Adaptive equalizer for controlling a step size in proportion to an estimated delay of received signals

An adaptive equalizer increases the estimation accuracy of a channel, and prevents the modulation characteristics from degrading. A sampling circuit generates a sampled sequence from a received signal. The sampled sequence is supplied from a buffer to an equalizing circuit, a channel estimating circuit and a delay amount estimating circuit. The equalizing circuit obtains an estimated transmitted symbol sequence from the sampled sequence and an estimated impulse response sequence. The channel estimating circuit obtains the estimated impulse response sequence from the sampled sequence, estimated transmitted symbol sequence and the estimated error. The delay amount estimating circuit supplies an estimated delay amount to a step-size control circuit. An error extracting circuit obtains the estimated error. The step-size control circuit controls a step-size parameter in response to the delay amount, and a tap coefficient update circuit updates the tap coefficients by the estimated transmitted symbol, estimated error e.sub.n and step-size parameter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an adaptive equalizer used for 
compensating the transmission delay of a received signal in a receiver of 
a digital communication system. 
2. Description of the Background Art 
Recently, digital mobile telecommunications have been intensively 
developed. One of the problems with the land mobile communications is 
gross distortion of waveforms of a received signal resulting from 
frequency selective fading due to fast movement of a mobile station and 
multiple interference waves accompanying delay. The adaptive equalizer is 
used for channels whose characteristics change with time to compensate for 
such distortion. 
A conventional equalizer is described, for example, in "Communication 
Systems Engineering" by J. G. Proakis et al., Prentice Hall, 1994, pp. 
577-595, which is incorporated here by reference. 
The adaptive equalizers track time variations in the channel response and 
adapt their coefficients to reduce the distortion. One of such 
conventional adaptive equalizers is based on an LMS (Least Mean Square) 
algorithm to estimate the channel characteristics. A received signal y(t) 
is sampled at a symbol period T to output a sampled sequence {y.sub.n }. 
The algorithm for optimizing the tap coefficients {Eht(0), Eht(1), . . . , 
Eh(M)} based on the LMS is expressed as 
EQU Eht(i).sup.n+1 =Eht(i).sup.n +.beta.*e.sub.n *Ex(n-i), (1) 
where Eht(i).sup.n+1 denotes tap coefficients at the (n+1)-th time instant, 
Eht(i).sup.n denotes tap coefficients at the n-th time instant, .beta. 
denotes a step-size parameter which is a fixed value, Ex(n) denotes an 
estimated transmitted symbol sequence, and e.sub.n denotes the difference 
obtained from the estimated transmitted symbol sequence Ex(n) and the 
sampled sequence {y.sub.n } where E placed at the initial position of the 
data names represents that they are an estimate. 
Another adaptive equalizer is based on the MLSE (Maximum Likelihood 
Sequence Estimation). It estimates, when a finite received sampled 
sequence Y.sub.k ={y.sub.1, y.sub.2, . . . , y.sub.n } is obtained, a 
transmitted symbol sequence x.sub.k ={x.sub.1, x.sub.2, . . . , x.sub.n } 
that provides the maximum likelihood of the finite received sampled 
sequence Y.sub.k ={y.sub.1, y.sub.2, . . . , y.sub.n } under the condition 
that the impulse response h(t) of the channel is known. This corresponds 
to obtaining the symbol sequence that maximizes the following expression 
(2), assuming that the channel noise is the white Gaussian noise. 
##EQU1## 
Expression (2) can be calculated using the Viterbi algorithm. 
The conventional adaptive equalizers, however, have the following problems. 
If there is no delayed wave on the channel, it would be ideal if the 
values of the delay term taps of the finite tap model would become zero. 
In practice, however, they have some non-zero amounts because of the 
estimated error e.sub.n. This decreases the estimation accuracy, which 
results in the degradation in the demodulation characteristics of a 
receiver. 
The estimation accuracy can be improved by reducing the tracking speed of 
the delay term taps of the finite tap model of the channel, or by updating 
the tap coefficients after leaking, that is, after reducing the weighting 
factors of the past tap coefficients when updating the delay term taps. 
However, if there is a delayed wave on the channel, the reduction of the 
tracking speed or the leakage of the tap coefficients presents a problem 
because the delay term taps themselves must also follow the time 
variations in the channel. There is another problem in that the 
demodulation characteristics of the receiver degrade owing to a temporary 
reduction in the power level of the received signal due to fading or the 
like. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an adaptive 
equalizer capable of achieving good demodulation characteristics 
regardless of the presence or absence of a delayed wave on the channel by 
improving the estimation accuracy of the channel. 
