Abstract:
Methods and systems for processing a Global Positioning System (GPS) signal are provided. A method includes: transforming a time domain GPS signal to a frequency domain GPS signal; storing a frequency domain pseudorandom noise (PRN) signal; correlating the frequency domain GPS signal with the PRN signal at a plurality of frequencies, said correlating including: shifting the frequency domain GPS signal by an amount corresponding to one of the plurality of frequencies; downsampling the shifted frequency domain GPS signal; and multiplying the shifted frequency domain GPS signal by the stored frequency domain PRN signal to produce a correlated signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/547,395, filed on Feb. 23, 2004, entitled “Method and System for Reducing and Sharing Computations While Searching for Multiple GPS Satellites at Multiple Frequencies,” the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The application relates generally to signal processing systems and more specifically to signal processing systems for satellite-based navigation systems. 
     Satellite-based navigation typically involves a device with a radio frequency (RF) receiver for receiving signals from satellites orbiting earth, and signal processing components for processing the received signals. The global positioning system (GPS) is a well-known satellite-based navigation system that includes a constellation of GPS satellites, each of which constantly transmits a unique pseudorandom noise (PRN) code. When a GPS device initially acquires satellite signals, it searches over many possible satellites and over many possible frequencies per satellite according to known methods. Typical search methods include modulating a received signal with a reference signal, correlation, and searching for signal peaks in the correlation output. Therefore, the initial search over many satellite/frequency pairs requires many computations. 
       FIG. 1  is a block diagram of a prior art GPS device  100  for receiving and processing GPS signals. The device includes an RF antenna  110  and receiver  120 . The RF signal is processed in multiple flows for a single satellite. Two of the multiple processing flows, flow  102  and flow  104  are shown. Each flow represents processing for one satellite/frequency pair. In the example of  FIG. 1 , flow  102  represents processing of the signal from satellite  1  at frequency  1  and flow  104  represents processing of the signal from satellite  1  at frequency f M , where there are N possible satellites and M possible frequencies. The flows  102  and  104  include blocks that represent hardware and/or software components for performing certain operations. Typically, the hardware and/or software are not actually duplicated for each flow, but are time-multiplexed between flows. 
     The satellite signal appears at the output of the RF component at some unknown satellite frequency. This unknown frequency consists of a known portion which is characteristic of the RF receiver  120  (such as location and velocity) and an unknown portion which is due to uncertainty in the satellite velocity and uncertainty in the local oscillator frequency. The output of the RF component is mixed in the time domain with an estimate of the satellite frequency. This mixing operation  121  is accomplished by multiplying with a complex exponential. 
     The signal is then correlated with a copy of the satellite&#39;s PRN code. This correlation can be performed in the time or frequency domains. One type of correlation is performed in the time domain using the three step correlation operation  124  shown in  FIG. 1 . If 1 msec of data is correlated, then the satellite search process can cover 1 kHz of the frequency uncertainty. In the device  100  shown in  FIG. 1 , 8 msec of the mixed data is stacked (or time-aliased) before the correlation operation. This stacking operation  122  gives more sensitivity than looking at 1 msec, but narrows the frequency search from 1 kHz to 125 Hz intervals, so more operations must be performed. 
     After correlation is performed, the output of the correlation operation  124  is examined to look for signal peaks. For strong signals, the peaks are obvious within a single correlation result. For weak signals, multiple correlation results are combined to increase the height of the peak. The traditional method of combining correlation results consists of squaring them and then averaging them together. This squaring operation doubles the effective bandwidth of the signal and thus reduces the frequency search interval from 125 to 62.5 Hz. This effectively doubles the required computation. 
     In one known method, for example, correlation is performed with fast Fourier transforms (FFTs). In effect this is a circular convolution operation. As shown in  FIG. 1 , this includes the three stages of: a forward FFT  131 ; a multiplication by a reference PRN (xPRN)  132 ; and an inverse FFT  133 . Correlation is typically computation intensive, involving for example, table lookups and calculation of sines and cosines. 
     For each satellite/frequency pair (a total of N×M) all of the operations shown are repeated. As a result, the amount of processing performed to acquire a signal can be quite high. Accordingly, it is desirable to provide a more efficient system for processing received signals. 
