High Capacity Radio over Fiber
Antonio Caballero Jambrina
Professor Idelfonso Tafur Monroy,
Assistant Professor Darko Zibar and
Associate Professor Kresten Yvind
Delivery Date: 1stAugust 2011
Department of Photonics Engineering
Technical University of Denmark
2800 Kgs. Lyngby
This thesis expands the state-of-the-art on the detection of high speed
wireless signals using optics. Signal detection at speeds over 1 Gbps at
carrier Radio Frequency (RF) ranging from 5 GHz to 100 GHz have been
achieved by applying novel concepts on optical digital coherent receivers.
This achievement has satisfied the requirements on transmission robustness
and high capacity of next generation hybrid optical fibre-wireless networks.
One important contribution of this thesis is the novel concept of pho-
tonic downconversion with free-running pulsed laser source for phase modu-
lated Radio-over-Fiber (RoF) links. This scheme operates without high
frequency electronics at the digital coherent receiver for the detection of
high bitrate wireless signals. Based on this concept, I have experimentally
demonstrated the recovery of up to 3.2 Gbps 16-QAM signal modulated at
40 GHz RF carrier. At that time, it was the highest bitrate reported of a
wireless signal, with complex modulation format, detected using photonic
means. I have developed an analytical model to support the experimental
results and performed a linearity characterization to establish engineering
design rules for this type of links. The results confirmed that this configu-
ration provides high linear end-to-end transmission links and is capable of
transparent transport of high spectral efficient modulation formats.
Furthermore, this thesis introduces a novel approach for the generation
and detection of high speed wireless signals in mm-wave frequencies at car-
rier frequencies exceeding 60 GHz, using photonic baseband technologies.
For signal generation, high spectral-efficient optical modulation technolo-
gies are used together with optical heterodyning. In the detection side,
the mm-wave signal is modulated in the optical domain and received us-
ing digital coherent detection. The experimental demonstration tested the
generation and detection in the 60 GHz and 75-110 GHz bands of signals
with capacity up to 40 Gbps. Those results reported the highest bitrate
at mm-wave frequencies for signal generation and detection using photonic
methods at the time of the writing of this thesis.
In conclusion, the results presented in this thesis demonstrate the feasi-
bility of photonic technologies for the generation, distribution and detection
of high speed wireless signals. Furthermore, it opens the prospects for next
generation hybrid wireless-wired access networks providing ultra-high ca-
Denne afhandling højner stadet indenfor detektion af tr˚ adløse, højhastigheds-
signaler ved brug af optik. Signaldetektion ved datahastigheder over 1 Gbps
p˚ a bærebølge radiofrekvenser mellem 5 GHz og 100 GHz er opn˚ aet ved at
anvende nye principper for optiske, digitale og kohærente modtagere. Disse
resultater tilfredsstiller krav om robust transmission med høj kapacitet i
næste generations hybride, optiske fiber-tr˚ adløse netværk.
Et vigtigt bidrag i afhandlingen er det nye princip for fotonisk ned-
konvertering til brug i fasemodulerede Radio-over-Fiber forbindelser ved
hjælp af en fritløbende, pulseret laser.
tr˚ adløse højhastighedssignaler f ungerer uden brug af højfrekvenselektronik
i den digitale, kohærente modtager. Baseret p˚ a denne metode demonstr-
eres eksperimentel gendannelsen af op til 3,2 Gbps 16-QAM sinaler, som er
moduleret op p˚ a en 40 GHz RF bærebølge. P˚ a det p˚ agældende tidspunkt
var det den højeste, rapporterede datahastighed af et tr˚ adløst signal med
avanceret modulationsformat, som blev detekteret ved brug af fotoniske
midler. I afhandlingen præsenteres ogs˚ a en analytisk model til fortolkning
af de eksperimentelle resultater, og der er udført en linearitetskarakteriser-
ing for derved at opstille ingeniørmæssige designregler for den p˚ agældende
type fiberforbindelse. Resulaterne bekræfter, at den nævnte konfiguration
muliggør højlinearitets, ende-til-ende transmissionsforbindelser og s˚ aledes
baner vej for transparent transmission af modulationsformater med høj
Denne metode til detektion af
Endvidere introducerer afhandlingen en ny tilgang til frembringelse og
detektion af tr˚ adløse, højhastighedssignaler p˚ a mm-bølge bærebølgefrekvenser
højere end 60 GHz ved brug af fotoniske, basisb˚ andsteknologier. Til frem-
bringelsen bruges optiske modulationsmetoder med høj spektral effektivitet
kombineret med optisk, heterodyn teknik. Til detektionen blev mm-bølge-
signalet moduleret over i det optiske domæne og dernæst modtaget ved
hjælp af digital, kohærent detektion. I den eksperimentelle demonstra-
iv Resum´ e
tion blev afprøvet frembringelse og detektion i 60 GHz og 75-110 GHz
frekvensb˚ andende af signaler med datahastigheder op til 40 Gbps. Disse
resultater udgjorde den højeste datahastighed p˚ a mm-bølgefrekvenser for
frembringelse og detektion ved hjælp af fotoniske metoder p˚ a det tidspunkt,
da afhandlingen blev skrevet.
Sammenfattende dokumenterer afhandlingens resultater, at fotoniske
metoder kan anvendes til frembringelse, distribution og detektion af tr˚ adløse
højhastighedssignaler. Endvidere ˚ abner resultaterne muligheder for næste
generations hybride, tr˚ adløse-tr˚ adbundne abonnentnet med ultra-høj ka-
First I would like to thank my supervisor, Professor Idelfonso Tafur Monroy,
Assistant professor Darko Zibar and Associate professor Kresten Yvind.
Idelfonso for believing in me since the first day I started my Master Thesis
under his supervision. Also for being a reference on what hard work and
dedication means. Darko, for all the time we have spent together in the lab
and for teaching me that it is important to be methodic and organized at
work. Kresten for instilling me the interest on the physics of what we work
on and open my mind to different research topics during my Ph.D.
I would also like to thank all my colleagues at Metro-access group, Neil,
Xianbin, Tim, Jesper, Kamau, Xu, Maisara, Robert, Xiaodan, Alexander,
Thang, Bomin.... and also my colleagues in Fotonik department.
Thanks to my closest people in Denmark: Valeria, Roberto and Diana.
Additionally, thanks to my salsa dancing friends. Thanks to the nice people
I met during this years in Denmark, specially the Spanish invasion and
Erasmus students. From my city, Zaragoza, to my closest friends, Daniel
and Paula and the Teleco’s troupe: Laura, Jorge, Paco, Felix and Sprint.
Thanks to my friends all over Spain and rest of you that are now living in
different parts of the world.
Finally, I would like to thank my family, for giving me support during
this time; specially thanks to my parents, my sister and my grandparents
for being there whenever I needed, supporting me in the distance.
Summary of Original Work
This thesis is based on the following original publications:
PAPER 1 A. Caballero, D. Zibar, C. G. Sch¨ affer, and I. Tafur Monroy,
“Photonic downconversion for coherent phase-modulated radio-over-
fiber links using free-running local oscillator,” Optical Fiber Technol-
ogy, 17, pp. 263—266, 2011.
PAPER 2 A. Caballero, D. Zibar, and I. Tafur Monroy, “Digital cohe-
rent detection of multi-gigabit 16-QAM signals at 40 GHz carrier
frequency using photonic downconversion,” in Proc. 35th European
Conference on Optical Communication, ECOC’09, Vienna, Austria,
2009, Post-Deadline Paper PDP3.4.
PAPER 3 A. Caballero, D. Zibar, and I. Tafur Monroy, “Digital coherent
detection of multi-gigabit 40 GHz carrier frequency radio-over-fibre
signals using photonic downconversion,” Electronics Letters, vol. 46,
no. 1, pp. 58–58, 2010.
PAPER 4 A. Caballero, D. Zibar, and I. Tafur Monroy, “Performance
evaluation of digital coherent receivers for phase modulated radio-
over-fiber links,” IEEE/OSA J. Lightw. Technol., accepted for pub-
PAPER 5 A. Caballero, N. Guerrero Gonzalez, F. Amaya, D. Zibar, and
I. Tafur Monroy, “Long reach and enhanced power budget DWDM
radio-over-fibre link supported by Raman amplification and coherent
viii Summary of Original Work
detection,” in Proc. 35th European Conference on Optical Commu-
nication, ECOC’09, Vienna, Austria, 2009, paper P6.09.
PAPER 6 A. Caballero, S. Wong, D. Zibar, L. G. Kazovsky and I. Tafur
Monroy, “Distributed MIMO antenna architecture for wireless-over-
fiber backhaul with multicarrier optical phase modulation,” in Proc.
Optical Fiber Communication Conference and Exposition, OFC/NFOEC,
Los Angeles, CA, 2011, paper OWT8.
PAPER 7 N. Guerrero Gonzalez, A. Caballero, R. Borkowski, V. Ar-
lunno, T. T. Pham, R. Rodes, X. Zhang, M. B. Othman, K. Prince,
X. Yu, J. B. