high rate techniques for papr reduction in ofdm systems
TRANSCRIPT
HIGH RATE TECHNIQUES FOR PAPR REDUCTION IN OFDM
SYSTEMS
_______________
A Thesis
Presented to the
Faculty of
San Diego State University
_______________
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
in
Electrical Engineering
_______________
by
Sudarshan M. Kannappan
Spring 2012
iii
Copyright © 2012
by
Sudarshan M. Kannappan
All Rights Reserved
iv
DEDICATION
This thesis is dedicated to my parents Sriram M K and Padmini M K, my sister
Smitha M K, my aunt Mythili M K and my cousins Kiran Kannappan and Kishore Mandyam
and also my pet Brownie.
I would also like to dedicate this to my best friend Srinivas Anand.
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ABSTRACT OF THE THESIS
High Rate Techniques for PAPR Reduction in OFDM Systems by
Sudarshan M. Kannappan Master of Science in Electrical Engineering
San Diego State University, 2012
Wireless communication has evolved so much today that it has greatly been integrated into everyone’s life. From listening to music using Bluetooth headset, accessing information on the web on the move using WiFi or 3G to calling our loved ones who are on the other side of the globe and locating our position when lost, can be done using a plethora of wireless devices available today.
This thesis addresses the problem of high peak-to-average power ratio (PAPR) found in orthogonal frequency division multiplexing (OFDM) modems. The reason for high PAPR in OFDM is the constructive addition of sinusoidal signals at different frequencies. High PAPR increases the dynamic range of power amplifier operation, thereby resulting in increased cost and chip area.
This thesis proposes several high rate techniques to reduce PAPR. One technique extends the efficiency of the well known complementary code keying (CCK) OFDM. The second technique eliminates high PAPR by removing the periodicity which may exist between the bits fed to the OFDM transmitter. This rate-12/16 technique is compared to other techniques such as traditional OFDM and carrier-interferometry (CI) OFDM. Further, this thesis integrates the proposed rate-12/16 technique and CCK to obtain an improved rate-12/16 technique, which brings the PAPR value down to a new low. Bit error rates (BER) are obtained for each coding technique for comparison. The proposed rate-12/16 technique achieves good BER performance due to the coding gain it provides. The improved rate-12/16 outperformed all the techniques discussed in terms of PAPR.
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TABLE OF CONTENTS
PAGE
ABSTRACT ...............................................................................................................................v
LIST OF TABLES ................................................................................................................. viii
LIST OF FIGURES ................................................................................................................. ix
ACKNOWLEDGEMENTS ..................................................................................................... xi
CHAPTER
1 INTRODUCTION .........................................................................................................1 2 CHALLENGES IN WIRELESS COMMUNICATION ................................................5 3 CHANNEL MODELS ...................................................................................................8
3.1 Additive White Gaussian Noise (AWGN) Channel ..........................................8 3.2 Fading Channels.................................................................................................9 3.3 Multipath Fading ..............................................................................................11
3.3.1 Rayleigh Fading ..................................................................................... 12 3.3.2 Rician Fading ......................................................................................... 13
4 ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING (OFDM) ................14 4.1 Cyclic Prefix ....................................................................................................17 4.2 OFDM Symbol.................................................................................................17 4.3 Performance of OFDM Systems ......................................................................17 4.4 PAPR in OFDM ...............................................................................................19
5 EXISTING PAPR REDUCTION TECHNIQUES ......................................................23 5.1 Carrier Interferometry-OFDM .........................................................................23 5.2 Performance of CI OFDM Systems .................................................................24 5.3 Complementary Code Keying (CCK) OFDM .................................................24 5.4 PAPR Performance of CCK OFDM Systems ..................................................27
6 PROPOSED PAPR REDUCTION TECHNIQUES ....................................................29 6.1 Extension of CCK OFDM ...............................................................................29 6.2 PAPR Performance of Proposed CCK OFDM System ...................................30 6.3 Rate-12/16 Technique ......................................................................................30
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6.4 Performance of Rate-12/16 Technique ............................................................31 6.5 Improved Rate-12/16 Technique: Combination of Rate-12/16 and CCK .......................................................................................................................33 6.6 PAPR Performance of Improved Rate-12/16 Technique.................................33 6.7 Comparison of Performance of All the Discussed Technologies ....................34
7 CONCLUSION AND FUTURE ENHANCEMENT ..................................................38 BIBLIOGRAPHY ....................................................................................................................39
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LIST OF TABLES
PAGE
Table 4.1. Comparison of OFDM Systems ..............................................................................18 Table 6.1. Comparison of OFDM, CI-OFDM, CCK, Rate-12/16 and Improved
Rate-12/16 Techniques ................................................................................................36
ix
LIST OF FIGURES
PAGE
Figure 3.1. Probability distribution function for Gaussian random variable for different variance values. ...............................................................................................9
Figure 3.2. Noise model in communication system. ................................................................10 Figure 3.3. Large scale fading v/s small scale fading. .............................................................11 Figure 3.4. Multipath fading. ...................................................................................................11 Figure 3.5. Probability distribution function for Rayleigh fading for different
variance. .......................................................................................................................12 Figure 3.6. Probability distribution function for Rician fading. ..............................................13 Figure 4.1. Comparison of the conventional FDM with OFDM. ............................................15 Figure 4.2. Block diagram of OFDM transmitter. ...................................................................16 Figure 4.3. Block diagram of OFDM receiver. ........................................................................16 Figure 4.4. Cyclic prefix in OFDM. ........................................................................................17 Figure 4.5. Typical OFDM symbol..........................................................................................18 Figure 4.6. Bit error rate of OFDM for AWGN channel. ........................................................19 Figure 4.7. (a) PAPR of OFDM and (b) is histogram of PAPR. .............................................21 Figure 4.8. Transfer function of a typical power amplifier......................................................22 Figure 5.1. Block diagram of CI-OFDM transmitter. ..............................................................24 Figure 5.2. Block diagram of CI-OFDM receiver. ..................................................................24 Figure 5.3. Bit error rate of CI-OFDM for AWGN channel. ...................................................25 Figure 5.4. (a) PAPR of CI-OFDM and (b) is histogram of PAPR. ........................................25 Figure 5.5. Block diagram of CCK OFDM transmitter. ..........................................................26 Figure 5.6. Block diagram of CCK OFDM receiver. ..............................................................26 Figure 5.7. (a) PAPR of CCK OFDM and (b) is histogram of PAPR. ....................................28 Figure 6.1. Details of the 7/16 spreading sequence block. ......................................................29 Figure 6.2. (a) PAPR of 7/16 CCK OFDM and (b) is histogram of PAPR. ............................30 Figure 6.3. Block diagram of rate-12/16 transmitter. ..............................................................31 Figure 6.4. Block diagram of rate-12/16 receiver. ...................................................................31 Figure 6.5. Bit error rate of rate-12/16 technique. ...................................................................32
x
Figure 6.6. (a) PAPR of rate-12/16 technique and (b) is histogram of PAPR. ........................32 Figure 6.7. Details of 12/16 mapper block. .............................................................................34 Figure 6.8. Block diagram of improved rate-12/16 transmitter. ..............................................34 Figure 6.9. Block diagram of improved rate-12/16 receiver. ..................................................35 Figure 6.10. (a) PAPR of improved rate-12/16 technique and (b) is histogram of
PAPR............................................................................................................................35 Figure 6.11. Bit error rate of OFDM, CI-OFDM, rate-12/16 techniques. ...............................36 Figure 6.12. PAPR of OFDM, CI-OFDM, rate-12/16 and improved rate-12/16
techniques. ...................................................................................................................37 Figure 6.13. Histogram of PAPR of OFDM, CI-OFDM, rate-12/16 and improved
rate-12/16 techniques. ..................................................................................................37
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ACKNOWLEDGEMENTS
I would like to take this opportunity to thank all of you who have supported me
throughout my thesis.
