124578363 Project Report Radio Over Fiber Network

DWDM BASED RADIO-OVER-FIBER

SOLUTION TO SUPPORT

ULTRA-WIDEBAND RADIO

Affan Hasan Khan 07-TE-36

Omer Khalid 07-TE-68

Project Supervisor

_______________

Prof. Dr. Muhammad Khawar Islam

DEPARTMENT OF TELCOM ENGINEERING

UNIVERSITY OF ENGINEERING AND TECHNOLOGY

TAXILA

July, 2011

i

ABSTRACT ________________________________________________________________

Wireless coverage of the end-user domain, be it outdoors or indoors (in-building),

is poised to become an essential part of broadband communication networks. In

order to offer integrated broadband services (combining voice, data, video,

multimedia services, and new value added services), these systems will need to

offer higher data transmission capacities well beyond the present-day standards of

wireless systems. Wireless LAN (IEEE802.11a/b/g) offering up-to 54 Mbps and

operating at 2.4 GHz and 5 GHz, and 3G mobile networks (IMT2000/UMTS)

offering up-to 2 Mbps and operating around 2 GHz, are some of today’s main

wireless standards. IEEE802.16 or WiMAX is another recent standard aiming to

bridge the last mile through mobile and fixed wireless access to the end user at

frequencies between 2 – 66 GHz.

The need for increased capacity per unit area leads to higher operating frequencies

(above 6 GHz) and smaller radio cells, especially in in-door applications where

the high operating frequencies encounter tremendously high losses through the

building walls. To reduce the system installation and maintenance costs of such

systems, it is imperative to make the radio antenna units as simple as possible.

This may be achieved by consolidating signal processing functions at a centralized

headend, through Radio-over-Fiber technology.

In this report we will discuss our design of a Radio-over-Fiber network to support

Ultra-wideband radio. Chapter 1 and 2 will provide a brief introduction to these

fascinating technologies i.e. Radio-over-Fiber and Ultra-wideband Radio. Chapter

3 will present a simple technique for optical generation of UWB pulses along with

the basic RoF network design to carry the pulses from centralized processing node

to the user premises. In chapter 4 we will study the effect of different optical

amplifiers on UWB pulses. Chapter 5 will introduce multiple access and the

procedure to embed CMDA in the proposed network design. Finally, chapter 6

will observe the effect of nonlinearities on the system.

ii

UNDERTAKING

I certify that research work titled “DWDM based Radio-over-Fiber solution to

support Ultra-Wideband Radio” is my own work. The work has not, in whole or

in part, been presented elsewhere for assessment. Where material has been used

from other sources it has been properly acknowledged/ referred.

Signature of Student

Affan Hasan Khan (2K7-TE-36)

Omer Khalid (2K7-TE-68)

iii

ACKNOWLEDGEMENTS

This thesis would not have been possible without the guidance and help of several

individuals who in one way or the other contributed and extended their valuable

assistance in the preparation and completion of this study. We are heartily

thankful to our supervisor, Prof. Dr. M Khawar Islam, whose encouragement,

guidance and support from the initial to the final level enabled us to develop an

understanding of the subject. Also, we owe our deepest gratitude to Mr. Ateeq

Mumtaz from Pakistan Telecommunications Company Limited (PTCL) for his

unselfish and unfailing support as our research adviser; He has made available

his support whenever we required. We offer our regards and blessings to all the

teachers and staff members who supported us in any respect during the

completion of the project. Last but not the least, our family and the one above all

of us, the Almighty God, for answering our prayers for giving us the strength to

move on despite our constitution wanting to give up, thank you so much Dear

Lord.

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TABLE OF CONTENTS

Undertaking......................................................................................................................... ii

Acknowledgements ............................................................................................................ iii

List of figures .................................................................................................................... vii

List of tables ....................................................................................................................... ix

Abbreviations ...................................................................................................................... x

CHAPTER 1: .................................................................................................................... 1

The Dawn Of Radio-Over-Fiber Technology ..................................................................... 1

1.1 Narrowband Wireless Communications Systems .............................................. 1

1.2 Broadband Wireless Communication Systems .................................................. 1

1.2.1 Challenges ..................................................................................................... 2

1.3 Radio-over-Fiber (RoF) Technology ................................................................. 4

1.3.1 General Concept ............................................................................................ 4

1.3.2 Advantages of RoF Technology .................................................................... 5

1.3.2.1 Very Low Attenuation Loss ..................................................................... 5

1.3.2.2 Huge Bandwidth ...................................................................................... 5

1.3.2.3 Immune to RF interference ...................................................................... 6

1.3.2.4 Easy Installation and Maintenance .......................................................... 6

1.3.2.5 Reduced Power Consumption .................................................................. 6

1.3.3 Limitations of RoF technology...................................................................... 7

1.3.4 Applications .................................................................................................. 7

CHAPTER 2: .................................................................................................................... 9

Introduction to Ultra-Wideband Technology ...................................................................... 8

2.1 Introduction to UWB Radio .............................................................................. 8

2.2 Regulatory ....................................................................................................... 8

2.2.1 Important UWB pulses .................................................................................. 9

2.3 Characteristics of UWB ................................................................................... 10

2.3.1 High Data Rates .......................................................................................... 10

2.3.2 Low Power Consumption ............................................................................ 10

2.3.3 Interference Immunity ................................................................................. 10

2.3.4 High Security .............................................................................................. 10

2.3.5 Reasonable Range ....................................................................................... 11

2.3.6 Low Complexity, Low Cost ........................................................................ 11

2.4 UWB over Fiber .............................................................................................. 11

2.5 Impulse Radio UWB ....................................................................................... 12

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2.5.1 Modulation Techniques ............................................................................... 12

2.5.1.1 On-Off Keying ....................................................................................... 12

2.5.1.2 Pulse Phase Modulation (PPM) ............................................................. 12

2.5.1.3 Pulse Amplitude Modulation (PAM) ..................................................... 13

2.6 OFDM Based UWB Radio .............................................................................. 13

CHAPTER 3: .................................................................................................................. 14

Basic System Design ......................................................................................................... 14

3.1 Methodology.................................................................................................... 14

3.1.1 UWB Generation Mechanism ..................................................................... 14

3.1.2 Dense Wavelength Division Multiplexing .................................................. 16

3.2 Simulations and Results................................................................................... 17

CHAPTER 4: .................................................................................................................. 22

Optical Amplifiers............................................................................................................. 22

4.1 Introduction ..................................................................................................... 22

4.2 Erbium Doped Fiber Amplifier ....................................................................... 23

4.2.1 Advantages of EDFAs:................................................................................ 29

4.2.2 Disadvantages of EDFAs: ........................................................................... 30

4.3 Semiconductor Optical Amplifier.................................................................... 30

4.3.1 SOA - Basic Description ............................................................................. 30

4.3.2 Principles Of Optical Amplification ............................................................ 31

4.3.3 Advantages: ................................................................................................. 33

4.3.4 Disadvantages: ............................................................................................ 34

4.3.5 Comparison of SOA and EDFA: ................................................................. 34

4.4 Raman Amplifier ............................................................................................. 34

CHAPTER 5: .................................................................................................................. 39

Multiple Access ................................................................................................................ 36

5.1 Introduction ..................................................................................................... 36

5.2 FDMA ............................................................................................................. 36

5.3 TDMA ............................................................................................................. 37

5.4 CDMA ............................................................................................................. 37

5.4.1 Embedding CDMA in UWB-over-Fiber System ........................................ 38

CHAPTER 6: .................................................................................................................. 42

Effect of Nonlinearities on Ultra-Wideband over Fiber Systems ...................................... 42

6.1 Introduction ..................................................................................................... 42

6.2 Self-Phase Modulation .................................................................................... 44

6.2.1 Analysis ....................................................................................................... 45

6.2.2 Theory ......................................................................................................... 45

6.2.3 Simulations .................................................................................................. 45

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6.3 Cross-Phase Modulation .................................................................................. 50

6.3.1 Analysis ....................................................................................................... 51

6.3.2 Theory ......................................................................................................... 51

6.3.3 Simulations .................................................................................................. 51

6.4 Four Wave Mixing .......................................................................................... 56

6.4.1 Analysis ....................................................................................................... 56

6.4.2 Theory ......................................................................................................... 56

6.4.3 Simulations .................................................................................................. 56

CONCLUSION ................................................................................................................. 61

REFERENCES ................................................................................................................. 62

GLOSSARY ................................................................................................................... 64

vii

LIST OF FIGURES

Figure 1.1 Overview of Wireless Communication System ................................................. 2 Figure 1. 2 Wireless Access Network ................................................................................. 3 Figure 1. 3 A Radio Over Fiber Network ............................................................................ 4 Figure 1. 4 Fiber Infrastructure for both Wired and Wireless Applications ........................ 7 Figure 2. 1 FCC Spectrum for Indoor Applications ............................................................ 9 Figure 2.2: Most used UWB Pulses .................................................................................... 9 Figure 3. 1 Generation Mechanism for UWB ................................................................... 14 Figure 3. 2 Working of a Differentiator ............................................................................ 15 Figure 3. 3 Gaussian Pulse and its Frequency Spectrum................................................... 15 Figure 3. 4 Ultra Wideband Monocycle with its Frequency Spectrum ............................. 16 Figure 3. 5 Dense Wavelength Division Multiplexing ...................................................... 17 Figure 3. 6 UWB Monocycle Generated at 2Gb/s ............................................................ 18 Figure 3. 7 Frequency spectrum of UWB monocycle at 2 GB/s. The spectrum is centered

at 5 GHz having a bandwidth of 6 GHz at -10dbm ........................................................... 18 Figure 3. 8 UWB Monocycle Generated at 1 Gb/s ........................................................... 19 Figure 3. 9 Frequency spectrum of UWB monocycle at 1 GB/s. The spectrum is centered

at 2 GHz having a bandwidth of 3 GHz at -10dbm ........................................................... 19 Figure 3. 10 Relation between input data rate and bandwidth for two different values of

