Ateeq Mumtaz 07F-PhD-EE-33

1

MICROWAVE SIGNAL GENERATION IN OPTICAL DOMAIN

FOR RADIO OVER OPTICAL FIBER BASED BROADBAND

WIRELESS ACCESS

Ateeq Mumtaz

07F-PhD-EE-33

A thesis submitted for the partial fulfillment of the requirement for the degree of Doctor of

Philosophy

Thesis Supervisor

Prof Dr. Muhammad Khawar Islam

Department of Electrical Engineering

University of Engineering and Technology, Taxila, Pakistan

April , 2012

i

ACKNOWLEDGEMENTS

I am first of all thankful to the Almighty Allah for giving me the quest for knowledge and

the ambition to make efforts to discover new facts. I am grateful to my supervisor Prof. Dr.

Muhammad Khawar Islam for his continuous guidance and support. This dissertation is

dedicated to my parents and family whose encouragement helped me in the achievement of this

task.

ii

UNDERTAKING

I hereby undertake that ―Microwave Signal Generation in Optical Domain For Radio Over

Optical Fiber Based Broadband Wireless Access‖ is my own work and it has not been presented

anywhere else for award of degree.

(Ateeq Mumtaz) 07F-PhD-EE-33

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ABSTRACT

Bandwidth requirements due to media applications are increasing and Radio over Fiber

(ROF) is becoming an attractive choice for design and implementation of high speed wireless

network. This technology increases the overall bandwidth and total number of users in a wireless

system. Radio signal can be generated in electrical domain; however, opt ical generation is more

efficient as it overcomes the electrical bandwidth limitations. In this thesis, radio signal has been

generated in optical domain by beating two closely spaced wavelengths. Two lasers are used in

this scheme and the output is Amplitude Shift Keying (ASK) signal. Optical switches have been

used to implement the frequency hopping and the operating frequency of the scheme can be

changed by controlling the optical switch. The DWDM ROF (Dense Wave Division Multiplexed

Radio over Fiber) system with capability to change the operating frequency is presented. DWDM

ring has also been simulated with frequency plan to assign the wavelength in DWDM ROF

scenario. Performance analysis of the system is done in the presence of different transmissio n

impairment and optimization is done to achieve the best performance of the system. A signal

with data rate of 4 Gbps has been successfully transmitted up to distance of 125 km.

Wavelength used for beating and generation of the down link signal is reused at Remote

Antenna Unit (RAU). Erbium Doped Fiber Amplifier (EDFA) has been used to amplify the light

signal; different algorithms have been developed and simulated to find the exact value of EDFA

gain and number of amplifiers to be used. These algorithms can provide the exact placement of

EDFAs in the DWDM ring. It has been verified that if gain is properly adjusted and EDFAs are

properly placed, the Continuous Wave (CW) can be reused and uplink signal can be transmitted

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to Central Station (CS) using this wavelength. After simulation of the algorithm, DWDM ring is

simulated to verify the algorithms and bit error rate and eye diagram analysis is done to compare

the performance of the system.

Ultra-wide band (UWB) signals have been generated using optical biasing, optical delay

line and optical subtractor. UWB impulses are generated and their bandwidth has been optimized

by controlling the relative delay. DWDM UWB rings are simulated and single UWB generator

with capability to generate monocycle; doublet and triplet are presented. The effects of

nonlinearities on UWB pulses are also studied. Multiple access technique is incorporated with

this solution.

A Next Generation Multi Service Access Gateway (NG-MSAG) is presented for Radio

over Fiber. The architecture of this NG-MSAG allows the convergence of fixed, wireless, voice

and data services. This convergence will decrease the overall deployment and operational cost of

telecom operators especially those operators which are new in the industry and interested in

deploying nation-wide fixed and wireless networks. The transport layer is based on IP/DWDM,

Radio layer on ROF and services core is based on Next Generation Networks (NGN). The

connectivity of NG-MSAG with telecom node is simulated. The Performance analysis is

conducted and results are optimized which reveal the best performance when dispersion is kept

in the range of +2 to +4 ps/(nm.km).

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS................................................................................................................. i

UNDERTAKING .............................................................................................................................. ii

ABSTRACT .................................................................................................................................... iii

TABLE OF CONTENTS ....................................................................................................................v

LIST OF ABBREVIATIONS ............................................................................................................vii

LIST OF FIGURES AND TABLES ...................................................................................................ix

Chapter 1 Introduction .....................................................................................................................1

Chapter 2 ROF signal generation in optical domain..........................................................................16

2.1 Introduction.......................................................................................................................16

2.2 Theory and operating principle of signal generation .............................................................17

2.3 DWDM ROF Unidirectional ring .......................................................................................25

2.4. Performance analysis ........................................................................................................29

2.5. Conclusion ........................................................................................................................32

Chapter 3 Wavelength Reuse for Uplink .........................................................................................34

3.1 Introduction ......................................................................................................................34

3.2 Operating Principle ............................................................................................................35

3.3 Architecture of single fiber DWDM ROF ring with wavelength reuse ...................................41

3.4 Routing and Wavelength Assignment ..................................................................................42

3.5 Simulation results ..............................................................................................................44

3.6 Signal power optimization using EDFA...............................................................................46

3.7 Conclusion ........................................................................................................................51

Chapter 4 UWB Signal Generation .................................................................................................54

4.1 Introduction.......................................................................................................................54

4.2 UWB Monocycle Generation..............................................................................................55

4.3 Simulations and results ......................................................................................................60

4.4 UWB Doublet and Triplet ..................................................................................................66

4.5 UWB DWDM Ring ...........................................................................................................68

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4.6 Effect of Fiber Non-linearity...............................................................................................70

4.7 Conclusions.......................................................................................................................73

Chapter 5 Next Generation Multi-Service Access Gateway.................................................................76

5.1 Introduction.......................................................................................................................76

5.2 Architecture of NG-MSAG.................................................................................................77

5.3 Connectivity of NG-MSAG with Core network ...................................................................78

5. 4 Simulation results and discussion ........................................................................................81

5. 6 Performance analysis .........................................................................................................83

5.7 Conclusion ........................................................................................................................84

Chapter 6 Conclusions ...................................................................................................................86

6.1 Summary of Accomplishments ...........................................................................................86

6.2 Future Work ......................................................................................................................88

List of Publications ...........................................................................................................................89

vii

LIST OF ABBREVIATIONS

ASK Amplitude Shift Keying

BER Bit Error Rate

CS Central Station

CW Continuous Wave

CWDM Coarse Wavelength Division Multiplexing

DGD Differential Group Delay

DSF Dispersion Shifted Fiber

DWDM Dense Wavelength Division Multiplexing

EDFA Erbium Doped Fiber Amplifier

EA Electrical/Electronic Amplifier

FBG Fiber Bragg Grating

FWM Four Wave Mixing

LD Laser Diode

MMF Multi Mode Fiber

MSAG Multi-Service Access Gateway

MSS Mobile Soft Switch

MZM MachZhender Modulator

NGN Next Generation Network

PSK Phase Shift Keying

RAU Remote Antenna Unit

ROF Radio over Fiber

RSM Radio Services Module

SBS Stimulated Brilluoin Scattering

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SMF Single Mode Fiber

SOA Semiconductor Optical Amplifier

SPM Self Phase Modulation

UWB Ultra-wide band

WDM Wavelength Division Multiplexing

XPM Cross Phase Modulation

ix

LIST OF FIGURES AND TABLES

Fig.2.1. Modulation of CW using MZM

Fig.2.2. Block Diagram of Proposed Solution

Fig.2.3. Simulation Results (a) Combined Signal (b) Input Data Stream

Fig.2.4. Simulation Results (a) Modulated Signal (b) RF Spectrum of

Signal

Fig.2.5. Schematic diagram of DWDM System

Fig.2. 6 (a) DWDM Signal (b) Combined RF Signal

Fig.2.7. Frequency Hopping Sequence

Fig.2.8. DWDM Ring for ROF System

Fig.2.9. Spectrum of Signal at different Sites

Fig.2.10. ASK Signal at Site A, B, C, D & E

Fig.2.11. (a) Eye Diagram (b) Demodulated Signal

Fig. 2.12. Eye Diagram of 6Gbps

Fig.2.13. (a) Distance vs. BER (b) Dispersion vs. BER

Fig. 3.1. Schematic Diagram of Proposed Solution for wavelength reuse at RAU

Fig.3. 2. Pin (uplink) and Pout (uplink) simulation using algorithm-1 with Pin (Laser)

at Central Station =0dBm

Fig..3 2. Pin (uplink) and Pout (uplink) simulation using algorithm-1 with Pin (Laser)

at Central Station =10dBm

Fig.3.1. Pin (uplink) and Pout (uplink) simulation using algorithm-2 with Pin (Laser)

at Central Station =0dBm.

Fig.3.2. RWA algorithm for photonic generation and wavelength reuse.

Fig.3.3. Input wavelengths launched in fiber

Fig.3.4. Input Signal used for modulation of reused wavelengths

Fig.3.5. Optical signal received at the input of DWDM De-MUX

Fig.3.6. Received signal at receiver of Central Station

Fig.3.7. Received signal at receiver of Central Station after deploying EDFA in

DWDM ring

Fig.3.8. Eye diagram of received signal at receiver after deploying EDFA in DWDM

ring without considering RAU insertion loss of Central Station

Fig.3.9. Eye diagram of received signal at CS EDFA in DWDM ring without gain

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optimization

Fig.3.10. Eye diagram of received signal at CS after EDFA is deployed with proper

gain adjustments at RAU and De-MUX preamplifiers.

Fig. 3.11. Effect of output power on Bit Error Rate, Gain (G1, G2, and G3) of RAU

Preamplifier is fixed (10dBm) where as Gain of pre -amplifier at De-Mux is increased

from 20 to 40dB.

Fig. 3.12. Effect of output power on Bit Error Rate, Gain of RAU preamplifier is

increased where as Gain of pre-amplifier at De-Mux is decreased from 20 to5 dB.

Fig. 3.13. Effect of output power on Eye height, Gain of RAU preamplifiers denoted as

G1, G2, G3 where as Gain of pre -amplifier at De-Mux is denoted as Gain.

Fig. 4.1. Schematic Diagram of the Proposed Solution

Fig. 4.2. Working of a Differentiator

Fig. 4.3. Summation of spread bit streams

Fig.4.4. Modulated Gaussian pulse train

Fig.4.5. Modulated UWB monocycle pulses

Fig.4. 6. UWB monocycle generated at 2 Gb/s.

Fig.4. 7. Frequency spectrum of UWB monocycle at 2Gb/s. The spectrum is centered

at 5 Ghz and has a bandwidth of 6 GHz at -10dbm.

Fig.4.8. UWB monocycle generated at 1 Gb/s. (Twice the pulse width at 2 Gb/s)

Fig. 4.9. Frequency spectrum of UWB monocycle at 1Gb/s. The spectrum is centered

at 2 GHz and has a bandwidth of 3 GHz at -10dbm.

Fig.4.10. Relationship between input data rate and bandwidth for two different values

of Gaussian pulse width used.

Fig.4.11. 32 Dense Wavelength Division Multiplexed channels

Fig.4.12. Shape of the UWB monocycle received after passing through 1 km of optical

fiber links

Fig.4.13. UWB pulse generator

Fig.4.14. Spectrum of Monocycle

Fig.4.15. Spectrum of Doublet

Fig.4.16. Spectrum of Triplet

Fig.4.17. DWDM UWB rings

Fig.4.18. Wavelengths in Ring#3

Fig.4.19. Wavelengths in Ring#2

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Fig.4.20. Spectrum with zero SPM

Fig.4.21. Spectrum with SPM effects.

