Method and apparatus for controlling tap coefficients of an adaptive matched filter in an automatic equaliser

Abstract: The automatic equalizer comprises a demodulator (101) for outputting a detection signal which indicates synchronous status as well as demodulating analog base band signals from received signals, an A/D converter (1) for outputting a first digital signal string by sample-quantizing the analog base band signals, an adaptive matched filter (201) for outputting a second digital signal string by converting the first digital signal string, a decision feedback equalizer (301) for outputting a decision signal by equalizing intersymbol interferences in the second digital signal string, a correlator (8-10) for correlating the decision signal with a signal on each tap of the adaptive matched filter, an integrator (11-13) for averaging outputs of the correlator, and a control means (401) for outputting each tap coefficient of the adaptive matched filter derived from multiplying an output of the integrator by a weighting coefficient based on the detection signal. This automatic equalizer has an excellent equalization characteristic to improve the pull-in operation of the control loop under fading.

Description: The present invention relates to an automatic equalizer used for a digital radio communication system, especially to an automatic equalizer comprising an adaptive matched filter and a decision feedback equalizer to remove an intersymbol interference due to radio channel fading and a method for controlling tap coefficients used therein. A conventional digital radio communication system has been so proposed to combine an adaptive matched filter (AMF) and a decision feedback equalizer (DFE) at a receiver side to compensate degradation of the channel quality due to frequency selective fading environment generated on a propagation path (See Paper No.B-929, Spring National Convention of Electronic Information Communication Society, 1989). Supposing that an amplitude ratio of a reflected (delayed) wave to a direct (preceding) wave is ρ, the DFE unity capable of equalizing two wave interference under fading functions in equalizing the intersymbol interference completely in case of ρ<1. While in case of ρ>1, its performance is deteriorated, implying that the equalization characteristic is degraded when the amplitude of the delayed wave becomes larger than that of the preceding wave (See Chapter 6 of "Digital radio communication" written by Muroya and Yamamoto, published by Sangyo Tosho). In order to improve the equalization characteristic in ρ>1, it has been proposed to place the AMF ahead of the DFE. Fig.4 is a block diagram of a conventional automatic equalizer as a first example of a prior art. A reference numeral 1 is an A/D converter. Reference numerals 2 and 3 are delay circuits which delay a symbol space by T. Reference numerals 4 to 6 are multipliers. Reference numeral 7 is an adder, 8 to 10 are correlators, 11 to 13 are integrators, 101 is a demodulator, 201 is a transversal filter, and 301 is a decision feedback equalizer (DFE). The adaptive matched filter of a T space 3 tap type is composed of the transversal filter 201, the correlators 8 to 10, and the integrators 11 to 13. The operation of the automatic equalizer of this type is hereinafter described. The demodulator 101 demodulates a received signal, and a demodulated signal is sample-quantized by the A/D converter 1. The sample-quantized digital signal string output from the A/D converter 1 is input to the transversal filter 201, then delayed by the delay circuits 2 and 3 sequentially. The input signals of the transversal filter 201 and output signals of the delay circuits 2 and 3 are respectively input to the multipliers 4 to 6 where they are multiplied by tap coefficients A -1 , A 0 , and A +1 , respectively. Outputs of the multipliers 4 to 6 are added through the adder 7. The output of the adder 7 is input to the DFE 301 where the intersymbol interference is equalized and decided. A decision signal 104 output from the DFE 301 is input to the correlators 8 to 10 where the decision signal 104, the input signal of the transversal filter 201 and output signals of the delay circuits 2 and 3 are correlated. The respective mean values of outputs of the correlators 8 to 10 are obtained through the integrators 11 to 13 to define new tap coefficients A -1 , A 0 , and A +1 of the transversal filter 201. Convolution of those tap coefficients and received signals provides outputs of the adaptive matched filter (AMF) corresponding to the impulse response on the propagation path. Fig.6 shows waveforms of the output response in case of ρ>1. Fig.7 shows waveforms of the output response in case of ρ<1. The adaptive matched filter (AMF) serves to disperse the interference wave component due to frequency selective fading generated on the propagation path ahead and behind the main wave component, respectively as shown by Figs. 6 and 7. In case of ρ>1, i.e., the amplitude of the delayed wave is larger than that of the preceding wave, the AMF enables the preceding wave as the interference