Sc fdma -an efficient technique for papr reduction in

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 03 Special Issue: 01 | NC-WiCOMET-2014 | Mar-2014, Available @ http://www.ijret.org 53
SC-FDMA -AN EFFICIENT TECHNIQUE FOR PAPR REDUCTION IN
UPLINK COMMUNICATION SYSTEMS -A SURVEY
Prittu Ann Thomas1
, M. Mathurakani2
1
PG Student [Wireless Technology], 2
Professor, Dept of ECE, Toc-H Institute of Science and Technology, Kerala, India
Abstract
In this paper, a survey on different modulation schemes having lowest peak to average power ratio (PAPR) value for use in uplink
communication has been studied. OFDM has become the popular modulation choice for high data rate wireless communication
systems. It can provide multi-path delay spread tolerance and is robust to channel dispersions. But one of the major limitations of
OFDM is the large variation in signal amplitude which gives it a high PAPR. OFDM signals with high PAPR will suffer from non-
linear distortion due to non-ideal behavior of High Power amplifiers as well as it is detrimental to battery powered devices like
mobile phones which are power limited .SC-FDMA was found to have a better PAPR reduction than OFDMA and has become
modulation choice for uplink communication in Long Term Evolution (LTE).
Keywords: OFDM, PAPR, SC-FDMA, Localized FDMA, Interleaved FDMA, SC-FDMA-CDMA
----------------------------------------------------------------------***------------------------------------------------------------------------
1. INTRODUCTION
The demand for a radio system with high data rate and
throughput has been increasing tremendously in recent years.
To support these requirements third generation partnership
project (3GPP) has developed a new radio access technology
called long term Evolution (LTE). LTE can provide a data rate
of 100Mbps in downlink and 50Mbps in uplink when
operating in a bandwidth of 20Mhz.The high data rate of LTE
not only demands a wider bandwidth but also a more
advanced modulation technique. OFDM was considered as an
optimum solution for downlink transmission requirement. But
this modulation scheme has some major limitations like high
PAPR which can increase complexity and power of
transmitter (mobile) in uplink. So an uplink scheme should be
considered with trade-off between low computation
complexity and system performance. A solution to this
problem was to use SC-FDMA which is similar to OFDM but
has an additional DFT pre-coding prior to OFDM modulation.
The survey is organized as follows. Section 2 provides a brief
overview on OFDM with its advantages and limitations.
Section 3 discusses the various PAPR reduction techniques.
Section 4 provides a detailed explanation on SC-FDMA.
Section 5 discusses the simulation results obtained from
different papers. The survey is finally concluded with
highlights on the main observations made in the study.
2. OFDM
The transmission bandwidth required for communication has
increased for taking care of increasing demand for higher data
rates. A wide transmission bandwidth can increase frequency
selectivity of the channel and thus inter-symbol interference
(ISI) becomes a major issue. An efficient way to mitigate
frequency-selective fading of wide band channel is to use
multicarrier modulation, which divides the entire channel into
a number of smaller sub-channels. Each sub-channel will
undergo a flat fading since the signal bandwidth is narrower
than coherence bandwidth of the channel.
Orthogonal frequency division multiplexing is a special case
of frequency division multiplexing where sub-carriers are
orthogonal to one another. This allows overlapping of
subcarriers which can ensure efficient bandwidth utilization
compared to conventional frequency division multiplexing
which requires guard bands between adjacent sub-bands.
Orthogonality of sub-carriers is achieved by using inverse-
discrete Fourier transformation. Thus discrete Fourier
transform at transmitter and its inverse at receiver form the
signal processing techniques at the heart of OFDM
implementations [1]. OFDM is basically a combination of
multiplexing and modulation. In OFDM the signals are first
split into independent channels modulated by data and then re-
multiplexed to create OFDM carrier. Since maximum of a
subcarrier corresponds to zeros of another subcarrier, each
subcarrier can be demodulated independently of the other.
