Detection of Time-Hopped DS-CDMA Signal for Pseuodolite-based Positioning System

J.W. Cheong, A.G. Dempster, C. Rizos

Abstract:	The requirement that signals must be continuously repeated in the time domain for the Fast Fourier Transform(FFT)-aided correlation to function results in considerable correlation loss when it is used on the Time Hopping / Direct Sequence – Code Division Multiple Access (TH/DS-CDMA) type signals. A proposed correlation technique utilising the Digital Passive Matched Filter (DPMF) based technique eliminates correlation losses due to TH/DS-CDMA’s non-timerepetitive property. However, direct implementation of DPMF is computationally expensive. A FFT-based overlap-add filtering technique is shown to overcome the shortcomings of the primitive FFT-based correlation technique. Also, an acquisition method for the time-hopping domain is presented, adapted from the concept of exhaustive search. The method is modified to take into account undetected ‘CDMA bursts’ in the code phase and frequency search process. As the exhaustive search requires signal acquisition to be performed continuously in time as new samples arrive, a large computational load is required. In the time-hopping domain search, a simplification to the DPMF-based acquisition is made in order to reduce the computational load by gradually reducing the Doppler frequency uncertainty range for each successful acquisition in the code phase and frequency search domains. Experimental results show that the peak-to-subpeak ratio was sustained at high values despite various delays being introduced. Successful acquisition of the signal collected via an over-the-air transmission demonstrates the viability of the algorithms presented. The paper comprises five sections. It begins with an introduction to the general concepts relevant to Pseudolite Positioning Systems, followed with an introduction to the TH/DS-CDMA signal. The third section describes the proposed acquisition model and compares it with an existing method. The last two sections describe the Time Hopping frame Starting Index (THSI) search methods and the experimental signal acquisition results based on real baseband signals collected from an operational LocataNet, followed by concluding remarks on the performance of the proposed signal acquisition methods.
Published in:	Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009) September 22 - 25, 2009 Savannah International Convention Center Savannah, GA
Pages:	881 - 891
Cite this article:	Cheong, J.W., Dempster, A.G., Rizos, C., "Detection of Time-Hopped DS-CDMA Signal for Pseuodolite-based Positioning System," Proceedings of the 22nd International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2009), Savannah, GA, September 2009, pp. 881-891.
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