component to be dispersed ahead and behind the main wave. With this mechanism, the interference wave component preceding the main wave that is difficult to be equalized by the DFE located behind the AMF is converted into that behind the main wave, thus the equalization characteristic of this case can be improved. In case of ρ<1, however, the construction of the first example of the conventional automatic equalizer generates the interference wave component preceding the main wave as Fig.7 shows. Accordingly the equalization characteristic is degraded compared with the construction using the DFE unity only. Aiming at overcoming the aforementioned degradation, another type of the equalizer has been proposed in JP-A-4 077 106. It is so constructed to operate the DFE unity only by suspending the AMF in case of ρ<1. While in case of ρ>1, the AMF and DFE are operated together. The operation of the AMF is suspended by fixing the tap coefficients of Fig.5 to initial values A 0 =1, A -1 =A +1 =0 (The DFE unit only). This mechanism is realized by adding a control circuit for controlling those tap coefficients to the construction shown by Fig.4. Fig.5 is a block diagram of the second example of the conventional automatic equalizer. The same construction items as those shown by Fig.4 have the same reference numerals, thus omitting their explanations. Referring to Fig.5, reference numerals 30 and 31 are multipliers, 32 is a control signal generator, and 402 is a control circuit. The control signal generator 32 monitors outputs of the integrators 11 to 13 which represent impulse responses on the propagation path to determine the status thereof. If the propagation path status is determined as p >1, the control signal generator 32 outputs 1 to the multipliers 15 and 16, respectively to operate the AMF and DFE in the same manner as described in the first example. While if determined as ρ<1, it gradually decreases the value output to the multipliers 30 and 31 to 0 so as to operate the DFE unity only. In this case, the AMF generates outputs without diverging the interference wave as Figs. 8 and 9 show. Adding the control circuit 402 provides higher equalization characteristic with the automatic equalizer compared with the first example on the propagation path status in case of both ρ>1 and ρ<1. When using this equalizer as the second example, in case the propagation path is determined as ρ<1 and the interference wave component becomes excessively large, it no longer performs its equalization characteristic appropriately, which causes output signals to be dispersed. In this case, the closer ρ approaches the value 1, the more difficult it becomes to perform pull-in operation, i.e., converging the location of output signals of the equalizer. If the propagation path is determined as ρ<1, the AMF is reset (suspended) and DFE unity is only operated. So the interference wave component is not dispersed by the AMF, requiring some DFE taps to have substantially a great value. When the propagation path status is in the vicinity of ρ=1, it may be further difficult to perform pull-in operation due to additional AMF reset control. In the first example of the prior art of Fig.4, the AMF disperses the interference wave component to require no great value of tap coefficient for the respective DFE taps even if the communication path status is in the vicinity of ρ=1, resulting in easy pull-in operation. In this case, however, as aforementioned, the equalization characteristic is degraded during pull-in status. EP-A-0 534 489 describes a matched filter receiver comprising a transversal filter having a tapped delay line for receiving an incoming digital signal to produce successively delayed signals at successive taps of the delay line, a plurality of tap-weight multipliers connected respectively to the successive taps of the delay line, and means for summing output signals of the tap-weight multipliers to produce a filter output, and supplying the filter output to a decision feedback equalizer. It is an object of the present invention to provide an automatic equalizer with excellent equalization characteristic to improve the pull-in operation of the control loop under fading for providing the solution to the aforementioned problem. This object is achieved with the features of the claims. The principle of the automatic equalizer of the present invention is described below. When the propagation path status is determined as ρ>1, the interference wave precedes the main wave, so the tap in the plus side (hereinafter referred to as "plus tap(s)") (more remotely from the input than the center tap) is correlated. At this time the tap in the minus side (hereinafter referred to as "minus tap(s)") (closer to the input than the center tap) is hardly correlated. When the propagation path status is determined as ρ<1, the interference wave is behind the main wave, so the minus tap among the AMF taps is correlated to increase the value of its tap coefficient. At this time the plus tap is hardly correlated. The operation of the AMF minus tap is reset when the control is loop pulled-in, i.e., the