3G UMTS HSPA - High Speed Packet Access Tutorial- UMTS HSPA, High Speed Packet Access, combines HSDPA and HSUPA for uplink and downlink to provide high speed data access.

3G HSPA, High Speed packet Access is the combination of two technologies, 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.

HSPA features

The system provides an enhancement on the basic 3G WCDMA / UMTS cellular system, providing data transfer rates that are considerably in excess of those originally envisaged for 3G as well as much greater levels of spectral efficiency.

Note on 3G UMTS / W-CDMA:

UMTS - Universal Mobile Telecommunications System is a 3G cellular system that uses Wideband CDMA, WCDMA as the format for the radio transmission. Its aim was to provide high speed data at much higher speeds than was previously possible. The basic system provided for speeds of 2 Mbps in the downlink and 384 kbps in the uplink.

Click for a UMTS / WCDMA tutorial

The system provides many advantages for users over the original UMTS system.

As the 3GPP standards evolved, so did the performance available.

3G HSPA benefits

The UMTS cellular system as defined under the 3GPP Release 99 standard was orientated more towards switched circuit operation and 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 HSPA were incorporated into many UMTS networks to enable them to operate more in the manner required for current applications.

HSPA provides a number of significant benefits that enable the new 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 the system offers many other improvements as well:

1. Use of higher order modulation: 16QAM is used in the downlink 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 rates quoted 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 a shorter TTI 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 resources enables greater levels of efficiency to be achieved and integrates with IP and packet data concepts.

4. Use of link adaptation: By adapting the link it is possible to maximize the channel usage.

5. Fast Node B scheduling: The use of fast scheduling 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 to provide 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, this is 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 its use brings significant benefits to user and operator alike.

3G UMTS HSPA constituents

There are two main components to 3G UMTS HSPA, each addressing 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 summarised below:

HSDPA - High Speed Downlink Packet Access: HSDPA provides 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. Read more about High speed downlink packet access, HSDPA

HSUPA - High Speed Uplink Packet Access: HSUPA provides improved uplink packet support, reduced delays and a peak raw data rate of 5.74 Mbps. This results in a capacity increase of around twice that provided by the Release 99 services. Read more about High speed uplink packet access, HSUPA

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 42 Mbps 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.

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 were introduced at successive releases of the 3GPP standard.

Release 4: This release of the 3GPP standard provided for the 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, a raw 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 an enhanced uplink with improved packet data support. This 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 broadcast services, i.e. Mobile TV.

Release 7: This release of the 3GPP standard included downlink 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 occurred during the course of 2008 and it 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.

Release 9: 3GPP Release 9 occurred during 2009 and included facilities for HPSA including 2x2MIMO in the uplink and a 10MHz bandwidth in the downlink. The uplink carriers may be from different bands.

Release 10: HSPA Release 10 utilises up to 4-carriers, i.e. 20 MHz bandwidth which may be from two separate bands. In addition to this 2x2 MIMO in the downlink provides data rates up to 168 Mbps. This figure equates to that obtained for LTE Release 8 when using comparable bandwidth and antennas configurations.

Release 11: Release 11 occurred during 2011 / 2012. It provided the facility for 40MHz bandwidth in the uplink along with up to 4x4 MIMO. The downlink was upgraded to accommodate 64-QAM modulation and MIMO.

Release 12: This 3GPP release is occurring in 2013 / 2014.

3G HSPA is able to provide very high speed data transmission, competing with the top performance of LTE and LTE-A. While its spectral efficiency is not as high, it is nevertheless a considerable improvement on previous systems.

3G UMTS HSDPA - High Speed Downlink Packet Access Tutorial- 3G UMTS HSDPA, High Speed Downlink Packet Access, provides the high speed downlink for HSPA. Using new data channels it enables speeds up to 14.4 Mbps to be provided.

3G HSDPA High Speed Downlink Packet Access is an upgrade to the 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 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: 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 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.

Additional channels: In order to be able to transport the data 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 modulation being 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.

