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Class Report
林宏穎:
OFDM Introduction
OFDM History
1957: Kineplex multi-carrier HF modem
1966: Chang, Bell Labs: OFDM paper & patent
1971: Weinstein & Ebert propose use of FFT and guard interval
1985: Cimini describes use of OFDM for mobile communications
1987 Alard & Lasalle: OFDM for digital broadcasting
1995: ETSI DAB standard: first OFDM-based standard
1997: DVB-T standard
1998: Magic WAND project demonstrates OFDM modems for
wireless LAN
1999: IEEE 802.11a and HIPERLAND/2 standards for wireless
LAN
2000: V-OFDM for fixed wireless access
2001: OFDM considered for new IEEE 802.11 and 802.16
standards
Introduction to OFDM
• Basic idea
– Using a large number of parallel narrow-band subcarrier instead of a single wide-band carrier to transport
information
• Advantages
– Very easy and efficient in dealing with multi-path
– Robust against narrow-band interference
• Disadvantages
– Sensitive to frequency offset and phase noise
– Peak-to-average problem reduces the power efficiency
of RF amplifier at the transmitter
• Adopted by various standards
– DSL, 802.11a, DAB, DVB, etc.
OFDM Definition
• The technique of OFDM is based on the wellknown technique of FDM
• FDM technique:
FDM
– Different streams of information
are mapped onto separate parallel
frequency channels
– Guard bands are inserted to reduce interference
between adjacent channels
• OFDM technique
– Multiple carriers carry the
information stream
– Carrier spectrum are are overlapped
and orthogonal to each other
– A guard time is added to each symbol
to combat the channel delay spread
frequency
OFDM
frequency
Concept of OFDM
• A type of multi-carrier modulation
• Single high-rate bit stream is converted to low-rate N
parallel bit stream
• Each parallel bit stream is modulated on one of N subcarriers
• Each sub-carrier can be modulated by QFSK or QAM
• Add a guard time to each OFDM symbol to avoid
inter-symbol interference of fading channel
• To achieve high bandwidth efficiency, the sub-carriers
are closely spaced and overlapped
• Sub-carriers are orthogonal over the symbol time
• Use coding to correct errors for sub-carriers in deep
fading environment
Advantages of OFDM
• Robust in multi-path propagation environment
• Successful Examples:
– DAB, DVB-T, Wireless LAN
• More tolerant of delay spread
– Due to the use of many sub-carriers, the symbol duration is increased,
relative to delay spread
– Inter-symbol interference is avoided through the use of guard interval
– Simplified or eliminate equalization needs, as compared to single carrier
modulation
• More resistant to fading
– Low symbol rate per carrier provides the robustness against frequency
selective fading or narrowband interference
– FEC is used to correct for sub-carriers that suffer from deep fade
• Multi-carrier with single frequency network (SFN)
OFDM Good for Broadband Systems
• Most broadband systems are subjects to
multipath transmission
• Conventional solution to multipath is an
equalizer in the receiver
– Equalizers are too complicated at high data
rates
• With OFDM there is a simple way of
dealing with multipath
– Relatively simple DSP algorithms
Modulation System
Single carrier modulation

Multi carrier modulation
N subchannels
S/P
quadrature
amplitude
modulation
(QAM)
encoder
N complex samples
N-IFFT
add
cyclic
prefix
P/S
D/A +
transmit
filter
TRANSMITTER
multipath channel
RECEIVER
N subchannels
P/S
QAM
decoder
channel
estimation &
equalizer
N complex samples
N-FFT
remove
S/P cyclic
prefix
Receive
filter
+
A/D
Multicarrier
Rate R
Mapping
Filter
f0
Rate R
Mapping
Filter
f1
Rate NR
Rate R
Mapping
Filter
fN-1
Bandlimited
signals
f0
f1
f2
fN-1
 The transmission bandwidth is divided into sub-bands which
are transmitted in parallel
 Ideally, each sub-band is narrow enough so that the fading
it experiences is flat (no ISI)
 Disadvantages
-- Requires filter bank at receiver
-- Spectrally inefficiency
OFDM Source of Impairment
Power Amplifier
Non-Linear
FEC
Coding
QAM
Mapping
Pilot
Insertion
Fixed-Point
Computation
Error
FEC
Decoding
IFFT
(TX)
FFT
(RX)
QAM DeMapping
Channel
Correction
Insert
Guard
Interval
DAC
IQ
Modulator
Frequency
Corrected
Signal ADC
noise
Remove
Guard
ADC
Interval
Symbol timing
HPA
Phase noise
AGC Response Time
AGC Amp
Timing
Frequency
Synchronization
Multi-path
Fading
Channel
LNA
Phase noise
Frequency offset
Performance Loss
• Detection Loss of synchronized Detection
– SNR (dB) required to achieve the performance of
perfect channel knowledge . (Infinite Precision
arithmetic assumed)
– Algorithms for channel model description
• Implementation Loss
– SNR (dB) resulting from finite precision arithmetic
– Computation complexity, architecture selection, cost
Problems of OFDM Modulation
• ICI (Inter-channel interference): interference
between symbol in adjacent frequencies
• ISI (inter-symbol interference): interference of
successive OFDM frames
• Highly vulnerable to synchronization errors and
frequency offsets
• Highly vulnerable to the non-linearity of the Pas
(in the RF analog front end)
Challenges for OFDM
• Synchronization challenges
– Transmitter frequency  Receiver frequency
• Mesochronous: same frequency, different phase
• Pleisochrnous: slightly different frequencies
• Asynchronous: totally different frequencies
– Transmitter sampling time  Receiver sampling time
– Symbol timing is unknown to receiver
• Peak-to Average Power Ratio (PAPR)
– Dynamic range at output of IFFT is much larger than at
input
– it is about 2 dB higher than that of the ATSC 8-VSB
system. A larger Tx (more dynamic range) might be
required or using pre-distortion and better filtering to
reduce the first adjacent channel interference
• Channel estimation for time varying environment
Impact of Symbol Duration
• The symbol duration of OFDM is much larger
than that of single carrier system under the similar
overall transmission bandwidth
• A larger symbol duration will enhance the
effective bit rate and power utilization if the delay
spread is about fixed
• The larger OFDM duration when compared with
the channel coherence time can reduce the ability
to combat the fast temporal fading
• The channel coherence time is inversely
proportional to the maximum Doppler shift
Impact of Sub-Carrier Spacing
• Because of the time-frequency duality, some of the
time-domain arguments can be translated to the
frequency domain
• The large number of OFDM sub-carriers makes
the bandwidth of the individual sub-carriers small
relative to the overall signal bandwidth and the
channel coherence bandwidth
• The fading on each sub-carrier is frequency flat
and can be better modeled as a constant complex
channel gain.
• The narrower sub-carrier spacing will be easier to
cause inter-carrier interference