demodulator is in synchronous status. In this case, the interference wave component is symmetrically dispersed through the AMF plus tap in case of ρ>1. While in case of ρ<1, the AMF outputs almost the same signals as input signals. Similar to the case shown by Figs. 8 and 9, therefore, in case of ρ>1, the AMF improves the equalization characteristic, while in case of ρ<1, the same equalization characteristic as that of the DFE unity can be obtained. When the control loop is divergent, i.e., the modulator is in asynchronous status, all taps of the AMF are enabled to improve the pull-in property. Once the control loop has been pulled-in, the AMF minus tap coefficient is gradually decreased at a rate lower than the tracking rate of DFE control. The operation of the AMF is, thus, suspended to enter into normal equalization. The AMF minus tap becomes active only in the re-pulling-in process after the control loop has diverged. Then the AMF operation is suspended so as to facilitate pull-in operation for the automatic equalizer. This and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and drawings, in which: Fig.1 is a block diagram of an embodiment of an automatic equalizer of the present invention; Fig.2 is a block diagram of a weighting coefficient control circuit; Fig.3 is an explanatory view of an operation of the weighting coefficient control circuit; Fig.4 is a block diagram of a first example of a prior art; Fig.5 is a block diagram of a second example of a prior art; Fig.6 is an explanatory view of an operation of an adaptive matched filter; Fig.7 is an explanatory view of an operation of the adaptive matched filter; Fig.8 is an explanatory view of an operation of the adaptive matched filter; and Fig.9 is an explanatory view of an operation of the adaptive matched filter. Fig.1 is a block diagram of an embodiment of the present invention. In Fig.1, the same construction items as those shown by Fig.4 have the same reference numerals, thus omitting their explanations. As Fig.1 shows, taps of a transversal filter 201 are divided into a center tap A 0 , minus taps A -1 located at signal input side, and plus taps A +1 located at signal output side. A demodulator 101 in Fig.1 comprises a carrier reproduction section 102 and a detector 103. The carrier reproduction section 102 reproduces the carrier by means of decision signals and error signals output from the DFE 301. It also outputs a detection signal 105 indicating synchronous status of carrier reproduction. More specifically when the carrier reproduction is in synchronous status, the output detection signal 105 indicates synchronous status. On the contrary when it is in asynchronous due to divergence in the carrier reproduction loop, the detection signal 105 indicates the asynchronous status. The detector 103 demodulates the reproduced carrier output from the carrier reproduction section 102 and output an analog base band signal to A/D converter 1. A control circuit 401 comprises a weighting coefficient generator 14, a weighting coefficient control circuit 16, and a multiplier 17. The detection signal 105 output from the carrier reproduction section 102 is input to the weighting coefficient generator 14. In case the detection signal 105 indicates the synchronous status, it outputs the weighting coefficient 0. While in case the detection signal 105 indicates asynchronous status, it outputs the weighting coefficient 1. The weighting coefficient control circuit 16 controls weighting coefficients output from the weighting coefficient generator 14 based on the detection signal 105. Fig.2 is a block diagram of the weighting coefficient control circuit 16. Referring to Fig.2, a reference numeral 21 is a change amount set circuit, 22 and 23 are selectors, 24 is a flip-flop, 25 is a subtracter, and 26 is a detector. In the change amount set circuit 21, change amount of the weighting coefficient is so set that the rate at which the weighting coefficient decreases from 1 to 0 is lower than the tracking rate of the DFE 301 control accompanied with the change in the status of the demodulator 101 from asynchronous to synchronous. The selector 22 receives a weighting coefficient output from the weighting coefficient generator 14 and a feedback value from the subtracter 25. In case the detection signal 105 indicates the synchronous status, the feedback value from the subtracter 25 is selected to be output. In case it indicates asynchronous status, the weighting coefficient output from the weighting coefficient generator 14 is selected to be outpuc. The selector 23 receives a change amount output from the change amount set circuit 21 and a value 0. In case the detecation signal 105 indicates synchronous status, the change amount output from the change amount set circuit 21 is selected to be output. In case it indicates asynchronous status, the value 0 is selected to be output. The flip-flop 24 retains the value output from the selector 22, and outputs it upon receiving the next value. The subtracter 25 subtracts an output value of the selector 22 from that of the flip-flop 24. The detector 26 monitors the