Orthogonal Frequency Division Multiplexed Access for ...

European Master of Research on Information and CommunicationTechnologies

Master Thesis

Orthogonal Frequency DivisionMultiplexed Access for Passive Optical

Networks

A hands-on training approach

Author: Angel PeraltaThesis Director: PhD. Marıa Concepcion Santos

July 2012

Abstract

Research activities for new alternatives in ultra fast transmission over optical fiber net-works are looking towards Orthogonal Frequency Division Multiplexing (OFDM) whichis used in different modern communication systems such as LTE, WiFi or WiMax mainlybecause of the following reasons: tolerance to channel limitations, better spectral ex-ploitation, scalability, flexibility and implementation feasibility with state of the artelectronics.

These benefits have been subject of major interest in Passive Optical Networks(PONs), which depend on reliable and low cost solutions to be commercialized. Theadaptation of OFDM as an access scheme in PONs with cost effective Intensity Mod-ulation with Direct Detection (IM/DD) can potentially deliver these benefits and be anext step in the evolution of high volume data access networks.

The flexibility given by OFDM is applied as bandwidth granularity for the usersin such a way that each of them will be guaranteed a certain level of service, yet thereconfigurability capabilities and bandwidth assignment for different users in a OpticalOrthogonal Frequency Division Multiplexed Access (O-OFDMA) PON must be properlyunderstood. This work reports the development of an experimental setup on IM/DDO-OFDMA for next generation PONs, in particular studying bandwidth assignment fordifferent users for future Dynamic Bandwidth Allocation strategies.

Keywords: Optical OFDM, PONs, IM/DD, Dynamic Bandwidth Allocation.

1

Resumen

La investigacion en nuevas alternativas para la transmision ultra rapida sobre redes defibra optica tiene interes en la aplicacion de la Multiplexacion por Division de Freque-cias Ortogonales (OFDM por sus siglas en ingles), la cual es utilizada en diferentessistemas de comunicacion modernas como son LTE, WiFi o WiMax, principalmente porlas siguientes razones: tolerancia a las limitaciones del canal, mejor uso del espectro,escalabilidad, flexibilidad y la factibilidad para la implementacion en las redes opticasdado el estado del arte en la electronica.

Estos beneficios han sido de mayor interes para Redes Opticas Pasivas (PONs, porsus siglas en ingles), que para poder ser comercializadas dependen de soluciones confi-ables y de bajo costo. La adaptacion de OFDM como un esquema de acceso en las redesPON con Modulacion de Intensidad y Deteccion Directa (IM/DD) potecialmente puedeproveer estos beneficios y ser el siguiente paso en la evolucion de las redes de datos dealta capacidad.

La flexibilidad que otorga OFDM es aplicada como granularidad en el ancho debanda asignado para los usuarios de tal manera que a cada uno se le garantiza ciertoservicio. Sin embargo, esta reconfigurabilidad y la asignacion de ancho de banda paradiferentes usuarios en Accesso por Multiplexacion de Frequencias Ortogonales OFDMApara redes PON deben ser ampliamente comprendidas. Este trabajo recoge los experi-mentos realizados en OFDMA para la nueva generacion de redes PON y en particularestudia estrategias para la asignacion de ancho de banda (Dynamic Bandwidth Alloca-tion). La implementacion esta basada en un sistema IM/DD por ser el de mas bajo costo.

Palabras clave: OFDM Optico, OFDM, Redes PON, IM/DD, Asignacion Dinamicade Ancho de Banda.

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Resum

La investigacio de noves alternatives per la transmissio ultra rapida sobre xarxes de fibraoptica te interes en l’aplicacio de la Multiplexacio per Divisio de Frequencies Ortogonals(OFDM per les sigles en angles), la qual es utilitzada en diferents sistemes de comu-nicacio modernes com son LTE, WiFi o WiMax principalment per les seguents raons:tolerancia a les limitacions del canal, millor us de l’espectre, escalabilitat, flexibilitati la factibilitat per a la implementacio en les xarxes optiques donat l’estat de l’art del’electronica.

Aquests beneficis son d’interes per les Xarxes Optiques Passives (PONS, per lessigles en angles), que per poder ser comercialitzades depenen de solucions fiables i debaix cost. L’adaptacio dels sistemas OFDM com un esquema d’acces a les xarxes PONamb Modulacio d’Intensitat i Deteccio Directa (IM/DD) potecialment pot proporcionaraquests beneficis i ser el seguent pas en l’evolucio de les xarxes de dades d’alta capacitat.

La flexibilitat que dona OFDM es aplicada com granularitat en l’ample de banda perals usuaris de tal manera que a cadascu se li garanteix cert servei. Aquesta reconfigura-bilitat i l’assignacio d’ample de banda per diferents usuaris en Acces per Multiplexaciode Frequencies Ortogonals OFDMA per les xarxes PON han de ser ampliament compre-ses. Aquest treball recull els experiments realitzats en OFDMA per a la nova generaciode xarxes PON i en particular estudia les estrategies per l’assignacio d’ample de banda(Dynamic Bandwidth Allocation). La implementacio esta basada en un sistema IM/DDper ser el de mes baix cost.

Paraules clau: OFDM Optic, Xarxes PON, IM/DD, Assignacio Dinamica d’Amplede Banda

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Contents

Abstract 1

Introduction 9

1 OFDM 121.1 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1.1.1 IFFT and FFT . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.1.2 Subcarriers and ICI . . . . . . . . . . . . . . . . . . . . . . . . . 151.1.3 Cyclic Prefix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.1.4 Peak Average Power Ratio . . . . . . . . . . . . . . . . . . . . . 161.1.5 Channel Estimation . . . . . . . . . . . . . . . . . . . . . . . . . 161.1.6 Equalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2 O-OFDM IM/DD Systems 182.1 Fiber channel impairments . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1.1 Chromatic Dispersion . . . . . . . . . . . . . . . . . . . . . . . . 182.1.2 Polarization Mode Dispersion . . . . . . . . . . . . . . . . . . . . 192.1.3 Fiber nonlinearities . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2 Intensity Modulation Direct Detection . . . . . . . . . . . . . . . . . . . 202.3 Optical Multiplexing Systems . . . . . . . . . . . . . . . . . . . . . . . . 21

2.3.1 Time Division Multiplexing . . . . . . . . . . . . . . . . . . . . . 212.3.2 Wavelength Division Multiplexing . . . . . . . . . . . . . . . . . 21

2.4 Optical Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222.4.1 Passive Optical Networks . . . . . . . . . . . . . . . . . . . . . . 22

2.5 Optical OFDM based on IM/DD . . . . . . . . . . . . . . . . . . . . . . 232.5.1 Transmission and reception . . . . . . . . . . . . . . . . . . . . . 232.5.2 Clipped OFDM Signal . . . . . . . . . . . . . . . . . . . . . . . . 26

2.6 IM/DD O-OFDM for Access Networks . . . . . . . . . . . . . . . . . . . 262.6.1 OFDMA in PONs . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3 O-OFDM Experimental Setup 313.1 Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313.2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.2.1 Electrical OFDM setup . . . . . . . . . . . . . . . . . . . . . . . 36

4

CONTENTS

3.2.2 Optical Back to Back scenario . . . . . . . . . . . . . . . . . . . 393.2.3 Single O-OFDM transmitter + 25km SSMF scenario . . . . . . . 413.2.4 O-OFDMA: 2 users with shared electrical bandwidth . . . . . . . 423.2.5 Bandwidth Allocation for O-OFDMA . . . . . . . . . . . . . . . 44

4 Conclusions 50

A Material Characterization 53

B Matlab Code 57

Acronyms 62

5

List of Figures

1.1 Orthogonal sinusoidals overlapping at zero crossing exploits the band-width without interference. (c)Jansen,S. . . . . . . . . . . . . . . . . . . 12

1.2 Generic OFDM system. . . . . . . . . . . . . . . . . . . . . . . . . . . . 131.3 IFFT applied to N=8 premodulated symbols s(n) to generate of the

time domain representation of the subcarriers u(n). (c)Jansen,S. ”Multi-Carrier Approaches for Next-Generation Transmission: Why, Where andHow?” 2012 OFC Tutorial. . . . . . . . . . . . . . . . . . . . . . . . . . 14

1.4 With the Cyclic Prefix addition, the extended OFDM symbol now ap-pears as a periodic signal when convolved with the channel. In case thesymbols are overlapped, we can still recover the complete symbol withthe redundant information. . . . . . . . . . . . . . . . . . . . . . . . . . 16

1.5 Training periods are composed of well know sequences at both the trans-mitter and receiver so estimation of the channel conditions is possible ifany change is present as a consequence of the channel effect. . . . . . . . 17

2.1 Lower Group Velocity of higher-frequency components causes ChromaticDispersion resulting in Pulse Broadening making the system susceptibleto ISI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.2 Pulse light components arrive at different times after propagating alongthe fiber. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.3 IM with direct modulation of the lightwave source applies the data intothe laser drive current. A laser diode provides a linear relationship be-tween optical emitted power Pout and drive current I above a thresholdcurrent Ith. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.4 In a PON, an Optical Distribution Network provides connectivity betweenthe OLT and the ONU. In a shared optical splitter the signal is multi-plexed to N number of customers (ONUs) tranporting different types ofservices. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

2.5 SSB helps detecting only one sideband avoiding amplitude fading. Byoffsetting the OFDM signal from DC with a spectral guard band of a sizeat least equal to the signal bandwidth the unwanted subcarrier mixingfrequencies do not overlap with the OFDM spectrum after direct detectionand the undesired mixing products are eliminated using a filter (black line). 24

6

LIST OF FIGURES

2.6 Hermitian Symmetry at the IFFT input generates a real valued output.By doing this we reduce the number of useful IFFT/FFT carriers by N/2.DC and Nyquist components are set to zero. . . . . . . . . . . . . . . . 25

2.7 Clipping the signal forcing negative values to zero to use only positivevalues reduces the PAPR and the required bias drive current at the lasermodulator providing an solution to PAPR. . . . . . . . . . . . . . . . . . 27

2.8 Signal Constellation before clipping (*) has amplitude of 1. After clipping(+) the signal the constellation will show amplitudes of 0.5. . . . . . . . 27

2.9 OFDMA PON to deliver transparent transport of arbitrary signals. (c)Cvijetic,N.; ”OFDM for Next Generation Optical Access Networks”; NEC labs. . 28

2.10 Scalability and Traffic shaping with reconfiguration capabilities for nextgeneration networks can be provided by OFDM. . . . . . . . . . . . . . 29

2.11 Optical OFDMA PON Models. . . . . . . . . . . . . . . . . . . . . . . . 30

3.1 Distributed Feedback Laser Diode scheme. (c) Mitsubishi . . . . . . . . 323.2 Thorlabs Laser Diode Mount Module layout. (c) Thorlabs Inc . . . . . 333.3 ILX LDC 3700C Series Controller. (c) ILX Lightwave Corporation . . . 333.4 Circuit with tunable lasers built at the UPC. . . . . . . . . . . . . . . . 343.5 OFDM Laboratory Test Procedure. . . . . . . . . . . . . . . . . . . . . . 353.6 Barker Code sequence [+1 + 1 − 1 + 1] at the input of the IFFT. After

