Cross correlation mitigation for C/A code GPS receivers

Many new applications of the Global Positioning System (GPS) are required to process weak GPS signals in the presence of other strong GPS signals. Unfortunately, the 1023 chip Gold code sequences used for the GPS C/A spreading codes limit the dynamic range of a typical GPS receiver such that the power difference between the strongest and weakest GPS signal is limited to less than 21 dB. When this condition is not satisfied, the multiple access interference (MAI) experienced can cause difficulties, ranging from poor measurement quality through to weak signal acquisition and tracking failures. This research investigates cross correlation mitigation (CCM) using digital signal processing performed within the GPS receiver, with the following research contributions:a) Development of a novel CCM technique called Adaptive Orthogonalisation Using Constraints in which the de-spreading code used to track the GPS signal is dynamically adjusted to reject MAI from other strong GPS signals. Computer simulations as well as a full C implementation using a software correlator were performed and the software correlator tested using GPS IF samples captured from a GPS receiver connected to a hardware GPS simulator. The limitations of this technique were also analysed.b) Development of a CCM technique called Delayed Parallel Interference Cancellation (DPIC) in which slave channels are employed to estimate the MAI between pairs of strong and weak signals, with the slave channel outputs subtracted from the weak signal correlations following appropriate scaling. Thorough performance characterisation of the technique was performed using computer simulations. The algorithm was validated using both a C software correlator and a Verilog hardware correlator by processing MAI affected signals generated using a hardware GPS simulator. A comparison of DPIC with other related methods is also provided.c) Development of 'on-the-fly' data bit estimation for data bit estimation as required for subtractive type CCM techniques such as DPIC and Successive Interference Cancellation (SIC).d) Development of a comb-filtering statistical technique to indicate the presence of MAI and continuous wave interference (CWI). Algorithms enabling calculation of detection thresholds using the comb-filter noise statistic (CFNS) are also presented and validated using computer simulations