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 when Hybrid 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 is changing rapidly.

Soft combining is a process whereby the user equipment or terminal does not discard information it cannot decode. Instead it 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 in retransmissions 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. 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.

HSDPA Channels- an overview of the HSDPA channels including HS-DSCH that are used to carry the high speed data in the HSPA downlink.

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 a common 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:

It carries the UE identity to allow specific addressing of individual UEs on the shared control channel.

The HS-SCCH carries the Hybrid ARQ to enable the combining process to proceed. This channel carries the Transport Format and Resource Indicator (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 scheduler and it is located in the uplink. The channel carries the following information:

Channel Quality Information which is used to provide instantaneous channel information to the scheduler.

HARQ ACK/NAK information which is used to provide information 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 adaptivity required to enable the much faster download speeds provided by 3G HSDPA.

HSDPA UE categories and data rates- an overview or summary of the HSDPA UE categories, their definitions and details including the HSDPA data rates.

The 3rd Generation Partnership Project (3GPP) has divided HSDPA UEs or mobile terminals into twelve categories. These HSDPA categories define the different characteristics including different HSDPA data rates.

These HSDPA categories are needed to cater for a number of implementations of the HSDPA standard. This allows for different levels of performance to be implemented including the maximum HSDPA data rate. 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.

HSDPA category definitions

The different HSDPA categories are outlined in the table below. From this it can be sent that the overall raw data rate and hence the category is determined by a number of elements including the maximum number of HS-DSCH codes, TTI, block size, etc.

Additional information about the HSDPA categories and HSDPA data rates table:

The maximum number of HS-DSCH codes is the number of multiplexed codes on the high-speed physical downlink shared channel (HS-PDSCH) that receive data.

The minimum transmission time interval (TTI) is the time interval allocated to the mobile terminal for receiving data. There is a balance between a short TTI for providing adaptively and while keeping the overhead to a minimum.

The maximum buffer size for hybrid automatic repeat request (H-ARQ) is determined considering signals received prior to resend and resent signals; it is the maximum number of bits in the receive buffer after demodulation.

Categories 1-10 support 16QAM (16 quadrature amplitude modulation) as well as QPSK (quadrature phase shift keying). Categories 11 and 12 only support QPSK.

3G HSUPA - High Speed Uplink Packet Access- a tutorial, description or information about the basics of 3G UMTS HSUPA, High Speed Uplink Packet Access, an integral part of HSPA.

High Speed Uplink Packet Access, HSUPA is the companion technology to HSDPA, but applied to the uplink from the UE or user equipment to the NodeB or base station. HSUPA uses many similar technologies to those found in HSDPA, but in view of the differences between the links, HSUPA is not identical.

HSUPA provides a considerable increase in speed for users in the uplink. Although lower data rates are normally required in the uplink direction, emails, two way data traffic and other uploads do require higher speeds than are often available with the basic 3G system, and accordingly the inclusion of HSUPA gives a significant improvement.

Although HSUPA provides a significant increase in the upload speed, it does not provide the same capacity as the downlink because in general the majority of the data flows in the downlink direction, i.e. towards the UE. In addition to this there are additional difficulties providing the same performance from the UE in view of some of the restrictions imposed by the fact that a large number of UEs are communicating with the NodeB.

3G UMTS HSUPA key characteristics

3G HSUPA brings enhanced performance through the addition of new features that sit on top of the existing UMTS / W-CDMA technology.

The key specification parameters that are introduced by the use of HSPA are:

Increased data rate: The use of HSUPA is able to provide a significant increase in the data rate available. It allows peak raw data rates of 5.74 Mbps.

Lower latency: The use of HSUPA introduces a TTI of 2 ms, although a 10ms TTI was originally used and is still supported.

Improved system capacity: In order to enable the large number 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 improved performance the Hybrid ARQ (Automatic Repeat reQuest) used for HSDPA is also employed for the uplink, HSUPA.

