CN1170486A - Method at OFDM-reception for correction of frequency, time window, sampling clock, and slow phase variation - Google Patents
Method at OFDM-reception for correction of frequency, time window, sampling clock, and slow phase variation Download PDFInfo
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- CN1170486A CN1170486A CN95196802A CN95196802A CN1170486A CN 1170486 A CN1170486 A CN 1170486A CN 95196802 A CN95196802 A CN 95196802A CN 95196802 A CN95196802 A CN 95196802A CN 1170486 A CN1170486 A CN 1170486A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L5/00—Arrangements affording multiple use of the transmission path
- H04L5/02—Channels characterised by the type of signal
- H04L5/06—Channels characterised by the type of signal the signals being represented by different frequencies
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2662—Symbol synchronisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2657—Carrier synchronisation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2668—Details of algorithms
- H04L27/2673—Details of algorithms characterised by synchronisation parameters
- H04L27/2675—Pilot or known symbols
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2602—Signal structure
- H04L27/261—Details of reference signals
- H04L27/2613—Structure of the reference signals
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- Computer Networks & Wireless Communication (AREA)
- Synchronisation In Digital Transmission Systems (AREA)
- Television Systems (AREA)
- Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
Abstract
The present invention relates to a method for correction of frequency, time window error, sampling clock and phase error at OFDM-receivers. The information which is transmitted in a signal which includes several carriers in digital form, includes consecutive frames comprising a number of symbols. In respective frames at least one up-chirp and one down-chirp is arranged. Correction of the frequency is made based on analysis of the up- and down-chirps. Further the main focus of the weighted impulse response is determined. The position of the main focus is used for correction of the time window and the sampling clock. Further the vectors from the location of the received carriers are registred in relation to their ideal location in a matrix. The angles between the received vector and the ideally located vector are determined and are weighed together with regard to the amplitude of the transmission function at the frequency of the carrier and the distance of the vector to the origin of coordinates. An average is after that made of the weighed angle distances. The obtained average is used for correction of the phase error. Further, the knowledge of the obtained phase error and previous phase errors is used for estimation of the coming phase error in the following reception.
Description
Technical field
The present invention relates to correction of frequency, sampling clock and the slow in time phase error that changes, the i.e. method of low frequency phase noise in digital system.Receiver is that reception is the so-called OFDM receiver of the digital information of unit with the frame.
Prior art
OFDM (Orthodoxy Frequency Division Multiplex) is a kind of modulator approach, and one of them digital signal is multiplexed on many narrowband carriers.Narrowband carrier is with high density compressed, because utilized a kind of like this fact, promptly equals 1 during divided by the symbol lengths of orthogonal symbols in the carrier spacing, and carrier wave is a quadrature.Usually realize OFDM with the special circuit of carrying out FFT (fast fourier transform).In ofdm signal sends,, adopt chnnel coding and so-called soft decoding (for example Viterbi decoding) usually in order to reduce the probability of error and to enable to tackle the selectivity decline of frequency.OFDM and above-mentioned chnnel coding combine and are known as COFDM (Orthodoxy Frequency Division Multiplex of coding).Adopt the system of sort signal send mode to be used for dissimilar broadcasting in recent years, promptly be used for the one-way system of digital broadcasting and Digital Television.
Patent document EP448493 has described the system that is used for transmitting digital TV.Image information is transferred to the mobile subscriber and is divided into two parts: a part is used for regenerating the normal tv picture and another part generates bigger picture with first.The digital radio signals receiver has been described in file EP441732.Receiver utilizes windowhood method to reduce to appear at the intersymbol interference in the multipath propagation.In order to reduce the negative effect that on receiving, brings, be equipped with on the receiver time window module that is used for from received signal, extracting useful sample by the loss of orthogonality of carrier wave.
U.S. file US5228025 has described the method for preferably digital data transfer being given mobile receiver by radio.This method is transmitted a synchronizing sequence in the mode of at least a frequency, and this synchronizing sequence changes by the known mode of receiver.Be used for tuning local oscillator at this synchronizing sequence of receiver end.
