Method and apparatus capable of quickly identifying an unknown system even on occurrence of a plurality of dispersive portions

• US 5,455,819 A
• Filed: 03/07/1994
• Issued: 10/03/1995
• Est. Priority Date: 03/05/1993
• Status: Expired due to Fees

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Sugiyama, Akihiko
• Primary Examiner(s): MARCELO, MELVIN C
• Application Number:
  US08/206,657
• Time in Patent Office: 575 Days
• Field of Search: 370/32, 370/32.1, 370/13, 370/17, 375/14, 375/232, 379/410, 379/411, 379/3
• US Class Current: 370/241
• CPC Class Codes: H04B 3/23 using a replica of transmitted signal in the time domain, e.g. echo cancellers
• CPC Class Codes: H04L 25/0202 Channel estimation

Abstract

In an identification apparatus for identifying an unknown system in response to a transmission input signal and an echo signal which includes multiple echoes sent through the unknown system, a plurality of adaptive filters each of which has a plurality of taps are supplied with the transmission signal in parallel to individually identify the multiple echoes by successively modifying tap coefficients in each adaptive filter. Information is transmitted from a j-th adaptive filter to a (j+1)-th adaptive filter to identify each of the multiple echoes. Each adaptive filter monitors modification times of tap coefficients or ERLE. Alternatively, a step size is changed from one to another to be delivered to tap coefficient generators and to accomplish high speed convergence.

Claims

1. A method of identifying an unknown system by the use of an adaptive filter having a plurality of taps assigned with consecutive tap numbers divided into a plurality of groups, said adaptive filter being supplied with a transmission input signal and a reception input signal which is sent through said unknown system and which exhibits an impulse response resulting from said unknown system, said impulse response being specified by a plurality of dispersive portions and a plurality of flat delay portions, the method comprising the steps of:

preparing first through K-th subadaptive filters which is obtained by dividing said adaptive filter and each of which has the taps having group tap numbers of each group; and

successively identifying at least one of the dispersive and the flat delay portions in said impulse response by successively controlling each of the first through the K-th subadaptive filters.

2. A method as claimed in claim 1, wherein said identifying step comprises the steps of:

selecting active ones of the taps in each group to determine active and inactive ones of the group tap numbers in each group;

calculating tap coefficients only in connection with the active taps of each group;

memorizing the active ones of the group tap numbers in each group with the inactive ones of the group tap numbers memorized in the form of a queue in each group;

modifying the tap coefficients in connection with the active taps of each group to predetermined times;

detecting a minimum absolute value of the tap coefficients in connection with the active taps of each group each time when the tap coefficients are modified the predetermined times, so as to render the tap of said minimum absolute value into an inactive tap having a new inactive tap number; and

successively substituting a new one of the active tap numbers for the new inactive tap number to change the active tap numbers from one to another and to adaptively control positions of the active taps in each of the subadaptive filters so that at least one of the dispersive and the flat delay portions is identified in each of the subadaptive filters.

3. A method as claimed in claim 2, wherein said modifying step comprises the steps of:

modifying the tap coefficients of the active taps in each group by the use of a first step size until the tap coefficients are modified a first predetermined time; and

thereafter modifying the tap coefficients of the active taps in each group by the use of a second step size greater than said first step size a second predetermined time, said predetermined times being equal to a sum of said first and said second predetermined times.

4. A method as claimed in claim 1, wherein the identifying step comprises the steps of:

detecting a maximum absolute value of the tap coefficients of the active taps in each group to obtain a specific tap number corresponding to the maximum absolute value; and

transmitting said specific tap number from an i-th one of the subadaptive filters to an (i+1)-th one of the subadaptive filters where i is not smaller than unity and is not greater than K-1.

