Abstract:
An apparatus for detecting lock status of a spread spectrum signal, having a first accumulator, a first calculation unit, a second calculation unit, a second accumulator, a multiplier and a comparator. The first accumulator accumulates an in-phase integration result and a quadrature integration result over a time period. The first calculation unit determines a first evaluation value based on the accumulated in-phase integration result and the accumulated quadrature integration result. The second calculation unit processes the in-phase integration result and the quadrature integration result. The second accumulator accumulates the output of the second calculation unit over the time period. The multiplier determines a second evaluation value by multiplying the accumulated result from the second accumulator with a predetermined value. The comparator compares the first and second evaluation results wherein the comparison result is an indicator of the lock status.

Description:
FIELD OF THE INVENTION 
     The invention relates to spread spectrum signal processing, and more particularly, to detecting a lock status of a GPS signal. 
     BACKGROUND OF THE INVENTION 
     Generally, a GPS receiver must first acquire GPS signals from a plurality of satellites and then track these signals. During the acquisition stage, the carrier frequency and initial phase of the pseudorandom noise code (PRN code) of a received signal are found. These two parameters are then used for tracking the signal. 
     Due to the motion of the satellites and the receiver, Doppler effect may occur and the carrier frequency and PRN code may vary over time. To overcome Doppler effect and maintain the availability of GPS signals, a tracking process needs to be performed based on the initial carrier frequency and initial PRN code which are acquired during the acquisition stage. When the variations of carrier frequency and PRN code are successfully tracked by the receiver, the GPS signal is referred to as “locked” by the receiver. When the receiver fails to track the variation, a GPS signal from certain satellite is referred to as “lost”. When a signal is lost, it can not be used by the receiver for further processing such as calculating the position of the receiver. The receiver may need to perform acquisition again to ensure there is enough number of signals acquired and locked for further processing. 
     Therefore, there is a need to detect if a GPS signal is locked or lost. When the receiver finds a signal lost, it may need to acquire the signal again or may need to acquire another signal from another satellite. The time used to detect the lock status of the signal is a critical parameter in positioning technology. The shorter the time is needed, the better the receiver performs. 
     Conventionally, in a GPS receiver, a bit synchronization method is employed to determine if the signal is locked or lost. More specifically, a bit synchronization module is needed to identify the bit boundaries of navigation data stream after the carrier signal and PRN code have been stripped off from the received GPS signal. The navigation data stream is formed by a sequence of navigation data bits. The lasting time of each data bit is 20 ms. The end of a data bit, which is also the beginning of another data bit, is referred to as a boundary of a navigation data bit (bit boundary). In the bit synchronization method, the lock status of the signal is detected at the same time when the bit boundaries of navigation data stream are determined. If the bit boundaries can not be determined after repeating the search process for a predetermined times, the signal is regarded as lost. The basic idea of this method is to check whether the data transitions always happen in the same position. 
     However, there are some drawbacks to this method. First, it is difficult to determine the bit boundaries and the lock status if there are no data transitions within a long bit sequence. In other words, this method is not efficient when there are long sequences of “0” or “1” in the navigation data stream. Second, it is time-consuming. It needs 800 ms to search 40 bits, and totally 4 seconds to confirm the result if the search process needs to be repeated 5 times. Third, the bit synchronization process needs to be performed from time to time in order to detect lock status even after the signal is acquired. 
     Therefore, it is to a spread spectrum receiver that is able to detect the lock status of a received signal quickly and efficiently, even if there are long sequences of “0” or “1” in the navigation data stream that the present invention is primarily directed. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for detecting the lock status of a GPS signal at the receiver. Advantageously, the status can be detected efficiently even if there are long sequences of “0” or “1” in the navigation data stream. 
     In one embodiment of the invention, there is provided a method for detecting the lock status of a spread spectrum signal. The method includes producing a first and second data stream by multiplying the spread spectrum signal with an in-phase carrier signal and a quadrature carrier signal, calculating a first integration result based on the first data stream and a predetermined PRN code, calculating a second integration result based on the second data stream and the PRN code, determining a first and second evaluation values in a lock status detector based on the first and second integration results, determining a ratio by dividing the first evaluation value by the second evaluation value, and determining a lock status result in the lock status detector by comparing the ratio with a predetermined value. The method may further include determining the lock status based on multiple lock status results over a time period with a state machine. 
     In another embodiment of the invention there is provided an apparatus for detecting lock status of a spread spectrum signal. The apparatus includes a first accumulator, a first calculation unit, a second calculation unit, a second accumulator, a multiplier, a comparator, and a state machine. The first accumulator accumulates an in-phase integration result and a quadrature integration result over a time period respectively. The first calculation unit is coupled to the first accumulator and is capable of determining a first evaluation value based on the accumulated in-phase integration result and the accumulated quadrature integration result. The second calculation unit processes the in-phase integration result and the quadrature integration result. The second accumulator is coupled to the second calculation unit and accumulates the output of the second calculation unit over the time period. At the multiplier, the accumulated result from the second accumulator is multiplied with a predetermined value to determine a second evaluation value. The comparator is coupled to the first calculation unit and the multiplier for comparing the first and second evaluation results. The state machine monitors the output of the comparator and makes a determination indicating whether the signal is locked or lost. 