First of all, I would like to thank my thesis advisor Dr. Santosh Nagaraj who is a
wonderful human being. His motivation, encouragement and support have helped me
complete my thesis successfully.
I would also like to thank Dr. Ashkan Ashrafi and Dr. Christopher Paolini for their
help in completing my thesis.
Thanks to my friends Vishak Neergund, Sagar Rao for having those technical
discussions which were thought provoking, this aided my thesis.
I would like to thank my parents, my sister, my aunt and my cousins who have been
with me giving their moral, emotional and financial support without which this thesis and
masters wouldn’t have been possible. I love you guys!
Lastly, thank you Srinivas Anand who has been with me and encouraged me when I
was feeling low during my thesis period.
1
CHAPTER 1
INTRODUCTION
Guglielmo Marconi in 1875, opened the way for modern wireless communications by
transmitting the three-dot Morse code for the letter ‘S’ over a distance of three kilometers
using electromagnetic waves. From satellite transmission, radio and television broadcasting
to the now ubiquitous mobile telephone, wireless communications has revolutionized the way
societies function [1].
Wireless Communication is used for a wide range of services which are categorized
as:
• Broadcasting services: AM, FM radio and terrestrial television.
• Mobile communications of voice and data: Maritime and aeronautical mobile for communications between ships, airplanes and land; Terrestrial mobile communications between a fixed base station and mobiles.
• Fixed Services: Point to point, Point to multipoint services.
• Satellite: Broadcasting, Communications and internet.
• Other Uses: Military, radio astronomy, meteorological and scientific uses [1].
The history of mobile telephones can be loosely broken into four periods. In first
(pre-cellular) period, mobile telephones used a frequency band exclusively in a particular
area. These telephones had severe problems with congestion and call completion [1].
The introduction of cellular technology expanded the efficiency of frequency use of
mobile phones. A geographic area was broken down into small areas called cells and a band
of frequency was allocated to a particular cell, rather than exclusively allocating a band of
frequency to one telephone call in a large geographic area. Different users in different
(non-adjacent) cells were able to use the same frequency for a call without interference [1].
First generation cellular mobile telephone (1G) refers to wireless telecommunication
technology developed in 1980s which was based on analog signals. In 1G, a voice call was
modulated to a higher frequency of about 150 MHz and up as it was transmitted between
radio towers. This was done using a technique called Frequency-Division Multiple Access
2
(FDMA). It had some disadvantages like low capacity, unreliable handoff, poor voice links
and no security at all.
Second generation (2G) mobile telephones used digital technology. It was developed
in 1990s. All phone conversations were digitally encrypted. 2G systems were significantly
more efficient on the spectrum and 2G also introduced data services for mobile with the
introduction of Short Message Service (SMS). 2G networks were built mainly for voice
services and slow data transmission. Groupe Speciale Mobile (GSM) was the first 2G
system. It was later standardized to Global System for Mobile Communication. GSM
allowed full international roaming, automatic location services, common encryption and
relatively high quality audio [1]. GSM is now the most widely used 2G system worldwide, in
more than 130 countries, using the 900 MHz frequency range. GSM uses Time Division
Multiple Access (TDMA) unlike FDMA scheme used in 1G.
The first major step in the evolution of GSM networks to 3G occurred with the
introduction of General Packet Radio Service (GPRS) which was called the 2.5G. It was
between 2G and 3G cellular wireless technologies. This technology implemented a
packet-switched domain in addition to the circuit-switched domain. GPRS could provide data
rates from 56 Kbit/s up to 115 Kbit/s. It was used for services such as Wireless Application
Protocol (WAP) access, Multimedia Messaging Service (MMS), and for Internet
communication services such as email and World Wide Web access.
GPRS networks evolved to EDGE networks with the introduction of 8PSK encoding.
EDGE was deployed on GSM networks beginning in 2003. Enhanced Data rates for GSM
Evolution (EDGE); Enhanced GPRS (EGPRS) was backward-compatible digital mobile
phone technology that allowed improved data transmission rates, as an extension on top of
standard GSM. EDGE provides a three-fold increase in capacity of GSM/GPRS networks.
The specification achieves higher data-rates (up to 236.8 Kbit/s) by switching to more
sophisticated methods of coding (8PSK), within existing GSM timeslots.
The third generation mobiles network uses spread spectrum technology, Code
Division Multiple Access (CDMA) in particular. These systems allow for significantly
increased speeds of transmission and are particularly useful for data services. WCDMA is
also found in 3G standard utilizes the DS-CDMA channel access method and the FDD
duplexing method to achieve higher speeds and support more users compared to most time
3
division multiple access (TDMA).W-CDMA differs from CDMA in many aspects. One
among them is WCDMA transmits on a pair of 5 MHz-wide radio channels, while CDMA
transmits on one or several pairs of 1.25 MHz radio channels. There are evolutionary
standards (EDGE and CDMA) that are backwards-compatible extensions to pre-existing 2G
networks as well as revolutionary standards that require all-new network hardware and
frequency allocations. 3G offers a minimum data rate of 2 Mbit/s for stationary or walking
users, and 384 Kbit/s when in a moving vehicle. Also 3G networks offer greater security than
their 2G predecessors. The bandwidth and location information available to 3G devices gives
rise to applications which were not previously available to mobile phone users. Some of them
are:
• Mobile TV
• Video on demand
• Video conferencing
• Tele-medicine
• Location-based services
High Speed Packet Access (HSPA) is combination of two mobile telephony
protocols, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet
Access (HSUPA) that improves the performance of existing WCDMA protocols. HSPA
supports increased peak data rates of up to 14 Mbit/s in the downlink and 5.8 Mbit/s in the
uplink. High-Speed Downlink Packet Access (HSDPA) is an enhanced 3G (third generation)
mobile telephony communications protocol which is called as 3.5G, which has increased data
transfer speeds and capacity. Current HSDPA support down-link speeds of 1.8, 3.6, 7.2 and
14.4 Megabits/s. Further speed increases are available with HSPA+, which provides speeds
of up to 42 Mbit/s downlink and 84 Mbit/s.
