IS95 and CDMA 2000

This chapter discusses the cellular standards IS-95 and CDMA 2000, often just called “CDMA” in the popular literature. The general system design (describing Base Station Subsystem (BSS), the Network and Switching Subsystem (NSS), and the Operation Support System (OSS) parallels that of GSM (see discussion there). The main chapter concentrates on the air interface of IS-95; the appendix discusses the enhancements of CDMA 2000 (including EV-DO). We first discuss coding, modulation, and spreading. Two different speech coders exist, the Qualcomm Code Excited Linear Prediction (QCELP), and the Enhanced Variable Rate Coder (EVRC). The output of the speech coder is FEC-encoded by addition of parity check bits (frame quality indicator, FQI) and convolutional encoding. The uplink uses M-ary orthogonal keying, where groups of encoded data are mapped onto Walsh codewords. These codewords are then spread by multiplication with a long spreading code, multiplied with short spreading sequences, and transmitted using Offset Quadrature Amplitude Modulation (OQAM). Databurst randomization and gating enable different data rates. The downlink data stream is first scrambled (without spreading by long spreading sequences). Spreading and channelization is done by multiplication with Walsh sequences. Finally, the output from the spreader is multiplied separately in the I- and Q-branch by short spreading sequence. IS-95 uses a combination of open-loop and closed-loop power control for the uplink, where the closed-loop consists of an outer and an inner loop. For the downlink, a coarse power control also exists. IS-95 distinguishes logical channels (types of data) and physical channels (time/frequency/code resources). Traffic channels, access channel, pilot channels, synchronization channel, paging channel, and power control channel are important examples. A discussion of the soft handover, another key feature of CDMA systems, concludes the chapter.