feedback value from the subtracter 25. In case the feedback value is 0, it outputs a detection signal to the selector 22 to cause the weighting coefficient generator 14 to output the weighting coefficient and outputs a detection signal to the selector 23 to output the value 0. The multiplier 17 multiplies an output of the correlator 11 by the output of the weighting coefficient control circuit 16 and output a result as a coefficient of the tap A -1 . An operation of the above-constructed automatic equalizer is explained. Assuming that the demodulator causes synchronization step out due to divergence of the carrier reproduction loop, the carrier reproduction section 102 outputs a detection signal 105 indicating asynchronous status to the weighting coefficient generator 14 and the weighting coefficient control circuit 16. Upon receiving the detection signal 105, the weighting coefficient generator 14 outputs the weighting coefficient 1. While, the selector 22 that receives the detection signal 105 selects the weighting coefficient 1 output from the weighting coefficient generator 14 to be output. The selector 23 selects 0 to be output to the subtracter 25. Then the subtracter 25 subtracts 0 from the weighting coefficient 1 that has been input via the flip-flop 24. It outputs the resultant weighting coefficient 1. The multiplier 17 multiplies the weighting coefficient 1 by an output of the correlator 11. The resultant value is output as the coefficient of the tap A -1 . When the detection signal 105 indicates asynchronous status, the weighting coefficient 1 is output to make the tap A -1 of the AMF active. Next, assuming that the demodulator currently in asynchronous status changes into the synchronous status, the carrier reproduction section 102 outputs a detection signal 105 indicating synchronous status to the weighting coefficient generator 14 and the weighting coefficient control circuit 16. Upon receiving the detection signal 105, the weighting coefficient generator 14 outputs the weighting coefficient 0. While, the selector 22 that has received the detection signal 105 selects the feedback value from the subtracter 25 (Since the flip-flop 24 retains the value 1, the initial value is derived form subtracting the change amount from 1, i.e., 1 - change amount.), and outputs it. The selector 23 selects the change amount output from the change amount set circuit 21 to be output to the subtracter 25. The subtracter 25 subtracts the change amount from the input value via the flip-flop 24. That is, the output value decreases by the change amount at every subtraction. The rate required for the change is lower than the tracking rate of the DFE 301 control. Fig.3 shows how this change has been done. The multiplier 17 multiplies the output value of the correlator 11 by the weighting coefficient to be output as the coefficient of the tap A -1 . When the feedback value from the subtracter 25 reaches 0, the detector 26 outputs detection signals to the selectors 22 and 23. The selector 22 selects the output of the weighting coefficient generator 14, i.e., 1, and the selector 23 selects 0 to be output, respectively. Then, the subtracter 25 subtracts the value 0 that is output by the weighting coefficient generator 14 from the value 0 that is output by the selector 23, i.e. "0". When indication of the detection signal 105 changes into synchronous status from asynchronous status, the weighting coefficient is decreased from 1 to 0 at a rate lower than the tracking rate of the DFE 301 control, the operation of tap A-1 of the AMF is stopped. With the above mechanism, the equalization characteristic is improved with the effect of the AMF tap A +1 in case of ρ>1. In addition almost the same equalization characteristic as that of the DFE unity can be obtained in case of ρ<1. When the control loop is in divergence, pull-in property is improved by operating all taps of the AMF. Once the control loop has been pulled-in, the tap coefficient of the AMF tap A -1 is gradually decreased at a rate lower than the tracking rate of the DFE control. It is further decreased to 0, i.e., the AMF is suspended, thus entering normal equalization operation. The above explanation relates to a transversal filter of 3 tap type as a simple example. However, it is obvious to be able to form a transversal filter using a plurality of taps. That is, the present invention is realized by controlling all the minus taps to the center tap using an asynchronous detection signal from the demodulator in the same manner as described with respect to this embodiment. According to the present invention as described above, all taps of the AMF are operated while the control loop is pulled-in. Once the control loop has been pulled-in, a tap coefficient for the minus tap of the AMF is gradually decreased at a rate lower than the tracking rate of the DFE control to stop operation of the AMF. The above construction realizes the automatic equalizer with excellent equalization characteristic and improved pull-in property for the control loop under fading on the propagation path status either in ρ>1 or ρ<1.

