WO2001011381A1 - Poursuite de boucle a retard de phase adaptee - Google Patents

Poursuite de boucle a retard de phase adaptee Download PDF

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Publication number
WO2001011381A1
WO2001011381A1 PCT/US2000/015849 US0015849W WO0111381A1 WO 2001011381 A1 WO2001011381 A1 WO 2001011381A1 US 0015849 W US0015849 W US 0015849W WO 0111381 A1 WO0111381 A1 WO 0111381A1
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WO
WIPO (PCT)
Prior art keywords
signal
reference code
code
taps
error
Prior art date
Application number
PCT/US2000/015849
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English (en)
Inventor
George Jeffrey Geier
Stanley Allen Bush
Christopher John Hunt
Carl Henry Voegtly
Original Assignee
Motorola Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Motorola Inc. filed Critical Motorola Inc.
Priority to AU56018/00A priority Critical patent/AU5601800A/en
Publication of WO2001011381A1 publication Critical patent/WO2001011381A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/7073—Synchronisation aspects
    • H04B1/7085—Synchronisation aspects using a code tracking loop, e.g. a delay-locked loop
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/22—Multipath-related issues
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S19/00—Satellite radio beacon positioning systems; Determining position, velocity or attitude using signals transmitted by such systems
    • G01S19/01—Satellite radio beacon positioning systems transmitting time-stamped messages, e.g. GPS [Global Positioning System], GLONASS [Global Orbiting Navigation Satellite System] or GALILEO
    • G01S19/13—Receivers
    • G01S19/24—Acquisition or tracking or demodulation of signals transmitted by the system
    • G01S19/30—Acquisition or tracking or demodulation of signals transmitted by the system code related
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/69—Spread spectrum techniques
    • H04B1/707—Spread spectrum techniques using direct sequence modulation
    • H04B1/709—Correlator structure
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B2201/00—Indexing scheme relating to details of transmission systems not covered by a single group of H04B3/00 - H04B13/00
    • H04B2201/69—Orthogonal indexing scheme relating to spread spectrum techniques in general
    • H04B2201/707—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation
    • H04B2201/70715—Orthogonal indexing scheme relating to spread spectrum techniques in general relating to direct sequence modulation with application-specific features