According to a first aspect of the present invention, there is provided an 
adaptive equalizer for compensating time-varying characteristics of a 
channel by estimating impulse response of the channel and by controlling 
tap coefficients of the adaptive equalizer in response to the estimated 
impulse response, the adaptive equalizer comprising: a sample-and-hold 
circuit for sampling a received signal fed through the channel, and for 
temporarily holding a sampled sequence generated by sampling; an equalizer 
circuit for estimating transmitted symbols in response to the sampled 
sequence and the estimated impulse response of the channel to generate an 
estimated transmitted symbol sequence; an error extracting circuit for 
extracting an estimated error sequence from the sampled sequence and the 
estimated transmitted symbol sequence; a tap coefficient update circuit 
for sequentially performing update calculations of the tap coefficients 
from the estimated error sequence and a step-size parameter by using a 
least mean square algorithm to sequentially generate the estimated impulse 
response of the channel; a detector circuit for obtaining from the sampled 
sequence one of an estimated delay amount of the received signal and an 
average amplitude of the received signal; and a control circuit for 
controlling one of the step-size parameter and leakage amounts of past tap 
coefficients in response to a detected value by the detector circuit when 
updating the tap coefficients. 
Here, the detector circuit may obtain the estimated delay amount of the 
received signal, and the control circuit may make the step-size parameter 
proportional to the estimated delay amount of the received signal. 
The detector circuit may obtain the estimated delay amount of the received 
signal, and the control circuit may make the leakage amounts of the past 
tap coefficients proportional to the estimated delay amount of the 
received signal. 
The detector circuit may obtain the average amplitude of the received 
signal, and the control circuit may make the step-size parameter 
proportional to the average value of the received signal. 
The detector circuit may obtain the average amplitude of the received 
signal, and the control circuit may make the leakage amounts of the past 
tap coefficients proportional to the average value of the received signal. 
According to a second aspect of the present invention, there is provided an 
adaptive equalizer for compensating time-varying characteristics of a 
channel by estimating impulse response of the channel and by controlling 
tap coefficients of the adaptive equalizer in response to the estimated 
impulse response, the adaptive equalizer comprising: a sample-and-hold 
circuit for sampling a received signal fed through the channel, and for 
temporarily holding a sampled sequence generated by sampling; an equalizer 
circuit for estimating transmitted symbols in response to the sampled 
sequence and the estimated impulse response of the channel to generate an 
estimated transmitted symbol sequence; a channel estimating circuit for 
estimating the impulse response of the channel in response to the sampled 
sequence and the estimated transmitted symbol sequence; and a delay amount 
estimating portion for estimating an estimated delay amount of the 
received signal from the sampled sequence, wherein the equalizer circuit 
carries out, when the delay amount of the received signal is smaller than 
a predetermined threshold, equalization processing by using values in 
which delayed wave term is reset of the estimated impulse response of the 
channel estimated by the channel estimating circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiment 1 
Before describing embodiments of an adaptive equalizer in accordance with 
the present invention, a transmitter and receiver will be described to 
which the adaptive equalizer is applied. 
FIG. 2 shows in a schematic block diagram a transmitter and a receiver for 
digital mobile communications to which the adaptive equalizer in 
accordance with the present invention is applied. In FIG. 2, a transmitter 
10 is connected to a receiver 30 through a channel 20. The transmitter 
comprises an encoder 11, a transmitting lowpass filter (LPF) 12 and a 
modulator 13. The receiver 30 comprises a demodulator 31, a receiving 
lowpass filter (LPF) 32, an adaptive equalizer 33, and a decoder 34. 
In operation, in the transmitter 10, the encoder 11 encodes the input data 
b.sub.m into transmitted symbols x.sub.n, which include I and Q components 
that are in quadrature to each other. The transmitter LPF 12 bandlimits 
the transmitted symbols x.sub.n to develop a complex baseband transmitted 
signal s(t) at its outpout port. The modulator 13 modulates a carrier with 
the complex baseband transmitted signal s(t), and transmits it over the 
channel 20 in the form of the transmitted signal s.sub.c (t). 