HSPA+ provides HSPA data rates up to 84 Megabits per second (Mbit/s) on the
downlink and 22 Mbit/s on the uplink through the use of a multiple-antenna technique known
as Multiple-Input Multiple-Output (MIMO) and higher order modulation (64QAM). MIMO
on CDMA based systems acts like virtual sectors to give extra capacity. The technology also
delivers significant battery life improvements.
With the introduction of OFDM, a new standard evolved called 4G which promises
very high data rates which increased data traffic by five-fold. A 4G system is expected to
4
provide a comprehensive and secure all-IP based mobile broadband solution to laptop
computer wireless modems, Smartphone and other mobile devices. Facilities such as
ultra-broadband Internet access, IP telephony, gaming services and streamed multimedia are
provided to users. 4G has two parts:
• WiMax (Wireless Interoperability for Microwave Access)
• LTE (Long Term Evolution)
WiMax is a telecommunications technology that provides fixed and fully mobile
internet access. It is based on IEEE 802.16 standard. Data rate of up to 70 Mbps can be
achieved. It uses frequency range of 10-66 GHz licensed bands, obtained at premium costs.
WiMax has two variants namely 802.16d and 802.16e which differs in their mobility. WiMax
offers higher speeds, extended range and supports greater number of users as compared to
WiFi standard, which is based on IEEE 802.11. WiMax has a scalable physical layer which
offers flexibility in choosing data rates. Unlike WiFi, WiMax is a connection oriented service
which establishes connection between the sender and receiver for offering the required
service.
On the other hand, LTE is a standard supports both voice and data traffic at high
speeds. LTE is often marketed as 4G even though they do not comply with the 4G standards.
The pre-4G standard is a step toward LTE Advanced, a 4th generation (4G) standard of radio
technologies designed to increase the capacity and speed of mobile telephone networks. LTE
Advanced is backwards compatible with LTE which uses the same frequency bands, while
LTE is not backwards compatible with 3G systems. LTE offers a peak data rate of 70 Mbps.
It uses SC-FDMA in the uplink and OFDMA in the downlink. OFDMA is a multiple access
technique for OFDM and SC-FDMA is single carrier frequency division multiple access
scheme used to overcome a high PAPR drawback in OFDM
This thesis’ contribution lies in reduction of PAPR (peak Average to Power Ratio) in
OFDM. The main short-coming of OFDM system is high PAPR. PAPR is the ratio of the
peak power and the average power of the OFDM signal. This consumes more power than
required and also forces one to use bulky power amplifiers. The issues might prove costly in
mobile devices as they are power hungry and portable. We have proposed couple of
techniques which deals with reducing PAPR by getting rid of periodicity which is the main
cause of high PAPR in OFDM systems.
5
CHAPTER 2
CHALLENGES IN WIRELESS
COMMUNICATION
Wireless Communication has many advantages and challenges too which are
discussed in this chapter. Some of the challenges of wireless communication are given below
[2].
• Developing reliable transmission and reception to send data through unfriendly wireless channel.
The most fundamental challenge for wireless communication comes from the
transmission medium itself. Wireless communication systems use radio wave propagation
mechanisms for transmission unlike wired communication channels which rely on a physical
connection such as copper wires etc. Several large and small obstructions, terrain
undulations, relative motion between the transmitter and the receiver, interference from other
signals, noise, and various other complicating factors together weaken, delay, and distort the
transmitted signal in an unpredictable and time-varying fashion [2]. It is a challenge to design
a digital communication system that performs well under these conditions, especially when
requirements are for very high data rates and high-speed mobility. Some of the impairments
contributed by the channel are:
1. Distance-dependent reduction in signal power
2. Inter-symbol interference (ISI) due to time dispersion
3. Doppler Spread due to frequency dispersion
4. Noise (AWGN)
5. Interference
• Achieving high spectral efficiency and coverage with limited available spectrum.
The second challenge to wireless communication comes from the scarcity of
bandwidth. The regulatory bodies around the world have allocated only a limited amount of
spectrum for commercial use. The need to accommodate an ever-increasing number of users
and offering bandwidth-rich applications using a limited spectrum challenges the system
6
designer to continuously search for solutions that use the spectrum more efficiently [2]. The
most significant tool used to achieve higher spectral efficiency is the concept of a cellular
architecture, where several lower-power transmitters are used to cover a smaller area, called
a cell. The cells are again subdivided into sectors. The available frequency spectrum is
divided among the cells to minimize interference. This method of allocation is called
frequency reuse.
• Supporting the required QoS (throughput, delay).
QoS refers to the “collective effect of service,” as perceived by the user. QoS actually
refers to meeting certain requirements such as throughput, packet error rate, delay, and jitter.
Wireless communication networks must support a diversity of applications, such as
voice, data, video, and multimedia, which has different traffic patterns and QoS requirements
[2]. The diversity in the QoS requirements makes it a challenge to accommodate all these on
a single-access wireless network, where bandwidth is precious. The perceived quality is
based on the end-to-end performance of the network from a user perspective. Therefore QoS
has to be delivered end-to-end across the network, which may include both wired and
wireless infrastructure.
• Supporting mobility through uninterrupted communication.
Mobility is one of the significant features offered by wireless communication. Two of
the main challenges are roaming and handoff which are critical in providing a good user
experience.
Roaming includes maintaining the communication link between two users when both
or either of them is travelling within the same base station area or outside that area or outside
the country.
Handoff is required to support roaming. When there is change of base station
or change of network, then the communication shouldn’t be hampered. Handoff takes
care of retaining the communication link by handing over the responsibilities to the
target base station without any interruption to the user. IP-based networks support
roaming and handovers across heterogeneous networks, such a WiMax network or a
WiFi network [2].
7
• Achieving low power consumption to handle mobile devices which are battery operated.
Portability is another unique aspect of wireless communication. Portability is desired
for full mobility and some nomadic applications. Portability requires the mobile device to be
battery powered and the battery to hold juice for a long time. Unfortunately, advances in
battery technology have been limited, especially when compared to processor technology.
The need for reducing power consumption translates to use of power-efficient modulation
and transmission schemes, computationally less intensive signal-processing algorithms,
low-power circuit-design, and battery technologies with longer life [2].
Unfortunately, there exists a trade-off between the power required and bandwidth.
Both are significant aspects for performance of any wireless communication link. This might
result in portable wireless systems offering asymmetric data rates on the downlink and the
uplink. The power-constrained uplink supports lower bits per second per Hertz than the
downlink.
• Providing security.
Security is an important consideration in wireless communication systems. The fact
that connections can be established in untethered fashion makes it easier to intrude in an
inconspicuous and undetectable manner [2]. Therefore, a robust level of security must exist.
From the perspective of an end user, the primary security concerns are privacy and data
integrity. Users need assurance that no one can eavesdrop on their sessions and that the data
sent across the communication link is not tampered. This is usually achieved through the use
of encryption. From the service provider’s perspective, an important security consideration is
preventing unauthorized use of the network services. This is usually done using strong
authentication and access control methods [2].
