Application Note

Practical Tips on
WCDMA Measurements
MT8222A MS272xB
BTS Master ™
Spectrum Master™

Introduction
This is a practical Wideband Code Division Multiple Access (WCDMA) measurement procedures note. The objective of
this note is to present measurement tips and procedures which will help a field-based network technician or RF engineer
conduct Node B measurements on WCDMA access networks.
Evolution To WCDMA
In the mid 1980’s a second generation (2G) digital system known as the Global System for Mobile Communications
(GSM) was introduced for mobile telephony. It significantly improved speech quality over the older analog-based systems
and, as it was an international standard, enabled a single telephone number and mobile phone to be used by consumers
around the world. It led to significantly improved connectivity and voice quality, as well as the introduction of a whole slew
of new digital services like low-speed data. Proving to be very successful, GSM was officially adopted by the European
Telecommunications Standardization Institute (ETSI) in 1991. It is now widely used in over 160 countries worldwide.
The success of GSM spurred the demand for further development in mobile telephony, and put it on an evolution-
ary path to third generation (3G) technology. Along the way, that development path has included 2G technologies like
Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). TDMA is similar in nature to GSM and
provides for a tripling of network capacity over the earlier AMPS analog system. In contrast, CDMA is based on the prin-
ciples of spread spectrum communication. Access to it is provided via a system of digital coding.
In 1997 a 2.5G system called the General Radio Packet Service (GPRS) was introduced to accommodate the grow-
ing demand for Internet applications. As opposed to the existing 2G systems, it offered higher data rates and Quality of
Service (QoS) features for mobile users by dynamically allocating multiple channels. GPRS installs a packet switch net-
work on top of the existing circuit switch network of GSM, without altering the radio interface.
In 1999, the International Telecommunications Union (ITU) began evaluating and accepting proposals for 3G protocols
in an effort to coordinate worldwide migration to 3G mobile networks. These proposals were known as International
Mobile Telecommunication 2000 (IMT-2000). One of the most important IMT-2000 proposals to emerge was Universal
Telecommunications Services (UMTS).
While GPRS is considered the first step in enhancing the GSM core network in preparation for EDGE and 3G, WCDMA is
a 3G technology according to the 3GPP standard (Figure 1). It is the digital access system for the UMTS network and is
today considered one of the world’s leading 3G wireless standards.

1G 2G 2G+ 3G

FDD/TDD
PDC ARIB (WCDMA)
WCDMA
GSM UTRA (WCDMA)
TD-SCDMA
EDGE

GPRS

AMPS IS-54 IS-136 IS-856

IxEV-DO

IS-95 CDMA2000
1xRTT

Figure 1. Evolution of cellular technologies.

Understanding WCDMA
WCDMA is an approved 3G technology which increases data transmission rates via the Code Division Multiplexing air
interface, rather than the Time Division Multiplexing air interface of GSM systems. It supports very high-speed multimedia
services such as full-motion video, Internet access and video conferencing. It can also easily handle bandwidth-intensive
applications such as data and image transmission via the Internet.
WCDMA is a direct spreading technology, it spreads its transmissions over a wide, 5 MHz, carrier and can carry both voice
and data simultaneously. It features a peak data rate of 384 kbps, a peak network downlink speed of 2 Mbps and average
user throughputs (for file downloads) of 220-320 kbps. In addition, WCDMA boasts increased capacity over EDGE for high-
bandwidth applications and features which include, among other things, enhanced security, QoS, multimedia support, and
reduced latency (Table 1).

Parameters WCDMA
Bandwidth 5 MHz

Chip Rate 3.84 Mcps

Power Control Frequency 1500 Hz up/down

Base Station Synchronization Not needed

Cell Search 3-step approach via primary, secondary search code and CPICH

Downlink Pilot CDM common (CPICH)

TDM dedicated (bits in DPCH)

User Separation CDM/TDM (shared channel)

2G Interoperability GSM-UMTS handover (Multi-mode terminals)

Table 1. System performance fo WCDMA

WCDMA networks offer a number of significant benefits. They are:

• High bandwidth and low latency which contributes significantly to a higher-quality user experience
and in turn increases data revenue and improves customer satisfaction.
• Support for a wide array of new and emerging multimedia services.
• Considered the most cost-effective means of adding significant capacity for both voice and data services.
•F
 ar better integration of RF components in the base station as compared to any other radio or mobile
technology. A WCDMA base station cabinet has several times the RF capacity of GSM cabinets.
• Extreme flexibility in allocating capacity to offer the optimal QoS for different traffic types.
To date, WCDMA has been adopted for 3G use as specified in the 3GPP standard by ETSI in Europe, and as an ITU
standard under the name “IMT-2000 direct spread.” NTT DoCoMo launched the first WCDMA service in 2001 and now
has millions of subscribers. WCDMA (BTS) is also the 3G technology of choice for many GSM/GPRS operators, with
dozens currently in trials. More than 100 GSM/GPRS operators have even licensed new spectrum with the intent to
launch WCDMA services in the coming years.

