High-Speed Downlink Packet Access (HSDPA) - Online Article

The next phase of evolution in the WCDMA line of wireless access technologiesis high-speed downlink packet access (HSDPA). Though it is too early to pen downthe success and efficiency of HSDPA in high-speed wireless broadband, thecuriosity it has generated merits a closer examination.

For GSM networks either UMTS or WCDMA is the way to 3G. Globally, there are64 WCDMA commercial deployments in 31 countries . In the 3G scenario, HSDPA isa similar enhancement to WCDMA as EDGE is to GSM/GPRS networks.

WCDMA in itself is capable of delivering data speeds up to 2 Mbps and HSDPApromises to hike data delivery by five fold to 10 Mbps in the 5 MHz channel.Theoretically, HSDPA can deliver data speeds between 10–14 Mbps. But it wouldbe realistic to expect 2–3 Mbps downlink on average. In a shared environment,with an adequate coverage, 300 kbps to 1 Mbps downlink speed is being targeted.

HSDPA also shortens the round-trip time between the network and terminals,and reduces variance in downlink transmission delay. This makes HSDPA comparableto any wireless LANs and fixed-line broadband in terms of data throughput.

What is HSDPA ?

HSDPA, short for High-Speed Downlink Packet Access, is a new protocol for mobile telephone data Transmission. It is known as a 3.5G (G stands for generation) technology. Essentially, the standard will provide download speeds on a mobile phone equivalent to an ADSL (Asymmetric Digital Subscriber Line) line in a home, removing any limitations placed on the use of your phone by a slow connection. It is an evolution and improvement on W-CDMA, or Wideband Code Division Multiple Access, a 3G protocol. HSDPA improves the data transfer rate by a factor of at least five over W-CDMA. HSDPA can achieve theoretical data transmission speeds of 8-10 Mbps (megabits per second). Though any data can be transmitted, applications with high data demands such as video and streaming music are the focus of HSDPA.

HSDPA improves on W-CDMA by using different techniques for modulation and coding. It creates a new channel within W-CDMA called HS-DSCH, or high-speed downlink shared channel. That channel performs differently than other channels and allows for faster downlink speeds. It is important to note that the channel is only used for downlink. That means that data is sent from the source to the phone. It isn't possible to send data from the phone to a source using HSDPA. The channel is shared between all users which lets the radio signals to be used most effectively for the fastest downloads.

The widespread availability of HSDPA may take a while to be realized, or it may never be achieved. Most countries did not have a widespread 3G network in place as of the end of 2005. Many mobile telecommunications providers are working quickly to deploy 3G networks which can be upgraded to 3.5G when the market demand exists. Early deployments of the service will be at speeds much lower than the theoretically possible rates. Early service will be at 1.8 Mbps, with upgrades to 3.6Mbps as devices are made available that can handle that increased speed.

The long-term acceptance and success of HSDPA is unclear, because it is not the only alternative for high speed data transmission. Standards like CDMA2000 1xEV-DO and WiMax are other potential high speed standards. Since HSDPA is an extension of W-CDMA, it is unlikely to succeed in locations where W-CDMA has not been deployed. Therefore, the eventual success of HSDPA as a 3.5G standard will first depend upon the success of W-CDMA as a 3G standard.


The High-Speed Downlink Shared Channel (HS-DSCH) lacks two basic features of other W-CDMA channels — variable spreading factor and fast power control. Instead, it delivers the improved downlink performance using adaptive modulation and coding (AMC), fast packet scheduling at the base station, and fast retransmissions from the base station, known as hybrid automatic repeat-request (HARQ). HARQ uses incremental redundancy, where user data is transmitted multiple times using different codings. When a corrupted packet is received, the user device saves it and later combines it with the retransmissions, to recover the error-free packet as efficiently as possible. Even if the retransmitted packets are corrupted, their combination can yield an error-free packet.

HSDPA User Equipment (UE) categories

HSDPA comprises various versions with different data speeds.


Why HSDPA for Operators

It is no mystery that voice ARPUs would not be enough for the operators tosustain themselves and only data over their networks would keep them fromclosing shop.

As the consumers become more mobile and get addicted to rich multimediacontent over handsets, the demand for better network would rise. HSDPA wouldenable operators to deliver advanced mobile broadband services and applications.For an operator, who has already rolled 3G plans on WCDMA, upgrading to HSDPAwould not be difficult. The HSDPA-ready WCDMA base stations require just asoftware upgrade and the network is ready to meet the demand for broadbandaccess over wireless.


As a technology, HSDPA has all the characteristics to lower the cost per bitfor data and can support services like interactive gaming, VoIP, and SIP-basedmultimedia. It also promises improved spectral efficiency delivering moreconsistent quality of service to a larger number of subscribers.

As it is being touted as an efficient technology that lowers the cost of theon-going network growth, HSDPA is all set to attract the attention of theoperators who have already deployed WCDMA or are planning to go for a 3Grollout. Though GPRS and EDGE have already made their entry in India, the 3Gplans none of the GSM operators is ready to come out in the open. It is also afact that they need to do a lot in their present offerings before they migrateto WCDMA, which in turn would usher in HSDPA. And, this seems a couple of yearsaway.

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