The various advantages of OFDM includes
• Tolerance to multi-path delay spread
• Robustness to channel distortion
• Throughput maximization
• Effective bandwidth utilization
• Frequency Diversity
OFDM can ensure a wide range of advantages in wireless
communication. But it has some major limitations.

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2.1 Limitations of OFDM
2.1.1 Peak to Average Power Ratio
OFDM signal is vector sum of a number of sub-carriers with
different phases. At some time instances, this sum will be
large and at other times it may be small. As a result the peak
value of the signal will be substantially larger than the average
value. The complex OFDM signal can be represented by
))((
1
0
)(1)( ttj
n
N
n
nn
etA
N
tx φω +
−
=
∑=
(1)
Where N is the number of subcarriers,
)(tAn is amplitude of
each sub-carrier, ω =2π f and
)(tn
φ
is phase of each sub-
carrier. The high envelope fluctuation in signal amplitude can
result in signal distortion which can result in out-of band
spectral re-growth of the signals. To accommodate the high
PAPR signal, the linear range of high power amplifier has to
be increased [2]. It can in turn increase the complexity and
cost of high power amplifiers. PAPR of a signal x (t) is given
by the following equation
}|)({|
|)(|max
2
2
txE
tx
PAPR =
(2)
The High PAPR value can also reduce the efficiency of HPA
because efficiency is given by,
PAPR
.
Efficiency
50
=
(3)
The PAPR problem can be reduced by providing a back-off
in input signal but it will reduce efficiency because HPA will
not be able achieve maximum efficiency when operating
below saturation point. Analog to digital converters are also
affected by this factor since quantization error will increase
with high PAPR.
Fig-1: A theoretical relationship between PAPR and transmit
power efficiency for ideal High power amplifier [7]
2.1.2 Carrier Frequency Offset
OFDM signal consist of a number of narrow sub-carriers
which require high degree of frequency synchronization
because an offset in carrier frequency can destroy the
orthogonality of sub-carriers which can in turn increase
multiple access interference among users.
2.1.3 Sensitivity to Spectral Null
The OFDM signal detection is done in frequency domain so it
is highly sensitive to spectral null. It requires an adaptive
coding scheme to overcome spectral nulls in the channel.
3. PAPR REDUCTION TECHNIQUES
In uplink communication PAPR becomes a major constraint in
the design of mobile terminals. PAPR affects uplink
communication than downlink because compared to base
station we cannot increase the complexity and power of
mobile terminals. So a number of PAPR reduction techniques
were developed. PAPR reduction techniques can be classified
into two:
3.1 Distortion Based Techniques
Distortion based technique will cause spectral re-growth of
signals. It is one of the most straightforward PAPR reduction
methods. Clipping is a distortion based technique in which
clipper limits the peak of the input OFDM signal to a
predetermined threshold value. But this is a non-linear
operation and can result in out-of band radiation of signals and
degradation in bit error rate performance of the system.
To overcome these limitations repeated clipping and filtering
of signals to several times can reduce out of-band radiation
and PAPR of signals to desired level [3]. But this technique
cannot eliminate in-band radiation of the signal.
3.2 Non-distortion Based Techniques
PAPR reduction techniques which do not distort shape and
cause any spectral re-growth of OFDM signals are called non-
distortion techniques. Tone reservation is one such technique
[5] in which a set of unused or reserved tones are selected to
design a peak cancelling signal. Since the reserved tones are
orthogonal chances of distortion due to additive noises are
eliminated. A major drawback of this method is the slow
convergence toward optimal solution and it has resulted in
bandwidth sacrifice as tones have to be reserved.
