CODE SHIFTED REFERENCE IMPULSE-BASED COOPERATIVE UWB

CODE SHIFTED REFERENCE IMPULSE-BASED COOPERATIVE UWB
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CODE SHIFTED REFERENCE
IMPULSE-BASED
COOPERATIVE UWB
COMMUNICATION SYSTEM
Pir Meher Ali Shah
Mohammed Abdul Rub
Ashik Gurung
This thesis is presented as part of Degree of
Master of Science in Electrical Engineering
Blekinge Institute of Technology
Sweden
September 2011
Blekinge Institute of Technology
School of Engineering
Department of Applied Signal Processing
Supervisor: Muhammad Gufran Khan
Examiner: Dr. Jörgen Nordberg
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Abstract
Ultra wideband (UWB) is a radio technology in which the transmission of information is
done over a large bandwidth with very short pulses at low energy levels. UWB technology has
gained a high popularity in the field of short-range wireless communications. UWB provides
significant benefits like position location capability, reduced fading effects, and higher channel
capacity. UWB technology is very desirable because of its certain characteristics like low power
consumption, cost reliability and simple architecture. However, UWB systems face challenges
regarding system design to achieve low complexity and low cost. UWB systems need high
sampling frequencies and face problems while using digital signal processing technology.
In this thesis, first, the comparison of transmitted reference (TR), multi-differential
frequency shifted reference (MD-FSR) and code shifted reference (CSR) is done in terms of
BER performance and system complexity. The simulation results validate that the CSR-UWB
system has better BER performance and lower complexity than MD FSR-UWB system.
Secondly, cooperative communication is implemented for CSR-UWB. The system BER
performance of the CSR impulse-based cooperative UWB communication system is evaluated
for different number of relays and different average distances between source node and
destination node. The simulations are carried out under both line of sight (LOS) and non-line of
sight (NLOS) environments. The simulation results show that the performance of the system
decreases with the increase in average source-to-destination distance. We also observe that the
system performs better under an environment of LOS channel than under an environment of
NLOS channel. Finally, the results validate that the system performs better as the number of
relay nodes increases until it reaches an adequately large number.
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Acknowledgement
First of all, we would like to express our warmest thankfulness to our supervisor Muhammad
Gufran Khan. We are highly indebted towards him for giving us his valuable time, effort and
guidance throughout our entire thesis period. We would really want to appreciate his clarity in
his direction and high dedication as a key of our motivation towards our thesis. Without his help
and support, this thesis would not have been possible. We would also like to convey our
gratitude to our examiner, Dr. Jörgen Nordberg. Our thanks also goes to our teachers in
Blekinge Institute of Technology. We would like to thank our classmates and friends for all
their help and for making our life here enjoyable. We are really grateful to our parents, brothers
and sisters for providing good education and good environment to us. Lastly, we would like to
thank God for giving us the courage and strength to move ahead towards our destiny.
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Table of Contents
ABSTRACT………………………………………………………………………………………………………………………………………………….………...3
ACKNOWLEDGEMENT…………………………………………………………………………………………………………………………………..……..5
CHAPTER ONE………………………………………………………………………………………………………………………………………………..…….9
INTRODUCTION………………………………………………………………………………………………………………………………..…..10
1.1 OVERVIEW…………………………………………………………………………………………………………………………10
1.2 HISTORY AND BACKGROUND…………………………………………………………………………………………….10
1.3 DEFINITION OF UWB………………………………………………………………………………………………………….11
1.4 FEATURES AND ADVANTAGES OF UWB……………………………………………………………………………..12
1.5 UWB CHALLENGES……………………………………………………………………………………………………………..14
1.6 THESIS CONTRIBUTION………………………………………………………………………………………………………15
CHAPTER TWO……………………………………………………………………………………………………………………………………………………..16
UWB SYSTEMS, MODULATION AND MULTIPLEXING TECHNIQUES……………………………………………………….17
2.1 UWB SYSTEM TYPES…………………………………………………………………………………………………………..17
2.1.1 MULTICARRIER UWB………………………………………………………………………………………………………..17
2.1.2 IMPULSE RADIO UWB………………………………………………………………………………………………………17
2.2 UWB PULSE SHAPE…………………………………………………………………………………………………………….18
2.3 MODULATION TECHNIQUES USED IN UWB SYSTEMS………………………………………………………..20
2.3.1 PULSE POSITION MODULATION……………………………………………………………………………………….20
2.3.2 PULSE AMPLITUDE MODULATION……………………………………………………………………………………21
2.3.3 BINARY PHASE SHIFT KEYING…………………………………………………………………………………………..22
2.3.4 ON-OFF KEYING……………………………………………………………………………………………………………….22
2.4 MULTIPLE ACCESS TECHNIQUES USED IN UWB SYSTEM……………………………………………………23
2.4.1 TIME-HOPPING UWB……………………………………………………………………………………………………….23
2.4.2 DIRECT SEQUENCE UWB………………………………………………………………………………………………….25
CHAPTER THREE…………………………………………………………………………………………………………………………………………………..26
ULTRA WIDEBAND WIRELESS CHANNELS………………………………………………………………………………………………27
3.1 PROPAGATION MECHANISMS AND CHANNEL CHARACTERISTICS……………………………………..27
3.1.1 MULTIPATH……………………………………………………………………………………………………………………..27
3.1.2 DELAY SPREAD…………………………………………………………………………………………………………………28
3.1.3 COHERENCE BANDWIDTH………………………………………………………………………………………………..29
3.2 CHANNEL FADING DISTRIBUTIONS………….…………………………………………………………………………29
3.2.1 GAUSSIAN CHANNEL………………………………………………………………………………………………..………29
3.2.2 RAYLEIGH CHANNEL…………………………………………………………………………………………………………30
3.2.3 RICEAN CHANNEL…………………………………………………………………………………………………………….30
3.3 ULTRA WIDEBAND CHANNELS……………………………………………………………………………………………30
3.4 IEEE 802.15.4A UWB CHANNEL MODEL………..……………………………………………………………………31
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3.4.1 PATH LOSS……………………………………………………………………………………………………………………….31
3.4.2 SHADOWING……………………………………………………………………………………………………………………32
3.4.3 POWER DELAY PROFILE……………………………………………………………………………………………………32
3.4.4 SALEH AND VALENZUELA…………………………………………………………………………………………………33
3.4.5 DELAY DISPERSION…………………………………………………………………………………………………………..35
3.4.6 SMALL SCALE FADING………………………………………………………………………………………………………36
CHAPTER FOUR……………………………………………………………………………………………………………………………………………………37
IR-UWB RECEIVERS………………………………………………………………………………………………………………………………..38
4.1 INTRODUCTION………………………………………………………………………………………………………………….38
4.2 TRANSMITTED REFERENCE (TR) UWB RECEIVER………………………………………………………………..38
4.3 FREQUENCY-SHIFTED REFERENCE (FSR) UWB RECEIVER……………………………………………………39
4.4 CODE-SHIFTED REFERENCE (CSR) UWB RECEIVER………………………………………………………………41
4.5 BER PERFORMANCE COMPARISON OF THE TR-UWB, THE FSR-UWB AND THE CSR-UWB
SYSTEMS……………………………………………………………………………………………………………………………..45
CHAPTER FIVE………………………………………………………………………………………………………………………………………………………48
COOPERATIVE UWB COMMUNICTION SYSTEM…………………………………………………………………………………….49
5.1 INTRODUCTION………………………………………………………………………………………………………………….49
5.2 WHAT IS COOPERATIVE COMMUNICATION?…………………………………………………………………….49
5.3 COOPERATIVE COMMUNICATION PROTOCOLS: PROCESSING MODES OF RELAYS…….…..…51
5.3.1 DECODE-AND-FORWARD………………………………………………………………………………………….……..51
5.3.2 AMPLIFY-AND-FORWARD………………………………………………………………………………………….…….51
5.3.3 COMPRESS-AND-FORWARD……………………………………………………………………………………….……52
5.3.4 ESTIMATE-AND-FORWARD………………………………………………………………………………………….…..52
5.3.5 CODED COOPERATIONS……………………………………………………………………………………………….….52
5.4 COOPERATIVE UWB SYSTEM MODEL…………………………………………………………………………………53
5.5 PERFORMANCE EVALUATION FOR RELAY POSITIONING…………………………………………………...55
5.6 PERFORMANCE EVALUATION OF THE COOPERATIVE CSR-UWB SYSTEM UNDER DIFFERENT
CHANNEL CONDITIONS………………………………………………………………………………………………….…..57
5.6.1 Case I: 4M (LOS) VS. 7M (LOS) WITH 5 RELAYS…………………………………………………………………58
5.6.2 Case II: 4M (LOS) AND 7M (LOS) VS. 4M (NLOS) WITH 5 RELAYS………………………..………..….59
5.6.3 Case III: 4M (LOS), 7M (LOS) AND 4M (NLOS) VS. 7M (NLOS) WITH 5 RELAYS.……………..….59
5.6.4 Case IV: 4M (LOS) VS. 7M (LOS) WITH 10 RELAYS…………………………………………………………….60
5.6.5 Case V: 4M (LOS) AND 7M (LOS) VS. 4M (NLOS) WITH 10 RELAYS…………………….……………..61
5.6.6 Case VI: 4M (LOS), 7M (LOS) AND 4M (NLOS) VS. 7M (NLOS) WITH 10 RELAYS….…………....62
CHAPTER SIX………………………………………………………………………………………………………………………………………………………..64
CONCLUSIONS………………………………………………………………………………………………………………………………………..65
FUTURE WORK…………………………………………………………………………………………………………………………………………………….66
REFERENCES…………………………………………………………………………………………………………………………………………………………67
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CHAPTER ONE
Introduction
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Chapter 1: Introduction
1.1 Overview
The tremendous growth in wireless technology and the huge demand in achieving successful
deployment of wireless communication have significant impact on our daily lives. Since 1990,
wireless communication has come to rise in the field of communication technology
throughout the whole world because of its undeniable applications. The cellular
communication growth from analog to digital, the development of third and fourth generation
radio systems and the transition of wired connection to Wi-Fi are enabling customers to
access a broad range of information at any time and from anywhere [1]. The need of every
customer is faster service, higher capacity, and more confidential wireless connections. The
Ultra Wideband (UWB) technology fulfills those demands by introducing exciting new
features in radio communications.
1.2 History and Background
Ultra wideband (UWB) differs from other communication techniques because it uses
tremendously narrow radio pulses for the communication between transmitters and receivers.
The utilization of short-duration pulses as building units for communication can produce a very
large bandwidth and provide many advantages [1] like immense throughput, robustness, along
with the co-existence of current radio features [1].
Ultra wideband communications was first introduced by Guglielmo Marconi in the early
th
19 century by employing spark gap radio transmitters for spreading Morse code sequences
over the Atlantic Ocean [2]. However, the advantage of a wide bandwidth and the potential of
implementation of multiuser systems presented by electromagnetic pulses were not taken in
account at that moment.
After about fifty years later, modern pulse based communication was introduced in the
form of radars. The technology was limited to military and defense departments for confidential
purposes like extremely secure communications. Development in the field of micro-processing
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and semiconductor technology has lifted UWB for commercial uses [1]. The demand for UWB
raised and developers of UWB system approached the Federal Communication Commission
(FCC) for an approval for commercial implementation. In 2002, FCC granted an approval to
UWB for commercial purpose [1].
1.3 Definition of UWB
Ultra wideband is a radio signal that can be employed with a very low energy in a high
bandwidth. FCC suggests a UWB system has a bandwidth that is larger than 500 MHz or it has
a fractional bandwidth more than 20% of the center frequency [3]. Fractional bandwidth is
defined as [3]
f BW =
fH − fL
fc
(1)
Here, fBW is the fractional bandwidth, fH is the highest cutoff frequency (at -10 dB emission
