166 Chapter 14 14.14 Higher Data Rates One advantage of CDMA is that multiple Walsh codes can be aggregated to a single user, to allow a higher data rate. Data rates in multiples of 9.6 kbps are available. Aggregating 16 channels together allows a data rate of 153.6 kbps. With 32 channels aggregated--the maximum because the pilot, sync, and paging channel are not available for user traffic--the data rate is 307.2 kbps. This rate is simply not possible in a TDMA system. However, the disadvantage is that a few data users can consume most of the downlink capacity, compromising the available capacity for voice users, who provide the bulk of the revenue to the wireless carriers. Therefore, a companion technology was developed to support high-speed data users. The market for this type of service is primarily businesspeople who need and are willing to pay for mobile high-speed service to allow remote Internet and email access. This service is known as Evolution for Data Only, or EVDO. It uses the same 1.25 MHz channel bandwidth and same uplink modulation techniques as CDMA2000. The downlink is completely redesigned and does not use CDMA techniques. It uses a very high speed form of QAM modulation, and the entire signal is devoted to one user at a time. The users share this high-speed link in a time-division duplex fashion. This link is suitable for high-speed data access, where the data access is often intermittent and packet based. The uplink typically requires much lower data rates and so retains the original CDMA uplink technology. Where EVDO service is offered, it uses a separate frequency spectrum from the CDMA2000 service. 14.15 Spectral Efficiency Considerations So how does CDMA stack up against TDMA when it comes to spectral efficiency? The original goal was for digital systems to provide a 10-fold increase in spectral efficiency. The IS-136 TDMA efficiency provided a threefold increase. · · · · · AMPS: One voice call per 30 kHz channel bandwidth. With seven-cell frequency reuse, the effective spectrum per user was 210 kHz. IS-136: Three voice calls per 30 kHz channel bandwidth. With seven-cell frequency reuse, the effective spectrum per user was 70 kHz. PDC: Six voice calls (half rate vocoder) per 25 kHz channel bandwidth. With seven-cell frequency reuse, the effective spectrum per user was 29 kHz. GSM: Eight voice calls per 200 kHz channel bandwidth. With seven-cell frequency reuse, the effective spectrum per user was 175 kHz. CDMA2000: CDMA is tougher to calculate capacity because this depends on network settings. In general, higher capacity is possible at the expense of voice quality, due to www.newnespress.com