Patent Publication Number: US-7719466-B2

Title: Communications systems that reduces auto-correlation or cross-correlation in weak signals

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 10/712,789, filed on Nov. 12, 2003 and issued as U.S. Pat. No. 7,183,972 B2, entitled “COMMUNICATIONS SYSTEM THAT REDUCES AUTO-CORRELATION OR CROSS-CORRELATION IN WEAK CDMA SIGNALS,” by Gregory B. Turetzky, et al.; 
     that is a continuation of U. S. patent application Ser. No. 09/910,404, filed on Jul. 20, 2001 and issued as U.S. Pat. No. 6,680,695 B2, entitled “COMMUNICATIONS SYSTEM THAT REDUCES AUTO-CORRELATION OR CROSS-CORRELATION IN WEAK CDMA SIGNALS,” by Gregory B. Turetzky, et al; 
     that claims priority under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 60/227,674, filed Aug. 24, 2000, entitled “METHOD AND APPARATUS FOR ELIMINATING AUTO-CORRELATION OR CROSS-CORRELATION IN WEAK CDMA SIGNALS,” by Gregory B. Turetzky, et al., which application are incorporated by reference herein. 
     U. S. patent application Ser. No. 09/910,404, filed on Jul. 20, 2001 and issued as U.S. Pat. No. 6,680,695 B2, entitled “COMMUNICATIONS SYSTEM THAT REDUCES AUTO-CORRELATION OR CROSS-CORRELATION IN WEAK CDMA SIGNALS,” by Gregory B. Turetzky, et al., also incorporated by reference the following patent applications: 
     U.S. patent application Ser. No. 09/909,091, filed on Jul. 20, 2001 and issued as U.S. Pat. No. 6,466,161, entitled “LOCATION SERVICES SYSTEM THAT REDUCES AUTO-CORRELATION OR CROSSCORRELATION IN WEAK SIGNAL,” by Gregory B. Turetzky, et al.; 
     U.S. patent Ser. No. 09/910,092 that issued as U.S. Pat. No. 7,197,305 B2, filed on Jul. 20, 2001, entitled “APPARATUS FOR REDUCING AUTO -CORRELATION OR CROSS-CORRELATION IN WEAK CDMA SIGNALS,” by Gregory B, Turetzky, et al.; 
     U.S. patent application Ser. No. 09/909,716, filed on Jul. 20, 2001, and issued as U.S. Pat. No. 7,106,786 B2 entitled “METHOD FOR REDUCING AUTO-CORRELATION OR CROSS-CORRELATION IN WEAK SIGNALS,” by Gregory B. Turetzky, et al.; and 
     U.S. patent application Ser. No. 09/909,717, filed on Jul. 20, 2001, and issued as U.S. Pat. No. 6,529,829, entitled “DEAD RECKONING SYSTEM THAT REDUCES AUTO-CORRELATION OR CROSS-CORRELATION IN WEAK SIGNAL ENVIRONMENTS,” by Gregory B. Turetzky, et at; which applications are all incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to Global Positioning System (GPS) receivers, and in particular to systems, methods, and apparatuses for reducing or eliminating auto-correlation or cross-correlation in weak Code Division Multiple Access (CDMA) signals in the presence of strong CDMA signals. 
     2. Description of the Related Art 
     Cellular telephony, including Personal Communication System (PCS) devices, has become commonplace. The use of such devices to provide voice, data, and other services, such as Internet access, has provided many conveniences to cellular system users. Further, other wireless communications systems, such as two-way paging, trunked radio, Specialized Mobile Radio (SMR) that is used by police, fire, and paramedic departments, have also become essential for mobile communications. 
     A current thrust in the cellular and PCS arena is the integration of Global Positioning System (GPS) technology into cellular telephone devices and other wireless transceivers. For example, U.S. Pat. No. 5,874,914, issued to Krasner, which is incorporated by reference herein, describes a method wherein the basestation (also known as the Mobile Telephone Switching Office (MTSO)) transmits GPS satellite information, including Doppler information, to a remote unit using a cellular data link, and computing pseudoranges to the in-view satellites without receiving or using satellite ephemeris information. 
     This current interest in Integrating GPS with cellular telephony stems from a new Federal Communications Commission (FCC) requirement that cellular telephones be locatable within 50 feet once an emergency call, such as a “911” call (also referred to as “Enhanced 911” or “E911”) is placed by a given cellular telephone. Such position data assists police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need or have legal rights to determine the cellular telephone&#39;s position. Further, GPS data that is supplied to the mobile telephone can be used by the mobile telephone user for directions, location of other locations that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular user via Internet maps or other GPS mapping techniques, etc. Such data can be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers. 