Gaussian pulse width used ................................................................................................ 20 Figure 3. 11 Dense Wavelength Division Multiplexed channels for 32 users .................. 20 Figure 3. 12 Received signal after travelling 1 km fiber span with 0.2 db/km attenuation

and 16.75 ps/nm/km dispersion. Photodetector noise has also been added....................... 21 Figure 3. 13 Received signal after travelling 1 km fiber span with 0.2 db/km attenuation

and 16.75 ps/nm/km dispersion excluding photodetector noise ........................................ 21 Figure 4.1: Gain Bandwidth of optical amplifier .............................................................. 23 Figure 4.2: Working of an EDFA ..................................................................................... 23 Figure 4.3: Erbium doped fiber amplifier components ..................................................... 24 Figure 4.6: UWB monocycle prior to passing through an EDF ........................................ 25 Figure 4.7: UWB monocycle after passing through an EDFA .......................................... 25 Figure 4.8: Graph showing the comparison of forward pump power and the output power

while keeping the length and input power of laser constant ............................................. 26 Figure 4.9: Graph showing the comparison of length of the fiber and the output power

while keeping the lforward pump power and input power of laser constant ..................... 27 Figure 4.10: Graph showing the comparison of fiber length and the output power while

keeping the Erbium ions concentration (m-3 )and Numerical Aperture (NA) constant .... 28 Figure 4.11: Graph showing the comparison of Erbium ions concentration (m-3) and the

output power while keeping the fiber length and Numerical Aperture (NA) constant ...... 29 Figure 4.12: Semiconductor optical amplifier basic architecture ...................................... 30 Figure 4.13: Fabry-Perot amplifier ................................................................................... 31 Figure 4.14: Travelling wave amplifier ............................................................................. 31 Figure 4.16: Graph showing the comparison of injection current and the output power

while keeping the dimensions of SOA and the input power of laser constant .................. 33 Figure 5.1: Multiple access aims at channel sharing without interference ........................ 36 Figure 5.2: Allocation of separate channels using FDMA ................................................ 37 Figure 5.3: Allocation of time slots in TDMA .................................................................. 37 Figure 5.4: Code division multiple access ........................................................................ 38 Figure 5.5: Summation of spread bit streams .................................................................... 39 Figure 5.6: Modulated Gaussian pulse train ...................................................................... 39

viii

Figure 5.7: Modulated UWB monocycle pulses ............................................................... 40 Figure 5.8: CDMA demodulation ..................................................................................... 40 Figure 5.9: Recovered data of User 3 ................................................................................ 41 Figure 6.1: Linear and Nonlinear interactions................................................................... 43 Figure 6.2: Nonlinear effects in optical fibers ................................................................... 43 Figure 6.3: Frequency chirping of pulse due to SPM. ....................................................... 44 Figure 6.4: Original Pulse before the effect of SPM ......................................................... 45 Figure 6.5: Effect of SPM ................................................................................................. 46 Figure 6.6: Effect of transmitted power ............................................................................ 47 Figure 6.7: Effect of fiber length ....................................................................................... 48 Figure 6.8: Effect of effective core area ............................................................................ 49 Figure 6.9: Dispersion to counter the effects of SPM ....................................................... 50 Figure 6.10: Effect of XPM .............................................................................................. 52 Figure 6.11: Effect of fiber length ..................................................................................... 53 Figure 6.12: Effect of dispersion ....................................................................................... 54 Figure 6.13: Effect of effective core area of the fiber ....................................................... 55 Figure 6.14: Original unaffected pulse .............................................................................. 57 Figure 6.15: Effect of FWM.............................................................................................. 57 Figure 6.16: Effect of dispersion on FWM ....................................................................... 58 Figure 6.16: Effect of increase dispersion ......................................................................... 59 Figure 6.18: Effect of fiber length on FWM ..................................................................... 59 Figure 6.19: Fiber length and optimum dispersion ........................................................... 60

ix

LIST OF TABLES

TABLE 1 ........................................................................................................................... 17 TABLE 2 ........................................................................................................................... 26 TABLE 3 ........................................................................................................................... 27 TABLE 4 ........................................................................................................................... 28 TABLE 5 ........................................................................................................................... 29 TABLE 6 ........................................................................................................................... 33 TABLE 7 ........................................................................................................................... 46 TABLE 8 ........................................................................................................................... 46 TABLE 9 ........................................................................................................................... 47 TABLE 10 ......................................................................................................................... 48 TABLE 11 ......................................................................................................................... 49 TABLE 12 ......................................................................................................................... 50 TABLE 13 ......................................................................................................................... 52 TABLE 14 ......................................................................................................................... 53 TABLE 15 ......................................................................................................................... 54 TABLE 16 ......................................................................................................................... 55 TABLE 17 ......................................................................................................................... 57 TABLE 18 ......................................................................................................................... 58 TABLE 19 ......................................................................................................................... 58 TABLE 20 ......................................................................................................................... 59 TABLE 21 ......................................................................................................................... 60

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ABBREVIATIONS

WLAN: Wireless Local Area Network

GSM: Global System for Mobiles

WPAN: Wireless Personnel Area Network

MS: Mobile Station

WTU: Wireless Terminal Unit

RAP: Radio Access Point

BS: Base Station

MU: Mobile Unit

RoF: Radio over Fiber

RAU: Remote Antenna Unit

SMF: Single Mode Fiber

EDFA: Erbium Doped Fiber Amplifier

DWDM: Dense Wavelength Division Multiplexing

EMI: Electromagnetic Interference

UMTS: Universal Mobile Telecommunication System

RF: Radio Frequency

FCC: Federal Communication Commission

Hiper LAN: High Performance Radio Local Area Network

IF: Intermediate Frequency

MAC: Media Access Control

DLP: Digital Light Processor

UWB: Ultra Wideband

CW: Continuous Wave

IR: Impulse Radio

OOK: On-Off Keying

PPM: Pulse Position Modulation

PAM: Pulse Amplitude Modulation

ASK: amplitude shift key

xi

TM: Time Modulation

PN: Pseudo-Noise

SOA: Semiconductor Optical Amplifier

EDFA: Erbium Doped Fiber Amplifier

TW: Travelling-Wave

FP-SOA: Fabry-Perot Semiconductor Optical Amplifier

VB: Valence Band

CB: Conduction Band

SRS: Stimulated Raman Scattering

FDMA: Frequency Division Multiple Access

TDMA: Time Division Multiple Access

CDMA: Code Division Multiple Access

SPM: Self-Phase Modulation

CPM: Cross-Phase Modulation

FWM: Four-Wave Mixing

SBS: Stimulated Brillouin-Scattering

GVD: Group Velocity dispersion

XPM: Cross-Phase Modulation

Chapter 1: The Dawn of Radio-over-Fiber Technology

DWDM based Radio-over-Fiber solution to support Ultra-Wideband Radio 1

CHAPTER 1:

The Dawn Of Radio-Over-Fiber Technology

1.1 Narrowband Wireless Communications Systems

The last two decades have seen enormous growth in the area of wireless

communications. Less than 1% of the world’s population had access to a mobile

phone in 1991. This percentage was increased to around 17% by the end of

2001.[1]

In 2002, the number of mobile subscribers in the world crossed 1 billion

and overtook the number of fixed-line subscribers. During the same period the

countries in the world having a mobile network increased from just three to over

90%. In February, 2010, United Nations published a report which indicates that

67% of the world’s populations are mobile subscribers representing 4.6 billion

people around the globe. [2]

The boom of wireless communications is not limited to telephones only, rather the

Wireless Local Area Networks (WLANs) which came on the scene less than one

and a half decade ago, have also experienced exceptional growth. The rapid

increase in the number of WLAN hotspots in public places, such as airport

terminals has been astonishing. It is expected that the number of wireless Internet

subscribers will overtake the number of wired Internet subscribers in the near

future.

This rapid growth of wireless communications can be attributed the ease of

installation as compared to fixed networks, technological advancements and fierce

competition among mobile operators.

1.2 Broadband Wireless Communication Systems

We live in the era of “communication anytime, anywhere, and with anything”. As

the optical fiber increases the data rates of wired communication substantially, the

ever growing demand to bring this broadband capacity in wireless communication

as well has put immense pressure on the wireless systems to increase their

transmission capacity and coverage.

There are two different issues to be addressed to clarify the primary difference

between mobile communications and wireless communications. The mobile

communication networks provide the user with high mobility. The user can move

around at high speed while communicating over the network. But they lack the

other important thing, i.e. capacity. On the other hand, the wireless

communication networks provide high capacity, high data rate wireless access but

they cannot allow the user to move around while accessing network services. As

the mobile networks struggle to increase capacity and the wireless networks, their



mobility, both of them tend to move towards a point of convergence, a broadband

wireless communications system. A comparison of different mobile and wireless

communication systems in this regard has been presented in figure 1.1.

Figure 1.1 Overview of Wireless Communication System[3][4]

On one side, we have the Global System for Mobile communications (GSM)

providing high mobility but the data rates restricted to just a few tens of kbps. On

the other hand, we find the Wireless Personal Area Networks (WPANs)

supporting data rates up to several tens of Mbps with almost no mobility.

1.2.1 Challenges[5]

To understand the challenges faced by broadband wireless systems, we need to

first look into the working principle of current widely deployed narrowband

wireless access systems. GSM can provide a good example. In these systems a

central office handles call processing and switching, while the base stations

provide the radio interface for Mobile Stations (MS) or Wireless Terminal Units

(WTU) held by users. The base stations are linked to the central processing office

either by the help of a point to point microwave link or digital optical fiber link.

The modulation of the signals onto the appropriate carrier is performed at the base

stations with all the mobile stations within the coverage radius of a single base

station sharing the radio frequency spectrum. Similar is the configuration of

Wireless Local Area Networks (WLANs) in which the radio interface is called the

Radio Access Point (RAP). The general architecture of narrowband wireless

communication systems has been demonstrated in figure 1.2.



Figure 1.2 Wireless Access Network

One of the reasons why narrowband wireless systems offer limited capacity is

their low frequency operation. Another reason is the stiff competition for

frequency spectrum among different wireless communication systems which

include, mobile communications, TV broadcasts, and vital communication

systems such as airports and rescue services, wireless LANs etc. Though the use

of low frequency for operation limits the capacity of a system, but it has its

advantages as well. Low frequency usage allows the vendors to manufacture low

cost radio front-ends both in the Base Stations (BSs) and the Mobile Units (MU).

Moreover, the RF active devices are more efficient at low frequencies than at high

frequencies. Furthermore, low frequency RF signals have longer ranges allowing

larger cell sizes to be deployed which in turn enhance mobility.

From the discussion above we can extract information on ways to increase the

capacity of wireless communication systems. One natural way to achieve this is to

deploy smaller cells. As the range of low frequency signals is large, the use of

smaller cells demands for reduced radiated power at the antenna. In this way we

may succeed in reducing the cell size somewhat.

Another option to enhance the capacity of a wireless system is to increase carrier

frequencies. Higher frequencies naturally offer greater modulation bandwidth. The

problem with this capacity improvement solution is that owing to the use high

frequency carriers, the cost of radio front-ends in the base stations and mobile

units is significantly increased. Moreover, smaller the cell size, more are the cells

required to cover an area and more are the base stations required one for each cell,

increasing the cost of the system further.



Hence, to increase the capacity of the system we can reduce the cell size and

increase the carrier frequency resulting in improved spectral efficiency through

increased frequency reuse. But, for the system-wide deployment of such a system

and the system to be economically feasible, the cost of a single base station must

be fairly low. This is where the Radio-over-Fiber technology plays its role. It

achieves the simplification of the Base Stations (BSs in case of mobile

communication) or Remote Antenna Units (RAUs in case of WLANs) by

integrating all the system functionalities at a centralized headend. These

functionalities are then shared by the RAUs. To reduce the cost of the extensive

feeder network, low-cost multimode fibers can be used to carry the signal from the

centralized headend to RAUs.

1.3 Radio-over-Fiber (RoF) Technology

1.3.1 General Concept

Radio-over-Fiber (RoF) technology involves the use of Optical fibers to distribute

RF signals from centralized headend to Remote Antenna Units (RAUs). RoF

technology enables processing functions like frequency up-conversion, carrier

modulation and multiplexing to be performed at the centralized headend unlike

the traditional narrowband communication where these functions are performed at

the base stations immediately before the signals are fed to the antenna. The use of

centralized processing location significantly simplifies the RAUs. The only

function performed at RAUs is optoelectronic conversion and amplification.