Definitions

  • the present invention pertains to delay lock loops and more specifically to adaptive delay lock loops for use in tracking GPS pseudo random codes and the like.
  • GPS Global Positioning System
  • SA Selective Availability
  • ionospheric refraction In applications where the positioning errors induced by Selective Availability (SA) and ionospheric refraction can be compensated, Global Positioning System (GPS) position error is generally determined by the magnitude of the residual noise and multipath error in the pseudo range measurements generated by the GPS receiver.
  • GPS Global Positioning System
  • Such applications include differential GPS, where the combined effects of SA and ionospheric delay are explicitly estimated using a separate GPS receiver at a known location, and attitude determination, where pseudo range measurements to an individual satellite can be differenced and filtered (in an effort to resolve integer ambiguities associated with differential carrier phase), thereby eliminating the error contributions of SA and the ionosphere.
  • DLL Delay Lock Loop
  • the noise and multipath content of the DLL used within the GPS receiver are strongly dependent upon the early/late spacing, i.e., the sampling of the code sequence relative to prompt, or on-time spacing within the loop. This motivates the use of narrow spacing: multipath errors cannot exceed the magnitude of the spacing, and the noise variance at the output of the discriminator is proportional to the spacing, as described in "Theory and Performance of Narrow Correlator Spacing in a GPS
  • DLL tracking parameters are preselected. Preselection is typically done conservatively, with large, fixed spacing (e.g., one-half chip) used during code acquisition and pull-in, and small, fixed spacing (e.g., one-tenth chip) used for code tracking.
  • large, fixed spacing e.g., one-half chip
  • small, fixed spacing e.g., one-tenth chip
  • Use of multiple weighted spacings for DLLs was examined to extend the tracking ability of the loop in degraded signal environments induced by jamming, as described in Correlation Tracking, W.M. Bowles, PhD Dissertation, MIT Department of Aeronautics and Astronautics, June 1980.
  • the DLL modification discussed in this dissertation degrade the noise and multipath performance of the loop.
  • FIG. 1 is a generalized system block diagram for a GPS receiver
  • FIG. 2 is a simplified block diagram of a GPS tracking loop
  • FIGs. 3A, 3B, 3C, and 3D illustrate the construction of early and late codes in a prior art DLL tracking loop
  • FIG. 4 illustrates a representative autocorrelation function for the prior art DLL tracking loop
  • FIG. 5 illustrates the discriminator function, or S curve, for the prior art DLL tracking loop
  • FIG. 6 is a simplified block diagram of a DLL code loop in accordance with the present invention.
  • FIG. 7 is a schematic representation of a reference code generator in accordance with the present invention
  • FIGs. 8A, 8B, and 8C illustrate optimal code generation in a DLL tracking loop in accordance with the present invention
  • FIG. 9 illustrates a representative autocorrelation function for the DLL tracking loop of the present invention
  • FIG. 10 illustrates the discriminator function, or S curve, for the DLL tracking loop in accordance with the present invention.
  • FIG. 1 a generalized system block diagram of a
  • Receiver 10 includes an antenna 11 for receiving GPS signals from a plurality of GPS satellites and supplying the received signals to an RF front end 12.
  • front end 12 provides the required filtering and mixing to supply the received signals as intermediate frequency (IF) signals to a channel signal processing block 15.
  • IF intermediate frequency
  • a GPS receiver can be in a position to receive signals from several satellites simultaneously and, as is known by those skilled in the art, the commercial GPS signals are all at the same carrier frequency, with each satellite identified by a unique code.
  • Channel signal processing block 15 communicates with a microprocessor 16 which includes software that controls and processes data from channel signal processing block 15 to measure or correlate a reference code signal with a desired one of the various received unique codes. In this fashion the software can, for example, determine the distance to a specific satellite and, eventually, the position of the GPS receiver.
  • Loop 20 receives an input signal s(t) at one input of a mixer 21 and a local oscillator signal, cos(w 0 t), from a carrier NCO
  • mixer 21 At a second input.
  • An output of mixer 21 is supplied through a carrier loop filter
  • the output of mixer 21 is also supplied to an input of an early minus late (E-L) mixer 25 and to an input of a prompt mixer 26.
  • the output of NCO 22 is also applied to a C/A code generator 27, representing an aiding signal supplied to the DLL from carrier tracking.
  • C/A code generator 27 provides an E-L signal to a second input of mixer 25, the output of which is applied through a code loop filter 28 to control C/A code generator 27.
  • C/A code generator 27 also supplies a prompt signal to a second input of mixer 26, the output of which is supplied to a data demodulator 30.
  • the output of data demodulator 30 is supplied as a data output to any subsequent equipment, such as microprocessor 16 in FIG. 1.
  • C/A code generator 27, mixer 25, and code loop filter 28 form the DLL which generates a reference code that is correlated or compared with the incoming or received signal, s(t).
  • DLLs such as those described in the above referenced patents
  • waveform (FIG. 3A) is a representation of a short portion of the received GPS pseudo random, spread spectrum code.
  • waveform (FIG. 3B) is a representation of a similar portion of the prompt generated reference code (input signal to mixer 26 from C/A code generator 27 in FIG. 2).
  • waveform (FIG. 3C) is a representation of the E-L generated code (input signal to mixer 25 from C/A code generator 27 in FIG. 2) with wide spacing utilized for tracking in prior art DLLs.
