Comparative performance analysis of different modulation techniques for papr reduction of ofdm signal

International Journal of Computer Networks & Communications (IJCNC) Vol.6, No.3, May 2014
DOI : 10.5121/ijcnc.2014.6306 59
COMPARATIVE PERFORMANCE ANALYSIS OF
DIFFERENT MODULATION TECHNIQUES FOR
PAPR REDUCTION OF OFDM SIGNAL
Md. Munjure Mowla1
, Liton Chandra Paul2
and Md. Rabiul Hasan3
1,2,3
Department of Electronics & Telecommunication Engineering,
Rajshahi University of Engineering & Technology, Rajshahi, Bangladesh
ABSTRACT
One of the most important multi-carrier transmission techniques used in the latest wireless communication
arena is known as Orthogonal Frequency Division Multiplexing (OFDM). It has several characteristics
such as providing greater immunity to multipath fading & impulse noise, eliminating Inter Symbol
Interference (ISI) & Inter Carrier Interference (ICI) using a guard interval known as Cyclic Prefix (CP). A
regular difficulty of OFDM signal is high peak to average power ratio (PAPR) which is defined as the ratio
of the peak power to the average power of OFDM Signal. An improved design of amplitude clipping &
filtering technique of us previously reduced significant amount of PAPR with slightly increase bit error rate
(BER) compare to an existing method in case of Quadrature Phase Shift Keying (QPSK) & Quadrature
Amplitude Modulation (QAM). This paper investigates a comparative performance analysis of the different
higher order modulation techniques on that design.
KEYWORDS
Bit Error rate (BER), Complementary Cumulative Distribution Function (CCDF), Long Term Evolution
(LTE), Orthogonal Frequency Division Multiplexing (OFDM) and Peak to Average Power Ratio (PAPR).
1. INTRODUCTION
The quick growth in multimedia controlled applications has triggered an insatiable thirst for high
data rates and resulted in an increased demand for technologies that support very high speed
transmission rates, mobility and efficiently utilize the available spectrum & network resources.
OFDM is one of the paramount resolutions to achieve this goal and it offers a promising choice
for future high speed data rate systems [1].OFDM has been standardized as part of the
IEEE802.11a and IEEE 802.11g for high bit rate data transmission over wireless LANs [2]. It is
incorporated in other applications and standards such as digital audio broadcasting (DAB), digital
video broadcasting (DVB), European HIPERLAN/2 and the Japanese multimedia mobile access
communications (MMAC). In addition, OFDM is also used now as the transmission scheme of
choice in the physical layer of the world wide interoperability for microwave access (WiMAX) &
long term evolution (LTE) standards. It has also been used by a variety of commercial
applications such as digital subscriber line (DSL), digital video broadcast- handheld (DVB-H)
and Media FLO[3]. As the data rates and mobility supported by the OFDM system raise, the
number of subcarriers also raise, which in turn leads to high PAPR. As future OFDM-based
systems may push the number of subcarriers up to meet the higher data rates and mobility
demands, there is a need to mitigate the high PAPR.
A number of attractive approaches have been proposed & implemented to reduce PAPR with the
expense of increase transmit signal power, bit error rate (BER), computational complexity and

