The introduction of WCDMA follows a natural evolution of 2G networks. As with the introduction of 2G networks, early 1990, it describes the first step in a continuous evolution of technologies and what they can provide for end-users and operators. The term "WCDMA Evolved" describes the evolution of WCDMA addressing both operator's need for efficiency and end-user's demand for enhanced experience and simplicity. WCDMA is evolving to handle higher bit rates. In the first step downlink was improved. WCDMA 3GPP Release 5 extends the specification with HSDPA, which is an enhancement of WCDMA. HSDPA improves the end-user experience by increasing peak data rates to 14 Mbps in the downlink, reducing delay; and providing 2-3 times more system capacity. In the second step the uplink has been enhanced. Through these capabilities, operators benefit from a technology that provides performance for improved end-user experience for Web access, file download and streaming services. Wireless broadband access to the Internet, intranet and corporate LAN benefits greatly from 'WCDMA Evolved.'

The Concept
In WCDMA 3GPP release 5, WCDMA has been extended with a new transport channel, the high-speed downlink shared channel (HS-DSCH), which provides enhanced support for interactive, background, and to some extent, streaming radio access bearer (RAB) services in the downlink. HS-DSCH transmission facilitates several new features. But to support them with minimum impact on the existing radio interface protocol architecture, a new MAC sub-layer, MAC-hs, has been introduced for HS-DSCH transmission. MAC-hs makes it possible to retain a functional split between layers and nodes from WCDMA 3GPP release 99. A minimum of architectural changes allows a smooth upgrade to HSDPA and ensures HSDPA operation in environments where not all cells have HSDPA functionality.

Benefits
The primary benefit of HSDPA is the improved end-user experience. In practice, this means shorter download times through higher bit rates (14 Mbps peak rate) and reduced roundtrip time over the air interface. HSDPA also provides advantages for operators by introducing greater system capacity.

Improved end-user experience: HSDPA can reduce the time it takes to download large files by a factor of 20. HDSPA opens up for enhanced end-user experience when using WCDMA for wireless broadband applications such as intranet and Internet access via laptop computers. Here the reduced delay improves the traditional web access. Download of emails and other heavy files are improved by the increased peak data rates.

Improved system capacity: A further benefit of HSDPA is greater system capacity. HSDPA increases capacity in several ways:

• Shared-channel transmission results in efficient use of available code and power resources in WCDMA

• The use of a shorter TTI reduces roundtrip time and improves the tracking of fast channel variations

• Link adaptation maximizes channel usage and enables the base station to operate close to maximum cell power

• Fast scheduling prioritizes users with the most favorable channel conditions

• Fast retransmission and soft-combining further increases capacity

• 16QAM yields higher bit rates

Depending on the deployment scenario, the combined gain in capacity is from two to three times that of WCDMA 3GPP release 99.

Another benefit of HSDPA is that it is an integral part of WCDMA. Wide-area mobile coverage can be provided with HSDPA. At present, WCDMA can provide simultaneous voice and data services (multi-services) to users on the same carrier. This also applies to HSDPA, which means that spectrum can be used efficiently. HSDPA also makes efficient use of power by employing unused power.

Wide Coverage
WCDMA will be evolved to handle higher bit rates. The first step is to improve the downlink in WCDMA. HSDPA will greatly improve the end-user experience by increasing bit rates to as much as 14 Mbps in the downlink, reducing delay, and increasing system capacity 200–300%.

No new spectrum / carrier is needed to roll out HSDPA in the network. At present, WCDMA can provide voice and data services on the same carrier simultaneously. This also applies to HSDPA.

With the advantages of HSDPA, 'WCDMA Evolved' will further enable operators to provide end-users with more advanced wireless broadband applications offering wide area coverage and mobility.

Nilabh Jha
nilabhj@cybermedia.co.in