Subject: [NEXTEL-1] Making 802.11G Transmitter Measurements

From: Marcel <[email-masked]>
Date: Tue, 14 Jan 2003 17:39:34 -0500

Excellent document worth a read URL for DWG's below
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Making 802.11G Transmitter Measurements

Application Note 1380-4

802.11g is the latest standard in wireless computer networking.It follows
on the developments of 802.11a and 802.11b,combining the speed of 802.11a
with the ow cost and installed base of 802.11b devices.The aim of 802.11g –
to be interoperable with 802.11b and have acceptable performance,cost,and
design time –leads to some unique measurement challenges.

Over the last few years,802.11b-based wireless networking has been an
enormous success,with sales growing at about 10%per quarter.This growth
has been driven by the immense growth in affordable high-speed Internet
access at home as well as the office.802.11b provides an excellent wireless
channel for Internet connections,but it often lacks the speed required
for transferring files and performing other large dataset networking tasks
within the intranet.802.11g expects to fix this by offering data rates as high
as 54 Mbps while maintaining compatibility with legacy 11-Mbps 802.11b
devices and networks.

802.11g will get increased data transfer rates by using the orthogonal
frequency division multiplexing (OFDM)modulation technique that is
used in 802.11a,while maintaining the timing and frequency arrangements
from 802.11b.This application note outlines the transmitter measurements
required for 802.11g and examines how they relate to measurements needed
for 802.11a and 802.11b testing.

?2
802.11g is best understood as the merging of two existing 802.11 wireless
networking standards,802.11a and 802.11b.802.11g will use the two different
modulation techniques of the a and b systems at 2.4 GHz,the current frequency
of the 802.11b standard.This will allow interoperability with 802.11b systems
while utilizing the data rate of the 802.11a standard.

To better understand 802.11g,a quick overview of the modulation techniques
used by 802.11a and 802.11b is useful background.
802.11a uses an orthogonal frequency division multiplexed (OFDM)multi-carrier
signal.In traditional frequency division multiplex (FDM)systems,the spacing
between channels is greater than the symbol rate to avoid overlapping the
spectrums.However,in OFDM systems,the carriers overlap,which conserves
bandwidth.Keeping the carriers orthogonal to each other controls sub-carrier
interference.Orthogonality means there is a mathematical relationship
between the sub-carriers.The 802.11a sub-carriers are bursted giving them a
sinX/X spectral contour (see figure 1).As long as each sub-carrier is placed
in a null of the sinX/X shape,the sub-carrier and its side-lobes will not
interfere with any other sub-carrier.

In 802.11a,all the sub-carriers are spaced 312.5 kHz apart.This spacing is
determined by dividing the Fast Fourier Transforms (FFT)sampling rate,
20 MHz,by the total number of bins required in the FFT,which is 64 points.
One 802.11a symbol is comprised of 48 data sub-carriers,four pilot
sub-carriers,and one nulled sub-carrier (zero sub-carrier).The remaining
11 sub-carriers are discarded.Thus the occupied bandwidth is 53 sub-carriers
multiplied by the spacing of 312.5 kHz,which is 16.56 MHz.The 802.11a
standard specifies that the occupied bandwidth is 16.6 MHz.

The high rate data OFDM signal is divided equally across the 48 carry data
carriers.This reduces the data rate and increases the symbol duration for
the sub-carriers,thus the relative amount of dispersion in time caused by
multi-path delay spread is reduced.Phase noise and non-linear distortion
contribute the most to loss of orthogonality,which results in inter-carrier
interference (ICI).A guard interval is added to help prevent ICI,as well as
inter-symbol interference (ISI).ICI will cause ISI.However,ISI will not cause
ICI.A signal with a slower data rate is more resistant to multi-path fading
and interference.

802.11g Basics
802.11a and OFDM

Figure 1: OFDM carriers and IM components do not overlap?3
802.11b has extended the generic 802.11 standard by adding physical layer
support of two speeds,5.5 Mbps and 11 Mbps.Frequency hopping (FHSS)
signaling was dropped and direct sequence spread spectrum (DSSS)signaling
was retained.Frequency hopping could not support the higher data rates
without violating FCC regulations.This means that 802.11b systems will
interoperate with DSSS systems,but will not work with the older 1-Mbps
and 2-Mbps 802.11 FHSS systems.