Multiple signal representation techniques like selected
mapping, partial transmit sequence are distortion less PAPR
reduction techniques [4].In selected mapping input data is
multiplied with a random series and resultant series with
lowest PAPR is chosen for transmission. This method has the
advantage that it doesn’t eliminate peaks and can handle any
number of subcarriers. But the limitation of this technique is

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that it requires additional side information that requires to be
transmitted to the receiver of the system in order to recover
information. Partial transmit sequence is flexible and effective
for OFDM system. In this method the input data frame is
divided into non-overlapping sub blocks and each sub block is
phase shifted by a constant factor to reduce PAPR. All these
techniques can provide PAPR reduction but are iterative in
nature. So the OFDM transmitter complexity will increase
with number of iterations.
An alternate approach to overcome PAPR problem is based on
signal transformation. This technique involves a signal
transformation prior to amplification at transmitter and an
inverse transformation prior to demodulation at the receiver.
Some of the transformations include companding, phase
modulation and discrete cosine transformation. Conversion of
OFDM signal into constant envelope by phase modulation is a
technique which can ensure a lowest achievable PAPR of 0dB.
It can provide a faster side-lobe roll-off than OFDM. But a
major limitation [6] of this technique was orthogonality of
subcarriers was not being maintained.
The major reason for PAPR in OFDM is due to its multi-
carrier structure. So conversion of multi-carrier OFDM signal
into single carrier can effectively reduce PAPR of the signals
[7]. Pre-coding of OFDMA signal with Discrete Fourier
transformation can convert it into single carrier frequency
division multiple access signal (SC-FDMA). Thus SC-FDMA
can overcome PAPR problem of OFDMA and many of its
limitations like sensitivity to carrier frequency offset, spectral
nulls (data detection is done in time domain) and can also
reduce complexity of transmitter.
4. SC-FDMA
Single carrier frequency division multiple access utilizes
single carrier modulation at the transmitter and frequency
domain equalization at the receiver. It has similar performance
and essentially the same overall complexity as that of an
OFDMA system [8-9].SC-FDMA can provide flexible
bandwidth assignment for different users and was adopted for
uplink in 3GPP Long Term Evolution (LTE) due to its low
PAPR property. The structure of SC-FDMA transmitter and
receiver are given below
Fig-2: SC-FDMA Transmitter [7]
Fig-3: SC-FDMA Receiver [7]
The information bits can be modulated using any modulation
techniques like QPSK, QAM or CPM. The aim of a
modulation scheme is not only to transmit message signals
through a radio channel but also to achieve this function with
best quality, power efficiency and minimum bandwidth.
Constant envelope modulation like QPSK can ensure high
power efficiency and can maintains low spectral side lobes.
Modulation scheme like CPM have constant envelope and
continuous phase which can provide high spectral efficiency
and better PAPR reduction. The modulation index can control
the spectral confinement of CPM signal, smaller the index
better is the spectral confinement. When modulation index
increases BER performance will be improved. But the
modulation and demodulation of CPM is complicated by the
fact that the initial phase of each symbol is determined by the
cumulative phase of all previous transmitted symbols which
requires the receiver to have memory. The PAPR value varies
with the modulation schemes used.
The complex modulated signal then undergoes discrete time
Fourier transformation. The number of DFT points (M) is
determined by the number of symbols to be transmitted. The
DFT output is then mapped on different sub-carrier. The
number of sub-carriers (N) is given by the following equation
N = M.Q (4)
Where, M represents the number of DFT points and Q
represents number of users.
Sub-carrier mapping can be divided into two. They are
1) Localized Mapping (LFDMA) and
2) Distributed Mapping.
