Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-09-17
2001-06-12
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S316000, C329S315000
Reexamination Certificate
active
06246732
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a demodulator including an adaptive equalizer and a demodulating method, in digital communications.
In land mobile communications, out-of-sight radio communications are generally used. Accordingly, a received wave has a complex characteristic of being constituted by multiple waves which are much subjected to reflection, diffraction and scattering. In addition, the communication characteristic of a path channel varies instantaneously in accordance with the movement of a mobile station, so that communication quality is deteriorated. It is known that the variation in the communication characteristic generally accords with a Rayleigh distribution. This phenomenon is called “Rayleigh fading”.
As a measure to cope with Rayleigh fading, particularly to cope with deterioration of communication quality caused by the instantaneous variation in amplitude, “diversity” is generally used. “Diversity” is a technique in which a received signal highest in received signal power among the several received signals is selected or synthesized from statistically independent received signals via a plurality of path channels to reduce the probability of lowering received signal power to thereby suppress the influence of Rayleigh fading. “Diversity” is classified into space diversity, directionality diversity, polarization diversity, etc., depending on the methods of selecting the independent received signal highest in received signal power. Further, “diversity” is classified into selection diversity, co-phasing combining diversity, maximal ratio combining diversity, etc., in accordance with the method of synthesizing independent received signals.
Because path channels constituting the above-mentioned plurality of path channels (multipath channels) have path channel lengths different from each other, the time of signal arrival at a reception point varies. The degree of this variation is called delay spread. The diversity is effective in the case where the delay spread is sufficiently small compared with transmission interval time per symbol, whereas bit error, so-called unrecoverable error, which is impossible to be compensated by only the diversity, occurs in the case where the delay spread is large.
FIG. 1
shows an example in which multipath channels in a base band are expressed by a model of time-variant filter. The reference numeral
1
designates a transmitted signal input terminal;
2
-
1
,
2
-
2
, . . . ,
2
-M, delay elements;
3
-
0
,
3
-
1
,
3
-
2
, . . . ,
3
-M, coefficient multipliers;
4
, an adder; and
5
, a received signal output terminal. In
FIG. 1
, a transmitted symbol sequence supplied to the transmitted signal input terminal
1
is sent to the delay element
2
-
1
and the coefficient multiplier
3
-
0
. Here, M delay elements
2
-
1
,
2
-
2
, . . . ,
2
-M (M is an integer) are series-connected and each has a delay time Ts equal to the transmission interval time of one symbol. The signals of the transmitted symbol sequence supplied to the transmitted signal input terminal
1
are sent to the delay elements
2
-
1
,
2
-
2
, . . . ,
2
-M successively. Signals are taken out from intermediate points between the delay elements
2
-
1
and
2
-
2
, between the delay elements
2
-
2
and
2
-
3
, . . . , between the delay elements
2
-(M-
1
) and
2
-M and from the last stage of the delay element
2
-M, respectively, and supplied to the coefficient multipliers
3
-
1
,
3
-
2
, . . . ,
3
-M, respectively. The coefficient multipliers
3
-
0
,
3
-
1
,
3
-
2
, . . . ,
3
-M have complex coefficients hi (i=0, 1, 2, . . . , M) which are set individually, so that the input signals are multiplied by the complex coefficients hi (i=0, 1, 2, . . . , M), respectively. Results of the multiplications are added up by the adder
4
, so that the resulting value is sent to the received signal output terminal
5
and is output therefrom. As represented by the model in
FIG. 1
, a received signal at a certain point of time is provided as a result of addition of a direct wave component based on transmitted symbols at a corresponding point of time and a delay wave component constituted by transmitted symbols before the corresponding point of time.
When the delay spread is large, the received signal is subjected to intersymbol interference by the delay wave component in accordance with the case where any one of the complex coefficients except the complex coefficient h0 has a value of amplitude near the amplitude of the complex coefficient h0 or a plurality of complex coefficients except the complex coefficient h0 have values of amplitude near the amplitude of the complex coefficient h0.
The intersymbol interference due to the delay spread in such a manner is a cause of bit error which brings serious deterioration of communication quality. To suppress this deterioration, an adaptive equalizer, for example, represented by a decision feedback equalizer or a Viterbi equalizer is required to be used.
Adaptive equalizers are classified into linear equalizers and maximum likelihood sequence estimators (MLSE). The decision feedback equalizer is known as a representative example of the linear equalizers and the Viterbi equalizer is known as a representative example of the maximum likelihood sequence estimators.
Incidentally, a conventional example of the decision feedback equalizer has been described, for example, in J. G. Proakis, “Digital Communications”, McGraw-Hill International Editions, 1989, pp. 593-600.
The decision feedback equalizer will be described below as an example of the adaptive equalizers.
FIG. 2
is a diagram showing an example of the configuration of the decision feedback equalizer. The reference numerals
6
and
8
designate multipliers;
7
, an adder;
11
, a digital received signal input terminal;
12
and
13
, delay elements;
15
, a symbol decision unit;
16
, a reference signal memory;
17
, a switch;
18
, an error estimator;
19
, a tap coefficient update unit;
20
, an equalization output terminal;
21
, a feed-forward portion (FF portion);
22
, a feedback portion (FB portion); and
23
, an equalization filter. In
FIG. 2
, each of the delay elements
13
has a delay time Ts equal to the transmission interval time per symbol, and each of the delay elements
12
has a delay time Tp equal to a value which is given by the delay time of the delay element
13
divided by an integer (generally, by 2). Further, the equalization filter
23
is constituted by an FF portion
21
, an FB portion
22
, and an adder
7
. The FF portion
21
is constituted by delay elements
12
and multipliers
6
. The FB portion
22
is constituted by delay elements
13
and multipliers
8
. Complex coefficients F-j, F-j+1, . . . , F0 called tap coefficients are set in the multipliers
6
respectively. Tap coefficients B
1
, B
2
, . . . , BK are set in the multipliers
8
respectively.
Received signals sampled at a sampling interval Tp are supplied to the digital received signal input terminal
11
, sent to the delay elements
12
of the FF portion
21
of the equalization filter
23
and successively multiplied by corresponding tap coefficients through the multipliers
6
respectively. At the same time, output signals of the switch
17
are sent to the FB portion
22
and successively multiplied by corresponding tap coefficients through the multipliers
8
respectively. All the results of these multiplications are sent to the adder
7
and added up by the adder
7
, so that the resulting signal of addition is provided as an output of the equalization filter
23
. Not only the output signal of the equalization filter
23
is sent to the decision unit
15
and the error estimator
18
but also the output signal is taken out from the equalization output terminal
20
. The decision unit
15
decides what symbol is expressed by the input signal and sends the decided symbol to one input of the switch
17
.
In digital communications, a fixed symbol sequence is generally inserted for the purpose of synchronization, or the like. Th
Fujiwara Yukinari
Kobayashi Takehiko
Antonelli Terry Stout & Kraus LLP
Hitachi Denshi Kabushiki Kaisha
Pham Chi
Tran Khai
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