IFFT transformation to time domain we obtain a raised cosine which isused as a synchronization sequence at the beginning and end of the trailof the OFDM symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.7 Correlation between the received data and the known synchronizationsequence. Peaks mark the beginning and the end of a series of OFDMsymbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.8 TM tool Instrument and Interface Objects created and connected. . . . 383.9 AWG and Oscilloscope serve as DAC and ADC so these must follow the

Nyquist sampling rule for synchronization and proper decoding. . . . . . 393.10 Back to Back O-OFDM IM/DD system. . . . . . . . . . . . . . . . . . . 403.11 OFDM received signal recorded in the oscilloscope. The raised cosine

used as reference for synchronization is considerably higher than the restof the signal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.12 Test bed layout for Dynamic Bandwidth assignment. Two ONUs transmitdifferent N subcarriers. A variable power output at the ONU 2 simulatesdistance affecting the OSNR and the respective BER. . . . . . . . . . . 43

3.13 BER monitoring under temperature control wavelength tunning for bothONUs with a bandwidth assignement of 64 subcarriers for each user(64sc/64sc). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.14 OSA capture of the spectrum of two ONUs with a distance of 0.5252nmbetween carriers necessary for different subcarriers assignment. . . . . . 45

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LIST OF FIGURES

3.15 Dynamic Bandwidth Allocation scenarios. Two ONUs transmit with dif-ferent number of subcarriers. The output power of ONU2 is modifiedbefore the coupler so the respective OSNR from ONU2 is modified incomparison with the OSNR from ONU1. . . . . . . . . . . . . . . . . . . 46

3.16 BER for two ONUs with 64sc/64sc,96sc/32sc,112sc/16sc bandwidth allo-cation schemes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.17 BER for two ONUs with 120sc/8sc, 124sc/4sc and 126sc/2sc bandwidthallocation schemes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.18 The OSA capture shows how the spectrum for ONU2 is noticeably broad-ened when reducing the number of subarriers for 126sc/2sc configuration. 49

A.1 Characterization of the coupler ports. . . . . . . . . . . . . . . . . . . . 53A.2 DFB lasers wavelenght tunning via temperature variation. . . . . . . . . 54A.3 Characterization of the p-i-n photodetector. . . . . . . . . . . . . . . . . 55A.4 BER for two users sharing the 256 subcarriers (w/ Hermitian Symmetry

in different bandwidth allocation schemes. . . . . . . . . . . . . . . . . . 56

8

Introduction

Optical Fiber is a huge capacity channel that provides efficient signal transport usingvery simple modulation formats (e.g. Single Carrier with OOK). This has allowed theoptical communications industry to cope with transmission issues with a lax pace anddelaying the adaptation of more complex developments years behind its equivalent inthe wireless domain where technology was pushed to overcome very harsh channel con-ditions and be able to guarantee the required performance.

Networks based on Optical Fiber evolved from SONET/SDH to networks capableof switching and routing at the optical layer level where Time Division Multiplexing(TDM) and Wavelength Division Multiplexing (WDM) offered the possibility to reachhigh bit rates over Standard Single Mode Fiber (SSMF). The limitations to go beyondlay mainly in combating Intersymbol Interference (ISI) caused by Chromatic Dispersion(CD) in TDM and the lack of granularity for bandwidth efficiency for WDM.

These restrictions should not be an impediment for Optical Networks to support andexceed the forecasted throughputs at the backbone transport and access networks froman end to end perspective, which means that these will need to provide the capacity toadd, switch and route a huge amount of traffic at the backhaul and provide the ability toguaranty services to thousands of users at the access network level with heterogeneousand high traffic volume demands. As an example, according to a 2011 data traffic reportfrom CISCO, IP traffic will grow annually at a rate of 32 percent in the period from2010 to 2015 and it seems that Wi-Fi and mobile devices will account for 54 percent ofIP traffic, where mobile data traffic is expected to grow at a a compound annual growthrate of 92 percent between the same 5 years period, reaching 6.3 exabytes per month by2015[18].

The reasons explained above have urged the need for more efficient and feasible opti-cal communication systems over fiber. In the search for different alternatives the OpticalCommunity has turned to Orthogonal Frequency Division Multiplexing (OFDM) whichhas been a very successful transmission scheme. Several communication systems such asDigital Video and Audio Broadcast, ADSL, WiFi or WiMax use OFDM technology forits robustness against channel impairments (e.g. multi-path and fast fading), its flexi-

9

Introduction

bility for bandwidth assignment and spectral efficiency. Optical-OFDM (O-OFDM) hasbeen projected as a promising technology for both Long-Haul Transmission or OpticalAccess Networks (OAN) because OFDM might offer a solution to counterbalance thetrade-offs between channel constraints, scalability, traffic shaping, cost, and backwardcompatibility in current optical network deployments in comparison or potentially inaddition to TDM and WDM.

Research in O-OFDM was introduced by Chanda et al. in [5] with the concept ofWireless on Fiber systems. In 2006 Armstrong et al. [1] proposed the first schemesfor Coherent Optical OFDM (CO-OFDM) for Long Haul as a substitute for SingleCarrier (SC) ultra fast systems. One of the most significant works for this thesis wasreported by Armstrong [2] in 2007 showing an Intensity Modulation Direct DetectionOFDM (IM/DD OFDM) experimental system. Publications in Optical OFDM saw anexponential growth and this lead to elaborate some guidelines from the OFC/NFOECmeetings for the research work to deploy OFDM as a long haul and/or access system.So far the most complete work for Access Networks was lead by the NEC Laboratories[7] which has served as a good reference for understanding the Passive Optical Networksbased on OFDM. Besides these publications, the starting point for this work is the in-ternal research activity at the UPC in Optical OFDM as part of the European project”Accordance: A Novel OFDMA-PON Paradigm for Ultra-High Capacity ConvergedWireline-Wireless Access Networks”, spanning from 2010 to 2013 and its main objectiveis the introduction of OFDMA architecture for optical back-haul that converges wirelessand copper based networks.

The motivation for this master thesis work is to provide for the first time in theUPC an experimental setup and create a hands-on training guide for research activitiesin OFDM signals applied to Optical Fiber communications in the UPC which is a hottopic in the optical communications community and complement the research activitiesin the Accordance project. More explicitly the implementation of Optical OrthogonalFrequency Division Multiplexed Access using Intensity Modulation / Direct Detection(IM/DD O-OFDMA) for next generation Passive Optical Networks (PONs) as a costeffective solution that could be applied to existing networks to provide a more granulartraffic shaping.

The rationale for this work is that similar to most of the current Optical transmissionsystems, Optical OFDM could be deployed using Intensity Modulation that implies alinear relation between the OFDM signal (electrical current) and the intensity of the op-tical signal (optical power) and at the receiver side Direct Detection with a photodiode.This makes IM/DD the cheapest and most commonly deployed detection system whichis a key factor to consider in a commercial cost-effective solution. By implementingIM/DD O-OFDMA we attempt to empirically understand how we can alleviate channelimpairments, enhance the spectral usage and provide different users a dynamic band-width allocation demanded for the next generation PONs.

10

Introduction

Objectives

• Deploy an experimental set-up of a OFDMA-PON Optical Network Unit ONU)based on a cost effective Intensity Modulation/Direct Detection (IM/DD) scheme.

• Implement scalable bandwidth assignment for 2 users to demonstrate the band-width granularity of IM/DD Optical OFDMA and understand its feasibility, lim-itations or difficulties for deployment.

• Elaborate a laboratory reference that could serve as starting point in academicactivity held by the Optical Communications Group to empower alumni and pro-fessors to understand, define and prove the benefits and constraints of Optical-OFDM and therefore reduce the learning curve time to acquire the know-how forlab tests for future research at the UPC in the matter.

Structure of the thesis

Chapter 1 contains a general OFDM system review where concepts like Frequency Or-thogonality using Fourier Transform, OFDM symbols, Cyclic Prefix (CP), and OFDMrelated issues such as Peak to Average Power Ratio (PAPR) are explained. In Chapter2 we review the fiber channel limitations like Chromatic Dispersion and PolarizationDispersion followed by general concepts of optical fiber transmission and multiplexingsystems (e.g. TDM and WDM). Intensity Modulation/Direct Detection systems andPassive Optical Networks are described since these are the basis of the optical OFDMsystem subject of this Thesis work. This chaper also summarizes the main elements,characteristics and limitations of an IM/DD Optical OFDM system. The end of thischapter includes a description of Orthogonal Frequency Division Multiplexed Access forfuture Passive Optical Networks that is currently being developed by the optical indus-try community. In Chapter 3 we report the test bed for key concepts of O-OFDMAusing IM/DD. This includes the methodology followed for the transmission and correctreception and decoding process of an OFDM signal in a back to back scenario follwedwith the addition of 25kms of fiber to simulate the link between a single user (Op-tical Network Unit) and the receiver (Optical Line Terminal). We also describe theimplementation of a multi-user topology with two OFDM transmitters simulating twousers (ONUs) transmitting to one OLT also known as Uplink channels at the same dis-tance. In a second stage of the experiment Dynamic Bandwidth Allocation for two userswas implemented by providing different bandwidth granularity and power level betweenusers. The results are based in the BER obtained. Finally Chapter 4 contains key con-clusions and suggestions for future lab work. Matlab Codes and the characterization ofthe laboratory material are included in the appendix.

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Chapter 1

OFDM

1.1 Fundamentals

Orthogonal Frequency Division Multiplexing is obtained when the information of Ndifferent symbols into time slots of temporal length T are assigned to a set of N parallel,narrow-band orthogonal subcarriers. After mapping the original information to this setof parallel subcarriers, the symbol period T extends to N ∗T so the possibility of havingIntersymbol Interference (ISI) is reduced. Moreover, the subcarriers are close to eachother without causing interference because of the orthogonality, achieved by using asubcarrier spacing such that (δf = 1

NT ). Under this condition the sinc-shaped spectrumof each subcarrier overlaps at the zero crossing point with the neighbour subcarrier. Seefigure 1.1. This allows better spectral occupation.

Figure 1.1: Orthogonal sinusoidals overlapping at zero crossing exploits the bandwidthwithout interference. (c)Jansen,S.

The ultimate goal is to increase the symbol duration until ISI affects just the neigh-bouring symbol. The spectral domain equivalent would be to have narrow enoughsubcahnnels so that the channel frequency response appears to be flat and single tap

12

1.1 Fundamentals

equalization can be used.

The figure 1.2 shows the OFDM signal generation process for the transmitter andthe receiver.

Figure 1.2: Generic OFDM system.

