Noise distribution shaping for signals, particularly spread spectrum signals like CDMA signals, with improved robustness

Abstract

Noise distribution shaping for signals, particularly for the application in receivers for CDMA signals. An embodiment provides a method for noise distribution shaping for signals comprising the acts of generating a blanking control signal by comparing a received signal comprising transitions with at least one blanking threshold, determining transitions of the received signal and zones around the determined transitions, and generating a transitions control signal comprising the determined transitions and zones, adapting the at least one blanking threshold or the received signal according to an offset value depending on the amplitude of the received signal and on the transitions control signal, and modifying the noise distribution of the received signal by applying blanking of the received signal under control of the blanking control signal.

Claims

1. A method for noise distribution shaping for signals comprising the acts of generating a blanking control signal by comparing a received signal comprising transitions with at least one blanking threshold, determining transitions of the received signal and zones around the determined transitions, and generating a transitions control signal comprising the determined transitions and zones, adapting the at least one blanking threshold or the received signal according to an offset value depending on the amplitude of the received signal and on the transitions control signal, and modifying the noise distribution of the received signal by applying blanking of the received signal under control of the blanking control signal.

2. The method of claim 1, wherein the transitions are determined by at least one tracking loop for tracking the received signal.

3. The method of claim 1, wherein the durations of the zones around the determined transitions are set to a constant value.

4. The method of claim 3, wherein the constant value is selected depending on the accuracy of a tracking loop for tracking the received signal.

5. The method of claim 1, wherein the durations of the zones around the determined transitions are adapted during the entire process of the noise distribution shaping.

6. The method of claim 1, wherein the generating of the blanking control signal comprises setting the at least one blanking threshold within the zones around the determined transitions to a predefined value, which is selected such that the adapting of the at least one blanking threshold is interrupted within the zones, or interrupting the adapting of the received signal within the zones around the determined transitions.

7. The method of claim 1, wherein transitions of the received signal and zones around the determined transitions are only determined for consecutive signal states of different polarity.

8. A device for noise distribution shaping for signals comprising means for generating a blanking control signal by comparing a received signal comprising transitions with at least one blanking threshold, means for generating a transitions control signal depending on determined transitions of the received signal and zones around the determined transitions, means for adapting the at least one blanking threshold or the received signal according to an offset value depending on the amplitude of the received signal and on the transitions control signal, and means for modifying the noise distribution of the received signal by applying blanking of the received signal under control of the blanking control signal.

9. The device of claim 8, wherein the means for generating a transitions control signal are configured to set the durations of the zones around the determined transitions to a constant value.

10. The device of claim 9, wherein the means for generating a transitions control signal are configured to select the constant value of the zones durations depending on the accuracy of a tracking loop for tracking the received signal.

11. The device of claim 8, wherein the means for generating a transitions control signal are configured to adapt the durations of the zones around the determined transitions during noise distribution shaping.

12. The device of claim 8, wherein the means for generating a blanking control signal are configured to set the at least one blanking threshold within the zones around the determined transitions to a predefined value, which is selected such that the adapting of the at least one blanking threshold is interrupted within the zones, or to interrupt the adapting of the received signal within the zones around determined transitions.

13. The device of claim 8, wherein the means for generating a transitions control signal are configured to only determine transitions of the received signal and zones around the determined transitions for consecutive signal states of different polarity.

14. The device of claim 8, wherein the received signal is a CDMA signal, specifically a BSPK or BOC modulated binary signal, and the means for adapting are configured to calculate the offset value by multiplying the square root of the estimated power Pest with a scaling factor being +1 or 1 depending on the chip values of the CDMA signal.

