3G HSDPA High Speed Downlink Packet Access provides additional capability to the basic 3G UMTS cellular telecommunications system.
HSDPA was the first upgrade along the path to HSPA which enabled high speed data to be carried in both directions. However as much more data was carried in the downlink direction, HSDPA was standardised and implemented first to provide the maximum benefit as soon as possible.
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.
- 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.
- 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.
- 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.
- 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.
HSDPA 16QAM use
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.
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 provided a significant improvement in performance for 3G. With peak user data rates of around 10 Mbps and peak raw data rates of 14.4 Mbps, the system gave a marked improvement over what was available with basic 3G UMTS. When combined with HSUPA and other HSPA upgrades, the system was able to provide performance that rivalled that of the next generation networks.
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