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3G UMTS HSPA - High Speed Packet
Access
3G HSPA of High Speed packet Access is the combination of twotechnologies, one of the downlink and the other for the uplink that
can be built onto the existing 3G UMTS or W-CDMA technology
to provide increased data transfer speeds.
The original 3G UMTS / W-CDMA standard provided a maximum
download speed of 384 kbps. With many users requiring much
high data transfer speeds to compete with fixed line broadband
services and also to support services that require higher data rates,
the need for an increase in the speeds obtainable became
necessary. This resulted in the development of the technologies for
3G HSPA.
3G HSPA benefits
The UMTS cellular system as defined under the 3GPP Release 99
standard was orientated more towards switched circuit operationand was not well suited to packet operation. Additionally greater
speeds were required by users than could be provided with the
original UMTS networks. Accordingly the changes required for 3G
HSPA were incorporated into many UMTS networks to enable
them to operate more in the manner required for current
applications.
3G HSPA provides a number of significant benefits that enable thenew service to provide a far better performance for the user. While
3G UMTS HSPA offers higher data transfer rates, this is not the
only benefit, as 3G HSPA offers many other improvements as
well:
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1.Use of higher order modulation: 16QAM is used in thedownlink instead of QPSK to enable data to be transmitted at
a higher rate. This provides for maximum data rates of 14
Mbps in the downlink. QPSK is still used in the uplink where
data rates of up to 5.8 Mbps are achieved. The data ratesquoted are for raw data rates and do not include reductions in
actual payload data resulting from the protocol overheads.
2.Shorter Transmission Time Interval (TTI): The use of ashorter TTI within 3G HSPA reduces the round trip time and
enables improvements in adapting to fast channel variations
and provides for reductions in latency.
3.Use of shared channel transmission: Sharing the resourcesenables greater levels of efficiency to be achieved andintegrates with IP and packet data concepts.
4.Use of link adaptation: By adapting the link it is possible tomaximize the channel usage.
5.Fast Node B scheduling: The use of fast scheduling within3G HSPA with adaptive coding and modulation (only
downlink) enables the system to respond to the varying radio
channel and interference conditions and to accommodate data
traffic which tends to be "bursty" in nature.
6.Node B based Hybrid ARQ: This enables 3G HSPA toprovide reduced retransmission round trip times and it adds
robustness to the system by allowing soft combining of
retransmissions.
For the network operator, the introduction of 3G HSPA technology
brings a cost reduction per bit carried as well as an increase in
system capacity. With the increase in data traffic, and operators
looking to bring in increased revenue from data transmission, thisis a particularly attractive proposition. A further advantage of the
introduction of 3G HSPA is that it can often be rolled out by
incorporating a software update into the system. This means that
the use of 3G HSPA brings significant benefits to user and
operator alike.
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3G UMTS HSPA constituents
There are two main components to 3G UMTS HSPA, eachaddressing one of the links between the base station and the user
equipment, i.e. one for the uplink, and one for the downlink.
Uplink and downlink transmission directions
The two technologies were released at different times through
3GPP. They also have different properties resulting from the
different modes of operation that are required. In view of these
facts they were often treated as almost separate entities. Now they
are generally rolled out together. The two technologies are
summarized below:
HSDPA - High Speed Downlink Packet Access: HSDPAprovides packet data support, reduced delays, and a peak raw
data rate (i.e. over the air) of 14 Mbps. It also provides
around three times the capacity of the 3G UMTS technology
defined in Release 99 of the 3GPP UMTS standard.
HSUPA - High Speed Uplink Packet Access: HSUPAprovides improved uplink packet support, reduced delays anda peak raw data rate of 5.74 Mbps. This results in a capacity
increase of around twice that provided by the Release 99
services.
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Beyond 3G HSPA
With the elements of 3G HSPA launched, further evolutions were
in the pipeline. The first of these was known as HSPA+ or Evolved
HSPA. The evolved HSPA or HSPA+ provides data rates up to 42Mbps in the downlink and 11 Mbps in the uplink (per 5MHz
carrier) which it achieves by using high order modulation and
MIMO (multiple input, multiple output) technologies.
In addition to 3G HSPA, and its evolutions, the next evolution for
3G UMTS is known as LTE - Long Term Evolution. This uses a
completely different air interface that is based around OFDM as
the modulation format. While many operators have opted to
migrate directly from UMTS to LTE, the majority are using 3G
HSPA to upgrade their existing 3G networks.
3G UMTS HSPA and 3GPP standards
The new high speed technology is part of the 3G UMTS evolution.
It provides additional facilities that are added on to t e basic 3GPP
UMTS standard. The upgrades and additional facilities wereintroduced at successive releases of the 3GPP standard.
