Seminar Presentation on
Power Allocation In SC-FDMA Systems
Presented By: Prabhash Kumar Singh Enrollment No: 12531012 Under the guidance of
Dr. Anshul Tyagi
Contents
1. Motivation 1.2 Reasons for using SC-FDMA
2.
Multicarrier modulation scheme 2.2 OFDMA
2.3 SC-FDMA Introduction to SC-FDMA 3.2 Subcarrier Mapping 3.3 Comparison of different mapping PAPR Conclusion
3.
4. 5.
Motivation Current Wireless Technologies
Standards Data Rates
3G 3G 3.5G
WCDMA/UMTS 384 Kbps CDMA-2000 384 Kbps
HSDPA/HSUPA 5-30 Mbps
What Next ?
Standards Data Rate
4G
LTE
4G
WiMAX
100-200 Mbps 100-200 Mbps
These are the New Access Technology
Demand for higher data rate is leading to utilization of wider transmission bandwidth Standards Transmission Bandwidth
2G 3G 4G
GSM
200 KHz
CDMA
WCDMA CDMA-2000 LTE,WiMAX
1.25 MHz
5 MHz 5 MHz Up to 20 MHz
LTE (Long Term Evolution)
Downlink OFDMA
Uplink SC-FDMA
Why Using SC-FDMA In Uplink Communication ??
SC-FDMA
Low PAPR
1. Better Peak Power Characteristics 2. To avoid Non Linear Distortion
1.Better Peak power Characteristic
SC-FDMA signal has better peak power characteristic due to Its inherent single carrier structure.It greatly benefit the mobile Terminals in terms of tranmit power efficiency and manufacturing cost.
2.Non Linear Distortion
Non Linearity in transmitter power amplifier most influences Performance of freqency division techniques.In FDMA systems Vulnerability to amplifier nonlinearity increases with high PAPR Of thr transmitted signal.SC-FDMA has PAPR lower than that Ofalternate tranmission scheme OFDMA.
Multi Carrier Modulation Scheme
Subcarrier Spacing=B/N N subcarrier
-(N/2 -1)B/N
-B/N
B/N
NB/2N
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
Multicarrier Composite transmitted signal
ith data stream modulated On ith subcarrier
This is the signal we are transmitting in to the air Now the received signal will be
In the absence of noise
To recover symbols corresponding to N subcarrier ,coherently demodulate each stream with corresponding subcarrier This scheme is called MCM The advantage of using the above scheme is, Since B/N <<coherence bandwidth, each subcarrier experiences freqyency flat fading hence there is no ISI in time domain. But implementing bank of N modulators and demodulators Is challenging!!!
use OFDMA !!
OFDMA
Consider the u th sample of MCM signal
T = sampling rate= 1/B IDFT of transmission symbols X(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.
PAPR IN OFDMA SYSTEMS X(0) X(1) X(N-1) IFFT x(0) x(1) x(N-1) IFFT samples of Information symbols
Information symbols
k th IFFT sample
Average Power =
Peak Power=
PAPR rises with N i.e the no. of subcarriers For 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
Typical OFDM Transmitter
S/P DEMUX N Point IFFT
Causing high PAPR in OFDM
P/S MUX
Modified OFDM Transmitter
FFT and IFFT cancel out each other
S/P DEMUX
N Point FFT
N Point IFFT
P/S MUX
New Blocks
Output is a single carrier output
However instead of using an N point FFT block,one can employ an M<N point FFT to reduce PAPR while still retaining the properties of OFDM system
Transmitter and Receiver structure of SC-FDMA system
No of Symbols M in FFT <
N the no of sub-carriers
The cyclic prefix is a copy of the last part of the block. It is Inserted 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 multipath propagation. 2. Since the CP is a copy of the last part of the block, it converts a discrete time Linear convolution into a discrete time circular convolution.
Subcarrier Mapping
x(0)
x(1) M point FFT
X(0) X(1) X(M-1)
Subcarrier Mapping
N Point IFFT
M to N subcarrier
.Subcarrier Mapping is a key operation in SC-FDMA
.Assigns M frequency domain modulation symbols to N subcarrier
Subcarrier Mapping Modes
1. In the localized subcarrier mapping mode, the modulation symbols are to M adjacent subcarriers.
2. In the distributedmode, the symbols are equally spaced across the entire channel bandwidth.
3. In both modes, the IDFT in the transmitter assigns zero amplitude to the N-M unoccupied subcarriers. 4. The distributed mode with equidistance between occupied Subcarriers is referred to as Interleaved FDMA (IFDMA)
An example of different subcarrier mapping schemes for M=4 N=12
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 equally spaced over all the subcarriers, Y0 = X0, Y2 = X1, Y4 = X2, Y6 = X3, and in the interleaved mode, Y0 = X0, Y3 = X1, Y6 = X2,Y9 = X3.
Comparison of Subcarrier Mapping Schemes
1.The IFDMA signal maintains the input time symbols in each sample whereas LFDMA and DFDMA have more complicated time samples because of the complexweighted sum of the input symbols. This implies that higher peak 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.
Peak Power Characteristic of SC-FDMA signal
1.PAPR relates to the power amplifier efficiency at the
transmitter, 2.maximum power efficiency is achieved when the amplifier operates at the saturation point. Lower PAPR allows operation of the power amplifier close to saturation resulting in higher efficiency. 3.We can express the theoretical relationship between PAPR [dB] and transmit power efficiency as follows
where n is the power efficiency and n max is the maximum power efficiency. For class A power amplifier, n max is 50%and for class B, 78.5%
PAPR is characterized using the CCDF function i.e COMPLEMENTARY CUMULATIVE DISTRIBUTION FUNCTION
We can see that all the cases for SC-FDMA have indeed lower PAPR than that of OFDMA. Also, IFDMA has the lowest PAPR, and DFDMA and LFDMA have very similar levels of PAPR.
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 low PAPR. Efficient transmitter Subcarrier mapping scheme has a significant impact on PAPR.
FINAL WORD
SC-FDMA
Low PAPR
Thank You
