Research On Weak GPS Signal Acquisition And Bit Synchronization Based On Differential Coherent

Considering the wide application of global positioning system(GPS) in various indoor environments, we studied some contents related to the signal receiving such as weak GPS signal acquisition, navigation data bit synchronization, the decision threshold setting and the efficiency of signal process algorithm.First, aiming at the theme of weak GPS signal acquisition, we considered the problem of the decision threshold setting according to GPS C/A code signal parallel acquisition scheme. Based on the weak GPS C/A code signal parallel acquisition scheme that adopted averaging correlation technique and differential coherent accumulation method, we proposed a decision threshold setting method –– the ratio method. In the ratio method, the decision variable is formed by the ratio between the maximum correlation energy value and the second maximum correlation energy value. Compared with the traditional threshold method, the probability of false alarm of the ratio method can be controlled nicely without calculation of noise power under weak signal condition. Then, for weak GPS C/A code signal parallel acquisition, we derived the calculation formula of detection probability and false alarm probability of the ratio method. According to the false alarm probability required by signal acquisition, the corresponding threshold values in accord with different received signal accumulation time were obtained through the Matlab numerical simulation. The simulation results showed that our threshold setting method was suitable for the parallel acquisition of weak GPS C/A code signal.Second, with the averaging correlation technique, differential coherent accumulation method and adaptive threshold setting technique, we proposed a weak GPS C/A code signal parallel acquisition scheme adjusted the decision threshold adaptively. We analyzed the statistical characteristic of the decision variable, deduced the detection probability and the false alarm probability of the acquisition scheme. Through the Matlab numerical simulation, we compared the performances of our scheme with those of two traditional acquisition schemes –– coherent non-cherent accumulation scheme and non-coherent accumulation scheme. Simulation results showed that the performances of our scheme were superior to those of two traditional acquisition schemes. We took into account both the processing gain and the complexity of the algorithm in our scheme according to the following measures: i. the combination of high sampling frequency and averaging correlation technique made the algorithm have limited increase in complexity and improved the C/A code primary acquisition accuracy largely, moreover, this measure also reduced the correlation loss between the received code and local C/A code replica and improved the processing gain; ii. the use of differential coherent accumulation avoided the square loss induced by the non-coherent accumulation, improved the processing gain and adapted to the Doppler frequency shift; iii. the adaptive decision threshold setting method was adopted, compared with the scheme using a fixed differential coherent accumulation time, the mean acquisition time was reduced.Third, we considered the problem of the navigation data bit synchronization after successfully acquiring GPS satellite signal. In order to solve the problem that the bit synchronization performance of histogram method and bit energy method used in existing commercial receiver decreased greatly or malfunctioned due to the greater remaining Doppler frequency offset after signal acquisition, based on the differential coherent process between the correlation output in one navigation data bit and the corresponding correlation output in another navigation data bit and the coherent accumulation of the differential coherent values within a navigation data bit, we proposed a navigation data bit synchronization method for the large Doppler frequency offset, high dynamic and weak GPS signal –– differential coherent accumulation algorithm. The statistical characteristics of the decision variable were analyzed in detail and the calculation formula of the edge detection rate was derived. On this basis, we modified the differential coherent accumulation algorithm and proposed a generalized differential coherent accumulation algorithm. The generalized differential coherent accumulation algorithm extended the delay time between two coherent integration outputs for differential coherent processing to arbitrary C/A code periods rather than any navigation data bit period. The generalized differential coherent accumulation algorithm improved the processing gain further.Finally, for the purpose of the real-time signal processing of GPS receiver, we proposed a real-time navigation data bit synchronization method for the weak GPS signal with large Doppler frequency offset –– efficient differential coherent accumulation algorithm. In this algorithm, only two adjacent correlation outputs within a C/A code period were processed with differential coherent manner, and the differential coherent values with the space of several navigation data bit length were accumulated coherently. Statistical properties of the decision variable of this algorithm were analyzed as before and the calculation formula of the edge detection rate was derived. Due to the dynamic range of GPS receiver under weak signal condition is usually low, while the real-time processing is very important in some occasions. In the efficient differential coherent accumulation algorithm, we had just considered these factors and used the following measures: i. the differential coherent processing adapted the algorithm to the large Doppler frequency offset; ii. the differential coherent outputs were only accumulated coherently to reduce the loss induced by non-coherent accumulation, so this algorithm was suitable to navigation data bit synchronization for weak GPS signal; iii. only two adjacent correlation outputs within a C/A code period were processed with differential coherent manner brings the lowest complexity and be beneficial to real-time processing. More and more GPS receivers adopt differential coherent accumulation method for signal acquisition, if this bit synchronization method is applied to such receivers, then the differential coherent outputs can be reused directly and reduce the computation of the algorithm greatly.