initializing, when an adaptive operation of the
`multipath eliminating adaptive filter is to be started, the level
`adjustment means of the multipath eliminating adaptive
`filter to an amount of level adjustment inversely proponional
`to a level of a digital signal detected by the kvcJ detection
`means.
`Still another aspect of the prescnt invention is character(cid:173)
`ized by comprising: level detection means for detecting a
`level of the digital signaiLO be inputted to the reflective wave
`detection means; and level adjustment amount initialization
`means for initializing, when an adaptive operation of the
`adaptive filter of the reflective wave detection means is to be
`2S started, the level adjustment means of the adaptive filter to
`an amount of level adjustment inversely proportional to a
`level of a digital signal detected by the level detection
`means.
`A still further aspect of the present invention is charac.-
`terized by comprising connection selector means for cascade
`connccting the adaptive filter of the reflective wave detec(cid:173)
`tion means to the multipath eliminating adaptive filter at a
`subsequent stage thereto after an adaptive operation of the
`multi path eliminating adaptive filter is started.
`A still funher aspect of the present invention is charac(cid:173)
`terized in that the digital filter of the multipath eliminating
`adaptive filter comprises an IIR (Infinite Impulsc Response)
`filter of first dcgree in single stage or llR filters of first
`degree which arc cascade connected in more than one stage.
`According to an aspe<:t of the prescnt invention, charac(cid:173)
`teristics of a reflected wave are detected from an input digilai
`signal or an output digital signal of a multipath eliminating
`adaptive filter. A digital fil ler of the multipath eliminating
`45 adaptive filter is initialized to initial filter characteristics
`corresponding to the detected characteristics of a renected
`wave before starting an adaptive operation of the mUltipath
`eliminating adaptive filter. Thus, it is possible for the mul(cid:173)
`tipath eliminating adaptive fil ter to stan a filter arithmetic
`so under an appropriate initial state suited 10 characteristics of
`a reflected wavc. Also, an amplitude of a digital signal
`output is converged quickly and securely to a predetermined
`value, thereby obtaining a n':oXpLioli outpul with a multi path
`component eliminated therefrom.
`According to another aspect of the present invention, an
`input digital signal or an output digital signal from the
`multipath eliminating adaptive filter is led to an adaptive
`filter for estimating characteristics of a reflected wave from
`filter characteristics of a digital filter of the adaptive fi lter at
`60 a point of time when an error between an output amplitude
`of the adaptive filter and a reference amplitude value
`becomes small to a certain extent. Since the adaptive filter
`used for estimating characteristics of a reflectcd wave is not
`intended to eliminate multipath distortion, but is intended 10
`6S estimate characteristics of a reflected wave, it adapts itself to
`a smaller circuit scale and can be implemented in a simple
`consuuetion.
`
`"
`
`l'
`
`A multipath eliminating filter shown in FIG. 17 is char(cid:173)
`acterized in that if convergence parameter a is sufficiently
`small, convergence is possible irrespective of an initial value
`of a filter coefficient. However, it had a problem that
`normally required degree N is very large, 128-256, causing
`a larger circuit scale and a longer time required for con vcr- 20
`gcnce.
`On the other hand, a multipath eliminating filter shown in
`FIG. 18 has a possibility of reducing a circuit scale and of
`quickening the convergence of filter characteristics. How·
`ever, it had a problem that a failure to select proper initial
`values may cause a fai lure in convergence due to a mutual
`influence of three parameters r, t, and g.
`In view of the above-mentioned prior art, it is an object of
`the present invention to attain secure convergence in a shon 30
`period of time.
`An aspect of the prescnt invention is characterized by
`comprising: a multipath eliminating adaptive filter compris(cid:173)
`ing a digital fill.cr having variable filter characteristics to
`which an FM modulated or phase modulated digital signal
`containing a multipath component is inpuncd and which
`performs a filter arithmetic process on the input d igital
`signal to eliminate the multipath component therefrom, error
`detection means for obtaining an error between an amplitude
`of a digital signal outputted from the digital fi lter and a 40
`reference amplitude value, level adjustment means allowing
`an amount of adjustment to be varied for adjusting an
`amplitude level of a digital signal, and update means which
`calculates filter characteristics and an amount of level
`adjustment for minimizing an error detected by !he error
`detection means and which updates filter characteristics for
`the digital filter and an amount of level adjustment for the
`level adjustment means; reflective wavc detection means for
`detecting characteristics of a reflected wave from the input
`digital signal or an output digital signal of the multipath
`eliminating adaptive filter; and control means which initial·
`izes the digital filler of the multipath eliminating adaptive
`filter to initial filter characteristics corresponding to charac(cid:173)
`teristics of a reflected wave detected by the renective wave
`detcction means and which then starts an adaptive operation 55
`at the multipath eliminating adaptive fi ller.