These features account for the major savings in system installation, operation and

maintenance especially in case of broadband wireless communication systems

where a large number of BS/RAPs are required due to smaller cell sizes. A Radio-

over-Fiber (RoF) system has been shown in figure 1.3. Processing is performed at

the optical core network as shown. The data for all the users is multiplexed and

carried over the fiber up to the Remote Access Node where it is distributed among

respective base stations.

Figure 1.3 A Radio Over Fiber Network



1.3.2 Advantages of RoF Technology

1.3.2.1 Very Low Attenuation Loss

Transmitting high frequency microwave signals electrically using either

transmission lines or free space is problematic and expensive due to high

attenuation offered by both of these media. Significant losses in free space include

absorption and reflection. In transmission lines, the attenuation is mainly offered

by the line impedance. All these factors increase with frequency, i.e. greater

attenuation is faced at higher frequencies. Hence, transmitting high frequency RF

signals electrically over long distances using transmission lines demands for

costly regenerating equipment. In case of mm-waves, transmission lines are not

feasible for their distribution even for short distances. A solution around this

problem can be to transmit baseband signals at an intermediate frequency from the

headend to base stations. Upon reaching the base station, frequency up-conversion

can be performed. The signals can then be amplified and radiated. This technique

leads to complex base stations because of the need for high performance local

oscillator at each base station that performs frequency up-conversion. A real

solution to this problem lies in the use of optical fibers to carry the signals from

the headend to base stations. Since optical fiber offers very low attenuation loss,

Radio-over-Fiber technology achieves low loss distribution as well as simplified

RAUs.

The standard Single Mode Fibers (SMFs) made up of glass have attenuation losses

as low as 0.2 dB.km and 0.5 dB/km in the 1550 nm and the 1300 nm windows,

respectively. These losses are much lower than those offered by the same length

of other media. For example, the losses in coaxial cable are higher than those in

optical fiber by three orders of magnitude at higher frequencies. Therefore, by

transmitting high frequency RF signals over optical fiber, not only the

transmission distances are increased several times but transmission powers are

also reduced considerably.

1.3.2.2 Huge Bandwidth

Optical fibers offer huge bandwidth. The three transmission windows offering low

attenuation include the windows at 850 nm, 1310 nm, and 1550 nm wavelengths.

In case of a standard single mode fiber the accumulated bandwidth of these three

windows is as high as 50 THz. Despite this enormous bandwidth available, the

commercial systems of the time use only a fraction of this capacity due to their

own limitations. Efforts are being made to make use of more bandwidth offered

by optical fibers. These efforts include the use of Erbium Doped Fiber Amplifiers

(EDFA) and advanced multiplexing techniques like Dense Wavelength Division

Multiplexing (DWDM).

The advantages of huge bandwidth offered by optical fibers are not limited to high

capacity only. In fact, this high optical bandwidth allows for very high speed



signal processing to be performed in optical domain which is by far impossible in

electronic systems. In other words, some of the demanding microwave functions

such as filtering, mixing, up- and down-conversion, can be implemented in

the optical domain.

The primary element that hampers the effective utilization of huge bandwidth

offered by optical fibers is the limitations in bandwidth of electronic systems

which are the primary sources and receivers in transmission systems. This

problem is referred to as “Electronic Bottleneck”.

1.3.2.3 Immune to RF interference

One of the most attractive features of optical fiber communications is its immunity

to Electromagnetic Interference (EMI). Optical fiber transmits signals in the form

of light and hence does not face any interference from the nearby EM radiators.

This property makes optical fiber a primary candidate even for short range mm-

wave transmission. Moreover, optical fiber communication is highly secure. It is

immune to eavesdropping. If the fiber is intercepted to extract the information

being transmitted, the signal drops. Hence optical fiber can provide private and

secure communication.

1.3.2.4 Easy Installation and Maintenance

A centralized headend makes RAUs simpler. All the complex processing is

performed at a central location while the RAUs comprise of a photo-detector, an

amplifier, and an antenna to transmit and receive the signals. In this way, the key

equipment is kept at the headend and is share by multiple RAUs. This kind of

arrangement adds another prominent quality to the overall system. It reduces

systems installation and maintenance cost substantially which is an imperative

requirement for mm-wave systems demanding large number of RAUs. Moreover,

there may applications where the RAUs might not be easily accessible, in such

cases, major operational cost can be saved owing to the reduction in maintenance

requirements.

1.3.2.5 Reduced Power Consumption

Reduced power consumption is a consequence of having simple RAUs with

reduced equipment. Most of the complex equipment is kept at the centralised

headend. In some applications, the RAUs are operated in passive mode. For

instance, some 5 GHz Fiber-Radio systems employing pico-cells can have the

RAUs operate in passive mode. Reduced power consumption at the RAU is

significant considering that RAUs are sometimes placed in remote locations not

fed by the power grid.



1.3.3 Limitations of RoF technology

Fundamentally, the Radio-over-Fiber technology is an analogue transmission

system as it entails the use of analogue modulation. Therefore, the transmission

impairments which have importance in analogue communication systems are

faced in RoF systems as well. Dynamic range of a system is the measure of its

tolerance when the received power is varying. The dynamic range of RoF systems

is limited. This means that RoF technology cannot be used for communication

systems like GSM where the received power at the BS from the MUs varies

extensively. The RF power received from a mobile unit closer to the BS is much

higher than that received from a mobile unit several kilometers away, within the

same cell.

1.3.4 Applications

Radio-over-Fiber distribution systems can be used for indoor distribution of

wireless signals of both mobile and data communication systems. The in-building

fiber infrastructure may then be used for both wired and wireless applications as

shown in Figure 1.4. RoF systems are also attractive for other present and future

applications where high dynamic range is not required. For instance, in Universal

Mobile Telecommunications System (UMTS), mobile units control their output

power so that same power is received at the base station from all mobile units.

Hence, UMTS does not require high dynamic range like GSM, so that RoF

distribution systems may be used for both indoor and outdoor UMTS signal

distribution. RoF technology can also be used to distribute WiMAX signals.

Optical fiber network can be used to carry WiMAX signals over long distances up

to the user premises, from where wireless links help to achieve broadband access,

in a cost effective way.

Figure 1.4 Fiber Infrastructure for both Wired and Wireless Applications

Chapter 2: Introduction to Ultra-Wideband Technology


CHAPTER 2:

Introduction to Ultra-Wideband Technology

2.1 Introduction to UWB Radio

Ultra-wideband is an up-and-coming technology in wireless communication that

has the potential to provide high data rate broadband wireless access. Though it is

an emerging technology at present, the concept is not new at all. The history of

using UWB for wireless communication goes back to early 1900s when Marconi

used UWB pulses in his spark-gap radio transmitter to transmit Morse code

sequences. UWB systems can transmit signals at a much broader frequency. UWB

pulses have a very wide fractional and absolute RF bandwidth. They have very

short pulses and are persistent to multipath reflections. Their transmission is

carrier-less.

UWB is a Radio Frequency (RF) technology that transmits binary data, using low

energy and extremely short duration impulses or bursts (in the order of

picoseconds) over a wide spectrum of frequencies. It delivers data over 15 to 100

meters and does not require a dedicated radio frequency, so is also known as

carrier-free, impulse or base-band radio.

Although the technology is old, its usage and consideration for commercial

applications such as home networking picked up after the Federal

Communications Commission (FCC) ruling in February 2002. This ruling

approved the limited use of unlicensed wireless systems that transmit high-speed

data across a broad portion of the UWB spectrum band. Technical standards and

operational restrictions accepted by FCC are intended to enable the co-existence

of UWB with existing radio technologies such as IEEE 802.11 (Wi-Fi), HomeRF,

and HiperLAN (High Performance Radio LAN).

People commonly refer to UWB as available spectrum rather than as a

technology. 7,500 MHz of unlicensed spectrum, in the 3.1-10.6 GHz band, is

currently available in the US for any communication system that occupies more

than 500 MHz.

2.2 Regulatory[6]

The primary issue in the usage of UWB in practical applications was that the

spectrum was not available and the chunk of spectrum required by UWB radio

was already occupied by other bands. In 2002, FCC authorized the unlicensed use

of UWB in the spectral range of 3.1 GHz to 10.6 GHz. As this band was already

filled, the requirement for UWB was that the transmission power level must be

kept below -41 dBm/MHz. There is no mystery about the number -41, it is just

that its the same power limit that applies to unintentional emitters and it does not



produce much noise to interfere with other licensed systems. However, the

emission limit for UWB emitters can be significantly lower (as low as -75

dBm/MHz) in other segments of the spectrum. Another requirement for a signal to

be classified as an UWB signal is that it must occupy a bandwidth equal to 20% of

the central frequency. For example, if a signal is centered around a frequency of 2

GHz then, to be called an Ultra-wideband signal, it must have a bandwidth of 500

MHz. A general method to generate such a pulse would be to trasmit pulses with a

time interval below 1 nanosecond. All these standards set by Federal

Communications commission are shown in the form of a graph in figure QWE.

Figure 2.1 FCC Spectrum for Indoor Applications

2.2.1 Important UWB pulses

The UWB pulses that are generally used in various applications are shown in the

figure below. It is the Gaussian pulse and the first, second derivatives of Gaussian

pulse. Third derivative of Gaussian pulse is also a frequently used UWB signal.

Fig 2.2: Most used UWB Pulses



2.3 Characteristics of UWB[7]

Ultra-wideband radio has the following prominent characteristics.

2.3.1 High Data Rates

UWB technology can do things that the existing wireless networking systems

cannot. Most importantly, UWB can handle more bandwidth-intensive

applications like streaming video, than either 802.11 or Bluetooth because it can

send data at much faster rates. UWB technology has a data rate of roughly 100

megabits per second, with speeds up to 500 megabits per second, This compares

with maximum speeds of 11 megabits per second for 802.11b (often referred to as

Wi-Fi) which is the technology currently used in most wireless LANs; and 54

megabits per second for 802.11a, which is Wi-Fi at 5MHz. Bluetooth has a data

rate of about 1 megabit per second.

2.3.2 Low Power Consumption

When transmitting data, UWB devices consume less than several tens of

microwatts. That is a huge saving and the reason is that UWB transmits short

impulses constantly instead of transmitting modulated waves continuously like

most narrowband systems do. UWB chipsets do not require Radio Frequency (RF)

to Intermediate Frequency (IF) conversion, local oscillators, mixers, and other

filters. The low power consumption makes UWB ideal for use in battery-powered

devices like cameras and cell phones.

2.3.3 Interference Immunity

Due to low power and high frequency transmission, UWB’s aggregate

interference is “undetected” by narrowband receivers. Its power spectral density is

at or below narrowband thermal noise floor. The low power level thus causes no

irritating interferences to existing home wireless systems. According to its First

Report and Order, the FCC requires that indoor UWB devices transmit only when

operating with a receiver. A device connected to AC power is not constrained to

reduce or conserve power by ceasing transmission, so this restriction will

eliminate unnecessary emissions. Additional tests conducted by the FCC have also

demonstrated conclusively that UWB devices may be permitted to operate under a

proper set of standards without causing harmful interference to other radio

operations.

2.3.4 High Security

UWB’s white-noise-like transmissions enhance security since receivers without

the specific code cannot decode it. Different coding schemes, algorithms, and

modulation techniques can be assigned to different users for data transmissions.



Although security standard is available for UWB, the study group IEEE 802.15.3

has defined AES-128 symmetric security for payload protection and integrity.

