WO1999059274A2 - Estimation de la qualite d'un canal radioelectrique - Google Patents

Estimation de la qualite d'un canal radioelectrique Download PDF

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Publication number
WO1999059274A2
WO1999059274A2 PCT/GB1999/001532 GB9901532W WO9959274A2 WO 1999059274 A2 WO1999059274 A2 WO 1999059274A2 GB 9901532 W GB9901532 W GB 9901532W WO 9959274 A2 WO9959274 A2 WO 9959274A2
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Prior art keywords
signal
quality
service
value
estimating
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PCT/GB1999/001532
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English (en)
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WO1999059274A3 (fr
Inventor
Adrian Paul Hillier
Robert Maxwell Frayling-Cork
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Simoco International Limited
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Publication date
Application filed by Simoco International Limited filed Critical Simoco International Limited
Priority to AU39417/99A priority Critical patent/AU3941799A/en
Publication of WO1999059274A2 publication Critical patent/WO1999059274A2/fr
Publication of WO1999059274A3 publication Critical patent/WO1999059274A3/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/004—Arrangements for detecting or preventing errors in the information received by using forward error control
    • H04L1/0056—Systems characterized by the type of code used
    • H04L1/0061—Error detection codes
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L1/00—Arrangements for detecting or preventing errors in the information received
    • H04L1/20—Arrangements for detecting or preventing errors in the information received using signal quality detector
    • H04L1/206—Arrangements for detecting or preventing errors in the information received using signal quality detector for modulated signals

Definitions

  • the present invention relates to a method of and apparatus for estimating the quality of service of a radio channel, and in particular to a method of and apparatus for estimating the quality of a phase-shift keying modulated radio channel, such as a TETRA (TErrestrial Trunked RAdio) channel.
  • a radio signal in a communications system may be viewed in terms of a radio-frequency wave (the RF carrier wave) which is altered in some way (modulated) to enable it to carry information.
  • the characteristics of the modulated signal are constrained to a limited set of states at periodic intervals (as opposed to an analogue system where there is a continuum of states) .
  • Each individual state is typically referred to as a "symbol", and the set of symbols forms the 'alphabet' by which information in the signal is described.
  • symbols In a binary modulation scheme, there are only two possible states, or symbols, denoting binary bits '0' and '1' .
  • the modulation scheme defines four valid signal states, or symbols, which are used to describe the four possible permutations of two binary digits, i.e. 00, 01, 10, 11.
  • the parameters used to distinguish between different symbols are usually the instantaneous amplitude and phase of the RF carrier for the duration of the symbol, relative to a reference amplitude and phase derived from the carrier.
  • the signal receiver determines which symbols have been transmitted by comparing the received amplitude and phase values of the carrier with the derived reference values and deciding to which symbol each pair of these values most closely corresponds .
  • the signal modulator is usually arranged to set the RF carrier to the correct amplitude and phase for an individual symbol as precisely as possible.
  • the phase and amplitude signal of a symbol will alter from the intended values, due to, for example, interference or signal fading. This makes it more difficult for the demodulator in the radio receiver to determine which symbol has been transmitted and can therefore degrade the quality of the radio signal as perceived by a user receiving it .
  • mobile radio communications systems such as private mobile radio systems and cellular telephone systems are arranged to provide estimates of the quality of service provided by a radio channel or signal.
  • quality estimates can be used, for example, to drive cell or base station reselection, radio channel acquisition, or radio channel handover processes, to try, for example, to improve received signal quality.
  • radio channel service quality One parameter traditionally used to estimate radio channel service quality is the received signal strength, with a high signal strength generally being equated to a good quality of service.
  • the quality of service of a radio channel can be degraded by high- energy interference (e.g. co-channel interference, adjacent channel interference, and narrow band jamming) which actually increases the received signal energy.
  • high- energy interference e.g. co-channel interference, adjacent channel interference, and narrow band jamming
  • Alternative quality of service estimation processes use a so-called 'training sequence' of known bits to estimate the quality of service.
  • a known bit-sequence is demodulated and compared with a reference correlation sequence. By measuring how closely the received bit- sequence resembles the reference sequence, it is possible to obtain an estimate of service quality which is sensitive to interference as well as noise.