1.1.1 IFFT and FFT

There are several ways to create a bank of orthogonal subcarriers that can be used fora OFDM multicarrier transmission system. Here we will use the extended technique ofthe Discrete Fourier Transform. In order to generate at the transmitter a continuoussignal, the Inverse Discrete Fourier Transform performs modulation and multiplexing ofthe source data block, where each data symbol will be represented as a sinusoidal. Thewell know IFFT equation is

xm =1√N

N−1∑k=0

Xkejπ2kmN for 0 ≤ m ≤ N − 1 (1.1)

where xm is the time domain set of N sinusoids and Xk are the symbols to transmit.Consider a block transmission of N pre-modulated data symbols X (e.g. M-QAM sym-bols that define the phase and amplitude of each subcarrier). These are mapped fromserial to N parallel channels and each output of the S/P block will be the correspondinginput to the Inverse Fast Fourier Transform (IFFT) block. At the output of the IFFTwe obtain a sampled waveform which is the time domain complex-valued representation

13

1.1 Fundamentals

of the subcarriers for transmission. This parallel block is then rearranged in a serialorder which is passed to a DAC to create a continuous time domain signal. The CyclicPrefix addition is explained later in this section.

At the receiver, the modulated time domain signal y is detected and during thedecoding process, to obtain the set of orthogonal subcarriers in frequency domain Ykthe signal goes through a serial to parallel operation and is decoded with the inverseoperation, the Fast Fourier Transform (FFT) given by:

Yk =1√N

N−1∑m=0

yme−jπ2km

N for 0 ≤ k ≤ N − 1 (1.2)

ym = xm + wm (1.3)

Note from equation 1.3 that the output ym represents the subcarriers in time domainwhich are composed of the subcarriers xm that represent the premodulated symbolsfrom the same or different sources plus Additive White Guassian Noise caused by thechannel given by wm.

In figure 1.3 we show the example of N=8 parallel symbols that can be representedas different sinusoidals (Orthogonal signals). By applying the IFFT we obtain the timerepresentation of the same information to generate an OFDM symbol by rearranging ina sequential order (P/S converter) the IFFT output.

Figure 1.3: IFFT applied to N=8 premodulated symbols s(n) to generate of the timedomain representation of the subcarriers u(n). (c)Jansen,S. ”Multi-Carrier Approachesfor Next-Generation Transmission: Why, Where and How?” 2012 OFC Tutorial.

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1.1 Fundamentals

1.1.2 Subcarriers and ICI

OFDM relies on the orthogonality between subcarriers to avoid ICI but each OFDMsubcarrier has side lobes so frequency offsets and phase noise can result on in phaseuncertainty in the constellation or extend across subcarriers spacing hence corruptingthe system as the orthogonality between signals is lost causing Inter Carrier Interference(ICI). Signal processing in the digital domain can provide a margin to compensate thisproblem as it will be explained further in this chapter.

For the signal not to suffer from aliasing caused by the sidelobes, theoretically, thesampling frequency has to fulfill the Nyquist criterion. In practice this is not quite accu-rate, so in order to filter the signal and not suffer the lost of information, the samplingfrequency must be higher or in the transmission side the IFFT can be stuffed with zerosat the edges of the signal spectrum to have a better defined spectrum (brick response)with aliasing away from main band. This relaxes the requirements in the analog filteringafter the Digital to Analog Converter (DAC) but at the cost of bandwidth loss.

1.1.3 Cyclic Prefix

When the bit rate increases, the pulse period reduces so in a dispersive channel thepulses transmitted can overlap one after another on the receiver. This effect is knownas Inter Symbol Interference (ISI).

In order to avoid this, we add guard time periods between data symbols with re-dundant information known as the Cyclic Prefix (CP) so these data symbols do notoverlap. Then the CP is later removed at the receiver side. In case the symbols overlapat the receiver, we have the copy of the information that it is lost at the overlapping.As shown in figure 1.4, assuming the serial transmission of the IFFT output over aperiod T is represented as x(n) = [x1(n), x2(n), ..., xN−1(n)], a portion of the last partof the sequence will be copied at the beginning of the sequence that will take the formof: xcp(n) = [xN−G(n), ..., xN−1(n), x1(n), x2(n), ..., xN−1(n)] which defines the lengthof the CP + the payload as the total length of the OFDM symbol .

ISI caused by channel impairments is mitigated by provisioning sufficient cyclic pre-fix. To compensate for the effects along the transmission period related to the channellength, the length of CP period Tcp should be larger than the channel response periodTc. In other words, the differential delay of the signal along the channel period is a smallfraction of the length of the OFDM symbol T + Tcp so the data that gets corrupted isthe redundant data.

Cyclic Prefix is quantified as a percentage of the payload according to the relatedperiod of the channel impulse response. It is important to understand that the lengthof the CP or ratio between CP and payload is a toll or trade-off between bandwidth,

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1.1 Fundamentals

Figure 1.4: With the Cyclic Prefix addition, the extended OFDM symbol now appears asa periodic signal when convolved with the channel. In case the symbols are overlapped,we can still recover the complete symbol with the redundant information.

power and protection against channel effects.

1.1.4 Peak Average Power Ratio

An OFDM signal is made up of multiple simultaneous signals that for a given averagehave a higher peak signal. PAPR is obtained by dividing the peak amplitude of theOFDM waveform by the RMS value.

PAPR = max0≤t≤T|S(t)|2

Prms(1.4)

As the number of subcarriers increase, the PAPR increases. Physically this means thatif we want to avoid distortion, many of components in the transmitter and receiver mustoffer a wide dynamic range, otherwise the signal peaks are clipped resulting in noise anderrors at the receiver.

The OFDM baseband components of the OFDM signal samples at the output of theIFFT, have approximately Gaussian distributions (central limit theorem). Through an-alyzing the cumulative distribution function of peak envelope to average symbol powerratio (PAPR) with a high number of FFT subcarriers, although OFDM has high sig-nal peaks, the probability for these peaks occur is low. This is the reason to expressthe PAPR in terms of statistics using the Cumulative Distribution Function because ofa sporadic appearance of high peaks, however in case of occurrence significant compo-nent saturation can occur and clip noise is added to the signal, for more reference see [5].

1.1.5 Channel Estimation

We have seen that ISI can be prevented if the CP is long enough in comparison withthe channel length which causes the pulse to be broadened. However we might also

16

1.1 Fundamentals

want to be aware of the total channel response for equalization purposes. Channelestimation is done by inserting a training sequence (TS) containing known informationso the channel response can be obtained. As shown in figure 1.5, this one or severaltraining sequences can be located at the beginning and the end of a trail of OFDMsymbols to help identifying the useful data. The required TS overhead depends onchannel dynamics and the OFDM symbol length.

Figure 1.5: Training periods are composed of well know sequences at both the trans-mitter and receiver so estimation of the channel conditions is possible if any change ispresent as a consequence of the channel effect.

1.1.6 Equalization

For a transmitted OFDM signal, because of the periodic nature of the cyclically-extendedOFDM symbol, the time-domain convolution between the channel and the OFDM signalcan be represented in frequency as the multiplication of the spectrum of the OFDM signalrepresented by X (e.g. the frequency-domain constellation points) with the frequencyresponse of the channel represented by H as represented in the equation below.

Y = HX +W (1.5)

The result is that each subcarrier symbol will be multiplied by a complex number equal tothe channel frequency response at that frequency of the subcarriers. Each received sub-carrier experiences a complex gain (amplitude and phase distortion) due to the channel.In order to undo these effects, a frequency-domain equalizer (tap-equalizer) is employedat the receiver. Equalization places the received symbols in the correct quadrants fordecision making.

The frequency domain equalizer consists of a single complex multiplication for eachsubcarrier. For the simple case of no noise, the ideal value of the equalizer response isthe inverse of the channel frequency response. That is Xk is multiplied by 1/Hk.

17

Chapter 2

O-OFDM IM/DD Systems

2.1 Fiber channel impairments

Even though an optical fiber does not have the effects of fast fading or multi-path as ina wireless system, the channel is still dispersive imposing limitations. We now brieflydescribe these dispersive effects and major limitations in current optical systems.

2.1.1 Chromatic Dispersion

It is described as the phenomenon by which different spectral components of an opticalpulse signal experience different delays when traveling through a single mode fiber. Thedifferential delay is proportional to the distance traveled by the pulse, so a ChromaticDispersion coefficient D [picoseconds/nm-km] can be defined to express the delay perspectral separation per fiber length. In terms of the coefficient D , CD is related to apulse transmitted that is subject to a broadening (See fig. 2.1) equal to τ proportionalto the total bandwidth in wavelengths and to the fiber length L traveled:

∆τ = |D|L∆λ = |D|L ∗Bd ∗ c/f2 (2.1)

Where ∆λ is the optical spectral width of the signal.

Chromatic Dispersion is one of the most detrimental impairments in optical fibercommunications. As the bit rate increases, the symbol period is reduced at the trans-mitter but because of the pulse broadening, the system becomes more susceptible toInter Symbol Interference at the receiver.

For Non-Dispersion Shifted Fiber (ITU-T G.652) typical CD at 1550nm is 17pm/nm*km.

CD impairments are alleviated using Dispersion Compensation Modules (DCM) alsoknown as Dispersion Compensation Units, which are not suitable for cost-sensitive sce-narios. Another alternative that has been proposed to diminish the effects of Chromatic

18

2.1 Fiber channel impairments

Figure 2.1: Lower Group Velocity of higher-frequency components causes ChromaticDispersion resulting in Pulse Broadening making the system susceptible to ISI.

Dispersion is Electronic Dispersion Compensation (EDC). EDC can compensate thedifference delay of spectral components and be deployed in two forms: Electronic Pre-Distortion at the transmitter or Electronic Post Compensation at the receiver. Theconstraint for this EDC is to have a feedback channel to provide the transmitter thechannel state information which makes the solution more costly. For more reference see[4, 17].

2.1.2 Polarization Mode Dispersion

Because of random imperfections or bending of the fibers, polarization changes can occurwhile the light travels along the fiber, and as a consequence the signal has two differentpolarized components with different group delays. Because these two light componentsarrive at different times (See fig. 2.2), it causes the original pulse to broaden andtherefore incorrect detections are obtained at the receiver. As opposed to ChromaticDispersion which shows a deterministic behavior, PMD has a random nature, whichcomplicates its treatment from an analytical point of view.

Figure 2.2: Pulse light components arrive at different times after propagating along thefiber.

19

2.2 Intensity Modulation Direct Detection

2.1.3 Fiber nonlinearities

Four Wave Mixing

Four-wave mixing (FWM) is a mixing process by which three waves, co-propagatingalong an optical fiber generate an optical wave at a fourth frequency. FWM productshave random, but distinct, phases. FWM is phase-sensitive for all beams and thereforeit can efficiently accumulate over long reach transmissions because it can cause cross-talk between different wavelength channels.

For this thesis work, we propose short reach scenarios with low bit rates and mod-erate launching power so the effects of PMD and FWM can be considered negligible.

