PEAK TO AVERAGE POWER RATIO (PAPR) REDUCTION IN OFDM BASED RADIO SYSTEMS Mohammad Zavid Parvez Md. Abdullah Al Baki This thesis is presented as part of Degree of Master of Science in Electrical Engineering Blekinge Institute of Technology May 2010 Blekinge Institute of Technology School of Engineering Department of Signal Processing Supervisor : Professor Abbas Mohammed Examiner : Professor Abbas Mohammed MEE10:12
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PEAK TO AVERAGE POWER RATIO (PAPR) REDUCTION IN
OFDM BASED RADIO SYSTEMS
Mohammad Zavid Parvez
Md. Abdullah Al Baki
This thesis is presented as part of Degree of Master of Science in Electrical Engineering
Blekinge Institute of Technology
May 2010
Blekinge Institute of Technology School of Engineering Department of Signal Processing Supervisor : Professor Abbas Mohammed Examiner : Professor Abbas Mohammed
MEE10:12
Blekinge Institute of Technology (BTH), Sweden Page 2
Dedication To
Our Parents And
Teachers
Blekinge Institute of Technology (BTH), Sweden Page 3
Abstract
High data rate wireless access is demanded by many applications. Usually,
more bandwidth is required for higher data rate transmission in any of the
system. With promising technology and ever-increasing wireless devices, the
spectrum is becoming scarcer day by day. In this case, using Orthogonal
Frequency Division Multiplexing (OFDM) and Cognitive Radio (CR) for
spectrally efficient transmission are an alternative solution.
OFDM is a bandwidth efficient multicarrier modulation where the available
spectrum is divided into subcarriers, with each subcarrier containing a low rate
data stream. OFDM has gained a tremendous interest in recent years because
of its robustness in the presence of severe multipath channel conditions with
simple equalization, robustness against Inter-symbol Interference (ISI),
multipath fading, in addition to its high spectral efficiency. However, the Peak-
to-Average Power Ratio (PAPR) is a major drawback of multicarrier
transmission system such as OFDM.
CR can be defined as an intelligent wireless system that is always alert about its
surrounding environment through sensing and should have ability to
dynamically adjust its radio operation parameters. The CR demands that the
physical layer (PHY) needs to be adaptable and flexible.
For flexibility and adaptability, the OFDM is an attractive candidate for CR
systems. This dissertation proposes a novel non-contiguous OFDM (NC-
OFDM) technique, where the implementation achieves high data rates of non-
contiguous subcarriers while simultaneously avoiding any interference to the
transmissions.
In this dissertation we apply different modulation techniques to reduce high
PAPR for non-contiguous bands spectrum of OFDM based CR. The simulation
results for PAPR reduction shows that higher modulation techniques are better
compared to lower modulation techniques.
Blekinge Institute of Technology (BTH), Sweden Page 4
Acknowledgement
All praises to almighty ALLAH who give us strength and abilities to complete
this thesis work successfully.
We would like to give our sincere gratitude to our honourable supervisor Prof.
Abbas Mohammed for his assistance and good guidance time after time which
made our thesis work become more precise and attractive. He supervises us and
gives his spare time to discuss more about the problem of thesis work.
We are grateful to our beloved parents for their love and continuous support
during the thesis work at BTH until finish.
Finally, we would like to thanks again to almighty ALLAH, for keeping us good
understanding and relationship in between us through this thesis work that is
more improved and constructive to do better.
