Blind startup of MMSE receivers for CDMA systems

This paper presents a novel technique for blindly starting up a minimum-mean-squared-error (MMSE) receiver for a user newly entering a code-division multiple-access (CDMA) system. It is assumed that only one user at a time is admitted into the system and that data before and after the onset of the new transmission are available. The technique is based on the generalized eigendecomposition of the two corresponding autocorrelation matrices (before and after the new user is admitted). In general, the two autocorrelation matrices differ by a low-rank matrix contributed by the autocorrelation of the new user signal. This fact yields the useful property that the generalized eigenvectors corresponding to the minimum eigenvalue span the noise subspace of the low-rank signal term. Exploiting this subspace relation, the signature of the new user can be identified. If, in particular, the difference between the two autocorrelation matrices is exactly a rank-one matrix, then there is only one maximum eigenvalue, and the corresponding eigenvector coincides with the MMSE receiver corresponding to the rank-one signal term. These properties are also applicable to long-code CDMA systems to provide an optimal multipath combining method.