WO1992014323A1 - Method for determining the optimum sampling point for three-level signals - Google Patents

Method for determining the optimum sampling point for three-level signals Download PDF

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Publication number
WO1992014323A1
WO1992014323A1 PCT/GB1992/000197 GB9200197W WO9214323A1 WO 1992014323 A1 WO1992014323 A1 WO 1992014323A1 GB 9200197 W GB9200197 W GB 9200197W WO 9214323 A1 WO9214323 A1 WO 9214323A1
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WO
WIPO (PCT)
Prior art keywords
value
sampling point
cycle
values
demodulated signal
Prior art date
Application number
PCT/GB1992/000197
Other languages
French (fr)
Inventor
Martin Robert Evans
Original Assignee
Cognito Group Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Cognito Group Limited filed Critical Cognito Group Limited
Publication of WO1992014323A1 publication Critical patent/WO1992014323A1/en

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/06—Dc level restoring means; Bias distortion correction ; Decision circuits providing symbol by symbol detection
    • H04L25/068—Dc level restoring means; Bias distortion correction ; Decision circuits providing symbol by symbol detection by sampling faster than the nominal bit rate
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L7/00—Arrangements for synchronising receiver with transmitter
    • H04L7/02—Speed or phase control by the received code signals, the signals containing no special synchronisation information
    • H04L7/033—Speed or phase control by the received code signals, the signals containing no special synchronisation information using the transitions of the received signal to control the phase of the synchronising-signal-generating means, e.g. using a phase-locked loop
    • H04L7/0334—Processing of samples having at least three levels, e.g. soft decisions

