Nearly Orthogonal Codes in GNSS Using Unequal Code Lengths

Qin Zhengdi and Seppo Turunen

Abstract:	Signal acquisition in a GNSS receiver is typically based on channel estimation done by correlating the received CDMA signal with a code replica. In order to achieve good sensitivity, the process is usually repeated several times and the results combined. A large number of repeated correlations is needed, especially for weak signal acquisition. The dynamic range for channel separation is, however, limited since co-channel interference cannot be attenuated by means of repeated correlations in a traditional CDMA system that uses short codes of equal length. This results from the fact that the cross-correlation pattern is stationary with respect to the correlation cycles. A coding method is given that provides an essential reduction of cross-correlation by allocating slightly different code lengths to different channels. The cross- correlation can drop to zero if the code lengths and the integration time in the receiver are carefully chosen and if data modulation is absent, as is the case with the pilot signals now being planned for both GPS and Galileo. Under these assumptions it looks as if the coded channels were orthogonal to each other. Theoretical analysis and simulations show that a group of short codes with unequal lengths can provide better cross-correlation distances than a group of longer codes with equal code length if the distances are calculated over several consecutive cycles of the short codes.
Published in:	Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004) June 7 - 9, 2004 Dayton Marriott Hotel Dayton, OH
Pages:	666 - 670
Cite this article:	Zhengdi, Qin, Turunen, Seppo, "Nearly Orthogonal Codes in GNSS Using Unequal Code Lengths," Proceedings of the 60th Annual Meeting of The Institute of Navigation (2004), Dayton, OH, June 2004, pp. 666-670.
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