2.3.5 Reasonable Range

IEEE 802.15.3a Study Group defined 10 meters as the minimum range at speed

100Mbps. However, UWB can go further. The Philips Company has used its

Digital Light Processor (DLP) technology in UWB device so it can operate

beyond 45 feet at 50 Mbps for four DVD screens.

2.3.6 Low Complexity, Low Cost

The most attractive of UWB’s advantages are of low system complexity and cost.

UWB radio is a cheaper technology as compared to traditional carrier based

technologies as UWB systems do no need to modulate and demodulate complex

analog carrier waveforms. In this way, minimal microwave electronics is required

to make a UWB system. Moreover, UWB system designs are highly frequency

adaptive and hence can be placed anywhere within the radio frequency spectrum.

Also home UWB wireless devices do not need transmitting power amplifier. This

is a great advantage over narrowband architectures that require amplifiers with

significant power back-off to support high-order modulation waveforms for high

data rates. The cost of placing UWB technology inside a consumer electronics

device is $20, compared with $40 for 802.11b and $65 for 802.11a.

2.4 UWB over Fiber

High frequency signals face more interference from the environment, get

attenuated fairly quick, and hence travel less distance. On the other hand a low

frequency signal with the same transmitting power can travel farther. Owing to

their high frequency the typical range of UWB signals in few tens of meters. A

system with such short range would mainly operate in stand-alone mode, with no

integrating option with existing wired or wireless wide-area infrastructures.

Therefore, the short range of the technology can hamper its use in numerous

useful wide area applications. But the issue is resolved when optical fibers come

into play. To increase the coverage area of UWB pulses and to give accessibility

of continuous service across different networks, a revolutionary technique was

introduced which uses optical fiber feeder network to carry high frequency UWB

signals to the user premises hence extending the range of UWB radio technology.

The ultra-wide band (UWB) radio over fiber technology is a fresh technology for

the transmission of UWB signals by making use of a carrier (optical) propagating

through optical fiber. In the UWB system, the UWB RF signal itself is

superimposed on the optical continuous wave (CW) carrier. This strategy makes

the conversion process transparent to the UWB's modulation method and allows

avoiding the high costs of additional electronic components required for

synchronization. In an UWB over fiber system, UWB signals are generated in the

centralized headend. These UWB signals are then distributed to the access points

via optical fiber. UWB signals can be generated directly in the optical domain

without the need of extra electrical to optical conversion which is rather



considered as a key advantage of a UWB over fiber system. In addition, the

generation of UWB signals in the optical domain provides other features such as

light weight, small size, large tunability, and immunity to electromagnetic

interference.

2.5 Impulse Radio UWB

Impulse- radio UWB (IR-UWB) is one of the most attractive techniques. Its

carrier-free impulse modulation not only keeps away from complicated frequency

mixer and filter circuits, but also has better pass-through characteristic due to

base-band transmission. Therefore UWB can also be used in radar imaging

technology, precision locating and tracking and precision time-of-arrival-based

localization approaches.

2.5.1 Modulation Techniques [8]

The following commercially useful UWB impulse modulation techniques

exemplify a wide range of implementation possibilities:

On-Off Keying (OOK)

Pulse Position Modulation (PPM)

Pulse Amplitude Modulation (PAM)

2.5.1.1 On-Off Keying

On-off keying (OOK) is the simplest form of amplitude-shift

keying (ASK) modulation that represents digital data as the presence or absence of

a carrier wave. In its simplest form, the presence of a carrier for a specific duration

represents a binary one, while its absence for the same duration represents a

binary zero. Some more sophisticated schemes vary these durations to convey

additional information. It is analogous to uni-polar encoding line code.

On-off keying is most commonly used to transmit Morse code over radio

frequencies (referred to as CW (continuous wave) operation), although in

principle any digital encoding scheme may be used. OOK has been used in

the ISM bands to transfer data between computers, for example. In addition to RF

carrier waves, OOK is also used in optical communication systems (e.g. IrDA). In

aviation, some possibly unmanned airports have equipment that let pilots key their

VHF radio a number of times in order to request an Automatic Terminal

Information Service broadcast, or turn on runway lights.

2.5.1.2 Pulse Phase Modulation (PPM)

PPM modulation has been developed in the form of Time Modulation (TM) and

was introduced by Time Domain Corporation in the late 1980s. It involves

transmitting impulses at high rates, in the millions to tens of millions of impulses

per second. However, the pulses are not necessarily evenly spaced in time, but

http://en.wikipedia.org/wiki/Amplitude-shift_keying

http://en.wikipedia.org/wiki/Amplitude-shift_keying

http://en.wikipedia.org/wiki/Modulation

http://en.wikipedia.org/wiki/Digital

http://en.wikipedia.org/wiki/Data

http://en.wikipedia.org/wiki/Carrier_wave

http://en.wikipedia.org/wiki/Binary_numeral_system

http://en.wikipedia.org/wiki/Unipolar_encoding

http://en.wikipedia.org/wiki/Line_code

http://en.wikipedia.org/wiki/Morse_code

http://en.wikipedia.org/wiki/Radio_frequency



http://en.wikipedia.org/wiki/Continuous_wave

http://en.wikipedia.org/wiki/ISM_band

http://en.wikipedia.org/wiki/Computer

http://en.wikipedia.org/wiki/Optical_communication

http://en.wikipedia.org/wiki/IrDA

http://en.wikipedia.org/wiki/Automatic_Terminal_Information_Service



http://en.wikipedia.org/wiki/Pilot_Controlled_Lighting



rather they are spaced at random or pseudo-noise (PN) time intervals. The process

creates a noise like signal in both the time and frequency domains. Time coding of

the pulses allows for channelization, while the time dithering, fine pulse position,

and signal polarity provide the modulation. UWB systems built around this

technique and operating at very low RF power levels have demonstrated very

impressive short- and long-range data links, positioning measurements accurate to

within a few centimeters, and high-performance through-wall motion sensing

radars.

2.5.1.3 Pulse Amplitude Modulation (PAM)

Pulse amplitude modulation is a scheme, which alters the amplitude of regularly

spaced rectangular pulses in accordance with the instantaneous values of a

continuous message signal. Then amplitude of the modulated pulses represents the

amplitude of the intelligence. A train of very short pulses of constant amplitude

and fast repetition rate is chosen. The amplitude of these pulse is made to vary in

accordance with that of a slower modulating signal the result is that of multiplying

the train by the modulating signal the envelope of the pulse height corresponds to

the modulating wave. The PAM wave contains upper and lower side band

frequencies besides the modulating and pulse signals.

2.6 OFDM Based UWB Radio

It is an FDM modulation technique for transmitting large amounts of digital data

over a radio wave. OFDM works by splitting the radio signal into multiple smaller

sub-signals that are then transmitted simultaneously at different frequencies to the

receiver. OFDM reduces the amount of crosstalk in signal

transmissions. 802.11a WLAN, 802.16 and WiMAX technologies use OFDM.

Ultra-wideband (UWB) wireless personal area network technology also utilizes

OFDM, such as in Multiband OFDM (MB-OFDM). This UWB specification is

advocated by the WiMedia Alliance(formerly by both the Multiband OFDM

Alliance [MBOA] and the WiMedia Alliance, but the two have now merged), and

is one of the competing UWB radio interfaces.

http://www.webopedia.com/TERM/F/FDM.html

http://www.webopedia.com/TERM/M/modulate.html

http://www.webopedia.com/TERM/C/crosstalk.html

http://www.webopedia.com/TERM/8/802_11.html

http://www.webopedia.com/TERM/W/WLAN.html

http://en.wikipedia.org/wiki/Ultra-wideband

http://en.wikipedia.org/wiki/WiMedia_Alliance

Chapter 3: Basic System Design


CHAPTER 3:

Basic System Design

3.1 Methodology

Figure 1 shows the schematics of the proposed design. The electrical Gaussian

pulses are generated from the incoming data by the help of a Gaussian pulse

generator. These electrical pulses modulate a Mach-Zehnder modulator and

optical Gaussian pulses are produced. The Gaussian pulse can be expressed as:

𝐺 = 1

𝜎 2𝜋 𝑒

− 𝑥2

2𝜎2

Figure 3.1 Generation Mechanism for UWB

3.1.1 UWB Generation Mechanism

Mathematically, the Ultra-wideband monocycle pulse is given by the first order

differential of a Gaussian pulse. The width of resultant UWB pulse can be

adjusted by manipulating the input Gaussian pulse. In our design, differentiation

was performed by splitting the optical Gaussian pulse into two equal components.

One component was provided with an optical bias, which lifts this component

higher than the peak of the other component. The second component is passed

through an optical delay line, delaying the signal in time by an amount equal to

the one half of the width of Gaussian pulse. Then, optical subtraction[9][10][11]

was used to subtract the later component from the former. In this way, during the

time in which elevated component goes through its ascending values, represented

by part 1 in figure 2, the delayed component is zero and the subtraction results in

nothing but the elevated component itself. Next, during part 2 in figure 2 the

subtraction results in the values forming part 5 in the UWB monocycle. And

finally, during the part 3 of subtraction, the elevated Gaussian pulse has a constant

positive value equal to the value of optical bias provided. Thus the subtraction of



the delayed component from this constant positive value results in an inverted

copy of the delayed component (part 6) producing UWB pulse in optical domain.

The working of a differentiator has been shown in figure 3.2. The elevated and

delayed component is shown and formation of UWB monocycle is depicted.

Figure 3.2 Working of a Differentiator

The equation for UWB monocycle can be deduced from equation (1):

𝑈 = 𝐴 𝑥

𝜎 𝑒−

𝑥𝜎

Figure 3.3 Gaussian Pulse and its Frequency Spectrum

Gaussian pulse along with its frequency spectra has been shown in figure 3.3.

Narrower the pulse width in time domain, broader is its frequency spectrum.

Ultra-wideband monocycle along with its frequency spectrum has been shown in

figure 3.3. It should be noted that the original Gaussian pulse used was a baseband

signal while the UWB pulse generated is a band-pass signal with its frequency

spectrum symmetric around a center frequency. Figure 3.3 and 3.4 has been

plotted using mathematical equations and will be used later to verify the

simulation results.



Figure 3.4 Ultra Wideband Monocycle with its Frequency Spectrum

3.1.2 Dense Wavelength Division Multiplexing

Dense wavelength division multiplexing, or DWDM for short, refers originally to

optical signals multiplexed within the 1550 nm band so as to leverage the

capabilities (and cost) of erbium doped fiber amplifiers (EDFAs), which are

effective for wavelengths between approximately 1525–1565 nm (C band), or

1570–1610 nm (L band). EDFAs were originally developed to replace

SONET/SDH optical-electrical-optical (OEO) regenerators, which they have

made practically obsolete. EDFAs can amplify any optical signal in their

operating range, regardless of the modulated bit rate. In terms of multi-wavelength

signals, so long as the EDFA has enough pump energy available to it, it can

amplify as many optical signals as can be multiplexed into its amplification band

(though signal densities are limited by choice of modulation format). EDFAs

therefore allow a single-channel optical link to be upgraded in bit rate by

replacing only equipment at the ends of the link, while retaining the existing

EDFA or series of EDFAs through a long haul route. Furthermore, single-

wavelength links using EDFAs can similarly be upgraded to WDM links at

reasonable cost. The EDFAs cost is thus leveraged across as many channels as can

be multiplexed into the 1550 nm band.

http://en.wikipedia.org/wiki/Erbium_doped_fiber_amplifier

http://en.wikipedia.org/wiki/Synchronous_optical_networking

http://en.wikipedia.org/wiki/Signal_regeneration



Figure 3.5 Dense Wavelength Division Multiplexing

3.2 Simulations and Results

The discussed design was simulated at an input data rate of 2Gb/s. The optical

delay line used for the differentiation of Gaussian pulses has an inversely

proportional relationship with input data rate. Table-1 lists the input data rates

corresponding to the optical delay required for proper differentiation. The value of

optical delay can be calculated from the data rate and width of the Gaussian pulse.