2.2 Intensity Modulation Direct Detection

The simplest and most straightforward way in communications to convert from electricalto optical signals and vice versa is based on one-to-one conversions between electronsand photons taking place in light emitting and photo detection diodes respectively. Thisis known as direct modulation with direct detection.

As shown in figure 2.3 the electrical to optical transfer function of a semiconductordiode presents a linear relationship between optical emitted power and drive currentabove a threshold current. By adding an offset bias to the laser, the optical power ofthe light wave follows the signal to transmit, which is recovered at the receiving side bythe reverse operation, getting the generated photons back to electrons.

Figure 2.3: IM with direct modulation of the lightwave source applies the data into thelaser drive current. A laser diode provides a linear relationship between optical emittedpower Pout and drive current I above a threshold current Ith.

When directly modulating a laser diode, due to the nonlinear nature of the electro-optical conversion, the optical spectrum is not an up-converted replica of the electrical

20

2.3 Optical Multiplexing Systems

spectrum. In presence of channel linear distortions that might alter the necessary am-plitudes and phases between spectral components, the detected signal could containnonlinear distortions and therefore new frequencies appear. This complicates the use oflinear equalization.

The advent of external modulation both in the form of electroabsorption (EAM) andMach Zehnder Modulators (MZM) provided independence between the light generationand modulation processes allowing better modulated signal quality and wider modula-tion bandwidth. However external modulation can result in a more expensive systemspecially when using MZM.

For the purpose of this work we have used Distributed Feedback Lasers (DFB). DFBlaser diodes are built with an interference grating that provides optical feedback so italso serves as the resonator for wavelength selection and produce a single longitudinalmode. These is a mature and relatively cheap technology. Its design includes the laserdiode and a built-in Thermal Electric Cooler plus Thermistor sensors that allow to ma-nipulate electrically through temperature variation the emission wavelength of the laserDiode and the laser configuration allows to connect a modulation source.

2.3 Optical Multiplexing Systems

2.3.1 Time Division Multiplexing

Single Carrier (SC) Time Division Multiplexing (TDM) is the simplest method for opti-cal data transmission where a complete wavelength is dedicated to the signal of interest.The modulation type can be as simple as On-Off-Keying which can be seen as a basicamplitude modulation applied for speeds up to 10Gbps while high level modulation for-mats are used for speeds up to 100Gbps. For TDM, the main limitation is ISI causedby Chromatic Dispersion.

2.3.2 Wavelength Division Multiplexing

Wavelength Division Multiplexing (WDM) uses parallel transmission on multiple wave-lengths (multi-carrier). In this way a single fiber carries many lightpaths. It is the cur-rent commercial technology in the optical networks known as Coarse WDM (CWDM)composed of a grid of 18 channels with 20 nm of separation between channels and DenseWDM (DWDM) which provides up to 20,40,80 or 160 channels with 200,100,50 or 25GHz channel spacing respectively.

The drawback of WDM is the spectral inefficiency as it requires a guard band be-tween carriers to avoid interference between adjacent carriers.

21

2.4 Optical Networks

2.4 Optical Networks

The evolution from the first generation of SHD/SONET technologies towards opticalrouting and switching in Optical Transport Networks (OTNs) searched for flexible andscalable backhaul links to provide ultra-long reach capabilities, while Optical AccessNetworks (OANs) -active or passive- searched for high capacity and granular infrastruc-ture. It is within the interest of this work to concentrate on the OAN services andtopologies.

The OAN physical architecture can be point-to-point or point-to-multipoint and pro-vides a set of access links while supporting different optical access multiplexing systems.An OAN can include active and passive (no electronic components) elements dependingon the distance to cover (20km up to 100km).

2.4.1 Passive Optical Networks

Passive Optical Networks (PON) provide a transport path between the central officeOptical Line Terminator (OLT) and the user equipment interface in the Optical Net-work Unit(ONU). The term Passive involves network elements that do not need to bepowered up to operate, leading to low power consumption and therefore cheaper equip-ment.

The function of the OLT is to adapt the PON Packet Data Units from/to the ONUsover the service provider interface. This type of OAN may include a number of OpticalDistribution Networks (ODN) connected to the same Optical Line Terminator (OLT).Additionally, the shared OLT provides management and maintenance functions for theODN and ONUs (Optical Network Units). See fig 2.4.

Historically Passive Optical Networks standards have evolved from APON (ITU-TG.983) to GPON (G.984) and 10G-PON (G.987). PON standards define different wave-lengths for Upstream and Downstream links as well as different throughputs (e.g. 10Gbit/s downstream and 2.5 Gbit/s upstream) which support dynamic bandwidth allo-cation for different users, more specifically, upstream time slots allocation to the ONUsbecause the ODN is shared between the different ONUs. The challenge for the follow-ing generation of PONs is to cope with the high throughputs required for each ONU,while providing granularity and reconfigurability capabilities. WDM-PON is a multi-access reconfigurable infrastructure proposal but lacks of granularity as it switches atthe lightpath level. SARDANA EU project proposed a ultrafast granular access schemethat combines WDM with TDM.

22

2.5 Optical OFDM based on IM/DD

Figure 2.4: In a PON, an Optical Distribution Network provides connectivity betweenthe OLT and the ONU. In a shared optical splitter the signal is multiplexed to N numberof customers (ONUs) tranporting different types of services.


The role projected for O-OFDM is to expand the capabilities of the next generationPONs with cheap and backwards compatible infrastructure. In this section we explainthe fundamental of O-OFDM based on IM/DD.

2.5.1 Transmission and reception

We know that the IFFT output is composed of a complex-valued (Inphase, I, andQuadrature, Q) data stream (Time Domain representation of several data symbols).This is a first key difference between RF and optical systems since OFDM signals arecomplex (real and imaginary components) and bipolar signals while the input signal tomodulate a laser must be real and positive. Two common methods to generate an OpticalOFDM signal from a complex-valued OFDM signal can be based on a)electrical IQ mod-ulation with RF upconversion also known as Offset Single Side Band and b)HermitianSymmetry

RF-Upconversion

This method involves an RF stage prior to IM where the real and imaginary parts ofan OFDM signal are mapped to I (in-phase) and Q (quadrature components of an RFreference carrier, creating a Double Side Band (DSB) signal. Chromatic Dispersioncauses phase changes between spectral components in the upper and lower sidebands of

23


the signal and destructive addition into the same electrical band occurs at the detector.This is know as amplitude fading and the required work around is to filter only oneband (Single Side Band) of the intensity modulated signal. In the receiver, when DD isapplied, RF downconversion stage is needed.

Another effect of CD over the IM systems is due to the nature of electrical-optical-electrical conversion. When CD alters the necessary phases changing the IM modulatedcomponents, Intermodulation distortion appears at the receiver side.

This can be avoided if a guard band of at least the same bandwidth as the signalis allocated between the signal band and the carrier. Intermodulation products will fallinto this guard band without impacting the decoded signal and can be discarded byusing a Low Pass Filter (LPF) as shown in figure 2.5. The toll to pay is that it reducesspectral efficiency because of the digital signal processing required (twice the size of theBW). For more details see [14]. This method is commonly known as Offset Single SideBand. However, in the absence of amplitude fading because of a short distance betweenend points, the filtering process could be eliminated.

Figure 2.5: SSB helps detecting only one sideband avoiding amplitude fading. Byoffsetting the OFDM signal from DC with a spectral guard band of a size at least equalto the signal bandwidth the unwanted subcarrier mixing frequencies do not overlapwith the OFDM spectrum after direct detection and the undesired mixing products areeliminated using a filter (black line).

With this method, in the PON scenario, we could allocate different users by mappingthe bandwdith assigned per ONU at different RF upconversion reference frequencies,with the users next to each other. OSSB-OFDM is not considered in this work because

24


we wanted to focus on a cost and low complexity approach.

Hermitian Symmetry

Another option to obtain a real signal avoiding extra equipment of RF up/downconver-sion at the cost of additional DSP and ADC/DAC computational power is the use ofHermitian Symmetry where the upper half of the subcarriers mapped to the complexconjugate of the lower half subcarriers. By constraining the input of the IFFT to haveHermitian symmetry we generate a real OFDM signal. This requires using only onehalf of the total number of FFT carriers available as shown in figure 2.6. HermitianSymmetry is the simplest and cheapest way to generate an OFDM signal that can beuse to directly modulate a laser. The bandwidth allocation can be based on the numberof subcarriers used by one or more users.

Figure 2.6: Hermitian Symmetry at the IFFT input generates a real valued output. Bydoing this we reduce the number of useful IFFT/FFT carriers by N/2. DC and Nyquistcomponents are set to zero.

25

2.6 IM/DD O-OFDM for Access Networks

2.5.2 Clipped OFDM Signal

In this section we describe an alternative to cope with PAPR limitations that couldimpact in an OFDM implementation.

In [5] it was proposed as a possible solution to O-OFDM PAPR to ”cut” the highestpeaks by designing a smooth clipping window. This leads to a clipping noise thataffects the signal and errors are present in the decoding process. A couple of yearslatter in [3] it was shown how clipping can be a memoryless operation so low noise orperfect reconstruction is feasible. The model makes use of the properties of the IFFTand FFT and the assumption of clipping distortion being a random noise process w(k)uncorrelated to the IFFT output of the signal x(k). When clipping is done after themodulation process all the clipping noise will fall in the DC component and the evensubcarriers, therefore only odd subcarriers are modulated (IFFT) to carry information.By eliminating this unused even subcarriers the original data is reconstructed at thereceiver without corruption using the clipped one xc(k). This provides an energy efficientsolution to PAPR but at the cost of using just 1/4 of the bandwidth. The method canbe summarized as follows:

• Generate a real valued OFDM base band signal by imposing the Hermitian sym-metry and mapping information symbols to the odd subcarriers only.

• Clip the resulting signal by forcing to zero all negative values[2.7]. This will reducethe signal amplitude by 1/2 [ 2.8].

• At the receiver the information is perfectly recovered from the clipped signal de-modulating the odd subcarriers and discarding the even ones.


Optical OFDM has been proposed for Optical Transport Networks in ultra longhaulsystems and short reach scenarios. The goal for this research work is bounded to theimplementation of O-OFDM as multiplexing access scheme in PONs, therefore in thissection we explain the physical and logical schemes proposed for OFDMA for PONs.

2.6.1 OFDMA in PONs

Network Description

Future PON technologies must be cost efficient and backward compatible to previousstandards in order to reuse as much as possible the existing optical distribution network.Since the legacy fiber distribution network might be already deployed, this accounts formost of the of PON investment cost. In the case of future PON OFDMA, it will bedesirable to centralize complexity as much as possible in the OLT providing service to alarge number of simpler, cost efficient ONU’s. The main challenge is to keep the low cost

26


Figure 2.7: Clipping the signal forcing negative values to zero to use only positive valuesreduces the PAPR and the required bias drive current at the laser modulator providingan solution to PAPR.

Figure 2.8: Signal Constellation before clipping (*) has amplitude of 1. After clipping(+) the signal the constellation will show amplitudes of 0.5.

