Analysis of Peak to Average Power Ratio Reduction Techniques in Sfbc Ofdm System

IOSR Journal of Electronics and Communication Engineering (IOSR-JECE)
e-ISSN: 2278-2834,p- ISSN: 2278-8735.Volume 7, Issue 5 (Sep. - Oct. 2013), PP 47-52
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www.iosrjournals.org 47 | Page
Analysis of Peak to Average Power Ratio Reduction Techniques
in Sfbc Ofdm System
T.Malathi1
, A.Sasipriya2
, S.Saravanan3
1
Asst.Prof., Muthayammal Engineering College, Rasipuram.
2
Asst.Prof., Muthayammal Engineering College, Rasipuram
3
Prof., Muthayammal Engineering College, Rasipuram.
Abstract: Orthogonal Frequency Division Multiplexing (OFDM) has become the most popular modulation
echnique for high speed data transmission. But the great disadvantage of the OFDM technique is its high Peak
to Average Power Ratio (PAPR). In this paper, the Selected Mapping (SLM) technique and Clipping and
Differential Scaling is applied to Space Frequency Block Coded (SFBC) OFDM systems to reduce the PAPR
with Alamouti coding scheme. In SLM technique, different representations of OFDM symbols are generated by
rotation of the original OFDM frame by different phase sequences, and the signal with minimum PAPR is
selected and transmitted. To compensate for the effect of the phase rotation at the receiver, it is necessary to
transmit the index of the selected phase sequence as side information (SI). Additionally, a suboptimum detection
method that does not need SI is introduced at the receiver side. In Clipping and Differential Scaling technique,
the amplitude of complex OFDM signal is clipped and then scaled in such a way so that the PAPR is reduced
without causing much degradation in bit error rate (BER). The threshold values for clipping and scaling is
determined using Monte Carlo Simulations. Simulation results show that the SLM method and Clipping and
Scaling method effectively reduce the PAPR.
Keywords: Orthogonal frequency division multiplexing (OFDM), peak to average power ratio (PAPR),
selected mapping (SLM), space frequency block coded (SFBC), Side information (SI), high power amplifiers
(HPA), complementary cumulative density function (CCDF), inter-symbol interference (ISI).
I. Introduction
Orthogonal Frequency Division Multiplexing (OFDM) has been recently seen rising popularity in
wireless applications. For wireless communications, an OFDM-based system can provide greater immunity to
multi-path fading and reduce the complexity of equalizers. Now OFDM have been included in digital
audio/video broad-casting (DAB/DVB) standard in Europe, and IEEE 802.11, IEEE 802.16 wireless broadband
access systems, etc. The basic principle of OFDM is to split a high-rate data stream into a number of lower rate
streams that are transmitted simultaneously over a number of subcarriers. These subcarriers are overlapped with
each other. Because the symbol duration increases for lower rate parallel subcarriers, the relative amount of
dispersion in time caused by multipath delay spread is decreased. Inter-symbol interference (ISI) is eliminated
almost completely by introducing a guard time in every OFDM symbol.
One of the main drawbacks of OFDM is its high Peak to Average Power Ratio (PAPR) because it is
inherently made up of so many subcarriers. The subcarriers are added constructively to form large peaks. High
peak power requires High Power Amplifiers (HPA), A/D and D/A converters. Peaks are distorted nonlinearly
due to amplifier imperfection in HPA. If HPA operates in nonlinear region, out of band and in-band spectrum
radiations are produced which appears as the adjacent channel interference. Moreover if HPA is not operated in
linear region with large power backs-offs, it would not be possible to keep the out-of-band power below the
certain limits. This further leads to inefficient amplification and expensive transmitters. To prevent all these
problems, power amplifiers has to be operated in its linear region. To overcome this problem, some algorithms
have been proposed, which reduce the PAPR of the baseband OFDM signal [2]–[12]. Some of these methods
need side information (SI) to be transmitted to the receiver, such as Partial Transmit Sequence (PTS) [8] and
Selected Mapping (SLM) [9]. Some other methods do not need SI, such as clipping and filtering [5], tone
reservation [6], [11], Polyphase Interleaving and Inversion [7] and Active Constellation Extension [2].
II. Papr Of An Ofdm Signal
Let us denote the data block of length N as a vector X = [X0, X1,… ,XN−1]T
where N is equal to the
number of sub-carriers. The duration of a symbol Xn in X is T. Each symbol in X modulates one of a set of sub-
carriers, {fn, n = 0, 1,…,N – 1}. The N sub-carriers are chosen to be orthogonal, that is, fn = nΔf, where Δf = 1/NT
and NT is the duration of an OFDM data block X. The complex envelope of the transmitted OFDM signal is
given by