Blekinge Institute of Technology (BTH), Sweden Page 5
Table of Contents .................................................................................................................................... 5
List of Figures .......................................................................................................................................... 8
List of Tables ........................................................................................................................................... 9
List of Abbreviations ............................................................................................................................. 10
Figure 6. 1- PAPR reduction using BPSK with clipping and filtering .............................. 54
Figure 6. 2- PAPR reduction using QPSK with clipping and filtering ............................. 55
Figure 6. 3- PAPR reduction using QAM16 with clipping and filtering .......................... 56
Figure 6. 4- BER Vs SNR using BPSK through AWGN channel ...................................... 57
Figure 6. 5- BER Vs SNR using QPSK through AWGN channel ..................................... 58
Figure 6. 6- BER Vs SNR using BPSK through Rayleigh fading channel ....................... 59
Figure 6. 7- BER Vs SNR using QPSK through Rayleigh fading channel ....................... 60
Figure 6. 8- BER Vs SNR using BPSK through two different channels ........................... 61
Figure 6. 9- BER Vs SNR using QPSK through two different channels .......................... 62
Blekinge Institute of Technology (BTH), Sweden Page 9
List of Tables
Table 2. 1-Comparison of PAPR Reduction Techniques ................................................... 29
Table 3. 1- Comparison different techniques of spectrum sensing ................................... 42
Table 4. 1- OFDM Based Wireless Standards ..................................................................... 46
Blekinge Institute of Technology (BTH), Sweden Page 10
List of Abbreviations
A
AWGN Additive White Gaussian Noise ATSC Advanced Television Systems Committee ADSL Asymmetric Digital Subscriber Line AI Adaptive Interleave ADC Analog to Digital Converter DAC Digital to Analog Converter
B
BPSK Binary Phase Shift Keying BER Bit Error Rate BSC Binary Symmetric Channel BO Back Off
C
CR Cognitive Radio COBSC Combination Optimized Sub-Block Coding Scheme CDMA Code Division Multiple Access CP Cyclic Prefix CDF Cumulative Distribution Function
D
DSP Digital Signal Processing DVB Digital Video Broadcasting DFS Dynamic Frequency Selection DARPA Defense Advanced Research Project Agency DFT Discrete Fourier Transform DTV Digital Television
E
Blekinge Institute of Technology (BTH), Sweden Page 11
EDGE Enhance Data GSM Environment
F
FFT Fast Fourier Transform FDM Frequency Division Multiplexing FDMA Frequency division Multiple Access FCC Federal Communication Commission FPGA Field Programmable Gate Arrays FEC Forward Error Correction
G
GPP General Purpose Processors GSM Global System for Mobile Communication
H
HiperLAN High Performance Radio LAN
I
ISI Inter Symbol Interference ICI Inter Carrier Interference IEEE Institute of Electrical and Electronic Engineers IFFT Inverse Fast Fourier Transform IDFT Inverse Discrete Fourier Transform
L
LTE Long Term Evolution LAN Local Area Network LO Local Oscillator LOS Line Of Sight
Blekinge Institute of Technology (BTH), Sweden Page 12
M
MIMO Multiple Input Multiple Output MISO Multiple Input Single Output MAC Media Access Control
N
NPRM Notice of Proposed Rule Making
O
OFDM Orthogonal Frequency Division Multiplexing OFDMA Orthogonal Frequency Division Multiple Access
P
PHY Physical Layer PTS Partial Transmit Sequences PSK Phase Shift Keying PEPs Peak Envelope Powers PA Power Amplifier LNA Low Noise Amplifiers PAPR Peak to Average Power Ratio
R RBLO-SBC Redundant Bit Location Optimized Sub-Block Coding RF Radio Frequency
Blekinge Institute of Technology (BTH), Sweden Page 13
S
SDR Software Defined Radio SNR Signal to Noise Ratio SLM Selective Level Mapping SoC System on Chip SISO Single Input Single Output SIMO Single Input Multiple Output
T
TR Tone Reservation TI Tone Injection TDMA Time Division Multiple Access
U
UWB Ultra Wide Band U-NII Unlicensed National Information Infrastructure
V
VLSI Very Large Scale Integration
W
WiMAX Worldwide Interoperability for Microwave Access Wi-Fi Wireless Fidelity WLAN Wide Area Network WRAN Wireless Regional Area Network WMAN Wireless Metropolitan Area Network WLAN Wireless Local Area Network WCDMA Wide Code Division Multiple Access WSS Wide Sense Stationary WLAN Wireless Local Area Network
Blekinge Institute of Technology (BTH), Sweden Page 14
Chapter One: Introduction
1.1 Motivation
With the advent of new high data rate wireless applications, demand of the spectrum is
rapidly increasing. Communications governmental and regulatory agencies impose
regulations on spectrum usage, such as control of allocations and priorities, as well as its
features. At this time, most of the prime spectrum has been assigned and it is difficult to find
spectrum for the new wireless applications. It can be made available for either expand
existing infrastructures or invent new services. Even though much of the spectrum has been
allocated and preliminary measurement is that the spectrum is unutilized by primary users.