27


of the ONUs and at the same time exploit as much as possible the spectrum available byreducing bandwidth idle time and traffic imbalances, without losing energy efficiency.Figure 2.9 shows an example of a possible topology where an OLT delivers transportservice to different types of users exploiting maximum transmission time while reducingspectral gaps by using OFDM.

Figure 2.9: OFDMA PON to deliver transparent transport of arbitrary signals.(c)Cvijetic, N.; ”OFDM for Next Generation Optical Access Networks”; NEC labs.

Dimensioning for Physical Access

From latest discussions and guidelines at the OFC/NFOEC congress [11, 8], the deploy-ment scenarios can be categorized at in Urban context with passive elements providinga maximum of 20km reach, a Passive Long Reach that is composed of only passive net-works with a 60km reach and an hybrid model known as Active Long Reach that impliesamplification with 100km reach. From this perspective suggested dimensioning consistsof:

• Around 10 segments per central Office)

• Up to 100 Gbps per segment

• Intermediate steps: 10G/40G/100G

• 256 ONUs per segment

O-OFDMA Logical Access Models

O-OFDMA technology is attractive in the context of point (OLT) to multipoint (ONUs)PONs because it can provide flexibility and scalability for bandwidth provisioning. Asshown in figure 2.10, multiuser traffic can be shaped by a)using different number ofsubcarriers, b)adjusting the constellation size by moving between different M-QAMlevels and c)increasing the symbol rate. These characteristics provide means for a moreefficient sharing of the network resources but also better network reconfiguration and

28


adaptability to changing conditions (bit loading and dynamic bandwidth assignment).

Figure 2.10: Scalability and Traffic shaping with reconfiguration capabilities for nextgeneration networks can be provided by OFDM.

For a more efficient network resource sharing, different outlines for multiplexed accesscan be deployed suiting the needs of the network as we enumerate here:

• OFDMA PON: Different users assigned different OFDM subcarriers within oneOFDM band of total N subcarriers.

• OFDMA/TDMA PON: Different users assigned different OFDM subcarriers andTDM slots within one OFDM band; 2 dimensional dynamic bandwidth allocation.

• OFDMA/TDMA/WDMA PON: Different users assigned different OFDM subcar-riers, and TDM slots and WDM λ.

The combination of these models potentially expands the benefits of OFDMA alone.Figure 2.11 shows how the possibilities to model the future logical access if OFDM isimplemented in OANs.

29


Figure 2.11: Optical OFDMA PON Models.

30

Chapter 3

O-OFDM Experimental Setup

In this chapter we report the laboratory tests for the O-OFDM transmitter (ONU)and receiver (OLT) in a point to point and O-OFDMA multipoint to single point sce-narios. Our target is to focus in the implementation of a cost effective ONUs usingOptical OFDM based on Intensity Modulation/Direct Detection (IM/DD) scheme. Themethodology proposed in this work provides guidance to the reader in a generic formand not constrained to any specific hardware or software platform.

We have performed several experiments. First, we deployed a pure electrical OFDMsetup to check that the coder and decoder where working correctly. Then we addedthe optical equipment consisting of a DFB laser and a p-i-n photodetector for a backto back experiment. Because of material limitations, off-line analysis was done in thereceiver to obtain the system performance quantified by the Bit Error Rate (BER). Thefollowing test bed included the addition of 25km of Standard Single Mode Fiber (SSMF)to check the impact of urban distances in the BER. A different experiment consisted oftwo OFDM transmitters simulating different ONUs sharing the bandwidth electricallywith a maximum distance of 25kms of SSMF to the OLT. The two ONUs where usingdifferent wavelengths to avoid interference between them and maintain an acceptableBER. The last test consisted on adjusting different bandwidth assignments per user andchanging optical power for one of the ONUs to simulate a different distance to the OLTand compare the obtained BERs per user.

3.1 Material

This subsection serves as a brief description of the equipment used which is not particu-lar to OFDM but to any optical system. Before attempting implementing the test all thematerial was characterized and this information is included in the Appendix A. A briefdescription of the equipment used is provided here in order to reinforce the proceduresection.

31

3.1 Material

Arbitrary Waveform Generator and Oscilloscope

The Waveform Generator can be thought as a DAC in our OFDM system with a sam-pling limit of 12GS/s. Once we have created the OFDM signal it is loaded in the AWGwhich can be controlled externally through a computer. During the experiments wehave set the AWG 2.5GS/s per channel with an 8-bit DAC. Just as the AWG can bethought as the DAC, the Oscilloscope serves as our ADC with a maximum samplingfrequency of 12.5GHz or 50GS/s.

Control for the Arbitrary Waveform Generator and the Oscilloscope is feasible usinga PC but physical and logical connections from the PC to the AWG and the Oscilloscopemust be present. This can be done via GPIB interfaces and logical control is done usingsoftware such as Matlab Test and Measurement Tool package which provides communi-cation between the hardware, drivers, interfaces accessible. For first time deployments,NI and specific drivers installation might be necessary in order to configure the GPIBports of each element depending on the manufacturer.

Laser Diodes operation and control

For Intensity Modulation we make use of Feedback Distributed Lasers, which are semi-conductor lases packed with thermoelectric cooler to maintain the laser at constantoperation at the desired wavelength and which can be directly modulated and that rep-resent a minor cost and less complexity.

Figure 3.1: Distributed Feedback Laser Diode scheme. (c) Mitsubishi

DFB Laser Diodes in Butterfly packages (BF) (See fig. 3.1) are powered, operatedand directly modulated though its pins via a Laser Diode Mount module (see fig. 3.2following operation values such as forward current, operating voltage, thermistor resis-tance and operation temperature given in the manufacturer datasheets. Some LaserDiode Mounts are reconfigurable in the internal circuitry to provide the user flexibilityto mount and use different PIN configurations from different manufacturers and addmodules for direct modulation of the lasers via add-on modules (e.g. BIAS T module

32

3.1 Material

for ILX Lightwave Diode Mount).

Figure 3.2: Thorlabs Laser Diode Mount Module layout. (c) Thorlabs Inc

The Laser diode mount is just our vehicle to connect the laser to the laser Thermo-Electric Controller equipment which provides means to operate the laser diode withreliability and precision. By regulating temperature,thermistor resistance, current andvoltage of the DFB laser is modified and stabilized (e.g. ONU that can variate tempera-ture in winter or summer from 18 Celsius degrees to 30 Celsius Degrees). We have useda Thermo-Electric Controller equipment (fig. 3.3 that connects to the diode mountusing 9-pin connectors for current source and 15 pin connector for connecting to theTE controller. If desired, the TECs can be manipulated remotely from a computer viaa USB or GPIB interfaces for tighter control. In our case we operated the equipmentmanually. For detailed guidance to set up the DFB butterfly lasers, modules and TECsplease refer to [16, 21, 25].

Figure 3.3: ILX LDC 3700C Series Controller. (c) ILX Lightwave Corporation

UPC GCO laser prototypes

The Optical Communications Group at the UPC developed 4 laser controllers that useDFB Fujitsu InGaAs butterfly laser diodes mounted in a circuit that includes tempera-ture control, current control and modulation input interface (fig 3.4. By manipulatingthe circuit with resistor switches the laser frequency can be manipulated to move in 2.5GHz hops. According to the datasheet the spectral bandwidth is 0.2nm. With these

33

3.2 Procedure

prototypes we bypassed the use of TECs and mount modules for the second stage of thetest.

Figure 3.4: Circuit with tunable lasers built at the UPC.

Additional equipment

Frequencies used in optical domain are out of the band limits of a Oscilloscopes andFrequency Analyzers so in order to monitor the optical bands during wavelength mea-surements we made use of Optical Spectrum Analysers (OSA) to obtain the distancebetween lambdas of the two transmitters and the difference between their respectivepeak powers.

For Direct Detection we have used a high speed PIN photodetector which transformsthe optical power that arrives into electrical current . This lightwave receiver must bepowered using an external Electrical Power Source (EPS).

3.2 Procedure

The flowchart from figure 3.5 describes how tests were structured for all experiments.Most of the OFDM transmitter tasks and data source are done in Matlab as well as thedecoding process. We recorded the received signal using the Oscilloscope, then off-lineanalysis was required to obtain the BER of the signal.

34

3.2 Procedure

Figure 3.5: OFDM Laboratory Test Procedure.

35

3.2 Procedure

3.2.1 Electrical OFDM setup

With this experiment we tested the correct OFDM signal generation and the decodingprocess connecting directly the output of the AWG channel to the oscilloscope.

OFDM signal generation

The data sequence was generated as a random sequence of binary symbols. The use M-QAM scheme to modulate the random binary source provides scalability in the amountof data symbols precoded for each orthogonal subcarrier. In our tests we used M=4.The set is re-arranged as a block transmission through a serial to parallel (S/P) stepand mapped as the input of the IFFT with N=256. The signal was constrained to haveHermitian Symmetry to obtain a real value signal for optical modulation. By doing thiswe reduced the number of useful IFFT/FFT carriers by N/2.

The output of the IFFT was re-arranged by a parallel to serial order and the cyclicprefix was added. Initially, in an attempt to test the correct system operation, an arbi-trarily CP=0.2 was added.

Syncronization sequence

We added a known sequence with high correlation properties so that the process ofdetecting the signal is based on identifying at the receiver the beginning and the endof OFDM symbols sequence that is bounded by a sequences with low cross-correlationwith the OFDM signal.

In this set up we have chosen a sequence with low cross correlation with othersequences (our data) known as Barker Code which is used in 802.11 OFDM standard.In figure 3.6 we show the use of a Barker sequence of length 4 which is inserted beforeand after the OFDM symbols. When passed through the IFFT we obtain a raised cosinein time domain at the output.

The raised cosine delimits the OFDM symbols and provides means to synchronize thetransmitter with the receiver. Since the receiver knows the synchronization sequence,the correlator at the receiver side should find two peaks at the beginning and the endof the OFDM valid data as shown in fig. 3.7.

Modulation and signal transmission

The output of the signal in the Matlab code is saved in the required format for the AWG(txt) and loaded in the Waverform Generator. Usually we need to provide the signaldata and the length of the signal (number of data points). The signal is loaded manuallyusing the supported media (e.g. floppy disk, USB pen drive). If remote control of the

36

3.2 Procedure

Figure 3.6: Barker Code sequence [+1 + 1 − 1 + 1] at the input of the IFFT. AfterIFFT transformation to time domain we obtain a raised cosine which is used as asynchronization sequence at the beginning and end of the trail of the OFDM symbols.

Figure 3.7: Correlation between the received data and the known synchronization se-quence. Peaks mark the beginning and the end of a series of OFDM symbols.

37

3.2 Procedure

AWG is done via the GPIB interfaces the file containg the signal is loaded using MatlabTest and Measurement tool which is initiated by typing:

>tmtool

in the command window. In the TM tool GUI, Instrument Objects (physical link)and Interfaces Objects (logical link) must be created and activated (connected) as shownin figure 3.8. Recent models of AWG have an integrated PC which includes a GUIenvironment to load the signal.