  • training sequence correlation as a service quality indicator, the radio receiver must know where the training sequence is located in the transmitted signal, i.e. the transmitter and receiver must be synchronised .
  • a method of estimating the quality of service provided by a radio channel comprising: determining the instantaneous value of a particular parameter for each of a plurality of samples of a modulated signal on the radio channel; and estimating the service quality on the basis of the variation of a set of two or more of the determined parameter values.
  • an apparatus for estimating the quality of service provided by a radio channel comprising: means for determining the instantaneous value of a particular parameter for each of a plurality of samples of a modulated signal on the radio channel; and means for estimating the service quality on the basis of the variation of a set of two or more of the determined parameter values .
  • the variation of a set of determined values for a given parameter for a signal on a radio channel is used to estimate the quality of service provided by the channel.
  • the variation of the values in the set gives an indication of the consistency of the signal over the set of samples, and can therefore be used as an indicator of service quality.
  • the variation will depend upon both signal fading and any high-energy interference which is increasing the received signal energy, and thus the method of the present invention is sensitive to the effects of interference (e.g. co-channel and adjacent channel interference and narrow band jamming) , in addition to additive white Gaussian noise.
  • the present invention does not involve the comparison of known signal sequences, and thus does not require any prior knowledge of what has been transmitted nor need any synchronisation between transmitter and receiver in order to estimate service quality.
  • the particular parameter, the instantaneous values of which are to be determined, can be selected as desired and could comprise a parameter normally used to estimate signal quality such as signal strength. It is preferably predetermined. In a particularly preferred embodiment the selected parameter is one that would, in an ideal signal, be substantially identical for each sample of the signal, as then any variation in the parameter value will be due substantially to signal degradation or distortion, i.e. quality loss.
  • the modulation technique used imparts to the carrier wave characteristics which at the instant of modulation are common to every sample or a particular set of samples of the signal on the radio channel.
  • the radio signal may be modulated to initially have one or more constant parameters.
  • FM frequency modulation
  • AM amplitude modulation
  • samples, and in particular symbols, of the signal are also initially modulated to have one or more identical, common characteristics or parameters.
  • each symbol may be modulated to have the same initial amplitude relative to the carrier wave.
  • each symbol may be initially modulated to have one of a set of predetermined values of a particular parameter or parameters, which values are varied relative to each other for one or more symbols in a predetermined manner by the modulation, such that removal of the known modulation induced differences would leave each symbol with the same value for that particular parameter or parameters .
  • An example of this could be the phase of each symbol relative to the carrier wave, which could be identical when the symbols are demodulated or mapped to a common phase quadrant so as to remove modulation- induced phase differences.
  • the particular parameter could, for example, comprise the amplitude of each sample, or a phase measurement for each sample, relative to the carrier wave .
  • the samples of which the parameter values are to be determined can be selected (e.g. as regards their spacing in time) as desired, as can the number of determined parameter values to be used in the set of which the variation is to be considered.
  • the set comprises a predetermined number of parameter values, and preferably at least sufficient values to average out variations caused by. Gaussian noise, and, where appropriate, most preferably sufficient values to average out Rayleigh fading.
  • the spacing and timing of the samples, and the number in a set should be such as to provide a practically useful and statistically reliable result (e.g. to avoid sampling over too long a period for a given set which might lead to errors induced by users deliberately changing the power of their signals) , as will be appreciated by those skilled in the art .
  • the samples, the parameter values of which are to be determined are selected to ensure that the parameter value being considered would be substantially identical for each sample in an ideal signal.
  • the samples preferably comprise symbols, and most preferably immediately successive symbols, of the digitally modulated signal .
  • the parameter value for each sample can be determined in any suitable manner known in the art .
  • the sampled symbols of a digitally modulated phase-shift keyed signal can be demodulated (or differentially demodulated, if differential phase-shift keying is used, such as would be the case in the TETRA system) or mapped to give them notionally common characteristics (e.g. phase values), i.e. to remove any; deliberate modulation induced parameter, e.g. phase differences, and the variation of one or more of the parameters, such as amplitude or phase of the so mapped or demodulated signals then analysed.