WCDMA Basics
Unlike GSM and GPRS, which rely on the use of the TDMA protocol, WCDMA – like CDMA - allows all users to
transmit at the same time and to share the same RF carrier. Each mobile user’s call is uniquely differentiated from
other calls by a set of specialized codes added to the transmission.
WCDMA base stations differ from some of the other CDMA systems Reverse WCDMA Channel
Control, Traffic, and Access
in that they do not have to be in system-wide time synchronization, nor
do they depend on a Global Positioning System (GPS) signal. Instead,
P-SCH
they work by transmitting a sync signal along with the downlink signal.
FWD Traffic FWD WCDMA
A downlink or forward link is defined as the RF signal transmitted from S-SCH
Channel
FWD Control
the base station to the subscriber mobile phone. It consists of the CPICH
RF channel, scrambling code (one per sector), an orthogonal variable
spreading factor (OVSF) channel for signaling (one per call), and one P-CCPCH

or more OVSF channels for data (Figure 2). It also contains the sync Others Reverse WCDMA Channel
signals (P-SCH and S-SCH), which are independent of OVSF and Control, Traffic, and Access

scrambling codes. The RF signal transmitted from the mobile phone is

referred to as the uplink or reverse channel.

Figure 2. WCDMA channel structure

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The WCDMA downlink and uplink data streams run at a constant 3.84 Mcps, are divided into time slots and grouped
as frames. The frame is the basic unit of data information that the system works with in the coding, interleaving and
transmitting processes.
Data transmitted via a WCDMA network – whether digitized voice or actual data – is spread using a code which
is running at a 3.84 Mbps code rate. Once the transmitted data is received by the subscriber’s mobile receiver, its
demodulator/correlator reapplies the code and recovers the original data (Figure 3). The signal received by the mobile
is a spread signal together with noise, interference and messages on other code channels in the same RF frequency
slot. The interference may emanate from multiple sources including other users in the same cell
or from neighboring cells.
30 kHz 3.84 MHz 3.84 MHz 30 kHz

DATA WIDEBAND
SPECTRUM CORRELATOR

DATA ENCODING & DE-INTERLEAVE DATA

DIGITAL
9.6 kbps INTERLEAVING FILTER & DECODE
BPF BPF
Variable Mbps 3.84 MHz 3.84 MHz Variable Mbps
OVSF, OVSF,
Scrambling Scrambling

CARRIER CARRIER

3.84 MHz 3.84 MHz

BACKGROUND EXTERNAL OTHER CELL OTHER USER

NOISE INTERFERENCE INTERFERENCE NOISE

Figure 3. Signal spreading and correlation in a WCDMA base station

WCDMA has two basic modes of operation:

•  Frequency Division Duplex (FDD) mode. Here separate frequencies are used for uplink and downlink.
FDD is currently being deployed and is usually referred to as WCDMA.
• Time Division Duplex (TDD) mode. In this mode, the uplink and downlink are carried in alternating bursts
on a single frequency.
Note that this Application Note focuses on FDD systems only.
One of the important features of a WCDMA system is its highly adaptive radio interface. WCDMA is designed to allow
many users to efficiently share the same RF carrier by dynamically reassigning data rates. The spreading factor (SF)
may be updated as often as every 10 ms, which in turn, permits the overall data capacity of the system to be used
more efficiently.
Some of the key things to remember about WCDMA are:
• In WCDMA, the RF signal from each base station sector is “scrambled” by multiplying the data and voice
channels by a unique pseudo-noise code, known as the Scrambling Code. The Scrambling Code is mixed prior
to the output of a base station or the output of a subscriber’s mobile unit. WCDMA base stations (Node B’s) use
one of 512 Scrambling Codes to uniquely identify each sector in the network.
• Adjacent base stations use the same RF frequency for spectral efficiency. WCDMA employs a frequency reuse
method in which the same frequency is used at every site, with forward links separated from one another by
Scrambling Codes.
• WCDMA uses channelization codes, known as OVSF codes or Spreading Codes, to uniquely identify a Dedicated
Physical Channel (DPCH) user channel. At the receiver, the received RF signal passes through the correlator, that
separates and identifies the code channels (pilot, signaling or user data/voice) of each WCDMA channel it sees.
Other spreading code channels are used for the pilot (P-CPICH), signaling, user voice or user data. Higher user
data rates can be achieved by shortening the spreading factor, thereby increasing the transmission rate.
Note that the synchronization channels, P-SCH and S-SCH, do not go through the OVSF spreading process. The
OVSF codes are orthogonal codes used to separate traffic in a WCDMA signal. Any mobile phone that receives a
transmitted data sequence and attempts to demodulate it using the “wrong” orthogonal code, would interpret the
information as noise. The noise, when integrated over time, will net to zero. As a result, interfering signals not intended
for a given mobile phone are effectively eliminated by signal processing in the mobile phone’s receiver. The OVSF codes
can be reused by each base station and mobile phone within the same location, since the scrambling codes identify the
transmitting device.

