Paraunitary generation/correlation of QAM complementary sequence pairs

• Authors: S. Z. Budišin;P. Spasojević
• Venue: Cryptography and Communications
• Year: 2014

A unique decomposition of arbitrary pairs of complementary sequences (including standard binary, polyphase and QAM sequences as well as non-standard sequences and kernels) based on paraunitary matrices is presented. This decomposition allows us to describe the internal structure of any sequence pair of length L using basic paraunitary matrices defined by an ordered set of L complex coefficients named the omega vector. When the omega vector is sparse, the canonic form is compact and leads to an efficient implementation of a generator/correlator. We show that sequences generated by the standard algorithm have the sparsest known omega vector (log2L non-zero elements out of L) and, thus, the most efficient generator/correlator. The equivalence of paraunitary matrices and Z transforms of complementary sequences allows us to apply the rich results from the theory of perfect reconstruction filter-banks to the field of sequence design. We introduce a new generator/correlator algorithm for sequences in standard and non-standard QAM constellations that is based on this equivalence. Both rectangular and hexagonal constellations are considered and the cardinality of the generated set of unique complementary sequences is either determined or estimated for a number of important cases. We show, in the case of the standard 16-QAM constellation, that the paraunitary algorithm generates the same number of sequences as the published algorithms based on generalized Boolean functions. In the case of 64-QAM, the proposed algorithm generates more sequences than known algorithms. We introduce an algorithm for generating 256-QAM sequences and derive a tight upper bound on the number of generated sequences.