Figure 3.8: TM tool Instrument and Interface Objects created and connected.

Decoding

The process of detecting the signal in order to perform the off-line analysis follows theflow chart in figure 3.9. If the AWG and the Oscilloscope are properly synchronized,recovering the original data is feasible. The captured signal can be recorded as a .csvfile in order to perform off-line processing.

Then we start the receiver decoder process by identifying the valid data which is inbetween the synchronization sequences, then the training sequences are removed. Oncewe know the length of the symbol, in case CP is used, it should be removed and weperform the inverse of the process at the transmitter: 1) Perform FFT and 2) IdentifyUseful modulated carriers considering if Zero Padding and/or Hermitian Symmetry wereperformed. Finally we re-arrange the trail so we can pass it through a QAM demodu-lator. When obtaining the bit sequence we calculate the Bit Error Rate.

38

3.2 Procedure

Figure 3.9: AWG and Oscilloscope serve as DAC and ADC so these must follow theNyquist sampling rule for synchronization and proper decoding.

Note that the symbol period, related to the bit rate in the lab set was limited inbandwidth by the Waveform Generator and Oscilloscope sampling capacity. Finallyfor our case, after all material was properly checked, the setup parameters were in theOscilloscope and AWG are optimized and once synchronization problems between thewere solved we obtained a BER equal to 0.

Applicability of Clipped OFDM signal

As we described in the previous chapter, the possibility of having PAPR in our OFDMsystem can be a limitation, for this reason we also tested succesfully the describedprocedure of generating and decoding a Clipped OFDM signal.

3.2.2 Optical Back to Back scenario

This experiment was based on the pure electrical test described above and the additionof the DFB laser that was modulated by the AWG output channel. For the receiverwe added a pin photodiode that provides an electrical output which is connected to theoscilloscope. The transmitter anf receiver are connected using a 1m long SSMF and theoptical carrier was monitored through the OSA. The set up is shown in figure 3.10 andthe lasers and photodiode characterizations are in the Appendix A.

Since the coder and decoder were working fine we explain only the main differencesfrom the previous test.

39

3.2 Procedure

Figure 3.10: Back to Back O-OFDM IM/DD system.

Resultion concerns

It is important to make emphasis that OFDM PAPR stopped being the main concernrelated to signal distortion and resolution capacity. Originally we developed and testeda Clipped OFDM coder and decoder assuming that the OFDM would present a highPAPR. Nevertheless with the laboratory implementation the signal PAPR is completelymodified by synchronization sequence used at the beginning and at the end of the OFDMsignal data trail causing the peak signal to go considerably higher than the rest of thetransmitted signal (fig. 3.11). At the receiver the ADC might not generate enoughquantization steps for the OFDM data signal and therefore loose resolution for thedecoding process.

In order to decrease the signal peak, either a different low cross-correlation signalshould be used from the one proposed here (Barker sequence) or reduce the peak am-plitude of the training signal, as we did in this case. For this purpose we have changedthe Barker sequence for an sequence of 1’s and 0’s which are not passed trough theIFFT but only added at the beginning of the OFDM trail. At this point the sequenceplays the same role of the raised cosine, but then we reduced the amplitude of thesynchronization sequence to the highest peak of the whole OFDM trail and confirmedthat this does not impact on the cross-correlation calculation at the receiver. PAPR po-tentially can cause distortion and peak clipping if the Oscilloscope is not scaled properly.

Another resolution concern is related to the sampling at the receiver (Oscilloscope).Inherently the addition of elements such as the photodiode attenuate the signal andadd some noise, for this reason the Oscilloscope was configure to oversample the signal,therefore the recorded data provides a higher number of points from the original signalgenerated at the AWG, so a workaround should be performed in the decoder such asdown sampling, averaging, euclidean distance. In this work we have used downsamplingusing the middle point out of 8 samples for computational efficiency. The results of thisexperiment showed that as long as the photodiode worked fine, and since the channel

40

3.2 Procedure

Figure 3.11: OFDM received signal recorded in the oscilloscope. The raised cosine usedas reference for synchronization is considerably higher than the rest of the signal

was only a 1m long fiber, no impairments or considerable attenuation damaged the sig-nal so the decoder provided a BER of 0.

3.2.3 Single O-OFDM transmitter + 25km SSMF scenario

With the procedure described above we added a 25km SSMF between the transmitterand the reciver. The modification of the channel imposed new considerations which aredescribed in this section.

Training Sequence

The 25km channel can induce some errors in the decoded symbols. As described in theprevious chapter a tap equalizer was added at the decoder to undo these effects. Byadding a training sequence at the beginning of the transmission we obtained the chan-nel response. Four (4) known series of symbols were randomly generated and passedto the receiver. During transmission the 4 series serve as a prelude to the payload. Atreception, the average of these 4 series determines the channel response and the requiredcoefficients for equalization.

41

3.2 Procedure

No need for Cyclic Prefix

We have worked under a short reach scheme or urban scenario with distance equal to25km, from eq. 2.2, the required cyclic prefix is given by:

τg =17 ps

nmkm ∗ 25km ∗ 50× 109Hz ∗ 3× 108

193.3× 1012Hz= 1.7× 10−11s (3.1)

While the Symbol Period is 1/5 × 109Hz = 2 × 10−10s and the OFDM Symbol periodis 2 × 10−10 ∗ 256 = 5.12 × 10−8s. This means that the CP required is negligible. Wecan say that because the bit rate and the fiber length used, no Chromatic Dispersion ispresent, therefore no CP prefix was used.

In this section we obtained a BER equal to 0 or in an acceptable margin below1× 10−3 if the signal arrives at a minimum of -27dBm.

3.2.4 O-OFDMA: 2 users with shared electrical bandwidth

OFDM offers the possibility to use different sources of pre-modulated symbols which rep-resent different users. In this part of the experiment we tested this feature for OFDMAunder the concept of Dynamic Bandwidth Allocation by changing the total number ofsubcarriers assigned to different users which can shape the traffic from each ONU to theOLT. According to their specific needs we could provide or remove bandwidth dynami-cally as long as the rest of the users sharing the medium are not affected.

The experimental setup is shown in figure 3.12. A total of 219 bits were randomlygenerated and mapped to a 4QAM modulation format to generate off-line the OFDMsignals. Using two channels of the AWG (@2.5GS/s each) we send two different OFDMsignals which electrically share the total number of N=256 subcarriers (sc) from whichonly 128 subcarrier are effectively used because of Hermitian symmetry. These subcar-riers were divided among the users, for instance in the case of 64sc/64sc, each ONUwas assigned 64 useful subcarriers (128 with its complex conjugate) while setting theremaining subcarrriers to zero. This means that ONU1 uses the lower part of the electri-cal spectrum while ONU2 utilizes the upper. These OFDM signals modulate separatelytwo different lasers directly over different central wavelengths (ONU1=1554.2nm andONU2=1554.9nm).

Both ONUs transmit simultaneously and pass through a coupler, and then 25km ofSSMF were added followed by an electronic attenuator reducing the signal optical powerby 9dB. This 9dB attenuation can be seen as adding 3 more couplers of 50/50 ratio (seeAppendix A), that could exemplify a network of 16 users. The receiver consists of justone PIN photodiode with a pre-amplification stage and detects both signals simultane-ously. This arrangement simulates the OLT end. The PIN transforms the optical signalsinto electrical and sends these signals to the Oscilloscope for detection and processing.At the decoder the FFT operation is performed, then signal is splitted assuming the

42

3.2 Procedure

Figure 3.12: Test bed layout for Dynamic Bandwidth assignment. Two ONUs transmitdifferent N subcarriers. A variable power output at the ONU 2 simulates distanceaffecting the OSNR and the respective BER.

receiver knows the assigned number of subcarriers assigned to each user and calculatesthe BER.

Wavelength Distance for Spectral Efficiency

Each ONU signal respectively modulates a laser with a close but different center wave-length (1554.2nm and 1552.9nm). In case these overlap, a temperature change canslightly tune the wavelength. In our set, with similar peak power at the output of thelasers (between -1 and -2.2dB), by manipulating the temperature control the distancein wavelength between the two ONUs was reduced as much as possible without im-pacting the BER of both ONUs (BER 1e-3). The figure 3.13 illustrates the BER forboth ONUs when modifying the temperature control to change the wavelength distancebetween transmitters. At this point we hadn’t realized that synchronization issues wereimpacting. the BER.

Sychronization Issues

A critical point for the correct detection of the signals was the synchronization of thetwo signals at the receiver. Special attention was to set almost identical distances fromeach ONU to the coupler. Nevertheless, a small delay was experienced in ONU2 withrespect to ONU1 (less than a data symbol period). The BER could be maintained forboth ONUs if the the sampling instance of the sychronization sequence for both ONUswas choosen properly. To do so, an upper MAC layer in the decoder and coder is neededto determine the distance of each ONU and synchronize accordingly, however this is be-yond the scope of this work.

43

3.2 Procedure

Figure 3.13: BER monitoring under temperature control wavelength tunning for bothONUs with a bandwidth assignement of 64 subcarriers for each user (64sc/64sc).

3.2.5 Bandwidth Allocation for O-OFDMA

In this test we found a minimum power difference acceptable from the ONU2 in compar-ison with the ONU1, and how much margin we have to modify the bandwidth assignedto each ONU under these circumstances without losing the minimum BER required forboth ONUs. The initial measurement was done with 64sc assigned to each user andgradually the power of the ONU2 was modified and BER calculations were made.

Since the number of subcarriers assigned to each user was progressively modified,the necessary wavelength separation for different bandwidth assignments changed withdifferent bandwidth allocation, so we decided to change the control current at the laserto separate the emitting wavelengths to achieve a minimum applicable for all cases(96sc/32sc, 112sc/16sc, 120sc/8sc, 124sc/4sc and 126sc/2sc) in contrast with the pre-vious test of 64sc/64sc. This re-tunning lead to a distance of 0.5252 nm, which couldbe compared to a DWDM 50GHz spacing. Figure 3.14 is a screen capture at theOSA showing this wavelength separation where the differential peak output power ofthe lasers was minimum with a 64sc/64sc configuration.

After modifying the wavelength distance the BER for both ONUs chaging the powerfrom ONU2 with respect to the power of ONU1 was obtained for: 64sc/64sc, 96sc/32sc,112sc/16sc, 120sc/8sc, 124sc/4sc and 126sc/2sc. This is represented in figure 3.15.

Because the energy in the spectrum is distributed differently when using 64sc/64scconfiguration from a 120sc/8sc or 126sc/2sc, the requiered optical power for ONU2changes the BER slope. It can be seen in figure 3.16 when both ONUs share the same

44

3.2 Procedure

Figure 3.14: OSA capture of the spectrum of two ONUs with a distance of 0.5252nmbetween carriers necessary for different subcarriers assignment.

bandwidth (64sc/64sc), the BER is acceptable (below 1×10−3) for both ONUs when thedifference of peak power between them is small, between -1 and 1 dB. If more bandwidthis assigned to ONU1 (96sc/32sc) the correct detection for both ONUs is between -3.5and 0 dB increasing the margin of difference between the peak power of both ONUs.This progressive effect is shown in in the 112sc/16sc scheme where the margin betweenpowers is bigger, in the range -6 to -1dB.

