What is High Speed Downlink Packet Access (HSDPA)?

The full form of HSDPA is High-Speed Downlink Packet Access. It is a new protocol used in mobile telephone data transmission. It is called a 3.5G technology.

Basically, this standard will provide download speeds on a mobile phone equivalent to an ADSL line in a home, by removing any limitations placed on the use of your phone by a slow connection.

More importantly, it is an evolution and improvement on W-CDMA, (Wideband Code Division Multiple Access) a 3G based protocol. It helps in improving the data transfer rate by a factor of at least five over W-CDMA. It can also achieve theoretical data transmission speeds of 8-10.

Even though any data can be transmitted, applications with high data demands like video and streaming music are the focus of HSDPA.

HSDPA Key Features

The techniques included in HSDPA are the use of higher order modulation, AMC (Adaptive Modulation and Coding), Hybrid Automatic Repeat reQuest (H-ARQ), and fast cell search.

Adaptive Modulation and Coding

It helps in changing the modulation and coding format based on the variations in the channel conditions, subject to system restrictions. This channel condition is estimated based on feedback from the receiver.

The advantages of AMC are as follows −

  • Basically, higher code rates are available for users in favourable positions which in turn increase the average throughput of the cell and Users close to the cell boundary are assigned lower order modulation with lower code rates.

  • Other one is reduced interference variation due to link adaptation based on variations in the modulation/coding scheme rather than variations in transmit power


H-ARQ is used for retransmission decisions instead of explicit measurements as in AMC. The software supports Chase combining, and incremental redundancy H-ARQ retransmission schemes.

The incremental redundancy provides UE feedback by sending different coded bits instead of the same coded packets if the decoding fails on the first attempt.

It also combines the additional parity bits from the retransmission with bits of the first transmission that gives better throughput performance because coding is effectively done across retransmissions, instead of sending simple repeats of the entire coded packet. It is mainly suited to lower data rates.