Analysis Of Peak To Average Power Ratio Reduction Techniques In Sfbc Ofdm System
x(t)=
1
√𝑁
𝑋 𝑘 𝑒 𝑗2𝜋 𝑓 𝑘 𝑡𝑁−1
𝑘=0 ,0 ≤ t ≤NT (1)
Presence of large number of independently modulated sub-carriers in an OFDM system the peak value
of the system can be very high as compared to the average of the whole system. This ratio of the peak to average
power value is termed as Peak-to-Average Power Ratio. Coherent addition of N signals of same phase produces
a peak which is N times the average signal. The PAPR of the transmitted signal in (1) is defined as
PAPR(x) =
max n⁡{ 𝑥(𝑛) 2}
𝐸 { 𝑥(𝑛) 2}
(2)
In principle, PAPR reduction techniques are concerned with reducing max|x(t)|. The Cumulative
Distribution Function (CDF) is one of the most regularly used parameters, which is used to measure the
efficiency of any PAPR technique.
III. Space Frequency Block Coded Orthogonal Frequency Division Multiplexing System
Model
In SFBC-OFDM systems with two transmitter antennas, the frequency-domain vector transmitted from
the pth antenna is denoted by
Xp = [Xp(0),Xp(1), . . . , Xp(Nc − 1)] (3)
It is noteworthy that the orthogonality of the space frequency matrix C leads to full diversity at the
receiver side. We have the space frequency matrix as follows. In this form will be the SFBC frames are made
for transmitting by two transmitter antennas. We convert a normal BPSK signal to a SFBC signal for two
transmitter ntennas.
input_bpsk1(1) = input_bpsk(1);
input_bpsk1(2) = -conj(input_bpsk(2));
input_bpsk2(1) = input_bpsk(2) ;
input_bpsk2(2) = conj(input_bpsk(1))
The sfbc inputs are as follows input_bpsk1(1) is the first input to the first antenna, input_bpsk1(2) is
the first input to the second antenna input_bpsk2(1) is the second input to the first antenna and input_bpsk2(1) is
the second input to the second antenna. This can be represent in matrix form as follows
𝑿𝟏(𝟐𝒗) 𝑿𝟏(𝟐𝒗 + 𝟏)
𝑿𝟐(𝟐𝒗) 𝑿𝟐(𝟐𝒗 + 𝟏)
=
𝑿(𝟐𝒗) 𝑿(𝟐𝒗 + 𝟏)
𝑿 ∗ (𝟐𝒗 + 𝟏) −𝑿 ∗ (𝟐𝒗)
(4)
where v = 0,1,…., Nc/ 2-1.
The vectors X1 and X2 can be generated from the original OFDM frame X. The vectors X1 and X2
are passed through the IFFT operation to yield the time-domain samples x1(n) and x2(n), 0 ≤ n ≤ N − 1. It is
noteworthy that the orthogonality of the space frequency matrix C in (4) leads to full diversity at the receiver
side, i.e.,
CCH
= |X(2ν)|2
+ |X(2ν + 1)|2
I2 (5)
where In is the n × n identity matrix. The PAPR of the pth antenna is defined by
PAPR{ xp } =
max n⁡{ 𝑥𝑝(𝑛) 2}
𝐸 { 𝑥𝑝(𝑛) 2}
p= 1, 2 (6)
where E{.} is the mathematical expectation. The overall PAPR of the SFBC-OFDM system is defined by
PAPR = max PAPR{xp} (7)
p € {1,2}
IV. Selected Mapping For Peak-To-Average Power Ratio Reduction Of Space-Frequency Block
Coded (Ofdm) Systems
In this paper, it is shown that the simplified SLM technique can be applied to SFBC-OFDM systems
with two transmitter antennas and Almouti coding scheme without changing the orthogonality of space
frequency coding. In this method, the optimum phase sequence is applied to the OFDM frames of two antennas
such that the SFBC structure remains constant. Then, it will be shown that, at the receiver side, the optimum
phase sequence can be detected without SI. Detection of phase sequence index does not need any extra
transmission power or extra computation complexity of the transmitter and only uses the intrinsic redundancy of
the SFBC code.

