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High Speed Downlink Packet Access An Introduction, Challenges and Protocol Test Examples

Application Note

Global deployment of 3G Networks and UMTS is finally underway and current estimates show that packet-switched traffic will overtake circuit–switched traffic in the not so far future. The growth of data used in communication will soon require networks which can handle increasing data rates. HSDPA offers high speed data rates of up to 14 Mbps.

This Application Note describes the technology and Challenges of High Speed DownlinkPacket Access (HSDPA) – an Evolution of the 3G UMTS networks to higher data rates. First describing the motivation and the different introduction phases of HSDPA, we will then focus on some challenges and typical protocol test issues. This paper is as well including an overview of the basic features and impacts to an existing UMTS network infrastructure.

High Speed Downlink Packet Access Application Note

2 www.tektronix.com/signaling

UMTS or W-CDMA Networks are developed within the

third Generation Partnership Project Workgroup (3GPP)

and consist of several releases that can be seen as

the evolution of the standard. In Release 99 the 5 MHz

UMTS carrier was defined to provide capacity and user

performance advantages over predecessor technologies

such as GSM, GPRS and EDGE.

R4 of UMTS provided nominal enhancements to the

transport, radio interface and features defined in R99 and

Release 5 extends the R99 and R4 specifications, offering

an enhancement called High Speed Downlink Packet

Access (HSDPA). This evolution of evolving UMTS delivers

more throughput and performance. HSDPA will provide

theoretical peak data rates of up to 14.4Mbps.

Different evolution steps ofHSDPAThe idea of HSDPA is to increase the possible downlink

data rate by increasing the spectral efficiency. The focus

on the downlink data rate is originated in High data rate

demanding Services such as Internet Access and file

downloads.

The First phase of HSDPA has been specified in 3GPP

release 5. Phase one introduces new basic functions and

is aimed to achieve peak data rates of 14.4 Mbps.

Newly introduced are the High Speed Downlink Shared

Channels (HS-DSCH), the adaptive modulation QPSK and

16QAM and the High Speed Medium Access protocol

(MAC-hs) in the Node-B

The second phase of HSDPA is currently being specified in

3GPP release 6 and is aimed to achieve data rates of up to

28.8 Mbps.

It will introduce antenna array technologies such as beam

forming and Multiple Input Multiple Output (MIMO).

Beam forming can be described as focussing the transmit-

ted power of an antenna in a beam towards the user’s

direction. Knowing that the limiting resources are the

transmission power of the base station sector, one can

understand that beam forming is a mean of increasing

this power. MIMO uses multiple antennas at the sending

and receiving side.

The third phase of HSDPA which still is a long way down

the road will concentrate on the air interface. It will intro-

duce a new Air Interface with Orthogonal Frequency

Division Multiplexing and higher modulation schemes.

Phase three of HSDPA aims at data rates of up to 50 Mbps

Additions to the UMTS 3GPPSpecificationIn order to support HSDPA, new physical channels, logical

channels as well as changes to protocols have been added

to the UMTS Specification.

Channel changes and additions

The two new physical channels introduced with HSDPA

are the High Speed Physical Downlink Shared Channel

(HS-PDSCH) as well as the High Speed Physical Control

Channel (HS-DPCCH).

The HS-PDSCH is the transport mechanism for the

newly introduced logical channels. It carries the actual

data, uses adaptive modulation and is power controlled

by the Node-B.

In addition to the code multiplexing of traditional

W-CDMA channels, where user data is transmitted

via dedicated channels, HSDPA also introduces time

multiplexing. This means that several user share the

same channel and at times where one user is not

using an available resource it is becoming available

to others. The reasoning behind this approach is

that user traffic is becoming more of a bursty nature,

so that a large number of users can use the same

time-multiplexed channel and efficiently use the available

radio network resources.

Figure 1. Mobile Networks Evolution


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The other newly introduced physical channel, the HS-

DPCCH is an uplink control channel. It carries signalling-

and channel quality information from the User Equipment

(UE) to the Node-B. This information is used by the

Node-B to perform the adaptive modulation, and coding

of the above described HS-PDSCH. We will talk about

the adaptive modulation at a later point in this paper.

The transmitted signalling information also contains

acknowledgements or non-acknowledgements for each

received user data block.

Logical Channel additions are the High Speed Downlink

Shared Channel (HS-DSCH) as well as the High Speed

Shared Control Channel (HS-SCCH).

