802.
11 a/g OFDM PHY
802.11 wireless networks, chapter 11
OFDM wireless LANs, part of chapter 3
S-72.333, Postgraduate Course in Radio
Communications
Juha Villanen, Radiolaboratory
Email: juha.villanen@hut.fi
Outline
Introduction
OFDM overview
OFDM in 802.11a
802.11a vs. 802.11g
Discussion
References
Homework
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Introduction
IEEE 802.11 standards:
Currently, 802.11b the most common. 802.11a/g,
however, are increasing their popularity.
Products of all three standards in the market
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OFDM overview
Available bandwidth divided into subcarriers
Subcarriers overlapping but orthogonal with
respect to each other
at the peak of each subcarrier, the other
subcarriers have zero amplitude
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OFDM overview
OFDM transmitter block diagram (N subcarries):
After modulation, N parallel symbol streams
(at 1/N of the original rate) are fed to the N-
point Inverse Fast Fourier Transformer (IFFT)
After IFFT, cyclic prefix (CP) is added (see
the next slide) at the beginning of the symbol
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OFDM overview
Problems in OFDM:
Frequency shift due to e.g. Doppler effect
Inter-carrier interference (ICI)
If large delay spread
Inter-symbol interference (ISI)
OFDM solution: guard time
and cyclic prefix.
Orthogonality maintained
Works well for delays
shorter than the guard time
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OFDM overview
Strictly speaking, convolution coding is not part of
OFDM. However, OFDM usually operated in
applications with deep fading.
Convolution coding often used for error
correction in conjunction with OFDM
(COFDM)
In OFDM, cosine windowing is often used to bring
the signal gradually up and down:
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OFDM in 802.11a
Wide variety of choices in modulation and coding
data rates from low and reliable to high and
more fragile can be realized
Data rate: 6, 9, 12, 18, 36, 48 or 54 Mbps
Modulation: BPSK, QPSK, 16-QAM or 64-QAM
Coding rates: 1/2, 9/16, 2/3 or 3/4
Number of Subcarriers: 52
Number of Pilot Tones: 4
OFDM Symbol Duration: 4 µ sec
Guard Interval: 800 η sec
Subcarrier Spacing: 312.5 kHz
Signal Bandwidth: 16.66 MHz
Channel Spacing: 20 MHz 8
OFDM PHY in 802.11a
Structure of an 20 MHz OFDM channel. Pilot
carriers are used for monitoring path shifts and ICI.
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OFDM PHY in 802.11a
Transmit spectrum mask for 802.11a. The center
frequency of the next carrier at 20 MHz.
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OFDM PHY in 802.11a
Encoding details for each data rate in 802.11a PHY:
Support is required for 6, 12 and 24 Mbps
Either ½, ¾ or 2/3 of the coded bits are redundant
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OFDM PHY in 802.11a
Constellation point labeling in 802.11a (16QAM):
Natural order Gray coded
In gray code, two-bit errors impossible
between neighboring points
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OFDM PHY in 802.11a
Hard and soft decision demodulators:
Hard decision: Output of the modulator zeros and ones.
Constellation point closest to the received symbol is
selected. For example, decision boundaries of QPSK
constellation:
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OFDM PHY in 802.11a
Soft decision: Output of the modulator retains information
about the reliability of the decision. The reliability of the
detected bits coded in the absolute value of the bits. The
absolute value is the distance to the decision boundary.
Soft decision can greatly improve the performance of
channel coding chemes
used in OFDM PHY of 802.11a!!
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OFDM PHY in 802.11a
PLCP (Physical Layer Convergence Precedure): Boundary between MAC
and wireless medium. OFDM 802.11a PLCP framing format:
PLCP preamble: used for synchronization of various timers between the
transmitter and the receiver.
Rate: Indicate the data rate applied in the DATA-field
Length: Number of bytes in the embedded MAC frame
Tail: 0-bits used to unwind the convolution code
Service: Transmitted in the data field at the data rate of the MAC frame.
Currently used to initialize the MAC frame scrambler
Pad Bits: Data field required to be integer multiple of block size => padding 15
OFDM 802.11a vs. 802.11g
Good sides of OFDM 802.11a compared to OFDM 802.11g
The unlicenced 5.2 GHz band provides more spectrum space than
the 2.4 GHz band. In addition, there are few devices on the market
operating at 5.2 GHz, whereas 2.4 GHz is heavily used.
Drawbacks of OFDM 802.11a compared to OFDM 802.11g
Higher frequencies have higher path losses
802.11a base stations have to be deployed more densely
than 802.11b/g base stations. At the highest data rates, line of
sight usually needed
Not compatible with the most popular standard 802.11b
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OFDM 802.11a vs. 802.11g
Data rates vs. operating distances of different 802.11
standards:
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Discussion
Both OFDM 802.11a and OFDM 802.11g
have their good sides and drawbacks.
802.11g, however, is more promising due to
larger operating distance and compatibility
with the widely used older 802.11 standards
The standardization of 802.11g is first step
towards dual-band WLANs capable of
operating at 2.4 GHz and 5.2 GHz using
OFDM (forward compatibility of 802.11g with
802.11a)
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References
[1] 802.11 Wireless Networks, The definitive guide, Matthew S. Gast, O'Reilly
2002.
[2] OFDM Wireless LANs: A Theorethical and Practical Guide, Juha Heiskala,
John Terry, Sams Publishing 2002.
[3] IEEE Standards, 802.11a:
http://standards.ieee.org/getieee802/download/802.11a-1999.pdf
[5] http://www.54g.org/docs/802.11g-WP104-RDS1.pdf
[6] http://nwest.nist.gov/mtg3/papers/chayat.pdf
[7] http://www.sss-mag.com/pdf/802_11g_whitepaper.pdf
[8] http://www.commsdesign.com/design_corner/OEG20020201S0035
[9] http://www.skydsp.com/publications/4thyrthesis/
[10] http://www.iec.org/online/tutorials/ofdm/topic04.html?Next.x=40&Next.y=18
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Homework
Compare the performance of different coherent modulations
schemes (ASK, PSK, QAM). Why FSK cannot be used in
OFDM applications? Justify the selection of BPSK, QPSK,
16-QAM and 64-QAM for 802.11a WLANs.
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Thank You!
Questions?
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