60
data rate loss etc. So, a system trade-off is required. These reduction techniques are basically
divided into three types of classes such as signal distortion, multiple signaling & probabilistic and
coding. In this paper, amplitude clipping & filtering based design (signal distortion) is used to
reduce PAPR with a little compromise of BER. The main objective of this paper is to investigate
the comparative performance analysis of different higher order modulation technique on that
particular design.
2. BASIC MODEL OF OFDM SYSTEM
OFDM is a special form of multicarrier modulation (MCM) with densely spaced subcarriers with
overlapping spectra, thus allowing multiple-access. MCM works on the criteria of transmitting
data by dividing the stream into several bit streams, each of which has a much lower bit rate and
by using these sub-streams to modulate several carriers.
Figure 1. Spectra of (a) An OFDM Sub-channel and (b) An OFDM Signal [4]
In multicarrier transmission, bandwidth divided in many non-overlapping subcarriers but not
necessary that all subcarriers are orthogonal to each other as shown in figure 1 (a). In OFDM the
sub-channels overlap each other to a certain extent as can be seen in figure 1 (b), which leads to a
proficient use of the total bandwidth. The information sequence is mapped into symbols, which
are distributed and sent over the N sub-channels, one symbol per channel. To permit dense
packing and still ensure that a minimum interference between the sub-channels is encountered,
the carrier frequencies must be chosen carefully according to their orthogonal properties. By
using orthogonal carriers, frequency domain can be viewed so as the frequency space between
two sub-carriers is given by the distance to their first spectral null [4].
2.1. Mathematical Explanation of OFDM Signals
Consider, a data stream with rate R bps where bits are mapped to some constellation points using
a digital modulation (QPSK or QAM). Let, N of these constellation points be stored for an
interval of Ts= N/R, referred to as the OFDM symbol interval. A serial-to-parallel converter is
used to achieve this. Now, each one of the N constellation points is used to modulate one of the
subcarriers. Then, all modulated subcarriers are transmitted simultaneously over the symbol
interval Ts to get the proper OFDM signal [2]. The OFDM signal )(tx can be expressed as,
∑
−
=
∆+=
1
0
))(2exp()(
N
k
ck tfkfjatx π
∑
−
=
∆=
1
0
)2exp()2exp(
N
k
kc ftkjatfj ππ
)()2exp( tatfj cπ= (1)

61
Where, ka , ,10 −≤≤ Nk are complex-valued constellation points representing data and
,fkff ck ∆+= ,10 −≤≤ Nk is the kth
subcarrier, with cf being the lowest subcarrier
frequency. f∆ is the frequency spacing between adjacent subcarriers, chosen to be sT/1 to ensure
that the subscribers are orthogonal. However, OFDM output symbols typically have large
dynamic envelope range due to the superposition process performed at the IFFT stage in the
transmitter.
3. SYNOPSIS OF PAPR
PAPR is extensively used to evaluate this variation of the output envelope. It is also an important
factor in the design of both high power amplifier (PA) and digital-to-analog (D/A) converter, for
generating error-free (minimum errors) transmitted OFDM symbols. As, there are large number
of independently modulated sub-carriers are existed in an OFDM system, the peak value of the
system can be very large as compared to the average value of the whole system. Coherent
addition of N signals of same phase produces a large peak which is N times of the average signal.
So, the ratio of peak power to average power is known as PAPR.
The PAPR of the transmitted signal is defined as [5],
(2)
4. AMPLITUDE CLIPPING AND FILTERING
Amplitude Clipping and Filtering is one of the easiest techniques which may be under taken for
PAPR reduction for an OFDM system. A threshold value of the amplitude is fixed in this case to
limit the peak envelope of the input signal [6].
c
k
x
A
A
k
x
Figure 2. Clipping Function
The clipping ratio (CR) is deﬁned as,
σ
A
CR = (3)
PowerAverage
PowerPeak
PAPR =
∫
≤≤
= NT
dttx
NT
NTt
PAPR
0
2
|)(|
1
0
max 2
|)(| tx
Clipped Off
Threshold Level

62
Where, A is the amplitude and σ is the root mean squared value of the unclipped OFDM signal.
The clipping function is performed in digital time domain, before the D/A conversion and the
process is described by the following expression,