The HS-DSCH provides the logical transfer mechanism

for the data that is transported on the physical channel

HS-PDSCH

The HS-SCCH is a downlink signalling channel providing

information to the UE. The information provided is around

timing and coding and amongst others contains the

channel code set, the modulation scheme, the transport

block size and the UE identity. This data enables the

user equipment to “listen” to the HS-DPCH in an opti-

mized way, at the right time and with the correct codec’s

in order to decode the received data. It enables a

connection without wasting precious radio resources.

Protocol changes and additions

The main changes where introduced for the Medium

Access Channel protocol (MAC). The MAC decides on

which channel the Protocol Data Units (PDU’s) will be

transmitted. The traditional MAC protocol resides in the

Radio Network Controller, whereas for HSDPA or more

precisely for the High Speed Physical Downlink Channel

the High Speed MAC (MAC-hs) has been introduced.

The MAC-hs resides in the Base Station. It takes care

of the transport block scheduling, channel allocation

and the transport format selection. Further tasks of the

MAC-hs amongst others are are:

Adaptive Modulation and Coding (AMC)

Fast packet scheduling mechanism

Hybrid Automatic Repeat Request (HARQ)

Adaptive Modulation and Coding (AMC) is one of the major

changes in HSDPA.

In UMTS release99 modulation techniques where applied to

provide a reliable connection under changing environmental

conditions. With decreasing signal to noise ratio more errors

are transmitted with the signal. The higher the coding rate

applied, the better the chances of an UE to decode the

original data. But on the other hand, the higher the coding

rate, the more bits are sent to transmit the information

which means that more bandwidth is used.

In HSDPA the UE is actively feeding back information about

the channel conditions which is used by the Node-B to

determine Modulation and coding scheme. For each

Transmit Time Interval (TTI) the UE feedback is taken into

account the best possible modulation and coding is chosen

and the highest possible transmission rate is obtained.

Figure 2. HS-DSCH data Frame Structure



The Fast scheduling mechanism handles the logical channel

resources and determines which particular user should

be served within a 2ms time interval. This mechanism also

takes into account the information sent by the individual

UE’s. The knowledge of the instantaneous quality of a

channel makes it possible to avoid sending data packets

during channel fades and rather schedule a UE in better

conditions. This might cause that certain UE’s will obtain a

lower throughput than others. The challenge for this packet

scheduling function is to optimize the cell capacity and at

the same time fulfil QoS requirements defined in Network

Operators policies.

Hybrid Automatic Repeat Request (HARQ) is an enhanced

form of the Automatic Repeat Request (ARQ) and is used

for the packet retransmission.

In Rel.99, whenever a packet was not properly decoded

by the UE, it was discarded and resent (ARQ).

Not so in HSDPA with HARQ. In HSDPA the erroneous

packet is not discarded but stored by the UE. The packet

is resent and both, the previous one as well as the newly

sent packets are used to extract the original information.

This approach has a much better error rate performance,

causes packets to be resent less often and therefore

enables to use the bandwidth more efficiently.

The retransmission is performed by the Node-B, so that

there is no retransmission via the Iub interface (figure 3).

Network Element Changes

The mainly affected network Element in HSDPA are the

Node-B and the RNC.

The Node-B in HSDPA, is taking over several functions

that where previously performed by the RNC. The MAC-hs

protocol with all its new functions such as flow control

towards the Iub, data buffering, the earlier described

scheduling the HARQ termination, link adaptation and

the power control are all functions that have not been

performed by Node-B’s so far.

Depending on the Node-B currently deployed, Network

Operators might face software or software and hardware

upgrades in order to deploy HSDPA. The maximum

achievable bit rate a Node-B can support depends on

the hardware.

The main new functionality for the RNC is the new Radio

Resource Management. The addition of shared channels

to the dedicated ones will have a major impact on this

function. The capacity of a cell needs to be split between

dedicated channels and HSDPA channels. New to the

Streaming services is the requirment of an intelligent algo-

rithm with a dynamic behaviour when sharing dedicated

and shared channels.

Also new to the RNC is the HS-DSCH Framing Protocol for

the user plane which has been added to the protocol stack.

Challenges for network test engineers As stated before, the Node-B as well as the RNC are

becoming key elements in HSDPA. And Test engineers

will be faced to test this new functionality. Decoding of

the new protocols, analysis of the obtained data as well

as the correlation of data over several Interfaces will play

an essential role for the deployment of HSDPA Networks.

The following chapter will describe specific test challenges

that Equipment Manufacturer (EM) and Network Operators

(NO) are facing today.