  • the wide spacing illustrated in waveform is a chip wide, i.e. one half chip wide on either side of the code edge. Once acquisition is achieved, the spacing is reduced to something close to one half chip, i.e. one quarter chip on either side of the code edge, as illustrated in waveform (FIG. 3D).
  • an ideal autocorrelation function for the C/A code is illustrated, indicating its peak value, corresponding to perfect correlation, and its linear decay to a value near zero for a full chip delay.
  • the E-L code wide spacing in the typical DLL discriminator is represented by points 35 and 36 on the autocorrelation function curve. Because a large amount of the autocorrelation function curve is utilized, acquisition of the GPS pseudo random, spread spectrum signal can occur, however, residual noise and multipath errors are maximum. Once acquisition is achieved, the spacing is reduced to points 37 and 38 for tracking, which reduces residual noise and multipath errors.
  • the discriminator or S curve for a prior art DLL tracking loop is illustrated, corresponding to a minimum E-L spacing; note the decay to zero at slightly less than one full chip of delay. It is this decay which achieves multipath suppression for delays outside of this range.
  • Code loop 40 includes a reference code generator 41 which, in this specific embodiment, supplies an E-L code and a prompt code to a signal correlator 42.
  • Signal correlator 42 supplies in-phase and quadrature E-L code signals and in-phase and quadrature prompt code signals to an error detector 45.
  • code generator 41 and correlator 42 are included as hardware on an ASIC chip, denoted by broken line box 43, and the remaining components (including error detector 45) are included in signal processing software within a microprocessor or the like, denoted by broken line box 46.
  • the error signal at the output of error detector 45 is supplied through an AGC circuit 47 and a loop filter 48 to a summing junction 50.
  • a carrier aiding signal is supplied to summing junction 50 to aid DLL code loop 40 in locking onto a correct or desired direct sequence, spread spectrum signal, such as a selected one of several GPS pseudo random, spread spectrum signals.
  • the output signal from summing junction 50 is supplied or fed back to an input of code generator 41 to control code generator 41 for acquisition and tracking.
  • the error signal at the output of error detector 45 is also supplied to the inputs of a pair of code loop error statistics generators 55 and 56.
  • Statistics generators 55 and 56 have different bandwidths, with statistics generator 55 having a bandwidth which is large or wide relative to the bandwidth of DLL code loop 40 and statistics generator 56 having a bandwidth which is narrow relative to the bandwidth of DLL code loop 40.
  • Code loop 40 incorporates gain adaption which is driven by the generation of code loop error statistics in the separate bandwidths of statistics generators 55 and 56.
  • Output signals from code loop error statistics generators 55 and 56 are supplied to a correlator weight selector 60, which supplies weight selector signals to code generator 41 , as will be described in more detail presently.
  • Large error statistics in the bandwidth of statistics generator 55 which is large or wide relative to the bandwidth of DLL code loop 40, are generally indicative of residual tracking errors in DLL code loop 40.
  • Error statistics in the bandwidth of statistics generator 56 which is narrow relative to the bandwidth of DLL code loop 40, are generally indicative of residual noise and multipath errors in DLL code loop 40.
  • Large error statistics in the narrow band (statistics generator 56) drive weight selector 60 to minimize residual noise and multipath errors.
  • FIG. 7 a schematic representation is illustrated of a portion of reference code generator 41 in accordance with the present invention.
  • reference code generator 41 includes a code synthesizer (not shown) which is controlled by the feedback signal from mixer 50.
  • the output or synthesized code signal is supplied to code input 65 in FIG. 6 and the various taps, designated L N through to and through tt M , are positioned at various points or times along a chip of the synthesized code signal.
  • t 0 is the prompt code output and there are a plurality (t N ) of early code taps and a similar plurality (L N ) of late code taps.
  • each early and late code tap has associated therewith a weighting function k, with the associated weighting functions being designated with a sub-numeral similar to the associated tap, i.e. k N through N.
  • Each of the early and late weighting functions k N through k. N are adjustable by signals from weight selectors 60.
  • the noise in a conventional (i.e. single E-L) DLL can be expressed as:
  • niE/L cos (T - Test) >ni [c(t - O st - 3 ) - c(t - ⁇ st + a ) ] dt
  • nQE/L sin (T - T est ) nQ[c(t - ⁇ +- Q st - 3 ) - c(t - ⁇ + st + 3 ) ] dt
  • T is the phase of the LI carrier
  • est is th e phase lock loop's estimate of carrier phase
  • ⁇ is the range delay induced in the C/A code
  • ni and nQ are the in-phase and quadrature components of the noise
  • noise variance is proportional to the spacing 3. In a conventional DLL, therefore, the residual noise is minimized with minimum spacing.
  • equations (1) and (2) become:
  • niE/L cos(T - T es t) / ki( ⁇ ni [c(t - ⁇ st - 3 i) - c(t - ⁇ f st + 3 i) ] dt ⁇
  • n Q E/L sin(T - T es t) / i ⁇ >nQ[c(t - O s t - i) - c(t - ⁇ Q st + 3 i) ] dt ⁇
  • the noise variances associated with the new reference code can be evaluated, and are given by:
  • T is the PreDetection integration Interval (PDI);
  • ⁇ 2 ni and ⁇ 2 n Q are the noise variances associated with ni and nQ
  • each early and late tap in FIG. 7 is assumed to be separated by less than 0.25 of a chip and, in this preferred embodiment by one-tenth of a C/A code chip interval, and, for simplicity, only three pairs of weightings will be allowed to be nonzero: k ⁇ k_ ⁇ , k 3 , k -3 , k 5 , and k -5 .
