Optimal QPSK modulated quasi-orthogonal functions for IS-2000

The forward link (FL) of a 3G CDMA system may be Walsh code limited in certain scenarios. Thus, other sets of orthogonal sequences, quasi-orthogonal functions (QOFs), which have low correlation with the original Walsh sequences are needed. A method of generating QOFs by multiplying each Walsh code by a vector of BPSK symbols, selected based on certain correlation criteria, was proposed by K. Yang et al. (see IEEE Trans. Info. Theory, 2000). In this paper, a method is provided for constructing vectors of QPSK symbols such that the QOFs have optimal mini-max correlation with Walsh code sets of variable length. This approach of using optimal QPSK-modulated QOFs has been adopted in IS-2000 standards.