High Speed Downlink Packet Access (HSDPA): A Tutorial

By Zahid Ghadialy

Introduction:
HSDPA is a major Release 5 feature. Some of the enhancements will be done in Release 6. HSDPA is designed to support data rates upto 10.8 Mbps and at the same time it will co-exist with R99 in the same frequency band of 5MHz. As could be observed this data rate is much higher than the Broadband speeds people generally use in their homes (512Kbps download). HSDPA will be able to satisfy the most demanding Multimedia applications.

The peak rate offered by HSDPA is about 10Mbps in 5MHz channel. One important thing to note is that the more important thing here is not the peak rate but the throughput capacity which increases significantly. This leads to more users being able to use high data rates on a single carrier.

Whats New in HSDPA:

3 Slot TTI (2ms)

CQI: Channel Quality Indication

AMC: Adaptive Modulation and Coding

Constellation Re-Arranging (16 QAM)

H-ARQ: Hybrid Automatic Repeat Request

N-channel SAW: N-Channel Stop and Wait Protocol

New MAC (High Speed) instance

New Stuff for HSDPA:
The implementation of HSDPA will require changes mainly in Physical layer. Layer 2 will be affected as well because new high speed MAC entity will be required. The impact to other layers will be minimum. The following sections will throw some light on some of the new features required for HSDPA.

HSDPA Physical and Transport Channels:
To support HSDPA new Physical channels have been defined:

HS-PDSCH or High Speed Physical Downlink Shared Channel: This is a downlink channel which is both time and code multiplexed. The channelisation codes have a fixed spreading factor, SF = 16. Multi-code transmissions are allowed that translates to UE being assigned multiple channelisation codes in the same TTI, depending on the UE capability. The same scrambling code sequence is applied to all the channelisation codes that form the single HS-DSCH CCTrCH. If there are multiple UE's then they may be assigned channelisation codes in the same TTI (multiplexing of multiple UE's in the code domain).
HS-DPCCH or High Speed Dedicated Physical Control Channel: This is an uplink channel that carries the Acknowledgements of the packet received on HS-PDSCH and also the CQI (Channel Quality Indication). THE CQI estimates have to be transmitted by the UE every 2.0 ms frame. This information is very important as it ensures reliability and impacts power capacity.
HS-SCCH or High Speed Shared Control CHannel: The HS-SCCH is a fixed rate (60 kbps, SF=128) downlink physical channel used to carry downlink signalling related to HS-DSCH transmission. This provides timing and coding information thus allowing the UE to listen to the HS-DSCH at the correct time and using the correct codes to allow successful decoding of UE data.

The main features of the physical channel are as follows:

Fixed Spreading Factor of 16 for HS-DSCH

QPSK and 16 QAM Modulation

Static TTI Length of 3 Time Slots = 2ms

Fixed CRC of 24 bits

Error Correction using 1/3 Turbo Coding


To support HSDPA the following new Transport channels have been defined:
HS-DSCH or High Speed Downlink Shared channel: The High Speed Downlink Shared Channel is a downlink transport channel shared by several UEs. The HS-DSCH is associated with one downlink DPCH, and one or several Shared Control Channels (HS-SCCH). The HS-DSCH is transmitted over the entire cell or over only part of the cell using e.g. beam-forming antennas.

Adaptive Modulation and Coding (AMC):

HSPDA standard ensures that highest possible data rate is acheived for all users regardless of whether they are close to the base station or far off. This is done using ACM. For HS-DSCH, the transport format, including the modulation scheme and code rate, can be selected based on the downlink channel quality. The selection of transport format is done by the MAC-HS located in Node B and is based on channel quality feedback reported by the UE. The spreading factor cannot change but the coding rate can change between 1/4 and 3/4. The higher coding rate reduces the number of errors. Also the standards support multicodes. This means that upto 15 codes can be allocated to a UE.

Hybrid Automatic Repeat Request (HARQ):

In case of ARQ, the receiving system on receipt of data checks the CRC. If the CRC is the same as that received in the message ACK is sent back to the sender. In case if CRC does not match then NACK is sent back and the packet discarded. In case of HARQ, this method of CRC checking is improved based on the following two things.

Chase Combining:In this when an error is detected in CRC, NACK is sent back but the packet is not discarded. It is stored. In case the re-transmitted packet is again erroneous then the previous and current poacket is combined in an attempt to recover from errors. Each time the packet is resent, the same scheme is applied. Eventually the error will be either resolved or maximum number of retries is reached. In that case higher layer protocols will deal with the error.

Incremental Redundancy (IR):IR is similar to Chase combining but the redundant information that was not transmitted earlier is also included to improve the chances of reception without errors or with enough errors removed so as to allow combining with the previously stored packet and resolve the errors.

Fast Cell Site Selection (FCSS):

When the UE moves between the cells, it is possible that it would be served by different cells. Hence the UE will construct a list of Active Set (the term Active Set is incorrect and the term that will be used eventually is "Eligible Set") Cells that it can use at any one time. The mobile will indicate on HS-DPCCH as to which one is the best cell for DL transmission. The serving cell then decides the modulation and coding scheme to be used for the mobile and in addition may code multiplex multiple mobiles within that HSDPA frame. To Simplify this procedure, it is further subdivided into Intra-Node B FCS and Inter-NodeB FCS.


REFERENCES:

[1] 3GPP TR 25.855 - HSDPA; Overall UTRAN Description; Release 5

[2] 3GPP TR 25.858 - HSDPA; Physical Layer Aspects; Release 5

[3] 3GPP TR 29.950 - UTRA High Speed Downlink Packet Access; Release 4

[4] UMTS Evolution to High Speed Downlink Packet Access - Douglas McCarthy

[5] UMTS - High Speed Downlink Packet Access - Rohde & Schwarz

[6] 3GPP TR 25.877 - HSDPA: Iub/Iur Protocol Aspects

[7] 3GPP TR 25.899 - HSDPA Enhancements; Release 5

[8] Nokia: HSDPA Solution White Paper

[9] 3GPP TS 25.211 - Physical Channels and mappin of Transport Channels onto Physical Channels; Release 5

[10] Motorola: Fast Cell Selection and Handovers in HSDPA















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