In first kind of prototype of transmission and reception DAB (digital audio broadcasting) signal, two synchronizing symbols have been adopted.First is called nil symbol and does not comprise any information, but the machine that is received is used for sign synchronization on the one hand, is used for estimating the interference in the channel on the other hand.Second symbol is by being called chirp or the sine sweep signal constitutes, and this swept-frequency signal is a sine-shaped signal, its frequency in time linear change and on whole channel width frequency sweep.Receiver is adjusted the position of time window with it on the one hand, promptly received signal is divided into to be used for the section that FFT handles; Be used for estimating the skew of the transfer function and the carrier frequency of channel on the other hand, if any.In last DAB explanation, the linear frequency modulation symbol by so-called TFPC signal (time-frequency phase control), is also referred to as the CAEAC symbol, and institute replaces, and receiver not only had been used for this signal timing, frequency adjustment but also had been used to estimate transfer function.The description of invention/technical problem
With OFDM technical transmission information the requirement very different with traditional system proposed precision.In traditional broadcast system, the normal synchronized between transmitter and the receiver is not enough for utilizing COFDM, transmitting programme information with digital form.Thereby, exist the demand of accurate estimation frequency, time window, sampling clock and phase noise compensation.Solution
The present invention relates to the method for correction of frequency, time window, sampling clock and time dependent phase error in the OFDM reception.The OFDM receiver receives the signal that transmitter sends.Signal contains predetermined content and the interval reference marks of determining, signal is divided into successive frame, and each frame in turn is divided into symbol again.Each frame is divided into continuous in time some symbols.Each allocation of symbols has a sequence number, and above-mentioned reference marks preferably sends in pairs.Receiver is analyzed different reference marks.Signal in the reference marks is made of so-called linear FM signal, promptly so-called sine sweep signal, they are sinusoidal signals, its frequency in time linear change and on whole channel width frequency sweep.The passing in time of one of linear FM signal changes to low-limit frequency from highest frequency, and another linear FM signal then changes to highest frequency from low-limit frequency.The frequency of receiver is adjusted in utilization about the relation between the content of the paired reference marks of Time And Frequency.In addition, the calculated signals according to the reference frame that is received goes out the impulse response that can correct time window and sampling clock.Determine the prime focus of impulse response, determine the actual position of time window in view of the above.The result that utilization obtains, by regulating sampling pulse according to the difference between above-mentioned prime focus and desired location, thereby the position adjustments of time window to desired location.Each symbol is made of the stack carrier wave that some frequencies have nothing in common with each other.Each carrier wave also is configured to such an extent that its phase place and amplitude are modulated by data message waiting for transmission, and its phase place and amplitude can also be described with real part and imaginary part.Above-mentioned reality and imaginary part are assigned definite position in matrix system.In matrix system, allow to have acceptable diverse location with imaginary part in fact.Point that received reality and imaginary part are indicated and the relation between the ideal position in the matrix indicate the angular difference with respect to ideal position, can utilize it to correct phase error when receiving.Advantage
The method of pointing out provided with former impossible mode and accurately adjusts the possibility of receiver, and had improved the intensity that OFDM receives.This method also makes necessity adjustment of carrying out receiver in simple mode become possibility.Thereby this method sends program and can carry out with desired high accuracy in connecting at these.
Accompanying drawing is described
How the schematically illustrated receiver of Fig. 1 at first locatees linear FM signal with one group of binary system correlator according to the present invention, accurately adjusts time window, carrier frequency and phase place then.
Fig. 2 illustrates how to use IFFT (fast fourier inverse transformation) generation ofdm signal.Each inlet on the IFFT is corresponding to a kind of carrier wave.
Fig. 3 illustrates the matrix of using according to the some evaluation phase error of 16QAM system, and the physical location of vector of reception and the angular dependence between the ideal position are shown.Detailed embodiment
A burst is sent and is received machine and receives by transmitter.This burst comprises the some symbols that dispose framing.Section start at each frame disposes one or more synchronizing symbols.At least two above-mentioned synchronization frames comprise so-called linear FM signal.Linear FM signal is a sinusoidal signal, and its frequency changes in time linearly.In order to detect the position of linear FM signal, adopted one group of correlator.The auto-correlation of linear FM signal has very sharp-pointed maximum.In receiver, the sign bit of linear FM signal is received and compares with the signal of storing.This signal be by some " XNOR (XNOR) " door processing compare.As from correlator output signal, draw the number of phase equipotential, i.e. the Hamming weight of result vector.Fig. 11 in the synchronizing signal that obtains be sent back to the frame structure generator, 2 among Fig. 1, this generator is a frame and symbol is controlled and to the single sample number in each symbol to the signal of cutting apart input.The frame structure generator is controlled splitting signal in this way in the suitable time window that is used for FFT (fast fourier transform).Division of signal in the reasonable time window is to FFT (fast fourier transform).After this symbol that comes out from FFT passes to a signal processor with other things, and 3 among Fig. 1 calculates the corrected value of carrier frequency and sampling clock frequency therein.Utilization generates the signal that transmitter sends according to the IFFT (fast fourier inverse transformation) of Fig. 2.For the linear FM signal that makes progress, a carrier wave is arranged according to formula