Fast Packet Scheduling: In order to reduce latency, fast packet 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 are a number of new technologies that are very similar to those used 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 because complete orthogonality cannot be maintained between all the UEs. 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 higher data 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 channelisation codes between users and the channel coding rates are therefore lower, although higher order modulation was introduced under Release 7.

HSUPA Category Definitions and Data Rates - a summary of the different HSUPA categories, the HSUPA category definitions and the different data rates.

In order to be able to cater for a number of variations in the level to which HSUPA is implemented, a number of different HSUPA categories have been defined. These HSUPA categories are equivalent in function to those used on the downlink, althought he actual parameters and speeds are naturally different.

The 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.

HSUPA category definitions

The HSUPA categories are detailed in the table below. This shows the different HSUPA categories with their data rates, and other required parameters.

Notes:*A 10 ms TTI is supported in all categories** Two E-DPDCHs at SF2 and two at SF4

Support for the E-DCH TTI (Transmission Time Interval) of 10 ms is required for all HSUPA categories. It is only some HSUPA categories that support a 2 ms TTI. Also the highest data rate supported with a 10 ms TTi is 2 Mbps. The reason for this is to limit the amount of buffer

memory required in the NodeB for soft combining because a larger block transport size means that a larger soft buffer is needed for retransmissions.

3G HSUPA channels- a tutorial, description or information about the basics of 3G UMTS HSUPA, High Speed Uplink Packet Access, an integral part of HSPA.

In order to provide the required data speeds and capabilities within HSUPA, further channels have been added to the basic 3G UMTS scheme that is used. These HSUPA channels provide additional signalling 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: This HSUPA uplink channel carries on block of data for each TTI (Transmission Time Interval). The E-DCH can be configured simultaneously with one or more DCHs. In this way high speed data transmission can occur at the same time and on the same UE as services that use the standard DCH.

As a low latency (delay) is one of the key requirements for the high speed uplink a short TTI (Transmission Time Interval) of 2 ms is supported in addition to one of 10 ms. The short TTI allows for rapid adaptation of transmission parameters and it reduces the end-user delays.

There is a balance to be determined for the TTI. It is found that the physical layer processing is proportional to the amount of data to be processed, and accordingly the shorter the TTI the lower the level of data per TTI. However for applications requiring relatively low data rates, the overheads required with a 2 ms TTI may be unduly high. In these circumstances a longer TTI is more appropriate.

The E-DCH is mapped to a set of E-DCH Dedicated Physical Data Channels. E-DPDCH (Enhanced Dedicated Physical Data Channel): This HSUPA uplink channel carries

uplink user data. Each UE can transmit up to four E-DPDCH channels at a spreading factor of SF256 to SF2. The number of E-DPDCHs s and their spreading factors are varied according to the

instantaneous data rate required. As an example of a typical scenario, to achieve a 2 Mbps rate - the raw data rate of early devices - two SF2 E-DPDCHs were required.

E-DPCCH (Enhanced Dedicated Physical Control Channel): This HSUPA channel carries the control data required by the Node B to decode the uplink channels including the E-DCH Transport Format Combination Indicator which indicates the block size, retransmission sequence number, etc.

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): This HSUPA channel provides the absolute limit of the power resources, i.e. the serving grant, that the UE may use. The channel is used to send scheduling grants from the scheduler to the UE to control when and what data rate the UE should be used. The E-AGCH is only sent by one NodeB regardless of the number that the UE is communicating with. The NodeB used is the one that has the main responsibility for the scheduling operation. The E-AGCH is typically used for large changes in data rate.

E-RGCH (Enhanced Relative Grant Channel): This channel is used to move the UE serving grant up, down or remain the same. This HSUPA channel is generally used for relatively small changes during an ongoing data transmission. Large changes are handled by the E-AGCH.

E-HICH (Enhanced DCH Hybrid ARQ Indicator Channel): This HSUPA channel is used to provide the acknowledgement of the UE data received by the Node B.

Evolved HSPA / HSPA+ - a tutorial or overview of the basics of 3G HSPA+ also called Evolved HSPA or HSPA Evolution.