point) and fL is the lowest cutoff frequency (at -10 dB emission point). fc is the center frequency
that can be calculated as fc = (fH + fL)/2. According to FCC, the UWB range for unlicensed
frequency is 3.1 GHz to 10.6 GHz for both outdoor and indoor environments [5]. Figure 1.1
shows the comparison between ultra wideband communication system and narrowband.
Power Spectral density
(dB)
Narrowband
UWB
-10 dB
fL
fC
fH
Frequency (Hz)
Figure 1.1: Comparison between ultra wideband communication system and narrowband
[4]
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1.4 Features and advantages of UWB
The main advantages of UWB are as follows:
• Channel capacity improvement
As mentioned earlier, UWB utilizes a very large frequency spectrum which improves the
channel capacity C (bits per seconds) or data rate. An increase in radio frequency (RF)
bandwidth also increases the capacity of band limited additive white Gaussian noise (AWGN)
channel [6].
C = BRF log 2 (1 +
Prec
),
BRF ! o
(2)
where C represents the capacity of the channel with its radio frequency (RF) bandwidth BRF.
Prec is the received power signal and !o is the noise of power spectral density (PSD).
• Ability to work with low SNR
The above channel capacity equation also denotes that it logarithmically depends on the signal
to noise ratio. Hence, the UWB system is able to work in rough communication channels with
low signal to noise ratio and provides better channel capacity which is the outcome of large
bandwidth [1].
• Accurate positioning and tracking or radar sensing
Larger the bandwidth, finer is the resolution. One of the key features of UWB technology is to
provide accurate positioning. This application is mostly used in radar sensing to detect the
targets [7].
• High Performance in Multipath Channels
Multipath is an unavoidable phenomena in wireless communication channels. Multipath
reflection of the transmitted signal can be caused when the transmitted signal gets reflected
from several surfaces like trees, buildings, people, etc as shown in figure 1.2. Multipath fading
is the variation in the attenuation of the signal caused when the signal reaches the destination
through multiple paths [1]. The effect of multipath fading is low in UWB communication
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system because UWB has short duration pulses, thus the effect of reflected pulses on the
signal is less degrading [1].
Figure 1.2: Multipath phenomenon in wireless transmission
• Simple Transceiver Architecture
The transmission in UWB communication system is carrier-less. That means the data need not
be modulated over the continuous waveform with any particular carrier frequency. Carrierless transmission needs smaller number of radio frequency components compared to carrierbased transmission [1]. Mixers and oscillators are not required in UWB transceiver to
translate carrier frequency to required frequency band [1]. Because of these reasons, UWB
transceiver system architecture has lower complexity compared to other narrowband
transceivers and is less expensive to design.
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• Resistance to jamming
Processing gain (PG) refers to the resistance of a radio system against jamming. It is identified
as the ratio of RF bandwidth to information bandwidth of the signal.
PG =
RF Bandwidth
Information Bandwidth
(3)
UWB spectrum accommodates a wide range of frequencies and provides a high processing
gain which in turn gives the UWB signals high resistance to jamming.
1.5 UWB Challenges
There are many challenges faced in the UWB technology by using very short pulses for
communications. Here we discuss only the important challenges observed in UWB
communication system [1].
• Channel estimation
The estimation of channel performance is very sensitive issue for designing a receiver in a
wireless communication system. Measuring the exact performance of each channel is not
possible in the field of wireless communication. To estimate channel parameters, it is essential
to employ training sequences such as delays and attenuations of the propagation path. Mostly
UWB receiver associate received signal with predefined signal model, which is not possible
without any prior knowledge of the wireless channel parameter. But, because of the large
bandwidth and lowered signal energy, UWB pulses face harsh distortion which makes channel
estimation very difficult [8].
• Multiple–Access Interference
In a multiuser environment, multiple users send information independently and
simultaneously through a shared transmission medium. On the receiving side, more than one
receiver should be set to separate users and receive information from the particular users. The
interference of multiple users leads to multiple-access interference (MAI). Increase in MAI
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tends to unavoidable noise that considerably degrades the UWB pulses and creates
complications in detection [1].
• Pulse Shape Distortion
UWB pulses can be distorted considerably by the transmission path. According to Friis
transmission formula, received signal power decreases when frequency is increased [1].
Because of the long range of frequencies of UWB spectrum, the received power immensely
changes and distorts the shape of the UWB pulse. This degrades the performance of the UWB
receivers [1].
• Synchronization of High Frequency
The synchronization of time is a very important challenge in UWB systems. But, as the UWB
pulses are tremendously short, flawless synchronization is hard to achieve. In such a case,
major issues can arise due to poor detection of the exact position of the received signal [1].
1.6 Thesis Contribution
This thesis presents introduction, back ground, features and challenges of UWB technology in
chapter 1.
Chapter 2 introduces types of UWB system such as Multicarrier UWB (MC-UWB), Impulse
Radio UWB (IR-UWB) and description of various modulation techniques
Chapter 3 gives brief outline about the different UWB channels and its mechanism as well as
equations for path loss. In addition, IEEE 802.15.4a channel model for low data rate UWB
systems is studied.
Chapter 4 presents the structure and implementation of different IR-UWB receivers such as
Transmitted Reference Shifting (TR) UWB, Frequency Shifted Reference (FSR) UWB and
Code Shifted Reference (CSR) UWB and the simulation results.
In chapter 5 we have discussed about Cooperative UWB communication system. We have
compared and analyzed computer simulation results of cooperative CSR-UWB systems under
different channels.
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CHAPTER TWO
UWB Systems, Modulation and Multiplexing
Techniques
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Chapter 2: UWB Systems, Modulation and
Multiplexing Techniques
2.1 UWB system types
UWB systems can be typically divided into two categories: one based on sending multiple
simultaneous carriers named as Multicarrier UWB and the other based on sending very short
duration pulses with relatively low energy called Impulse radio UWB [9]. These two systems
are discussed in the following sections.
2.1.1 Multicarrier UWB (MC-UWB)
The idea of using multiple carriers in sending UWB signal is to divide the channel bandwidth
into a number of small sub-channels with adequately small bandwidth for the efficient
utilization of the bandwidth of the system [10]. For multi-carrier transmission technique, OFDM
(orthogonal frequency division multiplexing) is employed which allows the sub-carriers to
overlap in frequency without interfering with each other that results an increase in spectral
efficiency [9]. Such a system is called Multi-band Orthogonal Frequency Division Multiplexing
(MB-OFDM) [11]. In such a technique, a spectrum is divided into further smaller subspectrums with a minimum bandwidth of 500 MHz each. The data is then interleaved on these
small sub-spectrums and transmitted into the air using multi-carrier OFDM technique [11].
Thus, by using this system, multiple users can also be supported by the allocation a group of
sub-channel to each user [9]. A high throughput can be obtained by a reliable communication
system by the transmission of multiple streams of data in parallel on separate carrier frequencies
[9].
2.1.2 Impulse Radio UWB (IR-UWB)
In impulse radio UWB systems, transmission is based on a series of discontinuous short pulses
or a pulse wave form which is generally known as monocycle pulses that have relatively low
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energy level [11]. The monocycle waveform could be any function that fulfills the requirements
of spectral mask regulations. Such common pulses comprise of Rayleigh, Laplacian, Gaussian
or Hermitean pulses [12]. A Gaussian monocycle waveform is employed along with Binary
Phase Shift Keying (BPSK) as a data modulation scheme in this thesis. In such systems,
because of the short length of the pulses which is nearly in nanoseconds, the bandwidth of the
transmitted signal is in gigahertz [9]. Such pulses have ultra-wide frequency domain features
which do not need any carrier modulation for propagation in the radio channel [12]. This
approach is usually employed for a single user, but it can also be employed on multiple users by
using the techniques of time-hopping or direct sequence spreading [12].
For the purpose of attaining a proper processing gain which can be employed to handle
noise and different interferences from the environment, a single symbol which has to be
transmitted is stretched over ! number of monocycle pulses [12]. This processing gain can be
expressed as
PG1 = 10 log10 ( ! )
(1)
With the help of pseudorandom (PR) time-hopping code, consecutive pulses are transmitted in
air interface in a discontinuous time-hopped scheme which provides UWB communication
resistance against severe multipath propagation [12]. The short pulse length and relatively
lengthy pulse repetition time helps in reducing the inter-pulse interference. This allows the
multipath components related to the transmitted pulse to be attenuated prior to sending the next
pulse [12]. The inter symbol interference (ISI) can be avoided between the pulses by increasing
the time in between the pulses so that it becomes greater than the channel delay spread [12].
2.2 UWB Pulse Shape
Usually, the pulse shapes implemented in UWB communications consist of Gaussian pulse,
Gaussian monocycle and Gaussian doublet as shown in figure 2.1.
A Gaussian pulse [12] is expressed as
PRe (t ) =
2
1
e − (1 / 2)((t − µ ) / σ )
2πσ
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where σ is the length of the pulse and µ represents the centre of the pulse. The Gaussian pulse
is practically not applicable in wireless communication systems because it consists of a DC
term. But, the higher derivatives of the Gaussian pulse are free from such type of DC terms, and
hence can be practically implemented in wireless communication systems.
The first derivative of Gaussian pulse is regarded as Gaussian monocycle. It is
commonly employed in impulse radio systems. A Gaussian monocycle in time domain can be
expressed as
PG (t ) =
2
 1  t − µ 
1   t − µ   −  2  σ 
 e
1 − 
2π σ   σ  
2
(3)
For Gaussian monocycle, µ = 3.5 σ and the effective time length Tp = 7 σ .
The second derivative of Gaussian pulse is regarded as the Gaussian doublet. It contains
two Gaussian pulses which are opposite in terms of amplitude. A Gaussian doublet in time
domain can be expressed as
1
PGD (t ) =
2π σ
 1   t − µ − Tw 
 − 1  t − µ  2
− 