Given amplitude and phase place, wherein k is corresponding to carrier number, and N is a number of carriers.Generate downward linear FM signal in a similar fashion, just phase place is opposite; Thereby formula becomes
Carry out fast fourier transform (FFT) in the signal receiver that receives.The correction of carrying out in the unit 3 of Fig. 1 is calculated the upwards chirp and the desirable downward chirp that show by receiving and is multiplied each other, and the downward chirp that received multiplies each other with the desirable chirp that makes progress, and finishes the multiplication of the carrier wave that is assigned reverse angle.The signal of Huo Deing (we are referred to as the racemization chirp) all constitutes the estimation of the transfer function of channel in this way.For desirable channel, these estimations of all carrier waves all equal 1.If the displacement of carrier frequency has appearred in the somewhere in channel, then the racemization chirp will have the residual phase shift with the sequence number linear correlation of this carrier wave.These changes upwards with in the downward chirp are obtaining different symbols.The phase position of racemization chirp also is subjected to the influence of frequency selective fading and time window misspecification, but this influence has identical symbol in two kinds of chirps.Therefore the carrier wave error can extract by the racemization phase position that deducts another from the racemization phase position of a chirp.The influence of carrier wave error is doubled like this, but has eliminated other influence.With known method, can estimate the numerical value of carrier wave error according to the linear dependence of phase position and carrier number.Utilize the numeral that draws with known method correction of frequency then.
The impulse response of channel is to utilize the IFFT conversion of racemization chirp to draw.In order to reduce the influence that noise calculates prime focus, before the position of calculating prime focus, a weighting function is multiply by in impulse response.Difference between this position and the desired location constitutes a correction signal, and its controls the clock frequency of A/D converter later on through filtering, and correction value will be by little by little to zero adjustment on this frequency.As a result, time window will arrive desirable position.
Can be according to upwards chirp that is received and downward chirp, the impulse response of estimation channel.It is favourable adopting these two kinds estimation sums in prime focus calculates, because depend on according to chirp upwards or according to chirp downwards and calculate, the error of carrier frequency will influence the position of prime focus with different symbols.Therefore, the prime focus position of two impulse response sums will be insensitive to the error in the carrier frequency, because error will be cancelled each other.
The carrier wave that is received is arranged in the matrix system according to their imaginary part and the corresponding relation of real part.On complex number plane, these points are dispensed in the zone of its existence of permission.The interior point of above-mentioned zone that appears at complex number plane is represented the data sequence of a certain transmission.The relation table of point and ideal position is shown the angular dependence between ideal position and the physical location.Phase error during above-mentioned angular difference is represented to receive.The angular difference mean value that calculates according to all different carriers of prosign constitutes the estimation of phase error.The phase error that obtains with this method just can combine with the phase error that obtains previously subsequently, is used for correcting the phase place of all carrier waves of symbol to be analyzed, and when receiving next symbol, the phase error of estimation expectation.Also it can be used to estimate little frequency shift (FS), because the phase error that they produce all is a constant variation for each symbol.
The above-mentioned method that is used for the evaluation phase error can also be carried out in a different manner.Had been found that two kinds are improved one's methods.One or both improvement can both be used.
First kind of improvement is based on a kind of like this fact, and promptly the amplitude of the signal that receives at synchronization can have varying strength for different carrier waves, frequency.This causes by following reason, i.e. the reflection interference of signal, so-called multipath propagation, or the interference that comes to send in the comfortable so-called Single Frequency Network other transmitter of same signal.The carrier wave of being damaged property interference dies down and draws letter/relation of making an uproar than other signal difference.Can be by in receiver, analyzing the transfer function relevant that the chirp that is received calculates channel with frequency.In the calculating of angular difference mean value, can be with the transfer function weighting that calculates from the value of different carrier, the weight that angular difference had of high decay carrier wave is lower than the angular difference of low decay carrier wave in the channel.Can make the influence minimum of the very noisy of self damping carrier wave whereby to the evaluation phase error.
Second kind of improvement is based on a kind of like this fact, i.e. same noise effects in the received signal, when the evaluation phase error, for away from or near the signal of the origin of coordinates, produce different uncertainties.In order to cross this state of things, above-mentioned angular dependence is according to the distance of the origin of coordinates and weighting.
The present invention is limited to the embodiment that illustrates as an example, but correct in the framework of patent claims that can be below and inventive concept.