     The approach in Krasner, however, is limited by the number of data links that can be connected to a GPS-dedicated data supply warehouse. The system hardware would need to be upgraded to manage the additional requirements of delivering GPS information to each of the cellular or PCS users that are requesting or requiring GPS data, which requirements would be layered on top of the requirements to handle the normal voice and data traffic being managed and delivered by the wireless system. 
     Krasner, however, does not discuss the problems of acquisition of a GPS satellite signal in difficult environments, such as urban areas, or where the mobile receiver has a limited or completely blocked view of the satellites. Inherent in such difficult environments is the ability of a sensitive receiver to acquire spurious signals in the electromagnetic spectrum. 
     Some of these spurious signals emanate from the GPS satellite that the mobile receiver is trying to acquire. If the mobile receiver sweeps through a subset of all of the possible codes, and finds a signal that is above the noise floor, the receiver will lock onto this signal. However, the receiver has no way of knowing if the signal it has chosen to lock onto is the proper signal, especially in weak signal environments. This type of event, where the receiver locks onto a spurious signal emanating from the GPS satellite of interest, is called “auto-correlation.” Auto-correlation can also occur in a strong signal environment, where the signal acquired is not the proper signal. 
     Other spurious signals emanate from other GPS satellites that are either within the line of sight of the mobile receiver, or, because of multi-path conditions, is not within the line of sight of the mobile receiver, and create the same problems as auto-correlation scenarios described above. However, when the spurious signal emanates from a GPS satellite other than the satellite of interest, the event is called “cross-correlation.” 
     Currently, there are no methods or devices designed to determine whether an auto-correlation or cross-correlation event has occurred. There are also no methods or devices designed to correct such events to ensure that the receiver is locked onto the proper signal. 
     It can be seen, then, that there is a need in the art for a method to determine whether an auto-correlation event or cross-correlation event has occurred. It can also be seen that there is a need in the art for a method to correct auto-correlation or cross-correlation events to allow the GPS receiver to lock onto the proper signal. It can also be seen that there is a need in the art for an apparatus to determine whether an auto-correlation event or cross-correlation event has occurred. It can also be seen that there is a need in the art for an apparatus to correct auto-correlation or cross-correlation events to allow the GPS receiver to lock onto the proper signal. 
     SUMMARY OF THE INVENTION 
     To minimize the limitations in the prior art, and to minimize other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses systems, methods and apparatuses for determining if an auto-correlation or cross-correlation event has occurred. The method and apparatus also provide the ability to correct the auto- or cross-correlation event to allow the GPS receiver to lock onto the proper signal. 
     The present invention also discloses methods and apparatuses for eliminating auto- and cross-correlation in weak signal CDMA systems, such as GPS systems. The invention uses parallel data paths that allow standard correlation of signals in parallel with verification of the lock signal to determine whether the system has locked onto the proper signal within the scanned signal window. The invention can be made with multiple CPUs, a single CPU with dual input modes, on multiple IC chips, or as a single IC chip solution for small, low cost reception, downconversion, correlation, and verification systems. 
     A system in accordance with the present invention comprises a transceiver capable of using a wireless communications link for transmission and reception of wireless signals, and a Global Positioning System (GPS) receiver. The GPS receiver can compute the position of the transceiver, and comprises a first data path and a second data path. The first data path correlates an incoming GPS signal located within a scanned signal window with a locally generated signal. The second data path verifies the incoming GPS signal against a lock signal, and determines whether the incoming GPS signal has at least one characteristic which differentiates the incoming GPS signal from an auto-correlated signal. The GPS receiver can change the locally generated signal to continue to search the scanned signal window for a second incoming GPS signal if the incoming GPS signal lacks the characteristic. 