There exist a lot of spectrums holes, which can be easily used by secondary users. FCC is
currently working on the concept of dynamic spectrum access, where secondary users can
borrow un-used portions of the spectrum from primary users. Cognitive Radio (CR) is
employing on proper spectrum utilization because of their rapid adaptability and flexibility.
Orthogonal Frequency Division Multiplexing (OFDM) is promising candidate for flexible
spectrum pooling in communication systems [31].
CR is an emerging technology, which intelligently detects a particular segment of the radio
spectrum currently in use and selects unused spectrum without interfering to licensed users.
One of the challenges of the OFDM is high peak-to-average power ratio (PAPR). A high
PAPR brings disadvantages like an increased complexity of the A/D and D/A converters and
reduced efficiency of radio frequency (RF) power amplifier [53]. OFDM signal consists of a
number of independent modulated subcarriers that leads to the problem of PAPR. If all
subcarriers come with same phase, the peak power is N times the average power of the signal
where N is the total number of symbols in an OFDM signal. Thus, it is not possible to send
this high peak amplitude signals to the transmitter without reducing peaks. Because power
amplifier used for the transmission has non-linear nature which causing inter-modulation and
out-of-band radiation. The high peak of OFDM signal can be reduced in several ways.
The focus of this dissertation is on OFDM based CR, which can handle the apparent spectrum
scarcity and enable high data rate communications. The proposed system exhibits high PAPR
reduction for non-contiguous bands spectrum of OFDM based CR.
Blekinge Institute of Technology (BTH), Sweden Page 15
1.2 Thesis out Lines
This thesis is organized as follows:
Chapter 2: Presents an introduction of OFDM and describe its principles, advantages and
disadvantages, the basic OFDM transceiver model, different techniques of PAPR reduction
and finally the comparison of PAPR reduction techniques in theoretical aspect.
Chapter 3: Presents the introduction of Cognitive Radio (CR), background, spectrum
sensing, licensed and unlicensed spectrum in CR. It also presents an introduction to software
defined radio (SDR), background, benefits, architectures, and the relationship between SDR
and CR.
Chapter 4: Presents the merits and challenge architectures of OFDM based CR and analysis
of its system block diagram.
Chapter 5: Presents in the different kinds of channel model; including AWGN, Rayleigh
fading channel.
Chapter 6: Presents the simulation results by using MATLAB simulator that is implemented
in different channels with BPSK, QPSK, and QAM16 modulation schemes and also BER
calculation.
Chapter 7: Presents the main objective of thesis with concluding remarks and proposes the
future work of this thesis for advance research.
Blekinge Institute of Technology (BTH), Sweden Page 16
Chapter Two: Orthogonal Frequency
Division Multiplexing (OFDM)
2.1 Introduction
Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique
that divides the available spectrum into subcarriers, with each subcarrier containing a low rate
data stream. The subcarriers have proper spacing and pass-band filter shape to satisfy
orthogonality as shown in Figure 2.1. OFDM will play an important role in realizing
Cognitive Radio (CR) concept by providing a proven, scalable, adaptive technology for
wireless communications [10]. Inter-symbol interference (ISI) is reduced completely by using
a guard band in every OFDM symbol. In OFDM, using guard band is cyclically extended in
order to avoid inter-carrier interference (ICI). The advantage of OFDM system is robustness
to channel fading in wireless communication environment. Frequency selective fading is
reduced by increasing the number of subcarriers. By choosing the coherence bandwidth is
greater than the subcarrier spacing of the channel, each subcarrier is going to be affected by a
flat channel and thus no or simple channel equalizer is needed.
OFDM is used in many wireless applications today. Already it is used in different WLAN
standards (e.g. HIPERLAN-2, IEEE 802.11a), Wireless Metropolitan Area Networks
(WMAN), Digital Video Broadcasting (DVB), 3GPP-LTE, Asymmetric Digital Subscriber
Line (ADSL) and power line communications.