In localized mapping each terminal is provided a set of
adjacent sub-carriers. LFDMA can potentially achieve multi-
user diversity in the presence of frequency selective fading by

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assigning each user to subcarriers in a portion of the signal
band where that user has favorable transmission characteristics
(high channel gain)[12].This demands channel-dependent
scheduling (CDS) of subcarriers. CDS requires the system
(base station) to monitor the channel quality as a function of
frequency for each terminal, and adapt subcarrier assignments
according to changes in channel frequency responses of all
terminals. In distributed mapping each terminal is provided a
set of sub-carriers that are distributed over the entire
bandwidth .This can ensure frequency diversity. Interleaved
sub-carrier mapping (IFDMA) is a special case of distributed
mapping where sub-carriers allocated for a user are equidistant
to one another. The time domain symbols of IFDMA [11] are
a simple repetition of input time symbols scaled by a factor
(1/Q).The time domain symbols of LFDMA have exact copies
of input time symbols in M multiple sample positions. It was
found that a system having many users with each transmitting
at a moderate bit rate is better off with IFDMA, while
LFDMA can work better in a system with high-bit-rate users.
Localized mapping are generally used in LTE uplink
applications.
The transmitter performs an important signal processing
operation before transmission. It inserts symbols referred to as
a cyclic prefix (CP) to provide guard time to prevent inter-
block interference (IBI) arising due to multipath propagation.
If the length of the CP is longer than the maximum delay
spread of the channel, or is equal to length of channel impulse
response, there will be no IBI. CP is a copy of the last portion
of the information block. It can convert discrete time linear
convolution into discrete time circular convolution. Thus data
transmitted through the channel can be modeled as a circular
convolution between channel impulse response and
transmitted data block, which in frequency domain is a point-
wise multiplication of the same. Thus the complexity of
equalization at the receiver can be reduced.
The transmitter performs a linear filtering operation referred to
as pulse shaping for transmission of signal through band
limited channel [13]. Raised-cosine filters (RC) are widely
used for pulse shaping in wireless communication systems. As
roll off factor of filter increase PAPR reduces significantly.
But a trade-off between PAPR and out of-band radiation is
required because out-of-band radiation can increase with
increase in roll-off. The performance of OFDM and different
sub carrier mapping of SC-FDMA were compared [14].It was
observed that PAPR was less when no pulse shaping was used
for IFDMA and LFDMA signals. But PAPR increases
significantly for IFDMA when raised cosine pulse shaping
was used and for LFDMA PAPR hardly increases with pulse
shaping. When pulse shaping was carried out using Root-
raised cosine filter (RRC), it was found that RRC pulse
shaping filter was effective in reducing PAPR than RC pulse
shaping filter. The table below gives the numerical analysis of
PAPR for different modulation schemes for IFDMA and
LFDMA for raised cosine filer with different roll off (α).
Table- 1: Comparison of PAPR for Different Modulation
Schemes and Filter Roll-Off [14]
In an SC-FDMA receiver the reverse operation of transmitter
is carried out. The receiver transforms the signal into
frequency domain via DFT, de-maps the subcarriers and then
performs frequency domain equalization. The equalization
process is done in frequency domain because in broadband
channels conventional time domain equalizers are impractical
as they have a very long channel impulse response in time
domain. In cellular systems with severe multipath propagation
SC-FDMA signals arrives the base station with substantial
inter-symbol interference. The base station uses an adaptive
frequency domain equalization to cancel this interference.
Most of the well-known time domain equalization techniques
such as minimum mean square error (MMSE) equalization,
decision feedback equalization (DFE) and turbo equalization
can be used. The equalized symbols are transformed back to
time domain via IDFT and then detection and decoding will
take place in the time domain.
In LTE uplink transmission it was observed [15] that OFDMA
turns out to have better performance with high-order
modulations and are typically used for users near base station
having favorable propagation conditions. SC-FDMA can
provide better performance with QPSK and lower code rates
and are used near cell edges for users with bad propagation
conditions. So OFDMA can provide higher cell capacity and
SC-FDMA can lead to cell range extension.