     In yet another embodiment of the invention there is also provided a system for processing a spread spectrum signal. The system includes an acquisition module and a tracking module. The acquisition module provides the tracking module with an initial carrier frequency and a PRN code. The tracking module tracks the spread spectrum signal. The tracking module includes a first integration unit which is capable of producing an in-phase integration result, a second integration unit which is capable of producing a quadrature integration result, a bit synchronization module for detecting boundaries of navigation data bits, a lock status detector coupled to the first and second integration unit, for detecting lock status of the spread spectrum signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages of embodiments of the invention will become apparent as the following Detailed Description proceeds, and upon reference to the Drawings, where like numerals depict like elements, and in which: 
         FIG. 1  illustrates an exemplary architecture of a GPS tracking module with a lock status detector; 
         FIG. 2  illustrates an exemplary architecture of the lock status detector; and 
         FIG. 3  illustrates a flow chart of a preferred embodiment of a state machine used to improve the performance of the lock status detector. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an exemplary architecture of a tracking module in a GPS receiver with a lock status detector for processing an intermediate frequency (IF) signal. The received GPS signal is first converted to a signal with a desired output frequency and then digitized at a predetermined sampling rate. The converted and digitized signal is known as an IF signal. 
     A carrier generator  102  generates two orthogonal carrier signals including an in-phase carrier signal  104  and a quadrature carrier signal  106 . The IF signal is multiplied with these two orthogonal carrier signals at multiplier  108 - 1  and multiplier  108 - 2  to generate a first data stream  110  and a second data stream  111 . A code generator  112  generates three PRN codes: an early code  114 , a late code  116 , and a prompt code  118 . The early code and the late code are both derived from the prompt code time-shifted by approximately one-half-chip more or less. The early code, late code, and prompt code are integrated with the first data stream and the second data stream respectively in a set of integrators, which are numbered consecutively form integrator  119 - 1  to integrator  119 - 6 , to generate six integration results. Integration herein refers to an operation of multiplying one of the data stream with one of the PRN code point by point and sum up the products over a time period. In a preferred embodiment, the time period of integration is 1 ms, which is the period of a PRN code. The in-phase integration result based on the prompt code and the first data stream is referred to as a first integration result Ii  120 . The quadrature integration result based on the prompt code and the second data stream is referred to as a second integration result Qi  122 . Ii and Qi are applied to a carrier tracking controller  124  which controls the carrier generator to generate the two orthogonal carrier signals. Ii and Qi are also applied to a lock status detector  126  to detect the lock status of the signal. The rest four integration results are applied to a code tracking controller  128  which controls the code generator to generate the early, late and prompt codes. 
       FIG. 2  illustrates architecture of the lock status detector  126  shown in  FIG. 1 . The lock status detector includes a first accumulator  202 , a first calculation unit  204 , a second calculation unit  206 , a second accumulator  208 , low-pass filter (LPF)  210 - 1 , LPF  210 - 2 , a multiplier  214 , a comparator  216  and a state machine  218 . Ii and Qi are divided into two paths: an S channel including the first accumulator  202 , the first calculation unit  204  and LPF  210 - 1 , and an N channel including the second calculation unit  206 , the second accumulator  208  and LPF  210 - 2 . 
     In the S channel, Ii and Qi are applied to the first accumulator  202  where Ii and Qi are accumulated over a time period to produce a first accumulated result Is  211  and a second accumulated result Qs  212 , respectively. In a preferred embodiment, each Is is generated by summing up all the Ii which are generated from a navigation data bit and each Qs is generated by summing up all the Qi which are generated from the same navigation data bit. If each Ii and each Qi are the integration results of 1 ms, then, Is and Qs are the accumulated results of 20 Ii and 20 Qi because the period of a navigation data bit is 20 ms. A bit synchronization module (not shown in this figure) is employed to determine the boundaries of the navigation data bits to ensure that Is and Qs can be generated from a complete navigation data bit. According to the present invention, the lock status detector, rather than the bit synchronization module, detects whether a signal is locked or lost. Advantageously, bit synchronization only needs to be performed once after acquisition of the signal compared with the conventional way using bit synchronization module to detect the lock status of the GPS signal from time to time. After a pair of Is and Qs is generated by the first accumulator  202 , a first calculation unit  204  determines a first evaluation value SL based on Is and Qs. 
     In the N channel, Ii and Qi are processed in a second calculation unit  206 . The processed results are then accumulated in a second accumulator  208  by summing up multiple processed results in a predetermined time period. The predetermined time period can be multiple of 1 ms. In a preferred embodiment, the predetermined time period is 20 ms which is consistent with the time used to produce Is and Qs in the S channel. The output of the second accumulator  208  is a second evaluation value NL. 
     There are various available embodiments of processing Is and Qs in the first calculation unit  204  and processing Ii and Qi in the second calculation unit  206 . Three preferred embodiments are provided herein. In a first embodiment of the invention, in the S channel, the first calculation unit  204  calculates the sum of squares of Is and Qs. In the N channel the second calculation unit  206  calculates the sum of squares of Ii and Qi. The first embodiment can be expressed as the following equation, where 
               ∑     i   =   1     M     ⁢     I   i           
is referred to as Is and
 
               ∑     i   =   1     M     ⁢     Q   i           
is referred to as Qs.
 
