Download - Peak-to-Average Power Ratio (PAPR)
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Peak-to-Average Power Ratio (PAPR)
One of the main problems in OFDM system is large PAPR /PAR(increased complexity of the ADC and DAC, and reduced efficiency of RF power amplifier, and etc.)
An OFDM signal consists of a number of independently modulated subcarriers, which can give a large PAPR /PAR when added up coherently.
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PAPR (Cont.)
S/Pconvertor
Encoder/modulator
N-pointIFFT
P/Sconvertor
D/ARF
converter
2
0
2
0
)(
)(max
tamean
taPAR
s
s
Tt
Tt
PAR
The crest factor
The probability that the PAR is above some threshold level; NP ))exp(1(1)( 2
00
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Reducing PAR techniques Signal distortion techniques
[Clipping (rectangular) and Peak windowing (Cosine, Kaiser, Hamming)]* window length increase -> reduce out of band radiation but increase BER
Probabilistic techniques(Partial transform Sequence (PTS), Selective Mapping (SLM))
Coding techniques (Block coding)* no good codes for practical value of N>64 and larger constellation size ( >4 )are known.
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Clipping
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Smart Clipping
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Selective Mapping (SLM)In SLM , transmitter selects one of the smallest PAROFDM signal by using phase rotation.
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Partial Transmit Sequence (PTS)In PTS, the data symbols are broken into several Sub-blocks. These sub-blocks are added and transmitted with optimized phase rotation factors.
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PTS (cont.)
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Drawbacks of techniques for reducing PAPR
Reducing data rate.
(the side information, coding rate) Increasing the out of band
radiation and BER. (clip the peak power signals)
Increasing systems complexity.
(PTS, SLM)
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OFDM SystemsSystem Transform
SizeNumberCarriers
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 3.2 0.8
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 Transceiver
Coding
Binary Input Data
InterleavingQAM
mappingPilot
InsertionS - P
IFFTFFT
DecodingDe-InterleavingQAM
demappingChannel
CorrectionP - S
Binary Output Data
S - P
P - S
Add Cyclic extension
& Windowing
DACRF Tx
Remove Cyclic
extension
Timing &Freq.Sync.
ADCRF Rx
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OFDM based Applications
Wireless LAN standards using OFDM are HiperLAN-2 in Europe IEEE 802.11a, .11g
OFDM based Broadband Access Standards are getting defined for MAN and WAN applications
802.16 Working Group of IEEE 802.16 -- single carrier, 10-66GHz band 802.16a, b -- 2-11GHz, MAN standard
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IEEE 802.11a Overview
Carrier frequency= 5 GHz Total allotted bandwidth= 20 MHz x 10 =
200MHz Size of the FFT= 64 Number of data subcarriers= 48 Number of Pilot subcarriers= 4 FFT period= 3.2 µs Channel bandwidth used= 64/3.2 µs => 20
MHz
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Typical Configuration
52 subcarriers, 64 point FT/IFFT Symbol time 4 µs Guard time 800 ns BPSK, QPSK, 16-QAM, 64-QAM Coding rates 1/2,3/4,2/3 Bit rates 6,12,18,24,36,48,54 Mbps Channel spacing 20 MHz Tolerable delay spread about 250 ns at 24
Mbps
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DFT (FFT) as Signal Generatorfor Complex Sinusoids
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DFT (FFT) As Signal Analyzer for Complex Sinusoids
1,...,2,1,0:)()(1
0
2
NkenhkH
N
n
nkNj
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Radix-2 FFT Flow Diagrams
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OFDM Modulation With IFFTand Interpolator
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OFDM Demodulation With FFT
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OFDM Transceiver
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Linear Versus Circular Convolution
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Fast Circular Convolution with the FFT
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Reserve Frequency Bins For Clipping Pulses
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Selecting Reserve Frequency Bins
-60 -40 -20 0 20 40 600
0.2
0.4
0.6
0.8
1
Spectrum 11-Adjacent Frequencies
-0.5 0 0.50
0.2
0.4
0.6
0.8
1
Time Series for 11-Adjacent Frequencies
-60 -40 -20 0 20 40 600
0.2
0.4
0.6
0.8
1
Spectrum 11-Equally Spaced Frequencies
-0.5 0 0.50
0.2
0.4
0.6
0.8
1
Time Series for 11-Equally Spaced Frequencies
-60 -40 -20 0 20 40 600
0.2
0.4
0.6
0.8
1
Spectrum 11-Randomly Spaced Frequencies
-0.5 0 0.50
0.2
0.4
0.6
0.8
1
Time Series for 11-Randomly Spaced Frequencies
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Reserve Bin Canceller Clipping at 2.5 (8 dB)
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, input to PAR control
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of first pass PAR control
dataclip leveldata std devaverage peak
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of second pass PAR control
dataclip leveldata std devaverage peak
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of third pass PAR control
dataclip leveldata std devaverage peak
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Statistics for Clip at 2.5 (8 dB)
0 1 2 3 40
0.005
0.01
0.015
0.02
0.025input histogram
0 1 2 3 40
0.005
0.01
0.015
0.02
0.025
std dev =0.928
clip level
output histogram
-5 0 5 1010
-6
10-5
10-4
10-3
10-2
10-1
100
average =-0.648 dB
prob of level crossing
PAR (dB)
inputpass-1pass-2pass-3
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Reserve Bin Canceller Clipping at 2.2 (6.9 dB)
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, input to PAR control
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of first pass PAR control
dataclip leveldata std devaverage peak
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of second pass PAR control
dataclip leveldata std devaverage peak
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of third pass PAR control
dataclip leveldata std devaverage peak
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Statistics for Clip at 2.2 (6.9 dB)
0 1 2 3 40
0.005
0.01
0.015
0.02
0.025input histogram
0 1 2 3 40
0.005
0.01
0.015
0.02
0.025
std dev =0.928
clip level
output histogram
-5 0 5 1010
-6
10-5
10-4
10-3
10-2
10-1
100
average =-0.653 dB
prob of level crossing
PAR (dB)
inputpass-1pass-2pass-3
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Reserve Bin Canceller Clipping at 2.0 (6 dB)
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, input to PAR control
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of first pass PAR control
dataclip leveldata std devaverage peak
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of second pass PAR control
dataclip leveldata std devaverage peak
0 50 100 150 200 2500
1
2
3
4
5Peak envelope, output of third pass PAR control
dataclip leveldata std devaverage peak
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Statistics for Clip at 2.0 (6 dB)
0 1 2 3 40
0.005
0.01
0.015
0.02
0.025input histogram
0 1 2 3 40
0.005
0.01
0.015
0.02
0.025
std dev =0.927
clip level
output histogram
-5 0 5 1010
-6
10-5
10-4
10-3
10-2
10-1
100
average =-0.659 dB
prob of level crossing
PAR (dB)
inputpass-1pass-2pass-3