15. A GNSS receiver circuitry comprising an analog to digital converter for generating samples of received CDMA signals, a plurality of noise distribution shapers for modifying the noise distribution of the sampled CDMA signals, each noise distribution shaper being a device for noise distribution shaping for signals according to claim 8, and a GNSS signal processor for processing the sampled CDMA signals output by the noise distribution shapers and for outputting chip values of the processed CDMA signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a diagram of a chip stream for a received and a replica signal, when the tracking error is non-zero;

(2) FIG. 2 shows a diagram of the chip stream for the received signal from FIG. 1 with the chip transitions between consecutive chips;

(3) FIG. 3 shows the diagram of FIG. 1 with an additional multipath signal which will superimpose onto the received signal;

(4) FIG. 4 shows the diagram of FIG. 1 with a modified blanking threshold, which is paused within zones around each chip transition according to the invention;

(5) FIG. 5 shows the diagram of FIG. 1 with a modified blanking threshold, which is paused within zones around the chip transitions between consecutive chips of different polarity according to the invention;

(6) FIG. 6 shows an embodiment of a GNSS receiver circuitry with a noise distribution shaper according to the invention; and

(7) FIG. 7 shows a flow chart of an embodiment of a noise distribution shaping method for CDMA signals according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(8) In the following, functionally similar or identical elements may have the same reference numerals. Absolute values are shown below by way of example only and should not be construed as limiting the invention. Even if the following description of the invention is applied to CDMA signals of a GNSS, it should be noted that the invention can generally be applied to any kind of signal comprising transitions and is, therefore, not restricted to CDMA signals.

(9) The blanking disclosed in the before mentioned European patent application no. 14290171.9 applies blanking thresholds BTH.sup.+ and BTH.sup. with the following values:
BTH.sup.+=(sqrt(P.sub.est))c(t)+B0
BTH.sup.=(sqrt(P.sub.est))c(t)B0

(10) Where c(t) equals +1 or 1 according to the chip polarity, is a scaling factor for example 1, P.sub.est is the estimated power, B0 the blanking threshold without offset (as for a conventional receiver). B0 can be set for example to 0.5.sub.noise where .sub.noise represents the standard deviation for the thermal noise.

(11) Both thresholds are synchronized to the received GNSS signal chip stream (whose amplitude is typically smaller than the standard deviation of the noise), which represents an ideal situation. In reality, a GNSS receiver does not know when the exact chip transition occurs, but can only estimate it thanks to tracking loops (DLL, possibly aided with a PLL). Such loops will consequently allow determining the positive or negative offset of both thresholds, according to the estimated polarity of the spreading sequence. As a consequence, in a real scenario the exact position of the chip transition will be known with the accuracy of the tracking loops.

(12) Now, at a low Carrier-to-Noise Spectral Density Ratio (C/N.sub.0) the corresponding tracking loop performance will consequently influence the setting of the thresholds.

(13) Furthermore, in case of high dynamic, the delay locked loop (and possibly the phase locked loop) might not react quickly enough and, as a consequence, the offset transition might be delayed with respect to the actual/true chip transition of the received signal. As a consequence, some of the decisions to blank or not some of the samples might not be optimal around those chip transitions.

(14) Finally, in presence of multipath and especially strong static multipath, the delayed multipath signal will also delay the chip transition, which might also degrade the optimal setting of the thresholds offset if the loops are sensitive to the corresponding additional multipath signal.

(15) FIG. 1 represents the chip stream for the received signal 100 and the replica signal 102, when the tracking error is non-zero. The two blanking thresholds BTH.sup.+ and BTH.sup. are also represented in dashed lines and are synchronized to the replica signal 102. The tracking error is caused either by thermal noise, or by the dynamic or both dynamic and thermal noise. FIG. 2 shows only the chip stream for the received signal 100 with the transitions between consecutive chips (chip transitions). As it can be seen, a chip transition occurs also between consecutive chips 112 and 114 of the same polarity.

(16) FIG. 3 additionally represents a multipath signal 104 which will superimpose onto the received signal 100 (but not onto the replica).