For example, if the input data rate is 2Gb/s, the width of a single bit comes out to

be 1/(2*109) = 0.5 ns. So, if the Gaussian pulses are configured to be 0.1 bit wide,

the width of the pulses in nanoseconds comes out to be 0.5*0.1 = 0.05 ns.

Practically, due to limitations of the Gaussian pulse generator, the actual width is

twice this value i.e. 2*0.05 = 0.1 ns. Hence, at a data rate of 2 Gb/s, an optical

delay line of 0.05 ns (one half of Gaussian pulse width) is required for proper

differentiation. From these calculations we can derive an equation to calculate the

value of optical delay line from the data rate and pulse width.

UWB pulses generated at 2Gb/s and the corresponding frequency spectrum is

shown in figure 5 and 6 respectively. The used modulation technique was On-Off

keying.

TABLE 1

DATA RATES CORRESPONDING TO THE REQUIRED OPTICAL DELAY

Sr.

No.

Data Rate

(Gb/s)

Optical

Delay (ns)

1. 0.5 0.2

2. 1 0.1

3. 2 0.05

4. 4 0.025



Figure 3.6 UWB Monocycle Generated at 2Gb/s

Figure 3.7 Frequency spectrum of UWB monocycle at 2 GB/s. The spectrum is centered

at 5 GHz having a bandwidth of 6 GHz at -10dbm

For comparison, the simulation has also been performed at a data rate of 1Gb/s.

With the data rate reduced to one half, the inverse proportional relationship

dictates us to double the optical delay offered. The required optical delay comes

out to be 0.05*2 = 0.1ns which can be verified from table. 1. Figure 7 and 8 shows

the UWB monocycle produced and the corresponding frequency spectrum

respectively, at an input data rate of 1Gb/s.



Figure 3.8 UWB Monocycle Generated at 1 Gb/s

Figure 3.9 Frequency spectrum of UWB monocycle at 1 GB/s. The spectrum is centered

at 2 GHz having a bandwidth of 3 GHz at -10dbm For comparison, the simulation has also been performed at a data rate of 1Gb/s.

With the data rate reduced to one half, the inverse proportional relationship

dictates us to double the optical delay offered. The required optical delay comes

out to be 0.05*2 = 0.1ns which can be verified from table. 1. Figure 7 and 8 shows

the UWB monocycle produced and the corresponding frequency spectrum

respectively, at an input data rate of 1Gb/s.

The width of the UWB monocycle in figure 5 is 0.2 ns while it is 0.4 ns in figure

7. These widths are twice the widths of the Gaussian pulses used to generate them.

With the data rate reduced to one half, the width of the pulse in time domain in

doubled. Intuitively, this should have an effect on the corresponding bandwidths

of the pulses as well. From figure 6 and 8 we can derive conclusions that there is a

direct proportionality between the input data rate and UWB pulse bandwidth.



Doubling the data rate doubles the bandwidth. Figure 9 shows this relationship in

the form a graph for two different values of Gaussian pulse width used for UWB

monocycle generation.

Figure 3.10 Relation between input data rate and bandwidth for two different values of

Gaussian pulse width used

From the graph, it is clear that increasing the width of Gaussian pulse used,

reduces the bandwidth of output UWB pulse, while the directly proportional

relationship between input data rate and bandwidth stays the same. This is in

compliance with the fact that, shorter the pulses in time domain, broader are their

frequency spectra.

Figure 10 shows the output of Dense Wavelength Division Multiplexing. The

UWB pulses carrying the data of 32 different users were multiplexed using a

frequency spacing of 100 GHz or a wavelength spacing of 0.8 nm.

Figure 3.11 Dense Wavelength Division Multiplexed channels for 32 users



Finally, figure 11 shows the shape of the signal for one user after travelling over a

1 km optical fiber link with an attenuation of 0.2 db/km and dispersion of 16.75

ps/nm/km. The Signal was received using a PIN photo diode.

Figure 3.12 Received signal after travelling 1 km fiber span with 0.2 db/km attenuation

and 16.75 ps/nm/km dispersion. Photodetector noise has also been added

A significant fraction of the noise as can be seen in the figure above is added by

the PIN photodetector. This noise includes the amplified spontaneous emission

(ASE) noise and thermal noise. For a comparison, these photodetector noises were

disabled and the received signal was plotted. Considerable reduction in the noise

can be seen in the figure below.

Figure 3.13 Received signal after travelling 1 km fiber span with 0.2 db/km attenuation

and 16.75 ps/nm/km dispersion excluding photodetector noise

Chapter 4: Optical Amplifiers


CHAPTER 4:

Optical Amplifiers

4.1 Introduction

An optical amplifier is a device that intensifies an optical signal, without the needs

to first convert it to an electrical signal. We can think of an optical amplifier as

a laser with the absence of an optical cavity, or one in which feedback from the

cavity is concealed. Optical amplifiers play an important role in optical

communication.

In order to transmit signals over long distances for example greater than 100km, it

is necessary to recompense for attenuation losses within the optical fiber. Firstly

this was achieved with an optoelectronic module consisting of an optical receiver,

regeneration and equalization system, and an optical transmitter to transmit the

data.

There are several different mechanisms that can be used to amplify an optical

signal, which matches to the major types of optical amplifiers.

Some types of OAs include:

Semiconductor optical amplifiers (SOAs)

Raman amplifiers

Earth doped fiber amplifiers

Amplification of incoming light in doped fiber amplifiers is a result of stimulated

emission in the amplifier's gain medium In semiconductor optical amplifiers

(SOAs), electron-hole combination occurs. In Raman amplifiers, Raman

scattering of incoming light with phonons is done in the lattice of the gain medium

to reduce photons coherent with the incoming photons. The most realistic optical

amplifiers to date include the SOA and EDFA kind. the performance of Raman

amplifiers is being improved by new pumping methods and materials. The figure

below shows Gain bandwidth of optical amplifiers.



1660 nm1640162016001580156015401520150014601440 1480

1660 nm1640162016001580156015401520150014601440 1480

Fluoride EDFA 62 nm

EDFA 52 nm

EDFA ~47 nm

Tellurite EDFA 76 nm]

TDFA 37 nm

TDFA 35 nm

Raman + Fluoride EDFA 80 nm

Dist. Raman + Fluoride EDFA 83 nm

Raman + TDFA 53 nm

Raman 18 nm

Raman 40 nm

Raman 100 nm

Raman 132 nm

C-Band L-BandS-Band U-BandE-Band

Fig 4.1: Gain Bandwidth of optical amplifier

4.2 Erbium Doped Fiber Amplifier

Erbium-doped fiber amplifier or EDFA is an optical or IR repeater that amplifies a

modulated laser beam directly, without conversion from electric to optical or vice

versa. This uses a short length of optical fiber doped with the rare-earth element

erbium. When the signal-carrying laser beams pass through this fiber, external

energy is applied, usually at IR wavelengths. This pumping excites the atoms in

the erbium-doped portion of optical fiber, raising the concentration of the laser

beams passing through. The beams rising from the EDFA keep hold of all of their

original modulation uniqueness, but are brighter than the input beams. Following

is a pictorial view of how an erbium doped fiber amplifier works.[12]

Fig 4.2: Working of an EDFA



Fig 4.3: Erbium doped fiber amplifier components

In fiber optic communications systems, problems arise from the fact that no fiber

material is perfectly transparent. The visible-light or infrared (IR) beams carried

by a fiber are attenuated as they travel through the material. This necessitates the

use of repeaters in spans of optical fiber longer than about 100 kilometers.

Erbium-doped fiber amplifiers are the very important components of the dense

wavelength division multiplexing (DWDM) optical communication systems.

Existing improvements of the DWDM systems and networks are entirely thankful

to the EDFAs. Virtues of EDFAs are high gain, low noise, broad bandwidth, high

output power, and high efficiency of the pump power. Major crucial factor of the

compact optical fiber amplifiers are gain, noise, bandwidth, and gain spectrum

flatness. Higher gain and lower noise let the continuation of the distance between

two repeaters. Wider bandwidth enables the DWDM networks to embrace more

channels that bring about higher capacities, and flatter gain spectrum causes

avoiding transmission impairments due to heterogeneous amplifications.

Physical Components of EDFA:

Biconical fused fiber couplers.

One or two (if high output required) laser pumps.

Polarization-insensitive optical isolator’s front and back. Allows only 1550

nm signals to pass. Pump radiation should not enter main fiber as well as

optical feedback from reflections.

Optical filter for gain flattening.

Photo detector system to monitor pump power or EDFA output power.

Factors controlling the degree of gain uniformity:



Concentrations of the active ion (erbium).

Optical gain flattening filter.

Additional (second) pump laser at each end of the fiber.

These factors were kept in mind and the gain of erbium doped fiber was noted.

Various results were observed by keeping the input power of the Laser and the

length of the erbium doped fiber amplifier to a constant value. It was seen that as

the pump laser power was increased the output gain or the output power from the

fiber was amplified and increased to a large extend. However there was no change

in the spectra of the output signal.

Fig 4.6: UWB monocycle prior to passing through an EDFA

Fig 4.7: UWB monocycle after passing through an EDFA



The Input Power (Pin) was kept equal to 1mW or 0dbm and also the length of the

fiber was kept 2m for all the readings while changing the Forward Pump Power

(FPP). We kept on changing the Forward Pump Power from 50mW to 200mW. It

was seen that there was an increase in the output power by an amount equal to

5dbm when the FPP was kept at 50mW. Similarly it increased by an amount of

7dbm when the forward pump power was kept at 100mW. It increased to 12mW.

TABLE 2

VALUES OF VARIOUS FACTORS THAT AFFECT EDFA OUTPUT

Input Power of

laser (mW)

Forward Pump

Power (mW)

Length (m) Output Power (dbm)

1 50 2 5

1 100 2 7

1 200 2 12

Fig 4.8: Graph showing the comparison of forward pump power and the output power

while keeping the length and input power of laser constant

So from the above results we conclude that as the forward pump of the erbium

doped fiber is increased, the output gain of the optical fiber also increases.

Now for the second simulation, the length of the Erbium doped fiber amplifier’s

length was changed while keeping the forward pump power same. It was seen that

as the length of the EDFA was increased, the output gain or the output power

increases but it was noted that with more increase of the length while keeping the

forward pump power same, results in a decrease of the output power.

The input power of the laser was kept at 1mw or 0dbm and also the forward pump

power was kept the same throughout the calculation of the output power that is

equal to 100mW. The length of the EDFA was increased from 1m to 20m in

regular steps.