= )( kxj
kc
k
Ae
x
x φ
Ax
Ax
k
k
>
≤
||
||
10 −≤≤ Nk (4)
Where, c
k
x is the clipped signal,
k
x is the transmitted signal, A is the amplitude and )(
k
xφ is the
phase of the transmitted signal,
k
x .
4.1. Limitations of Amplitude Clipping and Filtering
Clipping causes in-band signal distortion, resulting in BER performance degradation [7].
Clipping also causes out-of-band radiation, which imposes out-of-band interference
signals to adjacent channels. Although the out-of-band signals caused by clipping can
be reduced by filtering, it may affect high-frequency components of in-band signal
(aliasing) when the clipping is performed with the Nyquist sampling rate in the
discrete-time domain. However, if clipping is performed for the sufficiently-
oversampled OFDM signals (e.g., L ≥4) in the discrete-time domain before a low-pass
filter (LPF) and the signal passes through a band-pass filter (BPF), the BER performance
will be less degraded [7].
Filtering the clipped signal can reduce out-of-band radiation at the cost of peak regrowth.
The signal after filtering operation may exceed the clipping level specified for the
clipping operation [8].
5. PROPOSED CLIPPING AND FILTERING METHOD
Indicating the second point of limitation [8] that is clipped signal passed through the BPF causes
less BER degradation, we previously designed a scheme for clipping & filtering method where
clipped signal would pass through a high pass filter (HPF) [9]. The proposed method is now
shown in the figure 3. It shows a block diagram of a PAPR reduction scheme using clipping and
ﬁltering, where L is the oversampling factor and N is the number of subcarriers. The input of the
IFFT block is the interpolated signal introducing N(L −1) zeros in the middle of the original
signal is expressed as,




<<−≤≤
=′
Elsewhere
NLk
N
NLand
N
kforkX
kX
0
22
0],[
][ (5)
In this system, the L-times oversampled discrete-time signal is generated as,





 ∆
′=′ ∑
−
= LN
kfnj
kX
NL
mx
NL
k
π2
exp.][
.
1
][
1.
0
, m = 0,1,…NL – 1 (6)
and is then modulated with carrier frequency fc to yield a passband signal ][mxp
.

63
Proposed (Composed)Filter
Figure 3. Block Diagram of Proposed Clipping & Filtering Scheme.
Let, ][m
p
c
x denote the clipped version of ][m
p
x which is expressed as,







≥
<
−≤−
=
Am
p
xA
Am
p
xmPx
Am
p
xA
m
p
c
x
][
|][|][
][
][ (7)
Where, A is the pre-specified clipping level. After clipping, the signals are passed through the
proposed filter (Composed Filter). The filter itself consists on a set of FFT-IFFT operations where
filtering takes place in frequency domain after the FFT function. The FFT function transforms the
clipped signal ][m
p
c
x to frequency domain yielding ][k
p
c
X . The information components of
][k
p
c
X are passed to a high pass filter (HPF) producing ][
~
k
p
c
X . This filtered signal is passed to
the unchanged condition of IFFT block and the out-of-band radiation that fell in the zeros is set
back to zero. The IFFT block of the filter transforms the signal to time domain and thus
obtain ][~ m
p
c
x .
6. DESIGN AND SIMULATION PARAMETERS
In our previous research works, a linear-phase FIR filter using the Parks-McClellan algorithm was
used in the composed filtering [9]. Existing method [7] uses the band pass filter. But, using this
special type of high pass filter in the composed filter, significant improvement was observed in
the case of PAPR reduction. The Parks-McClellan algorithm uses the Remez exchange algorithm
and Chebyshev approximation theory to design filters with an optimal fit between the desired and
actual frequency responses. The filters are optimal in the sense that the maximum error between
the desired frequency response and the actual frequency response is minimized. The observations
were actually based on only QPSK & QAM. In this simulation, using this filter, the effects of
other higher order modulation techniques (8-PSK, 16-PSK, 32-PSK, 8-QAM, 16-QAM & 32-
QAM) will be analyzed.
Table 1 shows the values of parameters used in the different modulation systems for analyzing the
performance of clipping and filtering technique.
][kX ′ ][mx′ ][mx p′
][mx p
c
][kX p
c
][~ mx p
c ])[(~ ktx][
~
kX p
c
L.N
Point
IFFT
fc
Digital
up
Conver
sion
Clipping
High
Pass
Filter
Low
Pass
Filter
L.N
Point
FFT
L.N
Point
IFFT

