GB2347032A - Distortion measurement using a test signal with a spectral hole - Google Patents

Distortion measurement using a test signal with a spectral hole Download PDF

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Publication number
GB2347032A
GB2347032A GB9903792A GB9903792A GB2347032A GB 2347032 A GB2347032 A GB 2347032A GB 9903792 A GB9903792 A GB 9903792A GB 9903792 A GB9903792 A GB 9903792A GB 2347032 A GB2347032 A GB 2347032A
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United Kingdom
Prior art keywords
signal
test signal
processing path
distortion introduced
ifft
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GB9903792A
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GB9903792D0 (en
Inventor
John Stuart Curtis
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Synamedia Ltd
Ericsson Television AS
Original Assignee
NDS Ltd
Tandberg Television AS
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Publication date
Application filed by NDS Ltd, Tandberg Television AS filed Critical NDS Ltd
Priority to GB9903792A priority Critical patent/GB2347032A/en
Publication of GB9903792D0 publication Critical patent/GB9903792D0/en
Publication of GB2347032A publication Critical patent/GB2347032A/en
Withdrawn legal-status Critical Current

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    • HELECTRICITY
    • H03ELECTRONIC CIRCUITRY
    • H03FAMPLIFIERS
    • H03F1/00Details of amplifiers with only discharge tubes, only semiconductor devices or only unspecified devices as amplifying elements
    • H03F1/32Modifications of amplifiers to reduce non-linear distortion
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01RMEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R23/00Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
    • G01R23/16Spectrum analysis; Fourier analysis
    • G01R23/20Measurement of non-linear distortion

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  • Physics & Mathematics (AREA)
  • Nonlinear Science (AREA)
  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Mathematical Physics (AREA)
  • General Physics & Mathematics (AREA)
  • Monitoring And Testing Of Transmission In General (AREA)

Abstract

A method and apparatus for measuring distortion introduced into a signal by transmission through a processing path. The method and apparatus require the generation of a test signal comprising one or more time variable signals and at least one spectral hole. Preferred embodiments generate a set of sinewaves by means of an IFFT. The test signal is particularly suited to the measurement, detection and monitoring of in-band distortion introduced into an OFDM signal by non-linear processing components. This enables subsequent replacement or modification of any components or processing paths introducing sub-optimal amounts of distortion.