e  2   σ  − e  2   σ 


2




(4)
where Tw is the time gap between the maxima of each pulse. The effective time length is
T p = 14σ at Tw = 7σ .
1
Gaussian pulse
Gaussian monocycle
Gaussian doublet
Amplitude
0.5
0
-0.5
-1
-2
-1
0
Time [s]
1
X
2
10-9
Figure 2.1: Waveforms for Gaussian pulse, Gaussian monocycle and Gaussian doublet
[12]
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2.3 Modulation Techniques Used in UWB Systems
The process of changing the characteristics of periodic waveform with an external signal by
changing its amplitude, phase, or frequency is known as modulation. In UWB communication
system, different modulation techniques are employed. The most commonly used modulation
schemes are pulse-position modulation (PPM), pulse amplitude modulation (PAM), On-Off
Keying (OOK) and binary phase-shift keying (BPSK). These modulation schemes are discussed
below.
2.3.1 Pulse Position Modulation (PPM)
In Pulse Position Modulation (PPM), the selected bit that is to be transmitted controls the
position of UWB pulse. PPM is concerned with the nominal pulse position. In PPM, two or
more positions in time encode the information, which is shown in the figure 2.2 [12] [13].
Those pulses which are transmitted at nominal position are represented as 0 and the ones that
are transmitted beyond the nominal position are represented by 1 [13]. In figure 2.2, 2-ary PPM
modulation is shown in which one bit is encoded in every impulse [12] [13]. Adding more
positions allows more bits per symbol. In general, the time delay in between the position of
pulses is a fraction of a nanosecond, which is much shorter than the one in between the nominal
positions. This aids in avoiding the interferences among the impulses [13]. For PPM signals, the
signal model is generally expressed as
+∞
s (t ) = ∑ p (t − kT f ± Tpk )
(5)
k =0
where p(t) represents the UWB pulse and little shifts Tpk in pulse position performs the data
modulation [13].
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1
0
1
0
1
t
Figure 2.2: 2-ary PPM signal [14]
2.3.2 Pulse Amplitude Modulation (PAM)
Pulse Amplitude Modulation (PAM) is concerned with the transmission of data in a time
sequence of electromagnetic pulses by changing the power amplitudes or the voltage of
individual pulses. It can also be defined as a technique in which the data to be transmitted is
encoded in the amplitude of a series of signal pulses. The 2-ary PAM signal is illustrated in
figure 2.3 in which the pulse with higher amplitude is represented by 1 and the one with lower
amplitude is represented by 0 [14]. The M-ary PAM signal with different amplitude levels of M
that consists of sequences of modulated pulses is expressed as
s (t ) =
∞
∑a
m
(k ) p (t − kT f )
(6)
k = −∞
where am(k) is the amplitude of the kth pulse that depends on the M-ary information symbol m
{0,1,…,M-1}, Tf is the frame interval and Tp is the pulse duration [14].
1
0
1
0
1
t
Figure 2.3: 2-ary PAM Signal [14]
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2.2.3 Binary Phase Shift Keying (BPSK)
In Binary Phase Shift Keying (BPSK) which is also known as bi-phase modulation scheme, the
binary data is carried in the polarity of the pulses [14]. A positive polarity is given to the pulse
that represents the information bit 1 and a negative polarity is given to the pulse representing the
information bit 0, which is demonstrated in figure 2.4. BPSK is the simplest version of Phase
Shift Keying (PSK) [14]. The Binary Phase Shift keying can be expressed as
s (t ) =
∞
∑ d (k ) p(t − kT f )
(7)
k = −∞
where
1
d (k ) = 
− 1
1
if information bit is 1
if information bit is 0
0
1
(8)
0
1
t
Figure 2.4 BPSK Signal [14]
2.3.4 On-Off keying (OOK)
On-Off keying is also occasionally known as non-return-to-zero (NRZ) encoding. It is a binary
level modulation scheme that contains two symbols with equal probabilities [15]. It is a special
case of PAM, where m belongs to {0,1} with pulse amplitude as am (k) = m(k) [14].
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In OOK, a pulse or signal is transmitted only when the information bit is equal to 1 as shown in
figure 2.5. No pulse or signal is transmitted when the information bit is equal to 0. On-Off
keying is mathematically expressed as:
s (t ) =
∞
∑ m(k ) p(t − kT
f
)
(9)
k = −∞
where m(k) is the pulse amplitude and Tf is the frame time [14].
1
0
1
0
1
t
Figure 2.5 OOK Signal [14]
2.4 Multiple Access Techniques used in UWB System
In single band UWB systems, multiple users share a single UWB spectrum simultaneously. For
accommodating those multiple users, a suitable multiple access technique is required [16].
There are two common multiple access schemes: Time hopping (TH) and Direct Sequence
(DS) spreading, which are used to allow the users in a single band UWB system [16]. The
difference between the two systems is that the TH technique is concerned with the
randomization of the location of the transmitted UWB impulse in time, whereas the DS
technique is concerned with the continuous transmission of pulses comprising a single data bit
[17]. The TH-UWB and DS-UWB are explained in detail in the subsections below.
2.4.1 Time-Hopping UWB (TH-UWB)
In TH-based system, the information verified by the TH sequence is transferred with a time
offset for each pulse [16]. TH-UWB makes use of low duty cycle pulses, where users are time
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multiplexed by spreading the time spreading in between the pulses [16]. Every frame duration is
split into multiple smaller segments of which only one carries the transmitted monocycle of the
user [16]. The user is assigned a unique code known as TH sequence to designate the segment
employed for transmission in every frame interval [16]. As the position of every impulse is
verified by a pseudo-random (PR) code, extra energy is added to the symbol because of which
the range of the transmission is increased [17]. In such a way, the identification of different
users is done by their unique TH-code that allows them to be transmitted at the same time [17].
TH-UWB for the jth users for different modulation schemes of UWB can be expressed as
follows [17] [18]
For PAM modulation:
s ( j ) (t ) =
∞
! s −1
∑ ∑
p (t − kTs − lT f − cl( j )Tc )d k( j )
(10)
p (t − kTs − lT f − cl( j )Tc − d k( j )δ )
(11)
k = −∞ l = 0
For PPM modulation:
s
( j)
(t ) =
∞
! s −1
∑ ∑
k = −∞ l = 0
In these equations dk is the k-th data bit of jth user, !s is the number of impulses transmitted for
every information symbol, Ts is the total symbol transmission time that is divided into !s frames
each of duration Tf and each frame is itself subdivided into slots of duration Tc [17]. The PR TH
code sequence cl (unique for the j-th user) determines the position of one impulse in each frame
to be encoded as shown in the figure 2.6 [17]. Because of the blank transmission in case of 0th
bit, OOK cannot be employed in TH spreading [17].
Ts
t
Tc
Tp
Tf
Figure 2.6: TH-UWB Signals [17]
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2.4.2 Direct Sequence UWB
In Direct Sequence UWB system, data is carried by multiple pulses whose amplitudes are based
on a certain spreading code [16]. DS-UWB uses a train of high-duty-cycle pulses whose
polarities are based on pseudo-random code sequences [16]. Each user in the system is
specifically assigned a pseudo-random sequence that regulates pseudorandom inversions of the
UWB pulse train [16]. This sequence of UWB pulses uses a data bit to modulate. This results in
the transmission of continuous UWB pulses whose number depends on the length of the pulses
itself and a system defined bit rate [17]. DS-UWB scheme is only applicable in PAM, OOK and
PSM modulation. It is not suitable for PPM as it is a time-hopping technique [17] [16]. For
PAM and OOK modulation, DS-UWB can be expressed as following [17][18],
s
( j)
(t ) =
∞
! s −1
∑ ∑
p (t − kTs − lTc ) − cl( j ) d k( j ) )
(13)
k = −∞ l = 0
where dk is the k-th data bit, cl is the l-th chip of the PR code, p(t) is the pulse waveform of
duration Tp, Tc is the chip length which is equal to Tp as shown in figure 2.7, !s is the number of
pulses per data and j is the user index [17]. The PR sequence has values in {-1,+1} and length of
the bit is Ts = !sTc [17].
Ts
t
Tc = Tp
Figure 2.7: DS-UWB Signals [17]
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CHAPTER THREE
Ultra Wideband Wireless Channels
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Chapter 3: Ultra Wideband Wireless Channels
3.1 Propagation Mechanism and Channel Characteristics
The design and analysis of UWB communication systems are based on the propagation features
of UWB radio channels [18]. Reflection, diffraction and scattering are the main propagation
mechanism in communication [24]. Reflection occurs when a propagating signal impinges on
an object which has comparatively large dimensions than the propagating signal. Reflection
may occur at surfaces of the floor, walls and buildings [24]. Diffraction occurs when the radio
path between the transmitter and the receiver is obstructed by objects with sharp edges. This
results in the bending of signal around the obstacle, even when the line of sight exists between
transmitter and receiver. Scattering occurs when the signal passes through a medium which
contains objects that have very small dimensions compared to the wavelength, and when the
number of obstacles per unit volume is quite large [24].
In this chapter, we will discuss about UWB wireless channels. The natures of these UWB
wireless channels are very important and helpful while designing UWB communication system
to predict the coverage of the signal, to reach maximum data rate, to find optimal location of
antennas and for efficient modulation [27].
3.1.1 Multipath
The purpose of any communication system is to convey the message from transmitter to
receiver. A transmitted signal in wireless communication takes multiple paths to reach the
receiver which causes multipath effects because of reflections from objects like mountains,
buildings, water bodies, etc. Multipath effect includes constructive and destructive interference
at the receiver antenna and phase shifting of these multipath components of the signal causes
multipath fading [27]. Fading is a common problem that occurs in a propagating wireless signal.
Any fluctuation in the received signal is referred to as fading. If fading occurs due to multipath
then it is referred as multipath induced fading [19] [20]. Fading is classified into two types:
slow fading and fast fading. Slow fading arises when the coherence time of the channel is