Spreading in SC-FDMA using code-division multiple access
(SC-FDMA-CDMA) can provide better PAPR reduction than
OFDMA-CDMA [16]. It can combine both features of SC-
FDMA and Direct sequence CDMA. Single carrier nature of
SC-FDMA signal can effectively reduce PAPR while
spreading of signals can ensure easy frequency planning, high
immunity against interference and flexible data rate
adaptation.SC-FDMA-CDMA can ensure both IFDMA and
LFDMA mapping to co-exist (Hybrid mapping) which can
ensure higher data throughput than conventional sub-carrier
mapping techniques . But hybrid sub-carrier mapping can
destroy orthogonality of users which can be overcome by
using channel dependent scheduling and adaptive modulation.
The spreading code length is defined by DFT size and the

_______________________________________________________________________________________
number of spreading code will define the number of active
users in the system. It was observed that increase in spreading
gain can provide better BER performance for the system.
Fig-4: SC-FDMA-CDMA System [17]
5. SIMULATION RESULTS
The simulation of SC-FDMA system for QPSK and 16-QAM
modulation was carried out for the following parameters.
Table-2: Simulation Parameters
No of Symbol Block 100000
DFT Size 64
IFFT Size 256
No of users 4
Sub-carrier Frequency 15Khz
Filter Raised Cosine
Roll –Off 0.5
Oversampling Factor 8
The complementary cumulative distribution (CCDF) function
Vs PAPR plot of IFDMA and LFDMA was compared with
OFDMA. It was observed that SC-FDMA signal has
significant PAPR reduction compared to OFDMA and PAPR
of IFDMA was lower than LFDMA.
Fig-5: CCDF Vs PAPR of IFDMA, LFDMA and OFDMA
signals for (a) QPSK and (b) 16- QAM [9]
The CCDF Vs PAPR plot of IFDMA, LFDMA and OFDMA
for different roll-off of raised cosine filter was compared. It
was observed that PAPR will reduce with increase in filter
roll-off.
Fig- 6: CCDF Vs PAPR of IFDMA, LFDMA and OFDMA
signals for different roll-off (a) QPSK and (b) 16-QAM [9]

_______________________________________________________________________________________
The PAPR performance of SC-FDMA-CDMA and OFDM-
CDMA was compared for a frame length of 100, Spread
length of 16, for 8 users with 1000 data blocks .The PAPR of
SC-FDMA-CDMA was found to be lower than OFDM-
CDMA
Fig-7: PAPR performance of SC-FDMA-CDMA and OFDM-
CDMA [16]
6. CONCLUSIONS
In this paper we have done a survey on various modulation
schemes to determine their suitability in high data rate uplink
communication systems and it was found that SC-FDMA was
a better choice. SC-FDMA can provide better PAPR reduction
than OFDMA and can overcome many of its limitations.
Besides uplink communication SC-FDMA is used in various
applications like return-link of interactive broadband systems,
aeronautical telemetry, land mobile satellite communication
systems etc.SC-FDMA can also be used for multiuser
communication in downlink for obtaining a more energy
efficient and greener base station. It was also observed that
PAPR of signal will vary with the modulation used and a
trade-off between PAPR and out-of band signal energy has to
be considered while choosing the roll-off factor for pulse
shaping. Besides this, spreading in SC-FDMA can provide
both features of SC-FDMA and CDMA which has better
advantages than OFDM-CDMA.
REFERENCES
[1]. Ramjee Prasad, “OFDM for wireless communication
systems”, Artech House, Inc.2004
[2]. Mruthyunjaya, “Effect of Peak-to-Average Power Ratio
Reduction on the Multicarrier Communication System
Performance Parameters”, International Journal of Electrical
and Computer Engineering, 2009
[3]. J.Armstrong, “Peak-to-average power reduction for
OFDM by repeated clipping and frequency domain
filtering,"IEEE Electron. Lett, vol. 38, pp. 246-247, Feb. 2002
[4]. A. D. S. Jayalath and C. R. N. Athaudage, “On the PAR
Reduction of OFDM signals using multiple signal
representation,"IEEE commun. Lett, vol. 8, pp. 425- 427, July
2004
[5]. Brian S. Krongold, and Douglas L. Jones, “An Active-
Set Approach for OFDM PAR reduction via Tone
Reservation”, IEEE Transactions on Signal Processing, Vol.
