We have already discussed CDMA techniques in detail in Chapter 5. Before reading this chapter, readers should consult Chapter 5 for a better understanding. The present chapter discusses the WCDMA air interface (also referred as UMTS terrestrial radio access (UTRA), which has been developed by the third-generation partnership project – 3GPP, and the associated radio modem design aspects. 3GPP has the goal of harmonizing and standardizing, in detail, the similar types of proposals that have been prepared by ETSI, ARIB, TTC, TTA, and T1.
Direct-sequence code division multiple access (DS-CDMA) is used as the air medium multiple access technique. We have seen earlier that spread spectrum techniques can be used to achieve a greater data rate, but the chip rate needs to be increased, which in turn demands for more bandwidth. This is why in WCDMA the bandwidth is widened further to accommodate more users or to support an increased data rate. WCDMA has two modes characterized by the duplex method: FDD (frequency division duplex) and TDD (time division duplex), for operating with paired and unpaired bands, respectively. In the FDD version of UMTS, a physical channel is defined by its code and carrier frequency, while in TDD it is in terms of its code, carrier frequency, and time slot.