Jamming Tolerance of an
Orthogonal Frequency Division
Multiplexing Based System
kari.pietikainen@tkk.fi
5.4.2005
supervisor: Sven-Gustav Häggman, Communications Laboratory, TKK
Contents
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Background
OFDM basics
IEEE 802.11g
Measurement setup
Results
Conclusions
Summary
Background
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IEEE 802.11 based WLAN systems
gaining popularity
Flexible system, high data rates,
easy to use
Intented for indoor office and urban
outdoor use
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Possible to use in rural and hostile
(interference) environment?
OFDM Basics (1/3)
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Can be considered as multicarrier
modulation or multiplexing technique
Used in: WLAN, DAB, DVB, xDSL, PLC
Divides frequency selective channel into
several parallel flat fading subchannels
Data is serial-to-parallel converted before
transmission
Symbols are transmitted on parallel
subchannels
OFDM Basics (2/3)
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Signal processing made digitally in
the frequency domain
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Guard time is added between
consecutive
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FFT/IFFT pair
reduces the effects of multipath
propagation
Tolerant to frequency selective
fading
Flexible data rates
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convolutional coding
modulation methods
OFDM Basics (3/3)
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Advantages
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spectral efficiency
simple implementation
tolerant to ISI
Disadvantages
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bandwidth loss due guard time
prone to frequency and phase offset
errors (ICI)
peak-to-average power problem
Subchannels
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Frequency-selective channel is divided into flat
fading subchannels
Pilot subchannels are used to correct frequency
and phase offsets.
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Channel estimation.
4 pilot subchannels
48 data subchannels
IEEE 802.11g WLAN
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Currently IEEE 802.11b is the most
common WLAN standard.
Recent addition to the 802.11 family
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2,4 GHz frequency band (U-NII, unlicensed)
adds OFDM to the physical layer
faster data rates 6-54 Mbit/s
(1-11 Mbit/s in 802.11b)
MAC layer is the same for both 802.11b
and 802.11g standards.
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interoperability
Measurement setup (1/2)
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Ad-hoc WLAN
connection is
interfered with
single tone and
raised cosine
signal
10000 1000-byte
UDP-packets sent
(Client >> Server)
PER is calculated
desired signal
interference
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Measurement setup (2/2)
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Purpose is to test receiver
functionality.
Single tone interference in AWGN
channel
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’spectra’
Wideband interference in AWGN
channel
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50 MHz raised cosine signal
5 MHZ raised cosine signal
Results (1/4)
4 pilot subchannels 48 data subchannels
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Vendor 1’s
operation is
interesting.
Two ’lobes’.
Subchannels from
-XX to XX have no
influence on the
system.
Pilot subchannels
have minor
influence.
Results (2/4)
4 pilot subchannels 48 data subchannels
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Vendor 2’s
operation is erratic
Also two ’lobes’
Every Xth
subchannel has no
influence
Subchannels from
-X to X, no
influence
Results (3/4)
4 pilot subchannels 48 data subchannels
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Vendor 3’s
interface operates
as expected.
Data subchannels
have minor
influence.
Interfering the
pilots deteriorates
the connection.
Results (4/4)
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Under wideband
interference
Vendor 2 & 3
operate similarly
The effect of
modulation and
coding is clearly
visible
Conclusions
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When narrowband interference is present,
there are major differences between
interfaces.
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channel estimation, channel equalization,
receiver architechture.
Interfaces operate similarly when
wideband interference is applied
Vendor 1 operates erraticly
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used modulation or coding rate has no effect,
connection is easily clogged.
Summary
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OFDM is very effective physical layer
technology
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high data rates
tolerance to multipath propagation effects
Major differences between manufacturers
(transceiver chipsets)
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receiver implementation
price
Thank you.
Questions?