8
CHAPTER 3
CHANNEL MODELS
3.1 ADDITIVE WHITE GAUSSIAN NOISE (AWGN) CHANNEL
AWGN is a linear continuous memory-less and time invariant channel used to model
thermal noise in all communication links. Wideband Gaussian noise comes from many
natural sources, such as the thermal vibrations of atoms in conductors (referred to as thermal
noise or Johnson noise), shot noise, black body radiation from the earth and other warm
objects, and from celestial sources such as the sun.
The AWGN channel is a good model for many satellite and deep space
communication links. It is not a good model for most terrestrial links because of multipath,
terrain blocking, interference, etc. However, for terrestrial path modeling, AWGN is
commonly used to simulate background noise of the channel under study, in addition to
multipath, terrain blocking, interference, ground clutter and self interference that modern
radio systems encounter in terrestrial operation.
This model does not account for fading, frequency selectivity, interference,
nonlinearity or dispersion. However, it produces simple and tractable mathematical models
which are useful for gaining insight into the underlying behavior of a system before these
other phenomena are considered. Some of the set assumptions are:
• The noise is additive, i.e., the received signal equals the transmit signal plus some noise, where the noise is statistically independent of the signal.
• The noise is white, i.e., the power spectral density is flat, and so the autocorrelation of the noise in time domain is zero for any non-zero time offset.
• The noise samples have a Gaussian distribution.
The graph of the associated probability density function is “bell”-shaped, and is
known as the Gaussian function or bell curve.
f (x) =
9
where parameter μ is the mean (location of the peak) and σ2 is the variance (the measure of
the width of the distribution).
The distribution with μ = 0 and σ2 = 1 is called the standard normal. See Figure 3.1
for probability distribution function for Gaussian random variable.
Figure 3.1. Probability distribution function for Gaussian random variable for different variance values.
Communication systems use the system model shown in Figure 3.2. X is the
transmitted signal from the transmitter. N, which is assumed to be AWGN, is the thermal
noise present in the channel added to X which yields Y. Y is the received signal at the
receiver which contains the transmitted signal X and noise N. The goal of the receiver is to
deal with the noise and decode the information with limited error. N includes only thermal
noise and does not contain fading, multipath and other channel impairments.
3.2 FADING CHANNELS Fading is deviation of the attenuation that a carrier-modulated telecommunication
signal experiences over certain propagation media. The fading may vary with time,
geographical position and/or radio frequency, and is often modeled as a random process. A
10
Figure 3.2. Noise model in communication system.
fading channel is a communication channel that experiences fading. Fading occurs due to
many reasons. They are:
• Multipath propagation, referred to as multipath induced fading.
• Shadowing from obstacles affecting the wave propagation sometimes referred to as shadow fading.
Fading is classified into two types which are large scale fading and small scale
fading.
Large scale fading represents the average signal attenuation or path loss due to
motion over large areas. This is affected by prominent terrain contours such as hills, forest,
billboards etc. present between the transmitter and receiver. The statistics provide a way of
computing an estimate of path loss as a function of distance. This is described as mean path
loss and log normally distributed variation about the mean [3].
Small scale fading refers to dramatic changes in signal amplitude and phase that can
be experienced because of small changes. It manifests itself in two mechanisms- time
spreading of signal and time variation of channel. Time variation of channel is seen due to
motion of transmitter and receiver [3].
Figure 3.3 shows the comparison of large scale fading and small scale fading. m(t) is
large scale fading component and r0(t) is the small scale fading component. In Figure 3.3a,
the signal power received is a function of the multiplicative factor α(t). Small-scale fading
superimposed on large-scale fading can be readily identified. The typical antenna
displacement between adjacent signal-strength nulls due to small-scale fading is
approximately half of wavelength. In Figure 3.3b, the large-scale fading or local mean m(t)
has been removed in order to view the small-scale fading r0(t). The log-normal fading is a
N
X Y
11
Figure 3.3. Large scale fading v/s small scale fading.
relative slow varying function of position, while the Rayleigh fading is a relatively fast
varying function of position.
3.3 MULTIPATH FADING Multipath is a phenomenon that results in radio signals reaching the receiving antenna
by two or more paths. Some of the causes of multipath include atmospheric ducting,
ionospheric reflection and refraction, and reflection from water bodies and terrestrial objects
such as mountains and buildings. Figure 3.4 shows the phenomena of multipath fading.
Figure 3.4. Multipath fading.
TRANSMITTER RECEIVER
BUILDINGS or ANY
OBJECT
Line of Sight
Direct Path
12
Multipath can cause errors and affect the quality of communications. The errors are
due to introduction of inter-symbol interference (ISI) which causes smearing of the
neighboring symbol. Equalizers are used to correct the ISI. Alternatively, techniques such as
orthogonal frequency division modulation (OFDM) and rake receivers, which are used to
decode CDMA signals, may be used.
Multipath fading has two kinds viz Rayleigh and Rician which are discussed below.
3.3.1 Rayleigh Fading Rayleigh fading models assume that the magnitude of a signal that has passed through
a communications channel will fade according to a Rayleigh distribution shown in
Figure 3.5. Rayleigh fading is viewed as a reasonable model for signal propagation in urban
environments. Rayleigh fading is most applicable when there is no dominant propagation
along a line of sight between the transmitter and receiver. If there is a dominant line of sight,
Rician fading is more applicable.
Figure 3.5. Probability distribution function for Rayleigh fading for different variance.
13
The Rayleigh probability density function is:
f (x, ) = x ≥ 0
for parameter σ > 0.
3.3.2 Rician Fading Rician fading is a stochastic model for radio propagation anomaly caused by partial
cancellation of a radio signal by itself — the signal arrives at the receiver by several different
paths (hence exhibiting multipath interference), and at least one of the paths is changing.
Rician fading occurs when one of the paths, typically a line of sight signal, is much stronger
than the others. Rician fading is characterized by a Rician distribution which is discussed
below. Figure 3.6 shows Rician distribution.
Figure 3.6. Probability distribution function for Rician fading.
f (x│v, ) =
where I(z) is the modified Bessel function of the first kind with order zero. When v = 0, the
distribution reduces to a Rayleigh distribution.
14
CHAPTER 4
ORTHOGONAL FREQUENCY DIVISION
MULTIPLEXING (OFDM)
Orthogonal Frequency Division Multiplexing (OFDM) is a technique in which a
high-bit-rate data stream is divided into several parallel lower bit-rate streams, modulating
each stream on separate carriers called subcarriers [4]. OFDM belongs to a family of
transmission schemes called multicarrier modulation. Each sub-carrier is modulated with a
digital modulation scheme such as Phase-Shift Keying (PSK) or Quadrature Amplitude
Modulation (QAM). The total data rate is similar to the conventional single-carrier
modulation scheme with the same bandwidth. It is used for high speed data transmission over
multipath fading channels [5].
