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.

By Ian Poole

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