Claims
Modification according to the 19th of treaty
1. in the OFDM demodulation, correct carrier frequency, phase error, the method of time window and sampling clock frequency, wherein the signal of OFDM receiver reception is divided into symbol and distributes symbol and distribute reference marks with predetermined content on the interval of determining from the interval of determining, and this signal comprises some carrier waves, wherein each allocation of carriers has a sequence number and reference marks to transmit in pairs, in receiver, analyze the reference marks that is received, and the reference marks that is received is indicated the carrier frequency of how adjusting in the receiver about the content of Time And Frequency, according to the reference marks signal, calculate the impulse response of channel, utilize the correction of impulse response time of implementation window and sampling clock frequency, the complex vector position of receiving demodulation carrier wave and an ideal position compare, and utilize with respect to the skew of above-mentioned ideal position and correct phase error in the reception.It is characterized in that carrier amplitude and phase place in the reference marks is relevant with separately carrier frequency.
2. according to the method for claim 1, the complex vector position that it is characterized in that the receiving demodulation carrier wave is that the relation according to their imaginary part and real part is configured in the matrix system, each vector assignment has the zone of its existence of indication, and the vector that appears in the above-mentioned zone is equipped with certain angular distance apart from ideal bit, and above-mentioned angular distance is used for calculating phase error.
3. according to any one method in the claim of front, it is characterized in that, depend on whether receiver receives a plurality of signals, the amplitude of received signal is that intensity is different to different carrier, the estimation of the phase error of each carrier wave is to depend on transfer function amplitude on the current frequency and weighting, and wherein the amplitude higher signal has the signal higher weight lower than amplitude.
4. according to any one method in the claim of front, it is characterized in that, depend on the amplitude of vector, promptly they give vector different weights to the distance of the origin of coordinates, and these weights are according to the influence of noise to angle error, and amplitude is compensated.Wherein, distribute to apart from origin of coordinates angular distance farthest and have than those near the higher weight of angular distance of the origins of coordinates, the correction of phase error is carried out according to the weighting angle error.
5. according to any one method in the claim of front, it is characterized in that up-to-date phase error that obtains and the phase error that obtains previously compare, and calculate the expectation phase error when receiving next sequence.
6. according to any one method in the claim of front, it is characterized in that, determine the prime focus position of the impulse response of channel, the physical location of this position window instruction time, and according to the difference between physical location and desired location, by according to above-mentioned discrepancy adjustment sampling clock frequency, realize adjustment to the time window position.
7. according to any one method in the claim of front, it is characterized in that the content of reference marks is made of linear FM signal.
Claims (10)
1. in the OFDM demodulation, be used to correct carrier frequency, phase error, the method of time window and sampling clock frequency, wherein the signal of OFDM receiver reception is divided into symbol, and distribute symbol to reach on the interval of determining with the interval of determining and distribute reference marks with predetermined content, signal comprises some carrier waves, sequence number of each allocation of carriers wherein, and contrast signal can send in couples, it is characterized in that, in receiver, analyze the reference marks that is received, how the reference marks that is received adjusts carrier frequency in the receiver about the content representation of Time And Frequency, go out the impulse response of channel according to the reference marks calculated signals, utilize the correction of impulse response time of implementation window and sampling clock frequency, the complex vector position of receiving demodulation carrier wave and an ideal position compare, and utilize with respect to the skew of above-mentioned ideal position and correct phase error in the reception.
2. according to the method for claim 1, it is characterized in that, according to they real part and the relation of imaginary part with the complex vector position configuration of receiving demodulation carrier wave in a matrix system, each vector assignment has the zone of its existence of indication, the vector that appears in the above-mentioned zone is equipped with certain angular distance apart from ideal bit, and utilizes above-mentioned angular distance to calculate phase error.
3. according to any one method in the claim of front, it is characterized in that, depend on whether receiver receives a plurality of signals, the amplitude of received signal is that intensity is different for different carrier waves, the estimation of the phase error of each carrier wave is to depend on the transfer function amplitude of current frequency and weighting, and wherein the amplitude higher signal has the signal higher weight lower than amplitude.
4. according to any one method in the claim of front, it is characterized in that, the amplitude that depends on vector, it is the distance that they arrive the origin of coordinates, give vector different weights, these weights compensate amplitude the influence of angle error according to noise, have than those higher weights of angular distance near the origins of coordinates apart from origin of coordinates angular distance farthest, and the correction of phase error are carried out according to the weighting angle error.
5. according to any one method in the claim of front, it is characterized in that up-to-date phase error that obtains and the phase error that obtains previously compare, and calculate the expectation phase error when receiving next sequence.
6. according to any one method in the claim of front, it is characterized in that, determine the position of the impulse response prime focus of channel, the physical location of this position express time window, and according to the difference between physical location and desired location, by realize adjustment according to above-mentioned discrepancy adjustment sampling clock frequency to the time window position.