Despite of OFDM advantages, it has a major potential drawback in the form of high Peak-to-
Average Power Ratio (PAPR). The high PAPR has nonlinear nature in the transmitter and it
degrades the power efficiency of the system.
Blekinge Institute of Technology (BTH), Sweden Page 17
+∆f +∆f
f
Figure 2. 1-OFDM subcarriers in frequency domain
2.2 Advantages and Disadvantages of OFDM System
There are some advantages and disadvantages of OFDM are summarized below:
2.2.1 Advantages of OFDM
Some of the advantages of an OFDM system are as follows:-
OFDM is computationally efficient to employ the modulation and demodulation
techniques by using FFT.
The OFDM signal is robustness in multipath propagation environment and more
tolerant of delay spread.
1 Subcarrier
(f) ∆f=1/Ts
(f)
(f)
()
8 Subcarriers
Blekinge Institute of Technology (BTH), Sweden Page 18
OFDM is more resistant to frequency selective fading than single carrier transmission
systems.
OFDM system gives good protection against co-channel interference and impulsive
parasitic noise.
Pilot subcarriers are used in OFDM system to prevent frequency and phase shift
errors.
It is possible to use maximum likelihood detection with reasonable complexity.
OFDM is a good candidate for CR because of its flexibility and adaptability [10].
The orthogonality preservation procedures in OFDM are much simpler compared to
CDMA/TDMA technique in multipath conditions [8].
2.2.2 Disadvantages of OFDM
Some of the disadvantages of an OFDM system are as follows:-
The OFDM signal suffers high peak to average power ratios (PAPR) of transmitted
signal.
OFDM is very sensitive to carrier frequency offset.
It is difficult to synchronize when subcarriers are shared among different transmitters.
2.3 OFDM System Model
A Basic OFDM system is described in Figure 2.2. Here an input data symbols are supplied
into a channel encoder that data are mapped onto BPSK/QPSK/QAM constellation.
The data symbols are converted from serial to parallel and using Inverse Fast Fourier
Transform (IFFT) to achieve the time domain OFDM symbols. Time domain symbols can be
represented as:
(2.1)
where,
is the transmitted symbol on the subcarriers
N is the number of subcarriers
Blekinge Institute of Technology (BTH), Sweden Page 19
Time domain signal is cyclically extended to prevent Inter Symbol Interference (ISI) from the
former OFDM symbol using cyclic prefix (CP).
Figure 2. 2-A basic diagram of OFDM Transceiver
The Digital to Analog Converter (DAC) is performed to convert the baseband digital signal
into analog signal. This operation is executed in DAC block of diagram. Then, the analog
signal is proceeded to the Radio Frequency (RF) frontend. The RF frontend performs
operations after receiving the analog signal. The signal is up converted to RF frequencies
using mixer and amplified by using Power Amplifier (PAs) and then transmitted through
antennas. At the receiver side, the received signal is down converted to base band signal by
RF frontend.
The analog signal is digitized and re-sampled by the Analog to Digital Converter (ADC). The
ADC is used to digitize the analog signal and re-samples it. In the figure, frequency and time
synchronization block are not shown because of simplicity. Cyclic prefix is removed from the
signal in frequency domain. This step is done by the Fast Fourier Transform (FFT) block.
The received symbols in the frequency domain can be represented as:
Y (k) = H (k) Xm(k) +W (k) (2.2)
where, Y (k) is the received symbol on the subcarrier, H (k) is the frequency response of
the channel on the same subcarrier and W (k) is the additive noise added to , subcarrier
which is generally assumed to be Gaussian random variable with zero mean and variance of
. Thus, simple one tap frequency domain equalizers can be employed to get the transmitted
symbols. After FFT signals are de-interleaved and decoded to recover the original signal.
