High-Speed Downlink Packet Access (HSDPA) is a new mobile telephony protocol and is sometimes referred to as a 3.5G (or "3½G") technology. In this respect it extends WCDMA in the same way that EV-DO extends CDMA2000. HSDPA provides a smooth evolutionary path for Universal Mobile Telecommunications System (UMTS) networks allowing for higher data capacity (up to 14.4 Mbit/s in the downlink). It is an evolution of the W-CDMA standard, designed to increase the available data rate by a factor of 5 or more. HSDPA defines a new W-CDMA channel, the high-speed downlink shared channel (HS-DSCH) that operates in a different way from existing W-CDMA channels, but is only used for downlink communication to the mobile.
Technology

The HS-DSCH channel does away with two basic features of other WCDMA channels - the variable spreading factor and fast power control - and instead uses 1. Adaptive Modulation and Coding (AMC), 2. fast packet scheduling at the Node B (Base Station) and 3. fast retransmissions from Node B (known as HARQ-Hybrid Automatic Repeat Request) to deliver the improved downlink performance. The concept of "incremental redundancy" is used in HARQ, where retransmissions contain different codings of the user data, relative to the original transmission. When a corrupted packet is received, the user device saves it, and combines it with subsequent retransmissions, to formulate an error-free packet as quickly and efficiently as possible. Even if the retransmitted packet(s) is itself corrupted, the combination of the sum of the errored transmissions can yield an error-free packet.
The HS-DSCH downlink channel is shared between users using channel-dependent scheduling to take advantage of favourable channel conditions to make best use of available radio conditions. Each user device periodically transmits (as many as 500 times per second) an indication of the downlink signal quality. The Node B uses this information received from all user devices to decide which users will be sent data on the next 2 ms frame and, for each user, how much data should be attempted. More data can be sent to users which report high downlink signal quality.
The amount of the channelisation code tree, and thus network bandwidth, allocated to HSDPA users is determined by the network. The allocation is "semi-static" in that it can be modified while the network is operating, but not on a frame-by-frame basis. This allocation represents a tradeoff between bandwidth allocated for HSDPA users, versus that for voice and non-HSDPA data users. The allocation is in units of channelisation codes for Spreading Factor 16, of which 16 exist, and of which up to 15 can be allocated to HSDPA.

When the Node B decides which users will receive data on the next 2 ms frame, it also determines which channelisation code(s) will be employed for each user, and this information is sent to the user devices over one or more HSDPA "scheduling channels" (these scheduling channels are not part of the HSDPA allocation previously mentioned, but are allocated separately). Thus, for a given 2 ms frame, data may be sent to a number of users simultaneously, using different channelisation code. The maximum number of users to receive data on a given 2 ms frame is determined by the number of allocated channelisation codes. This differs from CDMA 1xEV-DO, where data is sent to only one user at a time.
As well as improved data rates that are associated with HSDPA one of the key benefits that are seen is the reduction on latency, improving the round trip time for applications.
HSDPA upgrades to networks is not one single step change - there are different evolutions of this network. QPSK is the initial modulation scheme, however, in good radio conditions the introduction on 16QAM modulation will improve data throughput rates by approximately double that of QPSK. QPSK with 5 Code allocation will typically offer up to 1.8 Mbit/s peak data rates. 16QAM with 5 Codes will increase this to 3.6 Mbit/s. Additional Codes (e.g. 10, 15) can also be used to improve these data rates or extend the network capacity throughput significantly. Theoretically, HSDPA can give throughput up to 10.8 Mbit/s.
HSDPA is part of release 5 UMTS networks, which also accompanies an improvement on the uplink providing a new bearer of 384 kbit/s (previous max bearer was 128 kbit/s).
Along with the HS-DSCH channel, two new physical channels are also introduced. One is Signaling Control Channel which informs the user that the data to be sent on HS-DSCH 2 slot ahead. The second one is a reverse channel, terminating at the Node B, which carries acknowledgement information and current channel quality of the user.
The First phase of HSDPA has been specified in 3GPP release 5. Phase one introduces new basic functions and is aimed to achieve peak data rates of 14.4 Mbps. Newly introduced are the High Speed Downlink Shared Channels (HS-DSCH), the adaptive modulation QPSK and 16QAM and the High Speed Medium Access protocol (MAC-hs) in the Node-B.

The Second phase of HSDPA is currently being specified in 3GPP release 6 and is aimed to achieve data rates of up to 28.8 Mbps. It will introduce antenna array technologies such as beamforming and Multiple Input Multiple Output (MIMO). Beam forming can be described as focussing the transmitted power of an antenna in a beam towards the user’s direction. Knowing that the limiting resources are the transmission power of the base station sector, one can understand that beam forming is a mean of increasing this power. MIMO uses multiple antennas at the sending and receiving side.
The Third phase of HSDPA which still is a long way down the road will concentrate on the air interface. It will introduce a new Air Interface with Orthogonal Frequency Division Multiplexing and higher modulation schemes. Phase three of HSDPA aims at data rates of up to 50 Mbps.
Although 3G is not available in most countries, this protocol does have a chance to become the follow up to 3G where WCDMA is deployed. Its success as a GSM-replacement, vis-à-vis other contenders like CDMA2000 1x or CDMA2000 1xEV-DO and cellular datacommunication standards like WiMAX (IEEE 802.16) is still unclear, especially considering that SK Telecom and KTF's CDMA2000 and CDMA2000 1xEV-DO is considered as successful introduction in South Korea and KDDI's CDMA2000 is generally considered as being much more successful and smooth than DoCoMo's and ex-Vodafone's (now: SoftBank's) UMTS/W-CDMA introduction in Japan, which are much slower than initially hoped.