Adaptive equalizer

Abstract: The adaptive equalizer of the present invention estimates a channel impulse response vector h in a channel impulse response estimator (121), and outputs a result to a channel impulse response convertor (125). The channel impulse response convertor (125) removes interference components from the channel impulse response vector h , and outputs a channel impulse response vector h' to an equalizer (110). The equalizer sets up, based on the channel impulse response vector h' , internal parameters, and equalizes, based on the internal parameters, the received signals. As a result of this configuration it is possible to minimise a distortion resulting from intersymbol interference and any reduction in the reception performance caused by Co-channel interference waves.

Description: The present invention relates generally to an adaptive equalizer, and more specifically to an adaptive equalizer enabling the deterioration of reception caused by intersymbol interference distortion and co-channel interference waves to be minimised. Conventionally, for example, when intersymbol interference and co-channel interference waves exist, a system to improve reception using space diversity and an equalizer is known, as described in a paper entitled "Co-channel Interference Performance of Diversity Equalization in Mobile Radio" by Hitoshi YOSHINO and Hiroshi SUZUKI in 1992, at The Autumn Meeting of The Electronic Data Academic Society at B-260. As shown in Fig. 5 of the accompanying drawings, this known system receives a plurality of signals from a plurality of antennas 500 and 510, equalizes and synthesizes these received signals in an equalizer 520, and then carries out their demodulation. By using this system, reception performance against co-channel interference waves is improved. The improvement of reception performance against co-channel interference waves, however, is obtained by space diversity. Therefore, in this case, there is the problem that the configuration of a receiver becomes complicated by the diversity requirement. On the other hand, in the case of equalizing signals while estimating a channel impulse response, when a co-channel interference wave exists, a channel impulse response estimator is locked to the co-channel interference wave and estimates a different channel impulse response from an actual channel impulse response. As a result, the system equalizes the received signals with the wrong channel impulse response. Therefore, there is a problem that the reception performance thereof is greatly impaired. In the specification of United States patent number 5,127,025, which was published on June 30 1992, there was described a space diversity receiver having multiple signal paths connected respectively to antennas for multipath reception. Each signal path included a digital demodulator coupled to the antenna, and an impulse response detector, or autocorrelator connected to the demodulator for deriving a channel impulse response of each one of the multiple paths. A maximum response detector was connected to each impulse response detector for detecting equalizable delay components which gave a maximum response value and unequalizable delay components. A power ratio of the unequalizable delay components to the equalizable delay components was derived for each signal path, and a minimum value of the power ratios was determined, and one of the signal paths in which the minimum value was detected was selected. The output signal of the demodulator of the selected signal path was applied to one input of an equalizer, or a maximum likelihood sequence estimator, and the equalizable delay components of the selected signal path were applied to the other input of the equalizer. An object of the present invention is to offer an adaptive equalizer which minimises the above problems, while not complicating its configuration. Another object of the present invention is to offer an adaptive equalizer which minimises distortion by intersymbol interference and the deterioration of reception preformance by co-channel interference waves, without using a diversity system. In an arrangement to be described below there is an equalizer which includes a channel impulse response vector calculator for estimating a channel impulse response vector h from received signals and calculating a channel impulse response vector h' by removing interference components from the estimated path impulse response vector h; and an equalizer for inputting the received signals and the channel impulse response vector h' , setting an internal parameter using the channel impulse response vector h' and then equalizing, based on the internal parameter, the received signals. It is desirable that the adaptive equalizer should include a maximum likelihood sequence estimator. It is desirable that the channel impulse response vector calculator should include means to input the received signals and the result of equalization of the received signals by the equalizer, to estimate the channel impulse response vector h using the equalization result and the received signals. Furthermore, it is desirable that the channel impulse response vector calculator should include a channel impulse response estimator for inputting the received signals and the equalization result and estimating a channel impulse response vector h , and a channel impulse response convertor for inputting the channel impulse response vector h and converting the channel impulse response vector h to the channel impulse response vector h' from which the interference components are removed. It is