In the receiver 30, the demodulator 31 demodulates a signal r.sub.c (t) 
received through the channel 20 into a complex baseband signal r(t). The 
receiving LPF 32 bandlimits the complex baseband signal r(t) to output a 
complex baseband received signal y(t). The adaptive equalizer 33 samples 
the signal y(t) at every symbol period T, and compensates for variations 
in the characteristics of the channel 20 due to frequency selective fading 
in response to the sampled sequence, thereby outputting estimated 
transmitted symbols Ex.sub.n. Finally, the decoder 34 decodes the 
estimated transmitted symbols Ex.sub.n to obtain the transmitted data 
Eb.sub.m, where E placed at the beginning of the data names indicates that 
they are estimated one. 
The invention will now be described specifically with reference to the 
accompanying drawings. FIG. 1 is a block diagram showing the embodiment 1 
of an adaptive equalizer in accordance with the present invention, which 
is employed in the receiver 30 as the adaptive equalizer 33. The adaptive 
equalizer is constructed with a DSP (Digital Signal Processor) controlled 
by program sequences in the illustrative embodiment. It includes a 
sampling processor or circuit 41 for sampling a received signal y(t) fed 
from the receiving LPF 32, FIG. 2, at every symbol period T to generate a 
sampled sequence {y.sub.n }. The sampled sequence {y.sub.n } output from 
the sampling processor 41 is supplied to a buffer 42 to be stored. The 
sampling processor 41 and buffer 42 constitute a sample-and-hold circuit. 
The output of the buffer 42 is delivered to the input of an equalizing 
processor or circuit 43 and a first input of an error extracting section 
44a in a channel estimating unit 44A. The equalizing processor 43 is 
adapted to carry out equalization based on the sampled sequence {y.sub.n } 
and an estimated impulse response sequence {Eht} of the channel 20, 
thereby generating estimated transmitted symbol sequence {Ex.sub.n }. The 
output of the equalizing processor 43 is supplied to a second input of the 
error extracting section or circuit 44a and a first input of a tap 
coefficient update section 44b in the channel estimating unit 44A. The 
error extracting section or circuit 44a extracts an estimated error 
sequence e.sub.n from the sampled sequence {y.sub.n } and the estimated 
transmitted symbol sequence {Ex.sub.n }. 
The output of the buffer 42 is further supplied to the input of a delay 
amount estimating section or circuit 45 which estimates a delay amount 
EDelay taking place on the channel 20. The delay amount estimation is 
carried out by finding peak positions of the output of a correlator for 
recovering a clock signal at every predetermined observation period (frame 
period, for example), and by observing the fluctuation of the peak 
positions. The output of the delay amount estimating section 45 is 
supplied to the input of a step-size control section 44c in the channel 
estimating unit 44A. The step-size control section or circuit 44c controls 
a step-size parameter in response to the delay amount EDelay. The output 
of the step-size control section 44c is supplied to a second input of the 
tap coefficient update section or circuit 44b which updates the tap 
coefficients of a finite tap model of the channel by applying an LMS 
(Least Mean Square) algorithm to the estimated error e.sub.n and the 
step-size parameter. 
FIG. 3 is a block diagram showing an example of a delay amount estimating 
section 45 in FIG. 1. The delay amount estimating section or circuit 45 
comprises a correlation calculator 45a, a maximum correlation detector 
45b, a detected position buffer 45c and a delay amount calculator 45d. The 
correlation calculator 45a obtains correlations S45a between a reference 
sequence (synchronized sequence) for clock recovery and a sub-sequence of 
the sampled sequence {y.sub.n } of the received signal at every 
observation period. The maximum correlation detector 45b searches for 
maximum correlation values S45b of the correlations S45a at every 
observation period. The detected position buffer 45c holds detected 
positions S45c of the correlation values S45b beginning from a 
predetermined period before up to the present time, which has been set for 
observing the fluctuation. The delay amount calculator 45d obtains the 
delay amount EDelay from the range of the fluctuations of the detected 
positions S45c held in the detected position buffer 45c. 
In operation, with reference to FIG. 1, the sampling processor 41 samples 
the received signal y(t) to generate the sampled sequence {y.sub.n }, and 
outputs it to the buffer 42. The sampled sequence {y.sub.n } input to the 
buffer 42 is temporarily stored there, and is supplied to the equalizing 
processor 43, channel estimating section 44A and delay amount estimating 
section 45. 
The equalizing processor 43 carries out equalization based on the sampled 
sequence {y.sub.n } and the estimated impulse response sequence {Eht} of 
the channel 20 estimated by the channel estimating section 44A. 
The channel estimating section 44A obtains the estimated impulse response 
sequence {Eht} of the channel 20 from the sampled sequence {y.sub.n }, 
estimated transmitted symbol sequence {Ex.sub.n } and estimated error 
e.sub.n. 