5. A method as claimed in claim 4, wherein the identifying step further comprises the steps of:

detecting a minimum absolute value of the tap coefficients in connection with the active taps of each group each time when the tap coefficients are modified the predetermined times, so as to render the tap of said minimum absolute value into an inactive tap having a new inactive tap number;

modifying the tap coefficients in connection with the active taps of each group to predetermined times with reference to the specific tap number and the new inactive tap number; and

successively substituting a new one of the active tap numbers for the new inactive tap number to change the active tap numbers from one to another and to adaptively control positions of the active taps in each of the subadaptive filters so that at least one of the dispersive and the flat delay portions is identified in each of the subadaptive filters.

6. A method of identifying an unknown system by the use of an adaptive filter having a plurality of taps assigned with consecutive tap numbers, said adaptive filter being supplied with a transmission input signal and a reception input signal which is sent through said unknown system and which exhibits an impulse response resulting from said unknown system, said impulse response being specified by a plurality of dispersive portions and a plurality of flat delay portions, the method comprising the steps of:

modifying tap coefficients of the taps by the use of a first step size until said tap coefficients are modified to obtain primary modified tap coefficients;

thereafter modifying the primary modified tap coefficients by the use of a second step size greater than said first step size to obtain secondary modified tap coefficients; and

successively identifying at least one of the dispersive and the flat delay portions in said impulse response by using the primary and the secondary modified tap coefficients.

7. A method as claimed in claim 6, further comprising the steps of:

counting modification times of the primary modified tap coefficients as count values to a predetermined count; and

switching the step sizes from the first step size to the second step size when said count values become equal to said predetermined count.

8. A method as claimed in claim 6, further comprising the steps of:

monitoring an echo return loss enhancement (ERLE) calculated in connection with said impulse response;

comparing said echo return loss enhancement with a preselected threshold level when the echo return loss enhancement reaches the preselected threshold level; and

switching the step sizes from the first step size to the second step size when the echo return loss enhancement reaches the preselected threshold level.

9. An identification apparatus for use in identifying an unknown system by responding to a transmission input signal and a reception input signal which is sent through said unknown system and which includes an impulse response of an echo signal specified by a plurality of dispersive portions and a plurality of flat delay portions, said identification apparatus comprising:

first through K-th adaptive filters which are supplied with said transmission input signal in parallel and a feedback signal and each of which identifies at least one of the dispersive portions to produce first through K-th identification signals representative of results of identification in the first through the K-th adaptive filters;

an adder circuit supplied with said first through said K-th identification signals for summing up said first through said K-th identification signals to produce an echo replica signal representative of a replica of said echo signal;

a subtracter for subtracting said echo replica signal from said echo signal to produce an error signal; and

means for supplying said first through said K-th adaptive filters with said error signal as said feedback signal.

10. An identification apparatus as claimed in claim 9, wherein each of said first through said K-th adaptive filters comprises:

a delay circuit, which defines a first predetermined number of taps and which is supplied with said transmission input signal, for successively delaying said transmission input signal to produce successively delayed signals through the taps, respectively, said taps being assigned with consecutive tap numbers of a group;

a path switch supplied with a path switching control signal and connected to said taps for selecting the taps of a second predetermined number smaller than said first predetermined number to selectively produce the successively delayed signals of the second predetermined number;

coefficient calculation means of the second predetermined number connected to said path switch and supplied with the second predetermined number of the successively delayed signals and said error signal for calculating tap coefficients of the second predetermined number;

production means connected to said path switch and said coefficient calculation means for producing each of the first through the K-th identification signals; and

a tap controller connected to said path switch and said coefficient calculation means for supplying said path switch and said coefficient calculation means with a path switch control signal and a coefficient control signal to make said path switch switch the taps from one to another in response to said path switch control signal and to make the coefficient calculation means modify the tap coefficients in accordance with said coefficient control signal, respectively.

11. An identification apparatus as claimed in claim 10, each of said coefficient calculation means modifying the tap coefficients in response to said coefficient control signal which defines a step size, wherein said tap controller comprises:

a counter for counting modification times of the tap coefficients to a predetermined count to produce a count signal when said modification times reach said predetermined count; and

a step size switching circuit, which has a first step size and a second step size greater than said first step size and which is connected to said counter, for switching the step size from said first step size to said second step size in response to said count signal; and

delivery means for selectively delivering said first and said second step sizes as said coefficient control signal to said coefficient calculation means.

