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UMTS/HSDPA Wireless Networks: Challenges in Maintenance and Testing
Meeting Your Network Goals
Cell phone users have come to depend on high quality
cellular service to conduct business and stay in contact.
The new 3G services are adding data delivery capabilities
that add to the complexity of deploying and maintaining
a high quality wireless network. At the same time the
addition of regulations requiring phone number portability,
and the ongoing competition between wireless service
providers worldwide, has made it easier for subscribers to
change carriers. On the other hand, the increasing cover-
age of mobile networks also raises popular concern about
the effects of RF radiation on human health. This concern
is being translated into regulatory requirements in many
countries. It is essential for wireless service providers to
engineer and maintain their cellular network to ensure the
fulfillment of such requirements as well as the high Quality
of Service. Dropped calls, non-availability, and poor
performance can lead to reduced revenue and customer
dissatisfaction, which in turn leads to fewer customers.
Pressure on operating and maintenance budgets is
forcing wireless service providers to make do with fewer
resources. Consequently, base station technicians and RF
engineers are reacting to the most urgent needs and have
difficulty performing proactive testing and maintenance.
This application note describes the areas of concern and
the important maintenance measurements to be made on
W-CDMA/UMTS base stations/Node Bs in order to be
able to ensure high Quality of Service. This 3
rd
edition now
also contains information about Electromagnetic field
(EMF) measurements and on how to measure the RF field
strength of a specific W-CDMA scrambling code as well
as references to Antenna testing. Examples of these meas-
urements will be made using the Tektronix NetTek
®
Base
Station Test field portable test tool. The NetTek
®
analyzer-
provides comprehensive W-CDMA and interference testing.
In addition, the NetTek
®
analyzers W-CDMA/UMTS Over-
the-Air (OTA) Scrambling Code Analysis capability further
enhances the state-of-the-art toolkit of testing capabilities
UMTS/HSDPA Wireless Networks: Challenges in
Maintenance and Testing
Application Note UMTS/HSDPA Wireless Networks: Challenges in Maintenance and Testing
Application Note
available for base station/Node B maintenance personnel.
This additional in-service testing capability can increase the
productivity and effectiveness of maintenance personnel
and reduce base station/Node B down time.
Evolution of GSM to W-CDMA/UMTS
GSM (Global System for Mobile Communications) was
introduced for mobile telephony in the mid-1980s. The new
digital system, GSM, improved speech quality over older
analog techniques. In addition, the uniform international
standard allowed a single telephone number and mobile
phone to be used around the world. GSM proved to be
very successful. The European Telecommunications
Standardization Institute (ETSI) adopted the GSM standard
in 1991 and it is now used in over 160 countries with over
350 million subscribers worldwide.
GSM, a second generation (2G) technology, improved
connectivity and voice quality while, at the same time, it
added a wide range of services including low-speed data.
Increased demand for higher speed data and additional
services has led to further development. Prominent among
these developments to be adopted was the 2.5G cellular
technology, GPRS (General Packet Radio Service) and the
3G cellular technology, W-CDMA (Wideband Code Division
Multiple Access). GPRS provides somewhat higher data
rates for mobile users. From a simplistic viewpoint, it installs
a packet-switch network on top of the existing GSM circuit
switched network without altering the radio interface. This
allows data calls to make more efficient use of the existing
air interface by operating in a manner closer to that of the
Internet, the now ubiquitous world-wide data network.
The development of W-CDMA and other 3G technologies
began under the auspices of the UN with its IMT-2000
(International Mobile Telephone of 2000) program. Early
W-CDMA technologies, such as ARIB in Japan (Association
for Radio Industry and Business) and ETSI (European
Telecommunications Standards Institude) in Europe
have been harmonized under the supervision of the
Third-Generation Partnership Project (3GPP) to create
UMTS (Universal Mobile Telephone Service). The 3GPP
is made up of industry representatives and members of
standards bodies from around the world.
W-CDMA defines the air interface access of the UMTS
network. W-CDMA has two modes defined: frequency
division duplex (FDD) and time division duplex (TDD). The
vast majority of systems being deployed are FDD, where the
downlink and uplink radio frequencies are different. For this
application note, W-CDMA will refer to UMTS/W-CDMA,
FDD mode. A W-CDMA base station is described in UMTS
specifications as a Node B. The terms Node B and base
station will be used interchangeably. UMTS specifications
also refer to mobile cell phones or mobile devices as
UE (User Equipment).
Overview of W-CDMA
W-CDMA is a wideband code-division multiple-access
system. It is an extremely flexible, and thus complex
system. Unlike GSM and GPRS, which use exclusively time-
division multiple access (TDMA), W-CDMA utilizes both
CDMA and TDMA to allow radio resources to be shared
more efficiently. CDMA is the primary method for users to
share access to the network. With CDMA each mobile
users call is uniquely differentiated from other calls by a
set of specialized codes added to the transmission which
allows all users to transmit at the same time and allows
these users to share the same RF carrier. Unlike some other
cdma systems, W-CDMA base stations do not have to be
in system wide time synchronization, nor do they depend
on a GPS (Global Positioning System) signal. Instead, they
transmit a sync signal along with the downlink signal.
For this discussion, key points to keep in mind are:
1. Adjacent base stations/Node Bs use the same
RF frequency.
2. W-CDMA base stations/Node Bs use one of 512 unique
scrambling code delays to uniquely identify themselves to
UE in their vicinity.
3. With W-CDMA, unlike 2G systems or CDMA-2000, there
is no operational distinction between cell and sector.
If directional antennas are used to divide the coverage of
a region then each division is its own Node B and is a cell
in itself with a unique scrambling code.
4. W-CDMA uses channelization codes, known as OVSF
(Orthogonal Variable Spreading Factor) codes, or simply
spreading codes, to uniquely identify a user channel. Other
spreading code channels are used for the pilot (P-CPICH),
signaling, user voice or user data. A significant advantage
of W-DCMA is that the spreading factor can be varied
dynamically to accommodate varying data rates. This
allows the system to efficiently provide services ranging
from SMS (short messaging service) to multi-megabit per
second data transfers.
5. GPS is not required for W-CDMA/UMTS systems.
2 www.tektronix.com/wireless W-CDMA/UMTS Wireless Networks
Application Note
HSDPA, Expanding
Data Capabilities
Unlike voice, where the amount
of information being transferred
is the same in both directions,
data demand is usually much
greater for the downlink than
the uplink.Also, each users data
needs are highly variable over
time.The Release 5 of W-CDMA
adds HSDPA (High Speed
Downlinkfor Packet Access)
to further expand the systems
datacapabilities. With HSDPA
peak data transfers at up to
12 Mbpscan be achieved, and
the system obtains a further
dimensionof flexibility to deal
with the intermittent and highly
varyingdemands of data users.
This new capability adds to the performance requirements
of the Node B transmitter; especially if it is equippedwith a
multi-channel power amplifier to provide more than oneRF
carrier in the cell. HSDPA makes use of advanced radio
resource management mechanisms in order to efficiently
exploit the available radio network capacity and quickly
adapt to changing radio network conditions. With HSDPA,
part of the network intelligence that in UMTS resides in the
Radio Network Controller (RNC) is now located at the nodeB;
this allows a faster interworking between the