Definitions

  • This invention relates to a method and apparatus for extracting data from a demodulated signal.
  • Eye patterns have therefore been used to monitor the performance of cable-based data transmission systems in which line attenuation is a problem that makes it necessary to employ signal compensation techniques.
  • An object of the present invention is to provide an improved method and apparatus for extracting data from a demodulated signal.
  • the value of a demodulated signal is sampled over successive cycles at successive predetermined intervals of time within each cycle to obtain sample values, the sample values so obtained are processed to determine an optimum sampling point for each cycle and threshold values to be applied at the sampling point to determine a decoded value for that cycle, and the value of the demodulated signal at the optimum sampling point is compared with the threshold values to determine a decoded value thereof.
  • a method comprises the following steps of sampling the value of the demodulated signal at successive predetermined intervals of time (phases) over successive cycles of the demodulated signal; determining for each sample value whether or not it corresponds to a zero value of the demodulated signal (a cross-over point); recording the occurrence of each cross-over point against the phase in which it occurs in a cycle so as to build up a histogram of cross-over points; selecting from the histogram the phase of a maximum number of cross-over points (the sampling point); recording the occurrence of successive sample values over successive cycles of the demodulated signal in an array of sample values against the phase of a cycle so as to build up the data equivalent to an eye-pattern; determining from the distribution of the recorded occurrences of sample values at the sampling point, occurrence minima (selection thresholds) both sides of the zero value; determining whether the sample value at the sampling point in successive cycles has a value within or outside the selection thresholds and allocating a corresponding decoded value to
  • This method therefore involves two processes which are preferably conducted simultaneously, one being to generate a histogram of the cross-over points from which a sampling point is determined, and the second being to generate a statistical eye-pattern from which the selection thresholds at the sampling point are determined for use in binary decoding.
  • the sampling point on the histogram is selected as that phase at which there is a maximum number of cross-over points, although irregularities in the shape of the histogram may lead to the median value being used rather than a simple maximum value.
  • Figure 1 shows an eye-pattern such as would be produced by a demodulated input signal as displayed on an oscilloscope
  • Figure 2 shows a histogram of cross-over points such as might correspond to the eye-pattern of Figure 1,
  • Figure 3 shows another histogram of a more irregular shape than that of Figure 2
  • Figure 4 shows a section through a statistical model of an eye-pattern on the line V-V in Figure 1
  • Figure 5 shows a schematic diagram of a system operating according to the invention.
  • the typical eye-pattern for a GMSK signal is shown in Figure 1, the pattern being produced by overlaying successive cycles of the signal on an oscilloscope.
  • the optimum point for binary decoding of the signal is selected as that point P in the cycle at which there is a maximum number of zero cross-overs.
  • point P there are maximum input values M both sides of zero which can be taken to represent a binary "1" value.
  • Threshold values T are selected between t v ase maxima M and zero in order to distinguish between binary "0" and binary "1" values, input values within the thresholds T being decoded as binary "0" and input values outside the thresholds T being decoded as binary "1".
  • the signal cycle is divided into sixteen phases and the input signal is analysed for zero cross-overs during successive phases over successive cycles so as to build up a record of the occurrence of zero cross-overs throughout these sixteen phases.
  • Zero cross-overs are detected by setting limits each side of the zero and comparing the value of the signal with these limits.
  • the signal is an oversampled signal so that comparison of sample values occurs once for each phase for each bit period. If a sampled value falls within the limits, it is taken as indicative of a zero cross-over, although more sophisticated tests can be employed for example to determine whether the signal actually passes through both limits.
  • a modulo sixteen address counter 6 operates in phase with the input signal and generates a corresponding address to identify each phase of the input cycle. If a cross-over is detected during a particular phase, the phase address generated by counter 6 is used to identify a corresponding cross-over count in a memory 4 which is read into a counter 5. Counter 5 is clocked to add one to the cross-over count, and the count is then written back into the memory 4.
  • the record of the accumulated cross-over counts for all sixteen phases represents a histogram such as shown in Figure 2.
  • the phase in which the maximum cross-over count occurs can be determined as the simple maximum.
  • the histogram may not be regular in shape and may possess more than one peak, as shown in Figure 3.
  • the median of the histogram is used, this being the phase at which the area of the histogram is bisected.
  • the median is determined by software using the following algorithm: Let the histogram be H[i]
  • A A - H[P] Optimum phase is P.
  • the sampled value of the input signal and its phase are used to progressively build up a statistical model of an eye-pattern.
  • the sampled value passes through a buffer 7 and is combined with the phase address from counter 6 and is used to read a corresponding count out of memory 4 into the counter 5. This . count is incremented by one and written back into the memory 4. This process is repeated for successive combinations of sampled value and phase over successive cycles of the input signal.
  • the associated data in memory 4 are analysed to determine the thresholds T.
  • the counts of the sampled values at the selected optimum phase P represent a section through the eye-pattern along the line V-V in Figure 1, and are shown in Figure 4.
  • Software is provided to identify the minima between the central maximum at zero and the two outer maxima M. The value of these minima correspond to the thresholds T which are then used for binary decoding of the input data.
  • the input data are passed through a buffer 8 and written into the memory 4 so that once the histogram has been produced to establish the sampling point P, and the model of the eye-pattern has been produced to establish the thresholds T, these input data can be decoded into binary data by comparison with the thresholds.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Power Engineering (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

Abstract

Data is extracted from a demodulated signal by sampling the signal in successive phases over successive cycles; determining whether each sample value corresponds to a zero cross-over point; recording the occurrence of each cross-over point against the phase in which it occurs in a cycle so as to build up a histogram of cross-over points; selecting a sampling point P from the histogram as the phase of a maximum number of cross-over points; recording the occurrence of successive sample values over successive cycles of the signal in an array of samples values against the phase of a cycle so as to build up the data equivalent to an eye-pattern; determining selection thresholds T both sides of the zero value as minima in the distribution of the recorded occurrences of sample values at the sampling point; and determining whether the sample value at the sampling point P in successive cycles has a value within or outside the selection thresholds T and allocating a corresponding decoded value to that cycle of the signal.

