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Seminar Presentation on
Power Allocation In
SC-FDMA SystemsPresented By: Prabhash Kumar Singh
Enrollment No: 12531012
Under the guidance ofDr. Anshul Tyagi
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1. Motivation
1.2 Reasons for using SC-FDMA
2. Multicarrier modulation scheme
2.2 OFDMA2.3 SC-FDMA
3. Introduction to SC-FDMA
3.2 Subcarrier Mapping
3.3 Comparison of different mapping4. PAPR
5. Conclusion
Contents
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MotivationCurrent Wireless Technologies
Standards Data Rates
3G WCDMA/UMTS 384 Kbps
3G CDMA-2000 384 Kbps
3.5G HSDPA/HSUPA 5-30 Mbps
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WhatNext ?
Standards Data Rate
4G LTE 100-200Mbps
4G WiMAX 100-200Mbps
These are the New Access Technology
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Demand for higher data rate is leading to utilization of wider
transmission bandwidth
Standards TransmissionBandwidth
2G GSM 200 KHzCDMA 1.25 MHz
3G WCDMA 5 MHz
CDMA-2000 5 MHz4G LTE,WiMAX Up to 20 MHz
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LTE (Long Term Evolution)
Downlink Uplink
OFDMA SC-FDMA
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Why Using SC-FDMA In Uplink Communication ??
SC-FDMA Low PAPR
1. Better Peak Power Characteristics2. To avoid Non Linear Distortion
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1.Better Peak power CharacteristicSC-FDMA signal has better peak power characteristic due toIts inherent single carrierstructure.It greatly benefitthe mobile Terminals in terms of tranmit power efficiencyand manufacturing cost.
2.Non Linear DistortionNon Linearity in transmitter power amplifier most influencesPerformance of freqency division techniques.In FDMA systems
Vulnerability to amplifier nonlinearity increases with high PAPROf thr transmitted signal.SC-FDMA has PAPR lower than thatOfalternate tranmission scheme OFDMA.
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Multi Carrier Modulation Scheme
-(N/2 -1)B/N -B/N 0 B/N NB/2N
SubcarrierSpacing=B/N
N subcarrier
Xi: data transmitted on i th subcarrier
Si(t)=Xi ej2fit
fi=iB/N is the center frequency of the ith subcarrier
There are now N subcarriers ,hence there are N data streams
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MulticarrierComposite transmittedsignal
ith data stream modulatedOn ith subcarrier
This is the signal we are transmitting in to the air
Now the received signal will be
In the absence of noise
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To recover symbols corresponding to N subcarrier ,coherentlydemodulate each stream with corresponding subcarrier
This scheme is called MCMThe advantage of using the above scheme is,Since B/N
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OFDMA
T = sampling rate= 1/B
Consider the u th sample of MCM signal
IDFT of transmission symbolsX(0)X(1)X(2).X(N-1)
At the receiver to recover the information symbols on can
Correspondingly Employ an FFT operation.Advantage: Much lower implementation complexity
compared to using bank of modulators.
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PAPR IN OFDMA SYSTEMS
IFFT
X(0)
X(1)
X(N-1)
x(0)
x(1)
x(N-1)
Information symbols
IFFT samples ofInformation symbols
k th IFFT sample
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Average Power =
Peak Power=
PAPR rises with N i.e the no. of subcarriersFor instance,In an OFDM system with N=512 ,and BPSK modulation,
The PAPR at the output can be as high as 10 db !!
TO REDUCE PAPR USE SC-FDMA
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Typical OFDM Transmitter
S/PDEMUX
N PointIFFT
P/SMUX
Causing high PAPR in OFDM
Modified OFDM Transmitter
S/PDEMUX
N PointFFT
N PointIFFT
P/SMUX
New Blocks Output is a singlecarrier output
FFT and IFFT cancel out each other
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However instead of using an N point FFT block,one canemploy an M
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Transmitter and Receiver structure of SC-FDMAsystem
No of Symbols M in FFT < N the no of sub-carriers
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The cyclic prefix is a copy of the last part of the block. It isInserted at the start of each block for two reasons.1. CP acts as a guard time between successive blocks to
prevent inter-block interference(IBI) due to multipathpropagation.
2. Since the CP is a copy of the last part of the block, it
converts a discrete time Linear convolution into a discretetime circular convolution.
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Subcarrier Mapping
M pointFFT
SubcarrierMapping
N PointIFFT
x(0)
x(1)
X(0)
X(1)
X(M-1)
M to N subcarrier
.Subcarrier Mapping is a key operation in SC-FDMA
.Assigns M frequency domain modulation symbols to N subcarrier
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Subcarrier Mapping Modes
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1. In the localized subcarrier mapping mode, the modulationsymbols are to M adjacent subcarriers.
2. In the distributedmode, the symbols are equally spacedacross the entire channel bandwidth.
3. In both modes, the IDFT in the transmitter assigns zeroamplitude to the N-M unoccupied subcarriers.
4. The distributed mode with equidistance between occupiedSubcarriers is referred to as Interleaved FDMA (IFDMA)
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An example of different subcarrier mapping schemes forM=4 N=12
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M=4 N = 12 subcarriers,In the localized mode, the four modulation
symbols occupy subcarriers 0, 1, 2, and 3: Y0 = X0, Y1 = X1, Y2 =X2, Y3 = X3, and Yi = 0 for i = 0, 1, 2, 3.
In the distributed mode with modulation symbols equallyspaced over all the subcarriers, Y0 = X0, Y2 = X1, Y4 = X2, Y6 =
X3, and in theinterleaved mode, Y0 = X0, Y3 = X1, Y6 = X2,Y9 = X3.
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Comparison of Subcarrier Mapping Schemes
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1.The IFDMA signal maintains the input time symbols ineach sample whereas LFDMA and DFDMA have more
complicated time samples because of the complex-weighted sum of the input symbols. This implies that higherpeak power is expected for LFDMA and DFDMA signals,which we will see through the numerical analysis of peak-to
average power ratio (PAPR)
2. All three single carrier subcarrier mapping schemes
exhibit lowerpeak power than OFDMA.
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Peak Power Characteristic of SC-FDMA signal
1.PAPR relates to the power amplifier efficiency at thetransmitter,
2.maximum power efficiency is achieved when the
amplifier operates at the saturation point. Lower PAPRallows operation of the power amplifier close tosaturation resulting in higher efficiency.
3.We can express the theoretical relationship between
PAPR [dB] and transmit power efficiency as follows
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where n is the power efficiency and n max is the maximumpower efficiency. For class A power amplifier, n max is50%and for class B, 78.5%
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PAPR is characterized using the CCDF function i.e COMPLEMENTARYCUMULATIVE DISTRIBUTION FUNCTION
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We can see that all the cases for SC-FDMA have indeed
lower PAPR than that of OFDMA. Also, IFDMA has thelowest PAPR, and DFDMA and LFDMA have verysimilar levels of PAPR.
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Conclusion SC-FDMA is a new single carrier multiple access technique
which has similar structure and performance to OFDMA. Currently adopted for uplink multiple access scheme for
3GPP LTE
Two types of subcarrier mapping, distributed and localized,give system design flexibility
A salient advantage of SC-FDMA over OFDMA is lowPAPR.
Efficient transmitter Subcarrier mapping scheme has a significant impact on PAPR.
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FINAL WORD
SC-FDMA Low PAPR
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Thank You