Release 4: This release of the 3GPP standard provided forthe efficient use of IP, a facility that was required because the
original Release 99 focused on circuit switched technology.
Accordingly this was a key enabler for 3G HSDPA.
Release 5: This release included the core of HSDPA itself.It provided for downlink packet support, reduced delays, araw data rate (i.e. including payload, protocols, error
correction, etc) of 14 Mbps and gave an overall increase of
around three over the 3GPP UMTS Release 99 standard.
Release 6: This included the core of HSUPA with anenhanced uplink with improved packet data support. This
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provided reduced delays, an uplink raw data rate of 5.74
Mbps and it gave an increase capacity of around twice that
offered by the original Release 99 UMTS standard. Also
included within this release was the MBMS, Multimedia
Broadcast Multicast Services providing improved broadcastservices, i.e. Mobile TV.
Release 7: This release of the 3GPP standard includeddownlink MIMO operation as well as support for higher
order modulation up to 64 QAM in the uplink and 16 QAM
in the downlink. However it only allows for either MIMO or
the higher order modulation. It also introduced protocol
enhancements to allow the support for Continuous Packet
Connectivity (CPC). Release 8: This release of the standard defines dual carrier
operation as well as allowing simultaneous operation of the
high order modulation schemes and MIMO. Further to this,
latency is improved to keep it in line with the requirements
for many new applications being used.
HSPA summary
3G HSPA is widely deployed and providing significantly increased
data transfer rates required for the variety of data applications
including mobile broadband for Internet connectivity now being
used by mobile users. As 3G UMTS HSPA is normally a relatively
straightforward upgrade based around a software change, its
incorporation involves a relatively low cost upgrade. As the use of
3G HSPA is able to increase the efficiency of the overall network,
reducing the cost per bit, then it is often a very cost effective
upgrade.
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3G UMTS HSDPA - High Speed
Downlink Packet Access
3G HSDPA High Speed Downlink Packet Access is an upgrade tothe original 3G UMTS cellular system that provides a much greater
download speeds for data. With more data being transferred across
the downlink than the uplink for data-centric applications, the
upgrade to the downlink was seen as a major priority. Accordingly
3G UMTS HSDPA was introduced into the 3GPP standards as
soon as was reasonably possible, the uplink upgrades following on
slightly later.
3G UMTS HSDPA significantly upgrades the download speeds
available, bring mobile broadband to the standards expected by
users. With more users than ever using cellular technology for
emails, Internet connectivity and many other applications, HSDPA
provides the performance that is necessary to make this viable for
the majority of users.
Key 3G HSDPA technologies
The 3G HSDPA upgrade includes several changes that are built
onto the basic 3GPP UMTS standard. While some are common to
the companion HSUPA technologies added to the uplink, others
are specific to HSDPA High Speed Downlink Packet Access,
because the requirements for the each direction differ.
Modulation:One of the keys to the operation of HSDPA isthe 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
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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: 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.
Improved scheduling: Further advances have been made inthe area of scheduling. By moving more intelligence into thebase 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.
Additional channels: In order to be able to transport thedata in the required fashion, and to provide the additional
responsiveness of the system, additional channels have been
added which are described in further detail below.
Use of 16QAM within HSDPA
The rate control within HSDPA is achieved dynamically by
adjusting both the modulation and the channel coding. Both
16WAM and QPSK are used, the higher order 16QAM modulationbeing used to provide a higher data rate, but it also requires a better
Eb/N0 (effectively signal to noise ratio). As a result the 16QAM
modulation format is normally used under high signal conditions,
e.g. when the mobile is close to the NodeB and in the clear.
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The coding rate as well as the modulation are then selected for
each 2ms TTI by the NodeB according to its assessment of the
conditions. In this way the rate control mechanism can rapidly
track the variations that may occur.
HSDPA Hybrid ARQ and soft combining
Hybrid ARQ or HARQ is hybrid automatic repeat request and it is
essentially a form of the more common ARQ error correction
methodology. When the basic ARQ format is used, error-detection
information bits are added to data to be transmitted. One form of
this may be a cyclic redundancy check, CRC. However whenHybrid ARQ is used, forward error correction (FEC) bits are also
added to the existing error detection bits. The added error detection
means that Hybrid ARQ performs better than ordinary ARQ in
poor signal conditions, but the additional overhead can reduce the
throughput in good signal conditions.
The combination of Fast Hybrid ARQ and soft combining enables
the terminal to request the retransmission of data that may be
received erroneously. This can be done within the adaptive
modulation and channel coding scheme so that when error-rates
rise the link can be modified accordingly.
