WO1999033203A1 - Method and apparatus for determining an operating point of a non-linear amplifier of a communication channel - Google Patents
Method and apparatus for determining an operating point of a non-linear amplifier of a communication channel Download PDFInfo
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- WO1999033203A1 WO1999033203A1 PCT/EP1998/008306 EP9808306W WO9933203A1 WO 1999033203 A1 WO1999033203 A1 WO 1999033203A1 EP 9808306 W EP9808306 W EP 9808306W WO 9933203 A1 WO9933203 A1 WO 9933203A1
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- signal
- communication channel
- pseudo noise
- level
- payload
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/14—Relay systems
- H04B7/15—Active relay systems
- H04B7/185—Space-based or airborne stations; Stations for satellite systems
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B17/00—Monitoring; Testing
- H04B17/20—Monitoring; Testing of receivers
- H04B17/24—Monitoring; Testing of receivers with feedback of measurements to the transmitter
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B17/00—Monitoring; Testing
- H04B17/10—Monitoring; Testing of transmitters
Definitions
- This invention relates to a method and an apparatus for determining an operating point of a non-linear amplifier of a communication channel, especially a transponder in a communication satellite under load.
- a non-linear high power amplifier In high frequency communication channels, a non-linear high power amplifier must often be driven at its saturation point in order to obtain the maximum possible output.
- a signal from an uplink station on the ground is received by an antenna, converted in frequency, filtered in an input multiplexer, and amplified by a driver limiter amplifier and a high power amplifier before being filtered in the output multiplexer and retransmitted to the ground.
- the high power amplifier In order to provide a sufficient signal everywhere within the satellite footprint, the high power amplifier must be driven in its saturation point, i.e. the point of maximum in the non-linear transfer curve representing output power vs. input power, as for example shown in Fig. 6a.
- the driver limiter amplifier is a preamplifier and can be set to run in one of two modes. In linear mode, it acts as a simple linear amplifier. In limiting mode, it provides the function of an automatic level control (ALC) .
- the DLA is normally operated in limiting mode in order to compensate short term level variations due to weather influences. In limiting mode, the DLA shall always provide the same output power to the high power amplifier (HPA) , such that the HPA is permanently operated in saturation.
- HPA high power amplifier
- the HPA is always operated in saturation and that the signal power from the uplink station is high enough in level at the satellite.
- the satellite operator is forced to monitor regularly the power flux density as received at the satellite transponder input.
- the aim is that the video signal from the uplink station is strong enough so that under clear weather conditions, the HPA on board of the satellite is driven in saturation with the DLA in linear mode. This criterion must also be met if the HPA transfer curve has changed due to aging.
- the operating point of the high power amplifier cannot be determined only from monitoring the downlink power. On one hand this is due to the fact that close to saturation, the input power may vary by a few dB, while the output power will only vary at most a few tenth of dB. On the other hand, if a certain amount of downlink power is measured, it cannot be determined whether the HPA is operated below or above saturation because the transfer curve is ambiguous in output power.
- the bitrate of the telemetry data stream may contain a maximum of a few kbit/s. Therefore, even if the satellite is equipped with a power monitoring system, it is also preferable to perform measurements from a ground station instead of onboard the satellite, for many reasons (i.e. failure, weight of the satellite etc.).
- a satellite operator In addition to measuring the received power at the satellite, a satellite operator is performing regularly 'In Orbit Tests' (IOT) in order to measure the power flux density required to drive the HPA in saturation.
- IOT In Orbit Tests'
- a first conventional method as decribed in International Journal of Satellite Communications, Special issue on In- orbit Testing of Communications Satellites, Volume 13, Number 5, Wiley 1995 or in DE-C-33 33 418, is known as AM nulling according to which an amplitude modulated (AM) signal in the uplink is used which is swept in power until the amplitude modulation disappears completely. This point is exactly at saturation.
- a second conventional method of determining the transfer curve of the HPA consists of measuring transmit and receive power of a clean carrier, where all path attenuations have to be cancelled out. Both IOT measurement methods require that the transponder under test is not operated. In other words, the payload signal has to be switched off during the tests .
