5091-7196E.pdf

Agilent PN 89400-3

CDMA Measurements with the 89400 Series

Vector Signal Analyzers

Product Note

Measurement challenges in CDMA

Code division multiple access (CDMA)

communications systems combine

digital modulation and spectrum-

spreading encoding techniques to

create broadband signals which are

immune to noise, and which them-

selves look like noise. Their natural

noise immunity allows multiple

CDMA signals to overlay each other

in a common band. Measurements of

CDMA signals present new challenges

to test instruments.

interest. Power in adjacent channels

makes it impossible to use power or

peak-power meters without external

filtering. You need an analyzer that

makes combined time- and frequency-

selective measurements.

In the precision RF front-end of

the Agilent Technologies 89400

Series VSAs, a block downconverter

tunes to a 7-MHz span anywhere

within a 2.65-GHz range for the

89441 and 1.8-GHz range for the

89440A. The down-converted sig-

nal is filtered and digitized by a

high-speed, high-resolution A to D

converter. Subsequent digital mix-

ers and filters allow the span to

be narrowed to as little as 1 Hz at

arbitrary center frequency. High-

performance processors provide

fast spectrum, time domain, and

demodulation analysis, with flexi-

ble measurement displays.

Other measurements of interest to

CDMA system designers include digital

modulation analysis, filter frequency

response, amplifier intermodulation

and compression, identification and

quantification of AM or PM spurious

sidebands, and phase noise character-

ization. These common measurements

of digital and analog radio systems

have heretofore required a number of

different analyzers.

In the world of CDMA, a key measure-

ment is the power spectrum within a

communications channel. This power

is broadband, noise-like, and varies

over time. Both short-term and long-

term average power spectra are of

The Agilent 89400 Series vector sig-

nal analyzers (VSAs) bring to bear on

these problems an impressive array

of analysis tools. Their primary con-

tribution is the ability to isolate and

digitize a frequency band up to 7 MHz

wide, in a range of 0 to 2.65 GHz on

the 89441A and 0 to 1.8 GHz on the

89440A. (Up to 10-MHz spans are

available in baseband mode.) The dig-

itized waveform can be displayed and

analyzed in time, the frequency spec-

trum can be calculated using the fast

Fourier transform (FFT), and inten-

tional or unintentional modulation

can be characterized using computed

demodulation. (AM, FM, and PM, and

optional digital formats can be ana-

lyzed.) Markers on the time display

can be used to limit the portion of the

time record for which the spectrum

or demodulation is calculated.

Flexible digital filtering eliminates

unwanted signals from time and

demodulation analyses.

These features are all available in the

analyzers’ vector mode. In scalar

mode, multiple spans are pieced

together by the analyzers to show

spectra with displayed spans of up to

2.65 GHz.

Let’s see how these features can be

used to make your CDMA measure-

ments:

Band-limited power measurements

One class of CDMA measurements for

which the 89400 Series VSAs are

right at home is power measurements

within and adjacent to a CDMA chan-

nel. Suppose you were to set up the

analyzer in vector mode with a fre-

quency span of 5 MHz, and set the

center frequency of the analyzer to

the carrier frequency of a CDMA cel-

lular base station. You would see a

spectrum display similar to Figure 1.

You are looking at the 1.23 MHz wide

CDMA modulation and some adjacent

sideband energy. The display would

change rapidly due to the pseudo-

random nature of a CDMA waveform.

To calculate average power over time,

you would turn on RMS averaging.

The speed of the 89400 Series VSAs

allows you to average hundreds of

spectra in less than a second and

display the result, which will look

similar to Figure 2.

Figure 1. CDMA spectrum

The FFT calculation used in the

89400 Series VSAs is like measuring

the output power of hundreds of nar-

rowband filters with center frequen-

cies distributed across the span, all

operating on the same signal. Because

of this, spectrum averages are accu-

rate at all frequency points regardless

of power changes during the measure-

ment. By contrast, a swept analyzer

calculates the average power at a sin-

gle frequency at a time, and moves

on. If the signal’s power changes dur-

ing the sweep, the resulting average is

inaccurate.

Band-power markers allow easy calcu-

lation of the power within the 1.23-MHz

band. The result is displayed at the

lower left of the screen.

Figure 2. Averaged CDMA spectra

The band-power markers can now be

moved to a span of 30 kHz centered

750 kHz above or below the CDMA

carrier frequency and the power in

that band calculated. The ratio of

these two band measurements, each

normalized to a 1-Hz bandwidth, con-

stitutes an occupied bandwidth test.

The 89400 Series VSAs make this test

accurately and with ease.

The spot marker on the time display

shows peak power versus time. You

can also place band-power markers

on a time display and display average

power during a selected time period.

response, which happens over the

course of 100 ms, you can use the

time capture feature of the 89400

Series VSAs. This simply stores the

output of the digitizer and digital fil-

ter in memory: up to 1-million sam-

ples with the extended capture mem-

ory (Option AY9).

The markers shown on the time dis-

play, called gate markers, demonstrate

an important feature of these analyz-

ers: the ability to do time-selective fre-

quency analysis. The spectrum shown

on the top of the display consists of

the FFT of the data within the gate

markers only. This calculation may be

windowed with any of the three avail-

able windows, or left unwindowed.

You can adjust the gate width and

position to observe the spectrum

before, during, and after the tran-

sient. For example, you may wish to

position the gate on the transient to

see if the spectrum splatters signifi-

cantly out of the assigned channel

during turn-on. In this display the

gate is shown well within the CDMA

burst, and band-power markers on

the top display show the in-channel

power.