Description

METHOD FOR DETERMINING THE OPTIMUM SAMPLING POINT FOR THREE-LEVEL SIGNALS
Technical Field
This invention relates to a method and apparatus for extracting data from a demodulated signal.
When a bit serial data stream is processed for transmission using G SK or any other partial response signalling method it is subject to line and differential coding and filtering in order to reduce the bandwidth of the transmitted signal". As a result of these processes, the data is distorted, especially through the interaction between successive bits, known as intersymbol distortion. There are therefore problems in extracting the data from the received signal after demodulation and filtering. However, it is known that the received data signal conforms to certain amplitude and cycle limits which are represented by an "eye pattern" formed by overlaying the oscilloscope traces of successive bit signals. From this pattern it is possible to identify a zero cross-over point corresponding to a cycle or bit synchronising point, and open areas or "eyes" corresponding to amplitude limits or thresholds between "0" and "1" bit levels. Eye patterns have therefore been used to monitor the performance of cable-based data transmission systems in which line attenuation is a problem that makes it necessary to employ signal compensation techniques.
Disclosure of the Invention
An object of the present invention is to provide an improved method and apparatus for extracting data from a demodulated signal.
According to the present invention the value of a demodulated signal is sampled over successive cycles at successive predetermined intervals of time within each cycle to obtain sample values, the sample values so obtained are processed to determine an optimum sampling point for each cycle and threshold values to be applied at the sampling point to determine a decoded value for that cycle, and the value of the demodulated signal at the optimum sampling point is compared with the threshold values to determine a decoded value thereof.
In particular, a method according to the present invention comprises the following steps of sampling the value of the demodulated signal at successive predetermined intervals of time (phases) over successive cycles of the demodulated signal; determining for each sample value whether or not it corresponds to a zero value of the demodulated signal (a cross-over point); recording the occurrence of each cross-over point against the phase in which it occurs in a cycle so as to build up a histogram of cross-over points; selecting from the histogram the phase of a maximum number of cross-over points (the sampling point); recording the occurrence of successive sample values over successive cycles of the demodulated signal in an array of sample values against the phase of a cycle so as to build up the data equivalent to an eye-pattern; determining from the distribution of the recorded occurrences of sample values at the sampling point, occurrence minima (selection thresholds) both sides of the zero value; determining whether the sample value at the sampling point in successive cycles has a value within or outside the selection thresholds and allocating a corresponding decoded value to that cycle of the demodulated signal.
This method therefore involves two processes which are preferably conducted simultaneously, one being to generate a histogram of the cross-over points from which a sampling point is determined, and the second being to generate a statistical eye-pattern from which the selection thresholds at the sampling point are determined for use in binary decoding.
The sampling point on the histogram is selected as that phase at which there is a maximum number of cross-over points, although irregularities in the shape of the histogram may lead to the median value being used rather than a simple maximum value.
Description of the Drawings
The invention will now be described by way of example with reference to the accompanying drawings, in which:
Figure 1 shows an eye-pattern such as would be produced by a demodulated input signal as displayed on an oscilloscope,
Figure 2 shows a histogram of cross-over points such as might correspond to the eye-pattern of Figure 1,
Figure 3 shows another histogram of a more irregular shape than that of Figure 2,
Figure 4 shows a section through a statistical model of an eye-pattern on the line V-V in Figure 1, and
Figure 5 shows a schematic diagram of a system operating according to the invention.