7. according to any one method in the claim of front, it is characterized in that the carrier amplitude in the reference marks and phase place are relevant with separately carrier frequency.
8. according to any one method in the claim of front, it is characterized in that the content of reference marks is made of linear FM signal.
9. according to any one method in the claim of front, it is characterized in that reference marks is to send in pairs, wherein distribute in the paired reference marks one upwards chirp and another downward chirp.
10. according to any one method in the claim of front, it is characterized in that carrier amplitude in the chirp upwards and phase place by
Definition, downwards in the chirp then by
Definition, wherein N represents number of carriers, k is the sequence number of each carrier wave, and j
2=-1.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| SE9404356-9 | 1994-12-14 | ||
| SE9404356A SE504787C2 (en) | 1994-12-14 | 1994-12-14 | Method of OFDM reception for correction of frequency, time window, sampling clock and slow phase variations |
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| Publication Number | Publication Date |
|---|---|
| CN1170486A true CN1170486A (en) | 1998-01-14 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CN95196802A Pending CN1170486A (en) | 1994-12-14 | 1995-11-27 | Method at OFDM-reception for correction of frequency, time window, sampling clock, and slow phase variation |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPH10510958A (en) |
| KR (1) | KR980700750A (en) |
| CN (1) | CN1170486A (en) |
| AU (1) | AU4275196A (en) |
| SE (1) | SE504787C2 (en) |
| WO (1) | WO1996019056A1 (en) |
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| EP3002884B1 (en) * | 2014-09-30 | 2018-04-18 | Semtech Corporation | Wireless communication method |
| JP6580289B2 (en) * | 2017-05-12 | 2019-09-25 | 三菱電機株式会社 | Wireless communication device |
| DE102017220828A1 (en) * | 2017-11-22 | 2019-05-23 | Robert Bosch Gmbh | Method and system for suppressing an interference signal in the detection of a chirp signal |
| CN115333160A (en) * | 2022-09-13 | 2022-11-11 | 国网福建省电力有限公司 | Microgrid robust frequency control method based on wind turbine generator and electric automobile |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB9020170D0 (en) * | 1990-09-14 | 1990-10-24 | Indep Broadcasting Authority | Orthogonal frequency division multiplexing |
| DE4128713A1 (en) * | 1991-08-29 | 1993-03-04 | Daimler Benz Ag | METHOD AND ARRANGEMENT FOR MEASURING THE CARRIER FREQUENCY STORAGE IN A MULTI-CHANNEL TRANSMISSION SYSTEM |
| DE4310031C2 (en) * | 1993-03-27 | 1997-07-17 | Grundig Emv | Method for correcting the phase and amplitude of a broadband received signal using reference signals |
-
1994
- 1994-12-14 SE SE9404356A patent/SE504787C2/en not_active IP Right Cessation
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1995
- 1995-11-27 WO PCT/SE1995/001413 patent/WO1996019056A1/en not_active Application Discontinuation
- 1995-11-27 JP JP8518667A patent/JPH10510958A/en active Pending
- 1995-11-27 CN CN95196802A patent/CN1170486A/en active Pending
- 1995-11-27 AU AU42751/96A patent/AU4275196A/en not_active Abandoned
- 1995-11-27 KR KR1019970703823A patent/KR980700750A/en not_active Application Discontinuation
Cited By (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN100440877C (en) * | 2001-08-30 | 2008-12-03 | 英特尔公司 | Technique for continuous ofdm demodulation |
| CN101939935B (en) * | 2007-12-11 | 2013-08-14 | 易兹安全网络公司 | Reduction of interference in OFDM signal with weighting vectors evolving in vector subspaces |
| CN102970034A (en) * | 2012-12-05 | 2013-03-13 | 天津光电通信技术有限公司 | High precision local oscillation output method used for short-wave receiver radio frequency module |
| CN102970034B (en) * | 2012-12-05 | 2014-11-05 | 天津光电通信技术有限公司 | High precision local oscillation output method used for short-wave receiver radio frequency module |
| WO2015161593A1 (en) * | 2014-04-23 | 2015-10-29 | 中兴通讯股份有限公司 | Method and device for measuring wireless channel response of td-lte system |
Also Published As
| Publication number | Publication date |
|---|---|
| SE9404356D0 (en) | 1994-12-14 |
| JPH10510958A (en) | 1998-10-20 |
| SE9404356L (en) | 1996-06-15 |
| SE504787C2 (en) | 1997-04-28 |
| WO1996019056A1 (en) | 1996-06-20 |
| KR980700750A (en) | 1998-03-30 |
| AU4275196A (en) | 1996-07-03 |
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