- the level of said second signal is approx. 20 dB or more below the level of said first signal.
- said second signal is a pseudo noise modulated clean carrier signal and said output signal of said communication channel (1) corresponding to said second signal is a recovered carrier signal.
- said second signal is a clean carrier signal and wherein said output signal of said communication channel corresponding to said second signal is a narrowband filtered carrier signal .
- reference values are used together with said output signal of said communication channel corresponding to said second signal to determine the operating point of said non-linear amplifier. These reference values can be pre-recorded for said nonlinear amplifier and correspond to a transfer curve of said non-linear amplifier.
- an apparatus for determining the operating point of a nonlinear amplifier of a communication channel comprising means for transmitting a second signal through said communication channel simultaneously with a first signal being transmitted through said communication channel and means for determining said operating point of said nonlinear amplifier on the basis of an output signal of said communication channel corresponding to said second signal, the input power of said first signal being such that said non-linear amplifier is operated in a non-linear mode and the input power of said second signal being below the input power of said first signal.
- said means for determining said operating point of said non-linear amplifier on the basis of an output signal of said communication channel corresponding to said second signal comprise means for storing reference values to be used together with said output signal of said communication channel corresponding to said second signal to determine the operating point of said non-linear amplifier.
- a first input signal is transmitted through the communication channel at a power level which drives the non-linear amplifier in a non-linear operation mode.
- a second input signal is transmitted through the communication channel simultaneously with the first input signal.
- the second input signal is transmitted at a level below the level of the first input signal. If the contribution of the second input signal to the total input of the non-linear amplifier is small, the operating point of the non-linear amplifier is determined almost only by the first input signal. Therefore, the output power corresponding to the second signal is determined most strongly by the input power of the first signal.
- the operating point of said non-linear amplifier is determined on the basis of an output signal of said communication channel corresponding to said second signal .
- the invention further provides a method for determining the operating point of a non-linear amplifier of a communication channel through which a payload signal is transmitted at a predetermined level, comprising: generating a first pseudo noise signal PN(t); modulating a clean carrier signal f (t) with said first pseudo noise signal PN(t) to generate a PN modulated clean carrier signal s(t); transmitting said PN modulated clean carrier signal s(t) simultaneously with said payload signal through said communication channel at a level below the level of said payload signal; receiving a receive signal s' (t) corresponding to said PN modulated clean carrier signal s(t) after having traveled through said communication channel; correlating said receive signal s' (t) with said first pseudo noise signal PN(t) to generate a recovered carrier signal f ' (t) ; and determining the operating point of said non-linear amplifier of the communication channel on the basis of said clean carrier signal f (t) and said recovered carrier signal f (t)
- the level of said PN modulated clean carrier signal s (t) is approx. 20 dB or even approx. 30 dB or more below the level of said payload signal.
- said first pseudo noise signal PN(t) is a binary pseudo noise sequence, said binary pseudo noise sequence being generated by means of a feed back shift register or a memory device in which a sequence of values of a pseudo noise signal is stored.
- Said correlating of said receive signal s' (t) and said first pseudo noise signal PN(t) can be achieved by delaying said first pseudo noise signal PN(t) and multiplying the delayed first pseudo noise signal PN(t) and said receive signal s' (t) .
- a gain is determined on the basis of said clean carrier signal f (t) and said recovered carrier signal f ' (t) and said gain is used to determine the input power of said payload signal.
- Reference values are used to derive from said gain the input power of said payload signal, said reference values having been prerecorded for said non-linear amplifier and representing a gain curve or transfer curve of said non-linear amplifier over the input power of said payload signal.
- the method according to the invention is advantageously applicable if said communication channel is a transponder of a communication satellite.
- the invention furthermore provides an apparatus for determining the operating point of a non-linear amplifier of a communication channel through which a payload signal is transmitted at a predetermined level, comprising first pseudo noise signal generating means for generating a pseudo noise signal PN(t); first modulating means for modulating a clean carrier signal f(t) with said first pseudo noise signal PN(t) to generate a PN modulated clean carrier signal s(t); transmitting means for transmitting said PN modulated clean carrier signal s (t) simultaneously with said payload signal through said communication channel at a level below the level of said payload signal; receiving means for receiving a receive signal s' (t) corresponding to said PN modulated clean carrier signal s (t) after having traveled through said communication channel; and first correlating means for correlating said receive signal s'(t) with said pseudo noise signal PN(t) to generate a recovered carrier signal f ' (t) .
