With data usage increasing greater levels of upgrade to the HSPA upgrade for 3G UMTS were introduced to further increase the data throughput.
The technique has a variety of names including DC-HSPA, Dual carrier HSPA, Dual Cell HSPA, and DC-HSDPA, Dual cell HSDPA.
Essentially DC-HSPA improves the utilisation of the available resources by multiplexing carriers in the CELL DCH state.
The DC-HSPA configuration makes much better use of the available resources and provides significantly improved performance under low signal conditions where normally it would be necessary to reduce the data rate by increasing the error correction and reducing the modulation order.
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.
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-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.
- 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.
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 HSPA UE Categories|
|UE category||3GPP Release||Max No of HS-DSCH Codes||Modulation||Maximum raw data rate||Comments|
|23||Rel 8||15||64 QAM||35.3||DC-HSDPA|
|24||Rel 8||15||64 QAM||42.2||DC-HSDPA|
|25||Rel 9||15||16 QAM||46.7||DC-HSDPA + MIMO|
|26||Rel 9||15||16 QAM||55.9||DC-HSDPA + MIMO|
|27||Rel 9||15||64 QAM||70.6||DC-HSDPA + MIMO|
|28||Rel 9||15||64 QAM||84.4||DC-HSDPA + MIMO|
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. By aggregating further carriers the improvements gained with DC-HSPA can be further improved along higher still peak data rates.
Dual carrier DC-HSPA provided some significant improvements for HSPA. It was one of the last major enhancements to 3G UMTS HSPA before operators migrated over to 4G LTE. It enabled very high levels of performance to be achieved with comparatively little operator investment as the infrastructure was already in place.
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