Efficient LDPC-Coded CCSK Links for Robust High Data Rates GNSS

Global navigation satellite system links may require increased data rates to accommodate future features and needs (e.g., precise positioning, authentication, reduction of time-to-first-fix data). A particular form of $M$-ary orthogonal modulation designed for direct-sequence spread-spectrum (DSSS) systems, the cyclic code-shift keying (CCSK) modulation, has been proposed for this purpose. This modulation inherently allows noncoherent processing at receiver side and has the potential to improve the energy efficiency of the data link with respect to classical DSSS/BPSK signals. In this article, $q$-ary ($q\in \lbrace 2,M\rbrace$) low-density parity-check (LDPC)-based channel coding for $M$-ary CCSK is analyzed, both in terms of robustness and computational complexity. $(q=M)$-ary LDPC-coded CCSK is compared to a bit-interleaved binary LDPC-coded CCSK strategy. Though both solutions provide very reliable links for practical decoding algorithms, it is shown that adequately designed bit-interleaved binary LDPC-coded CCSK signals can offer the additional flexibility inherent to bit-interleaved coded modulation (BICM) while remaining competitive from the point of view of both error rate performance and computational complexity. The latter can be adjusted through the use of incomplete iterative demapping schedules. The optimization of this performance/complexity tradeoff is discussed.