also desirable that the channel impulse response estimator should include a transversal filter. Furthermore, it is desirable that the channel impulse response estimator should include a channel impulse response estimator circuit inputting the received signals and the equalization result and estimating the channel impulse response vector h; a training signal detector for detecting training signals from the input received signals; an equalization result output circuit for outputting the equalization result corresponding to the training signals; and switching means for inputting the equalization result from the equalization result output means and the equalization result of the received signals and outputting an equalization result from the equalization result output circuit to the channel impulse response estimation circuit when training signals have been detected based on a detection signal from the training signal detector, and means outputting an equalization result of the received signals to the channel impulse response estimation circuit when training signals have not been detected. It is also desirable that the channel impulse response convertor means should include a plurality of absolute value operation means for inputting each component of the channel impulse response vector h , and calculating and outputting an absolute value of each component; a first maximum value detector for detecting and outputting a first maximum value among absolute values output from the plurality of absolute value operation means and specifying the absolute value operation means that has output the first maximum value; a gate for inputting absolute values output from the plurality of absolute value operation means and passing absolute values except for the first maximum value based on a specified signal from the first maximum value detector; a second maximum value detector for inputting the absolute values that have passed the gate, detecting a second maximum value among the absolute values that have passed the gate and outputting the second maximum value; a divider for inputting the first maximum value and the second maximum value and calculating the ratio between the first maximum value and the second maximum value; a weight controller for inputting the ratio calculated in the divider and the specified signal from the first maximum value detector and outputting a weight coefficient corresponding to each component of the channel impulse response vector h ; and a plurality of multiplier means for multiplying the weight coefficient that has been output from the weight controller and each component of the channel impulse response vector h . It is also desirable that the weight controller should include a weight coefficient memory for storing a plurality of weight coefficients; a selector for outputting only one weight coefficient among the plurality of weight coefficients; and a controller for inputting the specified signal from the first maximum value detector and the ratio output from the divider section and for outputting a control signal indicating a weight coefficient to be output to the selector. Furthermore, it is desirable that the weight controller should include a threshold value memory for storing a threshold value; a weight coefficient memory section for storing two weight coefficients; a selector for outputting a weight coefficient stored in the weight coefficient memory section; a comparison circuit for comparing the threshold value and the ratio output from the divider and for outputting a comparison signal; and a controller for inputting the comparison signal and the specified signal from the first maximum value detector and outputting a control signal indicating a weight coefficient to be output to the selector. As a result of this configuration, when transferring data via a communication path where co-channel interference waves exist and a distortion by intersymbol interference occurs, the problem that an equalizer is falsely-locked to co-channel interference waves when levels of the reception signal fall can be minimised and the reception performance can be improved without using a diversity system, thereby simplifying the configuration of the receiver. The following description and drawings disclose, by means of examples, both the invention which is characterised in the appended claims, whose terms determine the extent of the protection conferred hereby, and a previously proposed arrangement. In the drawings:- Fig. 1 is a block diagram showing one embodiment, Fig. 2 is a block diagram showing a configuration of the channel impulse response convertor section of the embodiment, Fig. 3 is a block diagram of a weight controller of the embodiment, Fig. 4 is a block diagram of a weight controller of another embodiment, and Fig. 5 is a block diagram illustrating a previously proposed arrangement. Fig. 1 is a block diagram showing an embodiment of the present invention. In this figure, 100 is an input terminal inputting reception signals, 110 is an equalizer, 120 is a channel impulse response vector calculator, 130 is an output terminal outputting equalized reception signals. The equalizer 110 is configured by a maximum likelihood sequence estimator as shown, for example, in the paper "Digital Communications" Second Edition, 1989 by J.G.Proakis and in a U.S. Patent No. 5,127,025 "Space Diversity TDMA Receiver" by Mr. OKANOUE, one of the inventors of the present invention. In this configuration, reception signals input therein are equalized based on internal parameters. More, each internal parameter is set by each component of