     It is an object of the present invention to provide a method to determine whether an auto-correlation event or cross-correlation event has occurred. It is another object of the present invention to provide a method to correct auto-correlation or cross-correlation events to allow the GPS receiver to lock onto the proper signal. It is another object of the present invention to provide an apparatus to determine whether an auto-correlation event or cross-correlation event has occurred. It is another object of the present invention to provide an apparatus to correct auto-correlation or cross-correlation events to allow the GPS receiver to lock onto the proper signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
         FIG. 1  illustrates a typical CDMA signal flow; 
         FIG. 2  illustrates an auto and cross correlation check in accordance with the present invention; 
         FIGS. 3A and 3B  illustrate an embodiment of the present invention; 
         FIG. 4  illustrates details of the sample block of the present invention; and 
         FIG. 5  is a flowchart illustrating the steps used to practice the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     In the following description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     Overview 
     In CDMA signal environments, cross- and/or auto-correlation problems occur that need to be corrected. The present invention performs a verification of the signal used for locking if the detected signal is weaker in signal strength than is expected by the receiver. If the expected determined (locking) signal is strong, or, at least, not weak, then no verification is required. However, if, by Signal-to-Noise Ratio measurements, or other methods, the determined signal is found to be below a predetermined signal strength, then the receiver may be receiving an improper locking signal, and thus, auto- or cross-correlation ghost signals may be the signal that the receiver is locking onto. The present invention discusses how to reduce or eliminate such auto and/or cross correlation problems. 
       FIG. 1  illustrates a typical Code Division Multiple Access (CDMA) signal flow. In many systems  100 , e.g., GPS receiver systems, cellular telephone systems, etc., CDMA input signals  102  enter an RF downconverter  104  for conversion to baseband signals. These baseband signals are then sampled in a sampler  106  to obtain digital samples of the CDMA input signals  102 . Typically, especially in a GPS receiver system  100 , these samples are then sent to correlator engine  108  and then on to Central Processing Unit (CPU)  110 . 
     The present invention allows for a separate path for the signals  112  to reach the CPU  110 . The signals  112 , which are the same samples that are used in the correlator engine  108 , are sent directly to the CPU  112 , or, optionally, through a buffer  114 . Although the signals  112  can be sent directly to the same CPU  112  for processing, which CPU is typically an ARM7, signals  112  can be sent to a separate Digital Signal Processor ASP), or, alternatively, to a CPU  112  that incorporates the DSP and ARM7 on a single integrated circuit (IC) chip. Further, the correlator engine  108 , CPU  110 , and optional buffer  114  can be on a single IC chip to allow for lower power consumption, smaller packaging of the system  100 , etc. The RF downconverter  104  can also be integrated with correlator engine  108 , CPU  110 , sampler  106 , and optional buffer  114  to provide a single IC chip system  100  if desired. Further, for ease of integration, CPU  110  can accept signals  116  and  118  at different ports, or signals  116  and  118  can be sent to separate CPUs  110 , e.g., signals  116  can be sent to a DSP, while signals  118  can be sent to an ARM7. Other configurations where single or multiple CPUs  110  can be realized with the present invention.  FIG. 1  is illustrative, but not exhaustive, of the possibilities of signal flow within the scope of the present invention. 
     Typically, in a communications system, the GPS receiver system  100  is co-located with another system that allows for transmission, such as a cellular telephone system  120 . The cellular telephone transceiver  122 , typically located in a cellular handset, can transmit and receive signals  124  on a wireless or hardwired link. Such a system  120  is embodied in the cellular telephone network, Personal Communications System (PCS) network, or can also be embodied as a Personal Data Assistant (PDA), laptop computer, or any other device that can transmit and/or receive data via wireless or hard-wired communications links. 
     Such a communications system  120 , when co-located with the GPS receiver system  100 , uses the GPS receiver system  100  to determine location and use the determined location for various purposes, e.g., location services, determining or computing the location of the wireless transceiver  122 , determining directions to a predetermined or desired location, pinpointing the location of the wireless transceiver  122  for emergency and/or law enforcement personnel, etc. 
     As such, the present invention is useful in a location services system, where users wish to use their mobile GPS receiver systems  100 , possibly located inside of a cellular telephone, to get directions, get assistance finding nearby points of interest, restaurants, or other physical locations that may be difficult to locate without some sort of mapping aid. A cellular telephone or other mobile device can display, either visually or otherwise, the user&#39;s location, the user&#39;s location on a map, a route or part of a route between the user&#39;s location and the desired destination, or any number of things that can be used for location services. 
     Further, the present invention is also useful in a dead reckoning system, wherein at least one sensor, such as a gyroscope, odometer, or other sensor, provides inputs to the GPS receiver system  100  to selectively assist in computing a position of the GPS receiver system  100 . Such systems are typically used in automobiles that travel into places where tunnels and other natural and man-made structures interfere with the receipt of GPS signals, but can also be used on or in conjunction with cellular telephones, wireless transceivers, or other devices. 
