SM technology family. These enhancements will enable GSM operators to improve their ability to provide mobile broadband multimedia services by supporting higher data transfer speeds and greater system capacity. Because this is an evolutionary path, the generic benefits associated with the GSM family are maintained, including global roaming, seamless billing, network com- patibility, and huge economies of scale. The 3G evolutionary path consists of a series of well-defined technology enhancements, including High-Speed Downlink Packet Access (HSDPA), High- Speed Uplink Packet Access (HSUPA), and High-Speed OFDM Packet Access (HSOPA). The first of these to be realized, HSDPA, provides performance improve- ment in the downlink channel. The following sections describe each of these standards. HSDPA HSDPA is part of the 3GPP/UTRAN (UMTS Terrestrial Radio Access Network) FDD Release 5 W-CDMA specifications. In 3GPP standards, HSDPA evolved from and is backward compatible with Release 99 W-CDMA systems. Release 4 specifications provide efficient IP support, enabling provision of services through an all-IP core network. Release 5 specifications focus on HSDPA to pro- vide data rates from 8Mbps to 14Mbps (and 20Mbps for MIMO systems) over a 5MHz bandwidth in W-CDMA downlink to support packet-based multimedia ser- vices. HSDPA makes use of QPSK for noisy channels and 16-QAM modulation for clearer channels. As discussed in Chapter 5, "Data Communications Basics," QPSK is more robust and can tolerate higher levels of interference than 16-QAM but has a lower transmission bit rate. 16-QAM, on the other hand, offers twice the bit rate but is more prone to errors than QPSK due to interference and noise, and hence it requires stronger forward error correction (FEC). Similar to the way EDGE enhances GSM/GPRS, HSDPA enhances W-CDMA. HSDPA doubles the air interface capacity and delivers a four- or five-fold increase 3G: Moving Toward Broadband Wireless in downlink data speeds. It also shortens the round-trip time between network and 