12. An identification apparatus as claimed in claim 11, wherein each of said coefficient calculation means comprises:

a first multiplier for multiplying each of the successively delayed signals by said error signal to produce a first product signal representative of a product of each of the successively delayed signals and said error signal;

a second multiplier for multiplying said first product signal by said coefficient control signal to produce a second product signal representative of a product of said first product signal and said coefficient control signal;

an adder circuit for summing up said second product signal and an additional signal to produce a sum signal representative of a sum of the second product signal and said additional signal; and

a memory circuit for memorizing said sum signal as each of the tap coefficients to feed each of the tap coefficients back to said adder circuit as said additional signal.

13. An identification apparatus as claimed in claim 11, wherein said tap controller of each of said first through said K-th adaptive filters further comprises:

a memory section for memorizing the tap numbers as inactive tap numbers assigned to the inactive taps;

a judging circuit coupled to said memory section for judging whether or not each tap number sent from said memory section satisfies a predetermined judgement condition to feed each tap number back to said memory section when said predetermined judgement condition is satisfied and, otherwise, to discard each tap number;

a tap number calculation circuit coupled to said memory section for supplying the tap numbers of the active taps as the active tap numbers to said path switch as the path switch control signal by removing the inactive tap numbers memorized in said memory section; and

a minimum coefficient calculation circuit supplied with said active tap numbers and the tap coefficients for obtaining a particular tap number of one of the active taps having a minimum absolute value of the tap coefficients to supply the particular tap number to said memory section.

14. An identification apparatus as claimed in claim 13, wherein said tap controller further comprises:

a maximum coefficient calculation circuit supplied with said active tap numbers and the tap coefficients for obtaining a specific tap number of another one of the active taps having a maximum absolute values of the tap coefficients to produce said specific tap number;

a control tap range calculation circuit connected to said maximum coefficient calculation circuit for calculating, as said predetermined judgement condition, a control tap range which defines a range of the active taps; and

means for delivering said predetermined judgement condition to said judging circuit.

15. An identification apparatus as claimed in claim 13, wherein said tap controller further comprises:

a combining circuit for combining each tap number fed back to said memory section and said particular tap number to produce a combined tap number; and

an estimation circuit connected to said control tap range calculation circuit and said counter for estimating whether or not the combined tap number satisfies said predetermined judgement condition;

said memory section comprising;

a first memory for memorizing the inactive tap numbers of the inactive taps adjacent to the specific tap number; and

a second memory for memorizing the inactive tap numbers of the inactive taps which are not adjacent to the specific tap number.

16. An identification apparatus as claimed in claim 9, wherein each of said first through said K-th adaptive filters is further supplied with said echo signal.

17. An identification apparatus as claimed in claim 16, wherein each of said first through said K-th adaptive filters comprises:

a delay circuit, which defines a first predetermined number of taps and which is supplied with said transmission input signal, for successively delaying said transmission input signal to produce successively delayed signals through the taps, respectively, said taps being assigned with consecutive tap numbers of a group;

a path switch supplied with a path switching control signal and connected to said taps for selecting the taps of a second predetermined number smaller than said first predetermined number to selectively produce the successively delayed signals of the second predetermined number;

coefficient calculation means of the second predetermined number connected to said path switch and supplied with the second predetermined number of the successively delayed signals and said error signal for calculating tap coefficients of the second predetermined number;

production means connected to said path switch and said coefficient calculation means for producing each of the first through the K-th identification signals; and

a tap controller connected to said path switch and said coefficient calculation means and supplied with said echo signal for supplying said path switch and said coefficient calculation means with a path switch control signal, a coefficient control signal, and said echo signal to make said path switch switch the taps from one to another in response to said path switch control signal dependent on said echo signal and to make the coefficient calculation means modify the tap coefficients in accordance with said coefficient control signal, respectively.