The delay amount estimating section 45, receiving the sampled sequence 
{y.sub.n } from the buffer 42, estimates the estimated delay amount EDelay 
over the channel 20, and outputs it to the step-size control section 44c 
in the channel estimating section 44A. 
In the channel estimating section 44A, the error extracting section 44a 
extracts the estimated error e.sub.n from the estimated transmitted symbol 
sequence {Ex.sub.n } and the sampled sequence {y.sub.n }, and the 
step-size control section 44c controls the step-size parameter .beta.(i) 
(i=0, 1, . . . , M) as follows in response to the estimated delay amount 
EDelay estimated by the delay amount estimating section 45. 
EQU .beta.(0)=.beta.0 
EQU .beta.(i)=.beta.0*EDelay/(i*T), 
when EDelay/(i*T).ltoreq.1, 
EQU .beta.(i)=.beta.0 
otherwise, 
where 
.beta.0: reference value of the step-size parameter, and 
T: tap interval. 
The tap coefficient update section 44b updates the tap coefficients 
{Eht(0), Eht(1), . . . , Eht(M)} in accordance with the following equation 
(3) in response to the estimated transmitted symbol sequence Ex(n), 
estimated error e.sub.n and the step-size parameter .beta.(i) set by the 
step-size control section 44c. 
EQU Eht(i).sup.+1 =Eht(i).sup.n +.beta.(i)*e.sub.n *Ex(n-i) (3) 
As shown in equation (3), the speed and accuracy of tracking of the delay 
term taps of the finite tap model of the channel 20 is controlled in 
response to the delay state of the channel 20. 
According to the first embodiment, since the adaptive equalizer is provided 
with the delay amount estimating section 45 for estimating the estimated 
delay amount EDelay of the received signal y(t), and the step-size control 
section 44c for making the step-size parameter proportional to the 
estimated delay amount EDelay when updating the tap coefficients, the 
tracking speed and accuracy of the delay term taps of the finite tap model 
of the channel 20 can be controlled in response to the delay state of the 
channel 20. This makes it possible to achieve good demodulation 
characteristics of the receiver regardless of the delay over the channel 
20. 
Embodiment 2 
FIG. 4 is a block diagram showing a second embodiment of the adaptive 
equalizer in accordance with the present invention, in which the like 
elements are designated by the same reference numerals. The second 
embodiment is the same as the first embodiment except that its channel 
estimating unit 44B has a configuration different from that of the channel 
estimating unit 44A. The channel estimating unit 44B includes a leakage 
coefficient control section or circuit 44d instead of the step-size 
control section 44c in the channel estimating unit 44A. The leakage 
coefficient control section 44d controls leakage coefficients in response 
to the estimated delay amount EDelay estimated by the delay amount 
estimating section 45, where the leakage coefficients are coefficients for 
reducing the weight of the past tap coefficients. The remaining 
configuration is the same as that of FIG. 1. 
The operation of the adaptive equalizer of FIG. 4 differs from that of FIG. 
1 in the following. Specifically, the leakage coefficient control section 
44d controls the leakage coefficients L(i) (i=0, 1, . . . , M) as follows 
in response to the estimated delay amount EDelay estimated by the delay 
amount estimating section 45. 
EQU L(0)=1 
EQU L(i)=EDelay/(i*T), 
when EDelay/(i*T).ltoreq.1, 
EQU L(i)=1 
otherwise, 
where T is a tap interval. 
The tap coefficient update section 44b updates the tap coefficients 
{Eht(0), Eht(1), . . . , Eht(M)} in accordance with the following equation 
(4) in response to the estimated transmitted symbols Ex(n), estimated 
error e.sub.n and the leakage coefficients L(i) set by the leakage 
coefficient control section 44d. 
EQU Eht(i).sup.n+1 =Eht(i).sup.n *L(i)+.beta.*e.sub.n *Ex(n-i) (4) 
where is a step-size parameter. 
As shown in equation (4), the speed and accuracy of tracking of the delay 
term taps of the finite tap model of the channel 20 is controlled in 
response to the delay state of the channel 20. 
According to the second embodiment, the adaptive equalizer is provided with 
the delay amount estimating section 45 for estimating the estimated delay 
amount EDelay of the received signal y(t), and the leakage coefficient 
control section 44d for making the leakage amounts of the past tap 
coefficients proportional to the estimated delay amount EDelay when 
updating the tap coefficients. The tracking speed and accuracy of the 
delay term taps of the finite tap model of the channel 20 can thus be 
controlled in response to the delay state of the channel 20 as in the 
first embodiment. This makes it possible to implement good demodulation 
characteristics of the receiver regardless of the delay over the channel 
20. 