The user equipment or terminal receives the data and decodes it,
reporting back the result to the NodeB after the reception of each
block, and in this way rapid retransmission of any blocks with
errors can be undertaken. This significantly reduces delays,
especially under poor radio link conditions or when the link ischanging rapidly.
Soft combining is a process whereby the user equipment or
terminal does not discard information it cannot decode. Instead it
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retains it to combine with any retransmission data to increase the
chance of successful decoding of the data.
A process called Incremental Redundancy (IR) is also used with
the retransmissions. This process adds additional parity bits inretransmissions to make the data retransmission more robust.
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. Itimproves 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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HSDPA Channels
A number of new channels were added to the downlink within
HSDPA to provide the additional data capacity as well as the
control required. The new HSDPA channels are used in the
downlink in addition to the existing 3G UMTS channels.
High Speed Downlink Shared Channel (HS-DSCH)
The HS DSCH channel is the data transport channel that all active
HSDPA users connected to the NodeB will use. The use of a
shared channel is a key characteristic of HSDPA and being acommon resource, the HS-DSCH is dynamically shared between
users.
The HS-DSCH supports adaptive coding and modulation changing
to adapt to the changing conditions within the system. The use of
the 2ms TTI means that scheduling delays are reduced and it also
enables fast tracking of the channel conditions allowing for the
optimum use of the available resource.
It is worth noting that the HS-DSCH is not power controlled but
rate controlled. This allows the remaining power, after the other
required channels have been serviced to be used for the HS-DSCH,
and this means that the overall power available is used efficiently.
High Speed Signaling Control Channel (HS-SCCH)
This HSDPA channel is used to signal the scheduling to the users
every 2 ms according to the TTI. The channel carries three main
elements of information:
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It carries the UE identity to allow specific addressing ofindividual UEs on the shared control channel.
The HS-SCCH carries the Hybrid ARQ to enable thecombining process to proceed.
This channel carries the Transport Format and ResourceIndicator (TFRI). This identifies the scheduled resource and
its transmission format.
High Speed Dedicated Physical Control Channel(HS-DPCCH)
This HSDPA channel is used to provide feedback to the schedulerand it is located in the uplink. The channel carries the following
information:
Channel Quality Information which is used to provideinstantaneous channel information to the scheduler.
HARQ ACK/NAK information which is used to provideinformation back about the successful receipt and decoding
of information and hence to request the resending
information that has not been successfully received.
These channels are added to the existing 3G UMTS channels and
provide the additional data capability and adaptively required to
enable the much faster download speeds provided by 3G HSDPA.
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HSDPA UE categories and data rates
Within HSDPA a number of categories are defined. UEs with
different HSDPA categories will have different characteristics
including different HSDPA data rates.
These HSDPA categories are needed to cater for a number of
different implementations of the HSDPA standard. This allows for
different levels of performance to be implemented. The
characteristics of the UE can then be easily communicated to the
network which can then communicate with the UE in a suitable
manner. Accordingly these HSDPA categories are widely used.
HS-DSCH
category
Max No of
HS-DSCH
codes
Min inter-
TTI interval
Data rate
(Mbps)
Transport
block size
Max no
soft bits
Supported mo
schemes
1 5 3 3.6 7298 19200 16QAM, QPSK
2 5 3 3.6 7298 28800 16QAM, QPSK
3 5 2 3.6 7298 28800 16QAM, QPSK
4 5 2 3.6 7298 38400 16QAM, QPSK
5 5 1 3.6 7298 57600 16QAM, QPSK
6 5 1 3.6 7298 67200 16QAM, QPSK
7 10 1 7.2 14411 115200 16QAM, QPSK
8 10 1 7.2 14411 134400 16QAM, QPSK
9 15 1 10.1 20251 172800 16QAM, QPSK
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10 15 1 14 27952 172800 16QAM, QPSK
11 5 2 1.8 3630 14400 QPSK
12 5 1 1.8 3630 28800 QPSK
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The key specification parameters that are introduced by the use of
HSPA are:
Increased data rate: The use of HSUPA is able to provide asignificant increase in the data rate available. It allows peakraw data rates of 5.74 Mbps.
Lower latency: The use of HSUPA introduces a TTI of 2ms, although a 10ms TTI was originally used and is still
supported.
Improved system capacity: In order to enable the largenumber of high data rate users, it has been necessary to
ensure that the overall capacity when using HSUPA is
higher. BPSK modulation: Originally only BPSK modulation that
adopted for UMTS was used. Accordingly it did not support
adaptive modulation schemes. Higher order modulation was
introduced in Release 7 of the 3GPP standards when 64QAM
was allowed.
Hybrid ARQ: In order to facilitate the improvedperformance the Hybrid ARQ (Automatic Repeat request)
used for HSDPA is also employed for the uplink, HSUPA.
