3G HSDPA - High Speed Downlink Packet Access
- a tutorial, description or information about the basics of HSDPA, High
Speed Downlink Packet Access, the enhancement to the UMTS / WCDMA standard for
3G cellular telecommunications
Improvements and enhancements are being made to the Wideband
CDMA or UMTS 3G telecommunications system. Called High Speed Downlink Packet
Access ( HSDPA ) the new technology promises to increase the download data rate
five fold. In addition to this HSDPA also provides a two fold increase in base
station capacity.
The introduction of HSDPA technology has come about as a
result of the need to drive down costs as well as increasing the data rates
possible. Current trends show the volume of IP / packet data rising and
overtaking the more traditional circuit switched traffic. By adopting a packet
based approach to the delivery of digital content as well as IP based person to
person digitized voice, a single session can be used for multiple purposes and
this can be used to drive revenues upwards. With this approach in mind the use
of HSDPA is a key element in providing the user with a better 3G service as well
as increasing revenues as a result of increased capacity and usage for the
service providers.
HSDPA and standards
The new high speed technology part of the 3G UMTS evolution.
Release 4 of the 3GPP W-CDMA standard provided the efficient IP support to
enable provision of services through an all IP core network. Then Release 5
included HSDPA itself with support for the packet-based multimedia services. A
further enhancement known as MIMO (Multiple Input Multiple Output) will then be
contained within Release 6. As HSDPA needs to work alongside the original
Release 99 systems, the new technology is completely backwards compatible.
Key HSDPA technologies
One of the keys to the operation of HSDPA is the use of an
additional form of modulation. Originally W-CDMA had used only QPSK as the
modulation scheme, however under the new system16-QAM which can carry a higher
data rate, but is less resilient to noise is also used when the link is
sufficiently robust. The robustness of the channel and its suitability to use
16-QAM instead of QPSK is determined by analyzing information fed back about a
variety of parameters. These include details of the channel physical layer
conditions, power control, Quality of Service (QoS), and information specific to
HSDPA.
Fast HARQ (hybrid automatic repeat request), has also been
implemented along with multi-code operation and this eliminates the need for a
variable spreading factor. By using these approaches all users, whether near or
far from the base station are able to receive the optimum available data rate.
Further advances have been made in the area of scheduling. By
moving more intelligence into the base station, data traffic scheduling can be
achieved in a more dynamic fashion. This enables variations arising from fast
fading can be accommodated and the cell is even able to allocate much of the
cell capacity for a short period of time to a particular user. In this way the
user is able to receive the data as fast as conditions allow.
A further channel known as the High Speed Downlink Shared
Channel (HS-DSCH) has been introduced. W-CDMA normally carries data over
dedicated transport channels (DCHs), several of which are multiplexed onto one
RF carrier. This approach has been adopted because it provides the optimum
performance with continuous user data. Under the new scheme the "bursty" nature
of the data has been accounted for and more efficient use of the available
spectrum has been made.
HSDPA performance
Using HSDPA scheme it will be possible to achieve peak user
data rates of 10 Mbps within the 5 MHz channel bandwidth offered under 3G UMTS.
The new scheme has a number of benefits. It improves the overall network packet
data capacity, improves the spectral efficiency and will enable networks to
achieve a lower delivery cost per bit. Users will see higher data speeds as well
as shorter service response times and better availability of services. However
new mobile designs will need to be able to handle the increased data throughput
rates. Reports indicate that handsets will need to have at least double the
memory currently contained within handsets. Nevertheless the advantages of 3G
HSDPA mean that it will be widely used as networks are upgraded and new phones
introduced.
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