Embodiment 3 
FIG. 5 is a block diagram showing a third embodiment of the adaptive 
equalizer in accordance with the present invention, in which the like 
elements are designated by the same reference numerals as in FIG. 1. The 
third embodiment is the same as the first embodiment except that the delay 
amount estimating section 45 in FIG. 1 is replaced by an average amplitude 
calculating section 46 for calculating an average amplitude Ave of the 
received signal y(t). The remaining configuration is the same as that of 
FIG. 1. 
More specifically, the sampled sequence {y.sub.n } stored in the buffer 42 
is delivered to the equalizing processor 43, channel estimating unit 44A 
and average amplitude calculating section 46. 
The average amplitude calculating section 46, receiving the sampled 
sequence {y.sub.n } from the buffer 42, obtains the average amplitude Ave 
of the received signal y(t), and outputs it to the channel estimating unit 
44A. 
In the channel estimating section 44A, the step-size control section 44c 
controls the step-size parameter .beta. as follows in response to the 
average amplitude Ave fed from the average amplitude calculating section 
46. 
EQU .beta.=.beta.0*Ave, 
when Ave.ltoreq.1.0, 
EQU .beta.=.beta.0, 
otherwise, 
where .beta.0 is a reference value of the step-size parameter. 
The tap coefficient update section 44b updates the tap coefficients 
{Eht(0), Eht(1), . . . , Eht(M)} in accordance with the following equation 
(5) in response to the estimated transmitted symbols Ex(n), the estimated 
error e.sub.n obtained by the error extracting section 44a, and the 
step-size parameter .beta. set by the channel estimating unit 44C. 
EQU Eht(i).sup.n+1 =Eht(i).sup.n +.beta.*e.sub.n *Ex(n-i) (5) 
As shown in equation (5), the tracking speed of the channel estimating unit 
44A is controlled in response to the power level of the received signal. 
According to the third embodiment, the adaptive equalizer is provided with 
the average amplitude calculator section 46 for calculating the average 
amplitude Ave of the received signal y(t), and the step-size control 
section 44c for making the step-size parameter .beta. proportional to the 
average amplitude Ave, when updating the tap coefficients. The tracking 
speed of the channel estimating section 44A can thus be controlled in 
response to the power level of the received signal y(t). This makes it 
possible to solve the conventional problem in that the error possibility 
of the estimation of the channel increases and the demodulation 
characteristics of the receiver degrade when the power level of the 
received signal y(t) falls temporarily due to fading or the like. 
Embodiment 4 
FIG. 6 is a block diagram showing a fourth embodiment of the adaptive 
equalizer in accordance with the present invention, in which also the like 
elements are designated by the same reference numerals as in FIGS. 4 and 
5. The fourth embodiment is the same as the second embodiment as shown in 
FIG. 4 except that the delay amount estimating section 45 in FIG. 4 is 
replaced by the average amplitude calculating section 46 as shown in FIG. 
5. The remaining configuration is the same as that of FIG. 4. 
More specifically, the leakage coefficient control section or circuit 44d 
controls the leakage coefficients L(i) as follows in response to the 
average amplitude Ave obtained by the average amplitude calculating 
section 46. 
EQU L(0)=1 
EQU L(i)=Ave, 
when Ave.ltoreq.1, 
EQU L(i)=1, 
otherwise. 
The tap coefficient update section 44b updates the tap coefficients 
{Eht(0), Eht(1), . . . , Eht(M)} as shown by the foregoing equation (4) in 
response to the estimated transmitted symbols Ex(n) output from the 
equalizing processor 43, the estimated error e.sub.n obtained by the error 
extracting section 44a, and the leakage coefficients L(i) set by the 
leakage coefficient control section 44d. 
As shown in equation (4), the tracking accuracy of the channel estimating 
unit 44B is controlled in response to the power level of the received 
signal y(t). 
According to the fourth embodiment, the adaptive equalizer is provided with 
the average amplitude calculating section 46 for calculating the average 
amplitude Ave of the received signal y(t), and the leakage coefficient 
control section 44d for making the leakage amount of the past tap 
coefficients proportional to the estimated delay amount EDelay when 
updating the tap coefficients. The tracking accuracy of the channel 
estimating section 44B can thus be controlled in response to the power 
level of the received signal y(t). This makes it possible to solve the 
conventional problem in that the error possibility of the estimation of 
the channel increases and the demodulation characteristics of the receiver 
degrade when the power level of the received signal y(t) falls temporarily 
due to fading or the like. 