- the level of said PN modulated clean carrier signal s (t) is at least 20 dB or even at least 30 dB below the level of said payload signal.
- said first pseudo noise signal generating means (9) is a feed back shift register or a memory device in which a sequence of values of a pseudo noise signal is stored.
- a clean carrier signal f (t) is modulated with a pseudo noise signal PN(t) and transmitted through the communication channel at a level below the level of a payload signal which is transmitted via the communication channel simultaneously.
- the received signal s'(t) is correlated with the same pseudo noise signal PN(t) to obtain a recovered carrier signal f ' (t) .
- the power of the clean carrier signal f (t) and of the recovered carrier signal f ' (t) are used to determine the gain of the signal and on the basis of reference values (calibration curves) the input power of the payload signal. Since the PN modulated clean carrier signal s(t) is transmitted at a low level, it is possible to perform measurements without switching off the payload signal, the input power of which defining the operating point of the non-linear amplifier.
- Fig. 1 shows a schematic diagram of a communication channel comprising a non-linear amplifier
- Fig. 2 shows transfer curves of a non-linear amplifier
- Fig. 3 shows a diagram of gain difference over input power of a non-linear amplifier
- Fig. 4 shows a schematic diagram of a transponder of a communication satellite
- Fig. 5 shows a schematic diagram of an embodiment of an apparatus according to the invention.
- Fig. 6a and 6b show transfer curves and gain curves of a non-linear amplifier for large and small signals.
- Fig. 1 shows a communication channel 1 comprising a nonlinear amplifier 2 for amplifying the signals transmitted through the communication channel. If a total input signal I is fed to an input 3 of the communication channel 1, the signal traveles through the communication channel 1, is amplified by the non-linear amplifier 2, and is output as a total output signal 0 at an output 4 of the communication channel 1.
- Fig. 2 shows a transfer curve A of a traveling wave tube amplifier (TWTA) , as an example of a non-linear amplifier, a non-linear mode of operation is effected if the input power Pj of the total input signal
- TWTA traveling wave tube amplifier
- each operating point of the nonlinear amplifier in the non-linear region (a) is defined by a specific input power P X of an input signal I and a corresponding output power P 0 of an output signal 0 of the communication channel. In saturation the input signal provides an input power of P IS corresponding to an output power of P os .
- a first input signal I ⁇ is transmitted through the communication channel 1 at a power level p ⁇ which drives the non-linear amplifier 2 in a nonlinear operation mode.
- a second input signal ⁇ 2 is transmitted through the communication channel 1 simultaneously with the first input signal II.
- the input power P I of the second signal ⁇ 2 i- s lower than the input power p ⁇ l of the first signal Ii . If the contribution of the second input signal ⁇ 2 to the total input of the non-linear amplifier is small, the operating point of the non-linear amplifier is determined almost only by the first input signal.
- the output power P 0 2 corresponding to the second signal i2 is determined most strongly by the input power P j ⁇ of the first signal I .
- any variation in input power of the first signal causes a variation in output power of the second signal.
- the second input signal should be some 15 to 30 dB or more, depending on the application, below the first input signal. This is indicated in the linear region (b) in Fig. 2 which shows a transfer curve B representing the output power of a small input signal plotted against the input power of a large input signal.
- the transfer curve B of the second input signal falls off much faster than the transfer curve of the first input signal so that any variation of the output power of the second input signal caused by a variation of the input power of the first input signal can be measured much easier as long as the part 02 of the output signal 0 corresponding to the second input signal ⁇ 2 can be separated from the part O ⁇ _ of the output signal 0 corresponding to the first input signal I ⁇ _, as indicated in Fig. 1.
- the separation of the contributions 0_ and 02 of the first and second input signals Ii and i2, respectively, in the output signal 0 may be achieved in several different ways .