Fig.4.22. SPM effects without adjusting the dispersion of fiber

Fig. 4.23. SPM effects by adjusting the dispersion

Fig.5.1. Architecture of NG-MSAG

Fig.5.2. Connectivity of NG-MSAG with Core Network

Fig.5.3. DWDM signal at the output of DWDM MUX

Fig.5,4. DWDM signal at the input of NG-MSAG OADM

Fig.5.5 Input data stream, simulated as IP data

Fig.5.6. ASK signal used for modulation of first laser

Fig.5.7. PSK signal used for modulation of third laser

Fig.5.8. ASK signal received at RSM of NG-MSAG

Fig. 5.9. Digital data received at NG-MSAG

Fig. 5.10 PSK signal received at RSM of NG-MSAG

Table 2.1. Frequency (THz) at the output of Optical Switches and -∆f (GHz) at output

of photodetector

Table 2.2-.Frequency plan for DWDM ring

Table 3.1 Algorithms for optical amplifier gain adjustment for wavelength reuse at

RAUs

Table 4.1 Data rates corresponding to required delay

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Chapter 1 Introduction

1


In the field of Radio over Fiber, different schemes have been theoretically and

experimentally presented for generation of radio signals in optical domain. J.He et al [1]

demonstrated that an input signal frequency can be multiplied by four times using beating of two

wavelengths. They presented two different solutions, one using single MZM and other using two

MZMs. In second scheme, one MZM was used for wavelength components generation while one

separate MZM was used for modulation of input signal. They have also presented an idea to

reuse the wavelengths at RAU for modulation of signal. They presented a full duplex system and

achieved quadruple frequency multiplication i.e 10 GHz was up-converted to 60 GHz. They have

also presented the performance analysis of the system in the presence of dispersion and proved

that two MZMs solution can give better performance. However, they were only able to achieve

quadruple multiplication of the frequency, moreover electrical oscillator of 10 GHz was still

required to generated the 60 GHz signal.

Hao Chi et al [2] presented a solution of pulse shaping for generation of microwave

photonic generation. In this scheme, Sagnac Loop Filter (SLF) and Femto-Second Pulse Laser

(FSPL) were used. The shape of pulses generated by FSPL is reshaped by SLF as the spectral

response of SLF is sinusoidal. SLF is composed of Polarization Maintaining Fiber (PMF) and

polarization controllers. Although very high frequency signals can be generated using this

scheme, the operating frequency of this system depends upon the dispersive element. Moreover,

it is very difficult to deploy this scheme in DWDM system. Any change in dispersive element of

the system will also disturb the operating frequency of the system.


2

Myunghun and Kumar [3] proposed and demonstrated a scheme for doubling the

optoelectronic oscillator. Reference signal of 10 GHz was used and the signal was up-converted

to 20 GHz and the signal with rate of 1.25Gbps was transmitted to the distance of 50 Km. This

system was very complex and was only able to achieve the double of input of frequency. The

scheme is very complex and requires lot of optical components along with light sources of 1310

and 1550 nm.

Chul So Park et al [4] presented an idea and experimentally demonstrated the up-

conversion of signal using Stimulated Brilluoin Scattering (SBS). The beating was used for up-

conversion of 1 GHz to 11.831 GHz. Although, the up-conversion ratio is very good, the scheme

is fully depending upon the SBS which is band dependent and also a source of signal

degradation. Very accurate control of SBS is required to exactly tune the operating frequency of

the system. Moreover, the scenario also becomes very complex for DWDM implementation.

Longer transmission distance and any addition of SBS will also create frequency stability issues.

In the work done by Jianjan Yu et al [5], proper dc biasing and filtering was required to

generate the signal in optical domain. The generated signal was up converted by four times. A 64

GHz signal was generated using 16 GHz signal and was transmitted up to distance of 20 km.

Again, in this case an electrical oscillator was required for up-conversion of the signal.

Wavelength reuse concept was also discussed in this paper, however, DWDM implementation

and proper power adjustment was not presented in this solution.

Zhensheng Jia et al [6] reported on simultaneous generation of wired and wireless signal.

In this work, single modulator was used for 10Gbps wired signal and 2.5Gbps modulated at 40

GHz or 60 GHz was used as wireless signal. This is a good idea and can be used in next


3

generation systems; however, it requires further work for its implementation incorporating

DWDM schemes. The photonic generation was not presented in this scheme, rather a dual

electrode MZM was used to achieve two different modulations at the same time.

Microwave signal generation based frequency multiplication using polarization

interferometer was demonstrated by M. Gracia Larrode [7]. The signals of 24 GHz and 40 GHz

were generated by this scheme. Polarization maintaining fiber was used to implement the FM-IM

conversion. The 120 Mbps signal was transmitted over distance of 50 km using SMF and 4.5 km

using MMF. This work generated very efficient signal distance and data rate which was limited

to 50 km and 120 Mbps.

Ultra-wideband (UWB) over Radio over Fiber can be used to extend the capacity and data

rate of the wireless systems. Electrical domain UWB signal can also be used, however, to

overcome the electrical bandwidth limitations photonic generation of signal is preferred. All

optical based UWB pulses were generated by using cross-phase modulation and frequency

discrimination [8]. The optical pulse train was generated by FSPL with spectrum slicing filter to

control the pulse width. XPM was generated by injecting CW into DSF which serves as non-

linear medium. UWB monocycle or doublet pulses are generated by converting phase modulated

pulses into intensity modulated pulses by using FBG.

H. Chen et al [9] generated UWB monocycle pulses by optical polarization delay method.

This scheme was based on birefringence in Polarization Maintaining Fiber (PMF). The optical

signal was sent into a differential group delay (DGD) component with two polarization

orientation along the principal axis. After optical to electrical conversion, monocycle was

received. This solution can also switch the polarity of monocycle. Although, it was very simple


4

method for generation of UWB signal, however, it was only limited to generation of monocycle

pulses.

A detailed theoretical and experimental work was presented by Shilong Pan and Jianping

Yao [10]. Chirped intensity modulator and an Asymmetric Mach-Zehnder Interferometer

(AMZI) were used for generation of UWB signals. The output of chirped intensity modulator

was forced to split into two AMZI and recombined at polarizer. The output current at the

detector was proportional to the first derivative of the input signal due to which monocycle was

generated. Polarization controller can be used to change the polarity of generated pulses. By

properly adjusting the phase modulation index, polarization angle of incident light and phase

difference doublet and triplet were generated. This scheme had capability to generate monocyle,

doublet and triplet however very complex architecture and very controlled parameters are

required to generate the required patterns and modulation schemes. Any change in parameters

like modulation index, polarization angle etc can change the shape of pulse. Simulation and

experimental work is also required to verify its performance when deployed with DWDM and

effect of different transmission impairments need to be explored.

Photonic up-conversion was experimentally demonstrated using SBS [11] for 1.25 Gbps ROF

systems. 11 GHz signal was generated and transmitted up to distance of 13 km. This was an

attempt to validate the already proposed system by the same group; however, the operating

frequency of the signal generated was 11 GHz and it is very difficult to change the frequency to

some higher range using this scheme. To increase the frequency multiplication factor, the same

group presented a scheme for frequency tripling [12]. In this method both up-conversion and

tripling was used and 32.493 GHz signal was generated from input signal of 1 GHz.


5

Harmonic millimeter wave generation and frequency up-conversion using passively mode

locked multi-section DFB laser under external optical injection was successfully demonstrated

by Lee et al [13]. 30.42 GHz and 60.84 GHz were generated with local frequency of 15.21 GHz.

This scheme was different from already presented solutions, however, stable operation of laser is

required as the operating frequency is again depending on the beating and any change in

harmonics will change the output frequency of ROF.

Optical up-conversion of UWB was successfully done for the first time [14] to up-convert

the monocycle from lower frequency to higher frequency using MZM in nonlinear regime.

Uplink signal was also recovered from UWB signal by down-converting the signal to the

baseband.

Jian Zang et al [15] demonstrated a microwave frequency multiplier using two cascaded

modulators and multiplied the input frequency with the factor of four i.e. quadruple up-

conversion. Two coherent carriers are obtained using optical intensity modulator and then

modulated with the phase shifted signal which again generates the frequency components. At the

output of diode beating generates the signal which is four times of input signal. Both theoretical

and experimental analysis has been done. The scheme presented an efficient method for up-

conversion, however, two MZM may create problem as any change in biasing may generate un-

necessary components which may results in change in operating frequency of the system.

Bidirectional ROF link was reported by Zhenshen Jia et al [16] using DPSK for

downstream and OOK for upstream signal. This work presents a link in which DPSK signal at 10

GHz was up-converted to 40 GHz and was used as downlink signal. For uplink, 2.5 Mbps signal


6

was used whereas in real scenario uplink will be analog signal and it is not feasible to

demodulate the signal at RAU.

In [17] Semiconductor Optical Amplifier (SOA) was used to generate the millimeter wave

by using Four Wave Mixing (FWM). A millimeter wave of 41.7 GHz was generated by this

scheme. Two sidebands are sent to SOA, and the output is then sent to RAU where due to

beating the millimeter wave signal is generated.

Heterodyne radio over fiber system was presented [18] for 10Gbps data rates, however,

most of the experimental work was only related to coherent radio over fiber system with effect of

laser line-width on system performance and bit error rate calculations.

Tang Yo et al [19] generated the PSK using two cascaded single drive MZMs. Non-linear

modulation scheme was used. They have numerically and experimentally verified the solution,

however, still a local oscillator was used for generation of the signal. The PSK signal frequency

is five times of the local oscillator frequency.

Wavelength used to transmit the signal can be used to modulate the uplink signal [20-22].

In this case no laser is required at RAU as the light transmitted from CS to RAU is reused. These

solutions present a method of light reuse however no optical power adjustment mechanism has

been presented. Different other works have been done to simulate and experimentally

demonstrate [1], [5], [16] the reuse however no work has been done to properly optimize the

power of the signal to achieve the optimized performance. Moreover, all these solutions are

using point to point links b/w CS and RAU. No scheme has been reported to reuse the

wavelength in DWDM. EDFA can be used to increase the power level of the signal, however, no

EDFA placement and power level adjustment procedures have been reported so far.


7

Routing and Wavelength assignment in DWDM when dealing with photonic generation is

different from the normal DWDM systems [23-26]. The wavelengths used for beating are to be

properly assigned to ensure that exactly required operating frequency is generated on the RAU.

In this thesis, a RWA algorithm has been proposed and simulated for proper assignment and

routing of wavelengths. ROADM can be used to reconfigure the wavelengths if required [27].

Radio over Fiber is now one of the most researched fields in the optical fiber. In recent

years lot of work has been done [28-38] in this field. DWDM effects in ROF system have been

studied by Ehsan et al [28]. Performance of 5 Gbps OOK ROF system over distance of 50 km

has been analyzed in this work. ROF transmission has been experimentally done and mitigation

of SBS has been done by proper selection of segmented single mode fiber [29]. ROF was also

implemented in IEEE 802.11 and its performance has been analyzed in the presence of multiple

users [30]. Similarly, uplink and downlink coverage improvements have been reported using

distributed antenna network in [31]. In [32], separate clock distribution is used to mitigate the

signal distortion. Error free transmission was achieved in a 20-km optical fiber system. In [33],

simulations have been done for modeling and performance analysis of the WCDMA Radio over

Fiber. In [34], an automatic planning model for ROF which used GIS data to automate the

planning for entire architecture. Transmission quality measurement for 60 GHZ ROF systems

were reported in [35]. Performance analysis of OOK and QPSK was done and both SMF and

MMF were simulated.