Mode of Carrying Out the Invention The typical eye-pattern for a GMSK signal is shown in Figure 1, the pattern being produced by overlaying successive cycles of the signal on an oscilloscope. The optimum point for binary decoding of the signal is selected as that point P in the cycle at which there is a maximum number of zero cross-overs. At this point P, there are maximum input values M both sides of zero which can be taken to represent a binary "1" value. Threshold values T are selected between tvase maxima M and zero in order to distinguish between binary "0" and binary "1" values, input values within the thresholds T being decoded as binary "0" and input values outside the thresholds T being decoded as binary "1".
In order to determine the zero cross-over maximum P, the signal cycle is divided into sixteen phases and the input signal is analysed for zero cross-overs during successive phases over successive cycles so as to build up a record of the occurrence of zero cross-overs throughout these sixteen phases. Zero cross-overs are detected by setting limits each side of the zero and comparing the value of the signal with these limits. The signal is an oversampled signal so that comparison of sample values occurs once for each phase for each bit period. If a sampled value falls within the limits, it is taken as indicative of a zero cross-over, although more sophisticated tests can be employed for example to determine whether the signal actually passes through both limits.
As shown in Figure 5, successive sampled values of the input signal are held in an input latch 1 and are compared with the two limits in the comparators 2 and 3. A modulo sixteen address counter 6 operates in phase with the input signal and generates a corresponding address to identify each phase of the input cycle. If a cross-over is detected during a particular phase, the phase address generated by counter 6 is used to identify a corresponding cross-over count in a memory 4 which is read into a counter 5. Counter 5 is clocked to add one to the cross-over count, and the count is then written back into the memory 4.
The record of the accumulated cross-over counts for all sixteen phases represents a histogram such as shown in Figure 2. The phase in which the maximum cross-over count occurs can be determined as the simple maximum. However, the histogram may not be regular in shape and may possess more than one peak, as shown in Figure 3. Preferably, therefore the median of the histogram is used, this being the phase at which the area of the histogram is bisected. The median is determined by software using the following algorithm: Let the histogram be H[i]
Let the total number of phases be N
Find the phase having the minimum number of cross-overs
Let this phase be P
Find the total area of the graph
Let half of this area be A
WHILE A = 0
P = (P + 1) mod N
A = A - H[P] Optimum phase is P.
At the same time that the input signal is analysed to produce the histogram record, the sampled value of the input signal and its phase are used to progressively build up a statistical model of an eye-pattern. The sampled value passes through a buffer 7 and is combined with the phase address from counter 6 and is used to read a corresponding count out of memory 4 into the counter 5. This . count is incremented by one and written back into the memory 4. This process is repeated for successive combinations of sampled value and phase over successive cycles of the input signal. Once the eye-pattern is complete, the associated data in memory 4 are analysed to determine the thresholds T. The counts of the sampled values at the selected optimum phase P represent a section through the eye-pattern along the line V-V in Figure 1, and are shown in Figure 4. Software is provided to identify the minima between the central maximum at zero and the two outer maxima M. The value of these minima correspond to the thresholds T which are then used for binary decoding of the input data.
The input data are passed through a buffer 8 and written into the memory 4 so that once the histogram has been produced to establish the sampling point P, and the model of the eye-pattern has been produced to establish the thresholds T, these input data can be decoded into binary data by comparison with the thresholds.
Although the invention has been described above by reference to a GMSK signal that produces a three level eye-pattern, it could equally well be applied to a system in which the eye-pattern has more than three levels.

