On the closeness of the space spanned by the lattice structures for a class of linear phase perfect reconstruction filter banks

The incompleteness of the existing lattice structures has been well established for MM-channel FIR linear phase perfect reconstruction filter banks (LPPRFBs) with filter length L>2ML>2M in the literature, and even the nonexistence of complete order-one lattice has been reported recently. Thus, a question arises naturally as to how large the space spanned by the existing lattice structure is, and about its closeness over some polynomial transformations. The study for such issue can reveal what sense of optimality the lattice based design for LPPRFBs possesses. Inspired from this perspective, this paper firstly studies the closeness of the space spanned by the existing lattice structures under the polynomial transformations for arbitrary equal-length LPPRFBs. We have shown that this space is closed under the popular polynomial transforms widely used in FB design, which establishes the suboptimality of the lattice based design methods for LPPRFBs. Furthermore, the explicit relationship between the lattice parameters before and after transformations has been shown for describing the closeness of the space spanned by those lattice structures.