Yin Je et al [36] also generated millimeter wave signal in the range of 40 GHz, however, 20

GHz clock was used as reference signal. In [37], photonic generation was experimentally

demonstrated using a polarization modulator. Next Generation Networks are now widely used in

telecom systems and most of the operators are migration to NGN based telecom core


8

infrastructure [38-41]. Multiservice gateways can be used to provide multiple services using

single gateways. These gateways are controlled by softswitches. In this thesis, these gateways

are also integrated with Radio over Fiber to present a new architecture of Next Generation Multi

Service Gateways.

In this thesis, Radio signal is completely generated in optical domain. Frequency hopping

was then simulated to shift the operating frequency. DWDM ROF systems are simulated and its

performance analysis is done to optimize the DWDM networks. Wavelength reuse concept for

DWDM ROF system is simulated and EDFA power is properly optimized to reuse the

wavelength in DWDM ring. UWB signals are generated in optical domain and effects of non-

linearities are studied. Architecture of NG-MSAG is proposed and simulated.

This dissertation is organized as follows:

Chapter 1 gives introduction to the issues in ROF and literature review

Chapter 2 presents photonic generation and frequency hopping and DWDM Rings

Chapter 3 demonstrates the wavelength reuse concept

Chapter 4 UWB signals are generated in optical domain

Chapter 5 NG-MSAG architecture is presented.

Chapter 6 Conclusions


9

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performance tradeoffs in the presence of wavelength converters‖ Optical Switching and

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based on wavelength selective switches : Architecture and Scalability‖, Optics

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Using a Polarization Modulator, IEEE Microwave And Wireless Components Letters, Vol.

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Technology, ICACT 2009







Chapter 2 Signal Generation

16

Chapter 2 ROF signal generation in optical domain

2.1 Introduction

The ROF has evolved as the most promising technique for the implementation of high

speed, broadband wireless access. The applications requiring higher bandwidth can be supported

by increasing the capacity of the system which can be acquired at the behest of reduction in the

size of the cell thus increasing the number of cells. In ROF system, the central station

modulates/demodulates the signal whereas remote station only converts optical signal into

electrical, amplify and transmit the signal using antenna. In recent years, trend in wireless access

has changed and different architectures of ROF system are proposed and experimentally

demonstrated [1-3]. The bandwidth demand per user of wireless systems is increasing due to

bandwidth consuming applications and increased number of users. The modern wireless access

systems for broadband services are using frequencies in the range of 60 GHz. The design of

transmitter, modulator and oscillator becomes complex due to the use of high operating

frequency and large bandwidth.

To generate the high frequency ROF signals, different schemes have been used [4-7]. Most

of these schemes employ the technique of beating different frequency signals to translate the

frequency spectrum to higher value which require electrical oscillators and modulators. The

oscillator free scheme is proposed by shaping the pulse to generate the modulated signal at

receiver; however, it is not possible to fully control the operating frequency due to its

dependence on dispersive elements [8]. Moreover, a femto-second pulse laser (FSPL) is also


17

required to implement such kind of solutions. A heterodyning ROF system is proposed and

experimentally demonstrated by beating two wavelengths to study the effect of linewidth of laser

and its impact on the system performance [9]. The DWDM systems have flexibility to increase

the number of wavelengths using single fiber thus increasing the overall capacity of the system.

In wireless systems, frequency hopping is required to ensure the quality of service in the

presence of interference along with increased security. It is imperative to generate the ROF

signal photonically incorporating the desired modulating format without using electrical

oscillator and modulator. The future ROF systems require DWDM support and frequency

hopping capability to fully exploit the capacity of fiber and increase the security and

performance of the system.

A complete scheme to optically generate Amplitude Shift Keying (ASK) Radio over fiber

DWDM signal incorporating frequency hopping capability is presented in this chapter. The

system is completely electrical oscillator free and can generate the signals in the range of 60-80

GHz. DWDM Ring is simulated to analyze the performance of the system.

2.2 Theory and operating principle of signal generation

In the proposed system, one laser is modulated with the input signal whereas second is used

as CW. In Mach-Zehnder Modulator (MZM) shown in Fig.2.1, when input electrical signal is

―1―, the output is the optical signal generated by laser whereas in the case of ―0‖ there is no

signal at the output. The second laser, without modulation is coupled with the modulated optical

signal which results in beating at the receiver. Suppose that input electrical signal at the input of

MZM is 0,1,0,1 and laser signal is E1 then at the output of MZM signal will be 0,E1,0,E1.


18

The equations for Laser 1 and Laser2 are given as:

Only E1 signal is modulated using MZM. E2 is fed to coupler directly without any

modulation as shown in fig. 2.2. 1 and 2 are operating frequencies of Laser 1 and 2. The phase

difference between two lasers is zero as phase synchronization is required for this scheme.

Optical switch can be used to change frequency 2 on the basis of control signal.

When light is received at photodiode the current is represented by

Now suppose that,

Fig.2.1. Modulation of CW using MZM

MZM E 1

0101

0 , E 1 , 0 , E 1


19

(2.9)

The photodiode will filter out components and resulting in the signal =0, 0, ∆f, 0, ∆f, 0,

∆f, where ∆f is proportional to . Thus ( ) can be used for the beating and can

generate the frequency components corresponding to difference of . The optical switch

can switch the frequency thus the final operating frequency can be changed by controlling the

control signal of optical switch. The control switch can be synchronized with some hopping

mechanism using slow frequency hopping system in both sequential and random manner.

I have proposed a scheme shown in Fig.2.2 for generation of Amplitude Shift Keying

signal (ASK). The E1 signal is modulated with digital signal using MZM whereas E2 signal is

un-modulated CW. Both these waves are launched in a single SMF using coupler. These two

wavelengths when received at photodetector generate the difference frequency component.

Amplitude modulated signal is received which can be fed to antenna using EA at Remote

Station. This solution has capability to dynamically change its operating frequency by changing

the operating wavelength of Laser2.

Base Station can simply use this method to modulate and transmit the data to remote stations.

No modulation is required at Base Station as the complete modulation will be done in optical

domain. The wavelength of Laser1 is 1551.239 nm whereas wavelength of Laser2 is 1551.72

nm. The signal is coupled into fiber using a coupler and spectrum of combined signal is shown in

Fig.2.3 (a). This figure shows two closely spaced signal with separation of approximately

0.48nm which corresponds to ∆f= (c/λ2 ) ∆λ ≈ 60 GHz.

Input data stream used for modulation of MZM is shown in Fig.2.3 (b). The signal generated

at photodiode after beating of these closely spaced wavelengths is simulated and shown in


20

Fig.2.4 (a). This signal is amplitude modulated signal and can easily be demodulated using

amplitude demodulator. The RF spectrum of this signal is shown in Fig.2.4 (b) which clearly

shows the 60 GHz signal.

Fig.2.2. Block Diagram of Proposed Solution

10100101001

MZM

Coupler

Photodiode

EA

Laser 1

Laser 2

SMF

E=E1,0,E1,0,0,E1,0,E1,0,0,E1

Signal at output of coupler

= (E2 + E)

I= (E1 +E2)2


21

2.3. WDM System with Frequency hopping and Simulation Results

Fig.2.3. Simulation Results (a) Combined Signal (b) Input Data Stream

Wavelength (m)

Po

we

r(d

Bm

)

Am

plitu

de

(v

)

Time (sec)

Fig.2.4. Simulation Results (a) Modulated Signal (b) RF S pectrum of Signal

Time (sec) Frequency (Hz)

Po

we

r(d

Bm

)

Am

plit

ud

e (

v)


22

Schematic diagram of DWDM based proposed solution is shown in Fig.2.5. In ROF CS

(Central Station), three optical switches and four MZM are used to support four channels on

different frequencies. Three channels are configured on frequency hopping whereas one channel

is kept as spare for broadcasting and control. Optical switches are connected to lasers and switch

the CW light from input port to the output port depending upon the hopping sequence. MZM

modulate the CW light from laser with input data stream and provides the optical signal to

DWDM Multiplexer.

Fig.2.5. Schematic diagram of DWDM System

L

A

S

E

R

A

R

R

A

Y

Switch-1

MZM

M

U

X

MZM

L

A

S

E

R

A

R

R

A

Y

MZM

SMF

MZM

Coupler

Coupler

PIN+PA

PIN+PA

Spare/Without

hopping

OSA

Switch-3

Switch-2

Hopping Sequence Controller

D

E

-

M

U

X

hopping

Spare/Without

Coupler PIN+PA

Coupler PIN+PA

Input Data Stream


23

Optical switches also provide the switched wavelength to Multiplexer. DWDM signal is

launched into single mode fiber and demultiplexed by DEMUX on remote antenna site. Two

ports are combined using coupler to generate the wavelength/frequency pairs for beating at

photodiode. Simulation parameters are shown in Table-2.1. Frequency pairs are adjusted to

ensure that the resultant frequency pairs generate the required operating frequency at

photodetector. Operating wavelength of master laser is fixed whereas optical switch switches the

frequency of secondary laser as per defined sequence in Table-1. For example, to generate the

operating frequency (∆f1), signal at master laser is fixed at 193.2 THz and optical switch

switches the f1 to ensure that 193.140 THz, 193.145 THz and 193.135 THz are received at input

port of DWDM MUX in a sequence. The DWDM signal launched in fiber is shown in Fig.

2.6(a), which clearly shows the closely spaced frequency pairs. The Fig.2.6 (b) shows combined

RF spectrum of signal generated at remote antenna stations. Frequency hopped between 60, 55

and 65 GHz signal is shown in Fig.2.7.

Fig.2. 6(a). DWDM Signal (b) Combined RF Signal

Frequency (Hz) Frequency (Hz)

Po

wer(

dB

m)

Pow

er(

dB

m)


24

Table 2.1 - Frequency (THz) at the output of Optical Switches and -∆f (GHz) at output of

photodetector

M-Laser

(THz) 193.2 195.2 197.2 Output at photodetector

Seq. f1 f2 f3 ∆f1 ∆f2 ∆f3

00 193.140 195.135 197.145 60 65 55

01 193.145 195.140 197.135 55 60 65

10 193.135 195.145 197.140 65 55 60

11 SPARE

Fig.2.7. Frequency Hopping Sequence

Frequency (Hz)

Pow

er(

dB

m)

S=01

S=10S=00


25

2.3 DWDM ROF Unidirectional ring

The DWDM system has been simulated for ROF ring with five OAMDs with each having

four transmitters. The schematic diagram is shown in Fig.2.8. Single fiber is used and

unidirectional ring is simulated. The simulation is conducted for different carriers and frequency

plan shown in the Table-2. A, B, C, D, E are five sites each having four carriers e.g. A1, A2, A3

and A4. A DWDM grid of 200GHz is used to ensure that there is no interfere nce between

adjacent wavelengths. 5 Km long fiber is used between each OAMD as this distance is enough

for metropolitan networks.

