ION GNSS 2012
Session E1: Receivers & Antennas 1

Title: Crosswise Reconstructing and Folding Based Rapid Acquisition Method for Time Division Data Modulation Long PN-code Signals
Author(s): H. Li, M. Lu, &, Z. Feng, Tsinghua University, China
Date/Time: Wednesday, September 19, 2012, 8:35 a.m.
Room: 205 (NCC)

In GNSS, to further improve anti-jamming and anti-spoofing capabilities, long PN-code signals rather than short ones are desired. Nevertheless, the navigation data modulated on every PN-code chip limit acquisition and tracking performances of the signals, since data reversal constrains coherent integration times of acquisition and tracking and the using of carrier tracking loop. Recently, a new technique called time division data modulation (TDDM) has been proposed and has been used in the modernized M-code signal. In contrast to conventional techniques where data modulate every PN-code chip, when the technique is used, data modulate odd chips but not even chips. The unmodulated even chips provide a dataless channel for PN-code acquisition and tracking. Consequently, longer coherent integration time is feasible to further improve acquisition and tracking performance, which enhances the survivals of GNSS signals in weak and jammed environments.
On the other hand, the time division data modulation technique challenges the conventional frequency-domain based long PN-code acquisition methods and makes them somewhat inapplicable. The conventional methods including the extended replica folding acquisition search technique (XFAST), the direct average method, the overlap average method, the dual-folding acquisition method, the double-block zero-padding method, are different in search strategies, but their common basis is that they do coherent integration on adjacent PN-code chips. The data modulated on even chips would be considered as a constant ´+1´, since there are no data on even chips. The data on odd chips are either ´+1´ or ´-1´. Thus, the data on odd chips and even chips are different with a probability 50% on average. Thus, when the data on odd chips are ´-1´, if adjacent odd and even chips are coherently integrated, an energy cancellation would happen and it would cause acquisition to fail. So the conventional frequency-domain based acquisition methods are inapplicable to TDDM long PN-code signals.

To this end, some pioneers have proposed a time-gate acquisition method for TDDM long PN-code signals. It conducts coherent integrations on odd chips and even chips respectively, by controlling a time gate to distinguish odd and even chips. The method is feasible for time-domain correlation technique, but is quite difficult for frequency-domain correlation technique. Compared with frequency-domain correlation technique, time-domain correlation technique requires much more computation requirements.

In the paper, a crosswise reconstructing and folding based acquisition method has been proposed for TDDM long PN-code signals. The method has two features. First, it crosswise reconstructs odd and even chips of local code replicas to break the intrinsic code phase relation of PN-codes. Consequently, it prevents the odd chip component and even chip component of received signal from aligning with local signal at the same time. As a result, it would avoid the possible energy cancellation of odd and even chips and makes frequency-domain correlation technique applicable to TDDM long PN-code signals. Second, the proposed method absorbs the advantages of previously proposed folding techniques, which have been widely used for conventional long PN-code signals such as P(Y) code signal. The method folds together several odd chips and several even chips of the crosswise reconstructed local codes, respectively, to directly reduce the code phases to test. Hence, it would accelerate the search process of PN-code acquisition. Therefore, compared with the previous methods, the proposed method not only makes frequency-domain correlation technique applicable to TDDM long PN-code signals, but also greatly decreases computation requirements and accelerates search process.

On the other hand, the gains are obtained by degrading the correlation properties of PN-code because of the crosswise reconstructing and folding. So compared with other methods, it has a little worse detection performance. In PN-code acquisition, acquisition performance, generally evaluated by mean acquisition time, depends on both detection performance and the capability of testing code phases in parallel. Since the proposed method saves computation requirements greatly by using frequency-domain correlation technique and directly reduces the code phases to test by folding together several odd chips and several even chips respectively, the proposed method has much stronger capability of testing code phases in parallel. The greatly enhanced capability would compensate the little degraded detection performance, and makes the proposed method outperforming other methods.

In the paper, the details, including how to crosswise reconstruct odd and even chips of local codes and how to fold the crosswise reconstructed local codes, will be introduced. The performance of the proposed method will be evaluated, and the factors of affecting the performance will be discussed as well.

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