64
2 4 6 8 10 12 14 16
10
-2
10
-1
10
0
PAPR0[dB]
CCDF=Probability(PAPR>PAPR0)
PAPR Distribution for CR=0.8,1.0,1.2,1.4,1.6[QPSK/N=128]
Unclipped
Clipped
Clipped & Filtering
CR=0.8
CR=0.8
2 4 6 8 10 12 14 16
10
-2
10
-1
10
0
PAPR0[dB]
PAPR Distribution for CR=0.8,1.0,1.2,1.4,1.6[ QAM / N=128]
Unclipped
Clipped
Clipped & Filtering
CR=0.8
CR=0.8
Table 1. Parameters Used for Simulation of Clipping and Filtering.
Parameters Value
Bandwidth ( BW) 1 MHz
Over sampling factor (L) 8
Sampling frequency, fs = BW*L 8 MHz
Carrier frequency, fc 2 MHz
No. of Subscribers (N) 128
CP / GI size 32
Clipping Ratio (CR) 0.8, 1.0, 1.2, 1.4, 1.6
Modulation Format
QPSK, 8-PSK, 16-PSK, 32-PSK,
QAM, 8-QAM, 16-QAM & 32-QAM)
6.1. Simulation Results for PAPR Reduction
In this first section, simulation is performed on our design for different higher order modulation
techniques and analyzed their performances in case of reducing PAPR. Here, we want to monitor
the effect of same number of symbol order (both for QPSK & QAM) step by step. It was analyzed
QPSK with QAM previously. Now, other comparative analysis will be discussed in the next
section.
6.1.1 Simulation Results:
In this section, PAPR distributions for different CR values are shown in the following figures.
Clipped & filtered signal are shown in red colours.
(a) (b)

65
2 4 6 8 10 12 14 16
10
-2
10
-1
10
0
PAPR0[dB]
PAPR Distribution for CR=0.8,1.0,1.2,1.4,1.6 [8-PSK/ N=128 ]
Unclipped
Clipped
Clipped & Filtering
CR=0.8CR=0.8
2 4 6 8 10 12 14 16
10
-2
10
-1
10
0
PAPR0[dB]
PAPR Distribution for CR=0.8,1.0,1.2,1.4,1.6[ 8-QAM / N=128]
Unclipped
Clipped
Clipped & Filtering
CR=0.8
CR=0.8
2 4 6 8 10 12 14 16
10
-2
10
-1
10
0
PAPR0[dB]
PAPR Distribution for CR=0.8,1.0,1.2,1.4,1.6[16-PSK / N=128]
Unclipped
Clipped
Clipped & Filtering
CR=0.8
CR=0.8
2 4 6 8 10 12 14 16
10
-2
10
-1
10
0
PAPR0[dB]
PAPR Distribution for CR=0.8,1.0,1.2,1.4,1.6[16-QAM / N=128]
Unclipped
Clipped
Clipped & Filtering
CR=0.8
CR=0.8
2 4 6 8 10 12 14 16
10
-2
10
-1
10
0
PAPR0[dB]
PAPR Distribution for CR=0.8,1.0,1.2,1.4,1.6 [32-PSK / N=128]
Unclipped
Clipped
Clipped & Filtering
CR=0.8
CR=0.8
2 4 6 8 10 12 14 16
10
-2
10
-1
10
0
PAPR0[dB]
PAPR Distribution for CR=0.8,1.0,1.2,1.4,1.6[32-QAM / N=128]
Unclipped
Clipped
Clipped & Filtering
CR=0.8
CR=0.8
( c ) (d)
Figure 4. PAPR distribution for CR=0.8, 1.0, 1.2, 1.4, 1.6;
(a) QPSK and N=128; (b) QAM and N=128
(c) 8-PSK and N=128; (d) 8- AM and N=128
(e) 16-PSK and N=128; (f) 16-QAM and N=128
(g) 32-PSK and N=128; (h) 32-QAM and N=128
In table 2, PAPR distribution for the above mentioned data are tabulated. The differences between
same order modulations are also shown.
(e) (f)
(g) (h)

