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7/30/2019 1. OFDM History
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OFDM History High-data-rate communications systems are limited not by noise,
but often more significantly by the intersymbol interference (ISI)due to the memory of the dispersive communications channel.
If the symbol rate exceeds the duration of channel impulse response(CIR), mechanisms must be implemented in order to combat the
effects of ISI. Channel equalization techniques can be used to suppress the echoes causedby the channel.
Significant research efforts have been invested into the development of suchchannel equalizers
Another approach is to utilize an FDM system which employs a set of subcarriers in order to transmit information in parallel subchannels over the same channel.
The data throughput of each channel is only a fraction of the data rate of thesingle-carrier system having the same throughput.
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OFDM is a considerable option when the channel introduces ISI
Applications: ADSL, DAB, DVB, Hiperlan/2, ...
Frequency Selective Channel
OFDM Orthogonal Frequency Division Multiplexing 2001-05-31
Uses the entire bandwidth
Splits bandwidth into subchannels Short symbol times
This causes ISI
Sends information in parallel
OFDM: orthogonal subcarriers
Single Carrier Multicarrier
Why OFDM?
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OFDM History In 1971, Weinstein suggested using a digital implementation based on the DFT.
The DFT is by its nature cyclically redundant in the frequency domain. The associated harmonically related frequencies can be used as a set of subchannels carriers
required by the OFDM system.
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Multipath can be described in two domains:time and frequency
time
time
Sinusoidal signal as input
time
time
Sinusoidal signal as output
f
Frequency response
Time domain: Impulse response
Frequency domain: Frequency response
time
Impulse response
timetime
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Modulation techniques:monocarrier vs. multicarrier
To improve the spectral efficiency :
To use orthogonal carriers (allowing overlapping ) Eliminate band guards between carriers
Selective Fading
Very short pulses
ISI is compartively long EQs are then very long
Poor spectral efficiencybecause of band guards
Drawbacks
It is easy to exploitFrequency diversity
Flat Fading per carrier
N long pulses
ISI is comparatively short N short EQs needed
Poor spectral efficiencybecause of band guards
Advantages Furthermore
It allows to deploy2D coding techniques
Dynamic signalling
N carriers
BPulse length ~ N/B
Similar toFDM technique
Data are shared among several carriersand simultaneously transmitted
BPulse length ~1/B
Data are transmited over only one carrier
Channel
Guard bands
Channelization
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Orthogonal Frequency Division Modulation
Data coded in frequency domain
N carriers
B
Transformation to time domain:each frequency is a sine wavein time, all added up.
f
Transmit
Symbol: 8 periods of f 0
Symbol: 4 periods of f 0
Symbol: 2 periods of f 0
+
Receivetime
B
Decode each frequencybin separately
Channel frequencyresponse
f
f
Time-domain signal Frequency-domain signal
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Each subcarrier is modulated at a low enough rate that dispersion(ISI) is not a problem. Subcarriers must be spaced so that they donot interfere.
f 0 f 1 f N-1Bandwidth, B
x
x
x
cos(2 p f 0 t )
cos(2 p f 1t )
cos(2 p f N-1t )
LPF
LPF
LPF
P/S
Demodulator
r(t)
Detector
Detector
Detector
S(f)
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Orthogonal Frequency Division Multiplexing (OFDM)
OFDM is a special case of multicarrier transmission that permitssubchannels to overlap in frequency without mutual interference increased spectral efficiency.
OFDM exploits signal processing technology to obtain cost-effective meansof implementation.
Mulitple users can be supported by allocating each user a group of
subcarriers.
Bandwidth, ~ B/2
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Spectrum of OFDM Signal
When N is large, the power spectral density (PSD) of the transmitted signal is
PSD of OFDM Signal
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Textbooks and References
Wireless OFDM Systems: How to Make Them WorkMarc Engels, Editor
OFDM Wireless LANs: A Theoretical and Practical Guide Juha Heiskala and John Terry
OFDM for Wireless Multimedia Communications
Richard Van Nee and Ramjee Prasad Single and Multi -Carrier Quadrature Amplitude Modulation Lajos Hanzo, William Webb, and Thomas Keller
ADSL, VDSL, and Multicarrier Modulation John Bingham
Implementing ADSL David Ginsburg
DSL Advances Massimo Sorbara, John Cioffi, and Peter Silverman
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OFDM OFDM also known as
Multi-Carrier or Multi-Tone Modulation DAB-OFDM
Digital Audio Broadcasting DVD-OFDM
Digital Video Broadcasting ADSL-OFDM
Asynchronous Digital Subscriber Line Wireless Local Area Network
IEEE-802.11a, IEEE-802.11gETSI BRAN (Hyperlan/2)
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OFDM Systems
System TransformSize
NumberCarriers
ChannelSpacing
kHz
BandwidthMHz
SampleRateMHz
SymbolDuration
sec
Data
Rate
Mbits/s
HyperLAN/2 64 524 312.5 16.25 20 3.20.8 6-54
802.11a 64 524
312.5 16.56 20 3.20.8
6-54
DVB-T 20481024
1712842
4.464 7.643 9.174 224 0.68-14.92
DAB 20488192
1536 1.00 1.536 2.048 24/48/96msec
3.072
ADSL 256 (down)64 (up)
36-1277-28
4.3125 1.104 1.104 231.9 0.64-8.192
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OFDM Advantages
Efficiently Deals With Multi-path Fading Efficiently Deals With Channel Delay Spread
Enhanced Channel Capacity Adaptively Modifies Modulation Density Robustness to Narrowband Interference
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OFDM Disadvantages
OFDM Sensitive toSmall Carrier Frequency Offsets
OFDM ExhibitsHigh Peak to Average Power Ratio
OFDM Sensitive toHigh Frequency Phase Noise
OFDM Sensitive toSampling Clock Offsets
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Adjacent Symbol Interference (ASI)
Symbol Smearing Due to Channel
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Guard Interval Inserted Between Adjacent Symbolsto Suppress ASI
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