Embodiment 5 
FIG. 7 is a schematic block diagram showing a fifth embodiment of the 
adaptive equalizer in accordance with the present invention, in which the 
like elements are designated by the same reference numerals as in FIG. 1. 
The fifth embodiment as shown in FIG. 7 is the same as the first 
embodiment as shown in FIG. 1 except for the following points. 
First, the fifth embodiment is provided with an equalizing processor 43A in 
place of the equalizing processor 43 in FIG. 1. The equalizing processor 
43A differs from the equalizing processor 43 in its configuration, and is 
supplied with the estimated delay amount EDelay from the delay amount 
estimating section 45. Second, a channel estimating unit 44C is placed 
instead of the channel estimating unit 44A. The channel estimating unit 
44C differs from the channel estimating unit 44A in its configuration. 
The equalizing processor 43A carries out, when the estimated delay amount 
EDelay is smaller than a predetermined threshold, the equalization by 
using values obtained by clearing the delayed wave term of the estimated 
impulse response sequence {Eht} of the channel 20 estimated by the channel 
estimating unit 44C. The channel estimating section 44C obtains the 
estimated impulse response sequence {Eht} of the channel 20 from the 
sampled sequence {Y.sub.n } and the estimated transmitted symbol sequence 
{Ex.sub.n }. The remaining portions are the same as those of FIG. 1. 
The operation of the fifth embodiment will now be described. In this 
embodiment, the equalizing processor 43A employs the MLSE (Maximum 
Likelihood Sequence Estimation). The delay amount estimating section 45, 
receiving the sampled sequence {y.sub.n } from the buffer 42, estimates 
the estimated delay amount EDelay (in terms of one symbol period T) over 
the channel 20, and outputs it to the equalizing processor 43A. The 
channel estimating section 44C obtains the estimated impulse response 
sequence {Eht} of the channel 20 from the sampled sequence {y.sub.n } and 
the estimated transmitted symbol sequence {Ex.sub.n }. 
The equalizing processor 43A carries out the maximum likelihood sequence 
estimation (MLSE) based on the sampled sequence {y.sub.n }, estimated 
impulse response sequence {Eht} and estimated delay amount EDelay to 
obtain the estimated transmitted symbol sequence {Ex.sub.n }. 
More specifically, the estimated transmitted symbol sequence {Ex.sub.n } 
can be obtained by calculating the symbol sequence {x.sub.1, x.sub.2, . . 
. , x.sub.n } that maximizes the following expression (6) when the 
estimated delay amount EDelay is smaller than a switching threshold value 
.theta.; that is, when no delayed wave is present. 
##EQU2## 
When the estimated delay amount EDelay is greater than the switching 
threshold .theta., the estimated transmitted symbol sequence {Ex.sub.n } 
can be obtained by calculating the symbol sequence {x.sub.1, x.sub.2, . . 
. , x.sub.n } that maximizes the following expression (7). 
##EQU3## 
Equations (6) and (7) can be operated effectively by the Viterbi algorithm. 
As shown by the expression (6), the delay term taps of the channel 
estimation section are forced to be reset, when the estimated delay amount 
EDelay is smaller than the switching threshold .theta., that is, when the 
delayed wave is absent. 
According to the fifth embodiment, the adaptive equalizer is provided with 
the delay amount estimating section 45 for estimating the estimated delay 
amount EDelay of the received signal y(t). The delay term taps of the 
channel estimating section 44C are thus forced to be reset, when the 
estimated delay amount EDelay is smaller than the switching threshold 
.theta., namely, when no delayed wave is present. This makes it possible 
to improve the demodulation characteristics of the receiver when the 
delayed wave is absent. 
Although the foregoing embodiments are explained in connection with the 
adaptive equalizer applied to the receiver for mobile communications, the 
present invention can also be applied to an adaptive equalizer for use in 
data communications or the like in a fixed network. 
The entire disclosure of Japanese patent application No. 106964/1996 filed 
on Apr. 26, 1996 including the specification, claims, accompanying 
drawings and abstract of the disclosure is incorporated herein by 
reference in its entirety. 
While the present invention has been described with reference to the 
particular illustrative embodiments, it is not to be restricted by those 
embodiments. It is to be appreciated that those skilled in the art can 
change or modify the embodiments without departing from the scope and 
spirit of the present invention.