     SUMMARY 
     In accordance with the present invention, a method is provided for processing GPS signals requiring significantly less computation than previous methods. In one embodiment, the method includes performing operations traditionally performed in the time domain in the frequency domain. The method further includes reusing the results of operations, thus avoiding the traditional repetition of some operations. 
     In accordance with embodiments of the present invention, a method of processing a Global Positioning System (GPS) signal is provided, comprising: transforming a time domain GPS signal to a frequency domain GPS signal; storing a frequency domain pseudorandom noise (PRN) signal; correlating the frequency domain GPS signal with the PRN signal at a plurality of frequencies, said correlating comprising: shifting the frequency domain GPS signal by an amount corresponding to one of the plurality of frequencies; downsampling the shifted frequency domain GPS signal; and multiplying the shifted frequency domain GPS signal by the stored frequency domain PRN signal to produce a correlated signal. 
     In accordance with other embodiments of the present invention, a receiver for processing a GPS signal is provided, comprising: a memory for storing a frequency domain GPS signal and a frequency domain pseudorandom noise (PRN) signal; a processor configured to: transform a time domain GPS signal to the frequency domain GPS signal and store the frequency domain GPS signal in the memory; store the frequency domain PRN signal in the memory; correlate the frequency domain signal with the PRN signal at a plurality of frequencies, said correlating comprising: shifting the frequency domain GPS signal by an amount corresponding to one of the plurality of frequencies; downsampling the shifted frequency domain GPS signal; and multiplying the shifted frequency domain GPS signal by the stored frequency domain PRN signal to produce a correlated signal. 
     In accordance with other embodiments of the present invention, a method of processing a Global Positioning System (GPS) signal is provided, comprising: producing a frequency domain representation of the GPS signal, the frequency domain GPS signal comprising a plurality of frequency bins having a bin frequency spacing of f b ; storing a frequency domain representation of a pseudorandom noise (PRN) signal; and correlating the GPS signal with the PRN signal at a plurality of M mixing frequencies with a mixing frequency spacing of f M =K×f b , wherein K is a positive integer, said correlating comprising for each mixing frequency: shifting the frequency domain signal by K×S samples, wherein S is an integer ranging from 0 to (M−1) to obtain a frequency domain shifted signal shifted by S×f M ; downsampling the frequency domain shifted signal; and multiplying the frequency domain shifted signal with the stored frequency domain representation of the PRN signal to produce a correlated signal. 
     Other features and aspects of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the features in accordance with embodiments of the invention. The summary is not intended to limit the scope of the invention, which is defined solely by the claims attached hereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a prior art GPS device for receiving and processing GPS signals. 
         FIG. 2  is a block diagram of a satellite-based navigation system, in accordance with embodiments of the present invention. 
         FIG. 3  is a block diagram of a routine call which generates the FFT output for N msec of received signal, in accordance with embodiments of the present invention. 
         FIG. 4  is a block diagram further illustrating a correlation operation, in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, reference is made to the accompanying drawings which illustrate several embodiments of the present invention. It is understood that other embodiments may be utilized and mechanical, compositional, structural, electrical, and operational changes may be made without departing from the spirit and scope of the present disclosure. The following detailed description is not to be taken in a limiting sense, and the scope of the embodiments of the present invention is defined only by the claims of the issued patent. 
     Some portions of the detailed description which follows are presented in terms of procedures, steps, logic blocks, processing, and other symbolic representations of operations on data bits that can be performed on computer memory. A procedure, computer executed step, logic block, process, etc., are here conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those utilizing physical manipulations of physical quantities. These quantities can take the form of electrical, magnetic, or radio signals capable of being stored, transferred, combined, compared, and otherwise manipulated in a computer system. These signals may be referred to at times as bits, values, elements, symbols, characters, terms, numbers, or the like. Each step may be performed by hardware, software, firmware, or combinations thereof. 
     Embodiments of the invention include a method for processing GPS signals requiring significantly less computation than previous methods. In one embodiment, the method includes performing operations traditionally performed in the time domain in the frequency domain. The method further includes reusing the results of operations, thus avoiding the traditional repetition of some operations. 
       FIG. 2  is a block diagram of a satellite-based navigation system  200 , in accordance with embodiments of the present invention. The system  200  includes an RF antenna  210  that receives GPS signals transmitted by multiple satellites. 