Fig.1. Block diagram of SFBC OFDM transmitter with two transmitter antennas and with SLM method for PAPR reduction.
The Block diagram of SFBC OFDM transmitter with two transmitter antennas and with SLM method
for PAPR reduction is shown in the Fig 1.Vectors X1 and X2 can be multiplied by D different phase sequences
to yield the minimum PAPR representation, but the SFBC structure must remain constant, so that full diversity
can be achieved. In simplified SLM for PAPR reduction of spatial multiplexed OFDM has been proposed. In
this scheme, the OFDM frames of the antennas are simultaneously modified with the same single-phase
sequence. This leads to reduction in the number of bits that must be transmitted as SI.
In this paper, this approach is used for SFBC-OFDM systems. It is shown that, if this method is used
for the SFBC-OFDM system, then the intrinsic redundancy of space frequency coding can be used to detect the
index of the phase sequence at the receiver side without SI. Based on this approach, D different representations
of the signals x1 and x2 are generated.
X1
d
= IFFTN {X1 ⊗bd
}
X2
d
= IFFTN {X2 ⊗ bd
}, 0 ≤ d D-1 (8)
It has been shown that a simple and optimal choice for the phase sequences is a random selection of 0
and π with equal probabilities. In this case, the complexity of the phase rotation is very low, because bd
(k) = ej0
= +1 or bd
(k) = ejπ = −1; thus, the multiplication of the frequency-domain vectors by the vector bd
is via the sign
change of some of the symbols.
By keeping the space frequency code orthogonal, full diversity can be achieved at the receiver side.
Consider an OFDM consisting of N subcarriers. Let a block of N symbol X = XK , k = 0,1,2,…,N-1 is formed
with each symbol modulating one of a set of subcarriers. { 𝑓𝑘, k=0,1,2,...N-1}. The N subcarriers are chosen to
be orthogonal, that is, 𝑓𝑘 = 𝑘∆𝑓, where ∆𝑓 =
1
𝑁𝑇
and T is the original symbol period. Therefore, the
complex baseband OFDM signal can be written as
x(t) =
1
√𝑁
𝑋𝑘 𝑒 𝑗2𝜋 𝑓 𝑘 𝑡𝑁−1
𝑘=0 ,0 ≤ t ≤NT (9)
In general, the PAPR of OFDM signal is defined as the ratio period between the maximum
instantaneous power and its average power during an OFDM symbol.
PAPR =
𝑥(𝑡) 2
0≤𝑡≤𝑁𝑇
𝑚𝑎𝑥
1/(𝑁𝑇) 𝑥(𝑡) 2 𝑑𝑡
𝑁𝑇
0
(10)
Reducing the max 𝑥(𝑡) is the principle goal of PAPR reduction techniques. In practice, most
systems deal with a discrete-time signal, therefore, we have to sample the continuous-time signal x(t).
To better approximate the PAPR of continuous-time OFDM signals, the OFDM signals samples are
obtained by times oversampling. By sampling x(t) defined in Equation , at frequency 𝑓𝑠 = 𝐿/𝑇, wher L is the
oversampling factor, the discrete-time OFDM symbol can be written as
x(n)=
1
√𝑁
𝑋𝑘 𝑒 𝑗
2𝜋
𝑁𝐿
𝑘𝑛
,𝑁−1
𝑘=0 0 ≤ n ≤ NL (11)
Equation (11) can be implemented by using a length (NL) IFFT operation. The new input vector X is
ex-tended from original X by using the so-called zero-padding scheme, i.e., by inserting zeros in the middle of
X. The PAPR computed form the L-times oversampled time domain OFDM signal samples can be defined as