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relatively larger than the delay constraints of the channel and fast fading arises when the
coherence time of the channel is relatively small than the delay constraint of the channel [21].
The fast fading transmitter takes the advantage of both channel variation and channel conditions
by the use of time diversity that helps to increase the strength of the communication signal.
Whereas in case of slow fading, time diversity cannot be used as an advantage due to single
realization of the channel within its delay constraint [22].
3.1.2 Delay Spread
When a signal transmits via a time-dispersive multipath channel, the signal arrives to the
receiver from different paths. This is the cause of delay spread. Delay spread depends on the
distance and the position of objects near the transmission path. Delay spread can be interpreted
as the difference between the arrival time of the first and last multipath components [23] [19].
Delay spread leads to inter symbol interference (ISI). ISI is a form of distortion in the
communication channels. In practice, communication channels have limited bandwidth, hence
the transmitted pulses spread during transmission. This spreading of pulses causes overlap over
the adjacent time slot that causes errors at the receiver. This phenomena is referred to as inter
symbol interference which is shown in figure 3.1.
Figure 3.1: Inter Symbol Interfernce (ISI) in digital transmission
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3.1.3 Coherence Bandwidth
Coherence Bandwidth is a range of frequencies that are allowed to pass through the wireless
channel without any distortion. It can be regarded as a statistical measurement of the frequency
ranges on the channel which can be assumed as “flat” [24]. Alternately, coherence bandwidth
can be suggested as an approximate highest bandwidth at which two frequencies of the signal
will possibly go through similar or correlated amplitude fading [25]. Multipath interference can
be avoided by decreasing the signal bandwidth so that it is less than the coherence bandwidth
[19].
BWC =
1
2πTd
(1)
3.2 Channel Fading Distributions
Channel fading distribution refers to the factors or conditions that distort the signal when it
transmits from source to destination through the channel. The performance of the channels plays
a very vital role in transmission [19].
3.2.1 Gaussian Channel
The Gaussian channel is particularly used in modeling the noise produced at the receiver when
the transmission path is ideal [19]. This model is moderately correct in few cases like wired
communications transmissions and space communications. This channel model is appropriate
for channels with single transmitter and single receiver. A condition when the information is
sent through a channel that can be subjected to an additive white Gaussian noise is
demonstrated in figure 3.2.
Xi
Channel
Encoder
+
Channel
Decoder
Continuous values
Noise (Zi)
Figure 3.2: Gaussian channel model
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Here, Yi is the output of the channel, Xi is the input of the channel and Zi is the noise which is
assumed to be independent of X. Zi is zero mean Gaussian with variance !: Zi ̴ N (0,!).
3.2.2 Rayleigh Channel
Rayleigh channel is a transmission channel which has a fading envelope in the form of Rayleigh
probability density function (pdf). Rayleigh fading occurs in an environment where there are
several obstacles that scatter the transmitted signal before it reaches the receiver [26]. Rayleigh
fading distribution is generally used to describe the statistical time. It varies in the nature of the
received envelope of flat fading. The envelope is a sum of two quadrature Gaussian noise signal
which follows the Rayleigh distribution [24].
3.2.3 Ricean Channel
Ricean channel is a transmission channel that has a line of sight (LOS) propagation path along
with a small scale fading envelope distribution [24]. In this channel, the signal reaches the
receiver at different angles or paths that result in multipath interference. Ricean fading takes
place when one LOS path signal is stronger than the others. The strong signal arriving with
several weak multipath signals results Ricean distribution [24]. Complex signals resemble noise
signals that have enveloped in Rayleigh channel. Ricean distribution degenerates to Rayleigh
distribution when the dominant signal fades away [26].
3.3 Ultra Wideband Channels
As mentioned in chapter 1, the UWB systems provide a promising technological application
across several commercial fields and military applications including radar, communication, and
medical instruments. This technology offers very high data rates to several users during short
range communication channels by allocating a large bandwidth. UWB channels demonstrate
two significant effects: pulse distortion and multipath propagation. Particularly, in the course of
propagation, each waveform can be discarded by any object that results in multipath
propagation. Pulse distortion is rather concerned with the variation in the original UWB pulse.
The main difference between narrow band channel and UWB channel is different radiation
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bandwidth ranges. The narrowband is used to cover less than 20 MHz of bandwidth, where as
UWB channels can cover more than 10 GHz of bandwidth [27] [29]. The standard UWB
channel models are designed by IEEE 802.15 groups: SG3a and SG4a which are known as
IEEE 802.15.3a and IEEE 802.15.4a channel models, respectively.
3.4 IEEE 802.15.4a UWB Channel Model
IEEE 802.15.4a UWB channel model provides a broad range of channel environments such as
industrial, residential, office and outdoor. It covers a frequency range from 2 GHz to 10 GHz.
[28]. This model suggests data rates from 1Kbps to several Mbps. The important features of
IEEE 80215.4a are:
3.4.1 Path Loss
Path loss is the ratio between transmit and receive signal power. In an end-to-end wireless
communication system, a transmitter communicates with a receiver by sending a signal over the
wireless medium. The signal strength attenuates when it travels through the medium. Thus it
becomes poorer or weaker as the propagation distance increases. The signal beyond a certain
distance becomes unacceptable. Then at a regular interval to reactivate the signal strength, we
need a booster or repeater. More challenging problems will occur when there are multiple
receivers in the communication and more complexity will arise when distance from transmitter
to receiver is varying [20]. Basically, path loss can be defined as:
PL =
Pt
Pr
(2)
where Pt is the transmitted power and Pr is the received power. Path loss in narrow band can be
defined as:
PL(d ) =
E{PRX (d , fc)}
PTX
(3)
where PTX is the transmit power and PRX is the receive power, d is the distance between the
transmitter and receiver, fc is the center frequency and E{} is expectation to averaging
shadowing and small scale fading [30].
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3.4.2 Shadowing
Shadowing is the effect that arises upon the received signal power when it is attenuated because
of the obstacles in the propagation path in between the transmitter and the receiver. The nature
of shadowing in UWB communication is similar to that of narrowband systems [29] [30]. The
average path loss evaluated over a small scale fading in dB is given as [30]:
d 
PL(d ) = PL0 + 10n log10   + S
 d0 
(4)
where, S denotes the shadowing Gaussian noise distributed random variable with zero mean and
the standard deviation ߪs.
3.4.3 Power delay Profile
Power delay profile demonstrates the quality of the received signal passing through a multipath
channel as a function of time delay. The time delay is the difference of travel time with
multipath arrivals. Power delay profile can be described as the squared magnitudes of impulse
response by spatial averaging along a local area [24],
PDP(τ ) = h(t;τ )
2
(5)
where, |h (t; )| is the modulus value of the impulse response of the signal. With the help of this
impulse response, we can get the received signal power as [27],
! −1
PDP (τ n ) = E{ h(t ) } = ∑ α n2δ (t − τ n )
2
n =0
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Generally, the paths which come at the later stage in the power delay profile go through more
attenuation. Consequently, the power delay profile is usually a descending function of the
excess delay. Figure 3.3 shows multipath components with different delays and attenuations.
Amplitude
Delay
Figure 3.3: Multipath components with different delays and attenuations
3.4.4 Saleh and Valenzuela
Saleh-Valenzula is a simple multipath model developed for indoor propagation measurements.
The basic postulation of this model is the arrival of multipath components (MPC) in the form of
clusters. The MPC amplitudes are independent random Rayleigh variables with variance. The
variance decays exponentially with both the cluster and excess delays within a cluster. The
forming of clusters is concerned with building structure. The components inside a cluster are
made from multiple reflections from objects. The clusters and MPC within the cluster that can
be derived according to Poisson arrival processes with different rates have exponentially
distributed inter-arrival times [24].
IEEE 802.15.4a model is based on Saleh-Valenzuela (SV) model which is shown in
figure 3.4. In complex baseband, the impulse response based on SV model is defined as [31].
L −1 K −1
hdiscr (t ) = ∑∑ α k ,l exp( jφ k ,l )δ (t − Tl − τ k ,l ),
l =0 k =0
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where, α k ,l is the tap weight of the kth component in the cluster, Tl is the arrival time of the lth
cluster, τ k ,l is the delay of the kth multipath component relative to lth cluster arrival time and
φk ,l denotes the uniformly distributed phases. For a band pass system, the phase angle is taken as
uniformly random distributed in a range from 0 to 2π [30].
An important component of the model which is the number of clusters is represent by L,
which is supposed as Poisson-distributed.
[
]
p(TL TL−1 ) = ∧ l exp − ∧ l (TL − TL−1) ,
l>0
(8)
where, ˄l is arrival rate of the cluster.
The arrival times of the ray are modeled with a mixture of two Poisson processes as follows,
[
]
[
]
P (τ k ,l τ ( k −1),l ) = βλ1 exp − λ1 (τ k ,l − τ ( k −1),l ) + ( β − 1)λ2 exp − λ2 (τ k ,l − τ ( k −1),l ) ,
k >0
where, β is the mixture probability, and λ1 and λ2 are the ray arrival rates [30].
The mean power of different component’s exponential within each cluster is given as [30]
2
1
E  α k ,l  = Ω1
exp( −τ k ,l / γ l )
γl [(1 − β )λ1 + βλ2 + 1]