Claims: An automatic equalizer comprising: a demodulator (101) for outputting a detection signal which indicates synchronous status as well as demodulating analog base band signals from received signals; an analog to digital converter (1) for converting said analog base band signals to a first digital signal string by sample-quantizing said analog base band signals; an adaptive matched filter (201) for converting said first digital signal string to a second digital signal string; a decision feedback equalizer(301) for equalizing intersymbol interferences in said second digital signal string and outputting a decision signal as an equalized result; a plurality of correlators (8 to 10), each of said correlators for correlating said decision signal with a signal on each tap of said adaptive matched filter, and a plurality of integrators (11 to 13), each of said integrators for averaging outputs of said correlators, and a control means (401) for multiplying outputs of said integrators by a weighting coefficient based on said detection signal and outputting a result for each tap coefficients of said adaptive matched filter, the automatic equalizer being characterized in that said control means (401) is adapted to: multiply outputs of said integrators corresponding to minus taps locating in an input side from a center, tap of said adaptive matched filter by a weighting coefficient 0 when said detection signal indicates synchronous status; multiply outputs of said integrators corresponding to said minus taps by a weighting coefficient 1 when said detection signal indicates asynchronous status, and gradually decrease said weighting coefficient from 1 to 0 at a rate lower than the tracking rate of said decision feedback equalizer when indication of said detection signal changes from asynchronous status to synchronous status, wherein said control means (401) comprises a first selector (22) and a second selector (23), each of them for switching output based on said detection signal, a flip-flop (24) connected with said first selector, and a subtracter (25) for subtracting an output of said second selector from an output of said flip-flop, said first selector (22) inputs an output of said subtracter and said weighting coefficient, selects said weighting coefficients when said detection signal indicates synchronous status, and selects an output of said subtracter when said detection signal indicates asynchronous status; and said second selector (23) inputs a tap coefficient change amount and 0, selects 0 when said detection signal indicates synchronous status, and selects said tap coefficient change amount when said detection signal indicates asynchronous status. The automatic equalizer of claim 1, wherein said adaptive matched filter (201) comprises a transversal filter. The automatic equalizer of claim 1 or 2, wherein said.control means (401) comprises: a plurality of multipliers (17), each of said multipliers for multiplying outputs of said integrators corresponding to said minus taps by said weighting coefficient; and a weighting coefficient generator (14) for outputting a weighting coefficient 0 to said multipliers when said detection signal indicates synchronous status and for outputting a weighting coefficient 1 to said multipliers when said detection signal indicates asynchronous status. The automatic equalizer of claim 1, 2 or 3, wherein said adaptive matched filter (201) comprises a transversal filter having a center tap, a plurality of minus taps locating in an input side from said center tap and a plurality of plus taps locating in an output side from said center tap. The automatic equalizer of claim 1, 2, 3 or 4, further comprising means for setting said tap coefficient change amount. A method for controlling tap coefficients of an automatic equalizer with the following steps: (a) demodulating analog base band signals front received signals using a demodulator and detecting synchronous status of carrier reproduction; (b) converting said analog base band signal string to a first digital signal string by sample-quantizing said analog base band signals using an analog to digital converter; (c) converting said first digital signal string to a second digital signal string by converting said first digital signal string using an adaptive matched filter; (d) outputting a decision signal by equalizing intersymbol interferences in said second digital signal string using a decision feedback equalizer; (e) correlating said decision signal with a signal on each tap of said adaptive matched filter using a plurality of correlators; (f) averaging outputs of said correlators using a plurality of integrators; said method being characterized by : (g) multiplying outputs of said integrators corresponding to minus taps locating in an input side from a center tap of said adaptive matched filter by a weighting coefficient 0 when said carrier reproduction is in synchronous status, and multiplying outputs of said integrators corresponding to said minus taps by a weighting coefficient 1 when said carrier reproduction is in asynchronous status, wherein said weighting coefficient is gradually decreased from 1 to 0 at a rate lower than the tracking rate of said decision feedback equalizer when the status of said carrier reproduction changes from asynchronous status to synchronous status, (h) switching an output based on said carrier reproduction using a first selector and a second selector, (i) subtracting an output of said second selector from an output of a flip-flop connected with said first selector using a subtracter, (j) inputting an output of said subtracter and said weighting coefficient into the first selector, selecting said weighting coefficient when said carrier reproduction is in synchronous status, and selecting an output of said subtracter when said carrier reproduction is in asynchronous status using said first selector; and (k) inputting a tap coefficient change amount and 0 into the second selector, selecting 0 when said carrier reproduction is in synchronous status, and selecting said tap coefficient change amount when said carrier reproduction is in asynchronous status using said second selector.

Inventors: YAMAMOTO TAKESHI, YAMAMOTO, TAKESHI, YAMAMOTO, TAKESHI, C/O NEC CORP.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H04B7
IPC codes: H04L25