PAPR Analysis MIMO-OFDM

Int. Journal of Electrical & Electronics Engg. Vol. 2, Spl. Issue 1 (2015) e-ISSN: 1694-2310 | p-ISSN: 1694-2426
123 NITTTR, Chandigarh EDIT-2015
A Review Paper on: The PAPR Analysis of
MIMO-OFDM Systems
1
Raj Lakshmi Shukla , 2
Garima Saini
1 2
ME Scholar, Assistant Professor, ECE Department
1,2
NITTTR Chandigarh
1
rajlakshmi.shukla@gmail.com,2
garimasaini_18@rediffmail.com
ABSTRACT:Orthogonal frequency division Multiplexing
(OFDM) is one of the most popular multicarrier or
multiplexing modulation techniques which transmits many
signals over a single path in high speed wireless
communication. OFDM convert high data rate stream in to
smaller data rate stream. Due to this high data rate and
ability to combat frequency selective fading, OFDM has a
strong candidate for 4G wireless networks. Because of
OFDM-MIMO advantages in multipath fading channel e.g.
stout against ISI, ICI and some other advantages like best
QoS for multiple users, efficient convention of bandwidth it is
suggested to be the modulation technique for next generation
4G networks e.g. LTE. OFDM combined with multiple-input
multiple-output (MIMO) to increase system capacity over the
time variant frequency-selective channels and the diversity
gain. The radio transmitter stations for covering and getting
enough transmitted power in their desired area has to use
High Power Amplifier (HPA) operable near to the saturation
region or else, an aspect memory-less nonlinear distortion will
affect the communication path. But along with all its
advantages there are some disadvantages also. e.g. High
PAPR (Peak to Average Power Ratio) at the transmitter end
and BER (Bit Error Rate) at the receiving end. As OFDM is
only used in the downlink of 4G networks. To reduce the
problems of OFDM-MIMO systems some procedures as
SLM, PTS, Clipping, Coding, & Pre-coding etc are suggested
but none of them is proficient enough to reduce the PAPR
and BER up to standard value. This Paper will discuss some
techniques of PAPR reduction, and both their advantages and
disadvantages.
Keywords: Orthogonal Frequency Division Multiplexing multiple
in multiple out (OFDM-MIMO), Peak–to-Average Power Ratio
(PAPR), High Power Amplifier (HPA), selective mapping (SLM)
Transmit sequence (PTS), Probability Distribution Function
(PDF).
I. INTRODUCTION
MIMO-OFDM is an attractive technique for high data rate
wireless communication systems, but it exhibits a large
peak-to-average power ratio (PAPR), due to the
superposition of the individual nonlinear region of High
Power Amplifier (HPA) [1-3], and the signal distortion
induces the degradation of Bit Error Rate (BER). High
PAPR causes signal distortions such as the in-band
deformation and out-of-band emission and induces the
degradation of bit error rate performance due to the
nonlinearity effects [1]. Therefore, RF power amplifiers
should operate in a very large linear region to avoid the
signal peaks from getting into the non-linear region of the
power amplifier causing in-band distortion. i.e., inter
modulation among the subcarriers and
out of band radiation. To surmount this, the power
amplifiers should be operated with a large power back-offs
and this indirectly leads to very inefficient amplification
and increases in transmitter power. There are several
reduction technique that have been proposed to reduce the
PAPR problem in MIMO-OFDM systems such as Clipping
And Filtering[2], Partial Transmit Sequences (PTS)[3],
Selected Mapping Technique[4], tone reservation (TR) and
tone injection (TI)[5]. And some coding techniques. SLM
and PTS are scrambling techniques which uses
probabilistic methods [4]. The principle of probabilistic
method depends on reducing the probability of high PAPR
by generating several OFDM symbols (multiple
candidates) carrying the same information and selecting
the one having the lowest PAPR [5]. The absence of the
PAPR reduction techniques will cause the increase in the
transmit power, the in Bit error rate at the receiver, the loss
in data rate, and the computational complexity. [6]
II SYSTEM MODEL
As in Fig 1 in a MIMO-OFDM system with N subcarriers
(or tones) the individual data streams are first passed
through OFDM modulators which perform an IFFT on
blocks of length N followed by a parallel-to-serial
conversion. A cyclic prefix (CP) of length Lcp _ L
containing a copy of the last Lcp samples of the parallel-
to-serial converted output of the N-point IFFT is then
prepended. The resulting OFDM symbols of length N +
Lcp are launched simultaneously from the individual
transmit antennas. The Cyclic Prefix is essentially a
security and a guard interval which serves to eliminate
interference between OFDM symbols and turns linear
convolution into circular convolution such that the channel
is diagonalized by the FFT. In the receiver the individual
signals are passed through OFDM demodulators which
MIMO-OFDM Systems
1
Garima Saini
1 2
1,2
NITTTR Chandigarh
1
disadvantages.
