hen the coded data block is received, the receiver first decodes the error correction code. If the channel quality is good enough, all transmission errors should be correctable, and the receiver can obtain the correct data block. If the channel quality is bad and not all transmission errors can be corrected, the receiver will detect this situation, using the error correction code, and then the received coded data block is discarded and a retransmission is requested by the receiver. Other features of HARQ include fast cell search and advanced receiver design. As Figure 14.7 illustrates, because HSDPA is built on a distributed architecture and implements key processing at the base station, closer to the air interface, HSDPA achieves low delay link adaptation. By taking advantage of techniques including a fast Layer 1 retransmission scheme (HARQ) and link adaptation, HSDPA delivers significantly improved packet data throughput performance. Chapter 14 Wireless WANs Device 1 Base Transceiver Station Device 2 Figure 14.7 HSDPA All operators are expected to evolve to HSDPA as the new baseline for mobile wireless broadband. HSDPA is widely embraced by the vendor community, and the majority of existing W-CDMA operators are expected to upgrade their 3GSM net- works to support HSDPA technology. Many operators have confirmed interest in and are considering, planning, deploying, or trialing HSDPA. HSDPA is expected to provide a user applications experience between 3 and 10 times faster than with traditional UMTS. HSDPA is often compared to WiMax (which is discussed in Chapter 15). Early HSDPA will focus on data, voice, and mobility from the handset perspective. WiMax will focus on broadband data, ini- tially to underserved areas because of its distance capability. An advantage of HSDPA is that, unlike WiMax, it requires no new infrastructure, just the down- loading of new software to the handset, using the existing cellular infrastructure. HSUPA HSUPA is the sibling of HSDPA but has taken longer to reach consensus. Enhancemen