Analysis of signal acquisition in GPS receiver software

Abstract

This paper presents a critical analysis of the flow signal processing carried out in GPS receiver software, which served as a basis for a critical comparison of different signal processing architectures within the GPS receiver. It is possible to achieve Increased flexibility and reduction of GPS device commercial costs, including those of mobile devices, by using radio technology software (SDR, Software Defined Radio). The SDR application can be realized when certain hardware components in a GPS receiver are replaced. Signal processing in the SDR is implemented using a programmable DSP (Digital Signal Processing) or FPGA (Field Programmable Gate Array) circuit, which allows a simple change of digital signal processing algorithms and a simple change of the receiver parameters. The starting point of the research is the signal generated on the satellite the structure of which is shown in the paper. Based on the GPS signal structure, a receiver is realized with a task to extract an appropriate signal from the spectrum and detect it. Based on collected navigation data, the receiver calculates the position of the end user. The signal coming from the satellite may be at the carrier frequencies of L1 and L2. Since the SPS is used in the civil service, all the tests shown in the work were performed on the L1 signal. The signal coming to the receiver is generated in the spread spectrum technology and is situated below the level of noise. Such signals often interfere with signals from the environment which presents a difficulty for a receiver to perform proper detection and signal processing. Therefore, signal processing technology is continually being improved, aiming at more accurate and faster signal processing. All tests were carried out on a signal acquired from the satellite using the SE4110 input circuit used for filtering, amplification and signal selection. The samples of the received signal were forwarded to a computer for data post processing, i. e. the whole receiver is software implemented in a MATLAB software package. One of the processes during the signal processing is the initial synchronization (acquisition), where a signal is detected and the carrier frequency is determined as well as the phase sequence code and the carrier Doppler frequency. The acquisition aim is to determine, in the shortest time possible, the parameters of the detected signals and forward them to the next block in synchronization. Depending on the speed and accuracy of the signal parameter determination, different methods of acquisition are applied in practice. The paper presents the methods of serial, parallel and cyclic convolution. For comparison purposes, the architectures of signal processing of particular methods for implementation in receiver software are shown. All measurements were performed on the same signal under the same conditions. On the basis of the tests performed, a detailed analysis of the collected data was carried out and the most acceptable acquisition method for implementation in software GPS receiver was proposed. Because of a relatively high level of noise at the receiver entrance and the received signal interference, the comparison of the results has been done on the basis of the analytical results and the mean time of signal synchronization. The measurement results are shown in tables for easy comparison. The results of measurements using the proposed method are presented as well. The technology of receiver software allows the user to access easily to the architecture of the receiver and therefore allows a simple change of parameters. The influence of the parameters on the process of signal acquisition is also shown in the paper. The graphic presentation shows how and to what extent some of the parameters affect the process of the receiver signal processing. All listed acquisition methods are used in practice. The proposed method is the most suitable for application in software receivers. Based on the analysis, a constructor can apply an adequate acquisition method, depending on the requirements of the final user.