terminals and reduces variance in downlink transmission delay. The combination of faster data rates and increased spectral efficiency should result in lower cost per data bit transmitted. The theoretical peak rate is 14.4Mbps, but the realistic end- user experience is initially likely to be 1.8Mbps or possibly up to 3.6Mbps. The high download speeds offered by HSDPA will greatly enhance a wide range of mobile services, from Web browsing to video downloads. HSDPA is a packet-based data service in the W-CDMA downlink channel. It was designed from the beginning to support IP and packet-based multimedia ser- vices. HSDPA offers many service-enhancing features, including Adaptive Modula- tion and Coding (AMC), which is a modulation technique that can be determined dynamically, depending on the conditions of the wireless channel at the time. Another key feature of HSDPA is the use of MIMO, which results in great improve- ments in capacity. (MIMO is discussed in Chapter 13.) MIMO antenna systems are the work item in the 3GPP Release 6 specifications, which support even higher data transmission rates (up to 20Mbps). Another important enhancement to HSDPA is that error control is greatly improved through the use of hybrid automatic repeat request (HARQ). HARQ is a variation of the ARQ error control method that gives better performance than ordi- nary ARQ, particularly over wireless channels, albeit at the cost of increased imple- mentation complexity. ARQ is an error control method for data transmission in which the receiver detects transmission errors in a message and automatically requests a retransmission from the transmitter. Generally, when the transmitter receives the ARQ, the transmitter retransmits the message until either it is correctly received or the error persists beyond a predetermined number of retransmissions. The simplest version of HARQ combines FEC (which is discussed in Chapter 13) and ARQ by encoding the data block plus error detection information with an error correction code prior to transmission. W