The original 802.11 DSSS standard specifies an 11-bit chipping —called a
Barker sequence —to encode all data sent over the air.Each 11-chip sequence
covers a single data bit (1 or 0)and is converted to a waveform,called a
symbol,that can be sent over the air.These symbols are transmitted at a
1 MSps (1 million symbols per second)symbol rate using binary phase shift
keying (BPSK)modulation.BPSK simply switches a carrier between two
different phases to represent a binary 1 or 0.This technique represents
one bit per symbol transmitted.

In the case of 2 Mbps,quadrature phase shift keying (QPSK)is used,doubling
the data rate available in BPSK.QPSK also modulates the phase of the signal,
but uses four phases instead of two.This increases the efficiency of the spectrum
used by transmitting two bits per symbol transmitted,but it reduces the system ’s
immunity to noise and other interference.As more information is placed in
a symbol,the states of the symbol become closer together allowing noise and
other interference to “push ”the signal into the wrong state and cause bit errors.

To increase the data rate in the 802.11b standard beyond 2 Mbps,advanced
coding techniques are employed.Rather than the 11-bit Barker sequence,
802.11b specifies complementary code keying (CCK),which consists of a set
of 64 eight-chip code words for the 5.5 Mbps and 11 Mbps data rates.As a set,
these code words have unique mathematical properties that allow them to
be correctly distinguished from one another by a receiver even in the
presence of substantial noise and multi-path interference (e.g.interference
caused by receiving multiple radio reflections within a building).The 5.5 Mbps
rate uses CCK to encode four bits per symbol,while the 11 Mbps rate encodes
eight bits per symbol.Both speeds use QPSK as the modulation technique and
signal at 1.375 MSps.This is how the higher data rates are obtained.
802.11b data rate specifications

Data rate Code length Modulation Symbol rate Bits/Symbol
1 Mbps 11 (Barker sequence) BPSK 1 MSps 1
2 Mbps 11 (Barker sequence) QPSK 1 MSps 2
5.5 Mbps 8 (CCK) QPSK 1.375 MSps 4
11 Mbps 8 (CCK) QPSK 1.375 MSps 8

Figure 2: 802.11b data rate versus modulation type
802.11b and BPSK, QPSK, CCK,
and Barker Codes?4
bits In 11 complex chips out
Modulation:
BPSK/QPSK
Barker sequence
+1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1, -1
Map
1 of 4/64
8-chip sequence
selector
DQPSK
rotate
8 complex
chips out
d2-d7
d0, d1
4/8 bits
d0-d3/d7
802.11g LONG preamble packets
Preamble/Header
(Barker words – 802.11-compatible)
OFDM
SYNC
PSDU selectable OFDM symbols
at 6, 9, 12, 18, 36, 48, or 54 Mbps
Signal
extension
192 µs (long)
96 µs (short)
12 usecs 6 µs

Uses OFDM modulation

Figure 3: 802.11b Barker modulator block diagram

Figure 4: 802.11b CCK modulator block diagram

Figure 5: Example 802.11g burst profile

In order to get the higher data rate of 802.11a and maintain compatibility
with 802.11b systems,an 802.11g system must let the older 802.11b systems
know that it is active and using the channel.802.11g achieves this by
transmitting an 802.11b-compatible header with the standard Barker code
modulation.This lets all nearby 802.11b systems know that a device is trans-
mitting and that the 802.11b devices should stay quiet for a given amount of
time.The 802.11g system then transmits the data part of the packet,which
may not be compatible with 802.11b systems.The 802.11b systems cannot
receive this signal and they do not interfere with it.With this method,
802.11g and 802.11b systems can co-exist without interference,and the
802.11g systems can take advantage of the higher data rates that OFDM
and PBCC can offer.
Merging 802.11b and 802.11a
to Get 802.11g?5