(PDF).
I. INTRODUCTION
II SYSTEM MODEL
MIMO-OFDM Systems
1
Garima Saini
1 2
1,2
NITTTR Chandigarh
1
disadvantages.
(PDF).
I. INTRODUCTION
II SYSTEM MODEL

NITTTR, Chandigarh EDIT -2015 124
first discard the CP and then perform an N-point FFT. The
outputs of the OFDM demodulators are finally separated
and decoded.
III: PEAK TO AVERAGE POWER RATIO
Peak-to-Average Power Ratio (PAPR) is defined as the
relation between the maximum power divided by the
average power of the OFDM system [6]. The PAPR is
defined as the ratio of the peak power of the signal to its
average power, which is in other words a measure of the
fluctuations or amplitude ebb and flow of the signal. The
transmit signals in an orthogonal frequency-division
multiplexing (OFDM) system can have high peak values in
the time domain since many subcarrier components are
added via an inverse fast Fourier transformation (IFFT)
operation. High PAPR results from the nature of the
modulation itself where multiple subcarriers /sinusoids are
added together to form the signal to be transmitted. When
total of N sinusoids add, the peak magnitude would have a
value of N, where the (rms) average might be quite low
due to disparaging interference between the sinusoids. In
fact, the high PAPR is one of the most detrimental aspects
in an OFDM system as it decreases the signal-to-
quantization noise ratio (SQNR) of the analog-digital
convertor (ADC) and digital-analog convertor (DAC)
while deteriorating the efficiency of the power amplifier in
the transmitter.
(1)
IV. DECISIVE FACTOR FOR PAPR REDUCTION
In this section a decisive factor is characterised for the
techniques used for both PAPR and BER reduction. There
are many different techniques and some hybrid techniques
used for PAPR and BER reduction. But still none gave
acceptable results. The technique must reduce the PAPR
and BER largely plus the following performance factors
must be considered for OFDM based system in order to be
acceptable.
a) Wherewithal of PAPR Reduction
The primary factor of selecting PAPR reduction technique
is the wherewithal and capability of PAPR reduction. A
technique is believed to be finest if it reduces PAPR
largely. OOB radiation and IB distortion are two of the
many considerable factors for selecting a technique.
b) Low Average Power
A procedure must not only reduce PAPR but also the
average power of the signal which should not be increased
from an acceptable region or else it will require a large
linear region for operation in HPA, which will enlarge the
BER rate of the OFDM system.
c) Low Complexity
The technique should also not increase the complexity of
the overall system. Complexity and intricacy includes both
time and hardware requirements for implementation and
thereby the functioning of the system.
d) Less Bandwidth Expansion
Some techniques like scrambling techniques needs side
information, which increase the use of the bandwidth.
Some coding techniques also enlarge the bandwidth due to
code rate generation. A technique must not be such which
enhances and increases the bandwidth to value which
ultimately causes degradation and filth in the throughput
and hence efficiency is reduced..
e) Less BER Performance Degradation
The main objective of the PAPR reduction technique is is
to achieve better performance including BER as compared
to usual and conventional OFDM system.
f) Less Additional Power Need
The technique must require no need of additional power
for PAPR reduction, as it will degrade BER performance
of the system plus power efficiency is the main goal of
wireless based systems.
h) Other Factors
Several previous factors like nonlinear devices such as
ADC/ DAC convertors and cost should be considered.