(10)
Amplitude
Clusters
Γ
Delay
1/ λ
1/∧
Figure 3.4: Principle of Saleh–Valenzuela model
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where, Ω1 is the integrated power of the lth cluster and γl is the intra-cluster decay time constant.
The mean (over small-scale fading) mean (over cluster-shadowing) energy (normalized to γl) of
the lth cluster adopts a general exponential decay which can be expressed as [30]
10 log(Ω1 ) = 10 log(exp(−T1 / Γ)) + M cluster
(11)
where, M cluster is a normal distribution random variables with ߪ cluster standard deviation around
it [30].
The scenarios for the non line of sight (NLOS) define the shapes of the power delay profiles
differently [29].
{
E α k ,l
2
}= (1 − χ .exp(−τ
( k ,l )
/ γ rise )). exp(−τ ( k ,l ) / γ 1 ).
γ 1 + γ rise
Ω1
γ 1 γ 1 + γ rise (1 − x)
(12)
The parameter χ represents the attenuation of the first component, γrise determines how fast the
power delay profile γ increases to its maximum and γ1 determines the decay at the last time.
3.4.5 Delay dispersion
Delay dispersion can be said to be occurring when the channel impulse response remains for a
finite quantity of time or the channel happens to be frequency-selective [32]. The effect of delay
dispersion can be expressed as the product of the delay spread and the bandwidth of the system.
If this product is below unity, then its delay dispersion effect will be low on the system design.
And, if the product is higher than unity, then it is said to have a strong delay dispersion effect in
the system performance [18]. In multipath channel, delay dispersion is featured by two
parameters: root mean square (rms) delays spread and mean excess delay [32]. The mean excess
delay is the first moment of the power delay profile (PDP) according to [30].
∞
τm =
∫ PDP(τ )τdτ
−∞
∞
∫ PDP (τ )dτ
−∞
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The rms delay spread is the second moment of the power delay profile (PDP) according to [30].
∞
τ rms =
 ∞ PDP(τ )τdτ
∫
∫−∞
−  −∞∞
∞
 ∫ PDP(τ )dτ
∫−∞ PDP(τ )dτ
 −∞
PDP(τ )τ 2 dτ





2
(14)
Delay spread depends upon the distance; nevertheless, for channel simplicity it is neglected.
3.4.6 Small scale fading
Small scale fading refers to the changes in amplitude, multipath delays or phase of the received
signals over a short period of time [29]. Small scale fading takes place because of destructive
and constructive interference of multipath components that reaches the receiver at fairly
different times [29]. The distribution of small scale amplitudes is Nakagami in this model [30].
pdf ( x ) =
2  m  m 2 m−1
 m 
exp − x 2 ,
  x
Γ ( m)  Ω 
 Ω 
(15)
where m ≥ 1/2 is the Nakagami m factor, gamma function is Γ (m), and mean square value of
amplitude is Ω. The parameter m modeled as a log generally random distributed variable. Both
values of logarithmic mean µm and standard deviation ߪm [30].
µ m (τ ) = mo − k mτ
^
(16)
^
σ m = mo − k m τ
Nakagami factor is deterministic and delay independent [30].
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CHAPTER FOUR
IR-UWB Receivers
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Chapter 4: IR-UWB Receivers
4.1 Introduction
The immense bandwidth of UWB systems can make the design of the receiver quite difficult in
conventional UWB systems that use antipodal or pulse-position modulation with very short
pulses [33] [34]. The analog to digital transformation of the whole signaling bandwidth in
simple low-power UWB receivers is very hard to implement [34]. Many digital UWB receivers
have certain number of analog correlators to accumulate signal energy in a front-end RAKE
receiver type architecture [34]. The efficient accumulation of energy in that kind of architecture
can be expensive because of a large number of resolvable paths in the standard UWB fading
environment. It can create problems in channel estimation even if allowable in the perspective
of circuit complexity [34]. Because of these problems encountered in traditional impulsive
UWB or DS-UWB, the approach regarding multiband UWB for short-range high data rate
applications has been favored [34]. In the following sections, three reference-based noncoherent IR-UWB systems, i.e., Transmit-Reference (TR), Frequency-Shifted Reference (FSR)
and Code-Shifted Reference (CSR) systems are discussed.
4.2 Transmitted Reference (TR) UWB System
As UWB systems have pulses of short duration and they are characterized by limited power,
these characters cause extreme dependence of these systems on timing requirements [44]. These
difficult timing requirements make receiver design complicated. In such a situation,
Transmitted-Reference (TR) UWB systems can offer a “simple” and cheap receiver that collects
the energy from various multipath components for the correct detection of UWB data [35].
In TR-UWB systems, some amount of the transmitted energy is used for measuring
channel [36]. Each frame of the transmitted signals contains two different pulses which are
reference and data [37]. The reference pulse has a fixed polarity. The polarity of the data pulse
indicates the data bit [38]. These two segments of the signal are separated in time domain. The
transmitted TR-UWB signal can be mathematically expressed as the following [39],
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x (t ) =
∞
! f −1
∑ ∑ ( p[t − ( j!
f
+ i )T f ] + b j p[t − ( j! f + i )T f − D ])
(1)
j = −∞ i = 0
where p (t ) is a UWB pulse with duration Tp , T f is the frame length, ! f is the number of
frames, ! f >>1, b j ∈ {1,-1} is the information bit transmitted during j th ! f T f time duration.
D is the delay between the reference and data pulse. There is a UWB pulse per each frame
interval [39]. At the standard TR-UWB receiver side shown in figure 4.1, the received signal is
filtered and correlated with a delayed version of itself. The correlated signal is integrated from
( j! f + i )T f to ( j! f + i )T f + TM to constitute a decision variable [48]. The value of TM ranges
from Tp to T f .
r (t )
∫
Matched
Filter
( j! f + i )T f +TM
( j! f + i )T f
! f −1
∑
r̂ j
sign (rˆj )
b̂ j
i =0
Delay
D
Figure 4.1: Block diagram of the TR-UWB receiver [34]
4.3 Frequency-Shifted Reference (FSR) UWB System
The TR-UWB architecture is able to provide a simple receiver design for a UWB system.
However, the implementation of TR-UWB receiver can be quite a challenge. In a low-power
integrated circuit environment desired by the TR-UWB receiver, it is hard to construct the delay
that handles a wideband signal [34]. To solve this complexity problem of the delay element, a
new system called Frequency-Shifted Reference Ultra-Wideband (FSR-UWB) is introduced
[34]. The main idea behind FSR-UWB is to propose a TR-UWB system in the frequency
domain which excludes the delay element of the standard TR-UWB receiver. Frequency
translation of a wideband signal is much easier than implementation of its delay element. In
FSR-UWB, the reference signal is translated in frequency (instead of time) to be orthogonal to
the data signal [40].
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The key principle proposed in [34] is to enforce the frequency shift of the data signal
relative to the reference signal over a symbol period rather than over a frame period. This
permits a significant overlapping of the frequency bands occupied by the data-bearing and
reference signal. The transmitted FSR-UWB signal can be mathematically expressed as the
following:
x(t ) =
∞ ! f −1
∑ ∑ (p[t − ( j!
f
)
+ i )T f ] + b j 2 cos( 2πf 0t ) p[t − ( j! f + i )T f ]
(2)
j =−∞ i =0
where p (t ) is a UWB pulse with duration Tp , T f is the frame length, ! f is the number of
frames, ! f >>1, f 0 = 1 / Ts is the frequency shift of the data signal relative to the reference signal,
b j ∈ {1,−1} is the information bit transmitted during j th ! f T f time duration [39]. Figure 4.2
shows the FSR-UWB receiver structure. The detailed explanation about FSR-UWB receiver is
given in [34].
r (t )
∫
Matched
Filter
( j! f + i )T f +TM
( j! f + i )T f
! f −1
∑
r̂ j
sign (rˆj )
b̂ j
i =0
2 cos( 2πf 0 t )
Figure 4.2: Block diagram of the FSR-UWB receiver [34]
A modified form of the traditional FSR-UWB is proposed in [41] which is called MultiDifferential (MD) FSR-UWB, where multiple data carriers use a single reference carrier. Every
data signal is a slightly frequency-shifted version of the reference signal [41]. The data carrier
frequencies are cautiously selected such that all data signals and the reference signal are
orthogonal to each other over the symbol period [41]. This adjustment expands the amount of
freedom accessible for signaling in the system [41]. The transmitted MD FSR-UWB signal for
M carriers can be mathematically expressed as following [41],
x(t ) =
∞ ! f −1

∑ ∑  p[t − ( j!
j =−∞ i =0
f
M

+ i )T f ] + ∑ b jk 2 cos(2πf k t ) p[t − ( j! f + i )T f ] 
k =1

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where b jk ∈ {1,-1} is the k th information bit transmitted over j th ! f T f time duration. The
carrier frequency of the k th data signal is expressed as f k = (2k + 1) / Ts .
Figure 4.3 shows the MD FSR-UWB receiver structure. The detailed explanation about
MD FSR-UWB receiver is given in [41]. For moderate data rate applications, the FSR-UWB
scheme performs better than the TR-UWB scheme. However, it is not preferable for high data
rate systems, because of the presence of intersymbol interference [41]. Apart from this, when
there are many users in the system, the frequency oscillator requires a lot of power [34].
cos(2πf1t )
! f −1
r (t )
Matched
Filter
rˆ(t )
∑
( . )2
rˆ j1
sign (rˆ j1 )
bˆ j1
i =0
cos(2πf k t )
∫
( j! f + i )T f +TM
! f −1
rij
∑
( j! f + i )T f
r̂ jk
sign (rˆ jk )
b̂ jk
i =0
cos(2πf M t )
! f −1
∑
r̂ jM
sign (rˆ jM )
b̂ jM
i =0
Figure 4.3: Block diagram of MD FSR-UWB receiver [41]
4.4 Code-Shifted Reference (CSR) UWB System
Recently a new scheme called Code-Shifted Reference (CSR) has been proposed for IR-UWB
systems. In this scheme, the reference and data pulse sequences are separated by a set of shifting
and detection codes instead of getting separated by time (TR) or frequency (FSR) [33]. In CSRUWB, a reference pulse sequence and a single or multiple data pulse sequences are
instantaneously transmitted [42]. Every pulse sequence is coded by a particular shifting code. A
set of detection codes are used to detect the information bits from the data pulse sequences at
the CSR receiver side. The CSR scheme has been able to remove the ultra wideband delay
element required in the TR-UWB transceiver because the separation of the reference pulse
sequence and the data pulse sequences is performed by the employment of code shifting rather
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than time shifting [43]. As the CSR-UWB transceiver separates pulse sequences with digital
codes in place of analog carriers used in FSR-UWB, it reduces a lot of degradation that is found
in the performance of the FSR-UWB system [43]. Moreover, it offers a lower system
complexity. Figure 4.4 shows the block diagram of the CSR-UWB transmitter proposed in [42].
b j1
1
ci 0
b jM
b jk
ci1
cik
ciM
|.|
UWB
Antenna
M
UWB
Pulse
Generator
Figure 4.4: Block diagram of the CSR-UWB transmitter [42]
The CSR-UWB transmitted signal can be mathematically expressed as following [42],
x(t ) =
∞
! f −1
∑ ∑ p[t − ( j!
j = −∞ i = 0
M
f
+ i )T f ] M ci 0 + ∑ b jk cik
(3)
k =1
where p (t ) is a UWB pulse with duration Tp , T f is the frame length, ! f is the number of
frames, b jk ∈ {1,-1} is the kth information bit transmitted over j th ! f T f time duration, and
cik ∈ {1,-1} is the ith bit of kth
shifting code selected from Walsh codes. M number of
information bits are transmitted instantaneously over !f number of frames of UWB pulses.
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Equation (4) gives the M+1 shifting codes that separate a reference pulse sequence and M data
pulse sequences.
c0   c00
  