Fig.2.8. DWDM Ring for ROF System

OADM OADM

COUPLE

R

PI

N

OADM OADM

COUPLE

R

PI

N

SITE-B

OADM OADM

COUPLE

R

PI

N

OADM OADM

COUPLE

R

PI

N

SITE-C

OADM OADM

COUPLE

R

PI

N

OADM OADM

COUPLE

R

PI

N

SITE-D

OADM OADM

COUPLE

R

PI

N

OADM OADM

COUPLE

R

PI

N

SITE-E

L

a

s

e

r

A

r

r

a

y

D

W

D

M

M

U

X

MZM

MZM

OADM OADM

COUPLER

PIN

OADM OADM

COUPLER

PIN

OADM OADM

COUPLER

PIN

OADM OADM

COUPLER

PIN

SITE-A


26

Overall length of fiber is 30 Km. Signal launched in fiber and respective signals after

dropping the required wavelengths are shown in fig.2.9. Different input data stream is used for

modulation of different sites, however, for same cells of a site same input data stream is used for

simulation. Spectrum at input of Site-A is shown in Fig.2.9 (a) whereas spectrum at input of

Site-B is shown in Fig. 2.9(b) which clearly shows that the wavelength has been dropped from

the main signal. Fig.2.9(c) is the spectrum of signal at input of Site-C and Fig. 2.9(d) is the final

signal after passing all the sites and dropping all the wavelengths.

Fig.2.9. S pectrum of Signal at di fferent Sites

Frequency (Hz)

P

o

w

e

r

-

d

B

m

Frequency (Hz)

Frequency (Hz) Frequency (Hz)

P

o

w

e

R

-

d

B

m

a b

dc


27

Detailed frequency plan is shown in the Table-2.2. In this plan, on the basis of operating

frequency the required wavelengths are selected for primary and secondary lasers. Primary laser

L1 is connected to DWDM directly whereas secondary laser is connected to MZM to modulate

the CW with input data stream. ASK signal generated on different sites is shown is Fig.2.10. All

these signals are shown without amplification to check the minimum level of signal and can be

amplified using RF amplifiers.

Fig.2.10. ASK Signal at Site A, B, C, D & E

Time (sec)Time (sec)

Time (sec) Time (sec)

Time (sec)

Am

plit

ud

e (

v)

Am

plit

ud

e (

v)

Am

plit

ud

e (

v)

Am

plit

ud

e (

v)

Am

plit

ud

e (

v)

X10 -8

X10 -8X10

-8

X10 -8

X10 -8


28

Table 2.2- Frequency plan for DWDM ring

Site/Cell

Frequency

(GHz) L1 (THz)

L2

(THz)

E4 88 195.6 195.688

E3 86 195.4 195.486

E2 84 195.2 195.284

E1 82 195 195.082

D4 80 194.8 194.88

D3 78 194.6 194.678

D2 76 194.4 194.476

D1 74 194.2 194.274

C4 72 194 194.072

C3 70 193.8 193.87

C2 68 193.6 193.668

C1 66 193.4 193.466

B4 64 193.2 193.264

B3 62 193 193.062

B2 60 192.8 192.86

B1 58 192.6 192.658

A4 56 192.4 192.456

A3 54 192.2 192.254

A2 52 192 192.052

A1 50 191.8 191.85


29

2.4. Performance analysis

The performance of the proposed system is analyzed for optimization. The received signal at

the output of photodiode is demodulated using amplitude demodulator and the output is shown in

the Fig. 2.11(b). This output is received when DWDM based ROF System is used with the data

rate of 1Gbps. The eye diagram is shown in Fig. 2.11(a).

The system performance is analyzed on following parameters:

Maximum Data rate: The maximum data rate supported by system is 4Gbps. The simulation

results show wide open eyes and low bit error rate for data transmission up to 4 Gbps. However,

when data rate is increased further, the demodulated signal cannot be detected properly as the

eyes get closed and resulting BER is high as shown in fig. 2.12.

Fig.2.11. (a) Eye Diagram (b) Demodulated Signal


30

Maximum Transmission Distance: The maximum distance achieved using Ideal modulator

is 175 Km. However, when real modulator is used, the error rate at 175 Km is very high and the

maximum transmission distance with good performance using real modulator is limited to 100

km as shown in Fig. 2.13(a).

Modulator type: Different types of modulators are simulated and results prove that LiNbO3

modulator gives better performance for the proposed scheme as compared to MZM. The

transmission distance can be increased to 25% by using LiNbO3 modulator. The 4 Gbps data

stream can be transmitted up to distance of 125 km using LiNbO3 MZM, whereas, simple MZM

can only transmit the same signal up to distance of 100 km. This increased transmission distance

is achieved by connecting both arms of LiNbO3 modulator with the same input signal.

Fig. 2.12 Eye Diagram of 6 Gbps


31

Dispersion: The simulation results for different values of fiber dispersion to optimize the

system performance are shown in Fig. 2.13(b). The good performance can be achieved when

dispersion is kept in the range of 2 ~ 4 ps/(nm.km) and using LiNbO3 modulator. In DWDM

systems, the limiting factor is Four Wave Mixing (FWM) which is high at zero dispersion

whereas higher dispersion will result in pulse spreading and hence increased BER.

Fig.2.13. (a) Distance vs. BER (b) Dis persion vs. BER


32

2.5. Conclusion

ASK signal is generated in optical domain using two lasers. The scheme is integrated with

DWDM and optical switching is used for hopping of operating frequency. Signal of 60 GHz is

successfully generated using this scheme and this signal is transmitted up to distance of 100 km.

Eye diagram and BER analysis proves that ROF system can support high data rate. Different

frequency planning schemes are successfully simulated in DWDM systems. The complete

modulation is done in optical domain without involving any electrical oscillator. It can switch the

operating frequency as per hopping sequence and can support data transmission up to 4Gbps.

Operating frequency is changed from 55 to 60 and then to 65 GHz successfully.

References

[1] Xiaoqiong Qi, Jiaming Liu, Xiaoping Zhang, and Liang Xie, IEEE Journal Of Quantum

Electronics, Vol. 46, No. 8, August 2010

[2] Toshiaki Kuri, Hiroyuki Toda, Juan Jose, Vegas Olmos and Ken-ichi Kitayama, Journal Of

Lightwave Technology, Vol. 28, No. 16, August 15, 2010.

[3] Marta Beltrán and Roberto, IEEE Transactions On Microwave Theory And Techniques, Vol.

58, No. 6, June 2010

[4] J. He , L. Chen, Z. Dong, S. Wen, J. Yu, Optical Fiber Technology, VOL. 15, 2009

[5] Chul Soo Park, Chung Ghiu Lee and Chang-Soo Park , IEEE Photonics Technology letters,

VOL. 19, NO. 10, MAY 15, 2007


33

[6] Myunghun Shin and Prem Kumar, IEEE Photonics Technology Letters, Vol. 19, No. 21,

November 1, 2007

[7] M. García Larrodé, A. M. J. Koonen, J. J. Vegas Olmos, and E. J. M. Verdurmen, Journal

Of Lightwave Technology, Vol. 25, No. 6, June 2007.

[8] Hao Chi, Fei Zeng, Jianping Yao, IEEE Photonics Technology Letters, Vol. 19, No. 9, May 1,

2007

[9] Ignacio Gonzalez Insua and Christian G. Schäffer, in National Fiber Engineer Conference,

2009

Chapter 3 Wavelength Reuse for Uplink

34


3.1 Introduction

In this chapter a single fiber DWDM ring scheme with wavelength reuse for uplink and

optical signal generation for downlink has been presented. Instead of using new laser for uplink,

the wavelength already used for wavelength beating to generate the downlink signal in optical

domain is reused for uplink. Algorithms for power optimization using Erbium-Doped Fiber

Amplifier (EDFA), wavelength routing and wavelength assignment are developed and simulated.

The DWDM-ROF scheme reusing the wavelength can support data rate in the range of 1Gbps.

Fifteen Remote Antenna Units (RAU) are simulated in this scheme, however, these can be

increased by adding more number of wavelengths. The proposed scheme simplifies the

architecture or RAUs, decreases the overall cost while enhancing the bandwidth and operational

flexibility of Radio over Fiber Systems. Wavelength reuse concept has been used in optical fiber

systems without using DWDM [6-9], whereas, these systems operate on single wavelength and

use separate fiber for uplink and downlink. The gain adjustment and EDFA placement method

has not been discussed in these systems. Moreover, all these methods use electrical local

oscillators to generate the signal at lower frequency and then up-convert it to some higher

operating range. A method for generation of radio signals in optical domain in DWDM systems

has already been presented [2]. This work only relates to generation of radio frequency (RF)

signal in optical domain with frequency hopping and transmission between point to point

DWDM systems for both uplink and downlink.


35

In this work, the uplink signal is modulated using Mach-Zehnder Modulator (MZM), where

as continuous wave (CW) used for photonic generation is reused as input CW of MZM. An

optical amplifier is used to optimize the performance of the system by achieving the required

output power. The algorithms for EDFA placement and gain adjustment are developed. The

proposed scheme results in a complete Central Station (CS) without any electrical oscillator and

RAU without any Laser. No electrical modulation/de-modulation is done at RAU as CS is

responsible for the same.

3.2 Operating Principle

A single fiber DWDM scheme without any local laser required at 15 RAUs, generates

Amplitude Shift Keying (ASK) signal by beating two wavelengths. One of these two dropped

wavelengths can be reused as CW at RAU to modulate the uplink signal. Architecture of all

RAUs is same; whereas, only one RAU-5 is shown in fig.3.1. The wavelength from first Re-

configurable Optical Add Drop Multiplexer (ROADM) is un-modulated CW and can be reused, if

power levels are optimized to ensure the appropriate power levels of wavelengths dropped at

RAU, inserted at RAU and received at CS, remains above the required power levels. At any RAU,

un-modulated light divided by 3-dB coupler is given by:

(3.1)

Where Pin is power of laser at CS, Loss at one RAU is estimated as 2dB, 3-dB is loss due

to coupler and L is the distance (length of fiber) of that specific RAU from CS.

(3.2)


36

Figure 3.1. Schematic diagram of proposed method for wavelength reuse at RAU

RAU-1

RAU-3

RAU-2

DWDM- MUX

Laser Array

Optical Swtiches

MZM block

Input

Data

Strea

m

ROADM-1 ROADM-2

Splitter Coupler

MZM PIN

Uplink RF

Singal

Downl

ink RF

singal

RAU-4

RAU-7

RAU-6

RAU-

10

RAU-8

RAU-9

RAU-

15

RAU-

13

RAU-

14

DE-MUX

Photodetectors (PIN) Upli

nk

(RF)

RAU-

11

RAU-

12

RAU-5


37

If received power, Pout (uplink) is greater than the sensitivity of PIN diode at CS, the

wavelength received at RAU can easily be reused for uplink. Typical value of the sensitivity of

PIN diodes is -20 dBm, so un-modulated wavelength dropped for photonic generation of

downlink signal can be reused if

If power of optical signal is low due to attenuation then optical amplifier can be used to

increase the power level. To optimize the gain of amplifier, two algorithms are shown in Table-

1. The algorithm-1 checks for Pin (uplink) as Pout is depending on this power. If Pin(uplink) is

less than -8dBm then one optical amplifier with 3dB gain is inserted.