As it shown in the plots from figure 3.17, the less number of subcarriers applied toONU2, the more energy is contained in that ONU2 in less symbols, therefore the decisionmaking for the constellation at the receiver is more efficient and the BER starts todecompose at poorer OSNR conditions in comparison with a more even distribution of sc,then making the slope more aggressive from 64sc/64sc towards 120sc/8sc configuration.This was also seen in the OSA (fig 3.18), as the spectrum is noticeably expanded forONU2 when reducing the number of subcarriers.

It is important to point that reducing the power at the output of the ONU2 viathe optical attenuator can be seen as locating the ONU2 at a larger distance. Underthe assumption that CD is not a considerable factor, then we see that in the 126sc/2scconfiguration the OSNR for ONU2 can be really low in comparison with ONU1 withoutdisrupting the BER.

45

3.2 Procedure

Figure 3.15: Dynamic Bandwidth Allocation scenarios. Two ONUs transmit with dif-ferent number of subcarriers. The output power of ONU2 is modified before the couplerso the respective OSNR from ONU2 is modified in comparison with the OSNR fromONU1.

46

3.2 Procedure

Figure 3.16: BER for two ONUs with 64sc/64sc,96sc/32sc,112sc/16sc bandwidth allo-cation schemes.

47

3.2 Procedure

Figure 3.17: BER for two ONUs with 120sc/8sc, 124sc/4sc and 126sc/2sc bandwidthallocation schemes

48

3.2 Procedure

Figure 3.18: The OSA capture shows how the spectrum for ONU2 is noticeably broad-ened when reducing the number of subarriers for 126sc/2sc configuration.

49

Chapter 4

Conclusions

We have reported the first Optical OFDM system implemented at the Optical Com-munications Laboratory of the Signal Theory and Communications Departement at theUPC. This test-bed was based on cost-effective IM/DD which can potentially be appliedas a multiplexed access scheme for future PONs. With different experimental set-ups wehave coped with fundamental issues and we have outlined minimum considerations thatshould be taken into account for the design and deployment of an IM/DD O-OFDMAscheme.

We have proved that IM/DD O-OFDMA is backwards complatible as it can relyon mature technologies such as DFB lasers and pin photodiodes which are likely to beused in existing PON networks so a big part of the investment for deployment would bealready covered and the missing portion would rely on cheap ONUs acquired by the users.

First, a back to back test from the ONU to the OLT was done. The source used4-QAM pre-modulated symbols to obtain 2 bits per symbol and the IFFT block of256 subcarriers was constrained with Hermitian Symmetry to generate a real signal tobe able to modulate the DFB laser. This was done at the cost of wasting half of thebandwidth. We used the AWG as DAC and the Oscilloscope as ADC so the bit rate(2.5Gbps) was constrained by the sampling rate of the AWG (2.5Ghz) and we developedthe code for generating a Clipped OFDM signal in case the PAPR could impact at thereceiver. The most sensitive points where 1) synchronization between the AWG at thetransmitter and Oscilloscope at the receiver to avoid clipping or cutting the signal and2) resolution at the DAC and ADC because the PAPR originated by the synchroniza-tion sequence in addition to the OFDM signal PAPR limited the quantification levels atthe DAC and the ADC which is extremely detrimental and caused the BER to drop tounacceptable values (above 1e− 3). The high amplitude was caused because originallywe attempted to use a 802.11 standard low cross-correlation sequence as synch referencewhich produced a high amplitude raised cosine. Then we used a simple set of 0’s and1’s which was still too high, so these were re-sized. By reducing the peak amplitudeto the maximum amplitude of the OFDM signal, a low cross-correlation between the

50

Conclusions

synchronization sequence and the OFDM signal was maintained. At the receiver andproper decoding was achieved without the need of using a Clipped OFDM method.

When adding 25km of SSMF to the set up, training sequences to estimate the aver-age channel response for equalization purposes were needed. With the lab tests we haveexperimentally found that for urban topologies below the 25km and low optical powers(between 0 and -15dB) neither Chromatic Dispersion or Polarization Mode Dispersionwill not be restrictive at a rate of 2.5Ghz, which is comparable with GEPON standard.Eventhough that mathematically we obtained a required CP period time negligible incomparison with the OFDM symbol length and channel dispersion, in case the OpticalDistribution Network is expanded for longer reach, the use of Cyclic Prefix should not bediscarded for further development of real systems with larger distances from the ONUto the OLT to reduce the ISI effects caused by CD. An acceptable BER (below 1e− 3)is obtained if the signal arrives at the detector a minimum of −27 dBm.

In a shared electrical bandwidth configuration, two O-OFDM transmitters weremounted at 2.5 GHz each simulating two different users transmitting from their respec-tive ONU to the OLT using 4-QAM pre-modulated symbols and sharing the electricalbandwidth of 256 subcarriers (N=128 for each user). With a distance of 25km SSMFwe used a wavelength separation of 0.5252 nm between lasers to avoid interference be-tween each user. Since there was a minimum differential fiber length between the twotransmitters (ONUs) to the receiver (OLT), a shift at the synchronization sequence -caused by different sampling instants of the ONUs at the receiver- was prejudicial forthe BER specially for small bandwidth assignments such as N=2. Optimization in thesynchronization part improved the BER and therefore the wavelength distance could bemodified in future.

With this test we have demonstrated that traffic shaping for reconfiguration capa-bilities for next generation networks can be provided using OFDM signals and dividingthe bandwidth for smaller allocation within the same channel. The key concept to em-phasize is that the users are allocated in the electrical part of the spectrum providingflexibility. One user (ONU1) using the lower part of the spectrum while the second user(ONU2) using the upper part of the spectrum. In our approach, because both userswere constrained to have Hermitian Symmetry, the bandwidth reduces by half leaving amaximum of 64 usable subcarriers for each user. In different allocation schemes such as64sc (ONU1) and 64sc (ONU2),96/32, 112/16 , 120/8 , 124/4 and 126/2. The possibilityto change the number of subcarriers assigned per user within an acceptable margin ofBER was proven.

As a different test we added 25km of SSMF plus 9dB attenuation between the ONUsand the OLT before the OLT pre-amp stage. This could be seen as the existence of 3couplers with 50/50 ratio to add 12 more users in the network to have a total of 16 pos-sible ONUs (4 couplers), proving flexibility for expansion purposes and more complextraffic management.

51

Conclusions

The output optical power of ONU2 was modified with respect to ONU1 and simulatea differential distance between the users to the OLT. The more attenuation, the moredistance to the ONU. We saw that if the two ONUs share the same amount of band-width, the BER is acceptable as long as the maintain a certain level of differential powerbetween users and a certain level of OSNR for the ONU with the weakest optical outputpower power. Moreover in the lab test results it is shown that if for the ONU that has alower OSNR, minimum service level can be assured by assigning less subcarriers to thatONU as the margin of reducing the OSNR for an acceptable BER is higher than in amore equally distributed subcarrier scheme.

Future work

The work reported here is a first approach to an IM/DD O-OFDMA system with elec-trical bandwidth assignment for users. This user allocation approach reduces the depen-dency on extremely accurate optical equipment however electronic circuit check couldbe performed to guarantee a more stable laser manipulation via temperature changesand depend less in drive current tunning.

For the transmission part some optimization should be done for the synchronizationpart between ONUs, but it is also important to start working on the required control atthe upper MAC layer. This can lead to find a minimum wavelength separation smallerthan the 0.5252 nm described here and to have more accurate frequency hops.

From the PON simulation this work can be expanded. Longer distances or if possiblehigher bit rates could be used for ISI and Cyclic Prefix tests. Given the lab material,with the current GCO laser prototypes we could grow the PON to 4 ONUs. This coulddeliver a better insight to be able to dynamically change the pre-modulated symbolsbetween 4-QAM to 16 QAM and tests at a deeper detail the granularity and understandlimitations given the power required at the receiver for larger constellations.

For the purpose of this work and with the limitations of the available material werelied on an AWG and Oscilloscope to serve as DAC and ADC in the transmitter and thereceiver performing off-line processing, however this work can serve for future referenceto integrate FPGAs to work with the laser control circuits developed at the UPC toprovide a more elaborated ONU prototype. By doing so an approximation to the realcost of the ONU can be calculated.

With a different approach for user allocation in IM/DD, in case IQ modulators areavailable, a bandwdith assignent could be designed using RF upconversion instead ofHermitian Symmetry.

52

Appendix A

Material Characterization

Figure A.1: Characterization of the coupler ports.

53

Figure A.2: DFB lasers wavelenght tunning via temperature variation.

54

Figure A.3: Characterization of the p-i-n photodetector.

55

Figure A.4: BER for two users sharing the 256 subcarriers (w/ Hermitian Symmetry indifferent bandwidth allocation schemes.

56

Appendix B

Matlab Code

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% O−OFDM Hermitian TX & RX System Code% Author : Angel P e r a l t a% Email : ange loopo l i s27@gmai l . com% Version : 1 .0% Date : 12 March 2012% Creat i ve Commons% You are f r e e :% to Share : to copy , d i s t r i b u t e and transmi t the work% to Remix : to adapt the work% Under the f o l l o w i n g c o n d i t i o n s :% A t t r i b u t i o n . You must a t t r i b u t e the work in the manner% s p e c i f i e d by the author or l i c e n s o r ( but not in any way% t h a t s u g g e s t s t h a t they endorse you or your use o f the work ) .% Noncommercial . You may not use t h i s work f o r commercial purposes .%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%clear a l l ;%% +++ Tx ++%% source %%BpS = 2 ;M=2ˆBpS ;Rb = 1e9 ;N bi t s = 1220 ;N FFT = 256 ;for F=0:3Oversampling = 1 ;CPlenght = 0 . 2 ;N b i t s per symbo l = log2 (M) ;N symbols = N bit s / N bi t s per symbo l ;%%%Source , M−QAM %%%

57

b i t s o u r c e = randint (1 , N bi t s ) ;b inary a lphabet = reshape ( b i t s o u r c e , N bit s per symbol , N symbols ) ;b inary a lphabet = binary a lphabet ’ ;dec ima l a lphabet = ( bi2de ( b inary a lphabet , ’ l e f t −msb ’ ) ) ’ ;modulat ion type = modem. qammod(M) ;QAM constel lat ion = modulate ( modulation type , dec ima l a lphabet ) ;%% FFT c a r r i e r s and Zeros f o r padding %%NyquistSC =N FFT/2+1;ModCarriers = ce i l (N FFT/ Oversampling ) ;i f Oversampling == 1