  • DLL code loop 40 To gain insight into the ability of DLL code loop 40 to attenuate multipath errors, assume a single, specular multipath reflection with a constant reflective coefficient. A detailed analytical treatment is not required, since the multipath content of the correlation performed by DLL code loop 40 can be viewed as the weighted sum of the multipath content of separate, DLLs with fixed spacings of 0.1 and 0.3 (for the example described above). Assuming that multipath delays within the chip spacing appear directly in the output of the correlation, the weightings derived previously to attenuate noise will also reduce multipath by 33%, since the multipath delay error induced by the 0.3 spacing will be subtracted from that induced by the 0.1 spacing.
  • DLL code loop 40 can be constructed with a plurality of fixed early and late spaced taps or the taps can be weighted as described. In environments where multipath reflections are expected to be within 0.1 chip (e.g. a spacecraft environment), DLL code loop 40 should outperform the multiple fixed 0.1 chip DLL.
  • the adaptation is based on the magnitude and frequency content of the code loop error signal, as computed below;
  • the code loop error signal computed by equation (10) represents the error which exists in DLL code loop 40, as measured by the loop itself. As such it represents the combined effects of noise, multipath, receiver clock induced error, and transient error induced by dynamics.
  • the presence of significant noise and/or multipath leads to a selection of gains as described above, i.e., with the largest gain applied to the smallest spacing, and the other gains selected to achieve the desired suppression. Errors induced by the receiver clock and/or dynamics, on the other hand, must be removed by the tracking loop, and so cannot be suppressed by the code loop discriminator.
  • the code loop bandwidth is properly selected, and maximum use is made of dynamic aiding sources (e.g., the carrier tracking loop will generally be used to aid code tracking, and remove its requirements to track dynamics), errors induced by the receiver clock and dynamics can be kept within the desired minimum spacing (e.g., 0.1 chip).
  • the primary adaptation of the weights will be as the loop approaches steady-state tracking conditions from an initial acquisition.
  • momentary loss of carrier lock due to excessive dynamics or degraded signal to noise ratio can be detected and used to adjust the weightings and maintain lock in situations where a conventional DLL could lose track.
  • This gain selection leads to a noise variance reduction of 71%.
  • the gain adaptation will be based on the mean and variance of the code loop error signal, as computed in two separate bandwidths.
  • the first bandwidth is large or wide with respect to the code loop bandwidth (e.g., 50 Hz to 100 Hz), and is used to measure the level of noise and multipath, i.e., sources of error with a frequency content which is high relative to the error sources which the loop can track.
  • the second bandwidth is small or narrow relative to the code loop bandwidth (e.g., 0.1 Hz to 1 Hz), and so measures residual error which the loop is expected to remove.
  • the gains are adapted to the set which minimizes these error sources.
  • the rate of gain adaptation is determined by code loop error statistics generators 55 and 56: as the residual error in the first bandwidth falls below a certain spacing, gains applied to larger spacings are set to small values.
  • the gains applied to larger spacings become nonzero.
  • waveform (FIG. 8A) is a representation of a short portion of a direct sequence, spread spectrum signal, such as a GPS pseudo random, spread spectrum signal.
  • Waveform (FIG. 8B) is a representation of a similar portion of the prompt generated reference code, generally as it appears on tap to of FIG. 7.
  • Waveform (FIG. 8C) is a representation of the E-L generated code in DLL code loop 40 with multiple 0.1 chip taps. As described above, in the preferred embodiment the taps are weighted and the weights are continuously adjusted to minimize residual noise and multipath errors in DLL code loop 40.
  • FIG. 9 a representative autocorrelation function for DLL code loop 40 is illustrated.
  • taps ti and ti are designated 70 and 71 , respectively; taps t 2 and L 2 being designated 72 and 73, respectively; and taps t and t N being designated 74 and 75, respectively. It should be understood by those skilled in the art that more or less taps can be utilized with the complexity of the design being determined by the number of taps or coder positions. Once an appropriate number of taps has been selected for the application/environment, an optimal set of weightings can be found.
  • FIG. 10 illustrates the shape of the discriminator or S curve, for three taps and the weight selection optimized for multipath and noise reduction; note the very significant differences with prior art (FIG. 5).
  • the improved signal tracking is accomplished by constructing an optimal reference code for correlation, and continuously adapting the optimal reference code as a function of tracking state.
  • the present invention has a substantial advantage over prior art tracking loops in that it avoids any preselection of the DLL tracking parameters, that is, the parameters which control the DLL are adapted continuously as a function of the statistics of the loop error signal (i.e., the output of the code loop discriminator).
  • the invention includes the generation and use of an "optimal" reference code for correlation using multiple, weighted spacings of a direct sequence, spread spectrum signal. Further, the invention includes the adaptation of the spacing weightings continuously as a function of the tracking state of the loop, as determined by error statistics generated in real-time. -l b- While we have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. We desire it to be understood, therefore, that this invention is not limited to the particular forms shown and we intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.