Evolved HSPA is also known as HSPA+ HSPA Evolution and even Internet HSPA (I-HSPA). By its name it can be seen that Evolved HSPA is an enhanced version of the 3G HSPA or High Speed Packet Access system that was used to increase the speeds of the basic 3G system. Using Evolved HSPA / HSPA+ the data transfer rates are enhanced further over those that could be achieved using HSPA and other factors such as latency and the backhaul have also been addressed.

The need for HSPA+ arose out of the increasing use of data and users wanting download speeds that were comparable with fixed broadband lines. Many other applications were also starting to need much faster data transfer rates and lower levels of latency. These are addressed by the use of HSPA+.

HSPA+ in 3GPP releases

The definition of HSPA+ / Evolved HSPA have been included in Releases 7 and 8 of the 3GPP standards.

3GPP Release 7: This release of the 3GPP standard included downlink 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 of more users that are in a "continuously on" state.

3GPP 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.

Evolved HSPA / HSPA+ highlight features

There are a number of major new features as well as some enhancements to existing capabilities that enable HSPA+ or Evolved HSPA to provide a significant improvement in performance over that provided by the standard HSPA systems.

Some of the major features include:

MIMO: many other systems have utilised MIMO and so too, HSPA+ is able to gain significant advantages from its use.

Higher Order Modulation: Although MIMO provides some significant improvements in throughput, where the multiple antennas needed for MIMO are not available, and where signal strength is relatively high, it is possible to increase the order of the modulation to enable higher throughput rates. However this can only be achieved when signal levels are sufficiently high otherwise data errors increase.

Continuous packet connectivity: With much of the data traffic being in the form of IP data, continuous connectivity is an increasing requirement. To achieve this the HS-DSCH and E0DCH channels have been reconfigured to enable them to be rapidly able to transmit user data.

Enhanced CELL_FACH operation: This enhanced operation is required to assist in maintaining the always-on packet connectivity during periods when there have been little or no activity.

Layer 2 protocol enhancements: In order to benefit from the higher data rates over the HS-DSCH enhancements to the RLC and MAC-hs protocols have been introduced.

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.

Although the basic comparisons appear to show that LTE will offer few advantages, there are several other features of LTE that mean that it is a preferable option for the long term. LTE enables wider bandwidths and the OFDM modulation enables data transmissions to be made more resilient to multipath and other propagation effects.

Evolved HSPA MIMO- notes or overview on the UMTS Evolved HSPA MIMO and HSDPA-MIMO used to provide significant improvements in data rate by utilising multipath propagation.

MIMO is one of the major features that have been incorporated into Evolved HSPA / HSPA+.

UMTS HSPA MIMO enables the peak data rates achievable to be increased through the use of what may be termed multi-stream transmission.

Evolved HSPA MIMO basics

MIMO used with HSPA+ is able to provide significant increases in data throughput. With data throughput being one of the key advantages of Evolved HSPA, MIMO is a significant element of the overall system.

Note on MIMO:

Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon's Law. MIMO provides a way of utilising the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.

Click on the link for a MIMO tutorial

Traditionally radio systems wanted to minimise the effects of multipath transmission as the multiple paths introduced phase delays that caused distortion and interference. However MIMO exploits the multiple paths in ways that enable the system to become more resilient to interference or to enable high data throughput rates by using spatial multiplexing.

MIMO is able to achieve high data rates - above those predicted by Shannon for a single channel because it is able to utilise the multiple paths to transmit multiple data streams in parallel. However to achieve these high data rates, a correspondingly high carrier to interference ratio must exist for the receiver. This means that spatial multiplexing is really applicable to small cells or larger ones where the receiver, in this case the mobile handset is relatively close to the base station.

In cases where a sufficiently high signal to noise ratio for spatial multiplexing cannot be achieved, the multiple antennas can be used to give receive diversity to improve the reception of a single data stream.