(3.3)

Pin is calculated again and if the out power is still less than -8dBm then gain is increased

by 3dB. Similar calculation is performed for all RAUs and optical amplifiers with required gain

are deployed. Output power depends upon the value of Pin(uplink), RAU insertion loss and

optical fiber loss. As shown in fig.3.2, Pout is less than -30dB for first 4 to 5 RAUs. The same

algorithm with laser input power of 10dBm is simulated; it cannot resolve the issue of power

loss, as evident from fig.3.3. The algorithm-2 has capability to optimize the system gain by

achieving a reasonable output power level at receiver. In this algorithm, an optical amplifier

with Gain G2 is added before De-Mux and its gain is calculated as per algorithm-2.


38

(3.4)

Figure 3.2. Pin (uplink) and Pout (uplink) simulation using algorithm-1 with Pin

(Laser) at CS =0dBm.

Figure 3.3. Pin (uplink) and Pout (uplink) simulation using algorithm-1 with Pin

(Laser) at CS =10dBm.


39

Simulation results shown in fig.3.4 clearly reveals that now wavelength can be reused on

all the RAUs. In section-3.6, different scenarios have been simulated to study the effect of using

EDFA on Bit Error Rate (BER) and eye-diagram. These simulations prove that by adjusting the

power of EDFA, the CW at RAU can be reused. However, both pre-amplifiers at RAUs and pre-

amplifiers at De-Mux are mandatory to adjust the power levels for wavelengths to avoid non-

linear effects. This reuse will simplify architecture of RAU, Routing and Wavelength assignment

problem for Radio over Fiber on single fiber DWDM system. These algorithms are simulated in

Matlab, and Optiwave is used for verification of the results by incorporating all required optical

components with their respective losses and transmission impairments.

Figure 3.4. Pin (uplink) and Pout (uplink) simulation using algorithm-2 with Pin (Laser) at

CS =0dBm.


40

Table.3.1. Algorithms for optical amplifier gain adjustment for wavelength reuse at RAUs

Algorithm-1 Algorithm-2

n= number of nodes

i=1;

k=current node

while (i<=n)

{

Calculate Pin(uplink)[k]= Eq.3

Calculate Pout(uplink)[k]= Eq.2

If ( Pin(uplink)[k] <-8)

{Gain=Gain+3

Repeat calculation of node k

}

k=k+1

}

n= number of nodes

i=1;

k=current node

while (i<=n)

{

Calculate Pin(=uplink)[k]= Eq.3

Calculate Pout(uplink)[k]= Eq.4

If ( Pin(uplink) <-8)

{Gain=Gain+3

while (pout[k] <-20)

{Gain2=Gain2+3

Calculate pout[k])=Eq.4

}

Repeat calculation of node k

}

k=k+1

}


41

3.3 Architecture of single fiber DWDM ROF ring with wavelength

reuse

The scheme proposed for single fiber DWDM ring is shown in fig.3.1. Fifteen RAUs, each

with single operating frequency are used in this ring. The laser array is used to generate 32

wavelengths, out of which 30 are used in this method. One wavelength from laser array is

connected with switch and other with MZM. Input data stream is connected to MZM to modulate

the light. The output of MZM and one wavelength from switch is connected to the Multiplexer.

Similarly, at CS the multiplexed uplink signal is de-multiplexed using De-Mux and fed to

Photodiodes for optical to electrical conversion. At each RAU, two wavelengths are dropped and

one is inserted. To make the block diagram simple, detailed architecture of only one RAU-5 is

shown. Remaining RAUs have the same architecture. ROADM is now widely used in DWDM

system to reconfigure wavelengths dynamically [10]. ROADMs are used at RAUs to ensure that

the operating wavelengths can be changed if required. Two drop/insert wavelength ROADMs

are cascaded to drop two wavelengths. The wavelength dropped, using first ROADM is passed

through splitter where it is divided into two equal parts. One is used as input to MZM and second

is used as input to the coupler where it is coupled with the wavelength dropped by second

ROADM. The two closely spaced wavelengths generate beating effect which results in ASK

signal at the output of PIN diode which is fed to the antenna.

Second part of divided wavelength, modulated by input RF signal at the output of MZM, is

launched into fiber using first ROADM. In this case, no laser is used on RAU to generate CW

and the wavelength already dropped by using ROADM is reused as uplink CW. The transmitting

and receiving nodes in CS, connected to RAU-1 and RAU-15 are shown separately in the fig.3.1.

These nodes are shown in simplified form, actually they are complex. Optical switc hes can be


42

used to make the system more flexible to generate and switch the operating wavelengths as per

requirements. The MZM block contains fifteen modulators for input data channels. The DWDM

32x1 Multiplexer is used to multiplex all generated wavelengths on single fiber. A de-

multiplexer is used to separate all wavelengths at the receiving node. The output ports of de-

multiplexer are connected to PIN detectors where the signal is converted into electrical RF

signal. In ROF systems, complete modulation and demodulation is done at Central Station.

RAUs are separated by 1km each and single mode fiber with 0.2 dB/km attenuation and

+4ps/(nm.km) dispersion is used. Initially, RAU insertion loss is considered as zero, however,

during EDFA optimization all losses including RAU insertion losses are considered.

3.4 Routing and Wavelength Assignment

Routing and Wavelength Assignment (RWA) problem in this DWDM scheme is different

as compared to normal DWDM [11-14] systems. In this scheme, λ1, λ2, .. λ30 are configured by

using RWA algorithm shown in fig.3.5. The first ROADM in all RAUs is configured on odd

wavelengths i.e. λ1, λ3, λ5, …., λ29. A 200 GHz grid is used between operating wavelengths of

first ROADM of two consecutive RAUs i.e. λ1 and λ3. However, operating wavelengths can be

changed if required. The wavelength configuration of second ROADM is complex as it is not

fixed on some specific value and can be changed immediately by changing the operating

frequency of the RAU. Wavelengths on second ROADM are denoted as λ2, λ4, λ6, …. , λ30.

There is no fixed grid between consecutive wavelengths. In general, the wavelength of any

RAUn can be represented as λ (2n-1) for first ROADM and λ2n for second ROADM. All

calculations of wavelengths, selection and routing of these wavelengths depend upon the

operating frequency. ∆λ, λ1, λ2 are calculated by


43

, (5)

∆f is operating frequency of RAUn. λ1 is selected from list of available wavelengths and

is calculated as:

(6)

Figure 3.5. RWA algorithm for photonic generation and wavelength reuse.

Select the

operating

frequency (f)

Calculate ∆λ

Scan list of free λ

Select primary λ1

Select secondary

λ2=λ1+∆λ

Tune the laser for

respective RAUs

Configure

ROADMs of

RAUs

All RAUs

configured

End

Block the request

Block the request

Block the request

No free

λ

No free

λ1

No free

λ2

Yes

No


44

3.5 Simulation results

DWDM spectrum of 15 RAUs with 30 wavelengths, launched in Single Mode Fiber (SMF) is

shown in fig.3.6. Initially, insertion loss of RAUs is ignored to keep the simulation simplified,

however, during analysis of EDFAs, insertion loss is included to gain the real picture of this

scheme. Two closely spaced wavelengths at each RAU are separated by a fixed difference.

The wavelength with higher power is first wavelength of RAU whereas the wavelength with

lower power is the second one. The Amplitude Shift Keying Signal at carrier frequency of 5GHz

with input data rate of 1Gbps is generated. The simulation results at RAU-5 operating at the first

wavelength of 1556.5 nm are discussed here. This wavelength is dropped and optical power is

divided by a 3 dB splitter. This CW is then modulated by MZM with the input ASK data shown

in Fig.3.7. The output of MZM is inserted at first ROADM where it is propagated in fiber and is

not dropped/de-multiplexed until the signal is de-multiplexed by DWDM de-multiplexer at

receiver of CS. The spectrum of the signal received at the input of de-mux is shown in Fig.3.8.

This spectrum confirms that the 1556.5 nm is propagated in the fiber, whereas, all remaining

wavelengths are dropped as their powers are very low as compared to the power of 1556.5nm

wave. This signal is de-multiplexed and optical to electrical conversion is done using PIN

photodiode. The converted signal is shown in Fig. 3.9 which can be easily demodulated to

retrieve the data signal. It can be observed that the received signal is distorted due to noise.


45

Figure 3.6. Input wavelengths launched in fiber

Figure 3.7. Input Signal used for modulation of reused

wavelengths

Figure 3.8. Optical signal received at the input of DWDM De -

MUX


46

3.6 Signal power optimization using EDFA

The performance of the system can be increased by using an EDFA to increase the signal

level. The amplified received signal after O/E conversion is show in Fig.3.10. To simulate and

compare the effect of EDFA, RAU-6 is configured and modulated with Gaussian pulse of 1Gbps.

The Eye diagrams of both RAU-5 (Return to zero - RZ) and RAU-6 (Gaussian) are shown in

Fig.3.11. For detailed analysis of EDFA‘s effects on system performance, we have to insert the

insertion loss of RAUs to simulate the exact loss and then simulate different gain values for

analysis of received power and its effect on bit error rate. The insertion loss at each RAU is

estimated as 2 dB. In section-3.2, detailed link budget analysis and algorithms for optimization

of output power have been proposed. Only inserting the EDFA at RAUs or just before De-Mux

cannot resolve the issue of output power. If only EDFA at De-Mux is used then the signal

already distorted due to noise is also amplified. Similarly, if EDFAs with very high gain are used

as pre-amplifier at RAUs then it will increase the power of optical signal in optical fiber and will

Figure 3.9. Received signal at receiver of CS


47

generate nonlinear effects and it will be very difficult to mitigate these effects. The proper

optimization is required and even if we add one EDFA as preamplifier before RAU5, the issue

cannot be resolved. When EDFA with 30-dB gain is deployed as pre-amplifier of De-Mux and

two EDFAs of 10-dB as pre-amplifiers of RAU5 and RAU7, the eye diagrams of RZ and

Gaussian pulses are shown in fig.3.12. The signal is still distorted and it is very difficult to

demodulate this signal as the eye opening is not wide enough. One more EDFA is used as pre-

amplifier at RAU-13 and gain of EDFA used as pre-amplifier of De-Mux is reduced from 30-dB

to 20-dB. The corresponding Eye diagram is shown in fig.3.13, which clearly reflects that with

the increase in gain of RAU preamplifiers the eye height starts increasing.

Figure 3.10. Received signal at receiver of CS after deploying EDFA in DWDM

ring

Figure 3.11. Eye diagram of received signal at receiver after deploying EDFA in

DWDM ring without considering RAU insertion loss of CS


48

Figure 3.12. Eye diagram of received signal at CS with EDFA in DWDM ring without gain

optimization

Figure 3.13. Eye diagram of received signal at CS after EDFA is deployed with proper gain

adjustments at RAU and De-MUX preamplifiers.

Bit period Bit period

Am

pli

tud

e (v

)

Bit period Bit period

Am

pli

tud

e (v

)


49

The performance of the system is increased by using EDFA with optimized gain at different

stages. The BER is also improved by amplifying the signal using RAUs pre-amplifier stages.

Comparison of effects on BER by increasing the Gain of pre-amplifier stage of De-Mux and pre-

amplifier are shown in the fig.3.14&15. These figures clearly reflects that the EDFA gain

distribution in pre-amplifier stages results in better performance as compared to increasing the

gain of pre-amplifier at De-Mux stage. A similar comparison of eye diagram height is shown in

fig.3.16, in which height of eye for increasing the gain of EDFA at RAU amplifier stages is

greater than that of increasing the De-Mux pre-amplifier gain and it almost becomes double

when output power is increased beyond -6dBm.