66
Table 2. PAPR Characteristics comparison of same symbol order modulation
CR value
QPSK
(dB)
QAM
(dB)
Difference
between QPSK
& QAM (dB)
8-PSK
(dB)
8-QAM
(dB)
Difference between
8-PSK & 8-QAM
(dB)
0.8 5.11 4.97 0.14 5.001 5.038 -0.037
1.0 5.18 5.25 -0.07 5.281 5.37 -0.089
1.2 5.65 5.67 -0.02 5.601 5.618 -0.017
1.4 6.04 6.09 -0.05 6.061 6.101 -0.04
1.6 6.51 6.51 0 6.570 6.569 0.001
CR value
16-
PSK
(dB)
16-
QAM
(dB)
Difference
between 16-
PSK & 16-
QAM (dB)
32-
PSK
(dB)
32-
QAM
(dB)
Difference between
32-QPSK & 32-
QAM
(dB)
0.8 4.959 5.021 -0.062 4.998 4.9 0.098
1.0 5.227 5.297 -0.07 5.219 5.267 -0.048
1.2 5.606 5.621 -0.015 5.615 5.7 -0.085
1.4 6.026 6.069 -0.043 6.064 6.174 -0.11
1.6 6.552 6.552 0 6.499 6.498 0.001
Performance Analysis:
Firstly, for the same number of subscribers (N=128) & low CR=0.8, QAM provides less PAPR
than QPSK. But, at the moderate CR value (1.0, 1.2, 1.4), QPSK results less PAPR than QAM. At
the high CR value (1.6), there is no difference between using QAM & QPSK. So, for lower CR
(More Amount of Clipping), QAM is more suitable than QPSK for this design.
Secondly, it is examined that for the symbol order (8), 8-PSK shows the less PAPR than 8-QAM
up to the CR value (1.4). But, at the higher CR value (Less Amount of Clipping), 8-QAM
provides the better results.
Thirdly, it is found that for the symbol order (16), 16-PSK shows the less PAPR than 16-QAM up
to the CR value (1.4). But, at the higher CR value (Less Amount of Clipping), both formats
provide the same results
Lastly, it is observed that for the higher symbol order (32), 32-PSK shows the less PAPR than 32-
QAM up to the CR value (1.4). But, at the higher CR value (Less Amount of Clipping), 32-QAM
provides the better results.
So, analyzing the simulated results by this design, it is clearly monitored that in case of higher CR
value (Less Amount of Clipping), QAM is more appropriate than PSK. On the other hand, PSK is
better suited than QAM in case of low CR value (More Amount of Clipping).
6.2. Simulation Results for BER Performance
The clipped & filtered signal is passed through the AWGN channel and BER are measured for
different modulation techniques. It is shown from these figures that the BER performance
becomes worse as the CR decreases. That means, for low value of CR, (More amount of
clipping), the BER is more.

67
0 1 2 3 4 5 6 7 8 9 10
10
-3
10
-2
10
-1
10
0
EbNo(dB)
BER
BER Performance [QPSK / N=128]
Analytical
Clipped
Clipped & Filtering
CR=1.6
CR=1.6
0 1 2 3 4 5 6 7 8 9 10
10
-4
10
-3
10
-2
10
-1
10
0
EbNo(dB)
BER
BER Performance [QAM / N=128 ]
Analytical
Clipped
Clipped & Filtering
CR=1.6
CR=1.6
0 1 2 3 4 5 6 7 8 9 10
10
-2
10
-1
10
0
EbNo(dB)
BER
BER Performance [8-PSK / N=128 ]
Analytical
Clipped
Clipped & Filtering
CR=1.6
CR=1.6
0 1 2 3 4 5 6 7 8 9 10
10
-0.8
10
-0.7
10
-0.6
EbNo(dB)
BER
BER Performance [ 8-QAM / N=128 ]
Analytical
Clipped
Clipped & Filtering
CR=1.6
CR=1.6
0 1 2 3 4 5 6 7 8 9 10
10
-0.7
10
-0.6
10
-0.5
10
-0.4
EbNo(dB)
BER
BER Performance [16-PSK / N=128 ]
Analytical
Clipped
Clipped & Filtering
CR=1.6
CR=1.6
0 1 2 3 4 5 6 7 8 9 10
10
-0.7
10
-0.6
10
-0.5
EbNo(dB)
BER
BER Performance [16-QAM / N=128]
Analytical
Clipped
Clipped & Filtering
CR=1.6
CR=1.6
6.2.1 Simulation Results:
In this section, BER Performance for different CR values is shown in the following figures.
(e) (f)
(a) (b)
(c) (d)