V. PEAK TO AVERAGE POWER REDUCTION
TECHNIQUE
Peak to Average Power ratio (PAPR) is the major problem
in the Orthogonal Frequency Division Multiplexing
(OFDM) [7]. For reduction or minimization of PAPR
various techniques are used [8]. Some of the techniques
used for the PAPR reduction like Clipping, Scrambling
Techniques, Tone Reservation and Injection techniques,
Active Constellation Extension, Adaptive Pre Distortion
Techniques, coding techniques are the foundation of PAPR
reduction. These techniques must change hardware of
transmitter and receiver according to the techniques which
is used for PAPR reduction. Also some techniques which
does not change hardware, are based on the software
programming and are called the channel coding.
1. CLIPPING AND FILTERING
This is simplest technique used for PAPR reduction of
OFDM signal. A Clip is also called as non linear saturation
which is employed around the peaks to reduce the peaks
before HPA to reduce PAPR and so is called Clipping
Technique. This is simple technique but it yet introduces
Out Of Band Radiation and In Band Distortion in OFDM
Signal. It also destroys the Orthogonality of OFDM
subcarriers. The part of the signal which are above the
allowed region are clipped in a simple clipping technique.
Joint flitering and clipping technique reduce the OOB
radiation but IB distortion are still there since this method
degrades OFDM system performance e.g. spectral
efficiency and BER. Peak Windowing improves spectral
efficiency but it increase OOB radiation and BER
Modulation. Envelop scaling is used for PAPR reduction
due to equality envelop properties of all subcarriers input
[9] Clipping means the amplitude of the signal is clipped at
the predefined values which limit the peak value of the
input signal to a predetermined or threshold value [10]. Let
X[n] denote the input signal and Xc [n] denote the clipped
signal of X[n], which can be represented as,
[ ] = −Y [ ] ≤−Y

[ ] [ ] <Y
Y [ ] ≥ Y (2)
Where Y is the threshold or predetermines value of
clipping level. Clipping is simple but yet it has some
drawback. Clipping cause signal distortion which increase
bit-error-rate performance. After the filtering operation
performed on the clipped signal clipping level may exceed
to the specified for the clipping operation.
2. CODING
Coding is the technique of reduction of PAPR by selecting
such a codeword that reduced the PAPR. Coding does not
cause any distortion and out of band radiation. But still it
has the drawback of bandwidth efficiency as the code rate
is reduced. It also suffer from the complexity of the
algorithm to find the optimal code and store large tables
for encoding and decoding for a multi-carrier system Pre-
coding based techniques are combined with scrambling
techniques, clipping techniques, ACE etc to get best
performance.
Scrambling Techniques uses the concept of phase rotation.
These techniques (Selective Mapping Technique) & PTS
(Partial Transmit Technique) are very popular technique
for PAPR reduction . As number of phase rotation
increases these techniques simply augment complexity.
Also the side information is needed for receiver to decipher
signal in these techniques
3. SELECTED MAPPIMG (SLM)
Selected Mapping (SLM) technique reduces PAPR of the
system by selected mapping so it is call Selected Mapping
technique [11]. A set of generated signal has the same
information in selected mapping, and the most favourable
signal is selected for transmitted. In the SLM the input data
stream is multiply by random weighted factor and the
lowest PAPR is chosen for transmission as can be seen
from Fig 2. To recover the original data at the receiver side
multiplying sequence can be sent as side information. SLM
method for reduction of the PAPR does not eliminate the
peak. The drawback of this technique is the side
information that requires to be transmitted to the receiver
of the system in order to recover the original data stream.
After IFFT operation select one with low PAPR for
transmits. For measurement the PAPR commutative
distributed function (CDF) and Complementary
commutative distributed function (CCDF) is used. CCDF
is measure the probability that the PAPR of a data block
exceed the given threshold value.
4. PARTIAL TRANSMIT SEQUENCE (PTS)
In this technique first step is the phase shifting of the sub-
block and then multiplication of data structure by random
vector [12]. The main idea behind this technique is that
input data block is divided into non-overlapping sub-block
and each sub-block is phase shifted by a constant factor to
reduce PAPR as can be seen from Fig 3. PTS technique is
modified version of the SLM technique. PTS method has a
capability that minimizes the PAPR more then SLM and
main advantage of the PTS technique is no need to send
side information. This advantage reduces the complexity of
the PTS technique .