Figure 3.14. Effect of output power on Bit Error Rate, Gain (G1, G2, and G3) of RAU Preampli fier is

fixed (10dBm) where as Gain of pre-ampli fier at De-Mux is increased from 20 to 40dB.


50

Figure 3.15. Effect of output power on Bit Error Rate, Gain of RAU preamplifier is

increased where as Gain of pre-amplifier at De-Mux is decreased from 20 to5 dB.

Figure 3.16. Effect of output power on Eye height, Gain of RAU preamplifiers denoted as

G1, G2, G3 where as Gain of pre -amplifier at De-Mux is denoted as Gain.


51

3.7 Conclusion

DWDM based Radio over Fiber down link based on photonic generation and uplink based

on wavelength reuse can be deployed to increase the capacity of the DWDM-ROF systems.

System is flexible in terms of operating frequency and number of RAUs as number of RAUs can

be increased or decreased by adding or decreasing the operating wavelengths. Similarly, as

operating frequency is depending on the DWDM wavelengths, same can be configured on any

RAU using optical layer. This method will simplify the architecture of RAUs, as the number of

lasers used to design the overall system is reduced, however, the gain adjustment of EDFAs is

the basic requirement and if the gain is not adjusted properly, the bit error rate will increase. It

has been verified that the proper placement of EDFAs and gain optimization can result in good

performance. Further work will be done to apply this scheme in existing Radio over Fiber

networks, UWB radio and Next Generation Optical Systems to verify the performance and

compatibility of the scheme.

References



Over-Fiber System‖ IEEE Journal of Quantum Electronics, Vol. 46, No. 8, 2010.

[2] Ateeq Mumtaz, M.K. Islam, M. Zafrullah, ―Photonic Signal Generation with Frequency

Hopping for DWDM based Radio over Fiber Systems‖, ICAIT, 2010.


52

[3] Toshiaki Kuri, Hiroyuki Toda, Juan Jose, Vegas Olmos and Ken- ichi Kitayama,

―Reconfigurable Dense Wavelength-Division-Multiplexing Millimeter-Waveband Radio-

Over-Fiber Access System Technologies‖ Journal Of Lightwave Technology, Vol. 28, No.

16, 2010.

[4] Stephen Pinter and Xavier Fernando, ―Radio over Fiber Systems for Broadband Wireless

Access‖, PHOTONS, the technical review magazine of the Canadian Institute of Photonic

Innovations (CIPI), 2004.

[5] Chun-Ting et. al , ―Hybrid Optical Access Network Integrating Fiber-to-the-Home and

Radio-Over-Fiber Systems‖, , IEEE Photonics Technology Letters, 2007


frequency quadruples for optical millimeter-wave generation‖ Optical Fiber Technology,

Vol. 15, 2009

[7] Lin Chen, Yufeng Shao, Xiaoyan Lei, Hong Wen, and Shuangchun Wen, ―A Novel Radio-

Over-Fiber System With Wavelength Reuse for Upstream Data Connection‖, IEEE

Photonics Technology Letters, Vol. 19, No. 6, 2007.

[8] L. Chen, J. Lu, J. He, Z. Dong and J. Y , ―A radio-over- fiber system with photonic generated

16QAM OFDM signals and wavelength reuse for upstream data connection‖, Optical Fiber

Technology, Vol. 15, Issue 3, 2009

[9] L. chen, J. Lu, Z.Dong, J.Yu, ―A radio-over-fiber system with photonics generated OFDM

signals and wavelength reuse for upstream data connection‖, ICAIT, 2008


53

[10] R.Shankar, M. Forjanczyk, T.J. Hall, A. Vukovic, Heng Hua ,‖Multi-degree ROADM

based on wavelength selective switches : Architecture and Scalability‖, Opt ics

Communications, 2007

[11] S. Azodolmolky, M. Klinkowski, Eva Marin, D.Careglio, J. Pareta, I.Tomkos, ―A survey

on physical layer impairments aware routing and wavelength assignment algorithms in

optical networks‖ Computer Networks, Vol.53, Issue.7, 13 May 2009

[12] Iyad Katib, Deep Medhi, ‖Adaptive alternate routing in WDM networks and its

performance tradeoffs in the presence of wavelength converters‖ Optical Switching and

Networking, Vol.6, Issue.3, 2009.

[13] Xiaowen Chu, Bo Li, ―Dynamic Routing and Wavelength Assignment in the Presence of

Wavelength Conversion for All-Optical Networks‖, IEEE/ACM Transactions on Networking

Vol. 13, No. 3, 2005.

[14] Poompat Saengudomlert, Eytan Modiano, and Robert G. Gallager, ―On-Line Routing

and Wavelength Assignment for Dynamic Traffic in WDM Ring and Torus Networks‖,

IEEE/ACM Transactions on Networking, Vol. 14, No. 2, 2006.







Chapter 4 UWB Signal Generation

54


4.1 Introduction This chapter presents a simple technique for generation of Impulse Radio Ultra-wideband pulses in

optical domain. This system can transmit the data in Giga-bits-per-second range for 32 users across an

optical fiber link using Dense Wavelength Division Multiplexing (DWDM). A multiple access

technique using Direct Sequence Ultra-wideband (DS UWB) pulses has also been presented. Moreover,

the relationship between the input data rate and signal bandwidth has been analyzed and also studied the

effect of pulse width on the bandwidth of a signal.

Ultra-wideband (UWB) 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. The prominent qualities of UWB attracting attention of researchers from around the

globe are its low complexity, low cost, reduced power consumption and increased data rate. The mere

limitation of UWB is its short range which can be overcome by the use of Radio over Fiber technology

[1]-[4] entailing the use of Optical Fibers to carry signals from the head-end to user premises. This

technology is commonly referred to as UWB-over-fiber.

The generation of UWB pulses has always been a challenge because the wide bandwidth requires

the signal to be considerably narrow in the time domain. Moreover, to fully exploit the advantages of

UWB, there is a need for optical generation of UWB pulses to avoid the use of high cost electrical


55

components, required to produce such narrow signals and to exceed the speed limitations offered by the

electrical components. Numerous solutions have been proposed in the past demonstrating all-optical

generation of UWB pulses. Recent approaches include the use of cross phase modulation [5], cross-gain

modulation in a semiconductor optical amplifier (SOA) [6], intensity modulator to generate polarity

switchable UWB pulses [7], LiNbO3 Intensity modulator using the transfer function‘s wavelength

dependant characteristics [8] and the use of Phase Modulator and asymmetric Mach-Zehnder

interferometer (AMZI) to generate UWB pulses [9]. The problem associated with phase modulation

and gain modulation [5] and [6] is that they require two laser sources resulting in increased system

complexity. In [7] and [8], the generation is limited to a single UWB pulse which confines the use o f

this technique to a limited number of applications. Also, the requirement of two wavelengths in [4]

renders the system expensive and complicated while [9] requires two Asymmetric Mach-Zehnder

interferometers (AMZIs) to be cascaded which causes the design to face stability issue.

A Mach-Zehnder modulator is used to modulate electrical Gaussian pulses. An optical delay line

followed by a subtractor is used to differentiate the optical Gaussian pulses and thus Ultra-wideband

monocycle pulses are generated. Owing to the use of optical components only, this technique results in

an ultrafast optical pulse source. In addition, we present a simple technique for the transmission of

optically generated Direct Sequence (DS) UWB and the provision of multiple-access capability in future

high speed wireless network.

4.2 UWB Monocycle Generation The schematics of the proposed design are shown in fig.4.1. 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.


56

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 is performed by splitting the optical Gaussian pulse into two equal

components. One component is passed through an optical delay line, delaying the signal in time by an

amount equal to one half of the width of Gaussian pulse. Then a subtractor is used to relate the later

component with the former. In this way, during the time in which the non-delayed component goes

through its ascending values, represented by section 1 in fig.4.2, the delayed component is zero and the

subtraction results in nothing but the non-delayed component itself.

Next, during section 2 in fig.4.2 the subtraction results in the values forming section 5 in the UWB

monocycle. During the section 3 of subtractor, the non-delayed Gaussian pulse has a constant value, thus

the correlation of the delayed component with this constant value results in an inverted copy of the

delayed component (section 6), producing UWB pulse in optical domain. The working of the

differentiator is demonstrated in fig.4.2. The delayed and non-delayed components are shown and

formation of UWB monocycle is depicted. As an application of the fore-mentioned technique we

provide an example of a 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, satellite communication and location among satellite formation. [10-

12].


57

Fig. 4.1. Schematic Diagram of the Proposed Solution

Fig. 42. Working of a Differentiator


58

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 obtained to be

transmitted over the optical fiber is shown in fig.4.3.

This waveform modulates a Gaussian pulse train as shown in fig 4.4. 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 p ulses carrying

the cumulative data of multiple users are shown in fig.4.5. 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.

Fig. 4.3. Summation of spread bit streams


59

Fig.4.4. Modulated Gaussian pulse train

Fig.4.5. Modulated UWB monocycle pulses


60

4.3 Simulations and results

The discussed design is simulated at an input data rate of 2 Gbps. The optical delay line

used for the differentiation of Gaussian pulses has an inversely proportional relationship with

input data rate. The input data rates corresponding to the optical delay required for proper

differentiation are listed in Table-4.1. 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 2 Gbps, 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 ‗D‘ from the data rate ‗R‘ and pulse width

‗W‘.

(4.1)

UWB pulses generated using On-Off keying at 2 Gbps and the corresponding frequency

spectrum are shown in fig.4. 6 & 7 respectively.

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


61

verified from table-4.1. The UWB monocycle produced and the corresponding frequency

spectrum, at an input data rate of 1 Gbps is shown in fig.4.8 & 9.

TABLE 4.1 DATA RATES CORRESPONDING TO REQUIRED 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

Fig.4. 6. UWB monocycle generated at 2 Gb/s.


62

The width of the UWB monocycle in figure 4.6 is 0.2 ns while it is 0.4 ns in figure 4.8. 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 fig.4.7 and 9 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. This relationship is shown in

Fig.4. 7. Frequency s pectrum of UWB monocycle at 2Gb/s. The spectrum is

centered at 5 Ghz and has a bandwidth of 6 GHz at -10dbm.


63

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

generation in fig.4.10.

Fig.4.8. UWB monocycle generated at 1 Gb/s. (Twice the pulse

width at 2 Gb/s)

Fig. 4.9. Frequency spectrum of UWB monocycle at

1Gb/s. The spectrum is centered at 2 GHz and has a

bandwidth of 3 GHz at -10dbm.


64

It is clear from the graph that increasing the width of Gaussian pulse, 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.

The output of Dense Wavelength Division Multiplexing is shown in fig.4.11. The UWB

pulses carrying the data of 32 different users are multiplexed using a frequency spacing of 100

GHz or a wavelength spacing of 0.8 nm. 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) is

shown in fig.4.12. The signal is received using a PIN photo diode.

Fig.4.10. Relationship between input data rate and bandwidth for two di fferent

values of Gaussian pulse width used.