a channel impulse response vector h' described later on. Based on reception signals and an equalization result of said reception signals, the channel impulse response vector calculator 120 calculates a channel impulse response vector h' consisted of N components, from which all interference components are removed. This channel impulse response vector calculator 120 is configured by a channel impulse response estimator 121, a training signal detector 122, an training sequence generating circuit 123, switching circuit 124 and a channel impulse response convertor 125. The channel impulse response estimator 121 is configured by a transversal filter as shown, for example, Fig.6.7.5 in the paper "Digital Communications" Second Edition 1989, by J.G.Proakis. Also, it is possible to estimate a channel impulse response h by a way to estimate basing only reception signals as shown, for example, in the paper "A Study of Blind Viterbi Equalization Algorithm" by FURUYA, USHIROKAWA, ISA and SATO, A-141 for The Spring Meeting of the Electronic data and communication Academic Society, 1991. However, to estimate more accurate channel impulse response vector h , this invention is configured to use not only reception signals but also equalization results of reception signals and training signals. Training signal detector 122 is configured to output a detection signal by detecting training signals in reception signals. The training sequence generating circuit 123, which stores an equalization result of preset training signals, is configured to output the predetermind training sequence to the switching circuit 124. The switching circuit 124 inputs an predetermined training sequence from the training sequence generating circuit 123 and another equalization result of reception signals, and outputs one of them based on a detection signal from the training signal detector 122. By the above configuration, an predetermined trainig sequence from the training sequence generating circuit 123 in case that the training signal detector 122 has detected a training signal or an equalization result of reception signals in case that the training signal detector 122 has not detected a training signal, is input to the channel impulse response estimator 121 by selected in switching circuit 124. Based on these equalization results, the channel impulse response estimator 121 estimates a channel impulse response h consisted of N components, then outputs it to the channel impulse response convertor 125. Next, a configuration of the channel impulse response convertor 125 is explained. Fig.2 is a block diagram showing a configuration of the channel impulse response convertor 125. In this figure, 200(1) to 200(N) are input terminals, 210(1) to 210(N) are absolute value operation circuit, 220 is a first maximum value detector, 230 is a second maximum value detector, 240 is a gate circuit, 250 is a divider, 260 is a weight controller, 270(1) to 270(N) are multipliers and 280(1) to 280(N) are output terminals. The channel impulse response convertor 125 removes interference components from a received channel impulse response h and converts this to a channel impulse response vector h' . This channel impulse response convertor 125 is configured by input terminals 200(1) to 200(N), absolute value operation circuits 210(1) to 210(N), a first maximum value detector 220, a second maximum value detector 230, a gate circuit 240, a divider 250, a weight controller 260 and multipliers 270(1) to 270(N). The input terminals 200(1) to 200(N) are configured to input each component of a channel impulse response h. The absolute value operation circuits 210(1) to 210(N) are configured to input each component of the channel impulse response h through the input terminals 200(1) to 200(N), calculate each absolute value of each component and output the results to the first maximum value detector 220 and the gate circuit 240. The first maximum value detector 220 is configured to detect the first maximum value (h max ) among N absolute values input therein and output it to the divider 250. More, it outputs a operation circuit specification signal specifying the absolute value operation section that have output the first maximum value to the gate circuit 240 and the weight controller 260. The gate circuit 240 is configured to gate only one absolute value that is the first maximum value (h max ) among absolute values of respective components input therein based on the operation circuit specification signal, and output (N-1) absolute values to the second maximum value detector 230. The maximum value detector 230 is configured to detect the largest value that is the second maximum value (h NEXT ) among (N-1) absolute values input therein, and output it to the divider 250. The divider 250 is configured to calculate a ratio of the first maximum value (h max ) and the second maximum value (h NEXT ) that is (h max /h NEXT ), and output the result to the weight controller 260. The weight controller 260 is configured to calculate each weight coefficient corresponding to each component of the channel impulse response h using the ratio (h max /h NEXT ) and the operation circuit specification signal, and output the results to the multipliers 270(1) to 270(N). The configuration of the weight controller 260 is explained later in detail. The multipliers 270(1) to 270(N) is configured to multiply each weight coefficient obtained in the weight controller 260 and each component of the channel impulse response h input through the input terminals 200(1) to 200(N), output the results to the output terminals 280(1) to 