     Further, since both the wireless transceiver  122  and GPS receiver system  100  are typically integrated circuits, for ease of packaging, lower power consumption, or other reasons, the GPS receiver system  100  and the wireless transceiver  122  can be located on a single integrated circuit, or can share circuitry between the wireless transceiver and the GPS receiver system  100 . For example, the GPS receiver system  100  can use the Central Processing Unit (CPU)  126  of the wireless transceiver  122  to perform mathematical calculations needed to determine the position of the wireless transceiver  122 , either instead of or in parallel with CPU  110 . Further, the wireless transceiver  122  can share other portions of the circuitry, such as a local oscillator to provide a reference frequency  128  to the GPS receiver system  100 , and the reference frequency  128  can either be the same as or different from the reference frequency used by the wireless transceiver  122 . 
     The wireless transceiver  122  can accept data  130  from the GPS receiver system  100 , as well as provide data  130  to the GPS receiver system  100 . Data  130  accepted by the wireless transceiver includes raw GPS data, pseudoranges, or a determined position. Data  130  provided by the wireless transceiver includes ephemeris information, time information, and coarse position information. 
       FIG. 2  illustrates an auto and cross correlation check in accordance with the present invention. 
     System  200  shows RF signal  202  entering system  200 , where it is decimated in block  204 . The result of decimate block  204  is the reduced bandwidth samples from RF signal  202 , shown as block  206 . These samples  208  are typically passed to a correlator engine  108  shown in  FIG. 1 . The local code  212  is then correlated against the incoming samples  208  in block  210 , which is then passed to the tracker  214  such that system  200  can track the RF input signal  202 . 
     Related art designs do not determine whether the tracker  214  is tracking the carrier or desired signal, or whether tracker  214  is tracking a spurious signal, which may be a cross-correlated spur or an auto-correlated spur. The present invention provides a method and apparatus for verifying whether the tracker  214  is tracking the correct or desired signal before the signal is validated for use in navigation. 
     The signal strength of the signal being tracked is checked in block  216 . If the signal strength is greater than a predetermined strength, e.g., greater than 35 dB-Hz, then the system  200  knows that the signal is strong enough that it is not a spurious signal, and the signal is validated in block  218 , and passed to the navigation system in block  220 . However, if the signal is not of sufficient strength, the auto-correlation check block  222  is entered. Block  222  can be the same block for a cross-correlation check, or can be a different block of computer code, hardware circuitry, or integration of hardware, software, firmware, or other devices and methods used to perform similar functions to those described herein. Further, block  222  can be a threshold Signal-to-Noise Ratio (SNR) verification, or other such verification to determine whether auto/cross correlation conditions exist. Such a check block can have one characteristic of the signal checked, can have multiple characteristics to check, or can select from one or more characteristics to be checked, either automatically or manually selected, depending on the design or desires of the user. 
     Samples  206  are stored in memory as shown by path  224  and block  226 . If the samples do not comprise enough data, e.g., if there is less data than a predetermined amount of data, block  228  will loop around until there is enough data in the system. As shown in  FIG. 2 , the system  200  continues to store sample data until there is enough data to process. For example, in the GPS system, 2 msec of data is desired to perform processing to determine whether the signal is the proper signal. 
     Block  230  shows processing the stored data to determine whether the signal that has been tracked (or locked onto) in block  214  is the proper signal within the signal window. In a cross-correlation situation, the proper signal can be determined by a correlation to a different satellite code being stronger than the correlation to a desired (or current) satellite code. In an auto-correlation situation, the proper signal can be determined by a correlation to a different delay of the same satellite code being stronger than the correlation to the locally generated code delay. Decision block  232  shows that the system  200  verifies that the signal is or is not the proper signal, again, via SNR verification or other methods. 
     The correlation methods used on the verification signal, which is on a second path relative to the incoming signal, include computing the correlation between the sample data and the same prn code and local reference frequency of the tracked signal (signal that has been locked to), computing the correlation between the sample data and a different prn code but the same local reference frequency as the tracked signal, computing the correlation between the sample data and the same prn code and local reference frequency that is a multiple of the prn repeat frequency of the tracked signal, computing the correlation between the sample data and a different prn code and different local reference frequency that is a multiple of the prn repeat frequency of the locked signal, and other correlations and methods. If the signal is the proper signal, the signal is verified and validated via path  234 . If the signal is not the proper signal, then the tracker  214  is redirected or otherwise controlled to the proper signal, which proper signal was determined in block  230 . 