65

Fig.4.11. 32 Dense Wavelength Division Multiplexed channels

Fig.4.12. Shape of the UWB monocycle received after passing through 1 km

of optical fiber links

Wavelength (m)


66

4.4 UWB Doublet and Triplet

UWB pulses are generated by a UWB pulse generator shown in fig.4.13. Monocycle,

doublet and triplet are generated by the UWB pulse generator stages. At first stage, monocycle is

received. This monocycle is generated by splitting input light into two equal parts and then

providing optical bias to one signal and delay to other. A subtractor is used to take the difference

of power. Doublet is generated by again splitting the signal genera ted by first stage and then

providing the bias and delay to one part of the spitted signal and then taking the difference of

both parts. Triplet is also generated from doublet using the same procedure as shown in fig.4.13.

Fig.4.13. UWB pulse generator


67

The spectrum of monocycle, doublet and triplet are shown in fig.4.14, 15 and 16

respectively, which clearly indicates the improvement in the spectrum of UWB signal.

Fig.4.14. S pectrum of Monocycle

Fig.4.15. S pectrum of Doublet

Fig.4.16 Spectrum of Triplet


68

4.5 UWB DWDM Ring

DWDM based UWB rings are shown in fig.17. Three rings are presented in this architecture.

In Ring#1 and 2, four nodes are configured; however, in ring 3, 8 nodes are designed. Each ring

is based on single mode fibers. Same fiber is used for uplink and downlink. In each ring, the

architecture of only one node is shown to keep the schematic simplified. Optical Add Drop

Multiplexers are used to drop and insert the wavelengths. At each site, one wavelength

modulated with downlink data is dropped and one with uplink data is inserted. The dropped

wavelength is converted into electrical signal using photodetector.

Uplink signal received from antenna is used for modulation of light using MZM. On CS, UWB

block is used to generate the modulated UWB pulses as per required pattern. Continuous light

generated by laser is modulated by MZM, which is then fed to UWB pulse generator. Output of

this pulse generator is connected to DWDM multiplexer where it is multiplexed and launched into

fiber.

DWDM block contains multiplexers and demultiplexers, which are operating independently for

different rings. Spectrum of signal launched in ring#2 and ring#3 is also shown in fig.4.18 & 19.

The spectrum of ring#1 is same as ring#2. In simple, this scenario, same wavelengths used in one

ring, can be reused in other rings as these wavelengths are being routed independently.


69

Fig.4.17. DWDM UWB rings

Fig.4.18 Wavelengths in Ring#3


70

4.6 Effect of Fiber Non-linearity

The effect of nonlinearities on Ultra-wideband pulses has been simulated. The ultra-wideband

technology is operated on low power. Therefore, the pulses are naturally immune to the effect of

nonlinearities. But, as the transmission distance is increased, the power of input pulses at the

transmitter needs to be increase to mitigate the effect attenuation. I have only simulated self

phase modulation and studied its effect on the optical signal bandwidth. When input power is

greater than or equal to 20mW, self-phase modulation causes a symmetric broadening of pulse

spectrum. The optical spectrum of UWB pulses ignoring all the nonlinear effects is show in

fig.4.20. The data rate is set to 40 Gbps and the transmitted power of the pulse is 20 mW.

Fig.4.19 Wavelengths in Ring#2


71

Fig.4.20. S pectrum with zero SPM

Fig.4.21. S pectrum with SPM effects.

Fig.4.22. SPM effects without adjusting the dis persion of fiber


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With the introduction of self phase modulation, the UWB optical pulse spectrum broadens

symmetrically on both sides as shown in fig.4.21. Careful selection of the factors like core

effective area and power can help to mitigate the effect of SPM. Increasing the transmitted

power or length of the optical fiber has an adverse effect on SPM. These effects are depicted in

fig.4.22.

The longer the transmission distance, more is the power required at the transmitter output. The

effect of SPM is dependent upon the intensity of the pulses propagating through fiber. Larger

core area also reduces the pulse intensity propagating through optical fiber and hence decreases

the effect of SPM, similarly dispersion in optical fiber can be adjusted to counter and further

reduce the effect of SPM. The spectrum of UWB pulse after the introduction of dispersion in

fiber is shown in figure 4.23.

Fig. 4.23. SPM effects by adjusting the dispersion of fiber


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4.7 Conclusions A simple technique for optical generation of Impulse radio UWB pulses was presented along

with the mechanism to transmit the data of 32 users over an optical fiber link using Dense

Wavelength Division Multiplexing. The use of optical components to generate UWB pulses

allows exceeding the limitations of electrical components and high data rates can be achieved. It

has been found that the bandwidth available is directly proportional to the input data rate.

UWB radio systems can be used with DWDM to effectively utilize the bandwidth and other

advantages of optical fiber. The presented method can be used with existing DWDM systems.

Single UWB method can be used to generated monocycle, doublet and triplet depending on the

requirement of the system. SPM effects in DWDM UWB can be decreased by selecting the

optimized dispersion profile. This scheme can be used to design an integrated DWDM ROF

UWB system.

References



Over-Fiber System‖ IEEE Journal of Quantum Electronics, Vol. 46, No. 8, 2010


Hopping for DWDM based Radio over Fiber Systems‖, ICAIT, 2010.


74

[3] Toshiaki Kuri, Hiroyuki Toda, Juan Jose, Vegas Olmos and Ken- ichi Kitayama,

―Reconfigurable Dense Wavelength-Division-Multiplexing Millimeter-Waveband Radio-Over-

Fiber Access System Technologies‖ Journal Of Lightwave Technology, Vol. 28, No. 16, 2010.


frequency quadruples for optical millimeter-wave generation‖, Optical Fiber Technology Vol.

15, 2009.

[5] F. Zeng, Q. Wang and J. Yao, ―All-optical UWB impulse generation based on cross-phase

modulation and frequency discrimination‖, Electronics Letters, 18th January 2007, Vol. 43 No.

2.

[6] Wang, Q., Zeng, F., Blais, S., and Yao, J., ―Optical ultrawideband monocycle pulse

generation based on cross-gain modulation in a semiconductor optical amplifier‖, Opt. Lett.,

2006, 31, pp. 3083–3085.

[7] Q. Wang and J. Yao, "UWB doublet generation using nonlinearly-biased electro-optic

intensity modulator," Electron. Lett. 42, (2006), 1304—1305.

[8] J. Q. Li, S. N. Fu, K. Xu, J. Wu, J. T. Lin, M. Tang, and P. Shum, "Photonic ultrawideband

monocycle pulse generation using a single electro-optic modulator," Opt. Lett., vol. 33, no. 3, pp.

288-290, Feb. 2008Opt. Lett. 33, 288 (2008).

[9] S. Pan and J. P. Yao: ―Switchable UWB pulse generation using a phase modulator and a

reconfigurable asymmetric Mach-Zehnder interferometer‖, Opt. Lett. 34, 160 (2009).

[10] R.J. Fontana, Microwave Theory and Techniques, 9,p.2087(2004).


75

[11] A. Widodo, T. Hasegawa, 5th Spread Spectrum Techniques and Applications,

p.82(1998).

[12] Wang shilian, Zhang eryang, ―Inter-satellite radio links and spread-spectrum ranging for

satellite formation flying‖ 3rd ICMMT 2002, p.233(2002).

Chapter 5 NEXT GENERATION MULTI-SERVICE ACCESS GATEWAY

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Chapter 5 Next Generation Multi-Service Access Gateway

5.1 Introduction

In this Chapter, we have presented a Next Generation Multi-Service Access Gateway (NG-

MSAG) for Radio over Fiber (ROF) systems. The architecture of this NG-MSAG allows the

convergence of fixed, wireless, voice and data services. This convergence will decrease the

overall deployment and operational cost of telecom operators especially those operators which

are new in the industry and interested in deploying nation-wide fixed and wireless networks.

Transport layer is based on IP/DWDM, Radio layer on ROF and services core is based on NGN.

Radio over Fiber can be used to completely centralize the radio resources like modulation,

demodulation and radio resource management to Central Station (CS) while deploying Remote

Antenna Units (RAU) at different locations [1-3]. We have presented solutions for generation of

radio signals [4-5] in optical domains for wireless ROF systems. However, the work was only

related to generation of ASK and UWB signals in optical fiber. We have presented an

integration of Radio over Fiber with Next Generation Networks (NGN). NGN is already

deployed by most of the telecom market leaders. In normal telecom NGN architecture, media

gateways (MG) and multi-service access gateways (MSAG) are used to provide fixed telecom

services [6-9], whereas for wireless services BTSs are deployed. These BTSs are connected with

BSC using optical fiber or microwave links.

In this chapter, we have incorporated a radio services capability in multi service access gateways.

This NG-MSAG architecture can be deployed with both TDM based MSC and Mobile Soft


77

Switch (MSS). It can also support fixed services like Basic Voice Service, Broadband Internet

(DSL) and IPTV. No additional BTS is required in this case as NG-MSAG will serve the

required area for both fixed and wireless services. NG-MSAG has remote antenna unit to feed

the antenna. Radio over fiber based Passive Optical Network (PON) can also be used to further

split the cells into pico-cells, if required. Optical transport layer is based on IP/DWDM which

can directly support TCP/IP traffic over wavelengths in addition to normal traffic. Erbium Doped

Fiber Amplifiers (EDFAs) can also be used to amplify the signal.

This NG-MSAG can be deployed in very short period of time as it is compact and does not

require a complex infrastructure as most of the complex functionalities are already shifted to CS.

NG-MSAG can also be connected with IMS by using SIP between NG-MSAG and IMS.

5.2 Architecture of NG-MSAG

In ROF, no modulator and demodulator is required at RAU. The architecture of NG-MSAG

is shown in fig.5.1. Radio Services Module (RSM) is used for wireless services. Control unit is

responsible for control, configuration and management of all modules. DSL/IPTV module is

used for provision of DSL and IPTV services. PSTN module is used for provision of traditional

voice and telephony services, however, it is based on VoIP and this module acts as VoIP

gateway. RSM incorporates the main functionality of wireless services which enables the NG-

MSAG to be used as BTS.

Optical Add Drop Multiplexer (OADM) with capability to add-drop two wavelengths is

used. One wavelength is routed to RSM, whereas, the second wavelength is connected to

router/switch which is further connected with DSL/IPTV and PSTN modules. In RSM, shown in


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the architecture, the dropped wavelength is connected to photodiode, which converts the optical

signal into electrical. The converted signal is filtered, amplified and transmitted by using

antenna. Uplink architecture of RSM is not shown in figure as it only converts the uplink signal

into light by using laser and MZM and then transmits to CS using OADM.

Fig.5.1 Architecture of NG-MSAG

5.3 Connectivity of NG-MSAG with Core network

NG-MSAG is connected with all telecom core network elements. It is connected with Soft-

Switch (SS) by using IP cloud and controlled by H.248. For DSL and IPTV services, it will have

connectivity with DSL/IPTV core platforms and for wireless services it will be connected with

Central Station which will be co- located with BSC or integrated with BSC. The connectivity

NG-MSAG

Control Unit

OADM

IPTV/

DSL

Router/Switch

Filter

RSM

PS

TN

(PO

TS

)

DS

L/IP

TV Voice

Photodiode

EA


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diagram of NG-MSAG is shown in the fig.5.2. For simplicity, only four nodes are shown

however they can be increased depending on the wavelength capacity of DWDM system. In this

scenario, only eight wavelengths are simulated. These wavelengths at the output of DWDM

MUX and input of first NG-MSAG are shown in fig.5.3&4. RSM module has an internal

amplifier and can directly feed antenna.

Antenna is connected with the output port of NG-MSAG and used for wireless services. If

the outgoing call belongs to some other operator or network it is routed via Media-Gateway

(MG).