- the first input signal I ⁇ is a FM or QPSK signal
- the second input signal ⁇ 2 ma Y be a pseudo noise modulated clean carrier signal as will be explained further below in greater detail.
- the recovered carrier signal represents the output signal 02 corresponding to the second input signal ⁇ 2 .
- the second input signal could be a clean carrier signal having a frequency which avoids deterioration of the second input signal by the first input signal, for example having a frequency outside the frequency band of the first input signal.
- narrowband filtering the output signal 0 at the frequency of the second input signal the part 02 of the second input signal i2 in the total output signal 0 can be determined.
- the operating point of the nonlinear amplifier can be determined in different ways. If the input power of the first and second input signal is known the output power corresponding to these signals can be measured and a transfer curve or a gain curve, an example being shown in Fig. 6b, can be obtained. If the transfer curve or the gain curve is known, the input power of the first input signal driving the non-linear amplifier in a non-linear mode of operation can be determined by transmitting a second input signal of a known input power through the communication channel and measuring the output power corresponding to the second input signal.
- the input power of the first signal and, therefore, the operating point of the non-linear amplifier can be determined on the basis of the input power of the second input signal and the transfer curve or the gain curve (or any other representation of the above described relation between the large and the small input signal) , if the second input signal is a small signal compared to the first input signal, as explained above.
- the transfer curve B of the second signal ⁇ 2 allows to determine the input power of the first signal I ⁇ to be P j ia without measuring the output power of the first signal at all.
- the transfer curve and the gain curve of the non-linear amplifier may change due to aging.
- such a change of the transfer curve can be detected by determining the operating point of the non-linear amplifier on the basis of first and second input signals I_ and i2 the individual input powers P- and P I2 of which are known.
- the operating point can be determined and compared to the operating point derived on the basis of the transfer curve (or any other representation thereof) .
- Fig. 3 shows a diagram representing a gain difference between the first signal and the second signal, i.e. gain sma ⁇ _]_ _ 9 a i n large' plotted over the input power of the first signal.
- an operating point Pla, Poa
- aging of the non-linear amplifier has caused a shift of the curve, indicated by curve C .
- a similar shift can also be seen in the transfer curve of the non-linear amplifier.
- Fig. 4 shows the components of a transponder in a communication satellite as an example for a communication channel .
- An output signal of said receiving antenna 11 is fed to an input demultiplexer (IMUX) 13 after frequency conversion in frequency converter 12.
- IMUX input demultiplexer
- Said input demultiplexer 13 comprises several first filters 14-1 to 14-n for separating individual signals within the signal from the antenna. Typically, one filter is provided for each signal to be separated from the other signals received via said receiving antenna 1 and corresponds to a communication channel.
- the n output signals of said input demultiplexer 13 are fed to a corresponding number of driver limiter amplifiers 15a-l to 15a-n and high power amplifiers 15b-l to 15b-n.
- a traveling wave tube (TWT) is employed for amplifying the output signals of said input demultiplexer 13.
- the high power amplifiers 15b-l to 15b-n are non-linear amplifiers having a transfer curve and gain curve as indicated by curves A in Fig. 6a and 6b, respectively. If not set to a linear mode the driver limiter amplifiers 15a- 1 to 15a-n are either limiting or amplifying the input signal received from the input demultiplexer 13 before being fed to the respective high power amplifier.
- the amplifier output signals are passed through second filters 16-1 to 16-n which are part of an output multiplexer (OMUX) 17 combining the n amplifier output signals.
- the output signal of said output multiplexer 17 is fed to a transmitting antenna 18 for being transmitted to the desired area on the ground.
- each of said high power amplifiers 15b-l to 15b-n depends on the payload signal (the first input signal) from the uplink ground station, which signal should be such that the amplifier is driven in saturation in order to achieve maximum output power.
- the driver limiter amplifiers 15a-l to 15a-n can be set such that each of said high power amplifiers is operated in its saturation point.
- the driver limiter amplifiers are set to linear operation.
- a pseudo noise signal PN(t) is generated by means of a pseudo noise signal generator 19, for example a feed back shift register or a memory device in which a sequence of values of a pseudo noise signal is stored.
