Telecommunications – Carrier wave repeater or relay system – Plural modulated carriers
Reexamination Certificate
1998-12-17
2001-08-14
Trost, William (Department: 2683)
Telecommunications
Carrier wave repeater or relay system
Plural modulated carriers
C455S334000, C455S341000, C330S002000, C330S010000, C375S367000
Reexamination Certificate
active
06275678
ABSTRACT:
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.
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. For example in a satellite, but without limiting the invention to this application, 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. 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. 6
a.
The driver limiter amplifier (DLA) 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. Even if the DLA is able to maintain the HPA in saturation if the power received from the uplink ground station is below specification, it is very important that the uplink ground station power is kept on a high level since if the DLA has to compensate for uplink power, the overall signal to noise ratio (SNR) is decreasing, as this is mainly driven by the SNR in the earliest stage of the signal path which is the uplink path in this case.
From the point of view of a satellite operator, it is important to know that the HPA is always operated in saturation and that the signal power from the uplink station is high enough in level at the satellite. Thus 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.
Since the uplink power is not accurately known (for instance if the uplink is not done from a site belonging to the satellite operator itself), 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.
In order to be able to monitor the total received signal power at the HPA input, many satellites are equipped with a power monitoring system. This data can be sent together with telemetry data to the satellite operator. Apart from not all satellites being necessarily equipped with a power monitoring system, the drawback of this method is that if such a system exists in the satellite the related data consumes a certain amount of the telemetry data stream from the satellite to the operator's ground station, which could be used for other vital data. Further, 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.).
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.
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 necessity of switching off the payload signal during in orbit tests represents a considerable drawback not only for the user of the transponder, since communication is interrupted, but also for the operator of the satellite since the tests have to be performed in an expedited manner to keep the interruption as short as possible. In some cases it is impossible to interrupt communications via the communication channel so that the conventional methods cannot be used for testing the HPA after the satellite has entered in operation.
It is an object of the present invention to provide a method and an apparatus for determining an operating point of a non-linear amplifier of a communication channel.
It is a further object of the invention to provide such a method and such an apparatus avoiding the necessity to interrupt traffic via the communication channel.
These objects and other objects are achieved by a method for determining the operating point of a non-linear amplifier of a communication channel, wherein a first signal is transmitted simultaneously with a second signal through said communication channel and said 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 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.
Preferably, the level of said second signal is approx. 20 dB or more below the level of said first signal.
In a preferred embodiment, 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.
In another preferred embodiment, 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.
Advantageously, 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 non-linear amplifier and correspond to a transfer curve of said non-linear amplifier.
The above objects and other objects are also achieved by an apparatus for determining the operating point of a non-linear 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 non-linear 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 th
Bethscheider Gerhard
Harles Guy
Societe Europeenne des Satellites S.A.
Tran Congvan
Trost William
Wolf Greenfield & Sacks P.C.
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