High-Speed Downlink Packet Access or HSDPA is a mobile telephony protocol. Also called 3.5G (or "3½G"). High Speed Downlink Packet Access (HSDPA) is a packet-based data service in W-CDMA downlink with data transmission up to 8-10 Mbit/s (and 20 Mbit/s for MIMO systems) over a 5MHz bandwidth in WCDMA downlink. HSDPA implementations includes Adaptive Modulation and Coding (AMC), Multiple-Input Multiple-Output (MIMO), Hybrid Automatic Request (HARQ ), fast cell search, and advanced receiver design.

HSDPA is beginning to reach deployment status in North America. Cingular has announced that they will begin to deploy UMTS with expansion to HSDPA in 2005.

In 3rd generation partnership project (3GPP) standards, Release 4 specifications provide efficient IP support enabling provision of services through an all-IP core network and Release 5 specifications focus on HSDPA to provide data rates up to approximately 10 Mbit/s to support packet-based multimedia services. MIMO systems are the work item in Release 6 specifications, which will support even higher data transmission rates up to 20 Mbit/s. HSDPA is evolved from and backward compatible with Release 99 WCDMA systems.