Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
2000-03-14
2003-09-02
Pham, Chi (Department: 2731)
Pulse or digital communications
Equalizers
Automatic
C375S229000, C375S233000, C375S376000
Reexamination Certificate
active
06614840
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an equalizer with a phase-locked loop which detects and eliminates a phase shift from a received signal which is subjected to a frequency offset or a phase variation generated at the beginning of a burst transmission and more particularly, to a delayed decision feedback sequence estimator with a phase-locked loop which can compensate a phase shift of a received signal which is terribly deteriorated by a transmission distortion.
2. Description of the Prior Art
A delayed decision feedback sequence estimation receiver as a first prior art which has been disclosed in JPA 11-8573 is shown in FIG.
1
.
Referring to
FIG. 1
, transmission line characteristic detector
41
detects discrete impulse responses from a received signal in a preamble, i.e. a training signal of an M-sequence. Absolute value calculator
42
calculates an amplitude of each impulse response. Accumulator
43
sets three regions, i.e. a maximum likelihood sequence estimation region, a decision feedback equalization region, and a non-estimation region, and obtains a sum
p
of amplitudes of impulse responses in the maximum likelihood sequence estimation region, a sum
q
of amplitudes of impulse responses in the decision feedback equalization region, and a sum
r
of amplitudes of impulse responses in the non-estimation region each time an amplitude of a succeeding impulse response is inputted. Maximum detector
44
calculates a value of p/(r+&agr;q) for each impulse response group among shifted groups and outputs a signal which indicates the impulse response group which maximize the value. Delayed decision feedback sequence estimator
45
estimates the maximum likelihood sequence from the received signal by using the impulse responses selected from the impulse responses inputted from transmission line characteristic detector
41
by the signal from maximum detector
44
.
Next, the calculation in maximum detector
44
which determines the optimum regions of impulse responses will be explained. Components in the decision feedback equalization region are ideally cancelled by feedback and do not contribute to improvement or deterioration of the estimation capability of delayed decision feedback sequence estimator
45
. Therefore, the estimation capability is determined by the ratio of p/r, i.e. the ratio of the sum
p
of amplitudes of impulse responses in the maximum likelihood sequence estimation region and the sum
r
of amplitudes of impulse responses in the non-estimation region, and the greater the ratio of p/r, the higher the estimation capability.
However, because of quantization errors and the like, components in the decision feedback equalization region are not canceled completely and remain as a distortion. Therefore, the greater the ratio of p/(r+&agr;q), i.e. the ratio of the sum
r
of amplitudes of impulse responses in the maximum likelihood sequence estimation region and the sum of the sum
r
of amplitudes of impulse responses in the non-estimation region and the weighted sum &agr;q of amplitudes of impulse responses in the decision feedback equalization region, the higher the estimation capability. That is, the impulse response group which maximizes the ratio of p/(r+&agr;q) indicates the optimum regions.
A phase-locked loop circuit using equalizer as a second prior art which as been disclosed in JPA 10-327204 is shown in FIG.
2
.
Impulse response detector
33
is similar to transmission line characteristic detector
41
of the first prior art. Delayed decision feedback sequence estimator
32
is similar to delayed decision feedback sequence estimator
45
of the first prior art. Region designator
34
is similar to a group of absolute value calculator
42
, accumulator
43
, and maximum detector
44
, each of which is of the prior art. Replica generator
35
convolutes impulse responses designated by region designator
34
among impulse responses of a transmission line obtained by impulse response detector
33
in a preamble period with a sequence signal estimated in delayed decision feedback sequence estimator
32
in order to generate a replica of a received signal. Delay circuit
36
delays the received signal which has been rotated in phase in phase rotator
31
to compensate the delay which is generated in delayed decision feedback sequence estimator
32
. Thus, an output from delay circuit
36
coincides with an output from replica generator
35
. Phase detector
37
detects a phase difference between the output from replica generator
35
and the output from delay circuit
36
. Here, if the received signal has a frequency offset, the phase difference between the transmission side and the reception side varies with a laps of time, and the phase difference detected by phase detector
37
varies with a laps of time. That is, because replica generator
35
outputs a signal with no phase variation as long as delayed decision feedback sequence estimator
32
does not cause errors as the impulse response of a transmission line used in replica generator
35
is constant, while delay circuit
36
outputs a signal with a phase variation, phase detector
37
detects a phase difference between the signals.
An output signal from phase detector
37
is in bandwidth restricted by filter
38
and inputted to VCO (Voltage Controlled Oscillator)
39
. Phase rotator
31
rotates a phase of a received signal by using an output of VCO
39
to reduce the phase difference detected by phase detector
37
, thereby absorbing a phase variation due to a frequency offset or the like.
The second prior art has a disadvantage that a replica generated in replica generator
35
may deteriorate in precision for some impulse responses of a transmission line. This disadvantage when a delayed decision feedback sequence estimator is used as a signal estimator will be explained below.
FIG. 3
shows an example of an impulse response and regions. In this example, the symbol length of maximum likelihood sequence estimation region
53
is 4, and the symbol length of decision feedback equalization region
54
is 3. There are direct wave
51
and delayed wave
52
delayed from direct wave by
5
T (T: symbol period), thereby constituting a two-wave model. In practical circumstances, there is a case that a level of direct wave
51
is extremely lowered because of fading. In such a case, as a result of calculation of regions as explained above, maximum likelihood sequence estimation region
53
includes a delayed wave
52
and both of maximum likelihood sequence estimation region
53
and a decision feedback equalization region
54
do not include direct wave
51
.
A delayed decision feedback sequence estimator has such a feature that a maximum likelihood sequence estimation region and a decision feedback equalization region
53
are determined in such a way that a direct wave or one or more delayed waves are not included in both of a maximum likelihood sequence estimation region and a decision feedback equalization region though the direct wave and all the delayed waves may be included in any of maximum likelihood sequence estimation region
53
and decision feedback equalization region
54
.
Delayed decision feedback sequence estimator
32
obtains a better estimation characteristics when executing a maximum likelihood estimation using a delayed wave high in level than using a direct wave low in level as shown in FIG.
3
. However, when obtaining a phase difference between a replica and a received signal, the replica does not include a direct wave which falls out of a maximum likelihood sequence estimation region and out of a decision feedback equalization region. Therefore, the phase difference includes an error composed of the direct wave. This error is not caused by a phase variation but the error is fedback to phase rotator
31
as if there is a phase variation, thereby deteriorating a estimation characteristics.
SUMMARY OF THE INVENTION
In order to overcome the aforementioned disadvantages, the present invention has been made and a
NEC Corporation
Pham Chi
Whitham Curtis & Christofferson, P.C.
Williams Demetria
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