Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-08-23
2001-12-11
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S147000, C375S213000
Reexamination Certificate
active
06330273
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This relates to receivers for direct sequence spread spectrum and is concerned with code tracking part of the receiving apparatus.
2. Description of the Related Art
The normal implementation of a code tracking device for spread spectrum consists of a so called early prompt late tracking loop and this involves energy detection of the correlated wave form for a correlation that is performed early, normally half a chip early, and a correlation of energy for a chip that is performed late, normally half a chip late. A timing discrimination curve is then generated by subtracting the early from the late and this is used to close a feedback loop to adjust the time, such that the prompt occurs at the correct time.
U.S. Pat. No. 5,402,450 discloses a method and apparatus for estimating the magnitude and phase of multipath induced error in the position in time of the peak of a composite autocorrelation function by synchronising a reference signal to the timing of a received composite signal is synchronised to a reference signal. Time variations between early and late gate timing points of corrupted and uncorrupted autocorrelation functions are detected by employing scanning correlators.
U.S. Pat. No. 5,414,729 discloses a pseudorandom noise ranging receiver which compensates for multipath distortion by making use of multiple correlator time delay spacing. The receiver consists of a sampling circuit, multiple carrier and code synchronising circuits and multiple digital correlators. The sampling circuit provides digital samples of a received composite signal to each of several receiver channel circuits. The synchronising circuits are preferably non-coherent and track any phase shifts in the received signal and adjust the frequency and phase of a locally generated carrier reference signal accordingly. The correlators provide signals which are fed to a parameter estimator from which the delay and phase parameters of the direct path signal from which a range measurement can be corrected.
U.S. Pat. No. 5,347,536 discloses a method of multipath noise reduction for spread spectrum signals in which phase variations among early and prompt lags are detected. Phase and pseudo-range observables are derived from earlier correlation lags and used to generate a multipath-free code cross-correlation shape. Deviations from that shape are used to infer the magnitude of multipath and to generate corrections for pseudo-range observables. Phase variations from early and prompt lags are used to generate corrections for carrier-phase observables.
An aim of the invention is to remove the need to have the early and late correlations taking place at half a chip but specifically, to enable them to be applied at plus or minus one chip. This results in particular difficulties in the normal implementation, in that at plus or minus one chip for a circuit that is operating at the normal timing, the correlation falls to zero and therefore energy detection of the signals will tend to be very noisy and thus a normal implementation would lead to a discrimination curve that is itself very noisy and would lead to a very jittery timing loop.
The apparatus uses spread spectrum signals, which have been Nyquist filtered and therefore it uses signals, the correlation functions of which still has a good slope on it through the zero points and the basic problems are solved by not performing energy detection on the measurements taken or the correlations taken at these intervals but rather, coherently compensating them against a phase reference derived from the prompt signal and thereby reducing the effect of the noise and thus enabling a discrimination signal to be derived which is of a similar signal to noise ratio as would obtained in the more conventional plus or minus half a chip early/late situation.
BRIEF SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided apparatus for code tracking in a direct sequence spread spectrum receiver, comprising means for receiving a signal, means for converting the received signal into an inphase and quadrature signal, and analogue to digital converter means for converting the inphase and quadrature signals into digital signals;
characterised in that said apparatus further comprises a first correlator for handling prompt signals, a second correlator for handling late signals, a third correlator for handling early signals, estimation means for obtaining estimates of the output signals from the first correlator, first and second multiplying and adding means for multiplying the estimates with the output signals from the second and third correlators respectively and adding the inphase and quadrature channels together, subtraction means for subtracting the result from said first and second adding means, and control means arranged to receive the output signals from said subtracting means and generate control signals for the analogue to digital converter means.
According to another aspect of the present invention, there is provided apparatus for code tracking in a direct sequence spread spectrum receiver, comprising means for receiving a signal, converting means for converting the received signal into an inphase and quadrature signal, and analogue to digital converter means for converting the inphase and quadrature signals into digital signals;
characterised in that said apparatus farther comprises a first correlator for handling prompt signals, a second correlator for handling late signals, a third correlator for handling early signals, a frequency phase discrimination means for receiving the signals from said first correlator and generating a signal for controlling said converting means in a manner such that a coherent signal is contained on one signal phase channel, subtractor means for subtracting the output signals from said second and third correlators and control means connected to said subtractor means for controlling said analogue to digital converters.
According to a further aspect of the present invention there is provided a method of acquiring code synchronisation between a received direct sequence spread spectrum signal and a locally generated reference code which locally generated reference code is representative of a spreading code used to spread the spectrum of said received signal, comprising the steps of:
converting said received signal into inphase and quadrature signals representative of said received signal,
analogue to digital converting said inphase and quadrature phase signals into digital representations, in accordance with a selectively controllable sampling rate,
correlating said digital inphase and said digital quadrature signals with said reference code in dependence upon a prompt temporal position;
characterised in that said method further comprises the steps of:
further correlating at least one of said digital inphase and said digital quadrature phase signals with said reference code in dependence upon a late temporal position,
further correlating at least one of said digital inphase and said digital quadrature phase signals with said reference code in dependence upon an early temporal position,
forming an error signal in accordance with a difference between said early further correlation and said late further correlation, and adjusting said controllable sampling rate in accordance with said error signal.
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Chandler David Peter
Hulbert Anthony Peter
Al-Beshrawi Tony
Chin Stephen
Crowell & Moring LLP
Roke Manor Research Limited
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