OFDM is a combination of modulation and multiplexing. Multiplexing refers to
independent signals, produced by different sources. In OFDM the signal itself is first split
into independent channels, modulated by data using suitable modulation technique and then
re-multiplexed to create the OFDM carrier. However, OFDM signals are known to suffer
from a high PAPR when a number of independently modulated subcarriers are added up
coherently [6].
The primary advantage of OFDM over single-carrier schemes is its ability to cope
with severe channel conditions, for example interference and frequency-selective fading due
to multipath, without complex equalization requirement. Inter-symbol interference (ISI) is
eliminated by use of guard intervals which allow the echoes to die down before the next
symbol’s arrival. Frequency selective fading is taken care because of the inherent property of
OFDM which uses multiple carriers. The entire OFDM symbol isn’t affected due to
frequency selective fading and hence only certain sub-carriers which are affected are
discarded. OFDM is the right fit for hostile channels.
Advantages of OFDM include:
• Reduction in computational complexity due to the use of FFT.
15
• Exploitation of frequency diversity.
• Robust against narrowband interference.
• Usage of simple frequency domain equalizers instead of complex time domain equalizers.
• Well handling of Multipath propagation.
Figure 4.1 shows the comparison of the conventional FDM with OFDM scheme.
Notice that there is 50% overlapping done in OFDM due to the presence of orthogonality.
Also notice a significant savings in bandwidth is achieved.
Figure 4.1. Comparison of the conventional FDM with OFDM.
OFDM has developed into a popular scheme for wideband digital communication. It
is used in variety of applications such as digital television and audio broadcasting, wireless
networking and broadband internet access. Some of them are listed below.
Applications of OFDM include:
• WLAN radio interfaces IEEE 802.11a/g/n.
• Digital Radio systems such as DAB, HD radio.
• The terrestrial digital TV systems DVB-T.
• The terrestrial mobile TV systems DVB-H.
• A variant of OFDM in 4G systems such as WiMax, LTE.
The block diagram of OFDM transmitter and receiver are shown in Figure 4.2 and
Figure 4.3. Bits arrive serially to the serial to parallel converter which is converted to parallel
16
Figure 4.2. Block diagram of OFDM transmitter.
Figure 4.3. Block diagram of OFDM receiver.
stream of data. The encoder encodes the data depending on the various modulation
techniques available such as PSK, QAM etc. The encoded data is fed to IFFT block
which takes the Inverse Fourier Transfer of the data. This is converted back again to
serial format which is later fed to Digital-to-Analog converter (DAC). The analog signal
obtained is fed to power amplifier and then to antenna for transmitting. The receiver
exactly does the reverse by taking the analog signal converting it to digital, taking FFT,
feeding it to the decoder which demodulates the data depending on various conditions and to
the parallel to serial converter to obtain the original data bits. Finally the data bits are
compared to get bit error rate. Noise adds on to the signal in the air which has to be taken
care at the receiver.
SERIAL
TO
PARALLEL
PARALLEL
TO
SERIAL
N
POINT
FFT
DECODER
DECODER
DECODER
DECODER
LOW NOISE AMPLIFIER
ADC •
SERIAL
TO
PARALLEL
PARALLEL
TO
SERIAL
N
POINT
IFFT
ENCODER
ENCODER
ENCODER
ENCODER
POWER
AMPLIFIER DAC
•
17
4.1 CYCLIC PREFIX Cyclic prefix refers to the prefixing of a symbol with a repetition of the end. Some
samples at the end of the symbol is prefix to the starting of the symbol shown in Figure 4.4.
The receiver discards the cyclic prefix samples before decoding. Cyclic prefix is used for
three reasons discussed below. In order for the cyclic prefix to be effective, the length of the
cyclic prefix must be at least equal to the length of the multipath channel.
Figure 4.4. Cyclic prefix in OFDM.
• To maintain orthogonality: Studies have shown that the echoes of the OFDM symbol last for 0.8 microseconds. Hence a gap called the guard interval of 0.8 microseconds is created after each OFDM symbol. Creation of gap results in loss of orthogonality and hence a cyclic prefix is added to compensate.
• Frequency domain equalization: As a repetition of the end of the symbol, it allows the linear convolution of a frequency-selective multipath channel to be modeled as circular convolution, which in turn may be transformed to the frequency domain using a Discrete Fourier transform. This approach allows for simple frequency-domain processing, such as channel estimation and equalization.
• Intersymbol interference (ISI): The creation of guard interval gets rid of ISI. ISI is caused due to multipath fading.
4.2 OFDM SYMBOL Figure 4.5 shows a typical OFDM symbol. As discussed above OFDM symbol
consists of a bunch of sinusoidal waves of different frequencies created by the IFFT block.
Comparison of OFDM systems is shown in Table 4.1.
4.3 PERFORMANCE OF OFDM SYSTEMS The performance of any OFDM system can be assessed using two parameters which
are Bit Error Rate (BER) and PAPR.
18
Figure 4.5. Typical OFDM symbol.
Table 4.1. Comparison of OFDM Systems
Standard Name DVB-T DVB-H IEEE 802.11a (WiFi)
Frequency Range (MHz) 470-862 174-230 470-862 4915-5825
Channel Spacing (MHz) 6,7,8 5,6,7,8 20
FFT (K=1024) 2K,8K 2K,4K,8K 64
No of sub-carriers 2K mode:1705 8K mode:6817
2K mode:1705 4K mode:3409 8K mode:6817
52
Sub-carrier Modulation technique
QPSK, 16 QAM 64 QAM
QPSK, 16 QAM 64 QAM
BPSK,QPSK 16QAM,64 QAM
Symbol length (microseconds)
2K mode:224 8K mode:896
2Kmode:224 4K mode:448 8K mode:896
3.2
Guard Interval (Fraction of symbol length) ¼, 1/8, 1/16, 1/32 ¼, 1/8, 1/16, 1/32 1/4
Sub-carrier spacing (kHz)
2K mode:4.464 8K mode:1.116
2K mode:4.464 4K mode:2.232 8K mode:1.116
312.5
19
The bit error rate is found from the errors generated by comparing the data bits both
at the transmitter and receiver. The bit error rate varies depending on the Eb/No value where
‘Eb’ is the energy per bit and No/2 is the power spectral density of noise. Figure 4.6 which is
plotted by adding AWGN channel only, resembles the typical waterfall model which is
expected.
Figure 4.6. Bit error rate of OFDM for AWGN channel.
Disadvantages of OFDM include:
• Sensitive to Doppler shift.
• Sensitive to frequency synchronization problems.
• Very susceptible to phase noise and frequency dispersion.
• High PAPR (Peak average to power ratio) which requires more power efficient amplifiers.
• Loss of efficiency caused by cyclic prefix.
4.4 PAPR IN OFDM Peak-to-Average Power Ratio (PAPR) is the ratio of the peak power and average
power of a signal. It is a dimensionless quantity.
20
where ‘s1’ contains signal voltages.