- the pseudo noise signal PN(t) has a very sharp autocorrelation function at zero delay. This allows to determine the time delay between the locally generated pseudo noise signal PN(t) and a received signal which is delayed due to the propagation time.
- the transponder remains usable during the test and can be continuously supplied with a payload signal.
- the level of the transmitted PN modulated clean carrier signal s(t) is sufficiently below the level of the payload signal, for example about 20 to 30 dB or more, such that the payload signal is not notably deteriorated.
- the PN modulated clean carrier signal s(t) can be transmitted while the communication channel is in use, i.e. simultaneously with a payload signal being transmitted to the transponder of the satellite from the same or from another ground station.
- antenna 23 is also used to receive the signal re-transmitted by the transponder of the satellite, in other words the signal which has traveled through the communication channel.
- the output signal of antenna 23 is passed through a downconverter 24 to obtain a receive signal s' (t) which is fed to a second multiplier 25 receiving also the same but delayed pseudo noise signal PN(t) .
- the delay is generated by delaying means 26 which are set such that the output of the second multiplier 25 becomes maximum.
- the receive signal s' (t) is multiplied, in other words correlated with the very same pseudo noise signal PN(t) which has been used for generating the PN modulated clean carrier signal s(t) and a recovered carrier signal f ' (t) is obtained which is only delayed and attenuated in comparison with the clean carrier signal f(t)
- the path attenuation is constant as free space loss does practically not vary with the distance between the satellite and the ground station. Since atmosperical attenuation can be measured with radiometers, it can be taken into account as well as the gain of the ground station antenna at the corresponding frequencies.
- the input power of the clean carrier signal f (t) and the output power of the recovered carrier signal f ' (t) can be measured to determine the gain of this signal.
- the input power of the payload signal is determined on the basis of said gain and of reference values or calibration curves, which are shown in Fig. 6a and 6b and which will be explained in greater detail further below.
- the gain of the small signal is measured to be -4 dB the input power of the large signal is -1 dB.
- this measurement is compared to measuring the output power: while the output power of the large signal changes by less than 0.05 dB for the input power varying from 0 dBW to -1 dB, the gain of the small signal varies by almost 2 dB.
- Fig. 6a transfer curves for a large signal (A) and three small signals (B ⁇ , B2, B3) over a traveling wave tube amplifier (TWTA) are shown. For simplicity, the values are given relative to the saturation point of the amplifier. This means that in Fig. 6a 0 dB input power corresponds to 0 dB output power.
- the three small signals (Bi, B2, B3) are 20 dB, 30 dB, and 40 dB below the large signal, respectively.
- Fig. 6b gain curves for the large signal (A) and the three small signals (B]_, B2, B3) are shown. Again, the values are given relative to the saturation point of the amplifier so that in Fig.
- 0 dB input power corresponds to 0 dB gain.
- the gain curves for the three small signals overlap completely.
- the above described transfer curves and gain curves, shown in Fig. 6a and 6b, are obtained as calibration curves for each amplifier in the satellite in order to determine the operating point of the individual amplifier later on.
- the calibration curves are recorded in the form of reference values which are stored in appropriate storage means for being used in determining the operating point of the non-linear amplifier.
- a large signal and a small signal are generated where the small signal is for example 20 dB, 30 dB, or 40 dB below the large signal.
- the large and the small signal may be a clean carrier or the large signal may be a FM or OPSK modulated signal to come as close as possible to real operation conditions and the small signal may be a pseudo noise modulated clean carrier signal.
- Both signals, i.e. the large and the small signal are combined and transmitted to the transponder.
- the total input signal received by antenna 11 is fed to the input of the high power amplifier (TWTA) .
- TWTA high power amplifier
- the combined signal is swept in power, thus the level difference between the large signal and the small signal at the input will always remain the same.
- the power of the small signal may be kept constant as it does not substantially influence the operating point of the nonlinear amplifier.
- the output signal of the high power amplifier (TWTA) is fed to antenna 18 via output demultiplexer 17 and the output levels corresponding to both input signals are measured separately.
- the output power of the large signal (which is almost equal to the total output power as the small signal has a negligible contribution) is given as a function of the input power of the large signal.
- the output power of the small signal is also given as a function of the input power of the large signal.
