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Orthogonal Complex Quadrature Phase Shift Keying (OCQPSK) Spreading for 3G W-CDMA Systems
Abstract
A variety of digital modulation techniques are currently being used in wireless communication systems. In 3G (third generation) spread-spectrum systems, such as W-CDMA (3GPP) and cdma2000 (3GPP2), the handset can transmit multiple channels at different amplitude levels. Modulation schemes such as OQPSK or GMSK do not prevent zero-crossings for multiple channels and are no longer suitable. There is a need for a modulation format or a spreading technique that can accommodate multiple channels at different power levels while producing signals with low peak-to-average power ratios. OCQPSK (Orthogonal Complex Quadrature Phase Shift Keying) has been proposed as the spreading technique for W-CDMA and cdma2000. OCQPSK is a complex spreading scheme that is very different from the modulation formats commonly used until now. The objective of this Chapter is to provide an overview of OCQPSK and explain how to start making modulation quality measurements on the reverse link (uplink) of 3G spread-spectrum systems. This chapter starts with the basic structure of the reverse link (uplink) for W-CDMA and cdma2000 with no scrambling, and explains the transition through complex scrambling to OCQPSK. The block diagrams shown are generic block diagrams for OCQPSK that are not particular to either W-CDMA or cdma2000. The chapter then describes: (1) why complex scrambling is used and how it works, and (2) why OCQPSK is used and how it works. Finally, this chapter provides how to measure modulation quality on the reverse link of 3G systems and a complete downlink physical layer model showing various results of BER and BLER calculation and also various time scopes and power spectrums.
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