PAPR is a major issue in OFDM. OFDM signals have high PAPR when compared to
the single carrier modulation signals. When the OFDM signal’s super-positioned sinusoidal
subcarriers to be in-phase at the input of the transmitter’s inverse fast Fourier transform
(IFFT) operator, these sinusoids would add constructively, producing a high magnitude at the
IFFT output [7]. PAPR increases exponentially with increase in the number sub-carriers. The
peak power of a signal is a critical design factor for band limited communication systems,
and it is necessary to reduce it as much as possible [8]. Because of the high PAPR, the
transmitter power amplifier may be driven into saturation. This potentially contaminates the
adjacent channels resulting in the co-channel interference [9].
The disadvantages of high PAPR are:
1. Power Amplifiers at high ranges need linearization.
2. Increases dynamic range of the power amplifiers which results in big, bulky and expensive power amplifiers.
3. PAPR generates out-of-band energy (spectral re-growth) and in-band distortion (constellation tilting and scattering) [10].
4. High PAPR requires high resolution for both the transmitter’s DAC and the receiver’s ADC [2].
5. High values of PAPR result in low efficient usage of the ADC and DAC word length [11].
6. Consumes more battery power in mobile devices which are power hungry [12].
7. Inefficient amplification which leads to out-of-band noise.
Figure 4.7 shows the graph of PAPR values versus the number of symbols in OFDM.
The first subplot shows various PAPR values for different OFDM symbols and the second
subplot show the histogram of PAPR in OFDM. Notice that the maximum PAPR appears to
be close to 16 which is very high.
Figure 4.8 shows the transfer function of a typical power amplifier. The region
between zero and peak is called the linear region and most of the functions of an amplifier
are carried out in this region. Exceeding the input voltage beyond peak introduces
non-linearity and causes non-linear distortion which should be avoided. One must make sure
21
Figure 4.7. (a) PAPR of OFDM and (b) is histogram of PAPR.
PAPR is well within the limits of the peak shown or increase the dynamic range (ratio of max
and min at the input) of the amplifier, which in turn increases the peak limit, at the expense
of high cost. By increasing the resolution of both DAC and ADC due to increase in PAPR,
the system gets more complex, costs high, and requires high power.
22
Figure 4.8. Transfer function of a typical power amplifier.
23
CHAPTER 5
EXISTING PAPR REDUCTION TECHNIQUES
5.1 CARRIER INTERFEROMETRY-OFDM Carrier Interferometry (CI) is a type of spread spectrum technology which uses a
unique orthogonal complex spreading sequence (applied in the frequency domain) to spread
parallel data streams over all sub carriers in orthogonal frequency-division multiplexing
(OFDM) [13]. This creates frequency diversity benefits for each symbol stream leading to
high performance.
Additionally, the use of carefully selected complex spreading sequences eliminates
large peaks in power, thus reducing the Peak-to-Average Power (PAPR) of the transmitted
signal.
The transmitter and receiver block diagram of CI OFDM are shown in Figures 5.1
and 5.2, respectively which has all the blocks from OFDM along with spreading blocks after
encoders. The first N/2 subcarriers are spread using the upper spread block and the next N/2
subcarriers are spread using the lower spread block as shown in Figure 5.1. The formulae for
spreading are:
First N/2 subcarriers:
Next N/2 subcarriers:
where A is the output from encoders and N is the number of subcarriers.
The receiver had a block dispreading to despread the bits similar to the one in the
transmitter as shown in Figure 5.1. The despreader did the reverse and the formulae were
First N/2 subcarriers:
Next N/2 subcarriers:
24
Figure 5.1. Block diagram of CI-OFDM transmitter.
Figure 5.2. Block diagram of CI-OFDM receiver.
5.2 PERFORMANCE OF CI OFDM SYSTEMS CI OFDM systems have bit error rate performance similar to traditional OFDM
systems. Figure 5.3 shows the bit error rate performance in AWGN channel only. Figure 5.4
shows the PAPR performance of CI OFDM. The reduction in PAPR can be seen as compared
to the traditional OFDM systems. In this case, the maximum PAPR value is limited to
10 which is certainly better when compared to traditional OFDM which had maximum PAPR
value of 16.
5.3 COMPLEMENTARY CODE KEYING (CCK) OFDM CCK is a modulation technique used in 802.11b WLAN standard. This technique is
based on polyphase complementary codes found by Golay. It was adopted to supplement
Barker code to increase the data rate in wireless networks. Golay sequences have the
property that the sum of their autocorrelation functions equals zero for all time shifts, except
SERIAL
TO
PARALLEL
PARALLEL
TO
SERIAL
N
POINT
FFT
DECODER
DECODER
DECODER
DECODER
LOW NOISE
DE-SPREADING FOR
1 TO N/2
DE-SPREADING FOR N/2+1
TO N
ADC • •
SERIAL
TO
PARALLEL
PARALL
EL
TO
SERIAL
N POINT
IFFT
ENCODER
ENCODER
ENCODER
ENCODER
POWER
AMPLIFIER
SPREADING
FOR 1 TO N/2
SPREADING
FOR N/2+1 TO
N
DAC • •
25
Figure 5.3. Bit error rate of CI-OFDM for AWGN channel.
Figure 5.4. (a) PAPR of CI-OFDM and (b) is histogram of PAPR.
26
zero. In other words, Golay sequences have a very good aperiodic autocorrelation property
which is useful to reduce the PAPR in OFDM system [14]. Wireless networks based on
802.11b specification employ CCK to operate at either 5.5 or 11 Mbit/s in band at 2.4 GHz.
A drastic reduction in PAPR can be seen by using complementary codes. The CCK
modulation used by 802.11b transmits data in symbols of eight chips, where each chip is a
BPSK at chip rate of 5.5 Mchip/s or a QPSK bit-pair at a chip rate of 11 Mchip/s. To
implement the 11 Mbps 802.1lb signal, a block of 8 data bits are mapped into a combination
of a unique 8-chip CCK codeword and a differential phase to form the nth symbol [15]. The
block diagram of transmitter and receiver is shown in Figures 5.5 and 5.6, respectively. The
spreading sequence block in the transmitter takes four bits as input giving eight values
(chips) which depend on the input bits as the following.
Figure 5.5. Block diagram of CCK OFDM transmitter.
Figure 5.6. Block diagram of CCK OFDM receiver.
SERIAL
TO
PARALLEL
PARALLEL
TO
SERIAL
DESPREADING SEQUENCE
DESPREADING SEQUENCE
DESPREADING SEQUENCE
DESPREADING SEQUENCE
LOW NOISE
AMPLIFIER
N
POINT
FFT •
SERIAL
TO
PARALLEL
PARALLEL
TO
SERIAL
SPREADING
SEQUENCE
SPREADING
SEQUENCE
SPREADING
SEQUENCE
SPREADING
SEQUENCE
POWER
AMPLIFIER
N POINT
IFFT • •
27
The eight output chips are [14]:
for BPSK
for BPSK
The above equation can be viewed as a form of generalized Hadamard transforms
encoding where the phase change φ1 is applied to every chip, φ2 is applied to all even code
chips, φ3 is applied to the first two of every four chips, and φ4 is applied to the first four of
the eight chips.