(17) All of the reasons motivated the inventors to introduce a zone around the chip transition where no offset, or even no blanking (to do that one solution comprises raising the blanking threshold to a very large value) shall be applied. FIGS. 4 and 5 represents the modified blanking thresholds, by introducing a zone where no blanking is applied. The width of this zone is 2. Hence during the period 2 around the chip transition, no offsetting of the blanking thresholds is used. It has to be highlighted that the de-activation of the threshold offsetting can also apply at the chip transitions even if the consecutive chip values are identical (same polarity) or not. Alternatively, it is also possible to de-active the threshold offsetting around the chip transition only when the polarity of consecutive chips is opposite. The width 2 can be adapted during receiver operation. For example, if the receiver contains any multipath detection function, which can monitor the multipath environment and confirm the absence of multipath, the 2 could be set very low or even suppressed (no offset de-activation). Similarly, if the receiver works at an higher (C/N.sub.0) or in absence of dynamic during a given period of time, again the 2 could be set very low or even suppressed. Consequently, the width 2 can be for example adapted by the tracking loops. Of course, it can also be set constant as a configuration parameter of the receiver and particularly the adaptive blanker. In FIG. 4, each chip transition comprises a zone 106, within the offsetting of the blanking threshold is paused or interrupted, also within the zone around the chip transition between two consecutive chips with the same polarity. However, it is also possible to not pause or interrupt the offsetting of the blanking threshold between two consecutive chips of the same polarity, as shown in the diagram of FIG. 5.

(18) FIG. 6 shows a block diagram of a GNSS receiver circuitry 10 comprising a noise distribution shaper according to the invention. The shown circuitry 10 comprises a RF front end circuitry 14 for filtering and down converting CDMA signals received from GNSS signals into a baseband for further processing, an ADC (Analog to Digital Converter) 16 for sampling the down converted CDMA signals (an AGC (Automatic Gain Control) (not shown) can be placed between the RF front end circuitry 14 and the ADC 16) and converting the analog samples of the CDMA signals into digital values, a noise distribution shaper 18 according to the invention for shaping the noise distribution of the digitized CDMA signal samples of one of the plurality of the received CDMA signals, and a GNSS signal processor 26 for correlating the received CDMA signals with spreading sequences or chip values used to encode the CDMA signals and processing the CDMA signals for determining and outputting positioning data 28. It should be noted that the noise distribution shaping can also be applied directly in RF, and is therefore not restricted to the baseband as described before.

(19) In the following, the implementation of the noise distribution shaper 18 is described in detail. The digitized CDMA samples output by the ADC 16 are input to a blanker 20 and an input of a comparator circuit 22. The comparator circuit 22 further receives a first adaptive blanking threshold BTH(i) and a second adaptive blanking threshold BTH+(i), which are both output from a blanking threshold generator 24. The adaptive blanking thresholds BTH(i) and BTH+(i) are assigned to one signal (index i) of the plurality of the I received CDMA signals. In other words, each received CDMA signal usually has its own dedicated adaptive blanking thresholds. Also, less or more than two adaptive blanking thresholds may be provided. Furthermore, for each chip value taken at instant k, the blanking thresholds BTH(i) and BTH+(i) will be modified according to this binary chip value. Now the time dependency is considered implicitly in the example to ease readability.

(20) The blanking threshold generator 24 receives from the GNSS signal processor 26 the chip values, which are used by the processor to decode and despread the received CDMA signals. For example, if the processor 26 is able to decode four (I=4) CDMA signals among the plurality of received CDMA signals, it outputs the four chip values corresponding to the spreading sequences assigned to the four received CDMA signals and used to decode them by the processor 26. It should be noted that in such case four different adaptive blanking threshold generators 24 are required since each block will be adapted to the chip value of each sequence (in FIG. 6, only one generator block 24 is shown).