You can then analyze any part of the

data after capture as if it were your

signal source. External triggering

allows you to synchronize the start of

the time-capture measurement with

the increase in base-station power.

Then you can look at the mobile

transmitted power at any time

between the start of the measurement

and the end of the captured data. The

base-station power is excluded from

the measurement because of the digi-

tal filtering. For more details on fre-

quency and time-selective measure-

ments, see product note 89400-1

(5091-7194E).

Time-variant power measurements

Another common measurement in

CDMA is to observe the power varia-

tions of a mobile phone over time.

Power in a mobile CDMA phone is

gated on and off in 1.25-ms bursts

when the data rate is less than the

full 9600 bits per second.

To measure the power during such a

burst, you set the span of the analyz-

er to 2 MHz and the center frequency

to the channel of interest. Increase

the number of measurement points

from the default of 401 to 3201. This

allows a maximum time record length

of 1.6 ms. IF triggering allows the

measurement to start based on when

the signal energy within the measure-

ment frequency span crosses a

threshold. You set the threshold to a

few dB below the peak power in the

burst and select pre-trigger delay to

view the transient. If the analyzer is

set to trigger continuously, this trig-

ger setup is interactive, much like set-

ting the triggering on an oscilloscope.

After setup, you can set the analyzer

to trigger once and retain the data for

post-measurement analysis.

Other system measurements

CDMA systems consist of digital mod-

ulators, filters, amplifiers, oscillators,

and a host of other components com-

mon to radios, all of which must work

together to successfully generate a

CDMA signal. If bit- and frame-error

tests reveal transmission of the

CDMA signal is impaired, the 89400

Series VSAs can help determine if

any of these analog components are

at fault.

Time capture for analysis of long

CDMA transients

A power control circuit in the mobile

unit is required to change its output

power in response to a change in the

power received from the base station.

To measure this power-control

Figure 3 shows a split display with

the time record on the bottom. The

ordinate of the display is the magni-

tude of the time record in dBm. Note

that this is similar to a zero-span

measurement done with a swept

spectrum analyzer, with an important

difference. The 89400 Series VSAs

capture all information within the

span, and weights it equally.

Information outside the span is

sharply attenuated. The 3-MHz wide

Gaussian resolution-bandwidth filter

of a swept analyzer attenuates the

CDMA signal at either side of the

channel because the filter is not flat,

and includes much more out-of-band

noise because its rolloff is more grad-

ual. This makes accurate power meas-

urements difficult.

Figure 3. Measurement of a time-variant CDMA signal, showing peak power versus time

on the lower trace and the spectrum of a selected time span on the upper trace.

Vector modulation analysis (Option

AYA) allows you to analyze QPSK,

and in future versions, offset QPSK

signals. These modulation formats are

used, for example, in a base-station

pilot signal or a mobile access pream-

ble. The analyzer acquires a time

record representing up to 4096 sym-

bols, determines the symbol clock,

and calculates the error between the

received vector trajectory and the

ideal. This analysis requires that the

ideal filter shape be defined.

The outstanding linearity of the

89400 Series VSAs makes them well

suited to perform amplifier compres-

sion and intermodulation tests. To

support such testing in vector mode

(within 7-MHz bandwidths), the RF

source (Option AY8) provides sinu-

soidal, pseudo-random, chirp, or user-

defined arbitrary signals.

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With a second 10-MHz input channel

(Option AY7), the analyzers can per-

form frequency response measure-

ments on two-port networks in the

frequency range of 0 to 10 MHz. The

internal source provides single tone

and several wideband signals to sup-

port such tests.

The analyzer can use standard raised-

cosine, root-raised-cosine, Gaussian,

and user-defined filter shapes. Note:

This analysis is not a complete CDMA

demodulation in that the bits recov-

ered are simply the PRN code, but it

is sufficient to determine the health

of the modulator. Available displays

include I/Q pattern, constellation, eye

diagrams on I or Q channel, vector

magnitude error, and vector phase

error.

Complex measurements can be easily

automated using Instrument BASIC

(Option 1C2). A “keystroke recording”

feature records your setups as you

make them and turns them into

BASIC I/O statements. A PC keyboard

attached to the analyzer allows you to

further edit the program, adding

branching, calculation, I/O, and dis-

play enhancements.

AM, FM, and PM demodulation of

oscillator signals can reveal the

nature and magnitude of spurious sig-

nals and noise. Time and frequency

displays of the demodulated data are

available. The use of the PM demodu-

lation analysis to characterize phase

noise is detailed in product note

89400-2 (5091-7193E).

The 89400 Series VSAs perform

noise-power measurements required

in CDMA with ease and calibrated

accuracy not found in swept analyz-

ers. Their combined time and fre-

quency selectivity, together with

demodulation analysis, allow you to

see details of signals in ways never

before possible. In addition, these

analyzers bring a broad array of tools

to assist you in CDMA system design

and troubleshooting.

Despite their advanced features, the

89400 Series VSAs were designed to

be familiar to users of traditional

spectrum analyzers. In scalar mode,

the 89441A can display a power spec-

trum over any frequency span up to

2.65 GHz (89440A up to 1.8 GHz).

With arbitrary resolution bandwidths

down to 312.5 mHz, you can easily

find low-level spurious signals that

otherwise would be obscured by

noise. You can use this mode, for

example, to determine compliance of

the CDMA system to FCC emission

standards.

Product specifications and descriptions in this

document subject to change without notice.

Printed in U.S.A. 10/00

5091-7196E

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