       FIGS. 3A and 3B  illustrate an embodiment of the present invention. 
     System  300  shows GPS Clear/Acquisition (also known as Coarse/Acquisition) (C/A) (CDMA formatted RF signals) data  102  entering the downconverter  104 , which downconverts the CDMA signal to baseband for processing. The downconverter passes the signals to the decimators  204 , which are part of the sample block  106 . These are passed to serial shift registers  302 , and then each placed in parallel into two additional registers, parallel register  304  and shift register  306 . Shift register  306  is loaded and then shifted out of the register  306 , whereas parallel register  304  is loaded and read directly by the CPU  110 . Parallel registers  304  provide signals  116  that are delivered directly to the CPU (microprocessor)  116 , whereas shift registers  306  provide signals to the correlator engine  108 . A Doppler rotator  308  is used to properly align in frequency the signals being fed into the correlator  108 . 
     Local code, emanating from coder  212 , is used to correlate against the incoming samples in determining the proper signal to lock onto within the sampled signals from sampler  106 . The signals are accumulated in the accumulator  310 , and a peak detector  312  determines the signal that is passed to the tracker, which is signal  118  shown in  FIG. 1 . Coder  212  is shifted in time and/or phase to assist in correlation. This shift is typically done by a separate circuit, and in the present invention, can be done by a data path executive when the incoming signal is determined to be an auto-correlated or cross-correlated signal. 
       FIG. 4  illustrates details of the sample block of the present invention. 
     System  400  shows the decimators  204  feeding the serial shift registers  302 , which each store, in parallel, their data in serial shift registers  304  and  306 . For clarity, parallel registers  304  and shift registers  306  have been shown as parallel registers  304 I and  304 Q, and shift registers  306 I and  306 Q, to indicate whether the registers contain I data or Q data, respectively. Shift registers  306  pass their data to the Doppler rotator  308 , whereas parallel registers  304  pass their parallel data, i.e., signals  116 , directly to the CPU  110 . Again, CPU  110  can be the same CPU that processes the Doppler rotated correlated signals, or a separate CPU. Further, even if the CPUs are separate, they can be co-located on a single IC chip if desired. 
     Additional control lines couple the CPU  110  to the capture block (sampler)  106 . Lines  402 I and  402 Q indicate when the CPU  110  has read the data from associated shift registers  306 I and  306 Q, respectively. Further, data available status lines  404 I and  404 Q are set to a known value, either high or low, to inform the CPU  110  that the parallel registers  304 I and/or  304 Q are available for reading. Once the parallel registers  304 I and/or  304 Q are read, the data available status registers  404 I and/or  404 Q can be cleared. 
     Process Chart 
       FIG. 5  is a flowchart illustrating the steps used to practice the present invention. 
     Block  500  illustrates correlating an incoming CDMA signal, located within a scanned signal window, with a locally generated signal on a first data path. 
     Block  502  illustrates verifying the incoming CDMA signal, on a second data path, located within the scanned signal window, against a lock signal of the first data path. 
     Block  504  illustrates determining, using the second data path, whether the incoming CDMA signal has at least one characteristic which differentiates the lock signal, or locally generated signal, from an auto-correlated or cross-correlated signal. 
     Block  506  illustrates continuing to search the scanned signal window for a second incoming CDMA signal if the lock signal lacks the at least one characteristic. 
     CONCLUSION 
     Although the description of the present invention herein describes specific embodiments of the present invention, the scope of the present invention includes other embodiments of the present invention not described herein. 
     In summary, the present invention describes systems, methods and apparatuses for reducing or eliminating the auto-correlation or cross-correlation events that occur during weak signal conditions. The devices in accordance with the present invention also provides the ability to correct the auto- or cross-correlation event to allow the GPS receiver to lock onto the proper signal. 
     A system in accordance with the present invention comprises a transceiver capable of using a wireless communications link for transmission and reception of wireless signals, and a Global Positioning System (GPS) receiver. The GPS receiver can compute the position of the transceiver, and comprises a first data path and a second data path. The first data path correlates an incoming GPS signal located within a scanned signal window with a locally generated signal. The second data path verifies the incoming GPS signal against a lock signal, and determines whether the incoming GPS signal has at least one characteristic which differentiates the incoming GPS signal from an auto-correlated signal. The GPS receiver can change the locally generated signal to continue to search the scanned signal window for a second incoming GPS signal if the incoming GPS signal lacks the characteristic. 
     The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention not be limited by his detailed description, but by the claims appended hereto.