The above code rate can be extended to rates 5/16, 6/32 and so on.
The drawback of this technique is that the efficiency is just 50% for code rate of
4/8 and decreases as we consider further variants such as 5/16 (31.25%), 6/32 (18.75%), etc.
The despreading sequence block at the receiver does the reverse of the one done by
the spreading sequence block at the transmitter. It takes in eight values and converts them
back into four bits which is desired.
5.4 PAPR PERFORMANCE OF CCK OFDM SYSTEMS Figure 5.7 shows the PAPR performance of Complementary Code Keying OFDM
technique. As we can notice, the maximum PAPR is close to 4 which is way less than
traditional OFDM and CI OFDM.
28
Figure 5.7. (a) PAPR of CCK OFDM and (b) is histogram of PAPR.
29
CHAPTER 6
PROPOSED PAPR REDUCTION TECHNIQUES
6.1 EXTENSION OF CCK OFDM The CCK technique proposed extends the rate from 5/16 to 7/16 while retaining the
same performance. This technique increases the efficiency by 40% which is very significant
in today’s data rate hungry systems. The same block diagram of CCK is used with replacing
the 5/16 spreading sequence with a 7/16 spreading sequence as shown in Figure 6.1. The rate
7/16 block consists of one rate 4/8 block as discussed above and a multiplexer which
contains two rate 4/8 with one of the bit replaced with either 0 or 1 depending on whichever
gives less PAPR value at the output. The upper block in the multiplexer takes 3 bits appends
a zero which results in 4 bits and acts as a rate 4/8 encoder, encoding 8 values given in the
CCK section above. The lower block in the multiplexer does the same but appends the 3 bits
with a one instead of a zero. The multiplexer makes sure that either of the blocks is switched
on depending on whichever gives least PAPR value at the output.
Figure 6.1. Details of the 7/16 spreading sequence block.
7-16 SPREADING
SEQUENCE
4-8
SPREADING SEQUENCE
MULTIPLEXER
3&ZERO-8 SPREADING SEQUENCE
3&ONE-8 SPREADING SEQUENCE
=
30
6.2 PAPR PERFORMANCE OF PROPOSED CCK OFDM SYSTEM
The simulation of the PAPR performance is shown in Figure 6.2. Notice that PAPR is
limited to a value of three which is very less compared to traditional OFDM. The PAPR
value is lesser than the 4/8 CCK OFDM system discussed earlier. The efficiency of the
system is sacrificed to obtain PAPR that low.
Figure 6.2. (a) PAPR of 7/16 CCK OFDM and (b) is histogram of PAPR.
6.3 RATE-12/16 TECHNIQUE The Rate-12/16 technique proposed can reduce PAPR while improving the
performance of bit error rate due to coding gain. This technique explorers the root cause of
PAPR which is periodicity of the bits which are fed to IFFT block after encoding and before
power amplifier. This technique eliminates the periodicity by introducing a block at the start
of the OFDM transmitter block diagram, which is the 3-4 mapper and demapper blocks as
shown in Figures 6.3 and 6.4, respectively. This block takes 3 bits as input and maps to one
of the 4 bit combinations which are non-periodic. This ensures elimination of periodicity of
the bits. For example: 000 can be mapped to either of the values such as 0001,0010,0100,
0110,0111,1000,1001,1011,1101, 1110 and not to 0000, 0011, 0101, 1010, 1100, 1111
because the latter values are periodic. PAPR is reduced by 55-60%, using this technique, as
compared to traditional OFDM whereas the efficiency is sacrificed by 25%. The tradeoff
31
Figure 6.3. Block diagram of rate-12/16 transmitter.
Figure 6.4. Block diagram of rate-12/16 receiver.
lies between the efficiency and the scale of reduction of PAPR. The block diagram of this
proposed technique is shown in Figures 6.3 and 6.4 which resembles the block diagram of
OFDM except presence of an additional block which is 3-4 mapper. The receiver has a block
called pre-decoder, which finds the correct 4 bit word if there is an error introduced due to
noise. The receiver knows the eight aperiodic combinations which are mapped at the
transmitter. Each received 4 bit word is multiplied by all the eight combinations known at the
receiver and sum is calculated for all. The 4 bit word corresponding to the one which gives
the highest sum is considered as the perfect match for further processing. This involves
demapping the mapped word into the original data bits which is done by 3-4 demapper
shown in the block diagram.
6.4 PERFORMANCE OF RATE-12/16 TECHNIQUE The performance of Rate-12/16 technique which consists of a Bit Error Rate (BER)
diagram and the PAPR diagram is shown in Figures 6.5 and 6.6, respectively. As we can see,
SERIAL TO
PARALLEL
PARALLEL
TO
SERIAL
N
POINT
FFT
DECODER
DECODER
DECODER
DECODER
LOW NOISE
AMPLIFIER
3-4 DEMAP
ADC
PRE-DECODER
SERIAL
TO
PARALLEL
PARA-
LLEL
TO
SERIAL
N
POINT
IFFT
ENCODER
ENCODER
ENCODER
ENCODER
POWER
AMPLIFIER
3-4
MAPPE
DAC •
32
Figure 6.5. Bit error rate of rate-12/16 technique.
Figure 6.6. (a) PAPR of rate-12/16 technique and (b) is histogram of PAPR.
33
the bit error rate figure is similar to the one obtained in traditional OFDM indicating
similarity in the error performance. The PAPR performance is well below traditional OFDM.
The PAPR value is limited to 7 which is a reduction of more than 50% compared to
traditional OFDM.
6.5 IMPROVED RATE-12/16 TECHNIQUE: COMBINATION OF RATE-12/16 AND CCK
Improved Rate-12/16 is a proposed technique which combines the previous
Rate-12/16 technique and the CCK (Complementary Code Keying) technique discussed
earlier. This combines the advantages obtained from both the techniques and reduces PAPR
even further giving the same performance. This technique replaces the 3-4 mapper block with
a 12/16 mapper which consists of two 3-4 mappers and 4-6 CCK encoder and a multiplexer
which contains four 2-2 encoders. The details of 12/16 codec is shown in Figure 6.7 between
the transmitter block diagram (Figure 6.8) and the receiver block diagram (Figure 6.9). The
3-4 mappers work similar followed by the 4-8 codec which also works similar as discussed.
The multiplexer, which has four 2-2 encoders, switches on any of the four encoders
depending on the one which gives least PAPR at the output. We get the best and the least
value of PAPR, which is the primary goal of all the techniques discussed. This technique also
has trade-off between the efficiency and the scale of PAPR reduction which existed in the
previous Rate-12/16 technique. This technique provides the same efficiency as the
Rate-12/16 technique but with reduced PAPR. The reduction when compared to the previous
Rate-12/16 technique is close to 30% which is pretty significant.
