Techniques and apparatus for identifying the target mobile stations for data transmission in a wireless communication network. Techniques and apparatus can include generating a first N-symbol bi-orthogonal sequence from Walsh functions based on a first portion of a MACIndex and generating a complex preamble signal based on a second portion of the MACIndex. Generating the complex preamble signal can include applying the first N-symbol bi-orthogonal sequence between an In-phase branch and a Quadrature branch channel of a complex signal, which can include switching between the In-phase and Quadrature branch channels based on the second portion of the MACIndex to communicate information.