HSDPA is a mouthful for speed

CANNES , FRANCE -- If the acronym HSDPA means nothing to you, read on because the technology behind this string of letters could soon significantly change the way you work and play on your mobile device.

High Speed Downlink Packet Access (HSDPA) is one of the new technologies being talked up and even demonstrated at the 3GMS World Congress in Cannes.

The technology, a rival to the EV-DO (evolution data optimized) standard in CDMA (Code Division Multiple Access) found in the U.S. and parts of Asia, promises something surprisingly missing in early 3G (third-generation) mobile broadband networks -- high data speeds similar to those of fixed-line ADSL (asymmetrical digital subscriber line) services.

Remember all the talk about 3G offering data downloads at speeds up to 2Mbps? Well, yes, those speeds are achievable -- in a lab with a handset held next to the base station. It's a theoretical speed that looks good in promotional brochures but is nearly impossible to achieve for the ordinary Joe Blow subscriber.

The first wave of 3G technology, Release 99 (R99), offers a maximum speed of 384Kbps. Even that's a peak rate, or something you might reach if you're alone in a mobile cell at 5 a.m. Most likely, you're going to have speeds between 150Kbps and 250Kbps, according to John Leonard, vice president business strategy at Lucent Technologies.

With HSDPA, expect initial throughput rates of between 400Kbps and 600Kbps, with a peak rate of 14.4Mbps.

HSDPA technology is the key feature of the Release 5 specification approved in 1999 by the 3GPP (Third Generation Partnership Project). The technology offers several significant improvements over R99 networks.

Unlike R99, HSDPA introduces an additional transport channel, called the high-speed downlink share channel. Up to 15 of these channels can operate in the WCDMA (Wideband CDMA) radio channel, allowing multiple users to share the entire downlink channel.

The additional channel is designed to take advantage of bursty data traffic, typical in data networks, through multiplexing. For example, once your data has been dispatched, other users can gain access to the network in much the same way they do with DSL technology.

Another feature is adaptive modulation and coding (AMC), a technique used to compensate for variations in radio conditions. With this technique, a network node schedules the transmission of data packets to a user by matching the user's priority and estimated channel operating environment with the appropriate coding and modulation scheme, thus increasing throughput under favorable conditions.

In addition, HSDPA offers a retransmission mechanism for quick error correction. In spread-spectrum networks such as WCDMA, handsets confirm when they receive data and communicate key information such as channel condition and power back to the network. While this process is handled by the Radio Network Control (RNC) system in R99 networks, with HSDPA, it is processed directly in the base station, enabling a much faster response.

Several operators have already begun HSDPA tests, including Vodafone Group in Japan and New Zealand, and mm02 PLC on the Isle of Man in the U.K.. Many others, such as T-Mobile International, are in the starting blocks.

"This is a first big step to give users of mobile devices an ADSL experience," said T-Mobile Chief Technology Officer Hamid Akhavan. "We're very excited."