PAPR 𝑥(𝑛) = 10𝑙𝑜𝑔
𝑥(𝑛) 2
0≤𝑡≤𝑁𝐿 −1
𝑚𝑎𝑥
𝐸 𝑥(𝑛) 2 (12)
We can evaluate the performance of PAPR using the cumulative distribution of PAPR of OFDM
signal. The cumulative distribution function (CDF) is one of the most regularly used parameters, which is used
to measure the efficiency of and PAPR technique. The CDF of the amplitude of a signal sample is given by
F(z)=1-exp(z) (13)
However, the complementary CDF (CCDF) is used instead of CDF, which helps us to measure the
probability that the PAPR of a certain data block exceeds the given threshold. The CCDF of the PAPR of the
data block is desired is our case to compare outputs of various reduction techniques. This is given by
P(PAPR>z) =1-P(PAPR>z)
=1-(F(z))N
=1-(1-exp(-z))N
(14)
V. Clipping And Differential Scaling
A new technique called Clipping and Differential Scaling is proposed. The probability distribution of
amplitudes of the OFDM signal follows Rayleigh distribution and thus the probability of high peaks is very less.
Select the clipping threshold at which the BER is degraded from 1.5 × 10−3 to 3.5 × 10−3 at SNR of 10dB and
the amplitudes above this clipping threshold are clipped. Differential Scaling is used to reduce the PAPR but not
deteriorate the BER. Since different ranges of amplitudes of the signal are scaled in a different manner, it is
called Differential Scaling. The three types of scaling are described below.
A. Scale Up
In this method, scale up the lower amplitudes of the signal by a factor of β. This leads to increase the
average value without affecting the peak values. Therefore, the resulting PAPR reduces. The PAPR reduction
function can be defined as
h(x) = αxp, if x > αxp
= βx, if x < A
= x, if A ≤ x ≤ αxp (15)
where xp is the amplitude peak value occurring in an OFDM symbol block, α is the factor deciding the clipping
threshold in terms of percentage of the peak value and β is the scaling factor for the range [0,A) whose value is
greater than one. The values of the parameters used are mentioned at the end of this section.
B. Scale Down
In this method, scale down the higher amplitudes of the signal by a factor of γ. This leads to decrease the
peak value. Although the average value would also fall down, the resulting PAPR reduces. Because the
reduction in peak power is greater than the reduction in the average power. The PAPR reduction function can be
defined as
= γx, if B ≤ x ≤ αxp
= x, if x < B (16)
where xp is the amplitude peak value occurring in an OFDM symbol block, α is the factor deciding the clipping
threshold in terms of percentage of the peak value and γ is the scaling factor for the range [β,αxp] whose value is
less than one. The values of the parameters used are mentioned at the end of this section.
C. Scale Up and Down
In this method, combine both the above-mentioned approaches i.e. up-scaling and down-scaling. This
method exploits the advantages of both the methods. Hence, a PAPR can be reduced considerably. The PAPR
reduction function can be defined as
= γx, if B ≤ x ≤ αxp
= βx, if x < A (17)
VI. Simulation Result
In this section, simulation results for the proposed techniques are presented. The simulation is based on
the system model in Fig. 1. The reduction of PAPR by Selected Mapping technique is shown in Fig.2.

Fig.2. PAPR Reduction using Seleted mapping
The original signal has the PAPR value of nearly 65db. The Clipping and Scaling Technique is applied
to that signal that is Scale Up, Scale Down and Scale Up Down techniques are applied.
Fig.3. PAPR Reduction using Scale up
Fig.4. PAPR Reduction using Scale down
Fig.5. PAPR Reduction using Scale up down
The Scale Up Technique is applied to that signal, the PAPR value is reduced to nearly 8db. The Scale
Down Technique is applied to that signal, the PAPR value is reduced to nearly 6.5db. The Scale Up Down
Technique is applied to that signal, the PAPR value is reduced to nearly 1.5 db. Among these three techniques,
the Scale Up Down Technique is the best method to reduce the PAPR value. The Scale Up, Scale Down and
Scale Up Down method is described separately in Fig.3., Fig.4., and Fig.5. Fig. 6 shows the CCDF with Scale
up, Scale own and Scale up-down by Clipping and Differential scaling.
Fig.6. PAPR Reduction using Clipping and Differential Scaling

VII. Conclusion
In this paper, a novel PAPR reduction technique based on Selected Mapping Technique is done. In this
method, the same phase sequence is concurrently applied to the frequency-domain signals for both antennas, and
the signal with minimum PAPR has been found and transmitted. A low-complexity suboptimum detector has
been proposed to detect the phase sequence index without SI. A simple approach based on Clipping and
Differential Scaling is proposed to reduce the PAPR of OFDM signals. Clipping along with three different
scaling methods, namely up-scaling, down-scaling, up-down scaling is described. Simulation results show that
the performance of the Selected Mapping method and Clipping and Differential Scaling in PAPR reduction of
the SFBC OFDM system with two transmitter antennas for BPSK modulation.
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Analysis of Peak to Average Power Ratio Reduction Techniques in Sfbc Ofdm System

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Analysis of Peak to Average Power Ratio Reduction Techniques in Sfbc Ofdm System