HSDPA MIMO

One of the main areas where the increase in data is required is within the downlink. For this the MIMO capability is applied to the HSDPA elements of the signal. The scheme used for HSDPA-MIMO is sometimes referred to as dual stream transmit adaptive arrays - there may be up to two streams of data. HSDPA MIMO is a multi-codeword scheme that uses rank adaptation and pre-coding.

The two streams of data within HSDPA-MIMO are subject to the same physical layer processing, spreading, etc. The same channelisation codes can used to save on channelization code resources.

After this has been completed, linear pre-coding is applied to the signal before the resulting signals are applied to the two antennas. The linear pre-coding attempts to make the two signals

nearly orthogonal to each other. This reduces the level of interference between the two signals and also reduces the level of receiver processing required.

In order to support dual stream transmission format of HSDPA-MIMO, the HS-DSCH is modified to support up to two transport blocks per TTI - one transport block per stream.

The use of pre-coding provides several benefits:

It is important in the case of single stream transmission where pre-coding provides both diversity gain and array gain. Two antennas are used at the transmitter and weights of the two streams adjusted so that the signals add coherently at the receiver.. This results in a higher signal to noise ratio and thereby increases the coverage area for a particular data rate.

In the case of two stream transmission pre-coding is used as to enable the receiver to separate the two data streams. If the pre-coding vectors are chosen to be orthogonal, then the two streams will not interfere.

In order to be able to demodulate the signals, the UE needs to be able to estimate the channel characteristics between all the antennas on the base station and those on the UE. This is achieved by transmitting common pilot channels on each physical transmit antenna. By decoding the pilot channel the UE is able to estimate the channel characteristics.

HSDPA-MIMO rate control

Rate control for HSDPA MIMO is very similar to the rate control used for the standard non-MIMO case. However the system needs know how many streams are being transmitted and also the pre-coding matrix being used. The rate control mechanism needs to determine the number of streams to transmit, the modulation scheme, and the pre-coding matrix. This information is transmitted to the UE on the HS-SCCH.

The multi-stream transmission provided by HSDPA-MIMO is only really beneficial when the signal channel is good, i.e. a high signal to noise ratio. It is therefore only used for the highest data rates. For lower data rates single stream transmission is generally used with the two transmission antennas providing diversity transmission to improve the fading, signal quality, etc.

Dual carrier HSPA: DC-HSPA, DC-HSDPA- notes or tutorial of the basics of Dual carrier HSPA, DC-HSPA which utilises two carriers on the downlink - DC-HSDPA, or Dual carrier HSDPA.

To further improve the HSPA performance a scheme utilising two HSDPA carriers to increase the peak data rates has been made available. The scheme known under a variety of names and acronyms - DC-HSPA, Dual carrier HSPA, Dual Cell HSPA, and DC-HSDPA, Dual cell

HSDPA, also better utilises the available resources by multiplexing carriers in the CELL DCH state.

DC-HSPA or DC-HSDPA enables better utilisation of the resources, especially under poor channel conditions where signal to noise ratios may not be as high as normally needed for high data rate links.

DC-HSPA / DC-HSDPA background

UMTS / W-CDMA was initially conceived as a circuit switched based system and was not well suited to IP packet based data traffic. Once the basics UMTS system was released and deployed, the need for better packet data capability became clear, especially with the rapidly increasing trend towards Internet style packet data services which are particularly bursty in nature.

The initial response to this was the development and introduction of HSDPA, followed by HSUPA for provide the combined HSPA service. These were defined in 3GPP Release 5 & 6. Later this was further developed and deployed in some areas to provide even higher data transfer rates as HSPA+ which occurred in Release 7.

A further release, Release 8 detailed the dual cell HSDPA, or HSPA, and then a combination of DC-HSDPA and MIMO being defined in Release 9.

DC-HSPA / DC-HSDPA basics

The concept behind DC-HSPA / DC-HSDPA is to provide the maximum efficiency and performance for data transfers that are bursty in nature - utilising high levels of capacity for a short time. As most of the traffic is in the downlink direction, dual carrier HSPA is applied to the downlink - i.e. HSDPA elements, and therefore dual carrier HSPA is also known as DC-HSDPA.