6.6 PAPR PERFORMANCE OF IMPROVED RATE-12/16 TECHNIQUE
The PAPR performance of the improved rate-12/16 technique is shown in
Figure 6.10. The reduction of PAPR seen is enormous when compared to traditional OFDM.
The reduction seen is more than 65% while retaining the BER performance. The highest
PAPR value seen in this technique is less than five on the histogram which is way less than
the traditional OFDM which is 16. This technique has lesser PAPR than the Rate-12/16
technique while the efficiencies remained the same which is 75%.
34
Figure 6.7. Details of 12/16 mapper block.
Figure 6.8. Block diagram of improved rate-12/16 transmitter.
6.7 COMPARISON OF PERFORMANCE OF ALL THE DISCUSSED TECHNOLOGIES
The bit error rate (BER) of all the technologies discussed so far is plotted in a single
graph for comparison. As seen in Figure 6.11, the BERs of traditional BPSK OFDM and
CIOFDM are similar but the Rate-12/16 technique is better because of coding involved.
See Table 6.1 for comparison of all the techniques discussed so far. There is a coding gain
which can be seen in Rate-12/16 technique. The PAPR comparison is shown in Figures 6.12
SERIAL
TO
PARALLEL
PARA-
LLEL
TO
SERIAL
N
POINT
IFFT
ENCODER
ENCODER
ENCODER
ENCODER
POWER AMPLIFIER
12/16 MAPPE
DAC •
3-4
MAPPER
3-4
MAPPER
MULTIPLEX
ER
4-6
CCK
12/16
MAPPER
2-2 ENCODER
2-2 ENCODER
2-2 ENCODER
=
2-2 ENCODER
35
Figure 6.9. Block diagram of improved rate-12/16 receiver.
Figure 6.10. (a) PAPR of improved rate-12/16 technique and (b) is histogram of PAPR.
and 6.13. Figure 6.12 shows comparison of the four technologies viz traditional OFDM,
CI-OFDM, Rate-12/16 technique and Improved Rate-12/16 technique. The Improved
Rate-12/16 technique wins the race as the PAPR is the least when compared to other
techniques discussed so far. The only drawback is the efficiency which is 75% whereas the
PAPR is reduced close to 70% compared to traditional OFDM. One can use Rate-12/16
technique for less complexity, if the obtained PAPR value is satisfied. The improved
rate-12/16 technique is a bit complex compared to Rate-12/16 technique but the PAPR
SERIAL
TO
PARALLEL
PARALLEL
TO
SERIAL
N
POINT
FFT
DECODER
DECODER
DECODER
DECODER
LOW NOISE
AMPLIFIER
12/16 DEMAP
ADC
•
36
Figure 6.11. Bit error rate of OFDM, CI-OFDM, rate-12/16 techniques.
Table 6.1. Comparison of OFDM, CI-OFDM, CCK, Rate-12/16 and Improved Rate-12/16 Techniques
Techniques BER @ Eb/No=5 Max PAPR Efficiency OFDM 10-3 16 1 CI-OFDM 10-3 11 1 5-16 CCK-OFDM - 2 0.3125 7-16 CCK-OFDM - 3 0.4375 Rate-12/16 10-4 7 0.75 Improved Rate-12/16 10-4 5 0.75
performance is up by 30%. One can chose either of the two techniques depending on the
requirements. Figure 6.13 shows the histogram of the same technologies and notice that no
value exist greater than 5 in case of improved rate-12/16 technique. One can conclude that,
there is a better performance in both BER and PAPR for a sacrificed efficiency.
37
Figure 6.12. PAPR of OFDM, CI-OFDM, rate-12/16 and improved rate-12/16 techniques.
Figure 6.13. Histogram of PAPR of OFDM, CI-OFDM, rate-12/16 and improved rate-12/16 techniques.
38
CHAPTER 7
CONCLUSION AND FUTURE ENHANCEMENT
High PAPR is the culprit for use of inefficient power amplifiers which lessens the
battery life in power hungry devices such as mobile phones. Even though many techniques
exist today which takes care of PAPR concerns, there is always scope for improvement. A
couple of improvements for PAPR were discussed and couple of them was proposed too. The
proposed techniques bring PAPR to a new low by making certain modifications. A trade-off
exists between the extent of reduction of PAPR and the efficiency. The trade-off also exists
between the complexity and extent of reduction of PAPR. All the simulations were carried
out in MATLAB.
Some of the future work involves:
• Increasing the efficiency while retaining the same or getting better PAPR performance.
• Reducing the complexity by using some efficient techniques.
39
BIBLIOGRAPHY
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[2] J. G. Andrews, A. Ghosh, and R. Muhamed, Fundamentals of WiMax, Understanding Broadband Wireless Networking. Upper Saddle River, NJ: Prentice Hall Inc., 2007.
[3] B. Sklar, Digital Communications-Fundamentals and Applications 2nd edition. Upper Saddle River, NJ: Prentice Hall Inc., 2001.
[4] Z. Ke and Z. Li Jun, “Reducing of peak to average power radio of OFDM system with pseudorandom sequence,” in IEEE Conf. Wireless Commune., Netw. & Mobile Comput., Dalian, China, 2008, pp. 1-4.
[5] R. Chackochan and H. Soni, “Peak to average power ratio (PAPR) reduction in OFDM for a WLAN network using SLM technique,” in 3rd Int. Conf. Electron. Comput. Tech. (ICECT), Kanyakumari, India, 2011, pp. 57-59.
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[10] V. Vijayarangan, R. Kalidoss, and R. Sukanesh, “Low crest mapping for PAPR reduction in OFDM systems,” in IET Int. Conf. Wireless, Mobile Multimedia Netw., Hangzhou, China, 2006, pp. 1-4.
[11] K. Kasiri, I. Hosseini, O. Taheri, M. J. Omidi, and P. G. Gulak, “A preprocessing method for PAPR reduction in OFDM systems by modifying FFT and IFFT matrices,” in 18th Int. Symposium on Personal, Indoor & Mobile Radio Commun., (PIMRC), Athens, Greece, 2007, pp. 1-5.
[12] Y. Hou and T. Hase, “PAPR reduction for OFDM system using partial symbol transmission,” in 4th Int. Conf. Circuits & Sys. Commun. (ICCSC), Shanghai, China, 2008, pp. 1-5.
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[13] D. A. Wiegandt, Z. Wu, and C. R. Nassar, “High-throughput, high- performance OFDM via pseudo-orthogonal carrier interferometry spreading codes,” IEEE Trans. Comms., vol. 51, pp. 1123-1134, July 2003.
[14] W. Jeong, H. Park, H. Lee, and S. Hwang, “Performance improvement techniques for CCK-OFDM WLAN modem,” IEEE Trans. Consumer Electron., vol. 49, pp. 602-605, Aug. 2003.
[15] A. Z. Al-Banna, T. R. Lee, J. L. LoCicero, and D. R. Ucci, “11 Mbps CCK-modulated 802.l lb WiFi: Spectral signature and interference,” in 2006 IEEE Int. Conf. Electro/Info. Tech., Chicago, 2006, pp. 313-317.