Precorrelation digital spread spectrum receiver
First Claim
1. A direct sequence spread spectrum RF receiver for receiving and processing an RF signal having plural modulated channel component RF signals broadcast from plural sources, each channel component RF signal being a spread spectrum signal modulated in accordance with channel code, wherein the receiver comprisesA. means for receiving said plural component RF signals and forming from said component RF signals a linear composite RF signal,B. baseband converter means for converting said linear composite RF signal to a baseband signal,N-bit digitizing means for converting said baseband signal to a succession of N-bit digital signals at a sampling frequency, each digital signal being representative of a corresponding value of said baseband signal,D. code generator means for generating a plurality of offset local code signals,E. a correlator for correlating said N-bit digital signals with said plurality of offset local code signals and for producing plural digital signal/code correlation signals, each said signal/code correlation signal being representative of the correlation of said N-bit digital signal with a local code signal having a different offset, andF. tracking means responsive to said correlation signals for tracking the code state and carrier of a component of said baseband signal derived from a modulated channel component RF signal broadcast from a single source.
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Abstract
A precorrelation digital spread spectrum receiver includes a signal converter for amplifying and directly converting RF band spread spectrum signals received by an antenna to a digitized baseband signal comprising a sequence of N-bit quantized baseband values. The baseband signal is a linear composite signal containing signals from all satellite channels as well as noise and jamming. An N-bit digital correlator correlates successive N-bit baseband signal values with successive local code values to produce plural correlation signals. A vector processor processes the correlation signals to derive early, late and on-time code correlation signals. The processed correlation signals are accumulated to provide process gain and are used to form error signals for carrier and code tracking, and measurement purposes.
A sequential-tracking multi-channel embodiment time shares a common set of circuit elements and digital processing instructions to successively track different channels.
In a preferred embodiment, the vector processor uses micro-code instructions to remove the carrier Dopler shift by multiplying the correlation value with a stored complex constant corresponding to the Doppler contribution. The receiver provides coarsely quantized code delays to the correlator, and uses digital interpolation routines to estimate correlation values for code delays of interest, thus providing fine resolution in the code delay without using either a numerical control oscillator or crystal oven.
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Citations
7 Claims
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1. A direct sequence spread spectrum RF receiver for receiving and processing an RF signal having plural modulated channel component RF signals broadcast from plural sources, each channel component RF signal being a spread spectrum signal modulated in accordance with channel code, wherein the receiver comprises
A. means for receiving said plural component RF signals and forming from said component RF signals a linear composite RF signal, B. baseband converter means for converting said linear composite RF signal to a baseband signal, N-bit digitizing means for converting said baseband signal to a succession of N-bit digital signals at a sampling frequency, each digital signal being representative of a corresponding value of said baseband signal, D. code generator means for generating a plurality of offset local code signals, E. a correlator for correlating said N-bit digital signals with said plurality of offset local code signals and for producing plural digital signal/code correlation signals, each said signal/code correlation signal being representative of the correlation of said N-bit digital signal with a local code signal having a different offset, and F. tracking means responsive to said correlation signals for tracking the code state and carrier of a component of said baseband signal derived from a modulated channel component RF signal broadcast from a single source.
Specification