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WCDMA Versus GSM
GSM was the first digital cellular system. It uses TDMA as its air interface standard and Gaussian Modulated Shift
Keying (GMSK) on the RF air interface. GSM systems in Europe operate in 900 and 1800 MHz bands, while in the
United States they operate in the 800 MHz (cellular) and 1900 MHz Personal Communications Services (PCS) bands.
There are many similarities between WCDMA and GSM systems including the fact that both the GSM Base Station
Subsystem (BSS) and the WCDMA Radio Access Network (RAN) provide a radio connection to the handset via the
same GSM core network (Figure 4). Both are also based on the principles of a cellular radio system. The GSM Base
Station Controller (BSC) corresponds to the WCDMA Radio Network Controller (RNC), while the GSM Radio Base
Station (RBS) corresponds to the WCDMA RBS.

GSM/WCDMA Architecture
Core Network
A A lu lu
GSM BSS WCDMA RAN

BSC BSC RNC lur

RNC
Abls Abls lub lub

BTS BTS BTS BTS RBS RBS RBS RBS

Um (the radio interface) Uu (the radio interface)

BSS: Base Station Subsystem WCDMA RAN: WCDMA
BSC: Base Station Controller Handset Radio Access Network Handset
BTS: Base Transceiver Station RNC: Radio Network Controller
RBS: Radio Base Station

Figure 4. Although GSM and WCDMA are different technologies, they both share the same core network.

The significant differences between the two standards, apart from the lack of an interface between the GSM BSCs
and an insufficiently specified GSM Abis-interface to provide multi-vendor operability, include the following:
•G
 SM uses TDMA technology with a lot of radio functionality based on managing the timeslots. WCDMA
systems use CDMA technology in which both the hardware and control functions are different.
• GSM was created with voice as the primary application. WCDMA includes support for voice, high-speed
packet data and multimedia applications.
•T
 he underlying WCDMA air interface is much more performance sensitive and its operation shares many
more similarities with its rival CDMA2000 than its predecessor GSM. To achieve link-level performance gains
over GSM’s equalization and frequency hopping techniques, WCDMA uses rake receiver technology for
diversity gain.
•W
 CDMA employs a fast power control scheme — 1500 Hz on both the up and downlink — to deal with
CDMA’s inherent near-far interference issues. GSM, which features a hard capacity due to its fixed frequency
reuse scheme, employs a very slow (2 Hz) power control scheme.

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Understanding WCDMA Measurements
Proper characterization of complex WCDMA signals requires field technicians to measure many different types
of parameters. The WCDMA measurements that can be made with BTS Master include:
• Carrier Frequency
Carrier frequency is defined as the selected trans-
mitter operating center frequency, entered by the
user or calculated from the signal standard, and
channel number, entered by the user.
• Carrier Feedthrough
Carrier Feedthrough measures the amount of
unmodulated signal that is leaking through the
transmitter and is displayed in the Code Domain
Power display. The WCDMA 3GPP specification
does not specify Carrier Feedthrough measure-
ment.
• Code Domain Power (CDP)
CDP displays how much of the power is in each
code channel (Figure 5). Power is normalized to the
Figure 5. CDP display example.
total power, so if a code reads –10dB, it means that
the code is one tenth of the channel power. Colors
are applied according to the following:

Parameter Description Color Viewable on Display

CPICH Common Pilot Channel Red All CDP views

 rimary Common Control Physical

P
P-CCPCH Magenta All CDP views
Channel

 econdary Common Control Physical

S
S-CCPCH Cyan All CDP views
Channel

PICH Paging Indicator Channel Green All CDP views

P-SCH Primary Sync Channel Navy Blue Control Channels

S-SCH Secondary Sync Channel Blue Control Channels

Traffic WCDMA Traffic Yellow All CDP views

Noise Noise Grey All CDP views

Table 2. Parameters description table

Note that in the WCDMA specification, the P-SCH and S-SCH signals are not assigned spreading codes and
therefore do not appear in the CDP display. The P-SCH and S-SCH signals are displayed in the control channel
table. They have special non-orthogonal scrambling codes and are on 10% of the time.
• Channel Power is the total power transmitted in the 3.84 MHz WCDMA channel specified. Channel power
measures the Node B/base station transmitting power across the entire 3.84 MHz WCMDA channel and is
measured in units of dBm and Watts. For Over The Air (OTA) measurements, the channel power will vary as
the signal path from the Node B transmitter to the BTS Master MT8222A varies.
• Scrambling Code
According to the WCDMA specification, the scrambling code can be from 0 to 511. If the scrambling code
is known, its value can be entered and the test set can decode and display the CDP of the signal. If the
scrambling code is unknown, BTS Master can be set to auto scrambling (automatically detect the scrambling
code) so that the test set can lock on to the strongest code to decode and display the CDP of the signal.
• Spreading Factor (OVSF Codes)
According to the 3GPP standard the spreading factor can vary from 4 to 512. BTS Master can be set to a
maximum spreading factor of either 256 or 512, depending upon the network requirements.
• Frequency Error
Frequency error is the difference between the received center frequency and the specified center
frequency. This value is tied to the external frequency or when the GPS option is installed it is tied to
the internal OCXO oscillator frequency accuracy. It is typically only useful with the GPS option or a good
external frequency reference.

5
 • Codogram
When codogram is selected the screen displays the changes in code power levels over time.
• Noise Floor
The average power of the unused scrambling codes, displayed in CDP and OTA measurement displays.
•  Threshold
The active channel threshold power level can be set to indicate which code channels are considered active.
Any code channels exceeding this power level are considered active traffic channels. Any code channels
below this power level are considered inactive (or noise). A horizontal red line on the screen represents the
threshold level. BTS Master can set this level automatically based on the received signal. The user can also
opt to manually enter a value in the threshold setup menu.
•  Occupied Bandwidth is the total integrated power occupied in a given signal bandwidth.
• Error Vector Magnitude (EVM) is the ratio, in percent, of the difference between the reference waveform
and the measured waveform. EVM metrics are used to measure the modulation quality of a transmitter. The
3GPP standard requires the EVM not to exceed 17.5%.
•  Symbol EVM (@EVM) is defined as the EVM for a single code channel.
•  Peak to Average Power is the ratio of the peak power and the RMS power of the signal calculated over one
frame interval and is measured in units of dB.
• Peak Code Domain Error (PCDE) takes the noise and projects the maximum impact it will have on all
OVSF codes. PCDE is the maximum value for the code domain error for all codes (both active and inactive).
Note that in the 3GPP standard, to address the possibility of uneven error power distribution in WCDMA,
the EVM measurement has been supplemented with PCDE. The 3GPP standard requires the PCDE not to
exceed –33 dB at a spreading factor of 256.
•  Ec is a measurement of chip energy for CPICH.
•  Ec/Io is the value of the pilot power compared to the total channel power.
• Pilot Dominance is the strength of the strongest pilot compared to the next strongest pilot from different
base stations or from different sectors of the same base station. This value should be >10 dB to make
good measurements.
•  Total Power is the sum of all the scrambling codes; also called Io. It is measured in units of dBm.
•  CPICH Abs Power is the absolute power of the common pilot channel power measured in units of dBm.
•  P-CCPCH Abs Power is the absolute Primary Common Control Physical Channel power measured in units
of dBm.
• S-CCPCH Abs Power is the absolute Primary Common Control Physical Channel power measured in units
of dBm.
•  P-SCH Abs Power is the absolute Primary Sync Channel power measured in units of dBm.
•  S-SCH Abs Power is the absolute Secondary Sync Channel Power measured in units of dBm.
•  PICH is the Paging Indicator Channel Power.

Making WCDMA Measurements

The Anritsu BTS Master MT8222A can measure WCDMA performance in one of two ways, either:
•  Over The Air (OTA) with an antenna.
•  Via Direct Connection of BTS Master to any Node B/WCDMA base station.