  • This mapping could be done, by e.g. taking the modulus of the real and imaginary parts of each symbol, and/or by changing the determined instantaneous parameter value in a predetermined manner based on the determined instantaneous value.
  • the determined parameter value for each sample can comprise the overall parameter value of the sample (i.e. of all the components in the sample) , or just the parameter value or values of one or particular components of the sample, such as the real and imaginary components of a phase-shift keyed signal. These separate values may provide additional information about the signal quality. For example, where the signal being analysed comprises in-phase and quadrature-phase components, the instantaneous amplitudes of each component for each sample can be determined and included as amplitude values in the set of which the variation is to be considered, such that the variation of the overall amplitude envelope is being considered.
  • the variation of the amplitude values of in-phase and quadrature phase components of the samples can be considered separately to give an in-phase component amplitude value variation and/or a quadrature-phase component amplitude value variation.
  • the in-phase components' and quadrature-phase components' amplitude value variations may be combined to give an overall amplitude value variation.
  • the variation of the set of determined parameter values can be determined in any suitable manner. For example, the extent of deviation of the values from a mean, modal or middle value, the range of the values, the difference between two selected values (e.g. the highest and lowest) , or the standard deviation of the set of values could be used to indicate and represent the variation.
  • the variation is determined by calculating the variance of the set of the determined parameter values, and the service quality is estimated on the basis of the calculated variance.
  • the apparatus of the present invention preferably comprises means for calculating the variance of the set of the determined parameter values, and means for estimating the service quality on the basis of the calculated variance .
  • the variance of the set of determined parameter values can be calculated using known statistical techniques.
  • the variation of the set of sampled values is determined by considering the error of each value from its expected value and then considering the errors of a set of sampled values. For example, by statistical analysis, such as taking the mean or modal error value, or considering the variation (e.g. variance or standard deviation) of the error values, a measure of the variation of the parameter values can be obtained.
  • the error value for each sample can be derived as desired. It is preferably the difference between the expected instantaneous parameter value (e.g. amplitude or phase) (which value would normally be predetermined by the modulation technique being used) and the observed instantaneous value of that parameter for that sample .
  • the samples should be symbols of the signal, and thus the error value is preferably the difference between the expected instantaneous amplitude or phase of a symbol of that type and the observed amplitude or phase value, respectively, for the symbol. It is believed that the error present in samples of a signal on a radio channel could in itself be useful as a quality of service estimation, since how closely the received signal resembles the modulation defined (i.e. expected) signal gives an indication of the degree of distortion caused by the radio channel at that instant. By considering this degree of distortion or error over a number of symbols, a more useful quality of service indication can be obtained.
  • a method of estimating the quality of service provided by a radio channel comprising: determining the instantaneous value of a particular parameter for each of a plurality of samples of a modulated signal on the radio channel; finding the difference between the expected value of the determined parameter for each sample and the determined value of each sample to determine a parameter error for each sample; and estimating the service quality on the basis of the parameter errors of two or more samples of the signal.
  • an apparatus for estimating the quality of service provided by a radio channel comprising: means for determining the instantaneous value of a particular parameter for each of a plurality of samples of a modulated signal on the radio channel; means for finding the difference between the expected instantaneous value of the particular parameter for each sample and the determined value of each sample to determine a parameter error for each sample; and means for estimating the service quality on the basis of the parameter errors of two or more samples of the signal.
  • the samples when a digitally modulated signal is being considered, the samples preferably comprises symbols of the signal, and most preferably the instantaneous amplitude and/or phase of each symbol is determined, and the difference between the expected amplitude and/or phase of each symbol and the observed determined value used as an amplitude error and/or phase error, and the service quality is estimated on the basis of two or more determined amplitude errors and/or phase errors.
  • the amplitude and phase errors can be considered in combination, to give a "vector" error for each symbol, if desired.
  • the service quality estimate is based on a statistical analysis of the errors . A small variation (e.g. value of the variance) will generally be indicative of a good quality of service (since it indicates very consistent parameter values) . Any additional in-band channel energy will increase the variation, indicating a channel quality degradation.