Input
Output
Transmitter
Receiver
IFFT S
/
P
RF frontend
DAC ADD CP Mod
P
/
S
ADC RF
frontend
Remove
CP
Demod
S
/
P FFT
P
/
S
Blekinge Institute of Technology (BTH), Sweden Page 20
2.4 Mathematical Definition of OFDM Signal
OFDM consists of multiple carriers. Each carrier can be presented as a complex waveform
like:
,
(2.3)
where,
is the amplitude of the signal
(t) is the phase of the signal
The complex signal can be described by
, (2.4)
This is a continuous signal. Each component of the signal over one symbol period can take
fixed values of the variables like:
,
,
where,
n is the number of OFDM block.
T is a time interval and the signal is sampled by 1/T then it can be represented by:
, (2.5)
Let ω0=0 then the signal becomes:
, (2.6)
The signal is compared with general Inverse Fourier Transform (IFT):
(2.7)
Here, is time frequency domain.
Blekinge Institute of Technology (BTH), Sweden Page 21
Both are equivalent if
∆f = = =
where,
τ is symbol duration period (2.8)
The OFDM signal can be defined by Fourier Transform. The Fast Fourier Transform (FFT)
can obtained frequency domain OFDM symbols and Inverse Fast Fourier Transform (IFFT)
can obtain time domain symbols. They can be written as:
Fast Fourier Transform
(2.9)
Inverse Fast Fourier Transform
(2.10)
where,
2.5 NC-OFDM System Model
In OFDM system, the achievement of large number of non-contiguous subcarriers by
collective usage for the high data rate transmission is referred as Non-Contiguous OFDM
(NC-OFDM) [31]. NC-OFDM can provide the necessary agile spectrum usage for the target
licensed spectrum if spectrum can be occupied by primary and secondary users. The spectrum
sensing measurements are deactivated during the subcarriers corresponding to the spectrum
occupied by primary user. Moreover, dynamic spectrum sensing can be determined when the
active subcarriers are located in the unoccupied spectrum bands.
Fundamentals of the NC-OFDM signal transmission and reception are quite similar to that of
the OFDM signal explained in Section 2.7.1. However, NC-OFDM techniques offer very
important advantage for growing scarcity of the large contiguous frequency spectrum, i.e. it
can support dynamic spectrum pooling for high data rate transmissions.
Blekinge Institute of Technology (BTH), Sweden Page 22
Figure 2. 3 NC-OFDM Transceiver [31]
2.6 Why PAPR reduction in OFDM system
The OFDM technique divides the total bandwidth into many narrow sub-channels and sends
data in parallel. It has various advantages, such as high spectral efficiency, immunity to
impulse interference and, frequency selective fading without having powerful channel
equalizer.
But one of the major drawbacks of the OFDM system is high PAPR. OFDM signal consists
of lot of independent modulated subcarriers, which are created the problem of PAPR. It is
impossible to send this high peak amplitude signals to the transmitter without reducing peaks.
So we have to reduce high peak amplitude of the signals before transmitting.
Input
Output
Null subcarrier
selection
Spectrum sensing measurement
Subcarrier ON/OFF information
Transmitter
Receiver
Subcarrier ON/OFF information
P
/
S IFFT
S
/
P
RF
frontend
DAC ADD CP Mod
ADC RF
frontend
Remove
CP
Demod
S
/
P FFT
P
/
S
Blekinge Institute of Technology (BTH), Sweden Page 23
2.7 Mathematical Definition of PAPR
The PAPR of the OFDM signal can be written as:
PAPR{ , τ} = (2.11)
where,
s(t) is the original signal
τ is the time interval
is the peak signal power
is the average signal power
2.8 PAPR Techniques
There have been many new approaches developed during the last few years. Several PAPR
reduction techniques have been proposed in the literature. These techniques are divided into
two groups. These are signal scrambling techniques and signal distortion techniques. The
signal scrambling techniques are:
Block coding
Selective Level Mapping (SLM)
Partial Transmit Sequences (PTS)
Signal scrambling techniques work with side information which minimized the effective
throughput since they commence redundancy. Signal distortion techniques introduce band
interference and system complexity also. Signal distortion techniques minimize high peak
dramatically by distorting signal before amplification.
