Pulse or digital communications – Spread spectrum – Frequency hopping
Reexamination Certificate
1999-11-29
2001-02-27
Pham, Chi H. (Department: 2631)
Pulse or digital communications
Spread spectrum
Frequency hopping
C375S350000
Reexamination Certificate
active
06195383
ABSTRACT:
This invention relates to digital signal processing apparatus.
The invention particularly relates to digital signal processing apparatus for frequency de-hopping narrowband received signals which have been transmitted on a carrier frequency which is bopped within a broadband range of frequencies. The invention especially relates to such digital signal processing apparatus for use on-board a communications satellite.
Current on-board processors use analogue signal processing apparatus for de-hopping the narrowband received signals. The de-hopped signal is demodulated, and then re-modulated and transmitted along a downlink. The on-board processor may be for communicating between different terminals which may be land, sea or air based, in a secure manner.
In the past, transparent on-board processors have been used, which do not demodulate the signal, but just amplify it and switch it to a desired downlink. However, any interfering signals would also be amplified and transmitted on the downlink.
One of the advantages of regenerative on-board processors is that this does not happen, because the received signal is demodulated, and the transmitted signal re-modulated,
The current analogue regenerative on-board processors are based on analogue frequency synthesisers, which produce a local oscillator frequency which is mixed with a received r.f. signal and which varies in step with the hopped carrier frequency, in order to effect the de-hopping. The tone produced by the agile frequency synthesiser at each hop is determined by a control word generated for that hop by a generator in the satellite or on the ground. A similar generator in the transmitting terminal determines the hopping pattern of the carrier frequency.
Problems associated with this approach include the mass volume and complexity of the synthesiser, and the difficulties associated with distributing the synthesiser output to multiple receive chains.
It might be expected that these problems would be alleviated by using digital processing. However, even if an analogue-to-digital (A-D) converter fast enough to digitise the incoming r.f. received signal was available, an interfering signal could cause sufficient incoming power for the A-D converter to saturate and therefore distort the incoming signal, or even for the A-D converter to burn out.
This could be avoided by providing a hybrid arrangement, using an analogue filter at the front end, and digitising a consequently reduced bandwidth. Using this approach, the filtered digitised received signal would be mixed digitally with a digital representation of the tone at each hop. However, the multiplication of the complex values entailed in the mixing would still represent a heavy processing overload.
The invention provides digital signal processing apparatus for frequency de-hopping a narrowband received signal which has been transmitted on a carrier frequency which is hopped within a broadband range of frequencies according to respective control words, including means for sampling the broadband at a first sampling rate, and means for using successive segments of the control word to select progressively narrower sub-bands which contain the narrowband signal, in which sub-bands processing is arranged to be performed at a reduced sampling rate.
This avoids having to perform explicit mixing operations at the higher first sampling rate.
REFERENCES:
patent: 5625641 (1997-04-01), Takakusaki
patent: 2 319 708 (1998-05-01), None
Kelly Peter John
Wishart Alexander Walker
Kinberg Robert
Matra Marconi Space UK Limited
Pham Chi H.
Phu Phuong
Venable
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