  • Predetermined ranges of variation (e.g. variance) values can be associated with particular channel service qualities, such as "good”, “bad” and “neither".
  • the quality of the sampled channel can then be estimated as being the quality associated with whichever predetermined range the variation of the set of sampled channel parameter values falls within.
  • the variation can be repeatedly considered at selected, e.g. regular, intervals for new sets of parameter values to monitor continuously the quality of service, or it could be calculated on demand (e.g. of a base-station or user) .
  • variation of successive sets of sampled parameter values is to be considered, preferably there is no duplication of the samples between the sets, although some of the samples can be used in more than one set, if desired.
  • the method and apparatus of the present invention is particularly suited to estimating the quality of a digitally modulated radio channel.
  • a method of estimating the quality of service provided by a digitally modulated radio channel comprising: measuring the amplitude and/or phase of each of a plurality of symbols in a digitally modulated signal on the radio channel; and estimating the service quality on the basis of the variation of a set of two or more of the measured amplitude and/or phase values.
  • an apparatus for estimating the quality of service provided by a digitally modulated radio channel comprising: means for measuring the amplitude and/or phase of each of a plurality of symbols in a digitally modulated signal on the radio channel; and means for estimating the service quality on the basis of the variation of a set of two or more of the measured amplitude and/or phase values.
  • These aspects of the present invention preferably further comprise removing any modulation induced phase and/or amplitude differences between the symbols in the set to provide a modified set of symbols having nominally identical amplitudes and phases, and then measuring the amplitude and/or phase of each symbol in the modified set of symbols and using these measurements for the service quality estimate.
  • the method and apparatus of the present invention is also particularly suited to estimating the quality of service of a differential quaternary phase- shift keying (DQPSK) modulated radio channel, such as is used in the TETRA system.
  • DQPSK differential quaternary phase- shift keying
  • a method of estimating the quality of service of a DQPSK modulated radio channel comprising: determining the variance of the instantaneous amplitude and/or phase values of a plurality of demodulated constellation points of a DQPSK modulated signal on the radio channel.
  • an apparatus for estimating the quality of service of a DQPSK modulated radio channel comprising means for determining the variance of the instantaneous amplitude and/or phase values of a plurality of demodulated constellation points of a DQPSK modulated signal on the radio channel.
  • the quality of service indication provided by the method and apparatus of the present invention can be used for all applications where quality of service estimates are useful, such as channel acquisition, cell or base station re-selection or handover, jamming detection, and channel quality reporting.
  • One preferred use of the quality of service indication is as a double check on the error check (e.g. parity or checksum check) that is often used on radio speech channels to ensure that only good quality speech is played back to a listener.
  • the error check e.g. parity or checksum check
  • a CRC or cyclic redundancy check is made on data received over the speech channel, and speech playback is inhibited or corrected if the CRC check fails, to ensure that only good quality speech is played to a listener.
  • a method of controlling the playing of speech by a mobile radio comprising: performing an error check on the received speech signal data; estimating the quality of service of the radio channel; and playing the speech signal only if the error check is met and the quality of service is estimated to be greater than a predetermined required quality of service for speech playback.
  • an apparatus for - controlling the playing of speech by a mobile radio comprising: means for performing an error check on the received speech signal data; means for estimating the quality of service of the radio channel; and means for playing the speech signal only if the error check is met and the quality of service is estimated to be greater than a predetermined required quality of service for speech playback.
  • the quality of service can be estimated using any suitable parameter, such as received signal strength, signal-to-noise ratio, bit error rate, or the variation or error value of the first to eighth aspects of the present invention.
  • the predetermined required quality of service for speech playback could, for example, correspond to the received signal strength or signal-to-noise ratio exceeding a predetermined value .
  • the methods in accordance with the present invention may be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from a further aspect the present invention provides computer software specifically adapted to carry out the methods hereinabove described when installed on data processing means.
  • the invention also extends to a computer software carrier comprising such software which when used to operate a radio system comprising a digital computer causes in conjunction with said computer said system to carry out the steps of the method of the present invention.
  • a computer software carrier could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.
