Orthogonal Frequency Division Multiplexing - OFDM
Multipath Radio Channel
Frequency-Selective Radio Channel
-10
-5
0
5
10
15
20
Frequency
Power response [dB]
Concept of parallel transmission
Channel impulse response
1 Channel (serial)
2 Channels
8 Channels
Time
In practice: 50 … 8000
Channels (sub-carriers)
Channels are transmitted at different frequencies (sub-carriers)
Concept of parallel transmission
Channel impulse
response
1 Channel (serial)
Channel
transfer function
Channels are
“narrowband”
2 ChannelsFrequency
Frequency
8 ChannelsFrequency
FrequencyTime
Signal is
“broadband”
5
Impact of fading
f
f
t
t
t
t
t
h(t)
s(t)
s(t)
S(f)
S(f)
r(t)
r(t)
High bit rate
Low bit rate
H(f)
f
6
p4(t)
Frequency Selective fading
�flat fading
t
t
t
th(t)
s(t)
p1(t)
r(t)
H(f)
f
p2(t)
t
p3(t)
p4(t)
p3(t)
Concept of an OFDM signal
Ch.1
Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10
Saving of bandwidth
Ch.3 Ch.5 Ch.7 Ch.9Ch.2 Ch.4 Ch.6 Ch.8 Ch.10
Ch.1
Conventional multicarrier techniques
Orthogonal multicarrier techniques
50% bandwidth saving
frequency
frequency
8
OFDM DEFINITION
� OFDM = Orthogonal FDM
� Carrier centers are put on orthogonal frequencies
� ORTHOGONALITY - The peak of each signal coincides with zero crossing of other signals
� Subcarriers are spaced by 1/ Ts
9
Orthogonal Subcarriers
10
A simplified view
S-P
Cos 2πf1t
Cos 2πf8t
+Input bitsInput bitsInput bitsInput bits
s(t)
P-S
Cos 2πf1t
Cos 2πf8t
r(t) Rep Output Output Output Output
bitsbitsbitsbits
11
Alternate View
12
OFDM Theory
13
Discrete Time Equivalent
� Inverse Discrete Fourier Transform �
s(n) = ∑ dk exp( j 2π k n/N)
� N-point IDFT � N2 complex multiplications
� Inverse Fast Fourier Transform
� Radix 2 N-point IFFT � (N/2). log2 N
� Radix 4 N-point IFFT � (3/8). N. ( log 2 N - 2)
N -1
k=0
14
15
Complexity of OFDM versus Single Carrier
� Key difference between OFDM and single carrier transmission is FFT versus equalizer
� Complexity of 64 point radix-4 FFT in IEEE 802.11a OFDM=96 million multiplications per second
� 16 taps OQPSK or GMSK Equalizer for same data rates above needs 768 million multiplications per second
� OFDM order of magnitude less complex
16
Cyclic Prefix
17
Generation of ICI
18
Cyclic Extension
19
Cyclic Prefix
20
Cyclic Extension
21
Windowing
22
Cyclic Extension & Windowing(filtering ?)
23
OFDM ADVANTAGES
� OFDM is spectrally efficient � Orthogonality & IFFT/FFT operation ensures that sub-carriers
do not interfere with each other.
� OFDM has an inherent robustness against narrowband interference.� Narrowband interference will affect at most a couple
of subchannels.� Information from the affected subchannels can be
erased and recovered via the forward error correction (FEC) codes.
� Equalization is very simple compared to Single-Carrier systems
24
OFDM ADVANTAGES
� OFDM has excellent robustness in multi-path environments.� Cyclic prefix preserves orthogonality between sub-
carriers.� Cyclic prefix allows the receiver to capture multi-
path energy more efficiently.
� Ability to comply with world-wide regulations:� Bands and tones can be dynamically turned on/off
to comply with changing regulations.
� Coexistence with current and future systems:� Bands and tones can be dynamically turned on/off
for enhanced coexistence with the other devices.