280(N). By the channel impulse response convertor 125 thus configured, each component of the channel impulse response h is multiplied by each weight coefficient and interference components are removed, thus a channel impulse response vector h' is calculated. Then, the channel impulse response vector h' is input to the equalizer 110. The equalizer 110 sets up internal parameters based on the channel impulse response vector h' input therein , and equalizes reception signals using the internal parameters. Next, the configuration of the weight controller 260 mentioned above is explained in detail. Fig.3 is a block diagram of the weight controller 260. In this figure, 300 and 310 are input terminals, 320 is a weight coefficient memory, 330 is a controller, 340(1) to 340(N) are selectors and 350(1) to 350(N) are output terminals. The input terminal 300 inputs the ratio (h max /h NEXT ) from the divider 250, the input terminal 310 inputs an operation circuit specification signal from the first maximum value detector 220. The weight coefficient memory 320 is configured to store a plurality of weight -coefficient and output these weight coefficients to the selectors 340(1) to 340(N). The controller 330 is configured to estimate interference components based on the ratio (h max /h NEXT ) input therein and output a control signal to the selectors 340(1) to 340(N) based on the operation circuit specification signal. The selectors 340(1) to 340(N) are configured to output, based on the control signal from the controller 330, only one weight coefficient among weight coefficients input therein. Next, the operation of the weight controller 260 thus configured is explained. The controller 330 estimates magnitude of interference components based on the ratio (h max /h NEXT ) input therein. That is, it decides that interference components are small when the ratio (h max /h NEXT ) is large, or it decides that interference components are large when the ratio (h max /h NEXT ) is small. On the other hand, it specifies, based on the operation circuit specification signal, a selector to be input the first maximum value (h max ). Then, the controller outputs a control signal that indicates the selector to output the maximum value among the weight coefficients input from the weight controller 330. Additionally, it outputs a control signal that indicates each selector to output the smaller value among the weight coefficients input from the weight controller 330 when the ratio (h max /h NEXT ) is large or the larger value among the weight coefficients input from the weight controller 330 when the ratio (h max /h NEXT ) is small. Thus, the respective weight coefficients corresponding to respective components of the channel impulse response h are output from the output terminals 350(1) to 350(N) and input to the multipliers 270(1) to 270(N). Next, another embodiment of the weight controller 260 is explained. Fig.4 is a block diagram showing another embodiment of the weight controller 260. In this figure, 400 and 410 are input terminals, 420 is a threshold value memory, 430 is a weight coefficient memory, 440 is a comparison circuit, 450 is a controller, 460(1) to 460(N) are selectors and 470(1) to 470(N) are output terminals. The input terminal 400 inputs the ratio (h max /h NEXT ) from the divider 250, the input terminal 410 inputs the operation circuit specification signal from the first maximum value detector 220. The threshold value memory 420 is configured to store a preset threshold value and output it to the comparison circuit 440. The weight coefficient memory 430 is configured to store 1 and 0 as weight coefficients, output them to the selectors 460(1) to 460(N). The comparison circuit 440 is configured to compare the threshold value and the ratio (h max /h NEXT ) input from the divider 250 through the input terminal 400, output the comparison result to the controller 450. The controller 450 is configured to output, based on the comparison result and the operation circuit specification signal from the first maximum value detector 220, a control signal to the selectors 460(1) to 460(N). Next, the operation of the weight controller 260 thus configured is explained. Firstly , the controller 450 compares a ratio (h max /h NEXT ) and a threshold value. Then, it decides that interference components are small when the ratio (h max /h NEXT ) is larger than the threshold value and outputs a control signal that indicates to output a weight coefficient not to convert each component of the channel impulse response h, that is 1, to the selectors 460(1) to 460(N). On the other hand, it decides that interference components are large when the ratio (h max /h NEXT ) input therein is smaller than the threshold value, outputs, based on the operation circuit specification signal, a control signal that indicates to output the weight coefficient 1 to a selector corresponding to the specified absolute operation and the weight coefficient 0 to the other selectors. The selectors 460(1) to 460(N) input the control signal, based on the control signal, outputs a weight coefficient corresponding to each component of the channel impulse response h to the output terminals 470(1) to 470(N). Then, the weight coefficients output from the weight controller 260 configured as above are input to the multipliers 270(1) to 270(N). Although the invention has been described, by way of example, with reference to particular embodiments, it will be appreciated that variations and modifications thereof, as well as other embodiments may be made within the scope of the appended claims.

