What is HSDPA?

Introduction

HSDPA is an acronym for High Speed Downlink Packet Access which is an advanced protocol for mobile telephone data transmission. HSDPA is an evolved form of W-CDMA (Wideband Code Division Multiple Access) technology. HSDPA promises to provide download speeds equivalent to an ADSL (Asymmetric Digital Subscriber Line) line on your mobile phone. The technology standard used in HSDPA is called 3.5G.

The Technology

As we saw, the 3.5 G standard used in HSDPA is an improvement upon the 3G protocol used in W-CDMA technology. With HADPA, the data transfer rate improves over W-CDMA by a factor of at least five. Theoretically speaking, it is possible to reach data transmission speeds of 8-10 Mbps with HSDPA. Therefore HSDPA is touted as highly favorable for transmission areas like video and streaming music where high data transfer rates are required.

Now the question is, in what way HSDPA has improved upon WCDMA. HSDPA employs different techniques for modulation and coding by creating a new channel within W-CDMA. This channel is called High Speed Downlink Shared Channel, or HS-DSCH. This channel is created in such a way that it makes higher downlink speeds possible. However this channel enables only downlinks, which means that the source can send data to the receptor but not vice versa. You cannot send data from your phone to the source using HSDPA. The HS-DSCH channel is shared among users, allowing the most efficient use of radio signals and thus helping faster download speeds. HSDPA uses 5 MHz bandwidth. The technology upgrade has enabled a smooth transition for Universal Mobile Telecommunications System (UMTS).

In the HS-DCH channel, the variable spreading factor and fast power control of the WCDMA channels are done away with. The new technology uses Adaptive Modulation and Coding (AMC), fast packet scheduling at the Base Station and fast retransmissions from the Base Station. The retransmissions are done through Hybrid Automatic Repeat Request (HARQ), where the concept of incremental redundancy comes into play. Here, the retransmissions have different codes of the data against the codes of the original transmission. In other words, the data received at the base station is ‘processed’ to manage any corruption the data may have gone through at the time of the original transmission. The net result of this process is that the user device can receive error free packets, thus ensuring better quality to the video and sound.

Is HSDPA Here to Stay?

Well, only time can answer that question. Many mobile telecommunication companies are working to deploy 3G (WCDMA) networks and this is a positive indication. It would be easy to upgrade to 3.5 G once a 3G network is in place. Some companies are expected to deploy HSDPA by the end of 2006. However the downlink speeds in reality at this moment are going to be much lower at 1.8-3.6 Mbps, than the theoretical potential of 8-10 Mbps.

The success of HSDPA will also depend on many other factors. There are other technologies like CDMA2000, 1xEV-DO and WiMAX that promise high data transmission speeds. Further, HSDPA is an improvement upon the W-CDMA standard, so it can gain acceptance only in places where W-CDMA has been deployed successfully.