Claims

1. A method of extracting data from a demodulated signal characterised in comprising the steps of sampling the value (V) of the demodulated signal over successive cycles at successive predetermined intervals of time within each cycle to obtain sample values, processing the sample values obtained to determine an optimum sampling point (P) for each cycle and threshold values (T) to be applied at the sampling point (P) to determine a decoded value for that cycle, and comparing the value (V) of the demodulated signal at the optimum sampling point with the threshold values (T) to determine a decoded value thereof.
2. A method as claimed in claim 1 in which the sample values (V) are processed by determining whether or not each sample value corresponds to a zero value of the demodulated signal, recording the occurrence of each zero value against the interval of time in which it occurs in its cycle so as to build up a histogram of zero values, and selecting from the histogram that interval of time for a maximum number of zero values as the sampling point (P) .
3. A method as claimed in claim 1 or 2 in which the sample values (V) are processed by recording the occurrence of successive sample values over successive cycles of the demodulated signal in an array of sample values against the intervals of time in which they occur in their cycles so as to build up a data equivalent to an eye-pattern; and selecting froir the distribution of the recorded occurrences of sample values (V) at the sampling point (P), threshold values (T) corresponding to occurrence minima both sides of the zero value.
4. A method as claimed in claim 3 in which the sample value (V) at the sampling point (P) in successive cycles is compared with the threshold values (T) to determine whether it is within or outside the threshold values (T), and a corresponding decoded value is allocated to that cycle of the demodulated signal.
5. Apparatus for extracting data from a demodulated signal characterised in comprising sampling means (1) to provide sample values of the demodulated signal over successive cycles at successive intervals of time within each cycle, and processing means (4, 5, 6) to determine an optimum sampling point (P) for each cycle and threshold values (T) to be applied at the sampling point to determine a decoded value for that cycle, and comparison means (4) to compare the value of the demodulated signal at the optimum sampling point (P) with the threshold values (T) to determine a decoded value thereof.
6. Apparatus as claimed in claim 5 in which the processing means (4, 5, 6) comprises zero determining means to determine whether or not each sample value corresponds to a zero value of the demodulated signal, zero recording means to record the occurrence of each zero value against the interval of time in which it occurs in its cycle so as to build up a histogram of zero values, and selecting means to select from the histogram that interval of time for a maximum number of zero values as the sampling point (P).
7. Apparatus as claimed in claim 5 or 6 in which the processing means (4, 5, 6) comprises eye-pattern recording means to record the occurrence of successive sample values over successive cycles of the demodulated signal in an array of sample values against the intervals of time in which they occur in their cycle so as to build up a data equivalent of an eye pattern; and threshold selecting means to select from the distribution of the recorded occurrences of sample values at the sampling point (P), threshold values (T) corresponding to occurrence minima both sides of the zero value.
8. Apparatus as claimed in claim 7 in which the comparison means (4) compares the sample value (V) at the sampling point (P) in successive cycles with the threshold values (T) to determine whether it is within or outside the threshold values, and allocate accordingly a corresponding decoded value to that cycle of the demodulated signal.
9. A method of extracting data from a demodulated signal substantially as herein described with reference to the accompanying drawings.
10. Apparatus for extracting data from a demodulated signal substantially as herein described with reference to the accompanying drawings.
PCT/GB1992/000197 1991-02-02 1992-02-03 Method for determining the optimum sampling point for three-level signals WO1992014323A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9102276A GB2253122B (en) 1991-02-02 1991-02-02 Decoding method and apparatus
GB9102276.4 1991-02-02

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Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0828239A2 (en) * 1996-09-04 1998-03-11 HE HOLDINGS, INC. dba HUGHES ELECTRONICS High-accuracy, low-distortion time-frequency analysis of signals using rotated-window spectrograms
EP1862777A2 (en) * 2006-05-30 2007-12-05 Fujitsu Ltd. System and method for adjusting offset compensation applied to a signal
EP1862776A2 (en) 2006-05-30 2007-12-05 Fujitsu Limited System and method for decoupling multiple control loops
EP1862778A3 (en) * 2006-05-30 2008-04-16 Fujitsu Limited System and method for the adjustment of offset compensation applied to a signal
US7760798B2 (en) 2006-05-30 2010-07-20 Fujitsu Limited System and method for adjusting compensation applied to a signal
US7801208B2 (en) 2006-05-30 2010-09-21 Fujitsu Limited System and method for adjusting compensation applied to a signal using filter patterns
US7804894B2 (en) 2006-05-30 2010-09-28 Fujitsu Limited System and method for the adjustment of compensation applied to a signal using filter patterns
US7817712B2 (en) 2006-05-30 2010-10-19 Fujitsu Limited System and method for independently adjusting multiple compensations applied to a signal
US7817757B2 (en) 2006-05-30 2010-10-19 Fujitsu Limited System and method for independently adjusting multiple offset compensations applied to a signal
US7839955B2 (en) 2006-05-30 2010-11-23 Fujitsu Limited System and method for the non-linear adjustment of compensation applied to a signal
US7839958B2 (en) 2006-05-30 2010-11-23 Fujitsu Limited System and method for the adjustment of compensation applied to a signal
US7848470B2 (en) 2006-05-30 2010-12-07 Fujitsu Limited System and method for asymmetrically adjusting compensation applied to a signal