The signal distortion techniques are:
Clipping
Peak windowing
Peak cancellation
Peak power suppression
Weighted multicarrier transmission
Blekinge Institute of Technology (BTH), Sweden Page 24
2.8.1 Signal Scrambling Techniques
2.8.1.1 Block Coding Techniques
Coding techniques can be applied for signal scrambling, M sequences, Golay complementary
sequences, Shapiro-Rudin sequences codes can be used to reduce the PAPR efficiently.
This Block coding technique has been proposed by Wilkinson and Jones in 1965 for the
minimization of the peak to mean envelope power ratio of multicarrier communication
system [1]. The key object in this paper is that PAPR can be minimized by block coding the
data. The block coding techniques have three stages for the development. The first stage
works with the collection of appropriate sets of code words for any number of carriers, any
M-ary phase modulation method, and any coding rate. The second stage works with the
collection of the sets of code words which enable proficient implementation of the
encoding/decoding. The third stage offers error deduction and correction potential.
There different methods for the collection of the sets of code words. The mainly insignificant
method, order to search the peak envelope power (PEP) for all possible code words for a
certain length of given number of carriers. This technique is simple and accurate for short
codes because it needs extreme computation. Natural algorithms are mainly sophisticated
searching techniques. It can be used for the collection of longer code words. A selection of
code words select from searches for encoding and decoding can be performed with a look up
table or using combinatorial logic exploiting the mathematical structure of the codes
minimization when the frame sixe is bigger.
Large PAPR reduction can be achieved if the long information sequence is separated into
different sub blocks, and all sub block encoded with System on a Programmable Chip
(SOPC). There are many likely spaces, where the odd parity checking bits can be put into
each frame to minimize PAPR. For further minimization of PAPR, redundant bit location
optimized sub-block coding (RBLO-SBC) optimizes these locations redundant Combination
optimized sub-block coding scheme (COSBC) optimizes the combination of the coded sub-
blocks, where two coding schemes instead of one is used to encode the same information
source.
2.8.1.2 Block Coding Scheme with Error Correction
This Block coding scheme with Error Correction has been proposed by Ahn and et.al in [12]
to introduce a new block coding proposal for minimization of peak to average power ratio
(PAPR) of an Orthogonal Frequency Division Multiplexing (OFDM) system. Block coding
has error correction capability. In block coding method, the OFDM symbol can be reduced by
selecting only those code words with lower PAPR. In this paper, the key object of the method
is proposed that properly designed block codes can not only minimize the PAPR, but also
give error correction capability. A k bit data block (e.g. 4-bit data) is encoded by a (n, k)
block code with a generator matrix ‘G’ in the transmitter of the system. Followed by the
phase rotator vector b to produce the encoded output x=a.G+b(mod 2).
Blekinge Institute of Technology (BTH), Sweden Page 25
To achieve the accurate generator matrix and phase rotator vector that make sure the
minimum PAPR for the OFDM system, check all the 2n codes and choose only 2k codes that
obtain the minimum PAPR. After that generator matrix ‘G’ and the phase rotator vector ‘b’
are produced; which are used mapping between these symbols combination and input data
vector ‘a’. The converse functions of the transmitter are executed in the receiver system. The
parity check matrix ‘H’ is achieved from the generator matrix ‘G’, with an exception that the
effect of the phase rotator vector b is removed before calculations of syndromes.
Contrasting the method in [1], which only presents error detection; this method can improve
the overall system performance and provides error correction capability.
2.8.1.3 Selected Mapping (SLM)
Selective Mapping (SLM) approaches have been proposed by Bauml in 1965 [13]. This
method is used for minimization of peak to average transmit power of multicarrier
transmission system with selected mapping. A complete set of candidate signal is generated
signifying the same information in selected mapping, and then concerning the most favorable
signal is selected as consider to PAPR and transmitted. In the SLM, the input data structure is
multiplied by random series and resultant series with the lowest PAPR is chosen for
transmission. To allow the receiver to recover the original data to the multiplying sequence
can be sent as ‘side information’.
One of the preliminary probabilistic methods is SLM method for reducing the PAPR
problem. The good side of selected mapping method is that it doesn’t eliminate the peaks,
and can handle any number of subcarriers. The drawback of this method is the overhead of
side information that requires to be transmitted to the receiver of the system in order to
recover information.