     
       
         
           
             
               
                 
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     In a second embodiment, in the S channel, the first calculation unit  204  calculates the sum of squares of Is and Qs and then calculates the square root of the sum of squares. In the N channel, the second calculation unit  206  calculates the sum of squares of Ii and Qi and then calculates the square root of the sum of squares. The second embodiment can be expressed as the following equation. 
     
       
         
           
             
               
                 
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     In a third embodiment, in the S channel, the first calculation unit  204  calculates the sum of absolute values of Is and Qs. In the N channel, the second calculation unit  206  calculates the sum of absolute values of Ii and Qi. The third method is expressed as the following equation. 
     
       
         
           
             
               
                 
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     The first evaluation value SL and second evaluation value NL are passed through LPF  210 - 1  and LPF  210 - 2  respectively to obtain smoother filtered results. The LPF can be a first order infinite impulse response filter (IIR filter). The filtered result of the first evaluation value SL is referred to as A and the filtered result of the second evaluation value NL is referred to as C. The ratio of A to C is compared with a threshold value TH to generate a lock status result. For easy implementation, in a preferred embodiment, C is first multiplied with TH at multiplier  214  to obtain a product which is referred to as B, and then A is compared with B at comparator  216  to generate a comparison result, which is an indicator of the lock status. If A is greater than B, it is suggested that the signal may be locked. If B is greater than A, it is suggested that the signal may be lost. To determine the lock status more accurately, the state machine  218  is provided to make a determination based on multiple pairs of A and B. 
       FIG. 3  illustrates a flow chart of a preferred embodiment of the state machine  218  used to make the determination if the signal is locked or lost based on multiple pairs of A and B. There are some parameters of the state machine including LOCKCNT, LOSTCNT, LP, LNA, and LO. The outputs of the state machine include a LOCKOUT signal and a LOSTOUT signal. LOCKOUT and LOSTOUT always have values of either “TRUE” or “FALSE”. When the state machine begins to operate, both LOCKOUT and LOSTOUT are initially set to “FALSE”; both LOCKCNT and LOSTCNT are initially set to 0. Each one of LP, LNA and LO is set to a predetermined integer respectively. LO is always greater than LNA. LP, LNA and LO are determined by system requirement such as detection probability, false alarm probability and the time needed to generate a lock status result. 
     The state machine receives a pair of A and B and makes a comparison, step  302 . If A is greater than B or if A is equal with B, LOSTOUT is set to FALSE and LOSTCNT is set to 0, step  304 . Then, the state machine checks the value of LOCKCNT, step  306 . If LOCKCNT is equal with LP, LOCKOUT is set to TRUE, step  308 ; else LOCKCNT is increased by 1, step  310 . 
     If A is smaller than B, then LOCKCNT is set to 0, step  312 . Then the state machine checks the value of LOSTCNT, step  314 . If LOSTCNT is equal with LO, then LOSTOUT is set to TRUE, step  316 , else LOSTCNT is increased by 1, step  318 . Then the value of LOSTCNT is compared with LNA, step  320 . If LOSTCNT is greater than LNA or LOSTCNT is equal with LNA, then LOCKOUT is set to FALSE, step  322 . 
     The lock status is determined by the value of LOCKOUT and LOSTOUT. If LOSTOUT is equal with FALSE and LOCKOUT is equal with TRUE, a status of “locked” is detected and the receiver will further process the information obtained from the signal. 
     If LOSTOUT is equal with TRUE and LOCKOUT is equal with FALSE, a status of “lost” is detected. The receiver may stop further processing the signal and may need to perform the acquisition process again. 
     If LOSTOUT is equal with FALSE and LOCKOUT is equal with FALSE, a status of “pre-lost” is detected. The status of “pre-lost” means the signal is not locked but may become locked through tracking process. Under this condition, the signal is held and not used for further processing, but acquisition is not performed. The state machine continues running until LOSTCNT is equal with LO to determine a status of “lost”, or until LOCKCNT is equal with LP to determine a status of “locked”. 
     In one embodiment of the invention, the state machine can be updated each time when a pair of Is and Qs is produced. To reduce the workload, in another embodiment of the invention, the state machine is updated each time when a predetermined number of Is and Qs have been produced. 
     By using the state machine, detection probability can be increased and false alarm probability can be decreased because the final result is based on the signal status which is obtained from a sequence of navigation data bits rather than from only one data bit. Occasional incidental or error has little effect on the final result. 
     It will be appreciated by those skilled in the art that apart from the state machine described above, there are different ways to design and implement the state machine, either by hardware or software. 
     The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described (or portions thereof), and it is recognized that various modifications are possible within the scope of the claims. Other modifications, variations, and alternatives are also possible. Accordingly, the claims are intended to cover all such equivalents.