Figure 7: Example 802.11g modulation types (right to left: BPSK, QPSK, 8PSK, and 64QAM)

The final modulation and encoding type allowed in 802.11g is packet binary
convolutional coding (PBCC).This method has recently been used in 802.11b
systems to increase the data rate from 11 Mbps to 22 Mbps.PBCC is basically
a more complex version of CCK,allowing twice as much data to be encoded
for a given bandwidth.Normally,a doubling of the data rate within a given
bandwidth requires the signal-to-noise ratio of the channel to increase by 3 dB.
By using PBCC,only a 0.5 dB increase is required for this data rate increase.
In PBCC,the real data rate increase comes from moving from QPSK to 8PSK
as the modulation technique,while the reduction in required signal-to-noise
ratio comes from using PBCC over CCK.

In 802.11g,PBCC is an optional mode.It is allowed by the standard,but not
required in any implementation.If implemented,it works exactly like the
OFDM mode of 802.11g;an 802.11b-compatible header is transmitted,then
the PBCC section is transmitted.

Function Specification Notes
Carrier frequency 2.400 to 2.4835 GHz 25 MHz channel spacing,
(ISM radio band) 3 non-overlapping channels
Modulation type OFDM – orthogonal frequency division multiplexing PBCC is optional
Division multiplexing
CCK – complementary code keying
PBCC – packet binary convolutional coding
Data rate 1, 2, 5.5, 6, 9, 11, 12, 18, 24, 36, 54 Mbps 11 Mbps CCK and 24 Mbps
OFDM are required
Output power Normally up to 100 MW, 1 W allowed Power level dependant on
modulation used
Figure 6: 802.11g WLAN standard specs
802.11g and PBCC?6

This section describes some of the common tests used when designing and
manufacturing 802.11g transmitters and modulators.As 802.11g is a system
designed for low cost and high data rates,care must be taken in the design
to ensure interoperability with other 802.11g and 802.11b equipment,while
also minimizing part cost and count.Much of the complexity in 802.11g systems
comes in the realm of modulation and amplification,and this section will
describe how to test these characteristics.

With these concerns in mind,the most powerful measurement device for
802.11g transmitters will be a vector signal analyzer (VSA).A VSA deals with
a signal in the time domain,measuring signals ’timing and amplitude.A VSA
can also apply Fast Fourier Transforms (FFTs)to analyze the signal in the
frequency domain or it can apply advanced digital signal processing (DSP)
techniques to provide modulation analysis capabilities.By analyzing the
802.11g signal in the time,frequency,and modulation domain,you can be
sure that your design will perform as you expect it to.

Transmitter Measurements

As a bursted and digitally modulated signal,802.11g has a range of timing
issues to handle.These issues range from transmitter turn-on time to
symbol timing.

Transmitter turn-on transient

Measuring the turn-on transient is crucial in a rapidly bursted signal such as
802.11g.Design issues such as the linearity of the amplifier,stability of the
voltage-controlled oscillator (VCO),slew rate of the overall system,and timing
alignment of the modulator come into play here.Looking at the turn-on
transient of an 802.11g burst provides insight in all these areas.
The time for a burst to go from the noise floor to a prescribed power level
or percent of final output power can be easily measured using the marker
functions of a VSA.This test also will quickly bring out any overshoot,under-
shoot,or ringing in the system by visually displaying the power versus time
curve of the burst.Additionally,the VSA can measure the phase of the signal
as it turns on,which is crucial to maintaining orthogonality in OFDM systems.
The duration of the turn-on transient is important to 802.11g because a short
transient is required to get high data throughput,while avoiding the inefficiency
of waiting for the transmitter to stabilize before starting the preamble and
synchronization sections of the burst.

The frequency spectrum of the signal during turn-on also is important.
Sometimes even moderate over-shoot can stimulate non-linearity in the
power amplifier,generating spurious emissions or a broadening of the
overall spectrum of the transmitted signal.
Time Domain Tests?7

The combination of FFT analysis and time gating capabilities offered in vec-
tor signal analyzers is designed for evaluating power-on/off of signal bursts.
A specific time portion of a signal can be selected and analyzed in the fre-
quency domain,showing the specific spectrum for that specific time.This
capability helps the designer narrow in on spectral problems and isolate
where in the burst they originate.See figure 8 for an example of time gating
on a WLAN signal.

