AUBRAUX

HSDPA (High Speed Downlink Packet Access)

(A simplistic view of HSDPA architecture and its applications)

By Bharathi Athinarayanan
Support Engineer,
Anritsu Corporation, India

Introduction & Motivation behind HSDPA:

High speed downlink packet access (HSDPA) is the new technology which is introduced in 3GPP Release 5. As the name suggests, HSDPA will enable the user to achieve high data rates in the downlink while on the move.

Primary goal of HSDPA is to enhance system throughput with minimum changes in network architecture,
Need of higher bit rates for sophisticated UE applications,

HSDPA Vs current UMTS system:

Various methods for packet data transmission in WCDMA downlink already exist in Release'99. The three different channels in Release'99/ Release 4 WCDMA specifications that can be used for downlink packet data are:

Dedicated Channel (DCH)
Downlink-shared Channel (DSCH)
Forward Access Channel (FACH).

The basic requirements for HSDPA are to carry high data rate in the downlink. The HSDPA technology will:

Increase the UTRAN network capacity
Reduce the round trip delay
Increase the peak data rates up to 14 Mbps

To achieve the above current UMTS architecture is maintained and some other features or functionalities are added on top of the existing architecture.

So the question arises is that to implement HSDPA which new features comes in, what goes out from the existing UMTS and what is added onto it.

Physical Channel Changes

To support HSDPA, new physical channels have been added to the UMTS specification:

High Speed Physical Downlink Shared Channel (HS-PDSCH) is the transport mechanism for the new HSDPA logical channels. This channel will be both time and code shared between users attached to a Node-B.
High Speed Dedicated Physical Control Channel (HS-DPCCH) is an uplink channel that carries packet acknowledgment signaling for each transport block and a Channel Quality Indicator (CHI) used by the Node-B to perform AMC.

Logical Channel Additions:

HS-DSCH - High Speed Downlink Shared Channel - provides the logical transport mechanism for data transfer. This is the transport channel carrying the user data with HSDPA operation. It is also known as downlink "fat pipe".

HS-SCCH - High Speed Shared Control Channel - provides timing and coding information to the User Equipment (UE). This allows the UE to listen to the HS-DSCH at the correct time and using the correct codes to allow successful decoding of received data.

With HSDPA two fundamental features of WCDMA are disabled which is:

Variable SF and
Fast Power Control

These two features are replaced by:

Adaptive Modulation and Coding (AMC)
Fast retransmission strategy (HARQ)
Scheduling Algorithm

Thus the comparison for the DSCH (UMTS) and HS_DSCH (HSDPA) can be made as:

Feature	DSCH	HS_DSCH
Variable spreading factor	Yes	No
Fast power control	Yes	No
Adaptive modulation and coding (AMC)	No	Yes
HARQ (Hybrid Automatic Repeat request )	No	Yes

Impact of HSDPA on Radio Access Network and UE Architecture:

In Release’99, all the transport channels are terminated at the RNC (Radio Network Controller) itself. RNC is the main element in the RNS (Radio Network Subsystem) that controls the use and the reliability of the radio resources. Three types of RNC exist, SRNC (Serving RNC), DRNC (Drift RNC) and CRNC (Controlling RNC). The retransmission procedure for the packet data is located in the SRNC, which also handles the connection for the particular user to the core network.

With the introduction of HS-DSCH, additional intelligence in the form of an HSDPA Medium Access Control (MAC) layer is installed in the Node B. This way, retransmissions can be controlled directly by the Node B, leading to faster retransmission. With HSDPA, the Iub interface between Node B and RNC requires a flow control mechanism to ensure that Node B buffers are used properly and that there is no data loss due to Node B buffer overflow.

Although there is a new MAC functionality added in the Node B, the RNC still retains the Release’99/Release 4 functionalities of the Radio Link Control (RLC), such as taking care of the retransmission in case the HS-DSCH transmission from the Node N would fail after, for instance, exceeding the maximum number of physical layer retransmissions.

The key functionality of the new Node B MAC functionality (MAC-hs) is to handle the Automatic Repeat Request (ARQ) functionality and scheduling as well as priority handling. Similar to Node B a new MAC entity, MAC-hs is added in the UE architecture. The functionality of the MAC-hs is same as on the Node B side.

Node B protocol stack in R99	Node B Protocol stack in R-5	UE protocol stack in R-5