For soft-phone users SIP will be used [9]. DSL and IPTV services are routed to DSL/IPTV

core platform depending on their VLANs. This scenario is already implemented in existing

networks however additional RSM will redesign the whole infrastructure and all wireless

calls/data are routed via NG-MSAG.

Wireless calls are received by antenna at the top of NG-MSAG, converted into light and

transmitted to CS on Single Mode Fiber (SMF). No de-modulation is done at NG-MSAG as the

signal is demodulated by CS. CS handovers the call to BSC where it is routed to the MSC for

further processing.

In case of wireless data services, like EVDO, again the same flow is adapted from RSM to

CS, however, the call is routed to PDSN using IP Core where after authentication from AAA

servers, the data is routed to Internet.


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Fig.5.2. Connectivity of NG-MSAG with Core Network

Fig.5.3. DWDM signal at the output of DWDM MUX

Fig.54. DWDM signal at the input of NG-MSAG OADM


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5. 4 Simulation results and discussion

We simulated the optical layer of NG-MSAG network shown in fig.5.2. In this simulation,

only four NG-MSAGs are simulated to keep the simulation simplified. Eight wavelengths are

used for DWDM with each having separation of 100 GHz. At each node two wavelengths are

dropped, one for radio services and one for fixed services. At central station 8 lasers are operated

at the required wavelengths with input power of 1mW. MZM is used to modulate the light by

electrical signal. Two wavelengths are reserved for each site, first wavelengths is used for RSM

whereas second one is shared by remaining modules of the NG-MSAG.

Fixed services are simulated by a stream of data whereas ASK and PSK are simulated for

wireless services. Input data stream for fixed services is shown in fig.5.5. In this simulation, first

laser is modulated with ASK signal, shown in fig.5.6, and second laser with digital data stream.

These two signals are required to be transmitted to first NG-MSAG. Third laser is simulated with

PSK shown in fig.5.7, and fourth again with the digital data stream of that respective MSAG and

required to be dropped at second NG-MSAG. DWDM MUX is used to multiplex all these eight

wavelengths. The spectrum of signal is shown in fig.5.3.

Fig.5.5 Input data stream, simulated as IP data

Fig.5.6. AS K signal used for modulation of first laser


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Single Mode Fiber of 10km is used between each node. The attenuation of this SMF is

0.2dB/km and dispersion is +4ps/(nm.km). The value of dispersion is selected to keep the Four

Wave Mixing at the minimum level. DWDM at the output of DWDM MUX and signal received

at the input of first node is already shown in fig.3&4. OADM is used to drop the required

wavelengths. First wavelength at the output of OADM is connected with RSM where it is

Fig.5.7. PS K signal used for modulation of third

laser

Fig.5.8. AS K signal received at RS M of NG-MSAG

Fig. 5.9. Digital data received at NG-MSAG

Fig. 5.10 PS K signal received at RS M of NG-MS AG


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converted into electrical signal by using Photodetector. Noise in the received signal is filter by

using a band-pass filter and the signal at the output of filter is shown in fig.5.8. Similarly, second

wavelength of first node is converted into electrical signal (fig.5.9.) and routed to the required

modules depending upon the IP addresses. On second node the same scheme is used to convert

the signal. The received PSK signal is also shown in fig.5.10. We have simulated ASK and PSK

modulation schemes, however, this solution is independent of modulation scheme and can be

deployed with all major wireless services.

5. 6 Performance analysis

In the simulation, only four NG-MSAGs are simulated, however, there will be no effect of

increasing the NG-MSAGs on performance of system. Total number of NG-MSAG depends on

the total available DWDM wavelenlenghts. This kind of architecture has not been presented so

far, so it is difficult to exactly compare the performance with any existing scenario, however, we

have done different simulation for different data rates and tranmission impairments to find the

optimized values for our scheme. Dispersion of Single Mode Fiber is incresaed from 0 to +17

ps/(nm.km) and it has been verified that the optimized value of dispersion for best performace is

+2 to +4 ps/(nm.km). Similarly, distance of last NG-MSAG has been incresaed from 10 km to

100km with fiber attenuation of 0.2 db/km, and the simulation results verify that the distance of

last NG-MSAG from CS can be easily increased upto distance of 100 km.


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5.7 Conclusion Integration of ROF with DWDM and media gateways results in a single node (NG-MSAG)

with capability to support all modern fixed and wireless telecom services. This solution is based

on NGN and can also be integrated with IMS. Deployment time of telecom infrastructure is

decreased and plannig of NGN access network is simplified which resluts in a cost effective

solution for telecom operators. This is a complete architecture which can be deployed by any

operator to provide all fixed and wireless, voice and data services.

References

[1] X.Qi, J.Liu, X.Zhang, and L..Xie, ―Fiber Dispersion and Nonlinearity Influences on

Transmissions of AM and FM Data Modulation Signals in Radio-Over-Fiber System‖ IEEE

Journal Of Quantum Electronics, Vol. 46, No. 8, 2010

[2] T.Kuri, Hi Toda, Juan Jose, V.Olmos and K.Kitayama, ,―Reconfigurable Dense Wavelength-

Division-Multiplexing Millimeter-Waveband Radio-Over-Fiber Access System

Technologies‖ Journal Of Lightwave Technology, Vol. 28, No. 16, 2010

[3] J. He , L. Chen, Z. Dong, S. Wen, J. Yu, ―Full-Duplex Radio-Over-Fiber System With

Photonics Frequency Quadruples For Optical Millimeter-Wave Generation‖ Optical Fiber

Technology, VOL. 15, 2009


Hopping for DWDM based Radio over Fiber Systems‖, ICAIT, 2010


85

[5] Affan Hasan Khan, Omer Khalid, Ateeq Mumtaz, M. Khawar Islam, ―DWDM Based System

for Optical Generation and Transmission of Impulse Radio UWB Signals‖, IEEE, ICET,

2011

[6] Morita. N, ―Introduction to NGN Functional Architecture‖, Network Operations and

Management Symposium, 2006

[7] 3GPP and ITU working groups recommendation on VOIP and NGN

[8] Mohapatra, S.K, ―Integrated planning for Next Generation Networks‖Integrated Network

Management-Workshops, 2009

[9] Pirhadi, M. Hemami, S.M.S. Tabrizipoor, A.I., ―Call set-up time modeling for SIP-based

stateless and stateful calls in Next Generation Networks‖, Advanced Communication

Technology, ICACT 2009











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Radio over Fiber provides an efficient optical layer for wireless services, by providing

increased bandwidth, reliability, network security, effective radio resource management,

increased capacity and flexible network architecture. In this thesis, different scheme for pho tonic

generation, frequency hopping, DWDM ROF system, Wavelength reuse, EDFA gain

optimization, UWB generation and Next Generation Radio over Fiber systems are presented.

Detailed simulations are carried out to verify these schemes and performance analysis is done to

verify the performance in the presence of transmission impairments. In this chapter, research

achievements have been summarized.

6.1 Summary of Accomplishments

1. The photonic generation of ROF signal by beating two wavelengths at photodiode is

simulated. Amplitude Shift Keying signal in the range of 60 GHz is successfully generated.

No electrical oscillator is used in this generation and the frequency of this signal can be

easily changed by changing the operating wavelength of one of the two lasers at the central

station.

2. Frequency hopping is proposed and simulated by using optical switch to shift the

wavelengths as per hopping sequence. Sequential and random slow hopping ca n easily be

achieved by this scheme. Different simulations are done to switch the operating frequencies

of the signals. This scheme is also compatible with DWDM as different scenarios are

successfully simulated.


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3. DWDM ROF rings using photonic generation are presented. Complete wavelength plan is

also presented which clearly provided the details of wavelengths management in DWDM

rings when photonic generation is used in DWDM systems.

4. Performance analysis of DWDM ROF system is done and optimized values of different

parameters have been achieved. Simulations results prove that 4Gbps can easily be

transmitted from CS to RAU up to distance of 100 km. Moreover, the best performance is

achieved when dispersion of fiber is optimized at 2 ~ 4 ps/(nm.km).

5. Wavelength reuses for uplink scheme at RAU is presented and power budget calculations

are done to optimize the gain. Different algorithms are proposed and simulated to adjust the

gain of EDFAs in DWDM rings. Extensive simulations are carried out to verify these

algorithms. Received power at RAU is successfully optimized using these algorithms and

wavelength is reused for uplink. In this scheme no laser is required at RAU. Wavelength

assignment and routing algorithm is also proposed.

6. DWDM rings with different RAUs are simulated by placing EDFA as per proposed

algorithms. Eye diagram analysis and bit error rate calculations are done to study the system

behavior, when wavelength is reused. Simulations results proved that by placing EDFA on

proper locations and gain adjustment the scheme can easily be used in DWDM ROF systems.

7. UWB signals are generated in photonic domain and pulse optimization is done to achieve the

monocycle for UWB signals. This generation is achieved by delaying one pulse and optical

biasing the other and then subtracting both signals. Application of this scheme in wireless

systems is also presented. Multi-UWB pulse generator for generation of monocycle, doublet

and triplet and the optimized bandwidth is also presented. Effect of non-linearities on the

systems is also studied.


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8. Next Generation Multi Service Access Gateway is presented for convergence of mobile and

fixed services. This NG-MSAG can provide both fixed value added and basis services along

with all mobile voice and data services. This is an integration of traditional optical fiber

system and radio over fiber system. This NG-MSAG is simulated and integrated with NG-

MSAG.

6.2 Future Work

Further work can be done to explore the field of photonic generation and methods for

generation of other modulation techniques and bandwidth optimization. Routing and

Wavelength assignment for radio over fiber is another field which required extensive research

work. The presented schemes in this thesis can be integrated with other schemes and existing

optical networks, however, it also requires further research and optimization in the presence of

transmission impairments.

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List of Publications

1. Ateeq Mumtaz, M.K. Islam, M. Zafrullah, ―Wavelength Reuse for Uplink on Dense

Wave Division Multiplexing (DWDM) Single Fiber Ring for Radio over Fiber (ROF)

Broadband Systems with Downlink Signal Generation in Optical Domain‖, Journal of

Optical Engineering, USA, Vol. 50(10), October 2011

2. Ateeq Mumtaz, M.K. Islam, M. Zafrullah, ―Photonic Signal Generation with Frequency

Hopping for DWDM based Radio over Fiber Systems‖, IEEE ICAIT, 2010

3. Ateeq Mumtaz, Muhammad Khawar Islam, ―Next Generation Multi-Service Access

Gateway for Radio over Fiber Wireless Systems‖, IEEE ICET, 2011

4. Affan Hasan Khan, Omer Khalid, Ateeq Mumtaz, M. Khawar Islam, ―DWDM Based

System for Optical Generation and Transmission of Impulse Radio UWB Signals‖, IEEE

ICET, 2011

5. Ateeq Mumtaz, M.K. Islam, M. Zafrullah, ―Amplitude Shift Keying Radio Over Fiber

Dense Wavelength Division Multiplexed Signal Generation For Broadband Wireless

Access‖, (submitted journal paper)

6. Ateeq Mumtaz, M. Khawar Islam, Affan Hasan Khan, Omer Khalid, ‖Ultra Wide Band

Radio DWDM rings with photonic UWB pulse generation and transmission in presence

of nonlinearities‖, IEEE IBCAST, 2012

Ateeq Mumtaz 07F-PhD-EE-33

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