Figure 3. Packet Retransmission with ARQ vs. HARQ



Signalling Test:In HSDPA Networks, new messages like the HS-DSCH

Capacity Request and Allocation messages are used on

the Node-B for data traffic control. The amount of traffic on

the Iub Interface, and therefore the achieved throughput is

depending on message-parameters like the Packed Data

Unit (PDU) size, the Interval, and the Repetition Period. The

Node-B buffering and the scheduling will also be affected

by several parameters like the conditions of the air interface

the retransmission control mechanism and others.

Monitoring the behaviour and development of each param-

eter helps to analyze the functionality of a signalling proce-

dure and the traffic flow control functionality. (See Fig-4).

The Tektronix K15 Monitor and K1297-G20 functional tester

are equipped with decoding functionalities to monitor and

test above parameters

User Data Test:HSDPA will provide a theoretical throughput of 14.4Mbps

per cell and is aimed to achieve over 1Mbps of actual

packet service speed. However, the actual traffic speed

is depending on several factors like the number of users,

the Node-B buffer condition, the air interface condition,

and the Node-B data transmission timing. Different types

of data user accesses, i.e. short packet data or long packet

data could also affect the traffic speed. If the traffic control

mechanism is not working properly, some user could

receive very high throughput rate (e.g. 4Mbps constantly)

whereas the rest would only receive low speed traffic.

Monitoring the actual IP throughput on the Iub interface

will help analyzing the actual traffic condition. Analyzing

multiple user’s traffic in one and the same cell will provide

information about the balance of throughput amongst

different user.

The IP packet retransmission ratio on the Iub interface is

one of the critical parameters for the reliability of a service.

This also means that monitoring and decoding of user-

plane IP data helps to analyze the network performance on

the Iub user-plane.

QoS:

Delay

The Node-B with its new functionality will perform data

buffering and scheduling at the Iub side. It will analyze the

air interface conditions and choose the optimal modulation

scheme at the air interface (Uu) side (i.e. 16QAM or QPSK).

One of the critical functions for an HSDPA enabled Node-B

would be the packet scheduling functionality and HARQ.

The packet process mechanism on both, the Iub and Uu

interface of any Node-B is a critical factor for minimizing

the latency. Monitoring of the delay between the Iub and

Uu will help analyzing the HSDPA scheduling and the

packet retransmission functionality. The delay measurement

is critical for HSDPA service performance.

Throughput

Data throughput on both Iub and Uu could give us some

key performance information of the Node-B. Analyzing

the data generation timing from Node-B and the related

timing information in NBAP, RRC message could be useful

for the performance analysis. Figure 5 illustrates a possible

test setup to analyze the air interface (Uu) using a spectrum

analyzer triggered by an event on the lub interfaces.

Figure 4. HSDPA Setup Signaling

Future test challenge:Once HSDPA will be deployed and become more mature,

the Node-B resource allocation will become a critical

factor for network optimization. HSDPA Networks will

require offering larger resources and quality values than

other mobile networks.

Key Performance Indicators (KPI) will help analyzing network

conditions and help optimizing the network deployment.

Following parameters are just some useful KPI’s that are

relevant in HSDPA and other mobile networks.

Number of Voice calls

Number of Video calls

Number of Packet calls

Spreading factor values

Number of non-success calls

- Released calls

- Rejected calls

- Failures

Power control information

The ability of mobile test equipment to measure above

parameters is becoming more and more important. It

enables Equipment Manufacturer and Network Operators

to deploy their networks more rapidly and maintain

their quality of service requirements. Mobile test equipment

like the Tektronix K15 or K1297-G20 are equipped to

meet these challenges and offer a large variety of protocol

decoding capabilities as well as applications that will

ensure to manage above mentioned challenges.



Figure 5. Uu, lub Test Example

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Last Update October 28, 2004

For Further InformationTektronix maintains a comprehensive, constantly expanding collection ofapplication notes, technical briefs and other resources to help engineersworking on the cutting edge of technology. Please visit www.tektronix.com

Copyright © 2004, Tektronix, Inc. All rights reserved. Tektronix products are covered by U.S. and foreignpatents, issued and pending. Information in this publication supersedes that in all previously published material. Specification and price change privileges reserved. TEKTRONIX and TEK areregistered trademarks of Tektronix, Inc. All other trade names referenced are the service marks,trademarks or registered trademarks of their respective companies. 10/04 DM/WOW 2FW-18269-0

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