The concept of packet data is that it data is split into packets with a destination tag, and these are sent over a common channel - sharing the channel as data traffic from one source is not there all the time.

DC-HSDPA seeks to take apply this principle to the multiple carriers that may be available to an operator. Often UMTS licences are issued in paired spectrum of either 10 MHz or 15 MHz blocks - two or three carriers, for uplink and downlink.

Using UMTS, HSPA, or even HSPA+ these carriers operate independently, and dependent upon the usage, one carrier could be fully utilised while the other is under used. Coordination between the carriers only takes place in terms of the connection management, and the dynamic load is not balanced. DC-HSDPA / DC-HSPA seeks to provide resource allocation and optimisation.

This joint resource allocation over multiple carriers requires dynamic allocation of resources to achieve the higher peak data-rates per HSDPA user within a single Transmission Time Interval (TTI), as well as enhancing the terminal capabilities. The use of DC-HSDPA is aimed at providing a consistent level of performance across the cell, and particularly at the edges where MIMO is not as effective.

Channels for DC-HSDPA

When implementing DC-HSDPA, the channels present within the system need to be modified to enable the system to operate as required.

HS-DPCCH: While it would have been possible to utilise two HS-DPCCHs, one on each carrier, only one is used - the feedback information being mapped to the single channel. There are either 5 or 10 CQI - channel Quality Indicator bits that are used. Five are used when only one channel is utilised, and ten when two are in use. The compound CQI is made up from two independent CQIs: one for each channel. New channel coding schemes are defined for the overall HARQ feedback format.

HS-SCCH: The HS-SCCH is transmitted on both the anchor, or primary carrier as well as the supplementary one, and the UE has to monitor up to four HS-SCCH codes on each carrier. However the UE is only required to be able to receive up to one HS-SCCH on the serving or main cell and one HS-SCCH on the secondary cell.

DC-HSDPA signalling & scheduling

One of the key processes required within DC-HSDPA is that of scheduling the data to be transmitted as this has to be achieved across the two carriers. The scheduling algorithms required developing in a manner that provided backwards compatibility for single carrier transmissions while providing throughput speed improvements for the dual carrier scenarios.

The queues for data to be transmitted are operated in a joint fashion to provide the optimum flexibility in operation - it enables the carrier with the least traffic queued to be used (not all UEs will have the dual carrier facility and therefore one carrier may be loaded more heavily than the other, etc..)

One area which did require addressing was the operation of the MAC-ehs entity within the Node-B stack. Within HSPA this was designed to support HS-DSCH operation in more than one cell served by the same Node-B and therefore extending this for dual carrier operation required only minor changes.

Separate HARQ entities are required for each HS-DSCH. In this way the transmission is effectively two separate transmissions over two separate HS-DSCHs - each one has its own uplink and downlink signalling.

Each carrier has a transport block that uses a Transport Format Resource Combination (TFRC) which is based on the HARQ and CQI feedback sent over the uplink HS-DPCCH. Any retransmissions required by HARQ will use the same modulation coding scheme as the first transmission.

UE categories for DC-HSPA

UE categories were developed to enable the base stations to be able to quickly determine the capabilities of different UEs. The numbers required extending for HSPA+ and DC-HSPA / DC-HSDPA.

DC-HSUPA and Multicarrier HSPA

The concepts behind DC-HSDPA can be taken further in a number of areas to provide further improvements in the performance of the overall HSPA+ system.

The first of these is to utilise a similar dual carrier system for the uplink. Using dual carrier HSUPA, DC-HSUPA, would provide similar gains in the uplink as DC-HSDPA provides for the downlink. The broad implementation would also be similar.

Another way in which performance of the system can be further pushed is to utilise multiple carriers, beyond the two used in DC-HSPA. Y aggregating further carriers the improvements gained with DC-HSPA can be further improved along higher still peak data rates.