18. An identification apparatus as claimed in claim 17, wherein said tap controller comprises:

an echo return loss enhancement estimator, which has a predetermined estimator threshold level and which is supplied with said echo signal and said error signal for estimating an echo return loss enhancement with reference to said echo signal and said error signal to compare said echo return loss enhancement with said predetermined estimator threshold level and to produce a switching signal when said echo return loss enhancement exceeds said predetermined estimator threshold level; and

a step size switching circuit, which has a first step size and a second step size greater than said first step size and which is connected to said counter, for switching the step size from the first step size to the second step size in response to said switching signal; and

delivery means for selectively delivering said first and said second step sizes as said coefficient control signal to said coefficient calculation means.

19. An identification apparatus as claimed in claim 18, wherein each of said coefficient calculation means comprises:

a first multiplier for multiplying each of the successively delayed signals by said error signal to produce a first product signal representative of each of the successively delayed signals and said error signal;

a second multiplier for multiplying said first product signal by said coefficient control signal to produce a second product signal representative of a product of said first product signal and said coefficient control signal;

an adder circuit for summing up said second product signal and an additional signal to produce a sum signal representative of a sum of the second product signal and said additional signal; and

a memory circuit for memorizing said sum signal as each of the tap coefficients to feed each of the tap coefficients back to said adder circuit as said additional signal.

20. An identification apparatus as claimed in claim 18, wherein said tap controller of each of said first through said K-th adaptive filters further comprises:

a memory section for memorizing the tap numbers as inactive tap numbers assigned to the inactive taps;

a judging circuit coupled to said memory section for judging whether or not each tap number sent from said memory section satisfies a predetermined judgement condition to feed each tap number back to said memory section when said predetermined judgement condition is satisfied and, otherwise, to discard each tap number;

a tap number calculation circuit coupled to said memory section for supplying the tap numbers of the active taps as the active tap numbers to said path switch as the path switch control signal by removing the inactive tap numbers memorized in said memory section; and

a minimum coefficient calculation circuit supplied with said active tap numbers and the tap coefficients for obtaining a particular tap number of one of the active taps having a minimum absolute value of the tap coefficients to supply the particular tap number to said memory section.

21. An identification apparatus as claimed in claim 20, wherein said tap controller further comprises:

a maximum coefficient calculation circuit supplied with said active tap numbers and the tap coefficients for obtaining a specific tap number of another one of the active taps having a maximum absolute values of the tap coefficients to produce said specific tap number;

a control tap range calculation circuit connected to said maximum coefficient calculation circuit for calculating, as said predetermined judgement condition, a control tap range which defines a range of the active taps; and

means for delivering said predetermined judgement condition to said judging circuit.

22. An identification apparatus for use in identifying an unknown system by responding to a transmission input signal and a reception input signal which is sent through said unknown system and which includes an impulse response of an echo signal to produce an identification signal representative of a result of identification, said identification apparatus comprising:

a delay circuit, which defines a first predetermined number of taps and which is supplied with said transmission input signal, for successively delaying said transmission input signal to produce successively delayed signals through the taps, respectively, said taps being assigned with consecutive tap numbers;

a path switch supplied with a path switching control signal and connected to said taps for selecting the taps of a second predetermined number smaller than said first predetermined number to selectively produce the successively delayed signals of the second predetermined number;

coefficient calculation means of the second predetermined number connected to said path switch and supplied with the second predetermined number of the successively delayed signals and said error signal for calculating tap coefficients of the second predetermined number;

production means connected to said path switch and said coefficient calculation means for producing the identification signals; and

a tap controller connected to said path switch and said coefficient calculation means for supplying said path switch and said coefficient calculation means with a path switch control signal and a coefficient control signal to make said path switch switch the taps from one to another in response to said path switch control signal and to make the coefficient calculation means modify the tap coefficients in accordance with said coefficient control signal, respectively;

said tap controller comprising;

coefficient control signal producing means for selectively producing the coefficient control signal which represent a selected one of a first step size and a second step size greater than said first step size.