5. TONE RESERVATION (TR)
Tone reservation technique is proposed for PAPR
reduction. In this technique small set of tone is used for
PAPR reduction. This can be originated as convex
problem. The amount of PAPR reduction is dependent on
the number of reserved tones, their location, quantity of
complexity and allowed power on them [13]. Tone
reservation shows that reserving a small fraction of tones
give larger minimization in PAPR even using simple
algorithm. The advantages of tone reservation method is
that it is less complex, no side information is sent, and also
no additional operation can be performed at the receiver
side.
6. TONE INJECTION (TI)
This technique is based on the increased size of the
constellation. Each point of the original constellation is
[ ] [ ] <Y
Y [ ] ≥ Y (2)
2. CODING
performance.
the PTS technique .
side.
[ ] [ ] <Y
Y [ ] ≥ Y (2)
2. CODING
performance.
the PTS technique .
side.

NITTTR, Chandigarh EDIT -2015 126
mapped into several equivalent points in the extended
constellation [13]. So that all the information can be
mapped into several equidistant constellation point. In
Tone injection the new point with an extended
constellation is replaced with a place of basic constellation
point which injects a tone with a proper phase and
frequency in multi carrier system. The drawback of the
tone reservation is need to decode side information at the
receiver side and so that needed extra IFFT operation
which increases the complexity of the system.
7. ACTIVE CONSTELLATION EXTENSION (ACE)
Here some of the outer signal constellation points in the
data blocks are extended towards the outside of the original
constellation such that PAPR of system is reduced.[14].
8. ADAPTIVE PRE-DISTORTION TECHNIQUE
These techniques can recompense for the non linear
consequences of HPA (High Power Amplifier). The non
linear deviation of the amplifier can be handled by the
usual adjustment of the input constellation with the help of
minimum usage of hardware. The convergence time of
predistorter and the MSE (Mean Square Error) can be
lessened via screening out and broadcasting techniques by
the training design assistance.
All these techniques are comparatively poles apart and
entail different restriction like multifaceted optimization,
(out-of-band) emission, IB (in-band) deformation, side
efficiency diminishing, computational complexity,
bandwidth expansion, BER (bit error rate) degradation and
data rate loss.
Relative
VI CONCLUSION
This paper investigates one of the bottleneck problems that
exist in OFDM wireless communication systems. High
Peak average power ratio (PAPR of OFDM signal), and
discuss how to reduce it by different effective algorithms.
Both scrambling and non scrambling techniques have been
discussed. Each technique of PAPR reduction in OFDM
based system are different from each other and impact
different constraints as in bandwidth expansion, OOB
radiation, IB distortion, reduction of spectral efficiency
radiation, BER reduction, high peak and average power,
and taken as a whole system’s intricacy and complexity.
Precoding based techniques results good with no need of
side information and works with less complexity. Clipping
cause signal distortion which increase bit-error-rate
performance. Coding increases bandwidth efficiency as the
code rate is reduced. It also suffer from the complexity of
the algorithm. SLM method for reduction of the PAPR
does not eliminate the peak. The drawback of this
technique is the side information that requires to be
transmitted to the receiver of the system in order to recover
the original data stream. Though PTS technique does not
need side information but still the implementation
complexity is increased due to number of IFFT operations
and due to complex multiplication and summation. The
drawback of the tone reservation is the need to decode side
information at the receiver side and so that extra IFFT
operation are needed which increases the complexity of the
system. Adaptive Predistortion technique can be dependent
upon the layout circuitry of hardware. Less is the hardware
the deviation can be handled of HPA. Precoding based
techniques results good with no need of side information
and works with less complexity Pre-coding plus other
techniques gives best result for PAPR reduction. Also
hybrid SLM and PTS allows further PAPR reduction even
though it requires too many calculations But it reduces the
efficiency of the use of the side bits.
VII FUTURE WORK
Though all the techniques in individual and hybrid effects
gives good results but still every method is having certain
drawbacks. So the future work will be discussing some
hybrid techniques amongst scrambling and non scrambling
and checking for The Cumulative Distribution Function
(CDF) which is the most important parameters of the
OFDM-MIMO systems to measure the PAPR.
VIII ACKNOWLEDGEMENT
Authors would like to thank to Dr.M.P.Puniya, Director
NITTTR,Chandigarh, Dr.S.B.L.Sachan,HOD-ECE,
NITTTR Dr.Prabal Chakravorty, Director VGI, Dadri,
without whose support and valuable guidance things
would not have been practically implemented.Last we
would thank our colleagues for giving their endless
support.
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