Another test of the initial portion of an 802.11g burst is determining when
the modulation actually starts.802.11g uses the very beginning portion of
the burst to align the receiver with the transmitter with respect to frequency
and symbol timing.If the modulation of the carrier begins before the VCO is
settled,the receiver may synchronize to the wrong frequency,or it may miss
the sync altogether,leading to a missed packet.If the modulation begins too
late,time is lost and frequency spectrum is wasted reducing system efficiency.

A: Ch1 Gate Spectrum

B: Ch1 Main Time

10 dBm

LogMag
10
dB
/div
-90
dBm
0 dBVpk
LogMag
10
dB
/div
-100
dBVpk
Center: 2.412 GHz
RBW: 811.845 kHz
Start: 0 sec
Range: -5dBm
Span: 34.375 MHz
GateLen: 4.704545 uSec
Range: 177.8279 mV
Stop: 23.25 uSec

TIME GATES

Figure 8: 802.11g burst with spectrum (top) and gated time (bottom)?8
A bursted system such as 802.11g also has to deal with the effects of turning
the carrier on and off and how it relates to the frequency of the carrier over
time.The spectrogram is a unique way of looking at signal frequency changes
over time.A spectrogram shows frequency,time,and amplitude on one
image.Frequency is displayed on the horizontal axis,much as it would be on
a spectrum analyzer.Amplitude is coded in color or shades of gray color.In
a spectrogram,elapsed time is shown on the vertical axis meaning that the
display scrolls upwards as time moves on.As shown below,this plot easily
shows the stability of an oscillator as it turns on.

This measurement is important to 802.11g systems because modulation
techniques such as OFDM require a highly stable carrier in terms of frequency.
Since the individual carriers in the OFDM modulation technique are only
312.5 kHz apart,a small misalignment in the 2.4 GHz carrier can cause seri-
ous problems in the radio link.It is also prudent to be aware of thermal and
power supply issues with your transmitter,especially as 802.11 systems move
into one chip solutions,with the power amplifier and VCO on the same chip
generating a high peak to average power ratio signals.

Frequency Versus time Tests

Figure 9: Spectrogram display of a VCO turn-on transient?9

As a standard that includes multiple modulation and signaling types,
802.11g requires a range of modulation tests to properly evaluate its
performance.
EVM

Many problems are possible in a digitally modulated signal.A good measure-
ment to start with is error vector magnitude (EVM).EVM is a measurement
of the real,transmitted signal against a perfect,theoretical signal.This
measurement gives an overall view of the quality of the modulation on the
signal,which in turn gives a sense of how well a receiver would be able to
receive and interpret the signal.Overall,low EVM is highly correlated with
a low bit-error rate (BER).

Many modulation issues will negatively impact the EVM of a signal.Anything
that affects the amplitude or phase of a complex modulated signal such as
OFDM or QPSK will show an increase in EVM.Some of the more common
sources of elevated EVM are non-linearity in of the power amplifier,the
overall quality of the modulator,the filtering applied to the signal,and
phase noise in the VCO.

The overall OFDM signal is a large composite and any impairment to this
signal will probably cause an overall composite EVM increase.Because of
this,one should look at the EVM of not just each sub-carrier by itself,but
the EVM of the signal as a whole.The ability of a VSA to show you EVM
versus symbol or EVM versus sub-carrier can be crucial in allowing a
designer to quickly locate the source of the impairment.
Modulation domain tests