(21) The blanking threshold generator 24 receives a chip transitions control signal, which is generated by a chip transitions control signal generator 17. The generator 17 receives from the GNSS signal processor 26 a signal indicating the estimated transitions of chips and a tracking error of the real GNSS signal. The GNSS signal processor 26 can estimate the chip transitions with a tracking loop, preferably a DLL as it may be used in the signal processor to track the received GNSS signal. In addition to the DLL, a PLL may be used, which can also be applied to estimate the chip transitions. From the signal indicating estimated transitions of chips in a received GNSS signal and the tracking error of an applied tracking loop, the generator 17 generates and outputs the chip transitions control signal containing the zone duration 2 around each estimated chip transition to the threshold generator 24, wherein the width of the zone can vary depending on the dynamic foreseen of the application type, for example it can differ for pedestrian and airborne users. Furthermore, in case two consecutive chips have the same polarity (+|+ or |), the width of the duration of zone around the chip transition can be set to 0, meaning that the offset of the blanking threshold is still applicable for such transitions with identical chip polarity or equivalently that the de-activation of the offset is cancelled.

(22) The blanking threshold generator 24 generates with the received chip values the first adaptive blanking threshold BTH(i) and the second adaptive blanking threshold BTH+(i). The blanking thresholds BTH+(i) and BTH(i) may be generated by offsetting predefined blanking thresholds BTH.sub.pre+ and BTH.sub.pre with the scaled amplitude of the respective CDMA signal i. The amplitude of the offsets applied to the predefined blanking thresholds BTH.sub.pre+ and BTH.sub.pre are calculated from the estimated power (P.sub.est) of the received signal based on the samples provided by the Analog-to-Digital Converter (ADC), from block 16, multiplied with a scaling factor also function of the binary chip value. Two scaling factors + and are respectively deduced for the positive and negative chip value. The predefined blanking thresholds BTH.sub.pre+ and BTH.sub.pre may be for example selected depending on the environment of operation of the GNSS receiver and particularly be configured depending on the occurrence of pulsed interferences such as generated by vehicle ignition systems, power lines, heavy current switches or microwave ovens. For example, in an environment with strong pulsed interferences, such as in the neighborhood of vehicle ignition systems or power lines, the predefined blanking thresholds may be set with a higher value than in environments with weaker pulsed interferences. For the selection of predefined blanking thresholds suitable for a certain environment, the strength and frequency of occurrence of pulsed interferences in the received CDMA signals may be determined, and depending thereon suitable predefined blanking thresholds may be determined.

(23) The offsetting of the predefined blanking thresholds BTH.sub.pre+ and BTH.sub.pre may comprise adding the estimated received power P.sub.est multiplied with a positive scaling factor .sub.+ for positive chip value, or adding the estimated received power P.sub.est multiplied with a scaling negative factor .sub. for negative chip value:

(24) Hence for positive chip values the adapted blanking thresholds are given by:
BTH(i)=BTH.sub.pre+.sub.+(i)(sqrt(P.sub.est))
BTH+(i)=BTH.sub.pre++.sub.+(i)(sqrt(P.sub.est))

(25) And for negative chip values the adapted blanking thresholds are given by:
BTH(i)=BTH.sub.pre+.sub.(i)(sqrt(P.sub.est))
BTH+(i)=BTH.sub.pre++.sub.(i)(sqrt(P.sub.est))

(26) The offsetting of the predefined blanking thresholds BTH.sub.pre+ and BTH.sub.pre is further controlled by the chip transitions control signal received from the chip transitions control signal generator 17. During the zone of duration 2 around each estimated chip transition as indicated by the chip transitions control signal, the blanking threshold generator 24 sets the adaptive blanking thresholds BTH+(i) and BTH(i) to BTH.sub.pre+ and BTH.sub.pre, where is a scaling factor. By doing this the blanking threshold is applied without offset.

(27) Hence during the zone duration 2 the blanking thresholds are given by:
BTH(i)=BTH.sub.pre and BTH+(i)=BTH.sub.pre+
for both positive and negative chip values

(28) An alternatively scheme for the zone around the chip transition could be:

(29) For positive chip values the blanking thresholds are given by:
BTH(i)=LVAL
BTH+(i)=+LVAL

(30) And for negative chip values the blanking thresholds are given by:
BTH(i)=LVAL
BTH+(i)=+LVAL

(31) LVAL is a large value, which is selected such that the amplitudes of CDMA samples of CDMA signal i do not exceed LVAL. By doing this the blanking is deactivated during the zone around the chip transition.