Fast Packet Scheduling: In order to reduce latency, fastpacket scheduling has been adopted again for the uplink as
for the downlink, although the implementation is slightly
different.
With these specification parameters enable HSUPA to complement
the performance of HSDPA, providing an overall performance
improvement for systems incorporating HSPA.
3G HSUPA basics
At the core of HSUPA, High Speed Uplink Packet Access is a
number of new technologies that are very similar to those used
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with HSDPA. However there are a few fundamental differences
resulting from the different conditions at either end of the link.
The uplink in UMTS, and HSUPA is non-orthogonal becausecomplete orthogonally cannot be maintained between all theUEs. As a result there is more interference between the
uplink transmissions within the same cells.
The scheduling buffers are located in a single location(NodeB) for the downlink, whereas for the uplink they are
distributed within several UEs for the uplink. This requires
the UEs requiring to send buffer information to the scheduler
in the NodeB so that it can then provide an overall schedule
for the data transmission. In the downlink, the shared resource is the transmission
power. In the uplink, the resource is limited by the level of
interference that can be tolerated and this depends upon the
transmission power of the multiple UEs.
High order modulation techniques are able to provide higherdata rates for high signal level links in the downlink. There is
not the same advantage in the uplink where as there is no
need to share canalization codes between users and the
channel coding rates are therefore lower, although higherorder modulation was introduced under Release 7.
HSUPA summary
HSUPA is an integral element of the overall HSPA upgrade for
UMTS. HSUPA enables much higher data rates to be achieved in
the uplink and thereby provides a significant improvement. As aresult, the overall UMTS system is able to support data rates more
in line with those being achieved by wired connections.
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HSUPA Category Definitions and Data
Rates
In order to be able to cater for a number of variations in the level towhich HSUPA is implemented, a number of different HSUPA
categories have been defined. These HSUPA categories allow for
different levels of performance within the UE. The characteristics
of the UE can then be easily communicated to the network which
can then communicate with it in a suitable manner.
HSUPAcategory
number
Maximumnumber E-
DPDCHs
Minimumspreading
factor
Supportfor 2 ms
TTI*
Maximumtransport
block size
(10 ms TTI)
Maximumtransport
block size
(2 ms TTI)
Maximumdata rate
(Mbps)
1 1 SF4 - 7110 - 0.7 Mbps
2 2 SF4 Y 14484 2798 1.4 Mbps
3 2 SF4 - 14484 - 1.4 Mbps
4 2 SF2 Y 20000 5837
2 Mbps for
10 ms TTI
2.9 Mbpsfor 2 ms
TTI
5 2 SF2 - 20000 - 2.00 Mbps
6 2 + 2** SF2 Y 20000 11520
2 Mbps for10 ms TTI
5.74 Mbps
for 2 msTTI
HSUPA category definitions
Notes:
*A 10 ms TTI is supported in all categories
** Two E-DPDCHs at SF2 and two at SF4
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3G HSUPA channels
In order to provide the required data speeds and capabilities withinHSUPA, further channels have been added to the basic 3G UMTS
scheme that is used. These HSUPA channels provide additional
signaling and data capabilities.
While HSUPA is effectively an uplink enhancement, channels
have been added to both the uplink and the downlink. The reason
for the downlink HSUPA channels is to provide the control, etc
needed for the uplink data.
Uplink HSUPA channels
A variety of new channels have been introduced for HSUPA to
enable the system to carry the high speed data. These new channels
are:
E-DCH, the Enhanced Dedicated Channel: Uplink E-DPCCH (Enhanced Dedicated Physical Control
Channel): Uplink
E-DPDCH (Enhanced Dedicated Physical Data Channel):Uplink
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Downlink HSUPA channels
A variety of new channels have been introduced for HSUPA to
enable the system to carry the high speed data. These new channels
are:
E-AGCH (Enhanced Absolute Grant Channel): Downlink F-DPCH (Fractional-Dedicated Physical Channel):
Downlink
E-HICH (Enhanced DCH Hybrid ARQ IndicatorChannel): Downlink
E-RGCH (Enhanced Relative Grant Channel): Downlink
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HSPA+ data rate comparison with LTE
The next migration of the cellular services beyond HSPA+ is
known as LTE. Using a completely new air interface based around
the use of OFDM rather than W-CDMA which is used for UMTS,
HSPA and HSPA+, it offers even higher data traffic rates. It is then
anticipated that it will be used as the basis for the next generation,
i.e. 4G systems.
It is however worth comparing the maximum data rates offered by
both HSPA+ and LTE.
Channel
Bandwidth
(MHz)
HSPA+ data rate
(Mbps)
LTE data rate
(Mbps)
5 42 37
10 84 73
20 -- 150