25
OFDM DRAWBACKS
� High sensitivity inter-channel interference, ICI
� OFDM is sensitive to frequency, clock and phase offset
� The OFDM time-domain signal has a relatively large peak-to-average power ratio
� tends to reduce the power efficiency of the RF amplifier
� non-linear amplification destroys the orthogonality of the OFDM signal and introduces out-of-band radiation
26
OFFSET ESTIMATION & SYNCHRONIZATION
27
OFDM Preamble
28
OFDM Transceiver
CodingCodingCodingCoding
Binary
Input
Data InterleavingInterleavingInterleavingInterleaving
QAM QAM QAM QAM
mappingmappingmappingmapping
PilotPilotPilotPilot
InsertionInsertionInsertionInsertionS S S S ---- PPPP
IFFTIFFTIFFTIFFT
FFTFFTFFTFFT
DecodingDecodingDecodingDecoding DeDeDeDe----InterleavingInterleavingInterleavingInterleavingQAM QAM QAM QAM
demappingdemappingdemappingdemapping
Channel Channel Channel Channel
CorrectionCorrectionCorrectionCorrectionP P P P ---- SSSS
Binary
Output
Data
S S S S ---- PPPP
P P P P ---- SSSS
Add Cyclic Add Cyclic Add Cyclic Add Cyclic
extension extension extension extension
& Windowing& Windowing& Windowing& Windowing
DACDACDACDACRF RF RF RF TxTxTxTx
Remove Remove Remove Remove
Cyclic Cyclic Cyclic Cyclic
extension extension extension extension
Timing &Timing &Timing &Timing &
Freq.Freq.Freq.Freq.
Sync.Sync.Sync.Sync.
ADCADCADCADCRF RxRF RxRF RxRF Rx
29
Selection of OFDM parameters
� Bandwidth, bit rate, delay spreadBandwidth, bit rate, delay spreadBandwidth, bit rate, delay spreadBandwidth, bit rate, delay spread
� Guard time Guard time Guard time Guard time TTTTgggg
� 2 to 4 times delay spread 2 to 4 times delay spread 2 to 4 times delay spread 2 to 4 times delay spread ���� 2222σσσσ to to to to 4444σσσσ
� depends on the order of modulation employeddepends on the order of modulation employeddepends on the order of modulation employeddepends on the order of modulation employed
� Symbol duration > Guard time to maximize SNRSymbol duration > Guard time to maximize SNRSymbol duration > Guard time to maximize SNRSymbol duration > Guard time to maximize SNR� More More More More subcarrierssubcarrierssubcarrierssubcarriers, smaller spacing, implementation complexity, more sensitivity , smaller spacing, implementation complexity, more sensitivity , smaller spacing, implementation complexity, more sensitivity , smaller spacing, implementation complexity, more sensitivity
to phase noise & frequency offset, high PAPRto phase noise & frequency offset, high PAPRto phase noise & frequency offset, high PAPRto phase noise & frequency offset, high PAPR
� Symbol duration Symbol duration Symbol duration Symbol duration ���� 5 x Guard time ( 15 x Guard time ( 15 x Guard time ( 15 x Guard time ( 1----dB SNR loss )dB SNR loss )dB SNR loss )dB SNR loss )
� TTTTFFTFFTFFTFFT = 5 x = 5 x = 5 x = 5 x TTTTgggg ���� TTTTofdmofdmofdmofdm = T= T= T= TFFT FFT FFT FFT + + + + TTTTgggg
