WCDMA evolved—High-speed packet-data services
Stefan Parkvall, Eva Englund, Peter Malm, Tomas Hedberg, Magnus Persson and Janne Peisa
Compared to second-generation systems, one of the most important aspects of third-generation mobile systems is enhanced packet-data access. WCDMA Release 99 provides data rates of 384 kbit/s for wide area coverage and up to 2 Mbit/s for hot-spot areas, which is sufficient for most existing packet-data applications. However, as the use of packetdata services increases and new services are introduced, greater capacity will be required. WCDMA Release 5 extends the specification with, among other things, a new downlink transport channel that enhances support for interactive, background, and to some extent, streaming services, yielding a considerable increase in capacity compared to Release 99. It also significantly reduces delay and provides peak data rates of up to 14 Mbit/s. This enhancement, which commonly goes under the abbreviation, HSDPA, is the first step of evolving WCDMA to provide even more outstanding performance. The authors describe the basic principles used by this enhancement and how they are incorporated into the specification and products. They also explain the associated system and end-user benefits.
High-speed downlink shared channel
WCDMA Release 5 extends the WCDMA specification with a new downlink transport channel, called high-speed downlink shared channel. With shared channel transmission, a certain amount of the channelization codes and transmission power in a cell are considered a common resource that is dynamically shared among users, primarily in the time
domain. Shared-channel transmission makes more efficient use of available code resources in WCDMA. HSDPA also supports new features that rely on, and are tightly coupled to, the rapid adaptation of transmission parameters to instantaneous radio conditions: • Fast link adaptation—instead of compensating for varying downlink radio conditions by means of power control, the transmission power is kept constant. Fast rate adjustment is used to adapt to the varying radio conditions. Commonly referred to as link adaptation, this method is more efficient than power control for services that tolerate short-term variations in the data rate. To further increase capacity and data rates, spectral-efficient 16-quadrature amplitude modulation (16QAM) can be used, channel conditions permitting. • Fast hybrid-ARQ with soft-combining— the terminal (user equipment, UE), can rapidly request retransmission of erroneously received data, substantially reducing delay and increasing capacity (compared to Release 99). Prior to decoding, the terminal combines information from the original transmission with that of later retransmissions. This practice, called soft-combining, increases capacity and robustness. • Fast channel-dependent scheduling— The scheduler determines to which terminal the shared channel transmission
BOX A, TERMS AND ABBREVIATIONS
3GPP Third-generation Partnership Project 16QAM 16-quadrature amplitude modulation ARQ Automatic repeat request CIR Carrier-to-interference ratio DCH Dedicated channel G-RAKE Generalized RAKE HSDPA High-speed downlink packet access HS-DSCH High-speed downlink shared channel HS-SCCH High-speed shared control channel MAC Medium access control PDCP Packet data convergence protocol PF Proportional-fair (scheduler) QPSK Quadrature phase-shift keying RAB RAN RBS RLC RNC RR RRC RTP SGSN TCP TTI UDP UE UMTS WCDMA Radio access bearer Radio access network Radio base station Radio link control Radio network controller Round-robin (scheduler) Radio resource control Real-time protocol Serving GPRS support node Transmission control protocol Transmission time interval User datagram protocol User equipment (mobile handset or terminal) Universal mobile telecommunications system Wideband code-division multiple access Ericsson Review No. 2, 2003