     The received signal is downsampled in a downsample operation  221  to one MHz, and processed in 8 msec blocks. An 8K-point FFT  222  is performed once and reused across all satellite/frequency pairs. At one MHz, if an 8K FFT is performed, then the frequency bins of the FFT are spaced by 125 Hz. This allows a mixing operation at 125 Hz to be performed by shifting one sample in the frequency domain. In addition, it is possible to mix by any multiple K of 125 Hz just by shifting K samples in the frequency domain. In the frequency domain, the mixing operation is a circular shift. This essentially eliminates the entire mixing step  121  described above with respect to  FIG. 1 . 
     The output of the 8K-point FFT is passed through a notch filter  223  to remove narrow band noise (NBN). This removes the interfering noise from the 8K complex points of data in the frequency domain. This is the initial processing performed on 8 msec of received data, and is satellite neutral and frequency neutral. 
     The output of the notch filter  223  is then processed for each satellite/frequency pair. This is shown in  FIG. 2  by the duplicated rows of processing flows  230 , each of which receives the output of the notch filter  223 . 
     The next operations shown in each of the processing flows  230  are a circular shift  231  in the frequency domain and downsampling  232 . As will be explained, the prior stacking  122  and correlation  124  operations, which involved multiple computations for each satellite/frequency pair, are replaced by a different set of operations, some of which are reused, or shared, among all satellite/frequency pairs. A correlation operation  124 , as previously discussed, can include three parts: a forward FFT  131 , multiplication in the frequency domain by a reference PRN signal (xPRN)  132 , and an inverse FFT (IFFT)  133 . Each correlation operation  124  in  FIG. 1  has these three components. According to an embodiment of the invention, the traditional mixing (or multiplication) operation  121  before the correlation  124  is eliminated and replaced by frequency domain shifts  231 , as shown in  FIG. 2 . The frequency domain shifts  231  are achieved by performing circular shifts by K samples, circular shifts by K×2 samples, etc. The forward FFTs are then shared among all of the different channels, or satellite/frequency pairs. So instead of one 1024 (“K pt”) FFT  131  that is repeated for each satellite/frequency pair, there is a single 8K point FFT  222  and that is reused later by each satellite/frequency pair. The circular shift  231  by K samples, as shown, is effectively a modulation, or mixing operation. 
     The next operation shown is downsampling  232  in the frequency domain. Downsampling in the frequency domain is equivalent to aliasing in the time domain by the downsampling factor. The 8 msec signal is aliased down to 1 msec. This frequency domain downsampling operation  232  replaces the traditional stacking operation  122 . 
     The next step in each process flow is multiplying by the PRN  233  in the frequency domain. The PRN  233  for each satellite is calculated in advance and stored in read only memory (ROM) in a software implementation. Multiplication is performed sample-by-sample. Traditionally, multiplication in the frequency domain by an 8K representation of the PRN would be performed before downsampling (e.g., taking one out of every 8 samples). In the embodiment shown in  FIG. 2 , downsampling  232  is performed first. Thus, the multiplication is on 1K-pt of data and 1K-pt of the reference PRN is stored. 
     After the multiplication, a “code phase adjustment”  234 , is performed as a fractional shift in the time domain. This fractional shift causes the correlation peak to shift slightly and allows multiple correlation peaks to be combined to form a sharp peak. In the frequency domain a fractional time shift is accomplished by multiplication by a linear phase term (complex modulation). After the code phase adjustment  234 , the processing returns to the time domain, and the IFFT  235  is performed. The number of IFFT performed is equal to the product of the total number of satellites and the number of Doppler bins. 
     The system  200  is applicable for one satellite, so the same FFT data is stored for the one satellite. To search for multiple satellites, the FFT  222  and the PRN  233  are changed. 
     In accordance with the embodiment described in  FIG. 2 , the time domain modulation is accomplished by a “free” circular shift in the frequency domain. A buffer is offset by the appropriate amount and read out. The stacking operation is also “free” and is accomplished by a downsampling operation  232  in the frequency domain. One out of every eight samples in the buffer is used. In effect, the forward FFT  222  is amortized over many satellite/frequency combinations, then multiplication by the reference PRN  233 , and the inverse FFT  235  are performed. Approximately ½ of the traditional computations are eliminated. 