52, No. 2, February 2004
[6]. Steve C. Thompson, Ahsen U. Ahmed, John G. Proakis,
James R. Zeidler and Michael J. Geile, “Constant Envelope
OFDM”, IEEE Transactions on Communications, Vol. 56, No.
8, August 2008
[7]. Hyung.G. Myung and David. Goodman, “Single carrier
FDMA - A new air interface for long term Evolution”,
Wiley Publications, 2008
[8]. Marilynn P. Wylie-Green, Erik Perrins and Tommy
Svensson, “Introduction to CPM-SC-FDMA: A Novel
Multiple-AccessPower-Efficient Transmission Scheme”, 2011
IEEE
[9]. Hyung G. Myung, Junsung Lim, and David J. Goodman,
“Single Carrier FDMA for uplink wireless transmission”,IEEE
Vehicular Technology Magazine, September 2006
[10]. Wafaa Radi, Hesham ElBadawy, Salwa ElRamly, “Peak
to Average Power Ratio Reduction Techniques for Long Term
Evolution- Single Carrier Frequency Division Multiple
Access System,” International Journal of Advanced
Engineering Sciences and Technologies, Vol No. 6, Issue No.
2, 230 – 236, 2011
[11]. Mohamed Salah, Gamal Abdel-Fadeel and Zaki B.
Nossair, “Peak to Average Power Ratio Reduction in Single
Carrier OFDMA Systems”, 13th International Conference on
Aerospace Sciences & Aviation Technology ASAT- 13, May
26 – 28, 2009
[12]. Hyung G. Myung, “Introduction to single carrier
FDMA”, 15th European Signal Processing Conference
(EUSIPCO 2007), Poznan, Poland, September 3-7, 2007,
[13]. Ken Gentile, “The care and feeding of digital, pulse-
shaping filters,” www.rfdesign.com April 2002
[14]. Hyung G. Myung Junsung Lim David J. Goodman, “
Peak-To-Average Power Ratio of Single carrier Fdma signals
with Pulse shaping”,17th Annual IEEE International
Symposium on Personal, Indoor and Mobile Radio
Communication, 2006
[15]. Christina Ciochina and Hikmet Sari, “A review of
OFDMA and Single–carrier FDMA and some recent results,
In Proc IEEE, vol.1, No1, 2010
[16]. Zhongqiang Luo, Xingzhong Xiong, “Performance
Comparison of SC-FDMA-CDMA and OFDM-CDMA
Systems for uplink”, 2011 IEEE
[17]. Dinesh N. Bhange, Pranita P. Dhakulkar, Trushna
G.Deotale, “Single Carrier Spread Spectrum Technique for
uplink”, International Journal of Electronics communication
and Computer Engineering Volume 4, Issue (2) REACT-2013

_______________________________________________________________________________________
BIOGRAPHIES
Prittu Ann Thomas has graduated from
Saintgits College of Engineering of Mahatma
Gandhi University in Electronics &
Communication Engineering in 2012. She is
currently pursuing her M-Tech Degree in
Wireless Technology from Toc H Institute of
Science & Technology, Arakunnam. Her research interest
includes Mobile Communication and Digital Communication
M. Mathurakani has graduated from
Alagappa Chettiar College of Engineering
and Technology of Madurai University and
completed his masters from PSG college of
Technology of Madras University. He has
worked as a Scientist in Defense Research
and development organization (DRDO) in the area of signal
processing and embedded system design. He was honoured
with the DRDO Scientist of the year award in 2003.Currently
he is a professor in Toc H Institute of Science and
Technology, Arakunnam. His area of research interest
includes signal processing algorithms, embedded system
implementations, reusable architecture and communication
systems.

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