`Another aspect of tile present invention is characterized in
`that the renective wave detection means comprises: an
`adaptive filter comprising a digital filter having variable
`filter characteristics which performs a fillCI" arithmetic pro(cid:173)
`cess on the input digital signal or the output digital signal of
`the multipath eliminating adaptive filter, error detection
`means for obtaining an error between an amplitude of II
`digitaJ signal outputled from the digital filter and a reference
`amplitude value, level adjustment means allowing an
`amount of adjustment to be varied for adjusting an ampli(cid:173)
`tude level of a digital signal, and update means which
`
`SUMMARY OF THE INVEI\"110N
`
`RTL898_1025-0018
`
`

`
`5,557,646
`
`6
`AG. 10 is a circuit diagram illustrating a modified lhird
`embodiment.
`FIG. 11 is a cireuit diagram illustrating another modified
`third embodiment.
`FIG. 12 is a circuit diagram illustrating a fuober modified
`third embodiment.
`FIG. 13 is a circuit diagram illustrating a multipath
`eliminating filter according to a fourth embodiment of the
`present invention.
`FIG. 14 is a circuit diagram illustrating a modified fourth
`embodiment.
`FIG. 15 is a grapb illustrating the result of experiment of
`the multipath eliminating filter of FIG. 14.
`AG. 16 is a graph illustrating the result of experiment of
`the multipath eliminating filter of FIG. 14.
`FIG. 17 is a circuit diagram illustrating a conventional
`multipath eliminating fillet.
`FIG. 18 is a circuit diagram illustrating another conven(cid:173)
`tional multipath eliminating fi lter.
`
`EMBODIMENTS
`
`s
`
`5
`According to a further aspect of the present invention, a
`level of a digital signal to be inputted to the muilipath
`eliminating adaptive filter is detected, and level adjustment
`means of the multipatb eliminating adaptive filter is to be
`initialized to a level adjustment amount inversely propor-
`tional 10 Ihe level of a digital signal when an adaptive
`operation of the multipath eliminating adaptive filter is
`started. Thus, an adaptive operation of the mwtipath elimi(cid:173)
`nating adaptive filter can be started at an appropriate initial
`amount of level adjusuncnt. Also, convergence can be 10
`aUained in a shoo period of time even when the difference
`between an amplitude of a direct wave and a reference
`amplilUdc is large.
`According to still another aspect of the present invention,
`a level of a digital signal to be inputted to reflective wave IS
`detection means is detected, and level adjusunc.ct means of
`an adaptive filter of the reflective wave detection means is
`initialized to a level adjustment amount inversely propor(cid:173)
`tional to the level of a digital signal when an adaptive
`operation of the adaptive filter of the reflective wave detec- 20
`tion means is 10 be staned. Thus, an adaptiveopcration of the
`adaptive filter can be stancd at an appropriate initial amount
`of level adjustment. Also, even when the difference between
`an amplitude of a direct wave and a reference amplitude is
`large, characteristics of a deflected wave can be detccted in '2S
`a shon period of time, and the multi path eliminating adap(cid:173)
`tive filter can be initialized accordingly before starting an
`adaptive operation_
`According to a still funher aspect of the present invention,
`the adaptive filter of the reflective wave detection means is 30
`cascade connected to the multipath eliminating adaptive
`filter at a subsequent stage thereto after an adaptive opera(cid:173)
`tion of the multipath eliminating adaptive filter is started.
`Thus, a remaining reflccted wave component which the
`multipath eliminating adaptive filter has failed to eliminate 35
`can be eliminated by the adaptive filter of the reflective wave
`detection means.
`According to a still further aspect of the present invention.
`the digital filter of the multipath eliminating adaptive filter
`comprises an UR filter of first degree in single stage or llR 40
`filters of first degree which art cascade conneclCd in more
`than one siage. Thus, a circuit scale ean be greatly reduced.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FlG. 1 is a circuit diagram showing a multipath eliminat(cid:173)
`ing fi lter according to a first embodiment of the present
`invention.
`Reference numeral 10 denotes an input tenninal to which
`a digital signal generalCd by AID converting an PM modu(cid:173)
`lated or phase modulated intermediate frequency signal
`containing a multipalh component is inputted; 11, a multi(cid:173)
`path eliminating adaptive filter which varies filter charac(cid:173)
`teristics thereof successively and adaptive\y using a propcny
`that an amplitude of an FM modulated or phase modulated
`wave is fixed and which eliminates a multipath component
`from the input digital signal before outputting the signal ; 12,
`an output terminal for outputting the digital signal with a
`multipath component eliminated therefrom to a subsequent
`stage; 13, a reflective wave detection unit whieh detects
`characteristics (reflection coefficient r indicative of a reflec(cid:173)
`tive wave level with reference to a direct wave and delay
`time t of a renectcd wave with referencc to a direct wave) of
`a reflccted wave component contained in a digital signal
`before the multipath eliminating adaptive filter is put in
`4S operation; and 14. a controller which controls the start of an
`adaptive operation of an adaptive filter, described later, and
`the multipath eliminating adaptive filter and which initial(cid:173)
`izes a digital fi lter of the multipatbeliminating adaptive filter
`to initial filter characteristics corresponding to eharacteris-
`tics of a reflected wave component.
