Orthogonal multi-spreading factor forward link for coherent DS-CDMA mobile radio

We present a method of generating orthogonal spreading codes with different spreading factors (SFs), which are called orthogonal multi-SF codes. The orthogonal multi-SF codes are applied to the forward link so that all users transmitting data at different rates are orthogonalized and a single RAKE combiner can be used at the mobile receiver. The generated orthogonal multi-SF codes have a tree structure. A group spreading modulator which simplifies the base station transmitter structure is also described. The forward link transmission performances under multiuser and frequency selective Rayleigh fading environments are evaluated by computer simulation to show that performance identical to that achieved by using multiple orthogonal codes in parallel can be achieved. Unlike the orthogonal multicode forward link, only a single RAKE combiner is required at a mobile receiver and this significantly simplifies the mobile receiver structure.