1. Application Note
High Speed Downlink Packet Access
An Introduction, Challenges and Protocol Test Examples
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 Downlink
Packet 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.

2. High Speed Downlink Packet Access
Application Note
UMTS or W-CDMA Networks are developed within the The second phase of HSDPA is currently being specified in
third Generation Partnership Project Workgroup (3GPP) 3GPP release 6 and is aimed to achieve data rates of up to
and consist of several releases that can be seen as 28.8 Mbps.
the evolution of the standard. In Release 99 the 5 MHz It will introduce antenna array technologies such as beam
UMTS carrier was defined to provide capacity and user forming and Multiple Input Multiple Output (MIMO).
performance advantages over predecessor technologies
Beam forming can be described as focussing the transmit-
such as GSM, GPRS and EDGE.
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 3GPP
Figure 1. Mobile Networks Evolution
Specification
In order to support HSDPA, new physical channels, logical
R4 of UMTS provided nominal enhancements to the channels as well as changes to protocols have been added
transport, radio interface and features defined in R99 and to the UMTS Specification.
Release 5 extends the R99 and R4 specifications, offering
Channel changes and additions
an enhancement called High Speed Downlink Packet
Access (HSDPA). This evolution of evolving UMTS delivers The two new physical channels introduced with HSDPA
more throughput and performance. HSDPA will provide are the High Speed Physical Downlink Shared Channel
theoretical peak data rates of up to 14.4Mbps. (HS-PDSCH) as well as the High Speed Physical Control
Channel (HS-DPCCH).
Different evolution steps of The HS-PDSCH is the transport mechanism for the
HSDPA newly introduced logical channels. It carries the actual
data, uses adaptive modulation and is power controlled
The idea of HSDPA is to increase the possible downlink
by the Node-B.
data rate by increasing the spectral efficiency. The focus
on the downlink data rate is originated in High data rate In addition to the code multiplexing of traditional
demanding Services such as Internet Access and file W-CDMA channels, where user data is transmitted
downloads. via dedicated channels, HSDPA also introduces time
multiplexing. This means that several user share the
The First phase of HSDPA has been specified in 3GPP
same channel and at times where one user is not
release 5. Phase one introduces new basic functions and
using an available resource it is becoming available
is aimed to achieve peak data rates of 14.4 Mbps.
to others. The reasoning behind this approach is
Newly introduced are the High Speed Downlink Shared that user traffic is becoming more of a bursty nature,
Channels (HS-DSCH), the adaptive modulation QPSK and so that a large number of users can use the same
16QAM and the High Speed Medium Access protocol time-multiplexed channel and efficiently use the available
(MAC-hs) in the Node-B radio network resources.
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3. High Speed Downlink Packet Access
Application Note
The other newly introduced physical channel, the HS- The HS-SCCH is a downlink signalling channel providing
DPCCH is an uplink control channel. It carries signalling- information to the UE. The information provided is around
and channel quality information from the User Equipment timing and coding and amongst others contains the
(UE) to the Node-B. This information is used by the channel code set, the modulation scheme, the transport
Node-B to perform the adaptive modulation, and coding block size and the UE identity. This data enables the
of the above described HS-PDSCH. We will talk about user equipment to “listen” to the HS-DPCH in an opti-
the adaptive modulation at a later point in this paper. mized way, at the right time and with the correct codec’s
The transmitted signalling information also contains in order to decode the received data. It enables a
acknowledgements or non-acknowledgements for each connection without wasting precious radio resources.
received user data block.
Protocol changes and additions
Logical Channel additions are the High Speed Downlink
The main changes where introduced for the Medium
Shared Channel (HS-DSCH) as well as the High Speed
Access Channel protocol (MAC). The MAC decides on
Shared Control Channel (HS-SCCH).
which channel the Protocol Data Units (PDU’s) will be
The HS-DSCH provides the logical transfer mechanism transmitted. The traditional MAC protocol resides in the
for the data that is transported on the physical channel Radio Network Controller, whereas for HSDPA or more
HS-PDSCH 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
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4. High Speed Downlink Packet Access
Application Note
The Fast scheduling mechanism handles the logical channel Network Element Changes
resources and determines which particular user should The mainly affected network Element in HSDPA are the
be served within a 2ms time interval. This mechanism also Node-B and the RNC.
takes into account the information sent by the individual
The Node-B in HSDPA, is taking over several functions
UE’s. The knowledge of the instantaneous quality of a
that where previously performed by the RNC. The MAC-hs
channel makes it possible to avoid sending data packets
protocol with all its new functions such as flow control
during channel fades and rather schedule a UE in better
towards the Iub, data buffering, the earlier described
conditions. This might cause that certain UE’s will obtain a
scheduling the HARQ termination, link adaptation and
lower throughput than others. The challenge for this packet
the power control are all functions that have not been
scheduling function is to optimize the cell capacity and at
performed by Node-B’s so far.
the same time fulfil QoS requirements defined in Network
Operators policies. Depending on the Node-B currently deployed, Network
Operators might face software or software and hardware
Hybrid Automatic Repeat Request (HARQ) is an enhanced
upgrades in order to deploy HSDPA. The maximum
form of the Automatic Repeat Request (ARQ) and is used
achievable bit rate a Node-B can support depends on
for the packet retransmission.
the hardware.
