Code Division Multiple Access (CDMA) is essentially a spread spectrum system which employs a set of "orthogonal" Walsh codes to distinguish between users. As a result, this system provides better immunity to multi-user interference than that provided by other wireless systems. CDMA employs both Quaternary Phase Shift Keying (QPSK) and Offset QPSK (OQPSK) modulation schemes, one for the down-link and the other for the up-link, respectively. The channel spacing in CDMA systems is 1.25 MHz. Next generation CDMA systems, known as Wide-band CDMA (W-CDMA) will employ wider bandwidths in order to support high speed data transfer.