Description

Method and Apparatus for Measurement of Distortion This invention relates to method and apparatus for measurement of distortion introduced into a signal. It is suited to measurement of distortion introduced into signals during non-liner processing, including distortion in signals such as those having a substantially Gaussian amplitude probability density function (pd.
It is well known that processing of a signal can give rise to distortion such as noise or non-liner distortions, e. g. intermodulation distortion. The presence of distortion degrades the performance of the signal. An increase in amplitude of the distortion above a given threshold for a particular application may give rise to an unacceptable level of performance of the signal.
Certain types of distortion, such as intermodulation distortion produce in-band, near-band and out-of-band distortion components. The out-of-band distortion can be measured easily, for example by means of a spectrum analyser and can be removed or minimised by filtering. However, the in-band distortion cannot be measured simply since this type of distortion normally has a much lower amplitude than the signal. It is thus masked by the signal. In addition, near-band distortion refers to distortion components that are very close in frequency of the signal. A signal having sidelobes can sometimes mask nearband distortion. In-band and near-band distortion components are hereinafter termed collectively as in-band distortion.
There is a need to provide means for measuring amplitude levels of distortion, and in particular in-band distortion, introduced into a signal by non-liner processing, to enable fault detection of the various processing steps applied to the signal. Processing components identifie as introducing sub-optimal levels of distortion into a signal can be subsequently modified as appropriate to reduce their contribution to the distortion introduced into the signal.
It is the aim of this invention to provide a means for measuring distortion that overcomes at least some of the problems of known means.
According to a first aspect of this invention there is provided a method of measuring distortion introduced into a first signal by transmission of the first signal through a predetermined processing path, the method comprising the steps of : creating a test signal comprising one or more time variable signals, wherein the test signal comprises at least one spectral hole ; applying the test signal to a representative processing path representing at least a portion of the predetermined processing path; and measuring the distortion introduced into the test signal by transmission of the test signal through the representative processing path.
According to a second aspect of this invention there is provided apparatus for generating a test signal capable of measuring distortion introduced into a first signal by transmission of the first signal through a predetermined processing path, the apparatus comprising: means for generating a test signal comprising one or more time variable signals and at least one spectral hole ; and means for supplying the test signal to apparatus capable of a measuring distortion introduced into the test signal by a representative processing path representing at least a portion of the predetermined processing path.
The method and apparatus of this invention generates a test signal, which allows measurement of distortion introduced into a signal by means of simple equipment. Additionally, the invention enables simple implementation of spectral holes at multiple and/or varying frequency points within the frequency spectrum of a signal.
It will be understood that a spectral hole is a zero, or substantially zero, amplitude point within the frequency spectrum of a test signal.
The invention will now be described by way of example only and with reference to the following drawings: Figure 1 is a schematic diagram of a frequency spectrum of a signal comprising a substantially Gaussian pdf; Figure 2 is a block diagram of a prior art transmission end of an orthogonal frequency division multiplex (OFDM) system; Figure 3 is a block diagram of a first embodiment of a test signal generator in accordance with this invention; Figure 4 is a block diagram of a second embodiment of a test signal generator in accordance with this invention; Figure 5 is a block diagram of a third embodiment of a test signal generator in accordance with this invention, and comprise within an OFDM system such as that of Figure 2; Figure 6 is a schematic representation of a test signal frequency spectrum.
Figure 1 is a schematic representation of a signal spectrum 1 of a signal having a substantially Gaussian amplitude pdf that has been transmitted through a non-linear channel. The signal has side-lobes 2. Such transmission has resulted in the introduction of a distortion spectrum 3 to signal spectrum 1. An orthogonal frequency division multiplex (OFDM) signal is an example of a signal having a substantially Gaussian pdf.
Figure 2 is a block diagram of a prior art system used to generate and process an OFDM signal prior to transmission across a broadcast transmission link processing path. It will be understood by those skilled in the art that such a transmission link can be used to transmit data for mobile telecommunications, television broadcast data distribution and general data distribution' An input signal comprising a digital bitstream representative of MPEG television data is supplied to modulator 20 at terminal 21. The modulator comprises a module 30, an N-point Inverse Fast Fourier Transform (IFFT) 22, a guard interval generator 23, an IQ modulator 31 and a digital to analogue converter 24. The output from the modulator 20 is in the form of an analogue OFDM baseband signal 25. The modulator output is provided as an input to frequency translator 26 that converts the signal to a radio frequency (RF) signal. The RF signal is amplifie by RF high power amplifier 28, and transmitted by antenna 29 across a transmission link.
The input from terminal 21 is processed by module 30 to perform forward error correction, inteneavingng and mapping. The IFFT 22 generates and adds together a set of sinewaves of equally spaced, but differing, frequencies.
The output of module 30 is mapped onto these sinewaves. The output of IFFT 22 comprises a series of signal blocks, each block comprising N time samples having I and Q complex components. Each block has a guard interval added by guard interval generator 23. The inclusion of a guard interval within an OFDM signal aids both synchronisation of the signal by receiver (s) (not shown) of the OFDM signal and multi-path radio-wave propagation performance. The complex input into IQ modulator 31 is converted into a real signal output. Following conversion of the carriers to an analogue OFDM signal by converter 24, the signal frequency is converted and amplified such that antenna 29 transmis an RF OFDM signal across a television broadcast transmission link.