- the gain of the large signal and the gain of the small signals are given as a function of the input power of the large signal.
- means 27 for determining a gain on the basis of said clean carrier signal f (t) and said recovered carrier signal f ' (t) are provided, receiving both the clean carrier signal f (t) and the recovered carrier signal f ' (t) .
- means 28 for deriving the input power of said payload signal from reference values and from said gain are provided.
- the output of said means 27 for determining a gain are supplied to said means 28 for deriving the input power of said payload signal.
- the reference values are stored in and supplied from means 29 for storing said reference values.
- the said reference values have been pre-recorded for said non-linear amplifier and represent a gain curve or transfer curve of said non-linear amplifier over the input power of said payload signal, as described with respect to Fig. 6a and 6b .
- true noise signals can be used in the method and the apparatus according to the invention. Properties of true and pseudo noise signals are well known to those skilled in the art and are described, for example in Bernard Sklar, "Digital Communications - Fundamentals and Applications", Prentice Hall, 1988.
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Priority Applications (9)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EA200000669A EA002214B1 (en) | 1997-12-18 | 1998-12-17 | Method and apparatus for determining an operating point of a non-linear amplifier of a communication channel |
JP2000525993A JP3639531B2 (en) | 1997-12-18 | 1998-12-17 | Method and apparatus for determining the operating point of a nonlinear amplifier in a communication channel |
PL98341217A PL190186B1 (en) | 1997-12-18 | 1998-12-17 | Method of and apparatus for determining operation point of a telecommunication channel non-linear amplifier |
AU25134/99A AU741481B2 (en) | 1997-12-18 | 1998-12-17 | Method and apparatus for determining an operating point of non-linear amplifier of a communication channel |
KR1020007006749A KR100609315B1 (en) | 1997-12-18 | 1998-12-17 | Method and apparatus for determining an operating point of a non-linear amplifier of a communication channel |
IL13676598A IL136765A (en) | 1997-12-18 | 1998-12-17 | Method and apparatus for determining an operating point of a non-linear amplifier of a communication channel |
BRPI9813708-5A BR9813708B1 (en) | 1997-12-18 | 1998-12-17 | method and apparatus for determining an operating point of a nonlinear amplifier of a communication channel. |
CA002315053A CA2315053C (en) | 1997-12-18 | 1998-12-17 | Method and apparatus for determining an operating point of a non-linear amplifier of a communication channel |
NO20003158A NO323273B1 (en) | 1997-12-18 | 2000-06-16 | Method and apparatus for determining an operating point of a linear amplifier in a communication channel |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
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EP97122420.9 | 1997-12-18 | ||
EP97122420A EP0929164B1 (en) | 1997-12-18 | 1997-12-18 | Method and apparatus for determining an operating point of a non-linear amplifier of a communication channel |
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WO1999033203A1 true WO1999033203A1 (en) | 1999-07-01 |
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PCT/EP1998/008306 WO1999033203A1 (en) | 1997-12-18 | 1998-12-17 | Method and apparatus for determining an operating point of a non-linear amplifier of a communication channel |
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US (1) | US6275678B1 (en) |
EP (1) | EP0929164B1 (en) |
JP (1) | JP3639531B2 (en) |
KR (1) | KR100609315B1 (en) |
CN (1) | CN1136682C (en) |
AR (1) | AR008353A1 (en) |
AT (1) | ATE190785T1 (en) |
AU (1) | AU741481B2 (en) |
BR (1) | BR9813708B1 (en) |
CA (1) | CA2315053C (en) |
DE (1) | DE69701473T2 (en) |
DK (1) | DK0929164T3 (en) |
EA (1) | EA002214B1 (en) |
ES (1) | ES2145552T3 (en) |
GR (1) | GR3033472T3 (en) |
HK (1) | HK1021455A1 (en) |
ID (1) | ID24961A (en) |
IL (1) | IL136765A (en) |
NO (1) | NO323273B1 (en) |
PL (1) | PL190186B1 (en) |
PT (1) | PT929164E (en) |
TR (1) | TR200001780T2 (en) |
WO (1) | WO1999033203A1 (en) |
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