Claims: An adaptive equalizer including a channel impulse response vector calculator (120) having an estimator (121) for estimating a channel impulse response vector h from received signals; and an equalizer (110) to an input of which the received signals are applied characterised in that there are provided means for removing interference components (125) from the estimated impulse response vector h to provide a channel impulse response vector h' , and in that the channel impulse response vector h' from which the interference components have been removed is connected to an input of the equalizer (110), and in that means (122-125) are provided for setting an internal parameter of the equalizer (110) using the channel impulse response vector h' , the received signals being equalized in the equalizer (110) on the basis of the internal parameter to produce an equalization result. An adaptive equalizer as claimed in claim 1, wherein the equalizer (110) is a maximum likelihood sequence estimator. An adaptive equalizer as claimed in either claim 1 or claim 2 wherein the means (122-125) for setting the internal parameter of the equalizer (110) includes a channel impulse response convertor (125) to which the channel impulse response vector h is input. An adaptive equalizer as claimed in claim 3, wherein the channel impulse response estimator (121) includes a transversal filter. An adaptive equalizer as claimed in claim 3, including a training signal detector (122) for detecting training signals from the received signals, a training sequence generating circuit (123) for storing an equalization result output corresponding to the training signals, and switching means (124) to which the equalization result is input, and from which the equalization result output corresponding to the training signals is output to the channel impulse response estimator (121) when training signals have been detected based on a detection signal from the training signal detector (122), an equalisation result of the received signals being output to the channel impulse response estimator (121) when training signals have not been detected. An adaptive equalizer as claimed in claim 3, wherein the channel impulse response convertor (125) includes a plurality of absolute value operation input means (200(1) to 200(N)) for inputting each component of the channel impulse response vector h , absolute value operation means (210(1) to 210(N)) for calculating and outputting an absolute value of each component, a first maximum value detector (220) for detecting and outputting a first maximum value among absolute values output from the plurality of the absolute value operation means (210(1) to 210(N)) and for specifying the absolute value operation means (210(1) to 210(N)) that has output the first maximum value, a gate (240) for inputting absolute values output from the plurality of absolute value operation means (210(1) to 210(N)) and passing absolute values except for the first maximum value based on a specification signal from the first maximum value detector (220), a second maximum value detector (230) for inputting the absolute values that have passed the gate (240) and for detecting and outputting a second maximum value from among the absolute values that have passed the gate (240), a divider (250) for inputting the first maximum value and the second maximum value and for calculating the ratio between the first maximum value and the second maximum value, a weight controller (260) for inputting the ratio calculated in the divider (250) and a specified signal from the first maximum value detector (220), and for outputting a weight coefficient corresponding to each component of the channel impulse response vector h , and a plurality of multiplier means (270(1) to 270(N)) for multiplying the weight coefficient that has been output from the weight controller (260) and each component of the channel impulse response vector h . An adaptive equalizer as claimed in claim 6, wherein the weight controller (260) includes a weight coefficient memory (430) for storing a plurality of weight coefficients, a selector (460(1) to 460(N)) for outputting only one weight coefficient among the plurality of weight coefficients, and a controller (450) for inputting a specified signal from the first maximum value detector (220) and the ratio output from the divider (250) and for outputting a control signal indicating a weight coefficient to be output to the selector (460(1) to 460(N)). An adaptive equalizer as claimed in claim 6, wherein the weight controller (260) includes a threshold value memory (420) for storing a threshold value, a weight coefficient memory (430) for storing two weight coefficients, a selector (460(1) to 460(N)) for outputting a weight coefficient stored in the weight coefficient memory (430), a comparison circuit (440) for comparing the threshold value and the ratio output from the divider (250) and outputting a comparison signal, and a controller (450) for inputting the comparison signal and a specified signal from the first maximum value detector (220) and outputting a control signal indicating a weight coefficient to be output to the selector (460(1) to 460(N)).

Inventors: OKANOUE KAZUHIRO, USHIROKAWA AKIHISA, OKANOUE, KAZUHIRO, USHIROKAWA, AKIHISA, OKANOUE, KAZUHIRO, C/O NEC CORPORATION, USHIROKAWA, AKIHISA, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: H04B7
IPC codes: H04L1, H04B1, H03H21, H03H17, H04B3, H03H15, H04B7