Figure 3 is a block diagram of an embodiment of this invention showing a test signal generator 35. The generator comprises m sinewave generators 36 (1) to 36 (m) that feed their individual outputs to adder 37. The sinewave generators are arranged to produce sinewaves of designated frequencies by module 38. The outputs of the sinewave generators are summed by adder 37 to provide a test signal comprising at least one spectral hole.
The inclusion of the or each spectral hole within the output of adder 37 is provided by way of frequency designation of the sinewave generators by module 38. The frequencies of sinewave generators 36 (3) and 36 (4) for example are designated such that they are sufficiently far apart to produce a spectral hole located between their frequencies. It will be understood that the number and frequency location of the spectral holes included within the output of adder 37 can be varied by module 38.
Where the output of adder 37 is used to subsequently measure distortion introduced into an OFDM signal during a particular processing path, then preferably there is no substantial change in phase and amplitude of each of the sinewaves within the test signal. This prevents modulation sidebands that might otherwise add an unwanted signal in the spectral hole. Additionally, it is preferable that the test signal comprises a total spectral power substantially similar to that of the OFDM signal in order to more closely represent the distortion introduced into the OFDM signal.
Figure 4 is a block diagram of an alternative embodiment of this invention.
Test signal generator 40 comprises data generator 41 and a zero value I and Q generator 42. The data from each of these generators is provided as an input to IFFT 43 by switch 44.
Generator 40 generates a test signal in the following way. Generator 42 provides an output of zero values of I and Q, resulting in the carrier or carriers to which the IFFT designates such values to have zero, or substantially zero, amplitude. Switch 44 operates to enable the IFFT to receive an input such that a predetermined number of adjacent carrier or carriers of a predetermined frequency are modulated by zero I and Q values. Thus, the spectral hole that is created may have a width of one or more carriers. Switch 44 is capable of operating to provide an input to IFFT 43 such that its output is a signal comprising at least one spectral hole.
Data from generators 41 and 42 is assigned by IFFT 43 onto sinewaves generated by the IFFT. Spectral holes are created at those frequencies to which the zero I and Q value data from generator 42 is assigned. The switch 44 is operated to allow input from generator 42 to create a spectral hole over the frequencies of one or more adjacent carriers and/or at a plurality of frequency locations.
The test signal output of IFFT 43 is a set of sinewaves comprising one or more spectral holes. This output can be in the form of a continuous output from the IFFT or altematively as one test block, which is stored in a repeater module 45. The repeater module stores the output block from the IFFT 43 and repeats this to form a continuous output. The repeated block is cyclically continuous (i. e. none of the sinewaves that make up the block has a discontinuity in phase or amplitude), and therefore has no associated sidelobes. The output of module 45 is thus a test signal comprising set of sinewaves and at least one spectral hole.
Advantageously, where the test signal output is used to measure distortion introduced into an OFDM signal, the amplitude of the non-spectral hole frequencies of the test signal are boosted such that the total spectral power of the test signal is substantially similar to that of the OFDM signal.
The time period length of each block generated by IFFT 43 is dependent upon the number of points of this IFFT. For measurement of distortion introduced into an OFDM signal it is preferred that the time period of the blocks of IFFT 43 is greater than those of the IFFT within the OFDM modulator. This creates the pdf of a test signal closer to a Gaussian distribution. The test signal can then be used for measurement of distortion introduced by transmission of the test signal through components such as amplifier 28 of Figure 2.
It will be understood by those skilled in the art that an altemative embodiment for generating a test signal for measuring distortion introduced into an OFDM signal can utilise the IFFT of the modulator to generate a test signal comprising at least one spectral hole. This can be seen in Figure 5.
Figure 5 is a block diagram of a modulator such as that of Figure 2 and additionally comprising zero I and Q generator 50 and repeater module 54.
IFFT 22 is arrange to receive an input from terminal 21 or zero I and Q value generator by means of switch 51. Where input from generator 50 is provided, then the output of IFFT 22 is a test block comprising a set of sinewaves and at least one spectral hole. In order to ensure cyclical continuity of the test block, guard interval generator 23 is by passed by means of switches 52 and 53.
The test block is routed to repeater module 54 by switch 55. The repeater module repeats the cyclically continuous test block to provide a continuous signal 56 suitable for use in measuring distortion introduced into a signal during particular processing path or paths. The test signal can be boosted such that its total spectral power is substantially similar to the OFDM signal generated by modulator 20.
It will be understood that test signals generated in accordance with this invention can be supplie to make measurements of distortion at any point during the processing of a datastream. This includes the points at which data is subjected to digital and analogue signal processing channes, and testing at both transmitter and receiver ends of a transmission link.
The frequency spectrum of the output of adder 37 of Figure 3, test signal generator 40 or repeater module 54 can be seen schematically in Figure 6.
Test signal spectrum 60 comprises five spectral holes 61.
Each of the embodiments of Figures 3,4 and 5 are described with reference a test signal comprising a set of sinewaves. However, it will be understood that the invention also includes a test signal derived from one or more time variable signals.'