UnModCarriers= 0 ;else

UnModCarriers = N FFT−ModCarriers ;i f mod( UnModCarriers , 2 ) == 1UnModCarriers = UnModCarriers + 1 ;end

endModCarriers = N FFT−UnModCarriers−2;Unmod FirstSet = NyquistSC−(UnModCarriers / 2 ) : NyquistSC ;Unmod SecondSet = NyquistSC : NyquistSC+(UnModCarriers / 2 ) ;ZeroPad=min( Unmod FirstSet ) :max( Unmod SecondSet ) ;N OFDM Symb = ce i l ( N symbols /( ModCarriers / 2 ) ) ;Symb Mtx = ones (N FFT,N OFDM Symb ) ;Symb Mtx ( 1 , : ) = 0 ;Symb Mtx( ZeroPad , : ) = 0 ;v a l i d p o s i t i o n = find (Symb Mtx ( 1 : NyquistSC , : ) ) ;C=zeros (1 , length ( v a l i d p o s i t i o n ) ) ;C( 1 : length ( QAM constel lat ion ) ) = QAM constel lat ion ;A = zeros (1 , NyquistSC∗N OFDM Symb ) ; %A( v a l i d p o s i t i o n ) = C;IFFT Symb Input=reshape (A, NyquistSC ,N OFDM Symb ) ;%% Hermitian Symmetry %%H Symb Mtx = zeros (N FFT,N OFDM Symb ) ;H Symb Mtx ( 1 : NyquistSC −1 , : ) = IFFT Symb Input ( 1 : NyquistSC −1 , : ) ;H Symb Mtx( NyquistSC : N FFT , : ) = fl ipud ( conj (H Symb Mtx ( 2 : NyquistSC , : ) ) ) ;IFFT Symb Out= i f f t (H Symb Mtx ) ;%% C a l c u l a t e & Add C y c l i c P r e f i x %%CP = ce i l ( CPlenght∗N FFT ) ;i f mod(CP, 2 ) == 1

CP = CP + 1 ;endOFDM symb length = CP + N FFT;%%%%% b u i l d OFDM symbol %%%%%%%%OFDM Symb Mtx = zeros ( OFDM symb length ,N OFDM Symb ) ;

58

OFDM Symb Mtx( (CP+1):OFDM symb length , : ) = IFFT Symb Out ( 1 : N FFT , : ) ;OFDM Symb Mtx( 1 :CP, : ) = OFDM Symb Mtx( ( ( OFDM symb length+1)−CP) : OFDM symb length , : ) ;OFDM Signal = reshape (OFDM Symb Mtx, 1 , (N OFDM Symb∗OFDM symb length ) ) ;OFDM SignalLength=length ( OFDM Signal ) ;%% DAC %%Rsym = Rb/ N bi t s per symbo l ;Tsym = 1/Rsym ;T OFDM Sym = OFDM symb length∗Tsym ;T CP=CP∗Tsym ;CP overhead=T CP/T OFDM Sym;TimeWindow=Tsym∗OFDM SignalLength ;t t o t a l = 0 :Tsym : TimeWindow−Tsym ;

%% Tx Training sequence genera tor wi th BakerCode = [+1 +1 −1 +1]TS=zeros (1 , OFDM symb length ) ;pu l s e=f loor ( OFDM symb length / 4 ) ;TS( 1 , 1 : 2∗ pu l s e )=+1;TS(1 ,2∗ pu l s e +1:3∗ pu l s e )=−1;TS(1 ,3∗ pu l s e +1:OFDM symb length)=+1;T TS=real ( ( i f f t (TS ) ) ) ;Tr a i l=zeros (1 , OFDM symb length+OFDM SignalLength ) ;Tr a i l ( 1 , 1 : OFDM symb length)=T TS ;Tr a i l (1 , length (T TS)+1: length (T TS)+OFDM SignalLength)=OFDM Signal ;Tr a i l (1 , length (T TS)+OFDM SignalLength+1: length (T TS)+OFDM SignalLength+OFDM symb length)=T TS ;TS OFDM Signal=T ra i l ;S i gna l=TS OFDM Signal ;SignalCont=zeros (1 ,5∗ length ( S i gna l ) ) ;SignalCont (1 ,F∗ length ( S i gna l )+1:(F+1)∗ length ( S i gna l ))= S igna l ;%% Expand w/ z e r o s f o r AWG v i s u a l i z a t i o n and %s y n c h r o n i z a t i o n%I n f i n i t e S e q u e n c e=z e r o s (1 ,3∗ l e n g t h ( TS OFDM Signal ) ) ;%Vector wi th z e r o s to have 0−>Trai l−>0%I n f i n i t e S e q u e n c e (1 , l e n g t h ( TS OFDM Signal)+%1:2∗ l e n g t h ( TS OFDM Signal))=TS OFDM Signal ; % Add t r a i l%S i g n a l=I n f i n i t e S e q u e n c e ;%Signa lLength=l e n g t h ( S i g n a l ) ;%SignalAwgMtx=z e r o s ( Signa lLength ,3);% Create Matrix%SignalAwgMtx ( 1 : SignalLength ,2)= S i g n a l ;% I n s e r t S i g n a l in the matrix%SignalAwgMtx (1:11 ,3)=1;%AwgInput=SignalAwgMtx ;%% +++ Rx ++%% to c a l c u l a t e the C y c l i c p r e f i x percentage , zero padding l o c a t i o nN bits per symbol RX = log2 (M) ;N symbols RX = N bit s / N bits per symbol RX ;

59

NyquistSC RX =N FFT/2+1;%%%%%%%%% C a l c u l a t e C y c l i c P r e f i x %%%%%%%%%CPlenght =0.2;CP = ce i l ( CPlenght∗N FFT ) ;i f mod(CP, 2 ) == 1

CP = CP + 1 ;endOFDM symb length RX = CP + N FFT;%%%%%%%%% Determine the Zero Padding and modulated c a r r i e r s%%%%%%%%%ModCarriers = ce i l (N FFT/ Oversampling ) ;i f Oversampling == 1

UnModCarriers= 0 ;else

UnModCarriers = N FFT−ModCarriers ;i f mod( UnModCarriers , 2 ) == 1

UnModCarriers = UnModCarriers + 1 ;end

endModCarriers = N FFT−UnModCarriers−2;Unmod FirstSet = NyquistSC−(UnModCarriers / 2 ) : NyquistSC ;Unmod SecondSet = NyquistSC : NyquistSC+(UnModCarriers / 2 ) ;ZeroPad=min( Unmod FirstSet ) :max( Unmod SecondSet ) ;%% Rx Training sequence genera tor wi th BakerCode = [+1 +1 −1 +1]TSRX=zeros (1 , OFDM symb length RX ) ;pu l s e=f loor ( OFDM symb length RX / 4 ) ;TSRX( 1 , 1 : 2∗ pu l s e )=+1;TSRX(1 ,2∗ pu l s e +1:3∗ pu l s e )=−1;TSRX(1 ,3∗ pu l s e +1:OFDM symb length RX)=+1;T TSRX=real ( ( i f f t (TSRX) ) ) ;MeasureLength=length ( S i gna l ) ;R=zeros (1 , MeasureLength−OFDM symb length RX+1);for n=1: length (R)

Evaluate=S igna l (1 , n : n+OFDM symb length RX−1);R(n)= cor r (T TSRX’ , Evaluate ’ ) ;n=n+1;

endmatch=max(R ( : ) ) ;so r tedValues=unique (R ( : ) ) ;maxValues=sortedValues (end−1:end ) ;maxIndex=ismember (R, maxValues ) ;DEF=find ( maxIndex ) ;YYY=min(DEF)+OFDM symb length RX :max(DEF)−1;OFDM SignalLength RX=length (YYY) ;OFDM Signal RX=Signa l (1 ,YYY( : ) ) ;

60

N OFDM Symb RX=length (OFDM Signal RX)/OFDM symb length RX ;%% Perform the demodulation p roc ess to a r e c e i v e d OFDM s i g n a lDecoder=reshape (OFDM Signal RX , OFDM symb length RX ,N OFDM Symb RX ) ;%S/PDecoder ( 1 :CP, : ) = [ ] ;FFT Decoder out=f f t ( Decoder ) ;Non H Symb Mtx=FFT Decoder out ( 1 : NyquistSC , : ) ;Non H Symb Mtx( Unmod FirstSet , : ) = [ ] ;Non H Symb Mtx ( 1 , : ) = [ ] ;Vectores=reshape (Non H Symb Mtx , 1 ,min( s ize (Non H Symb Mtx ) )∗max( s ize (Non H Symb Mtx ) ) ) ;QAM constel lation Rx=Vectores ( 1 : N symbols ) ;demodulat ion type = modem. qamdemod(M) ;dec imal a lphabet Rx = demodulate ( demodulation type , QAM constel lation Rx ) ;b ina ry a lphabe t rx = de2bi ( dec imal alphabet Rx , ’ l e f t −msb ’ ) ;b ina ry a lphabe t rx=binary a lphabet rx ’ ;b i t d e c o d e r = reshape ( b inary a lphabet rx , 1 ,min( s ize ( b ina ry a lphabe t rx ) )∗max( s ize ( b ina ry a lphabe t rx ) ) ) ;b i n c o r r=cor r ( b i t decoder ’ , b i t s o u r c e ’ ) ;ErrorVector=xor ( b i t s o u r c e , b i t d e c o d e r ) ;Errors=sum( ErrorVector ) ;BER=Errors / length ( b i t s o u r c e ) ;plot ( SignalCont ) ; hold on ;xlabel ( ’ Data Symbols ’ ) ;grid on ,t i t l e ( ’OFDM Transmiss ion ’ , ’ FontWeight ’ , ’ bold ’ ) ;

61

Acronyms

ADC: Analog to Digital ConverterADSL: Asymmetric Digital Subscriber LineAPON: ATM-Based Broadband PONATM: Asynchronous Transfer ModeBER: Bit Error RateCD: Chromatic DispersionCP: Cyclic PrefixCWDM: Coarse Wavelength Division MultiplexingDAC: Digital to Analog ConverterDFB: Distributed FeedbackDWDM: Dense Wavelength Division MultiplexingDSB: Double Side BandEDC: Electronic Dispersion CompensationFFT: Fast Fourier TransformFPGA: Field Programmable Gate ArrayFWM: Four-Wave MixingIFFT: Inverse Fast Fourier TransformIM/DD: Intensity Modulaution with Direct DetectionLASER: Light Amplification by Stimulated Emission or RadiationLTE: Long Term EvolutionMZM: Mach-Zehnder ModulatorOAN: Optical Access NetworkOFDM: Orthogonal Frequency Division MultiplexingOFDMA: Orthogonal Frequency Division Multiplexed AccessO-OFDM: Optical Orthogonal Frequency Division MultiplexingOLT: Optical Line TerminatorONU: Optical Network UnitPAPR: Peak to Average Power RatioPMD: Polarization Mode DispersionPON: Passive Optical NetworkTDM: Time Division MultiplexingWDM: Wavelength Division Multiplexing

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