seminar presentation - copy

7/30/2019 Seminar Presentation - Copy

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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

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