The output power was 8dbm at 2m of EDFA length and it became 9dbm at the

length of 3m. When the length of the EDFA was changed to 5m the output gain

also increased to 10dbm. But now the downfall of the output power starts as the

length was increased to 10m the power at the output decreased to 9dbm. It did

increase but not at the same step as it should be. Similarly when the length was

further increased, the output gain decreased in the sense that the amplification

took place but not up to the level that it should be.

TABLE 3


Input Power of

laser (mW)

Forward Pump

Power (mW)

Length (m) Output Power

(dbm)

1 100 1 8

1 100 3 9

1 100 5 10

1 100 10 9

1 100 12 8

1 100 15 7

Fig 4.9: Graph showing the comparison of length of the fiber and the output power while

keeping the lforward pump power and input power of laser constant

When the Erbium Doped Fiber Amplifier’s length is increased while keeping the

erbium ions concentration constant and also the numerical aperture of the optical

fiber is kept the same. It has been seen that as the fiber length is increased the

output gain is also increased.

When the Erbium ions concentration is kept at 1*1024

m-3

and the numerical

aperture is kept at 0.22, the output power increases. The output power was 1dbm

at 2m of EDFA length and it became 2dbm at the length of 4m. When the length



of the EDFA was changed to 10m the output gain also increased to 4dbm. Further

increasing the length of the EDFA, the gain of the fiber also increases.

TABLE 4


Fiber Length (m) Erbium ions

concentration (m-3

)

Numerical

Aperture (NA)

Output Gain

(dbm)

2 1*1023

0.22 1

4 1*1023

0.22 2

10 1*1023

0.22 4

15 1*1023

0.22 6

20 1*1023

0.22 8

Fig 4.10: Graph showing the comparison of fiber length and the output power while

keeping the Erbium ions concentration (m-3 )and Numerical Aperture (NA)

constant

Various results were observed by changing the erbium ions concentration of

erbium doped fiber while keeping the length of the fiber same and also the

numerical aperture. It was noted that as the concentration of the erbium ions was

increased the gain was also increased in the same ratio. The input power was kept

the same for the calculation of output power.

When the Erbium ions concentration is kept at 8.3*1024

m-3

and the numerical

aperture is reserved at 0.22, the output power increases. The output power was

8dbm at 2m of EDFA length and it became 6dbm when the concentration of the

erbium ions was decreased from 8.3*1024

m-3

to 6.3*1024

m-3

while keeping the

length and the numerical aperture same. Similarly when the erbium ions

concentration was further decreased to 6.3*1023

m-3

the output gain of the fiber

decreased to 2dbm. It should be noted that amplification was still done but the

amount of increase in output power was reduced as the ions concentration of



erbium was decreased. The spectral width of the output signal however showed no

change

TABLE 5


Fiber Length (m) Erbium ions

concentration (m-3

)

Numerical

Aperture (NA) Output Gain (dbm)

2 8.3*1024

0.22 8

2 6.3*1024

0.22 6

2 6.3*1023

0.22 2

Fig 4.11: Graph showing the comparison of Erbium ions concentration (m-3) and the

output power while keeping the fiber length and Numerical Aperture (NA) constant

4.2.1 Advantages of EDFAs:

High power transfer efficiency from pump to signal power (>

50%).

Wide spectral band amplification with relative flat gain (>20 dB) –

useful for WDM applications.

Large dynamic range.

Low noise figure.

Suitable for long-haul applications.



4.2.2 Disadvantages of EDFAs:

Relatively large devices (km lengths of fiber) – not easily

integrated with other devices.

ASE – amplified spontaneous emission. There is always some

output even with no signal input due to some excitation of ions in

the fiber – spontaneous noise.

Cross-talk effects.

Gain saturation effects.

4.3 Semiconductor Optical Amplifier The semiconductor optical amplifier (SOA) can be an substitute to costly

wavelength-flattened Erbium doped optical amplifiers to be used in optical

processing as wavelength switching. However, present day SOAs are costly and

the accessibility of a reliable and flexible computer-aided design programs is an

important task to cut designing costs and also to predict the performance of very

high speed based data links. Theoretical models with different complexity levels

were being applied to predict SOA-based functionalities, and complex models can

provide more accurate knowledge of what happens inside the semiconductor

optical active region.[13]

4.3.1 SOA - Basic Description[14]

An SOA is an optoelectronic device that under suitable operating conditions can

amplify a light signal. The active region in the device communicates gain to an

input signal. An external electric current provides the energy source that enables

gain to take place. An embedded waveguide is used to confine the propagating

signal wave to the active region, However, the optical confinement is weak so

some of the signal will leak into the surrounding lossy cladding regions. The

output signal is accompanied by noise. This additive noise is produced by the

amplification process itself and so cannot be entirely avoided. The amplifier facets

are reflective causing ripples in the gain spectrum.

Fig 4.12: Semiconductor optical amplifier basic

architecture



SOAs can be classified into two main types shown in the below figures. The

Fabry-Perot SOA (FP-SOA) where reflections from the end facets are significant

(i.e. the signal undergoes many passes through the amplifier) and the travelling-

wave SOA (TW-SOA) where reflections are negligible (i.e. the signal undergoes a

single-pass of the amplifier).

Fig 4.13: Fabry-Perot amplifier

Fig 4.14: Travelling wave amplifier

4.3.2 Principles Of Optical Amplification

In an SOA electrons (more commonly referred to as carriers) are injected from an

external current source into the active region. These energized carriers occupy

energy states in the conduction band (CB) of the active region material, leaving

holes in the valence band (VB). Three radiative mechanisms are possible in the

semiconductor. These are shown in the figure ahead, for a material with an energy

band structure consisting of two discrete energy levels.



Fig 4.15: Three radiated mechanisms possible in semiconductor

In stimulated absorption an incident light photon of sufficient energy can

stimulate a carrier from the VB to the CB. This is a loss process as the incident

photon is extinguished.

If a photon of light of suitable energy is incident on the semiconductor, it can

cause stimulated recombination of a CB carrier with a VB hole. The recombining

carrier loses its energy in the form of a photon of light. This new stimulated

photon will be identical in all respects to the inducing photon. Both the original

photon and stimulated photon can give rise to more stimulated transitions. If the

injected current is sufficiently high then a population inversion is created when the

carrier population in the CB exceeds that in the VB. In this case the likelihood of

stimulated emission is greater than stimulated absorption and so semiconductor

will exhibit optical gain.

In the spontaneous emission process, there is a non-zero probability per unit time

that a CB carrier will spontaneously recombine with a VB hole and thereby emit a

photon with random phase and direction. Spontaneously emitted photons have a

wide range of frequencies. Spontaneous emission is a direct consequence of the

amplification process and cannot be avoided; hence a noiseless SOA cannot be

created. Stimulated processes are proportional to the intensity of the inducing

radiation whereas the spontaneous emission process is independent of it.

The degree of gain uniformity can be controlled by the injection current. This was

kept in mind and the gain of semiconductor optical amplifier was observed.

Various results were viewed by keeping the input power of the Laser and physical

dimensions of the semiconductor amplifier to a constant value. It was seen that as

the injection current was increased the output gain or the output power from the

fiber was amplified and increased to some extent. However the spectra of the

output signal was distorted to some degree.



The Input Power (Pin) was kept equal to 1mW or 0dbm while we kept altering the

injection current. The injection current was changed from 0.15A to 1A. It was

seen that there was an increase in the output power by an amount equal to 15dbm

when the injection current was raised to 0.15A. Similarly it increased by an

amount of 18dbm when the injection current was kept at 1A.

TABLE 6

VALUES OF VARIOUS FACTORS THAT AFFECT SOA OUTPUT

Input Power of laser

(mW)

Injection Current

(A)

Dimensions of SOA

(length, width, height) m

Output Power

(dbm)

1 0.15 0.0005, 3e-006, 8e-008 15

1 0.50 0.0005, 3e-006, 8e-008 17

1 1 0.0005, 3e-006, 8e-008 19

Fig 4.16: Graph showing the comparison of injection current and the output power while

keeping the dimensions of SOA and the input power of laser constant

So from the above results we conclude that as the injection current of the

Semiconductor amplifier is increased, the output gain of the optical fiber also

increases.

4.3.3 Advantages:

Is the right size to be integrated with waveguide photonic devices

(short path length requirement).

Can easily be integrated as preamplifiers at the receiver end.

Use same technology as diode lasers.

Gain relatively independent of wavelength.



Are pumped with current, not another laser.

4.3.4 Disadvantages:

Polarization dependence.

Self-phase modulation leading to chirp.

Cross-phase modulation.

Four-wave mixing and crosstalk

Extremely short (ns) excited state lifetimes

4.3.5 Comparison of SOA and EDFA:

The technology of semiconductor amplifiers competes with that of erbium-doped

fiber amplifiers (EDFAs). The main differences compared with EDFAs are:

The setup is much more compact, containing only a small

semiconductor chip with electrical and fiber connections.

The output powers are significantly smaller.

The gain bandwidth is smaller, but devices operating in different

wavelength regions can be made.

SOAs exhibit much stronger nonlinear distortions in the form of

self-phase modulation and four-wave mixing.

The noise figure is typically higher.

The amplification is normally polarization-sensitive.

4.4 Raman Amplifier

Unlike the EDFA and SOA the amplification effect in Raman amplifier is

achieved by a nonlinear interaction between the signal and a pump laser within an

optical fiber. There are two types of Raman amplifier: distributed and lumped. A

distributed Raman amplifier is one in which the transmission fiber is utilized as

the gain medium by multiplexing a pump wavelength with signal wavelength,

while a lumped Raman amplifier utilizes a dedicated, shorter length of fiber to

provide amplification.

The pump light may be coupled into the transmission fiber in the same direction

as the signal (co-directional pumping), in the opposite direction (contra-directional

pumping) or both. Contra-directional pumping is more common as the transfer of

noise from the pump to the signal is reduced. The pump power required for

Raman amplification is higher than that required by the EDFA, with in excess of

500mW being required to achieve useful levels of gain in a distributed amplifier.

Lumped amplifiers, where the pump light can be safely contained to avoid safety

implications of high optical powers, may use over 1W of optical power.



Fig 4.17: Typical raman amplifier configuration

The principal advantage of Raman amplification is its ability to provide

distributed amplification within the transmission fiber, thereby increasing the

length of spans between amplifier and regeneration sites. The amplification

bandwidth of Raman amplifiers is defined by the pump wavelengths utilized and

so amplification can be provided over wider, and different, regions than may be

possible with other amplifier types which rely on dopants and device design to

define the amplification window.

Raman optical amplifiers differ in principle from EDFAs or conventional lasers in

that they utilize stimulated Raman scattering (SRS) to create optical gain.

A Raman optical amplifier is little more than a high-power pump laser, and a

WDM or directional coupler. The optical amplification occurs in the transmission

fiber itself, distributed along the transmission path. Optical signals are amplified

up to 10 dB in the network optical fiber. The Raman optical amplifiers have a

wide gain bandwidth (up to 10 nm). They can use any installed transmission

optical fiber. Consequently, they reduce the effective span loss to improve noise

performance by boosting the optical signal in travel.

Chapter 5: Multiple Access


CHAPTER 5:

Multiple Access

5.1 Introduction

In any communication system, multiple users need to share the medium.

Therefore, algorithms are required to ensure that all the users get their share of the

medium without causing interference to other users. These algorithms are called

multiple access techniques or multiple access schemes. Two major techniques that

allow the bandwidth in a communications system to be shared are FDMA and

TDMA. These techniques enable two or more signals to share the channel in such

a way that each signal can be received without interference from another.

