Rank-2-optimal Binary Spreading Codes

Over the real/complex field, the spreading code that maximizes the output signal-to-interference-plus-noise ratio (SINR) of the linear maximum-SINR receiver is the minimum-eigenvalue eigenvector of the interference autocovariance matrix. In the context of binary spreading codes, the maximization problem is NP-hard with complexity exponential in the code length. A new method for the optimization of binary spreading codes under a rank-2 approximation of the inverse interference autocovariance matrix is presented where the rank-2-optimal binary code is obtained in lower than quadratic complexity. Significant SINR performance improvement is demonstrated over the common binary hard-limited eigenvector design which is seen to be equivalent to the rank-1-optimal solution.