Block-Accumulating Coherent Integration over Extended Interval (BACIX) For Weak GPS Signal Acquisition

摘要

Extending the coherent integration interval is a viable technique for acquisition and ultimate tracking of a weak (e.g., in-door) GPS signal. However, even though a signal is within a time-frequency searchrncell, its long coherent integration still faces several technical challenges, notably, (1) possible data bit sign reversal every 20 ms and (2) tolerable frequency error inversely proportional to the integration interval. Brute force search over all possible combinations/values is prohibitive computationally. Aided GPS relies on external infrastructure for data bit and frequency error information, costly, too.rnIn this paper, we present an unaided acquisition technique for weak GPS signals based on three ideas. The first idea of successive sign reversal of 1 ms correlations per block (i.e., 20 ms) enables data bit transition detection and data bit sign correction within a data bit interval. This is in sharp contrast to conventional methods that only process half blocks at a time (undesired SNR loss) or are stretched over two blocks (excessive latency). In addition, it allows for efficient use of Fast Fourier Transform (FFT), which only needs to be calculated once and manipulated simply for all tentative bit alignments. As the second idea, the FFT is implemented as a bank of bandpass filters to integrate blocks of correlations in a coherent manner, as opposed to conventional phase rotation that can only handle one bandpass filter at a time and requires excessive frequency search. Finally, the third idea optimizes the overall computation by pruning unlikely branches of the search paths in partial sums. Such a block accumulating coherent integration boosts the signal power while reducing noise despite unknown data bit transitions and other variations that may occur during the extended coherent integration intervals.rnThe block accumulating coherent integration over extended interval (BACIX) is first formulated in the paper. Computer simulation results are analyzed to illustrate the unaided weak signal acquisition process.