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JPH10215289A (en) 1996-06-04 1998-08-11 Matsushita Electric Ind Co Ltd Synchronization device
GB9712019D0 (en) * 1997-06-09 1997-08-06 Northern Telecom Ltd Eye measurement of optical sampling
GB2326230A (en) * 1997-06-09 1998-12-16 Northern Telecom Ltd Eye measurement of optical signals by optical sampling
US6084931A (en) * 1997-10-31 2000-07-04 Motorola, Inc. Symbol synchronizer based on eye pattern characteristics having variable adaptation rate and adjustable jitter control, and method therefor

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Cited By (18)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0828239A2 (en) * 1996-09-04 1998-03-11 HE HOLDINGS, INC. dba HUGHES ELECTRONICS High-accuracy, low-distortion time-frequency analysis of signals using rotated-window spectrograms
EP0828239A3 (en) * 1996-09-04 1998-11-25 Hughes Electronics Corporation High-accuracy, low-distortion time-frequency analysis of signals using rotated-window spectrograms
EP1862777A2 (en) * 2006-05-30 2007-12-05 Fujitsu Ltd. System and method for adjusting offset compensation applied to a signal
EP1862776A2 (en) 2006-05-30 2007-12-05 Fujitsu Limited System and method for decoupling multiple control loops
EP1862778A3 (en) * 2006-05-30 2008-04-16 Fujitsu Limited System and method for the adjustment of offset compensation applied to a signal
EP1862776A3 (en) * 2006-05-30 2008-04-23 Fujitsu Limited System and method for decoupling multiple control loops
EP1862777A3 (en) * 2006-05-30 2008-04-23 Fujitsu Ltd. System and method for adjusting offset compensation applied to a signal
US7760798B2 (en) 2006-05-30 2010-07-20 Fujitsu Limited System and method for adjusting compensation applied to a signal
US7764757B2 (en) 2006-05-30 2010-07-27 Fujitsu Limited System and method for the adjustment of offset compensation applied to a signal
US7787534B2 (en) 2006-05-30 2010-08-31 Fujitsu Limited System and method for adjusting offset compensation applied to a signal
US7801208B2 (en) 2006-05-30 2010-09-21 Fujitsu Limited System and method for adjusting compensation applied to a signal using filter patterns
US7804894B2 (en) 2006-05-30 2010-09-28 Fujitsu Limited System and method for the adjustment of compensation applied to a signal using filter patterns
US7804921B2 (en) 2006-05-30 2010-09-28 Fujitsu Limited System and method for decoupling multiple control loops
US7817712B2 (en) 2006-05-30 2010-10-19 Fujitsu Limited System and method for independently adjusting multiple compensations applied to a signal
US7817757B2 (en) 2006-05-30 2010-10-19 Fujitsu Limited System and method for independently adjusting multiple offset compensations applied to a signal
US7839955B2 (en) 2006-05-30 2010-11-23 Fujitsu Limited System and method for the non-linear adjustment of compensation applied to a signal
US7839958B2 (en) 2006-05-30 2010-11-23 Fujitsu Limited System and method for the adjustment of compensation applied to a signal
US7848470B2 (en) 2006-05-30 2010-12-07 Fujitsu Limited System and method for asymmetrically adjusting compensation applied to a signal

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NZ241489A (en) 1995-07-26
AU1186192A (en) 1992-09-07
GB9102276D0 (en) 1991-03-20
GB2253122B (en) 1995-08-16
GB2253122A (en) 1992-08-26

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