The Spreading and Overlay Codes for the L1C Signal

Joseph J. Rushanan

Abstract:	This paper describes the rationale and construction of the family of L1C spreading codes and the associated family of overlay codes. The L1C spreading codes comprise 210 pilot/data pairs of length-10230 sequences. The codes are perfectly balanced and exhibit good auto- and crosscorrelation in both the even and odd cases. The length 10230 precluded the immediate adaptation of wellknown spreading code families. Instead, the relatively new prime-length Weil sequence construction was adapted. Specifically, selected length-10223 Weil-codes were padded with a fixed 7-bit pad to yield the L1C spreading codes. The selection process involved a search over all Weil codes and insertion points. The search criteria were derived from threshold bounds on the sidelobes for both auto- and cross-correlation and for both the even and odd cases. The overall search yielded a set of 739 codes from which the final set of 420 codes was chosen. Because L1C is currently considering two separate modulation (BOC(1,1) and MBOC), two separate families of codes were constructed that are optimized for their respective modulations. The 210 L1C overlay codes are length 1800, which corresponds to a frame length of 1800 symbols. The search to construct the overlay codes used two types of criteria. First, full period even auto- and cross-correlation sidelobe bounds were specified. Second, criteria were given for the correlation sidelobes when a small subsequence of the code is correlated against the full code. For this case, lengths of 100 (one second) and 200 (two seconds) were used. The overlay codes are based on truncated linear feedback shift register sequences of length 2047, including both m-sequences and Gold sequences.
Published in:	Proceedings of the 2007 National Technical Meeting of The Institute of Navigation January 22 - 24, 2007 The Catamaran Resort Hotel San Diego, CA
Pages:	539 - 547
Cite this article:	Updated citation: Published in NAVIGATION: Journal of the Institute of Navigation
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In