(32) Alternatively to considering the chip transitions control signal supplied to the threshold generator 24, the chip transitions control signal may also be directly supplied to the comparator circuit 22, as outlined in FIG. 6 by the dashed arrow from block 17 to block 22. Then the supply of chip transition supplied from block 17 to block 24 would not be necessary. Then, the comparator circuit 22 directly processes the chip transitions control signal when generating a blanking control signal NBCTRL(i) as described in the following.

(33) The adaptive blanking thresholds BTH+(i) and BTH(i) are sent to second inputs of the comparator circuit 22, which receives on its first input the digitized CDMA samples from the ADC 16. The comparator circuit 22 compares the amplitudes (algebraic values) of the digitized CDMA samples with the adaptive blanking thresholds BTH+(i) and BTH(i) and generates depending on the comparison the blanking control signal NBCTRL(i), which controls the blanker 20.

(34) The comparator circuit 22 may generate the blanking control signal NBCTRL(i) for example as follows:
amplitudes of CDMA samples of CDMA signal i>BTH+(i)->NBCTRL(i)=+1
amplitudes of CDMA samples of CDMA signal i<BTH(i)->NBCTRL(i)=1
amplitudes of CDMA samples of CDMA signal i<BTH+(i) and >BTH(i)->NBCTRL(i)=0

(35) The blanker 20 sets the samples of the CDMA signals received at its input to a predefined value such as zero as long as the noise blanking control signal NBCTRL(i) is +1 or 1, and passes the received digitized CDMA signals from its input to its output if the blanking control signal NBCTRL(i) is 0.

(36) During an indicated chip transition (for example by means of the chip transitions control signal from the generator 17 or immediately via the adaptive blanking thresholds BTH+(i) and BTH(i) from the threshold generator 24 if the comparator 22 does not receive the chip transitions control signal), the comparator circuit 22 outputs a blanking control signal NBCTRL(i), which is either 0 without any blanking (which is equivalent to setting the adaptive blanking thresholds BTH+(i) and BTH(i) to a very large value) or +1/1 with blanking (in the case of the offsetting of the blanking thresholds BTH+(i) and BTH(i) is not applied, and those thresholds equals BTH(i)=BTH.sub.pre and BTH+(i)=BTH.sub.pre+).

(37) The above example is given for two blanking thresholds BTH+ and BTH (N=2). In the following, a brief example for the case of a larger N=4 for a CDMA signal i is given (hereby BTH(n, i) means blanking threshold n for CDMA signal i):
CDMA signal i<BTH(1,i).fwdarw.NBCTRL(i)=+1.
BTH(1,i)<CDMA signal i<BTH(2,i).fwdarw.NBCTRL(i)=0.
BTH(2,i)<CDMA signal i<BTH(3,i).fwdarw.NBCTRL(i)=+1.
CDMA signal i>BTH(4,i).fwdarw.NBCTRL(i)=+1.

(38) Again the proposed control low for the NBCTRL(i) based on a comparison with the 4 thresholds is just one example. The blanking control signal for the blanking thresholds BTH(n, i) is adapted for the i.sup.th tracked CDMA signal among I signals. As a consequence, as many blanking blocks as there are tracked signals (I) are required. Alternatively, a single control block fed with the different chip values for the I CDMA signals can be provided. However, this single control block must then be able to handle the blanking thresholds BTH(n, i) for the i.sup.th CDMA signal, for example by applying multiplexing inside the block.

(39) FIG. 7 shows a flowchart of a method for noise distribution shaping for I CDMA signals according to the invention, which can be implemented as a computer program to be executed by a processor. The term steps used in the following description does not mean that the steps are performed in a certain sequence, since the single steps can also be performed in parallel. Thus, steps must be understood as acts performed by the method.