     The initial 8K point FFT  222  shown in  FIG. 2  can be shared among different satellites. This is illustrated in  FIG. 3  which shows a call to a single routine  300  (setSignal) which generates the FFT output for N msec of received signal. In this figure, the 8K-point FFT is calculated using 8 smaller 1K-point FFTs together with mixing and stacking operations. The buffers at the output of  FIG. 3  are reused for all satellite/frequency combinations. These steps are independent of a specific satellite/frequency pair. This has the same effect as the modulation  121  and stacking  122  operations in the prior art  FIG. 1 , but is at a fixed set of frequencies: e.g., 0 Hz, 125 Hz, 250 Hz, 375 Hz, 500 Hz, 625 Hz, 750 Hz, and 875 Hz. Narrowband noise components are then removed from each FFT result. 
       FIG. 4  is a block diagram further illustrating a correlation operation, in accordance with an embodiment of the present invention. The SARAM buffers on the left correspond to the SARAM buffers on the right side of  FIG. 3 . A coarse Doppler mixing is performed in which the samples are circularly shifted and code phase adjustment is performed, as previously described. For a signal represented by a discrete FFT, coarse Doppler mixing by a mixing frequency f M  can be performed by circular shifting of the values. For example, if a signal is represented by x1, x2, x3 . . . xN, which corresponds to a discrete FFT with frequency spacing f b =1000 Hz, then to mix the signal by f m =1000 Hz, the points are shifted to provide a new mixed signal of x2, x3 . . . xN, x1. 
     Code phase adjustment is performed by a multiplication by a complex exponential. The adjustment depends upon the center frequency of each bin and the number of milliseconds into the received signal. Then, the samples are multiplied by the PRN. Alternatively, the code phase adjustment can be performed after the multiplication by the PRN. Each row in the diagram corresponds to a single satellite/frequency pair, and the overall diagram represents the processing for one satellite. This processing is then repeated for each satellite. 
     Another embodiment of the invention is that correlations can be calculated at a 125 Hz interval even when squaring and combining correlation results to improve sensitivity. First, correlations are calculated at 125 Hz intervals as described above. Then, every other point is interpolated to obtain a finer sampling of 62.5 Hz. These interpolated results are then squared and averaged. Once the averaging is complete, another interpolation process occurs and this yields the final ambiguity function. This function is then searched for a peak. 
     Overall, embodiments of the present invention may reduce computation time by roughly a factor of four as compared to traditional frequency domain correlation methods. First, reusing the FFT of the signal for many satellite/frequency pairs saves about half the processing. If the signal processing is performed for only a single satellite, the conventional processing methods may be more efficient. However, when processing for multiple satellites, the reuse of the FFT can produce significant efficiencies. Second, because the correlations are computed at 125 Hz intervals rather than 62.5 Hz intervals, the processing is reduced by half again. Some additional processing is required to perform the interpolation, but this is small compared to the overall savings. 
     Various embodiments of the present invention take advantage of the efficiency of modulating frequency in the frequency domain. Modulation of frequency in the time domain typically requires multiplication by a complex number, thereby involving a substantial amount of processing time. On the other hand, performing a discrete FFT in the frequency domain simply requires a shift in the starting point by one bin. Thus, performing a frequency shift is relatively trivial in the frequency domain. This can be particularly advantageous when utilizing software-based GPS signal detection algorithms, as opposed to conventional time domain based hardware correlators. 
     Embodiments of the invention have been described with reference to particular examples, which are not intended to be limiting. The invention is applicable to many variations of signal processing systems not specifically described. 
     The program logic described indicates certain events occurring in a certain order. Those of ordinary skill in the art will recognize that the ordering of certain programming steps or program flow may be modified without affecting the overall operation performed by the preferred embodiment logic, and such modifications are in accordance with the various embodiments of the invention. Additionally, certain of the steps may be performed concurrently in a parallel process when possible, as well as performed sequentially as described above. 
     The figures provided are merely representational and are intended to illustrate various implementations of the invention that can be understood and appropriately carried out by those of ordinary skill in the art. 
     Therefore, it should be understood that the invention can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be understood that the invention can be practiced with modification and alteration and that the invention be limited only by the claims and the equivalents thereof.