 expected to make its commercial debut in late 2007 or early 2008. UMTS TDD/TD-CDMA TDD is specifically intended for high-data-rate services. Unlike W-CDMA, which uses FDD, UMTS TDD uses TDD and is designed to work in a single, unpaired fre- quency band. TDD uses a combined TDD and CDMA scheme. It was first defined by 3GPP in Release 99, and it has continued to evolve in the later releases. UMTS TDD operates in the unpaired frequency bands 1,900MHz to 1,920MHz and 2,010MHz to 2,025MHz, using TDD transmission. To provide flexibility to ser- vice providers, UMTS TDD has also been rebanded to allow operation in other licensed spectrum bands, even up to 3.6GHz. UMTS TDD makes use of Direct Sequence Spread Spectrum (DSSS) and the QPSK modulation technique. It oper- ates on a single 5MHz or 10MHz channel, and 20MHz channels are planned as well. The frequency reuse is N = 1, meaning the frequencies can be reused in all the cells. Peak download speeds of up to 12Mbps are promised. One of the largest benefits of using TDD is that it supports variable asymmetry. This means than an operator can decide how much capacity is allocated to the downlink versus the uplink. Because data traffic patterns generally favor the down- link, the result is much better use of spectrum and higher efficiency. Another fea- ture promoted by UMTS TDD proponents is mobility support, with claims that UMTS TDD allows subscribers to stay connected while traveling in excess of 75 mph (120 kph), as long as they remain within the network footprint. It also supports tower-to-tower handoff and network-to-network roaming. Finally, TD-CDMA devices consume significantly less power than other devices, resulting in improved talk times and standby times. For more information, see the UMTS TDD Alliance Web site (www.umtstdd.org). Chapter 14 Wireless WANs TD-SCDMA TD-SCDMA was proposed by the China Communications Stan- dards Association (CCSA; www.cwts.org/english/index.php) and approved by the ITU in 1999. The TD-SCDMA Forum (www.tdscdma-forum.org) includes eig