Coded data generation or conversion – Digital code to digital code converters
Patent
1999-08-24
2000-08-15
Young, Brian
Coded data generation or conversion
Digital code to digital code converters
375 1, H03M 700
Patent
active
061043222
DESCRIPTION:
BRIEF SUMMARY
This invention relates to direct sequence spread spectrum systems and in particular the initial acquisition of code phase lock.
In such systems a spread spectrum transmitter transmits a code consisting of a sequence which may be of considerable length. A receiver, which has not acquired time synchronisation, performs correlations against the received sequence until such time as energy is found exceeding a threshold. These correlations are performed for a variety of phase shifts of the code and in general these phase shifts consist of whole numbers of chips but also the incoming signal is sampled usually several times per chip. This ensures that when the various phases of the code are tested the peak of the correlation function or a place close to it is found avoiding losses associated with sampling of the code away from the correlation peak.
EP-A-0 668 663 discloses a sliding correlation detector in which stored signals are multiplied with a replica of the spread series codes and integrated over time. A threshold judging circuit determines whether the integrated value exceeds a threshold limit. If the limit is not exceeded, the phase of the replica of the spread series codes is changed and the process repeated.
U.S. Pat. No. 4,894,842 discloses a precorrelation digital spread spectrum receiver in which RF band spread signals are converted to a digitised baseband signal comprising a sequence of N-bit quantised baseband values. A correlator correlates successive baseband signal values with successive local code values to produce the correlation signals which are accumulated to provide process gain and form error signals for tracking. The receiver is intended for use as part of a GPS navigation system.
U.S. Pat. No. 5,105,437 discloses a programmable digital acquisition and tracking controller which provides an output signal which is indicative of the correlation between the received PN code and the locally generated PN code as compared with a threshold. Programmable detector logic detects acquisition correlation and tracking correlation and switches from acquisition to tracking once the I and Q data channels have been acquired.
In prior systems the transmitted signal consisted of a rectangular pulse for each chip and this led to an auto correlation function which was triangular for this case. If one was to sample as much as a half of a chip out of phase this would result in a degradation of six decibels. More recently band limiting functions have been applied to the transmitted spectrum, typically raised cosine pulses, and here the attenuation through sampling half a chip away from the optimum point is reduced of the order 4.2 decibels. Thus, one may consider the possibility of sampling as infrequently as once per chip, however 4.2 decibels is still an unacceptably large degradation. An aim of the present invention is to significantly reduce the loss in this case.
According to one aspect of the present invention, there is provided apparatus for acquiring a code phase lock in a direct sequence spread spectrum system comprising:--means for receiving an input signal; means for converting the received input signal into inphase and quadrature components of a complex baseband signal; means for performing a modulus squared function on said complex baseband signal; and comparing means for comparing said modulus squared function with a threshold value; characterised in that the apparatus further comprises: first and second correlators for respectively receiving said inphase and said quadrature components of said complex baseband signal; storage means for storing the results of said modulus squared function prior to said comparing means; adding means for adding together inputs to said storage means with outputs therefrom which represent correlations spaced apart by one chip difference in code phase; and control means for controlling said correlators and said storage means.
The principle is to apply effective interpolation to the outputs of the correlators. This means that the number of correlations which have to be pe
REFERENCES:
patent: 4903279 (1990-02-01), Murakami
patent: 5778023 (1998-07-01), Ideta
Chandler David Peter
Hulbert Anthony Peter
Roke Manor Research Limited
Young Brian
LandOfFree
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