Combined binary pulse position modulation/biorthogonal modulation for direct-sequence code division multiple access

A novel modulation format is proposed for cellular direct-sequence CDMA systems where a user-specific spreading sequence is binary pulse position and biorthogonally modulated to form a set of biorthogonal spreading sequences. The modulation scheme trades the signal space used for spreading sequences with that for modulation while a global space is fixed. The interference is mainly determined by the cross correlation properties among sequences, but also affected by modulation. The effect is taken into account to evaluate the multi-user performance of the combined modulation. Compared to M -ary orthogonal modulation, the performance is shown to be almost the same while resulting in a simpler receiver structure.