  
c  =  c
 k   0k
  
  
cM  c0 M



c( ! −1) k 



c( ! −1) M 

ci 0
c( !
cik
f
−1) 0
(4)
f
ciM
f
Figure 4.5 shows the CSR-UWB receiver. The received UWB signal after being filtered is
squared. It is then integrated over the limits of (j!f+i)Tf to (j!f+i)Tf+TM. TM ranges from Tp to
Tf. Larger the TM, larger will be the collection of signal energy, but with larger amount of added
noise and interference. The integrated signal is respectively correlated with M number of
detection codes to detect the M number of information bits.
~
ci1
! f −1
r (t )
Matched
Filter
∑
rˆ(t )
(.)
rˆ j1
sign (rˆ j1 )
bˆ j1
i =0
2
~
cik
∫
( j! f + i )T f +TM
! f −1
rij
∑
( j! f + i )T f
r̂ jk
sign (rˆ jk )
b̂ jk
i =0
~
ciM
! f −1
∑
r̂ jM
sign (rˆ jM )
i =0
Figure 4.5: Block diagram of the CSR-UWB receiver [42]
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Equation (5) gives the M detecting codes in the CSR-UWB receiver.
c~0   c~
   00
  
~   ~
ck  =  c0 k
  
  ~
c~M  c0 M
  
~
ci 0
~
cik
~
ciM
~
c( !



