High Speed Downlink Packet Access (HSDPA) is an enhanced 3G mobile telephony communications custom in the High-Speed Packet Access family, also dubbed 3.5G, 3G+ or turbo 3G, which permits networks primarily based on Universal Mobile Telecommunications System to have higher information transference speeds and capacity.

Current HSDPA deployments support down-link speeds of 1.8, 3.6, 7.2 and 14.0 Mbit / s. Further speed increases are available with HSPA+, which gives speeds of almost 42 Mbit / s downlink and 84 Mbit / s with Release nine of the 3GPP standards HSDPA is a custom for info communications on cellular networks.

HSDPA supports unproven data rates of 1.8 Mbps and up to 14.4 Mbps maximum, though standard speeds are nearer to 5 hundred Kbps. Some telecommunication corporations like Cingular which have been using the slower UMTS custom are upgrading to HSDPA as a move to supposed "3G" mobile networks. Besides being built into new cell phones, certain cellular cards (network adapters) for portables also support HSDPA. Technology standards contesting with HSDPA include EV-DO and WiMAX. For a new channel layer, High Speed Downlink Shared Channel (HS-DSCH) has been added to W-CDMA design.

It is implemented by introducing 3 new physical layer channels: HS-SCCH, HS-DPCCH and HS-PDSCH. The High Speed-Shared Control Channel (HS-SCCH) alerts the user that information will be dispatched on the HS-DSCH two slots ahead. The High Speed-Dedicated Physical Control Channel (HS-DPCCH) carries admission info and current channel quality indicator (CQI) of the user. This value is then employed by the base station to work out how much information to send to the user devices on the following transmission.

The High Speed Physical Downlink Shared Channel is the channel mapped to the above HS-DSCH transport channel that carries tangible user info. HSDPA is part of the UMTS standards since its release five years ago, which also accompanies an improvement on the uplink providing a new bearer of 384 kbit / s. The maximum bearer prior to this one was 128 kbit / s. In addition to improving information rates, HSDPA also decreases latency and the round trip time for applications. In later 3GPP design releases HSPA+ increases information rates further by adding 64QAM modulation, MIMO and Dual-Cell HSDPA operation.