  • Figures la and lb show schematically two example modulation schemes;
  • Figure 2 shows a remapped quaternary phase-shift keyed symbol sequence;
  • FIG. 3 shows an apparatus in accordance with the present invention
  • Figure 4 shows a modulated ⁇ /4 differential quaternary phase-shift keying modulated TETRA signal
  • Figure 5 shows the demodulated form of the signal shown in Figure 4 ;
  • Figure 6 shows the demodulated constellation points of Figure 5 mapped to the first quadrant of the graph
  • Figure 7 shows the probability density function of the points on the graph in Figure 6.
  • Figure 8 is a graph of signal power against time for an exemplary radio signal subjected to a Rayleigh fading envelope.
  • quality of service estimation is based upon identifying one or more characteristics of the symbol set of a digitally modulated signal which are common to each and every symbol in the alphabet.
  • Figure la shows a demodulated QPSK symbol sequence
  • Figure lb a demodulated 16PSK symbol sequence.
  • the quality of service estimation technique described below is based upon an analysis of one or more of these common characteristics, and specifically an analysis of the degree of variation of these characteristics observed across a set of received symbols (since an ideal received signal would exhibit zero variation over a set of mapped symbols in each of these characteristics, and the degree of variation will increase as the quality of service degrades) .
  • Figure 3 shows an apparatus 1 in accordance with the present invention suitable for estimating the quality of a physical TETRA channel, and in particular of a ⁇ /4 differential quaternary phase-shift keying ( ⁇ /4 DQPSK) modulated signal, such as might be present on that channel .
  • ⁇ /4 DQPSK differential quaternary phase-shift keying
  • the points in Figure 5 are the symbols of the signal (i.e. the parts of the signal which represent the data in the signal) and form the samples whose instantaneous parameter values are to be determined.
  • the radius of the circle on which the constellation points lie is proportional to the mean symbol energy ⁇ p .
  • the sequence 4 of demodulated constellation points or symbols, y is given, as is known in the art, by:
  • Each demodulated constellation point comprises two orthogonal 'soft decisions' whose magnitude represents the instantaneous energy in the in-phase and quadrature- phase components of the signal samples.
  • the variance ⁇ 2 of a set of the values in the scalar amplitude sequence can then be determined using standard statistical analysis:
  • a quality of service indication is given by the variance :
  • ⁇ 2 ⁇ (p n 2 /N) - ( ⁇ (p n /N)) 2
  • a low value of ⁇ 2 indicates a good quality of service. Any additional in-band channel energy will increase ⁇ 2 , indicating a channel quality degradation.
  • the quality of service indication therefore responds to co-channel interference, adjacent channel interference, and narrow band jamming in addition to additive white Gaussian noise .
  • Figure 7 shows an exemplary probability density function for a symbol scalar amplitude sequence derived as described above.
  • IQ energy sequence comprising the instantaneous energies of both the in-phase and the quadrature phase components of the received signal was determined, it is possible to consider the in-phase and quadrature phase components separately, i.e. to determine the variance of the in- phase component energy sequence alone :
  • the sum of the two variances is the overall scalar energy sequence variance.
  • the above example shows the determination of the variance for a single sequence of constellation points (i.e. set of signal samples) .
  • the method can be repeated for plural sequences to monitor the quality of service over time.
  • the above example uses the variance of the set of parameter values as an indicator of signal quality.
  • Other metrics indicative of the sampled values ' variation can be used.
  • One preferred such metric is to divide the mean of the set of values, e.g. symbol energy ⁇ p , by the standard deviation of the set, ⁇ . This gives a metric which is proportional to the mean symbol energy (Es) and inversely proportional to both the background noise (N 0 )and the interference (C ⁇ ) levels, which will each serve to increase the standard deviation of the energy distribution:
  • Es mean symbol energy
  • N 0 background noise
  • C ⁇ interference
  • Q IND is preferably used as the quality of service indicator:
  • the value Q IND may be readily calculated and gets smaller as the quality of service improves .
  • the mean symbol energy, ⁇ P/ is preferably substantially constant throughout the set of symbols, as then any variation will be due to signal distortion. This has important implications when considering a practical radio communications system which maybe subjected to a Rayleigh fading envelope, as shown for example, in Figure 8.