68
0 1 2 3 4 5 6 7 8 9 10
10
-0.5
10
-0.4
EbNo(dB)
BER
BER Performance [32-PSK / N=128]
Analytical
Clipped
Clipped & Filtering
CR=1.6
CR=1.6
0 1 2 3 4 5 6 7 8 9 10
10
-0.7
10
-0.6
10
-0.5
EbNo(dB)
BER
BER Performance [32-QAM / N=128 ]
Analytical
Clipped
Clipped & Filtering
CR=1.6
CR=1.6
Figure 5. BER performance for CR=0.8, 1.0, 1.2, 1.4, 1.6;
(a) QPSK and N=128; (b) QAM and N=128
(c) 8-PSK and N=128; (d) 8- AM and N=128
(e) 16-PSK and N=128; (f) 16-QAM and N=128
(g) 32-PSK and N=128; (h) 32-QAM and N=128
Table 3. BER Performance comparison of same symbol order modulation
CR
value
QPSK QAM Difference
between QPSK &
QAM
8-PSK 8-QAM Difference
between 8-PSK &
8-QAM
0.8 0.0752 0.07602 -0.00082 0.2445 0.1896 0.0549
1.0 0.0616 0.06256 -0.00096 0.2356 0.1865 0.0491
1.2 0.0492 0.05091 -0.00171 0.2166 0.1827 0.0339
1.4 0.04025 0.04089 -0.00064 0.2007 0.1815 0.0192
1.6 0.0339 0.03642 -0.00252 0.1876 0.1803 0.0073
CR
value
16-PSK
16-
QAM
Difference
between 16-PSK
& 16-QAM
32-PSK
32-
QAM
Difference
between 32-
QPSK & 32-
QAM
0.8 0.3279 0.2137 0.1142 0.3617 0.2618 0.0999
1.0 0.3176 0.2129 0.1047 0.3583 0.2506 0.1077
1.2 0.3071 0.2088 0.0983 0.3482 0.2436 0.1046
1.4 0.2939 0.2067 0.0872 0.3452 0.2408 0.1044
1.6 0.2914 0.2053 0.0861 0.3349 0.2339 0.1010
(g) (h)

69
Performance Analysis:
It is observed from the table 3 that, for all CR values, QAM results more BER than QPSK. But
interestingly, in case of higher order modulation, M-QAM provides less BER than M-PSK (M=8,
16 and 32). As stated earlier, that for low CR means more amount of clipping that consequences
more amount of BER, so, it is also monitored that for all cases of modulation.
As, PAPR reduction using amplitude clipping & filtering is a distortion method, so, there is a
need for system trade-off. Here, we reduce PAPR with a little sacrifice of BER.
From data, it is also analyzed that, in case of QPSK & QAM, for almost 3% reduction of PAPR
causes maximum 4% increment of BER i.e:~ that is acceptable. But, for this design, using higher
order modulation, almost 3% PAPR reduction, it causes more than 15% BER i.e:~ that is too
much.
Another viewpoint is the bit error rate per dB (BER/dB) shows that for the differences between
same order modulations (M-PSK & M-QAM) gradually increases as M increases in case of a
particular CR.
6. CONCLUSION
In this paper, a comparative performance is analyzed using higher order modulation techniques. It
is observed from the simulated result that using this design, in case of higher CR value (Less
Amount of Clipping), QAM is more appropriate than PSK. On the other hand, PSK is better
suited than QAM in case of low CR value (More Amount of Clipping). As QPSK provides less
PAPR than QAM, so, it causes high BER compare to QAM. The rational amount of BER is quite
more than rational amount of PAPR reduction in case of all higher order modulations. So, in this
design, lower order modulation (M=4) is better than higher order modulations (M=8, 16 and 32).
In the present simulation study, ideal channel characteristics have been considered. In order to
estimate the OFDM system performance in real world, multipath fading will be the next concern.
The increase number of subscribers (N) & other parameters can be another assumption for further
study.
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