5.2 FDMA FDMA stands for Frequency Division Multiple Access. In FDMA, signals from

various users are assigned different frequencies i.e. total bandwidth of the channel

is divided amongst all the users.[15]

Two users cannot operate on a same frequency

unless they are separated from each other by a specified distance to avoid

significant interference. The bandwidths of FDMA channels are relatively narrow

(25-30 kHz)

Fig 5.1: Multiple access aims at channel sharing without interference



Fig 5.2: Allocation of separate channels using FDMA

5.3 TDMA

TDMA stands for Time Division Multiple Access. TDMA divides the channel on

the basis of the time and each slot is known as time slot. The whole bandwidth is

available for each time slot.[16]

Information from each user is carried within one

time slot. Users using different time slots may share the same frequency. When all

the available time slots in a given frequency are occupied a new user connecting

to a system must be assigned a time slot on a different frequency.

Fig 5.3: Allocation of time slots in TDMA

5.4 CDMA

FDMA and TDMA are decent schemes that allow the medium to be shared among

users. But with the ever increasing demand for more bandwidth from both new

technology and the users of technology, these techniques fail to fulfill the

requirements. A faster and more advanced way of distinguishing users on a

common medium is Code Division Multiple Access (CDMA). CDMA allows full

spectrum to be used by all users (unlike FDMA) for the entire period of time

(unlike TDMA). It differentiates between communicators using orthogonal codes

assigned to all the users.[17]

The receiver extracts the data of any user, from the



medium, using the unique code of that user. All the users are on a single broad

frequency of operation.

Fig 5.4: Code division multiple access

5.4.1 Embedding CDMA in UWB-over-Fiber System

As an application of the fore-mentioned technique we provide an example of high

speed wireless communication system where multiple-access technique is required

to distinguish users on a common medium. Sharing signals have important applied

applications in various disciplines such as automobile anti-collision system for

expressway, inter-vehicle communication and satellite communication and

location among satellite formation.

Assume that user 1 is transmitting a 0, user 2 is also transmitting a 0 and user 3

transmitting a 1, represented by a -1, -1 and 1 respectively. The spreading code for

user 1, user 2 and user 3 is [1 1 1 1], [1 0 1 0] and [1 0 0 1] respectively. Upon

multiplication of user input with their corresponding orthogonal codes and

subsequent summation, the waveform received to be transmitted over the optical

fiber is shown in fig. 3.



Figure 5.5: Summation of spread bit streams

This waveform modulates a Gaussian pulse train as shown in fig 4. The Gaussian

pulse train must be synchronized with the data rate of the input for proper

amplitude modulation of the pulses.

The modulated Gaussian pulse train is then used to generate UWB monocycle

pulses in the optical domain through the above discussed generation mechanism.

The UWB pulses carrying the cumulative data of multiple users are shown in fig.

5. The amplitude of the UWB monocycles corresponds to the amplitude of

cumulative waveform carrying user data while the occurrence of positive and

negative lobes corresponds to the polarity of the cumulative waveform.

Figure 5.6: Modulated Gaussian pulse train



Figure 5.7: Modulated UWB monocycle pulses

This modulated Gaussian pulse train is transmitted over the optical fiber. To

recover the pulse, an M-ary threshold detector is configured and the waveform

carrying the sum of data from all the users if obtained.

Figure 5.8: CDMA demodulation

Figure shows part of the network that recovers the cumulative waveform. To

recover the data of individual users, the cumulative waveform is again multiplied

with the code of that user as can be seen in the figure above. Subsequently, the

different levels of the waveform after multiplication are added together and the



resulting value is divided by the total number of users to obtain the bit transmitted

by the user. Figure shows the recovered bit for user 3.

Figure 5.9: Recovered data of User 3

Chapter 6: Effect of Nonlinearities on Ultra-Wideband over Fiber Systems


CHAPTER 6:

Effect of Nonlinearities on Ultra-Wideband

over Fiber Systems

6.1 Introduction

The development of low loss optical fibers, highly efficient lasers, optical

detectors and amplifiers has brought a revolution to telecommunications. While

propagating through an optical fiber, light pulses are confronted with various

impairments degrading the quality of the signal. The effects such as attenuation,

temporal broadening and nonlinearities tend to distort the original signal and

result in loss of information.[18]

The problems get worse when multiple

wavelengths constituting independent channels are multiplexed (WDM) and travel

simultaneously through an optical fiber. Another issue of interest is the power

contained in the pulses. For the system to be economical, increased spacing is

required between optical repeaters in the link which in turn requires higher

launched power per wavelength to achieve the desired signal to noise ratio. With

several wavelengths multiplexed over the same fiber, increase optical power

launched per wavelength, and the ever growing desire to increase the data rate, the

total optical power propagating through the optical fiber increases leading to

nonlinearities in optical fiber.

The terms linear and nonlinear (Figure 4.1), in optics, mean intensity independent

and intensity-dependent phenomena respectively. Nonlinear effects in optical

fibers occur due to two main reasons:

Change in the refractive index of the medium with optical intensity

Inelastic scattering phenomenon.

The power dependence of the refractive index leads to optical Kerr-effect.

Depending upon the type of input signal, the Kerr nonlinearity can be observed in

three different effects.

Self-Phase Modulation (SPM),

Cross-Phase Modulation (CPM)

Four-Wave Mixing (FWM).

At high power level, the inelastic scattering can provoke stimulated effects such as

Stimulated Brillouin-Scattering (SBS) and Stimulated Raman-Scattering (SRS).

Above a certain threshold value the intensity of the scattered light grows

exponentially. The difference between Brillouin and Raman scattering is that the

phonons generated in Brillouin scattering are coherent and generate a macroscopic



acoustic wave in the fiber, while Raman scattering is characterized by incoherent

phonons and no macroscopic wave is generated.

Figure 6.1: Linear and Nonlinear interactions

Figure 6.2: Nonlinear effects in optical fibers

All nonlinear effects except SPM and CPM provide gains to some channel at the

cost of reducing power from other channels. SPM and CPM affect only the phase

of signals and can cause spectral broadening, leadings to increased dispersion.

Due to their importance in designing optimum systems, nonlinearities in optical

fiber is an area of academic research.[19-24]



6.2 Self-Phase Modulation

The higher intensity portions of an optical pulse face a higher refractive index

while propagating through the optical fiber compared with the lower intensity

portions. A time varying signal intensity produces a time varying refractive index

in a medium that has an intensity-dependant refractive index. The leading edge

will experience a positive refractive index gradient while the trailing edge will

experience a negative refractive index gradient. This time varying index change

results in a temporally varying phase change, as shown in Figure. This causes the

optical phase of the pulse to vary in exactly the same manner as the optical signal.

Since, this nonlinear phase modulation is induced by the intensity of the pulse

itself, it is called as self-phase modulation.

Owing to the intensity dependence of phase fluctuations, different parts of the

pulse undergo different phase shift resulting in frequency chirping. The rising

edge of the pulse finds frequency shift in upper side whereas the trailing edge

faces shift in lower side. Hence primary effect of SPM is to broaden the spectrum

of the pulse, keeping the shape of the signal in time domain unaltered. The SPM

effects are more pronounced in systems with high-transmitted power because the

chirping effect is proportional to transmitted signal power.

Figure 6.3: Frequency chirping of pulse due to SPM



There is broadening of the spectrum without any change in distribution of the

signal in time domain in case of self-phase modulation while in case of dispersion,

there is broadening of the pulse in time domain and spectral contents remain

unaltered. In other words, the SPM by itself leads only to chirping, regardless of

the pulse shape. It is dispersion that is responsible for pulse broadening. The SPM

induced chirp varies the pulse broadening effects of dispersion.

6.2.1 Analysis

6.2.2 Theory

SPM arises due to intensity dependence of refractive index. Fluctuation in signal

intensity causes change in phase of the signal. This change in phase induces

additional chirp, which leads to dispersion penalty. This penalty will be small if

input power is less than certain threshold value. The input power should be kept

below 19.6mW. The appropriate chirping of the input pulses can also be beneficial

for reducing the SPM effects.

6.2.3 Simulations

Case 1: Original Spectrum

The original spectrum of Ultra-wideband monocycle in optical domain is shown

in figure 6.4.

Figure 6.4: Original Pulse before the effect of SPM

Figure 1 corresponds to the optical spectrum without any nonlinearities in place.

The data rate is set to 40 Gbps and the transmitted power of the pulse is 20 mW.

Several other important parameters are listed in table 1.



TABLE 7

SPM CASE 1

Length of the fiber 50 Km

Effective core area 20 um2

Self-Phase modulation Off

Dispersion Off

Data rate 40 Gbps

Transmitted power 20 mW

Case 2: Effect of SPM

Figure 6.5: Effect of SPM

TABLE 8

SPM CASE 2



Self-Phase modulation On

Dispersion Off

Data rate 40 Gbps


Observations:

Self-Phase modulation causes a symmetric broadening of pulse spectrum. The

effect is more prominent with transmitted power greater than or equal to 20 mW.



Case 3: Effect of transmitted power

Figure 6.6: Effect of transmitted power

TABLE 9

SPM CASE 3




Dispersion Off

Data rate 40 Gbps


Observations:

The higher the transmitted power of the pulse, the more prominent is SPM

induced spectral broadening.



Case 4: Effect of fiber length

Figure 6.7: Effect of fiber length

TABLE 10

SPM CASE 4




Dispersion Off

Data rate 40 Gbps


Observations:

Longer the fiber, greater is the impact of SPM on pulse spectrum.



Case 5: Effect of core area

Figure 6.8: Effect of effective core area

TABLE 11

SPM CASE 5




Dispersion Off

Data rate 40 Gbps


Observations:

The intensity of a pulse is inversely proportional to the effective core area.

Increasing the core area reduces the intensity of the pulse propagating through the

fiber which in turn reducing the effect of SPM.



Case 6: Dispersion to counter the effects of SPM

Figure 6.9: Dispersion to counter the effects of SPM

TABLE 12

SPM CASE 6




Dispersion On

Data rate 40 Gbps


Observations:

The effect of GVD on the pulse propagation depends on whether or not the pulse

is chirped. With the proper relation between the initial chirp and the GVD

parameters, the pulse broadening (which occurs in the absence of any initial chirp)

will be preceded by a narrowing stage (pulse compression). On the other hand, the

SPM alone leads to pulse chirping, with the sign of the SPM-induced chirp being

opposite to that induced by anomalous GVD. This means that in the presence of

SPM, the GVD induced pulse-broadening will be reduced (in the case of

anomalous), while extra broadening will occur in the case of normal GVD.

6.3 Cross-Phase Modulation

SPM is the major nonlinear limitation in a single channel system. The intensity

dependence of refractive index leads to another nonlinear phenomenon known as

cross-phase modulation (CPM). When two or more optical pulses propagate

simultaneously, the cross-phase modulation is always accompanied by SPM and

occurs because the nonlinear refractive index seen by an optical beam depends not

only on the intensity of that beam but also on the intensity of the other co-



propagating beams. In fact CPM converts power fluctuations in a particular

wavelength channel to phase fluctuations in other co-propagating channels. The

result of CPM may be asymmetric spectral broadening and distortion of the pulse

shape.