`The reflective wave detection unit 13 comprises an adap(cid:173)
`tive filter 15 which varies filter characteristics thereof sue(cid:173)
`ccuively and atlarli vely using a property that an amplitude
`of an FM modulated or phase modulated wave is fixed and
`55 which performs an operation of eliminating a multipath
`component from the input digital signal, and a reflective
`wave estimation unit 16 to which filter characteristics at a
`point of time when the adaptive filter IS has carried out an
`adaptive operation for a predetermined time are inputted to
`60 estimate reflected wave characteristics (r, t).
`In the adaptive filter 15, reference numeral 17 denotes a
`multiplier allowing an adjustment amount (=gain coefficient
`g) for adjusting an amplitude level of the input digital signal
`to be varied; 18, a digital fi lter comprising an FIR filter
`6S having a variable filter coefficient; 19, an operator which
`squares an absolute value of an OUlput from the digital fi lter
`18; 20, a subtracter which subtracts a reference amplitude
`
`so
`
`AG. 1 is a cireuit diagram illustrating a multipath elimi(cid:173)
`nating filter according to a first embodiment of the present
`invention.
`FIG. 2 is a circuit diagram showing an PM tuner fitted
`with a multipath eliminating filter.
`FIG. 3 is a graph illustrating the result of experiment of
`tnc multipath diminating filter of FIG. 1.
`FIG. 4 i~ 1\ grnph illunrating the result of experiment of
`the multipath eliminating filter of FIG. 1.
`FIG. 5 is a graph illustrating the result of experiment of
`the multipath eliminating filler of FIG. 1.
`AG. 6 is a circuit diagram illustrating a modified first
`embodiment.
`FIG. 7 is a circuit diagram illustrating a mullipath elimi(cid:173)
`nating filter accord.i.ng to a second embodiment of the
`present invention.
`FIG. 8 is a circuit diagram specifically illustrating a level
`detection unit.
`FIG. 9 is a cireuit diagram illustrating a multipath elimi(cid:173)
`nating filtcr aCCOrding to a third embodiment of the present
`invention.
`
`RTL898_1025-0019
`
`

`
`5,557,646
`
`7
`value of I from an output from the operator 19 to obtaln an
`error; and 21. a filter update unit which obtains update
`values of gain coefficient and filtercocflicicnt from an output
`from the subtracter 20 and the like by a method of steepest
`gradient and which sets thus obtained update values for the S
`digital filter 18.
`Operations of the adaptive filter 15 and the reflective
`wave estimation unit 16 will now be described. With a value
`at time n of an input digital signal taken as x". a gain
`coefficient of the multiplier 17 as g, do;:grcc uf the digilal
`fil ter 18 as N, a coefficient of the digital filter 18 as ck (k=O
`lo N). and a value at time n of a digital signal outputted from
`the digital filter 18 as y". the following expression will bold.
`
`"
`
`20
`
`25
`
`where CT=[co• Ct .~, ... ,eN]' and XT=[x", Xn_1, X"_l • . . .
`, x"-Nl.
`Error En at a reference amplitude value of 1 is expressed
`by
`
`Evaluating
`expressed by
`
`function F in an adaptive algorithm is
`
`When a method of steepest gradient (LMS algorithm) is
`applied with an inslantancous value arithmetic substituted
`for an expected value arithmetic, update expressions for gain
`coefficient g and filter coefficient C are as follows:
`
`(I)
`
`(t)' 3S
`
`8
`The reflective wave estimation unit 16 takes in fi lter
`coefficient C which is set in the digital filter 18, from the
`filter update unit 21 after the elapse of a predetennined time
`after the adaptive filter 15 has started an adaptive operation.
`Also, the reflective wave estimation unit 16 selects one filter
`coefficient cm having a maximum absolute value except &0-
`The reflective wave estimation unit 16, then, estimates
`characteristics of a reflected wave taking an absolute value
`of em as reflection coefficient r and mu as delay time t, and
`10 outputs the estimation data to the controller 14.
`Since the digital filter 18 is not intended to eliminate
`multipath distortion, but is intended to estimate character(cid:173)
`istics of a reflected wave, degree N of 16-32 is sufficient. In
`comparison with the prior art shown in FIG. 17, degrec N is
`far smaller, and hence a circuit scale bccomes smaller.
`According to the present embodiment, the reflective wave
`estimation unit 16 takes in filter coeffieicnts of the digital
`fi lter 18 aftcr the elapse of a predetermined time after the
`adaptive filter 15 has started an adaptive operation, for
`estimating characteristics of a reflected way. However, the
`rcflective wave estimation unit 16 may monitor error Elt
`outputted from the subtractcr 20 of the adaptive filter 15 and
`may take in filter coefficients of the digital filter 18 when Elt
`becomes a certain predetermined value or less, for estimat(cid:173)
`ing characteristics of a reflected wave.
`In the multipath eliminating adaptive filter 11, reference
`numeral 22 denotes a multiplier allowing an adjuslment
`amount (=gain coefficient g) for adjusting an amplitude level
`of an input digital signal to be varied; 23. a digital filter
`comprising an IIR filter of first degree allowing a delay time
`30 of a delay element and a multiplication coefficient of a
`multiplication element to be varied; 24, an operator which
`squares an absolute value of an output from the digital filter
`23; 25, a subtracter which subtracts a reference amplitude
`valuc