2.8.1.4 Partial Transmit Sequence (PTS)
Partial Transmit Sequence (PTS) technique has been proposed by Muller and Hubber in 1997
[14]. This proposed method is based on the phase shifting of sub-blocks of data and
multiplication of data structure by random vectors. This method is flexible and effective for
OFDM system. The main purpose behind this method is that 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.
PTS is probabilistic method for reducing the PAPR problem. It can be said that PTS method
is a modified method of SLM. PTS method works better than SLM method. The main
advantage of this scheme is that there is no need to send any side information to the receiver
of the system, when differential modulation is applied in all sub blocks.
2.8.1.5 Interleaving Technique
Interleaving technique has been proposed by Jayalath and Tellambura [2], for reduction peak
to average power ratio of an OFDM transmission. A data randomization technique has
proposed for the minimization of the PAPR in this paper.
Blekinge Institute of Technology (BTH), Sweden Page 26
The notion that highly correlated data structures have large PAPR can be reduced, if long
correlation pattern is broken down. Also, this paper proposes an additive method to minimize
the complexity.
The basic idea in adaptive interleaving is to set up an initial terminating threshold. PAPR
value goes below the threshold rather than seeking each interleaved sequences. The minimal
threshold will compel the adaptive interleaving (AL) to look for all the interleaved sequences.
The main important of the scheme is that it is less complex than the PTS technique but
obtains comparable result. This method does not give the assurance result for PAPR
reduction. In this circumstance, higher order error correction method could be used in
addition to this method.
2.8.1.6 Tone Reservation (TR)
Tone Reservation (TR) method is proposed for PAPR reduction [15]. The main idea of this
method is to keep a small set of tones for PAPR reduction. This can be originated as a convex
problem and this problem can be solved accurately. The amount of PAPR reduction depends
on some factors such as number of reserved tones, location of the reserved tones, amount of
complexity and allowed power on reserved tones.
This method explains an additive scheme for minimizing PAPR in the multicarrier
communication system. It shows that reserving a small fraction of tones leads to large
minimization in PAPR ever using with simple algorithm at the transmitter of the system
without any additional complexity at the receiver end. Here, N is the small number of tones,
reserving tones for PAPR reduction may present a non–negligible fraction of the available
bandwidth and resulting in a reduction in data rate. The advantage of TR method is that it is
less complex, no side information and also no additional operation is required at the receiver
of the system. Tone reservation method is based on adding a data block and time domain
signal. A data block is dependent time domain signal to the original multicarrier signal to
minimize the high peak. This time domain signal can be calculated simply at the transmitter
of system and stripped off at the receiver.
2.8.1.7 Tone Injection (TI)
Tone Injection (TI) method has been recommended by Muller, S.H., and Huber, J.B. [14].
This technique is based on general additive method for PAR reduction. Using an additive
method achieves PAPR reduction of multicarrier signal without any data rate loss. Note that
Tone injection (TI) uses a set of equivalent constellation points for an original constellation
points to reduce PAPR. The main idea behind this method is to increase the constellation
size. Then, each point in the original basic constellation can be mapped into several
equivalent points in the extended constellation, since all information elements can be mapped
into several equivalent constellation points. These additional amounts of freedom can be
utilized for PAPR reduction. This method is called Tone Injection method because of
replacing the points in the basic constellation for the new points in the larger constellation
which corresponds to injecting a tone of the proper phase and frequency in the multi-carrier
symbol. The drawbacks of this method are; need to side information for decoding signal at
the receiver side, and cause extra IFFT operation which is more complex.
Blekinge Institute of Technology (BTH), Sweden Page 27
2.8.2 Signal Distortion Techniques
2.8.2.1 Peak Windowing
The peak windowing method has been suggested by Van Nee and Wild [17]. This method,
proposes that it is possible to remove large peaks at the cost of a slight amount of self
interference when large peaks arise infrequently. Peak windowing reduces PAPRs at the cost
of increasing the BER and out-of-band radiation. Clipping is a one kind of simple introduces
PAPR reduction technique which is self interference. The technique of peak windowing