Figure 10: Impaired OFDM constellation and EVM?10
Constellation diagram

Locating problems within a modulator is often expedited through the use of
a constellation diagram.This display quickly shows you where all the symbols
are landing,giving a visual representation of the quality of the signal.
An example of a problem a constellation diagram can help the designer
identify is amplifier clipping.In single carrier modulation schemes,clipping
is characterized by the outermost symbols falling short of their theoretically
correct point.This indicates that the output section does not have sufficient
range to meet the demands of the modulator.In OFDM,symbols are spread
across many carriers.Individual carriers are independent of one another,but
sometimes all of the carriers can add together and require a very high output
from the power amplifier.If this brief peak is high enough,the amplifier will
clip and many carriers will be affected at once.A large,transient error in the
constellation is often indicative of amplifier clipping on OFDM systems.As
figure 11 shows,this error manifests itself as random high EVM points in the
EVM versus time (i.e.symbol)measurement and as an imperfect CCDF curve
as well.

Clipping in QPSK-modulated systems is not as common because they typically
use broad filters that limit overshoot of the constellation.However,issues
such as phase noise or incorrect filtering are readily apparent in a QPSK
constellation.Phase noise in a QPSK signal will spread out the constellation
points around their targets and produce a line of points rather than a single
point.Filtering problems often present as multiple stable points around the
target point.These stable points are caused by the ISI that can be present
with the wrong filtering coefficients.They are stable due to the deterministic
error caused by the previous transmitted bit(s).
Figure 11: Clipped OFDM EVM and CCDF?11

Error versus frequency or time

One of the biggest problems you have after measuring a signal is figuring
out why it does not perform as expected.Often the issue turns out to be a
spur in the signal or an issue that only appears during certain time periods.
These problems can be very hard to spot with a traditional spectrum analyzer.
A VSA becomes a very powerful tool in finding these frequency or time
related problems.By giving the designer the option to see errors versus
time,or errors versus frequency,problems and their causes can be easily
identified.A spurious emission in the middle of the signal may not show up
much above the peak of the signal on a swept frequency measurement,but
in error versus frequency it can easily be seen that most of the error in the
signal comes from one specific frequency.By knowing this the designer can
focus in on that problem that would have been invisible to other instruments.
Errors versus time problems can be especially hard to find.They can be
serious enough to cause high bit error rates,but may not occur often
enough to modify the spectrum in any serious way.By looking at error
versus time,it becomes easy to spot when the errors occur and if they are
periodic in any way.Knowing this allows a designer to look for issues in
the circuit that may be able to cause errors during these times and start
the focus in these areas.Often times these problems can be traced back
to power supply noise or feedback in the system
Capture and post-analysis

As with almost all measurements on a VSA,the above modulation domain
measurements can be made on a specific segment of the burst.The capability
of a VSA to capture a signal to memory and then post-analyze it means
that it is possible to analyze a subset of the captured data.In the case of
802.11g device analysis,it is often very useful to look only at the turn-on
transient,preamble,or data portions of the packet.802.11g may use
different modulation techniques in the preamble and data burst,and
it may be prudent to examine these areas separately.

It is also worth noting that the large memory and PC-connectivity of some
VSAs allows the capture of many bursts at once,and the archiving of these
recordings for analysis or comparison at a later date.By applying DSP to
this data,it is even possible to shift center frequencies and reduce the
span of captured data to fit the analysis needed.Capturing more than is
needed in the beginning and then reducing the data afterwards helps
ensure that you will have the signal you want to analyze and that you
will be able to do as much analysis as is needed.

Figure 12: OFDM EVM versus frequency with spur?By internet, phone, or fax, get assistance
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Product specifications and descriptions in this
document subject to change without notice.
© Agilent Technologies, Inc. 2002
Printed in USA October 31, 2002
5988-7813EN
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In the future,wireless LAN systems
may move to higher data rates,wider
bandwidths,and more complex
modulation techniques.When
choosing a measurement system,
the future developments of WLAN
should be taken into account since
these standards continue to evo ve.
802.11g is already an extension
and homologation of the previous
802.11a and 802.11b standards.
With the past standards already
being updated,current standards
being quickly deployed,and future
standards requiring even more
advanced measurement techniques,
choosing a f exible,upgradeable
measurement platform will help
make WLAN measurements easy,
fast,accurate,and informative
today and in the future.
Final Analysis

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