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  • Engineering & Computer Science (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Signal Processing (AREA)
  • Position Fixing By Use Of Radio Waves (AREA)

Abstract

L'invention concerne une boucle DLL adaptative pour assurer le suivi d'un signal séquentiel direct à spectre étendu comprenant un générateur de code de référence (41) dont le premier et le dernier point d'entrée réglables sont pondérés. On combine les signaux des premiers et derniers points, ce qui donne un signal de sortie de code de référence premier moins dernier (E-L). Un corrélateur de signal (42) et un détecteur d'erreur (45) sont couplés pour recevoir et corréler un signal séquentiel direct à spectre étendu objet du suivi et traité et des signaux de sortie de code référence de E-L (41). Les premiers et seconds générateurs de statistiques d'erreurs de boucle de code (55,56) sont connectés afin de recevoir un signal d'erreur issu du corrélateur de signal (42) et un détecteur d'erreur (45) et de générer des premiers et seconds signaux de statistiques d'erreur afin de régler le poids des premiers et derniers points pondérés. Des premiers et derniers points supplémentaires peuvent être ajoutés et réglés ou initialisés à un poids initial.
PCT/US2000/015849 1999-08-09 2000-06-09 Poursuite de boucle a retard de phase adaptee WO2001011381A1 (fr)

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AU56018/00A AU5601800A (en) 1999-08-09 2000-06-09 Adaptive delay lock loop tracking

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US37019199A 1999-08-09 1999-08-09
US09/370,191 1999-08-09

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Cited By (3)

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EP1543631A1 (fr) * 2002-09-23 2005-06-22 Thomson Licensing S.A. Correlateur a longueur variable pour communications a spectre disperse
US7006556B2 (en) 2001-05-18 2006-02-28 Global Locate, Inc. Method and apparatus for performing signal correlation at multiple resolutions to mitigate multipath interference
US8989236B2 (en) 2003-10-22 2015-03-24 Global Locate, Inc. Method and apparatus for performing frequency synchronization

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7006556B2 (en) 2001-05-18 2006-02-28 Global Locate, Inc. Method and apparatus for performing signal correlation at multiple resolutions to mitigate multipath interference
EP1543631A1 (fr) * 2002-09-23 2005-06-22 Thomson Licensing S.A. Correlateur a longueur variable pour communications a spectre disperse
EP1543631A4 (fr) * 2002-09-23 2011-07-27 Thomson Licensing Correlateur a longueur variable pour communications a spectre disperse
US8989236B2 (en) 2003-10-22 2015-03-24 Global Locate, Inc. Method and apparatus for performing frequency synchronization

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