  • the number of successive symbols N, over which Q IND is evaluated should be less than T Q /T S (where T Q is the period over which the mean signal amplitude ⁇ p is substantially constant and T s is the symbol period) , since over this set of symbols the mean symbol amplitude remains substantially constant .
  • T Q is the period over which the mean signal amplitude ⁇ p is substantially constant and T s is the symbol period
  • the Q IND metric will show how the service quality is degraded by moving through a deep fade. If Q IND is evaluated over an extended period T v , by taking one long symbol sequence of length (T v /T s ) then the value ⁇ p is no longer constant, and its variation would contribute to the service quality metric Q IND and make it less reliable.
  • the time T Q over which to evaluate the metric Q IND can be selected as desired with these factors in mind.
  • the method of the present invention can be implemented in hardware and/or software, as will be appreciated by those skilled in the art. It can be implemented in the mobile units of a mobile radio communications system to allow those units themselves to estimate service quality.
  • the quality of service indication provided by the present invention can be used for applications where quality of service estimates are useful, such as channel acquisition, cell or base station re-selection and handover, jamming detection, and channel quality reporting.
  • the quality of service indication is as a double check on the error check, such as the TETRA CRC or cyclic redundancy check, that is often used on radio speech channels to ensure that only good quality speech is played back to a listener.
  • received speech is only played if the error check is met and the variance is less than a predetermined value indicative of a satisfactory quality of service . This arrangement avoids spurious playback induced by random noise received over the speech channel .
  • the present invention is equally applicable to other radio communication systems, such as GMSK, nPSK (where n is any number ⁇ 2 ) , or a system which uses amplitude variations as well as phase variations to differentiate between symbols, and analogue systems such as FM and AM.

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

Abstract

L'invention concerne un appareil (1) permettant d'estimer la qualité d'un signal radioélectrique différentiel à modulation numérique par déplacement de phase à quatre états. Ce signal (2) est introduit dans un démodulateur différentiel (3), une séquence de points de constellation à modulation numérique étant démodulée de manière différentielle afin de produire une séquence (4) de points de constellation ou de symboles démodulés dudit signal démodulé. On calcule ensuite l'intensité des énergies instantanées des composants en phase et de phase en quadrature de chaque symbole de cette séquence (4), ce qui permet d'obtenir la valeur instantanée de l'énergie scalaire de chaque symbole échantillonné dudit signal à modulation numérique. On peut donc estimer la qualité de service d'un canal radioélectrique en calculant la variation d'un ensemble d'au moins deux valeurs relatives à l'énergie des symboles, par exemple en considérant la variation de l'ensemble de valeurs de la séquence d'amplitude scalaire.
PCT/GB1999/001532 1998-05-14 1999-05-14 Estimation de la qualite d'un canal radioelectrique WO1999059274A2 (fr)

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Publication number Priority date Publication date Assignee Title
WO2007002417A1 (fr) * 2005-06-22 2007-01-04 Qualcomm Incorporated Estimation d'une probabilite d'erreur sur les bits (bep) dans un systeme sans fil edge
JP2008547327A (ja) * 2005-06-22 2008-12-25 クゥアルコム・インコーポレイテッド Ｅｄｇｅ無線システムにおけるビット誤り確率（ｂｅｐ）の推定
EP2182668A1 (fr) * 2005-06-22 2010-05-05 QUALCOMM Incorporated Estimation d'une probabilité d'erreur sur les bits (BEP) dans un système sans fils EDGE.
JP4834088B2 (ja) * 2005-06-22 2011-12-07 クゥアルコム・インコーポレイテッド Ｅｄｇｅ無線システムにおけるビット誤り確率（ｂｅｐ）の推定
US8532232B2 (en) 2005-06-22 2013-09-10 Qualcomm Incorporated Estimating bit error probability (BEP) in an edge wireless system

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AU3941799A (en) 1999-11-29
GB2337424B (en) 2000-12-13
WO1999059274A3 (fr) 2000-03-16
GB9810395D0 (en) 1998-07-15
GB9911284D0 (en) 1999-07-14
GB2337424A (en) 1999-11-17

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