In Rel.99, whenever a packet was not properly decoded
The main new functionality for the RNC is the new Radio
by the UE, it was discarded and resent (ARQ).
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
Figure 3. Packet Retransmission with ARQ vs. HARQ becoming key elements in HSDPA. And Test engineers
will be faced to test this new functionality. Decoding of
Not so in HSDPA with HARQ. In HSDPA the erroneous the new protocols, analysis of the obtained data as well
packet is not discarded but stored by the UE. The packet as the correlation of data over several Interfaces will play
is resent and both, the previous one as well as the newly an essential role for the deployment of HSDPA Networks.
sent packets are used to extract the original information. The following chapter will describe specific test challenges
This approach has a much better error rate performance, that Equipment Manufacturer (EM) and Network Operators
causes packets to be resent less often and therefore (NO) are facing today.
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).
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5. High Speed Downlink Packet Access
Application Note
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-
Figure 4. HSDPA Setup Signaling
plane IP data helps to analyze the network performance on
the Iub user-plane.
Signalling Test:
QoS:
In HSDPA Networks, new messages like the HS-DSCH
Capacity Request and Allocation messages are used on Delay
the Node-B for data traffic control. The amount of traffic on The Node-B with its new functionality will perform data
the Iub Interface, and therefore the achieved throughput is buffering and scheduling at the Iub side. It will analyze the
depending on message-parameters like the Packed Data air interface conditions and choose the optimal modulation
Unit (PDU) size, the Interval, and the Repetition Period. The scheme at the air interface (Uu) side (i.e. 16QAM or QPSK).
Node-B buffering and the scheduling will also be affected
One of the critical functions for an HSDPA enabled Node-B
by several parameters like the conditions of the air interface
would be the packet scheduling functionality and HARQ.
the retransmission control mechanism and others.
The packet process mechanism on both, the Iub and Uu
Monitoring the behaviour and development of each param- interface of any Node-B is a critical factor for minimizing
eter helps to analyze the functionality of a signalling proce- the latency. Monitoring of the delay between the Iub and
dure and the traffic flow control functionality. (See Fig-4). Uu will help analyzing the HSDPA scheduling and the
The Tektronix K15 Monitor and K1297-G20 functional tester packet retransmission functionality. The delay measurement
are equipped with decoding functionalities to monitor and is critical for HSDPA service performance.
test above parameters
Throughput
User Data Test: Data throughput on both Iub and Uu could give us some
key performance information of the Node-B. Analyzing
HSDPA will provide a theoretical throughput of 14.4Mbps
the data generation timing from Node-B and the related
per cell and is aimed to achieve over 1Mbps of actual
timing information in NBAP, RRC message could be useful
packet service speed. However, the actual traffic speed
for the performance analysis. Figure 5 illustrates a possible
is depending on several factors like the number of users,
test setup to analyze the air interface (Uu) using a spectrum
the Node-B buffer condition, the air interface condition,
analyzer triggered by an event on the lub interfaces.
and the Node-B data transmission timing. Different types
of data user accesses, i.e. short packet data or long packet
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6. High Speed Downlink Packet Access
Application Note
Figure 5. Uu, lub Test Example
Future test challenge: The ability of mobile test equipment to measure above
parameters is becoming more and more important. It
Once HSDPA will be deployed and become more mature,
enables Equipment Manufacturer and Network Operators
the Node-B resource allocation will become a critical
to deploy their networks more rapidly and maintain
factor for network optimization. HSDPA Networks will
their quality of service requirements. Mobile test equipment
require offering larger resources and quality values than
like the Tektronix K15 or K1297-G20 are equipped to
other mobile networks.
meet these challenges and offer a large variety of protocol
Key Performance Indicators (KPI) will help analyzing network
decoding capabilities as well as applications that will
conditions and help optimizing the network deployment.
ensure to manage above mentioned challenges.
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
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7. High Speed Downlink Packet Access
Application Note
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Last Update October 28, 2004
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