~
c( ! −1) k 


~
c( ! −1) M 

f
−1) 0
(5)
f
f
A maximum of M=2!-1 number of information bits can be simultaneously transmitted for !f =2!
number of frames. Table 1 shows an example of the selection of the M+1 shifting codes and M
detection codes from Walsh codes [42].
Code Length
!f=2
Shifting Codes
c0 = [1,1]
Detection Codes
c~1 = [1,-1]
c1 = [1,-1]
!f=4
c0 = [1,1,1,1]
c1 = [1,-1,1,-1]
c~1 = [1,-1,1,-1]
c~ = [1,1,-1,-1]
2
c2 = [1,1,-1,-1]
!f=8
c0 = [1,1,1,1,1,1,1,1]
c1 = [1,-1,1,-1,1,-1,1,-1]
c2 = [1,1,-1,-1,1,1,-1,-1]
c3 = [1,1,1,1,-1,-1,-1,-1]
c~1 = [1,-1,1,-1,1,-1,1,-1]
c~ = [1,1,-1,-1,1,1,-1,-1]
2
c~3 = [1,1,1,1,-1,-1,-1,-1]
c~ = [1,-1,-1,1,-1,1,1,-1]
4
c4 = [1,-1,-1,1,-1,1,1,-1]
Table 1: Example of shifting and detection codes selected from Walsh Codes [42]
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4.5 BER performance comparison of the TR-UWB, the FSR-UWB
and the CSR-UWB systems
The simulations have been performed with bit error rate (BER) of the transceivers as a function
of Eb/N0 under the IEEE 802.15.4a industrial LOS channel environment. The fame duration of
TR-UWB is Tf = TM = 120ns with Td = 60ns. The fame duration of FSR-UWB and CSR-UWB
is Tf = TM = 60ns. The number of frames is fixed at !f = 8. The pulse duration is Tp = 1ns. The
data rates at M=1, 2, 3 and 4 are 2Mbps, 4Mbps, 6Mbps and 8Mbps respectively. Figure 4.6
shows the BER performance of TR-UWB, MD FSR-UWB and CSR-UWB. Two observations
that can be obtained by looking at the plot are:
I. When the value of !f is fixed, the BER performance of the CSR-UWB system becomes
better with the increasing value of M. The reason for this is that the selection of the shifting
and detecting codes assures that all of the information bits are transmitted orthogonally to
each other. Thus, when the value of M is incremented, there is also an increment in the
power of reference pulse sequence. The power of the reference pulse sequence is shared
between all the data pulse sequences, but this does not bring in any additional interference
amongst the data pulse sequences. The CSR-UWB transceiver attains its best BER
performance at M=!f/2. At this point, the BER performance of the CSR-UWB system is
equivalent to that of the TR-UWB system.
II. While TM=Tf, the BER performance of CSR-UWB transceiver under a multipath channel is
equivalent to that of the FSR-UWB transceiver under the AWGN channel. Hence the BER
performance of CSR-UWB transceiver under a multipath channel is better than that of the
FSR-UWB transceiver under the multipath channel. The reason behind this is the existence
of phase offsets in between the analog carriers that are reproduced by the FSR-UWB
receiver because of the delay spread of the multipath channel, and the corruption of the
analog carriers in the received data pulse sequences by multipath channel [34] [41]. In the
case of CSR-UWB transceiver, there is the absence of phase offsets in between the detection
codes produced by the receivers and the shifting codes in the received data pulse sequences
as the shifting and detection codes are always constant (i.e, either 1 or -1) within a pulse
duration.
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0
10
-1
BER
10
-2
10
-3
10
CSR-UWB (M=1)
CSR-UWB (M=2)
CSR-UWB (M=3)
CSR-UWB (M=4)
FSR-UWB (M=1)
FSR-UWB (M=2)
FSR-UWB (M=3)
FSR-UWB (M=4)
TR-UWB
-4
10
13
14
15
16
17
18
19
Eb/No (dB)
Figure 4.6: BER performance comparison of TR-UWB, FSR-UWB and CSR-UWB at
TM=Tf and f=8
Better BER performance can be attained by differing the value of TM in between Tp and Tf
rather than fixing it at Tf. Figure 4.7 shows the plot for the BER performances of CSR-UWB
transceiver under an IEEE 802.15.4a LOS industrial channel for Tf=60ns, M=4, !f=8 and
different values of TM. It can be observed that the BER performance of the CSR-UWB
transceiver improves with the decrease in the value of TM until it reaches 5ns. This is because,
decreasing the value of TM avoids unnecessary noise introduced by the channel. But after a
point, when TM is significantly small, there is loss in the original information itself. On the other
hand, the integration time should be fixed at Tf in the case of the FSR-UWB transceiver, for the
correlation of the reproduced analog carriers at the receiver with the ones in the received data
pulse sequences [34] [41]. Eventually, the BER performance of the CSR-UWB transceiver is
much higher than the BER performance of the FSR-UWB transceiver except when TM is near to
Tf , in a multipath channel having serious delay spread.
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0
10
-1
10
-2
BER
10
-3
10
FSR-UWB, Tm=60ns
CSR-UWB, Tm=60ns
CSR-UWB, Tm=30ns
CSR-UWB, Tm=15ns
CSR-UWB, Tm=7.5ns
CSR-UWB, Tm=5ns
-4
10
-5
10
13
13.5
14
14.5
15
15.5
16
16.5
17
17.5
18
Eb/No (dB)
Figure 4.7: Improvement in BER performance of the CSR-UWB transceiver for TM<Tf
It has been seen that CSR-UWB transceiver has numerous benefits over TR-UWB
transceiver and FSR-UWB transceiver. The system complexity of CSR-UWB transceiver is low
in compare to TR-UWB and FSR-UWB transceivers. The CSR-UWB transceiver does not use
any delay elements of ultra wide bandwidth or any analog carriers. The CSR-UWB transceiver
has a higher BER performance than FSR-UWB transceiver. The CSR-UWB transceiver has a
BER performance which is equivalent to that of the TR-UWB transceiver when M=!f/2. The
CSR-UWB transceiver can use more than two pulses to transmit information bits without
degrading its BER performance. Thus it is much more flexible regarding frame design than TRUWB transceiver. CSR-UWB transceiver shifts the UWB pulses by the use of digital codes
with various discrete values rather than analog carriers with continuous values. Therefore, less
power is required by the CSR-UWB transceiver than the FSR-UWB transceiver. Ultimately, all
of these benefits prove the CSR-UWB as an extremely desirable scheme to build less complex
and low power transceivers for wireless communications.
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CHAPTER FIVE
Cooperative UWB Communication System
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Chapter 5: Cooperative UWB Communication
System
5.1 Introduction
Impulse Radio UWB (IR-UWB) is capable of high speed information transmission,
immense multipath resolution, low power expenditure and is highly cost efficient [45]. These
features have made IR-UWB very popular in wireless communication. Federal Commission of
communication (FCC) has set a standard according to which the average transmitted power of
the UWB signal is pretty low [46]. The power of the received signal decreases after its
transmission through multipath fading channel, which makes it difficult to detect and
demodulate the UWB signals [45]. Therefore, cooperative communication technique has been
introduced in UWB system for efficiently increasing the power at the receiver side and upgrades
the performance of the UWB system [47]. Here, based on IEEE 802.15.4a channel model, we
have implemented cooperative communication with the CSR-UWB system using decode and
forward (DF) relay method, and the BER performance of the system is evaluated using different
scenarios.
5.2 What is Cooperative Communication?
The benefits of multiple-input multiple-output (MIMO) systems have been so largely
recognized that certain transmit diversity techniques have become a very important part of
wireless standards [48]. However, transmit diversity might not be a practical scheme for other
scenarios, even though it is highly beneficial for cellular base stations [49]. Wireless agents
might not be capable of supporting multiple transmit antennas because of certain factors like
cost, size and limitations of hardware [49]. This is the reason why cooperative communication
was introduced. Cooperative communication allows single-antenna mobiles to possess some of
the advantages of MIMO communication systems [50]. The basic concept of cooperative
communication is that single antenna systems within a multi-user set-up can share their
antennas in such a style that a virtual MIMO system is created [51].
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It has been observed that channels in a wireless scenario are subjected to fading which
means that the signal strength can decay noticeably during the course of transmission [52].
Diversity can be generated by transmitting independent copies of the signal, and this can
efficiently reduce the injurious effects of fading [52]. Particularly, by the transmission of signals
from different locations, spatial diversity is generated. This gives different independent faded
copies of the signal at the receiver [53]. This diversity can be generated in a new and exciting
fashion by cooperative communication.
The idea of cooperative communication is to promote the broadcast feature of wireless
communication networks, in which the neighboring nodes “overhear” the signal from the source
and then relay the information to the destination [54]. In figure 5.1, A third-party terminal acts
as a relay by receiving the signals from the source and forwarding the overheard information to
the destination to expand the capacity and upgrade the reliability of the direct communication
[53]. The end-to-end transmission is separated into two different phases in time domain which
are: broadcasting and relaying [55]. In the broadcasting stage, all receiving terminals (i.e. relays
and destination) operate in the same channel (i.e. time or frequency). In the relaying stage, the
transmitting terminals (relay nodes) may work in separate channels to dodge co-channel
interference [55].
Relay
R
S
D
Source
Destination
Figure 5.1: Basic cooperative communication comprising a single relay
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5.3 Cooperative Communication Protocols:
Processing Modes of Relays
The basic concept of cooperative relaying is that the signal is transmitted by the source to both
the relay and destination [51]. The relay receives the same signal from the source and then
retransmits it to the destination. The destination merges the received signal from both the relay
and source to boost reliability. This whole process can be carried out by various methods of
relaying protocols which are discussed in the following subsections.
5.3.1 Decode-and-Forward (DF)
In decode-and-forward scheme, using regenerative method, the relay node decodes the signal
received from the source, and then re-encodes it prior to forwarding it to the destination [51].
Possibly wrongly decoded information at the relay can considerably lower the performance of
the system because of error propagation [56]. Therefore, it is supposed that, relays helps direct
communication only if the source signal has been detected correctly. It is assumed that cyclic
redundancy check (CRC) code to be capable of perfectly decoding the information. Such a relay
using the approach of CRC can be called as adaptive DF [57]. Nevertheless, this approach is not
always practical because the relay is sometimes not capable of correctly detecting the signal
from the source. Hence, another approach called fixed DF mode is introduced where the relay
always forwards the decoded information to the destination irrespective of the received signal
quality [57]. When the quality of the channel between the source and relay is very fine, the relay
is capable of decoding very quickly and correctly.
5.3.2 Amplify-and-Forward (AF)
In amplify-and-forward scheme, using non-regenerative method, the relay node amplifies the
signal received from the source without decoding, and then forwards it to the destination [59].
The noisy form of the signal from the source is multiplied by the relay with the amplifying gain
with a constraint (e.g. power constraint) and the resulting version of the signal is transmitted to
the destination [59]. The complexity of hardware is lower in AF than DF as the decoding
section is excluded in AF [51]. Even though the noise is also amplified along with the signal,
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the destination can still make a better detection of the information as it receives two
independent faded versions of the signal [60]. AF relay can be further divided into two
subcategories. If the relay has complete awareness about the channel state information (CSI),
the amplify gain can be changed [51]. Such a relay is called variable-gain AF relay or CSIassisted AF relay. Whereas, if the relay needs only the statistical characteristics of the channel
in between source and relay, the relay is called fixed gain AF relay or semi-blind AF relay [51].
The latter has less complexity, but lacks behind from the former with respect to performance
regarding error-rate.
5.3.3 Compress-and-Forward (CF)
Compress-and-Forward is another technique of relaying which does not require decoding in the
relay. In Compress-and-Forward relaying method, the signal received from the source is
quantized and compressed by the relay with the aid of Wyner-Ziv lossy source coding [61]. The
compressed version of the signal is then transmitted to the destination by the relay. The received
information from the source and the quantized and compressed form of that information from
the relay is merged by the destination. CF performs better than DF on the basis of achievable
rate when the relay is near to the destination and vice versa [61].
5.3.4 Estimate-and-Forward (EF)
Estimate-and-Forward is also another relaying method where decoding is not needed in the
relay. In Estimate-and-Forward, an analog estimate of the signal received from the source is
forwarded by the relay to the destination [62]. This estimation is done by entropy constrained
scalar quantization of the signal received from the source or with the help of an unconstrained
minimum mean square error (MMSE) technique [66]. DF performs better than EF with regards
to achievable rate when the relay is far from the destination and vice versa [62].
5.3.5 Coded Cooperations
Coded cooperation is distinct from other relaying techniques because in this scheme, the
channel coding is integrated into cooperation [51]. The data (codeword) of every user is divided
into two parts. At first, every user transfers the former segment of its own codeword and tries to
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decode the other segment of its corresponding communication partner [63]. If the information is
successfully decoded as verified by the Cyclic Redundancy Check (CRC) code, the user creates
the left over portion of its partner’s codeword and sends it to the destination. Else, the user
sends the left over portion of its own codeword [63]. The user and its corresponding
communication partner should work in an environment of orthogonal channels. In coded
cooperation, various channel coding techniques can be assigned [63].
5.4 Cooperative UWB System Model
Cooperative UWB system generally follows ad-hoc network structure [64]. This is for reducing
the complexity of the system. In that sort of structure, every node can play any of the following
three types of role of nodes: (i) source node (SN), (ii) destination node (DN) and (iii) relay node
(RN) [65]. However, in a specific process of communication, every node can only play a single
role. In a process of communication, a cooperative UWB system comprises of a source node, a
destination node and some relay nodes. Figure 5.2 shows a communication process in a
cooperative UWB system where “M” represents the number of relay nodes.
Every communication process in cooperative UWB system model consists of following
three different stages [65]:
i. At first, the source node transmits pilot symbol to all of the relays. At this phase, because of
the obstructions in the links in between the source node and the relay nodes, the links aren’t
confirmed.
ii. From among all the relay nodes, only the relay with the best bit error rate (BER)
performance is chosen as the relay of that communication process. For the purpose of
minimizing the power consumption of the network, only a single relay node is selected for
each communication process.
iii. The communication in between the source node and the destination node takes place via the
selected relay node.
The second step is of the highest significance among all the three steps of