� SubcarrierSubcarrierSubcarrierSubcarrier spacing spacing spacing spacing ∆∆∆∆f = 1 / Tf = 1 / Tf = 1 / Tf = 1 / TFFTFFTFFTFFT
� Number of Number of Number of Number of subcarrierssubcarrierssubcarrierssubcarriers = 3= 3= 3= 3----dB BW / dB BW / dB BW / dB BW / ∆∆∆∆f f f f
30
Example :� Bit rate = 20 MbpsBit rate = 20 MbpsBit rate = 20 MbpsBit rate = 20 Mbps
� Tolerable delay spread = 200 nsTolerable delay spread = 200 nsTolerable delay spread = 200 nsTolerable delay spread = 200 ns
� Bandwidth < 15 MHzBandwidth < 15 MHzBandwidth < 15 MHzBandwidth < 15 MHz� Tg = 800 ns
� Tofdm = 5 x Tg + Tg = 4.8 µµµµsec
� ∆∆∆∆f = 1 / 4 µµµµsec = 250 KHz
� Number of bits in one OFDM symbol = 20 Mbps x 4.8 µµµµsec = 96
� 16-QAM with rate ½ Conv. Coding ���� 2 bits / symbol / subcarrier ���� 48 subcarriers ���� 48 x 250 KHz = 12 MHz < 15 MHz
� QPSK with rate ¾ coding ���� 1.5 bits / symbol / subcarrier ���� 64 subcarriers
���� 64 x 250 KHz ���� 16 MHz > 15 MHz
� 64 point IFFT / FFT ���� 16 zero subcarriers ���� oversampling
Given requirements
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OFDM based ApplicationsOFDM based ApplicationsOFDM based ApplicationsOFDM based Applications
� DABDABDABDAB----OFDM OFDM OFDM OFDM ���� Digital Audio BroadcastingDigital Audio BroadcastingDigital Audio BroadcastingDigital Audio Broadcasting
� DVDDVDDVDDVD----OFDM OFDM OFDM OFDM ���� Digital Video Broadcasting Digital Video Broadcasting Digital Video Broadcasting Digital Video Broadcasting
� ADSLADSLADSLADSL----OFDM OFDM OFDM OFDM ���� Asynchronous Digital Subscriber LineAsynchronous Digital Subscriber LineAsynchronous Digital Subscriber LineAsynchronous Digital Subscriber Line� Wireless Local Area NetworkWireless Local Area NetworkWireless Local Area NetworkWireless Local Area Network
� IEEEIEEEIEEEIEEE----802.11a, IEEE802.11a, IEEE802.11a, IEEE802.11a, IEEE----802.11g, .11n802.11g, .11n802.11g, .11n802.11g, .11n
� ETSI BRAN (Hyperlan/2)ETSI BRAN (Hyperlan/2)ETSI BRAN (Hyperlan/2)ETSI BRAN (Hyperlan/2)
� Wireless MAN 802.16 Working Group of IEEEWireless MAN 802.16 Working Group of IEEEWireless MAN 802.16 Working Group of IEEEWireless MAN 802.16 Working Group of IEEE
� 802.16 a, b 802.16 a, b 802.16 a, b 802.16 a, b -------- 2 to 11 GHz2 to 11 GHz2 to 11 GHz2 to 11 GHz
� Mobile Mobile Mobile Mobile WiMAXWiMAXWiMAXWiMAX
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OFDM Systems
0.64-8.192231.91.1041.1044.312536-127
7-28
256 (down)
64 (up)
ADSL
3.07224/48/96
msec
2.0481.5361.0015362048
8192
DAB
0.68-14.922249.1747.6434.4641712
842
2048
1024
DVB-T
6-543.2
0.8
20 16.56 312.5 52
4
64802.11a
6-543.2
0.8
2016.25312.552
4
64HyperLAN/2
Data
Rate
Mbits/s
Symbol
Duration
µµµµsec
Sample
Rate
MHz
Bandwidth
MHz
Channel
Spacing
kHz
Number
Carriers
Transform
Size
System
33
Create Multiple Channel
FDM
POTSPOTS
UpstreamUpstream
DownstreamDownstream
Echo Cancellation
FrequencyFrequency
1 Mhz1 Mhz
ADSL
Thank You