Claims (19)

  1. CLAIMS 1 A method of measuring distortion introduced into a first signal by transmission of the first signal through a predetermined processing path, the method comprising the steps of : creating a test signal comprising one or more time variable signals, wherein the test signal comprises at least one spectral hole ; applying the test signal to a representative processing path representing at least a portion of the predetermined processing path; and measuring the distortion introduced into the test signal by transmission of the test signal through the representative processing path.
  2. 2 The method of Claim 1 further comprising the step of determining the distortion introduced into the first signal.
  3. 3 The method of Claims 1 or 2 wherein the step of creating a test signal comprises combining a set of sinewaves.
  4. 4 The method of Claim 3 wherein the step of combining a set of sinewaves comprises generating at least one output block from an IFFT.
  5. 5 The method of Claim 4 further comprising the step of repeating one output block.
  6. 6 The method of any of the preceding Claims and further comprising the step of detecting an increase above a predetermined threshold of distortion introduced into the test signal by the representative processing path.
  7. 7 The method of Claim 6 and further comprising the step of modifying the predetermined processing path to decrease the distortion introduced into the first signal.
  8. 8 The method of any of the preceding Claims further comprising the step of boosting the test signal to provide a test signal comprising a total spectrum power substantially the same as the total spectrum power of the first signal.
  9. 9 Apparatus for generating a test signal capable of measuring distortion introduced into a first signal by transmission of the first signal through a predetermined processing path, the apparatus comprising: means for generating a test signal comprising one or more time variable signals and at least one spectral hole ; and means for supplying the test signal to apparatus capable of a measuring distortion introduced into the test signal by a representative processing path representing at least a portion of the predetermined processing path.
  10. 10 The apparatus of Claim 10 wherein the test signal is a set of combined sinewaves.
  11. 11 The apparatus of Claims 9 or 10 wherein the first signal is a signal having a substantially Gaussian pdf.
  12. 12 The apparatus of Claim 12 wherein the signal is an OFDM signal.
  13. 13 The apparatus of any of Claims 9 to 12 wherein the test signal generator comprises an IFFT capable of generating at least one output block.
  14. 14 The apparatus of Claim 13 further comprising a repeater capable of repeating one output block from the IFFT to form a continuous test signal output.
  15. 15 The apparatus of any of Claims 9 to 14 further comprising a booster capable of boosting the total frequency spectrum power to substantially the same as the total frequency spectrum power of the first signal.
  16. 16 The apparatus of any of Claims 9 to 15 further comprising means for measuring distortion introduced into the test signal by transmission of the test signal through the representative path.
  17. 17 An OFDM modulator comprising apparatus according to any of Claims
    12 to 16.
  18. 18 A method as substantially described herein and with reference to Figure 3, Figure 4 and Figure 5 of the accompanying drawings.
  19. 19 Apparatus as substantially described herein and with reference to Figure 3, Figure 4 and Figure 5 of the accompanying drawings.
GB9903792A 1999-02-17 1999-02-17 Distortion measurement using a test signal with a spectral hole Withdrawn GB2347032A (en)

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GB2347032A true GB2347032A (en) 2000-08-23

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102007010868A1 (en) * 2007-03-06 2008-09-11 Rohde & Schwarz Gmbh & Co. Kg Device and method for determining a transmission behavior of subchannels

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4258314A (en) * 1979-03-30 1981-03-24 Yoshimutsu Hirata Nonlinear distortion measurement using composite pulse waveform
EP0522706A2 (en) * 1991-07-06 1993-01-13 Racal Communications Systems Limited Amplification systems
US5872814A (en) * 1997-02-24 1999-02-16 At&T Wireless Services Inc. Method for linearization of RF transmission electronics using baseband pre-distortion in T/R compensation pilot signals

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4258314A (en) * 1979-03-30 1981-03-24 Yoshimutsu Hirata Nonlinear distortion measurement using composite pulse waveform
EP0522706A2 (en) * 1991-07-06 1993-01-13 Racal Communications Systems Limited Amplification systems
US5872814A (en) * 1997-02-24 1999-02-16 At&T Wireless Services Inc. Method for linearization of RF transmission electronics using baseband pre-distortion in T/R compensation pilot signals

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102007010868A1 (en) * 2007-03-06 2008-09-11 Rohde & Schwarz Gmbh & Co. Kg Device and method for determining a transmission behavior of subchannels
US8774020B2 (en) 2007-03-06 2014-07-08 Rohde & Schwarz Gmbh & Co. Kg Device and method for determining a transmission behavior of sub-channels

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