(40) The method receives in step S0 digitized CDMA signal samples of the i.sup.th CDMA signal from the ADC 16. In step S11, the method receives chip transitions and a tracking error of the i.sup.th CDMA signal and generates a chip transitions control signal. In step S12, the chip values of the i.sup.th CDMA signal are provided, which are known a priori and stored in a memory. In step S13 the i.sup.th CDMA signal from the ADC 16 is provided in order to estimate the power of the i.sup.th received signal. The scaled estimated chip amplitudes are then used in step S14 to offset predefined blanking thresholds BTH.sub.pre(i) to generate a set of blanking thresholds BTH+(i) and BTH(i) for each CDMA signal. The offsetting in step S14 can be performed depending on the chip transitions control signal provided through step S11: during each estimated chip transition, a zone with a duration 2 is defined around each estimated chip transition and, during each zone, the blanking thresholds BTH+(i) and BTH(i) for each CDMA signal i are set to a predefined value, which is selected such that the adapting of the at least one blanking threshold is interrupted within the zones, while out of the chip transition zones the blanking thresholds BTH(i) for each CDMA signal are offset with the scaled estimated amplitude. With the set of blanking thresholds BTH(i) and the received digitized CDMA signal samples, the method generates in step S10 a blanking control signal for each CDMA signal i, for example by a comparison as described above. Alternatively, the offsetting in step S14 can be performed without considering the chip transitions control signal (in fact, the chip transitions control signal generated in step S11 must not be provided to step S14) and the chip transitions control signal can be directly provided through step S11 to step S10 (dashed arrow from step S11 to step S10 in FIG. 7). Then, the chip transitions control signal is used for the generation of the blanking control signal, namely for interrupting or pausing the adapting of the received spread spectrum coded signal within the zones around the determined chip transitions as described above. In step S16, the noise distribution of the received samples of the i.sup.th CDMA signal is shaped by a blanking process controlled by the i.sup.th blanking control signal generated in step S10. The CDMA signal samples with shaped noise distribution are finally output to the GNSS signal processor 26 for further processing.

(41) Simulations performed by the inventors have shown that by introducing the corresponding zones around the chip transitions for the blanker, the improvement in (C/N.sub.0) is reduced with respect to the case with blanking thresholds permanently offset but still much better than the case without adaptive blanking thresholds. Despite this reduction in absolute (C/N.sub.0) improvement, the corresponding receiver will gain in robustness for low (C/N.sub.0), high dynamic and strong multipath environments.

(42) The present invention constitutes an extension of the blanking approach disclosed in the European patent application no. 14290171.9. It extends the range of applicability, especially for typical environment with multipath or for receiver having large dynamic.

(43) The present invention allows reducing the degradation of the SNIR of a received signal, particularly a CMDA signal, when there are no pulsed interferences, by offsetting the blanking thresholds or the signal depending on the amplitude of the signal, for example with the scaled amplitude (the amplitude can be positive or negative according to the chip value) of a received CDMA signal, and by comparing the amplitude (algebraic value) of samples of the received signal to the blanking thresholds. The present invention can be used for receivers for navigation and communication applications. In general, the invention is of interest for all terrestrial or space based applications in telecommunication, or more generally for all kinds of signal transmissions where the signal is buried in high-level noise.

(44) While at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms comprise or comprising do not exclude other elements or steps, the terms a or one do not exclude a plural number, and the term or means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.

REFERENCE NUMERALS AND ACRONYMS

(45) 10 GNSS receiver circuitry 12 antenna 14 RF front end circuitry 16 ADC 17 chip transitions control signal generator 18 noise distribution shaper 20 blanker 22 comparator circuit 24 blanking threshold generator 26 GNSS signal processor 28 GNSS output data 100 received GNSS signal 102 replica GNSS signal 104 multipath signal 106 zones around chip transitions for pausing or interrupting of the offset of the blanking threshold 108-114 consecutive chips ADC Analog to Digital Converter BTH Blanking Threshold CDMA Code Division Multiple Access C/N.sub.0 Carrier-to-Noise Spectral Density Ratio GNSS Global Navigation Satellite System DLL Delay Locked Loop NBCTRL Noise Blanking Control PLL Phase Locked Loop RF Radio Frequency SNIR Signal-to-Noise and Interference Ratio