Communication satellite system having an increased power output density per unit of bandwidth
First Claim
1. An earth station transmitter for transmitting a spread spectrum modulated data signal to a satellite comprising:
- a source of data signals;
a spreading sequence generator for generating a spreading signal at a spreading signal chip rate f(c) synchronized with said data signals;
a multiplier for multiplying said data signals with said spreading signal to produce a spread spectrum signal having an amplitude versus frequency function defined by ##EQU5## where X is frequency;
a filter having an amplitude versus frequency characteristic defined by a 6-pole Butterworth function and by X/SIN X for filtering said spread spectrum signal to thereby produce a filtered spread spectrum signal which is amplitude normalized over at least a portion of said spread spectrum signal bandwidth; and
,a converter for frequency upconverting said filtered spread spectrum signal for transmission to the satellite.
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Accused Products
Abstract
A satellite communication system for providing maximum power output in a spread spectrum signal transmission is disclosed. Spectral shaping is provided at a satellite transmitter to shape a spread spectrum so that over a bandwidth of interest, the spectrum has a maximally flat characteristic. By maximizing the spectrum amplitude flatness, the required power density for a bandwidth limitation imposed to prevent interference with other communication facilities is maintained. In each earth station receiving the transmission from the satellite, a filtering is provided of the demodulated baseband satellite signal such that the spectrum is restored to its original shape. The filters on the receive side may be analog, having the same amplitude function as the transmit spectrum shaping filter, but having an inverse phase response. This insures a matched filter condition at transmitter and receiver, maximizing signal transfer. A programmable demodulator is provided to select a demodulation mode. The programmable demodulator may receive microcode from a microprocessor configuring the PLA into a despreader, demodulator for spread spectrum applications or as a straight BPSK demodulator where the data signal has not been subject to spectrum spreading.
71 Citations
15 Claims
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1. An earth station transmitter for transmitting a spread spectrum modulated data signal to a satellite comprising:
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a source of data signals; a spreading sequence generator for generating a spreading signal at a spreading signal chip rate f(c) synchronized with said data signals; a multiplier for multiplying said data signals with said spreading signal to produce a spread spectrum signal having an amplitude versus frequency function defined by ##EQU5## where X is frequency;
a filter having an amplitude versus frequency characteristic defined by a 6-pole Butterworth function and by X/SIN X for filtering said spread spectrum signal to thereby produce a filtered spread spectrum signal which is amplitude normalized over at least a portion of said spread spectrum signal bandwidth; and
,a converter for frequency upconverting said filtered spread spectrum signal for transmission to the satellite. - View Dependent Claims (2, 3, 4)
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5. A satellite earth station receiver system for receiving a spread spectrum modulated data signal spectrum shaped to have a maximally flat Butterworth filtered characteristic comprising:
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a down converter for frequency translating said modulated spread spectrum signal to an intermediate frequency signal; a baseband signal processor for dividing said intermediate frequency signal into I and Q phase quadrature signals; first and second filters for filtering respective of said I and Q signals, said filters having an impulse response time inverse to said Butterworth filtered characteristic; and
,demodulator means connected to receive said I and Q signals demodulating data contained in said spread spectrum modulated data signal. - View Dependent Claims (6, 7, 8, 9, 10, 11)
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12. A satellite communications system comprising:
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means for producing a spread spectrum signal from a data signal with a spreading signal having a chip rate f(c) and having an amplitude of ##EQU6## where X is a frequency;
an FIR filter for producing from said spread spectrum signal a signal having a flat amplitude function over a substantial portion of said filter 3 db bandwidth which is substantially equal to one half of said spreading signal chip rate f(c);modulation means for modulating said signal having said substantially flat amplitude function on a carrier signal; and
,antenna means for transmitting said carrier signal to a satellite, wherein it is rebroadcast on a different carrier signal; and
,a receiver including a down converter for producing an intermediate frequency signal from said satellite rebroadcast carrier signal; a baseband demodulation means for removing said spread spectrum signal from said intermediate frequency signal, producing I and Q quadrature baseband signals; first and second filter means for filtering said I and Q signals, said filter means having an amplitude frequency response substantially identical to said FIR filter response; and
,demodulator means for removing said data signal from said I and Q signals.
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13. In a satellite communications system for transmitting and receiving a spread spectrum data signal which is filtered by an FIR filter to spectrally shape said spread spectrum data signal, a receiver comprising:
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a down converter for converting a carrier signal having said spread spectrum data signal modulated thereon to an intermediate frequency signal; a baseband demodulator means for removing said spread spectrum data signal from said intermediate frequency signal, and forming I and Q quadrature signals; first and second analog Butterworth filters for filtering said I and Q signals having an amplitude frequency response matched to said FIR filter which spectrally shapes said spread spectrum data signal; and
,demodulation means for despreading said spread spectrum signal and producing therefrom said data signal. - View Dependent Claims (14, 15)
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Specification