communication of cooperative UWB system model. It is essential to know the channel fading of
the source to relay link and relay to destination link respectively to decide the route with the
best BER performance [64].
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In figure 5.2, the channel fading of the source to relay link is represented by hi(t), where
i=1, 2, …, M. The number of relays is represented by “M”. The channel fading can be calculated
once the pilot symbols are received by the relays. The pilot symbols along with the achieved
signal-to-noise ratio (SNR) are then retransmitted to the destination node from every relay node
in separate time slots [65]. The pilot symbols from separate relay nodes are demodulated at the
receiving side. At this point, the channel fading of the relay to destination link is evaluated. In
figure 2, the channel fading of the source to relay link is represented by gi(t), where i=1, 2, …,
M.
RN1
g1(t)
selection of only a
single relay
h1(t)
h2(t)
RN2
g2(t)
SN
DN
hM-1(t)
hM(t)
gM-1(t)
RNM-1
gM(t)
RNM
Figure 5.2: Cooperative UWB system model
After the selection of the relay node with the best BER performance, the source node
transmits the data signal to the destination node via this path [65]. Generally, RAKE receiver is
implemented for the collection of multipath energy, and better performance is achieved at the
expense of the complexity of hardware [64]. Because a UWB system requires to be simple and
low in cost, RAKE receiver is hardly employed in the adoption of UWB systems. Energy
detection receivers are capable of decreasing the complexity of the system [53]. However, in
that case, UWB system’s performance is degraded. Hence, we have used Code-shifted reference
(TR) receiver, which is based on the energy detection receiver and it is capable of balancing
system complexity with system performance.
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If amplify-and-forward (AF) cooperative communication protocol is implemented, the
multipath component at the source-relay link is amplified and forwarded towards the destination
[59]. Several multipath components are resulted after passing via the dense multipath channel in
between the relay nodes and the destination node. These multipath components interfere with
each other and decrease the SNR at the destination node [60]. Therefore, taking the dense
multipath feature of UWB channel under consideration, we have implemented decode-andforward (DF) protocol in our cooperative UWB system model to transmit the data from the
source node to the destination node through the relay nodes. This decreases the complexity of
the system as well as avoids the distortion of waveform that results from multipath expansion.
5.5 Performance Evaluation for Relay Positioning
It has been said that the relay nodes are placed in between the source node and
destination node with an aim to give better BER performance of the UWB system [51]. But it is
important to know the particular position of a relay in between the source node and the
destination node that gives the best BER performance [53]. By the term “position”, here we can
relate to the distance at which a relay is located from the source and destination. Distance is an
important factor in signal transmission. The signal quality decreases with the increase in
distance because of factors like path-loss, power-loss, noise and interference [53]. We have
considered Dxi as the distance between the source node and the relay node, and, Dyi as the
distance between the relay node and the destination node, where i=1,2,…,M. The number of
relays is represented as M. Let D be the distance between the source node and the relay node.
Figure 5.3 shows the BER performance of the UWB system as a function of Eb/N0 under
the IEEE 802.15.4a office LOS channel environment with the relay node at different distances
from the source and destination. The source node and the destination node are kept at a distance
(D) of 10m. It is assumed that the relays are kept at certain points over a straight line in between
the source node and the relay node, so as to keep the overall transmission distance constant
(10m) for all cases to ease performance comparison, i.e. D = Dxi + Dyi. Simulations are done for
the BER performances of 5 relays which are kept at a distance (Dxi) of 2m, 4m, 5m, 7m and 9m
from the source node. Thus, the corresponding distances (Dyi) of these relays from the
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destination node are 8m, 6m, 5m , 3m and 1m respectively. It can be noted that the third relay is
at an equal distance from the source node and destination node, ie. Dxi = Dyi = 5m. For
comparison, the BER performance for a case with no relay is also simulated, i.e. the direct
transmission of the data signal from the source node to the destination node without any relay.
The fame duration the CSR-UWB is taken as Tf = 60ns with the number of frames as !f = 8.
The data rate is Rb=8Mbps.
0
10
-1
BER
10
-2
10
Dxi=2, Dyi=8
Dxi=4, Dyi=6
Dxi=5, Dyi=5
Dxi=7, Dyi=3
Dxi=9, Dyi=1
!o Relay
-3
10
-4
10
18
18.5
19
19.5
20
20.5
21
21.5
22
Eb/No (dB)
Figure 5.3: BER performances with relays at different positions
The simulation results clearly show that the CSR-UWB system model performs better in
the presence of a relay than in the absence of a relay. It can be observed that the BER
performance of the cooperative UWB system model increases as the relay gets closer to the
centre point in between the source node and the destination node. Thus, we can say that for a
given channel model, the BER performance of the system depends on the distance between the
source node and the relay node (Dxi) and the distance between the relay node and the destination
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node (Dyi). The BER performance of the cooperative CSR-UWB system model is the best when
Dxi = Dyi = 5m at D =10m. Thus, we can conclude that the BER of the cooperative CSR-UWB
system is the minimum when the relay is equidistant from the source node and the destination
node.
5.6 Performance Evaluation of the cooperative CSR-UWB
system under different channel conditions
In the previous section, it has been assumed that the relay is positioned just anywhere
over the straight line in between the source and the destination for performance comparison
purposes. However, in a practical situation, the relays may not exactly lie in the straight line
between the source node and the destination node. Therefore, we can assume the angle made by
that line with the line between the source node and relay node as θ , that is evenly distributed
from 0 to π [65]. We have come to know from the previous section that the BER performance
of the cooperative CSR-UWB system is the best when the relay is equidistant from the source
node and the destination node. So, let us suppose that Di= Dxi = Dyi (where i=1,2,…,M) is the
distance between the source node and relay node as well as the distance between the relay node
and the destination node. Hence, for a given value of Di , the average distance between the
source node and the destination node can be specified as the following [65]:
Di =
1
π
π∫
0
Di2 + Di2 − 2 Di2 cosθ dθ =
4 Di
π
(1)
We evaluated the performances of cooperative CSR-UWB system for different number
of relays, in LOS (Line of Sight) and NLOS (Non-Line of Sight) environments and different
average distance between source node and destination node. The simulations have been done for
the BER performances of cooperative CSR-UWB system for the number of relays M=5 and
M=10. We have taken the CM3 and the CM4 with 100 channels from IEEE 802.15.4a channel
model in our simulations. The CM3 channel represent the LOS (Line of Sight) channels and the
CM4 channel represent the NLOS (Non-Line of Sight) scenario. We have assumed two
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different values for the average distance between the source and the destination which are Di =
4m and 7m. The frame duration of CSR-UWB is taken as Tf = 60ns with the number of frames
as !f = 8. The data rate is Rb=8Mbps.
The simulation of the cooperative CSR-UWB system with 5 and 10 relays is performed
under LOS and NLOS channel environments with 4m and 7m average distance between source
and destination. Evaluation of the performances is given in the following subsections.
5.6.1 Case I: 4m (LOS) vs. 7m (LOS) with 5 relays
First, the performance of the system is compared between scenarios of average sourcedestination distance 4m and 7m. Both simulations are performed with an IEEE 802.15.4a LOS
channel CM3. From figure 5.4, we can observe that, in LOS channel environment, at a BER
requirement of 10-3, the cooperative CSR-UWB system with average source-to-destination
distance of 4m outperforms the one with average source-to-destination distance of 7m by 4dB.
0
10
-1
BER
10
-2
10
LOS (4m)
LOS (7m)
-3
10
18
20
22
24
26
28
30
32
34
36
Eb/No (dB)
Figure 5.4: System BER performance at LOS (4m) and LOS (7m) for M=5
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5.6.2 Case II: 4m (LOS) and 7m (LOS) vs. 4m (NLOS) with 5 relays
The performance of the system with 4m average source-destination distance in an IEEE
802.15.4a NLOS channel CM4 is compared with the ones with 4m and 7m source-destination
distance in an IEEE 802.15.4a LOS channel CM3. We can observe in figure 5.5 that, for the
same distance of 4m, at a BER requirement of 10-3, the performance of the system in LOS
channel environment is 9dB better than that in NLOS channel environment.
0
10
-1
BER
10
-2
10
LOS (4m)
LOS (7m)
NLOS (4m)
-3
10
18
20
22
24
26
28
30
32
34
36
38
40
Eb/No (dB)
Figure 5.5: System BER performance at LOS (4m), LOS (7m) and ALOS (4m) for M=5
5.6.3 Case III: 4m (LOS), 7m (LOS) and 4m (NLOS) vs. 7m (NLOS) with 5
relays
Simulations are done to compare the performance of the system with average sourcedestination distance 4m and 7m for both LOS and NLOS channel environments. IEEE 802.15.4a
CM3 channels are used for LOS environment and IEEE 802.15.4a CM4 channels are used for
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NLOS environment. Figure 5.6 shows that the BER performance of the system is the worst at
7m average source-to-destination distance for NLOS channel environment.
0
10
-1
BER
10
-2
10
LOS (4m)
LOS (7m)
NLOS (4m)
NLOS (7m)
-3
10
18
20
22
24
26
28
30
32
34
36
38
40
Eb/No (dB)
Figure 5.6: System BER performance at LOS (4m), LOS (7m), ALOS (4m) and ALOS
(7m) for M=5
5.6.4 Case IV: 4m (LOS) vs. 7m (LOS) with 10 relays
The performance of the system is compared between scenarios of average source-destination
distance 4m and 7m. Both simulations are performed with an IEEE 802.15.4a LOS channel
CM3. We can observe that, figure 5.7 also shows similar results as in figure 5.4. Here also, at
10-3 BER requirement under LOS channel environment, the BER performance of the
cooperative CSR-UWB system with average source-to-destination distance of 7m is 4dB less
than the one with average source-to-destination distance of 4m.
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0
10
-1
BER
10
-2
10
LOS (4m)
LOS (7m)
-3
10
18
20
22
24
26
28
30
32
Eb/No (dB)
Figure 5.7: System BER performance at LOS (4m) and LOS (7m) for M=10
5.6.5 Case V: 4m (LOS) and 7m (LOS) vs. 4m (NLOS) with 10 relays
The performance of the system with 4m average source-destination distance in an IEEE
802.15.4a NLOS channel CM4 is compared with the ones with 4m and 7m source-destination
distance in an IEEE 802.15.4a LOS channel CM3 with 10 relays. We can observe in figure 5.8
that, at the same average source-to-destination distance of 4m, the cooperative CSR-UWB
system under LOS channel environment outperforms the one under NLOS channel environment
by about 9dB at a BER requirement of 10-3.
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0
10
-1
BER
10
-2
10
LOS (4m)
LOS (7m)
NLOS (4m)
-3
10
18
20
22
24
26
28
30
32
34
36
Eb/No (dB)
Figure 5.8: System BER performance at LOS (4m), LOS (7m) and ALOS (4m) for M=10
5.6.6 Case VI: 4m (LOS), 7m (LOS) and 4m (NLOS) vs. 7m (NLOS) with 10
relays
Simulations are done to compare the performance of the system with average source-destination
distance 4m and 7m for both LOS and NLOS channel environments with 10 relays. IEEE
802.15.4a CM3 channels are used for LOS environment and IEEE 802.15.4a CM4 channels are
used for NLOS environment. Figure 5.9 shows that the channel with average source-todestination distance of 7m under LOS channel environment gives the poorest performance.
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0
10
-1
BER
10
-2
10
LOS (4m)
LOS (7m)
NLOS (4m)
NLOS (7m)
-3
10
18
20
22
24
26
28
30
32
34
36
38
40
Eb/No (dB)
Figure 5.9: System BER performance at LOS (4m), LOS (7m), ALOS (4m) and ALOS
(7m) for M=10
By the comparison of figure 5.6 and figure 5.9, we can observe that the cooperative
CSR-UWB system with 10 relays outperforms the cooperative CSR-UWB system with 5 relays
by about 4dB under a BER requirement of 10-3 for both LOS and NLOS channel environments.
This is because more number of relay nodes opens greater possibilities of getting the relay node
with the highest BER performance. Mostly, the relay node lying nearest or just in the straight
line between the source node and destination node gives the best BER performance for the
system. Nevertheless, at a point when the number of relays (M) becomes adequately huge,
adding more number of relay nodes does not make the BER performance of the system any
better.
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CHAPTER SIX
Conclusions
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Chapter 6: Conclusions
In this thesis, first, the CSR-UWB system has been compared with the TR-UWB and the
FSR-UWB systems in terms of complexity and BER performance. The system complexity of
CSR-UWB system is low in compare to TR-UWB and FSR-UWB system since it does not use
any delay elements of ultra wide bandwidth or any analog carriers. The BER performance
comparison shows that, the CSR-UWB system has a BER performance which is equivalent to
that of TR-UWB system when M=!f/2. When the value of TM is decreased in the CSR-UWB
system, it has a higher BER performance than the FSR-UWB system. Secondly, a cooperative
CSR-UWB communication system has been investigated and its BER performance is presented.
The BER performance of the cooperative CSR-UWB system has been evaluated for different
number of relays under different channel environments using IEEE 802.15.4a channel model.
The simulation results show that, under a LOS channel at a BER requirement of 10-3, the
performance of the cooperative CSR-UWB system with 4m average source-to-destination
distance is approximately 4dB better in SNR than the one with 7m. This means the performance
of the system decreases with the increase in average source-to-destination distance. It has been
observed that with the same average-to-destination distance of 4m, the performance of the
system under a LOS channel environment is about 9dB better than that under a NLOS channel
environment. Hence, it can be said that the system performs better under an environment of
LOS channel than NLOS channel. It can also be observed that the system with 10 relays
outperforms the system with 5 relays by 4dB. This means that the cooperative CSR-UWB
communication system performs better as the number of relay nodes increases until it reaches
an adequately large number.
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Future work
In this thesis, the BER performance of the Code Shifted Reference impulse-based
Cooperative UWB Communication System has been evaluated only under IEEE 802.15.4a LOS
channel CM3 and NLOS channel CM4. The system can also be tested using other IEEE
802.15.4a channel environments in future.
The CSR-UWB system spends half of its power to transmit the reference pulse
sequence. Differential CSR (DCSR) is another version of CSR-UWB which reduces the power
spent in transmitting the reference pulse sequence so as to improve the performance of the CSR
UWB system. For future work, cooperative communication can be implemented over DCSRUWB which can perform better than the Code Shifted Reference impulse-based Cooperative
UWB Communication System.
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