Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1998-12-17
2003-03-18
Liu, Shuwang (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S130000, C375S142000, C375S150000, C375S297000
Reexamination Certificate
active
06535546
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and an apparatus for determining characteristics of components of a communication channel, especially a transponder in a communication satellite under load.
BACKGROUND OF THE INVENTION
The characteristics of a communication channel may change during the lifetime of the equipment used. Various tests may be performed not only at the beginning of but also repeatedly during the lifetime to verify that the communication channel meets predetermined specifications. Usually, these tests are performed without normal traffic, i.e. without the communication channel being used for transmission of a communication signal. This scenario will be explained in the following in greater detail with reference to communication satellites but without limiting the invention disclosed further below to only this application although the invention is specifically applicable in this field.
In a communication satellite, a communication channel is set up by a transponder of the satellite comprising several components like a receiving antenna, an input demultiplexer, a power amplifier, an output multiplexer, and a transmitting antenna. Transponder characteristics, such as amplitude response and group delay, are measured not only at the beginning of life of the spacecraft on the ground and, after launch, in orbit but also during the lifetime. These measurements are conventionally carried out without normal traffic on the transponder, i.e. without a payload signal being transmitted to and being re-transmitted by the transponder.
The necessity of switching off the payload signal during 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 components of these channels cannot be tested after having enters into operation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method and an apparatus for determining characteristics of components of a communication channel, especially a transponder of a satellite, without the necessity to interrupt traffic via the communication channel.
This object and other objects are achieved by a method for determining characteristics of components of a communication channel which is designed for transmitting a payload signal 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 characteristics of components of the communication channel on the basis of a comparison of said clean carrier signal f(t) and said recovered carrier signal f′(t).
Advantageously, the level of said PN modulated clean carrier signal s(t) is at least 15 dB, preferably 25 dB or more below the level of said payload signal.
In a further embodiment, said first pseudo noise signal PN(t) is a binary pseudo noise sequence, which is preferably generated by means of a feed back shift register.
A chiprate of said first pseudo noise signal PN(t) is less than 5 MChip/s and preferably less than or equal to 2.5 MChip/s.
In a further embodiment, said correlating of said receive signal s′(t) and said first pseudo noise signal PN(t) is 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).
To generate a reference, the method according to the invention further comprises: generating a second pseudo noise signal PN
R
(t); modulating a reference carrier signal f
R
(t) with said second pseudo noise signal PN
R
(t) to generate a PN modulated reference carrier signal s
R
(t); transmitting said PN modulated reference carrier signal S
R
(t) through said communication channel at a level below the level of said payload signal; receiving a reference receive signal s
R
′(t) corresponding to said PN modulated reference carrier signal s
R
(t) after having traveled through said communication channel; correlating said reference receive signal s
R
′(t) with said second pseudo noise signal PN
R
(t) to generate a recovered reference carrier signal f
R
′(t); and determining characteristics of components of the communication channel also on the basis of a comparison of said reference carrier signal f
R
(t) and said recovered carrier signal f
R
′(t).
Advantageously, the level of said PN modulated reference carrier signal s
R
(t) is at least 15 dB, preferably 25 dB or more below the level of said payload signal.
In a further embodiment, said second pseudo noise signal PN(t) is a binary pseudo noise sequence which is preferably generated by means of a feed back shift register.
In a further embodiment, said correlating of said reference receive signal s
R
′(t) and said second pseudo noise signal PN
R
(t) is achieved by delaying said second pseudo noise signal PN
R
(t) and multiplying the delayed second pseudo noise signal PN
R
(t) and said reference receive signal s
R
(t) can also be transmitte through a different transponder of the satellite.
The method of the invention as characterized above is especially applicable when said communication channel is a transponder of a communication satellite. Said PN modulated reference signal s
R
(t) may be transmitted through the same transponder of the satellite, but then said second pseudo noise signal PN
R
(t) must not correlated with said pseudo noise signal PN(t). Said PN modulated reference signal s
R
(t) can also be transmitted through a different transponder of the satellite.
The characteristics of said communication channel may be group delay and amplitude response.
The above objects and other objects are also achieved by an apparatus for determining characteristics of components of a communication channel which is designed for transmitting a payload signal 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) 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).
Advantageously, the level of said PN modulated clean carrier signal s(t) is at least 15 dB, preferably 25 dB or more below the level of said payload signal.
In a further embodiment, said first pseudo noise signal generating means is a feed back shift register.
A chiprate of said first pseudo noise signal PN(t) is less than 5 MChip/s preferably less than or equal to 2.5 MChip/s.
In a further embodiment, the above apparatus comprises first delaying means for delaying said first pseudo noise signal PN(t).
To obtain a reference, the above apparatus further comprises second pseudo noise generating means for generating a second pseudo noise signal PN
R
(t); second modulating means for modulating a
Bethscheider Gerhard
Harles Guy
Liu Shuwang
Societe Europeenne des Satellites S.A.
Wolf Greenfield & Sacks P.C.
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