CPM hampers the system performance through the same mechanism as SPM:

chirping frequency and chromatic dispersion, but CPM can damage the system

performance even more than SPM. CPM influences the system severely when

number of channels is large. Theoretically, for a 100-channels system, CPM

imposes a power limit of 0.1mW per channel.

6.3.1 Analysis

6.3.2 Theory

The CPM-induced phase shift can occur only when two pulses overlap in time.

This overlap enhances the frequency dependant phase shift which in turn enhances

the broadening of the pulse significantly limiting the performance of optical

systems. The effects of CPM can be reduced by increasing the wavelength spacing

between channels. With an increase in wavelength spacing, the pulses overlap for

a very period of time and hence reduce the effects of CPM. In fact, the sufficiently

different propagation constants of the channels introduced due to fiber dispersion

cause the pulses corresponding to individual channels to walk away from each

other. Due to this pulse walk-off phenomenon the pulses, which were initially

overlapping in the time domain, cease to be so after propagating for some distance

and cannot affect each other further. Thus, effect of CPM is reduced. In a WDM

system, CPM converts power fluctuations in a particular wavelength channel to

phase fluctuations in other co-propagating channels. This leads to broadening of

pulse. It can be greatly reduced in WDM systems operating over standard non-

dispersion shifted single mode fiber. One more advantage of this kind of fiber is

its effective core area, which is typically 80 μm2. This large effective area is

helpful in reducing nonlinear effects. Like SPM, the CPM also depends on

interaction length of fiber. The long interaction length is always helpful in

building up this effect up to a significant level. Keeping interaction length small,

one can reduce this kind of nonlinearity.

6.3.3 Simulations

Case 1: Effect of XPM

We simulate two pulses with spectra centered at 193.1 THz and 210 THz and

having a transmit power of 20 mW and 0.5 mW respectively. Although the

concepts of SPM and XPM are often overlapping, they can be observed keeping in

mind the thresholds above which they are prominent. In our case, the pulse with

20 mW power is prone to the effects of both SPM and XPM while for the pulse

with 2 mW power, the effect of SPM is negligible and hence asymmetric spectral

broadening caused by XPM can be observed.



Figure 6.10: Effect of XPM

TABLE 13

XPM CASE 1




Cross Phase Modulation On

Dispersion Off




Observations:

The intensity of the high power pulse modulates the intensity of low power pulse

which is unaffected by self phase modulation. This phenomenon is called cross

phase modulation and it leads to asymmetric pulse broadening.

Case 2: Effect of fiber length:

Figure 6.11: Effect of fiber length

TABLE 14

XPM CASE 2





Dispersion Off


Observations:

Increasing the fiber length, increasing the effect of cross phase modulation

because of increased interaction of the pulses.



Case 3: Effect of dispersion:

Figure 6.12: Effect of dispersion

TABLE 15

XPM CASE 3





Dispersion On


Observations:

Although turning on the dispersion has mitigated the effect of SPM as expected,

no effect can be observed on XPM.



Case 4: Effect of effective core area:

Figure 6.13: Effect of effective core area of the fiber

TABLE 16

XPM CASE 4





Dispersion On




Observations:

From figure 1 and figure 2, it can be observed that the effect of XPM decreases as

the effective core area of the fiber increases. This is again due to the decrease in

pulse intensities increasing core area.

6.4 Four Wave Mixing

The origin of FWM process lies in the nonlinear response of bound electrons of a

material to an applied optical field. The FWM process originates from third order

nonlinear susceptibility (χ(3)). If three optical fields with carrier frequencies ω1,

ω2 and ω3, co-propagate inside the fiber simultaneously, (χ(3)) generates a fourth

field with frequency ω4, which is related to other frequencies by a relation,

. In quantum-mechanical context, FWM occurs when

photons from one or more waves are annihilated and new photons are created at

different frequencies keeping net energy and momentum conserved during the

interaction. Unlike SPM and CPM, that are significant mainly for high bit rate

systems, the FWM effect is independent of the bit rate and is dependent on the

channel spacing and fiber dispersion. Decreasing the channel spacing increases

the four-wave mixing effect and so does decreasing the dispersion.

6.4.1 Analysis

6.4.2 Theory

Four-wave mixing process results in power transfer from one channel to other.

This phenomenon results in power depletion of the channel, which degrades the

performance of that channel (i.e., BER is increased). In order to achieve original

BER, some additional power is required which is termed as power penalty. Since,

FWM itself is interchannel crosstalk it induces interference of information from

one channel with another channel. This interference again degrades the system

performance. To reduce this degradation, channel spacing must be increased. This

increases the group velocity mismatch between channels and hence FWM penalty

is reduced.

6.4.3 Simulations

Case 1: Original spectra of the pulses

Two pulses are transmitted with spectra centered at 193.1 THz and 193.15 THz

and a transmitted power of 2 mW each. The original optical spectra of the pulses

are shown in figure 1. The data rate is set to 2.5 Gbps.



Figure 6.14: Original unaffected pulse

TABLE 17

FWM CASE 1



Dispersion Off

Data rate 2.5 Gbps

Transmitted power 2 mW each

Case 2: Effect of FWM

The effect of Four Wave Mixing can be observed after the pulses traverse a 75

Km long fiber span.

Figure 6.15: Effect of FWM



TABLE 18

FWM CASE 2



Dispersion 4 ps/nm/km

Data rate 2.5 Gbps


Case 3: Effect of dispersion on FWM

Figure 6.16: Effect of dispersion on FWM

TABLE 19

FWM CASE 3



Dispersion 16.75 ps/nm/km

Data rate 2.5 Gbps


Observations:

Increasing the dispersion from 4 ps/nm/km to 16.75 ps/nm/km has significantly

minimized the effect of FWM. But this does not prove that increasing the

dispersion always has a positive effect of FWM as we can see in the next figure

with the following values for parameters.



TABLE 20

FWM CASE 3B



Dispersion 25 ps/nm/km

Data rate 2.5 Gbps


Figure 6.16: Effect of increase dispersion

Hence, further increasing the dispersion to 25 ps/nm/km has resulted in increase in

FWM effect. We can conclude that 16.75 ps/nm/km is an optimum value of

dispersion to minimize FWM for a fiber length of 75 Km. Next we shall how

changing the length of the fiber can make 25 ps/nm/km an optimum value for

dispersion instead of 16.75 ps/nm/km.

Case 4: Effect of fiber length on FWM

Figure 6.18: Effect of fiber length on FWM



TABLE 21

FWM CASE 4



Dispersion 16.75 ps/nm/km

Data rate 2.5 Gbps


Observations:

We reduced the length of the fiber keeping dispersion same. It is observed that

16.75 ps/nm/km is no more the optimum value to avoid the effects of FWM and

significantly better results can be obtained at 25 ps/nm/km as shown in the figure.

Figure 6.19: Fiber length and optimum dispersion



CONCLUSION

We designed a complete and economical optical fiber backbone network to

support very high data rate Ultra-Wideband Radio. The network carried out all

complex processing at a single centralized location considerably simplifying the

Radio Antenna Units (RAUs). For an even better and faster performance, the

network made use of all-optical components wherever possible. The use of all-

optical components allows exceeding the limitations of electrical components and

higher data rates can be achieved. A simple technique for optical generation of

Impulse radio UWB pulses was introduced along with the mechanism to transmit

the data of 32 different sites over the optical fiber backbone network using Dense

Wavelength Division Multiplexing. Furthermore, to differentiate users over the

common wireless medium and to allow the users of any site to share the medium

with each other, multiple access technique was employed in the system. Code

Division Multiple Access technique was embedded in the system, simulated and

found to be fulfilling the requirement decently. A comparison of different optical

amplifiers was conducted and the interest merit in choosing one in the face of the

other was studied. Finally, the effects of index based nonlinearities posing

limitations to system operation were studied and the system was optimized for

better performance.



REFERENCES

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http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=3984



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GLOSSARY

Radio-over-Fiber:

A technology where radio signals are transmitted over optical fibers from the

centralized headend to remote antenna units

Ultra-wideband radio:

A radio technology where user data is transmitted using very narrow pulses

thereby providing large modulation bandwidth.

W-PAN:

A short range Wireless Personal Area Network with a typical range of few

meters.

On-Off Keying:

A modulation technique that represents digital data as the presence or absence of the

pulse.

Pulse Position Modulation:

This modulation technique involves transmitting impulses at high rates, in the

millions to tens of millions of impulses per second. However, the pulses are not

necessarily evenly spaced in time, but rather they are spaced at random or pseudo-

noise (PN) time intervals.

Pulse Amplitude Modulation:

Pulse amplitude modulation is a scheme, which alters the amplitude of regularly

spaced rectangular pulses in accordance with the instantaneous values of a

continuous message signal.

Semiconductor Optical Amplifier

SOA are amplifiers which use a semiconductor to provide the gain medium

Erbium Doped Fiber Amplifier

EDFA is an optical repeater device that is used to boost the intensity of optical

signals being carried through a fiber optic communications system. An optical

http://en.wikipedia.org/wiki/Modulation

http://en.wikipedia.org/wiki/Digital

http://en.wikipedia.org/wiki/Data

http://en.wikipedia.org/wiki/Carrier_wave

http://www.webopedia.com/TERM/R/repeater.html

http://www.webopedia.com/TERM/F/fiber_optics.html



fiber is doped with the rare earth element erbium so that the glass fiber can absorb

light at one frequency and emit light at another frequency.

Raman Amplifier

Raman amplifier is achieved by a nonlinear interaction between the signal and a

pump laser within an optical fiber.

Raman scattering

Raman scattering or the Raman effect is the inelastic scattering of a photon.

Frequency Division Multiple Access

In FDMA, signals from various users are assigned different frequencies i.e. total

bandwidth of the channel is divided amongst all the users

Time Division Multiple Access

TDMA divides the channel on the basis of the time and each slot is known as time

slot. The whole bandwidth is available for each time slot.

Code Division Multiple Access

It is an access method that operates on Spread Spectrum Technique and

differentiates various communicators on the basis of codes. All the users are on a

single broad frequency of operation.

Self-Phase Modulation

SPM is a nonlinear optical effect of light-matter interaction

Cross-Phase Modulation

XPM or CPM is a nonlinear optical effect where one wavelength of light can

affect the phase of another wavelength of light through the optical Kerr effect.

Four-Wave Mixing

It is an intermodulation phenomenon in optical systems, whereby interactions

between 3 wavelengths produce a 4th wavelength in the signal.

http://www.webopedia.com/TERM/D/dope.html

http://en.wikipedia.org/wiki/Inelastic_scattering

http://en.wikipedia.org/wiki/Photon

http://en.wikipedia.org/wiki/Nonlinear_optics

http://en.wikipedia.org/wiki/Light

http://en.wikipedia.org/wiki/Matter

http://en.wikipedia.org/wiki/Kerr_effect

http://en.wikipedia.org/wiki/Intermodulation

http://en.wikipedia.org/wiki/Optical



Brillouin Scattering

It occurs when light in a medium (such as air, water or a crystal) interacts with

time dependent optical density variations and changes its energy (frequency) and

path.

http://en.wikipedia.org/wiki/Light

http://en.wikipedia.org/wiki/Water

http://en.wikipedia.org/wiki/Crystal

http://en.wikipedia.org/wiki/Density

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