23. An identification apparatus as claimed in claim 22, wherein said coefficient control signal producing means comprises:

a counter for counting modification times of the tap coefficients to a predetermined count to produce a count signal when said modification times reach said predetermined count; and

a step size switching circuit, which has a first step size and a second step size greater than said first step size and which is connected to said counter, for switching the step size from said first step size to said second step size in response to said count signal; and

delivery means for selectively delivering said first and said second step sizes as said coefficient control signal to said coefficient calculation means.

24. An identification apparatus as claimed in claim 22, wherein said coefficient control signal producing means comprises:

an echo return loss enhancement estimator supplied with said error signal and said echo signal and said tap coefficients for estimating an echo return loss enhancement from said error signal and said echo signal to produce said coefficient control signal representative of either of said first and said second step sizes;

a step size switching circuit, which has a first step size and a second step size greater than said first step size and which is connected to said echo return loss enhancement estimator, for switching the step size from said first step size to said second step size in response to said count signal; and

delivery means for selectively delivering said first and said second step sizes as said coefficient control signal to said coefficient calculation means.

25. A method of identifying an unknown system by the use of an adaptive filter having a plurality of taps which are assigned with consecutively tap numbers and which are divisible into active and inactive taps, said method comprising the steps of:

classifying said taps into a plurality of tap control subgroups;

preparing a plurality of queue memories which are in one-to-one correspondence to said tap control subgroups and which are for memorizing, in the form of queues, inactive tap numbers of the inactive taps which belong to the tap control subgroups, respectively;

selecting, as a selected tap control subgroup, one of the tap control subgroups that includes the inactive tap numbers memorized in the corresponding queue memory; and

successively modifying tap coefficients of the selected tap control subgroup by the use of a step size determined by active tap information related to each of tap coefficients assigned to the active taps.

26. An identification apparatus for use in identifying an unknown system by responding to a transmission input signal and a reception input signal which is sent through said unknown system and which includes an impulse response of an echo signal, said identification apparatus producing an identification signal representative of a result of identification and comprising:

a delay circuit, which defines a first predetermined number of taps divided into a plurality of subgroups and which is supplied with said transmission input signal, for successively delaying said transmission input signal to produce successively delayed signals through the taps, respectively, said taps being assigned with consecutive tap numbers and classified into the subgroups each of which includes active and inactive taps;

a path switch supplied with a path switching control signal and connected to said taps for selecting the taps of a second predetermined number smaller than said first predetermined number to selectively produce the successively delayed signals of the second predetermined number;

coefficient calculation means of the second predetermined number connected to said path switch and supplied with the second predetermined number of the successively delayed signals and said error signal for calculating tap coefficients of the second predetermined number;

production means connected to said path switch and said coefficient calculation means for producing the identification signal; and

a tap controller connected to said path switch and said coefficient calculation means for supplying said path switch and said coefficient calculation means with a path switch control signal and a coefficient control signal to make said path switch switch the taps from one to another in response to said path switch control signal and to make the coefficient calculation means modify the tap coefficients in accordance with said coefficient control signal, respectively;

said tap controller comprising;

monitoring means for monitoring active tap information determined by the tap coefficients of the active taps in each subgroup;

varying means coupled to said monitoring means for varying a step size with reference to said active tap information; and

supplying means for supplying said step size to the coefficient calculation means to modify the tap coefficients.

27. An identification apparatus as claimed in claim 26, wherein said active tap information is specified by a maximum absolute value of the tap coefficients sent from said coefficient calculation means.

28. An identification apparatus as claimed in claim 26, wherein said active tap information is specified by a total sum of absolute values of the tap coefficients in each subgroup.

29. An identification apparatus as claimed in claim 26, wherein said active tap information is specified by a total sum of coefficient square values of the tap coefficients in each subgroup.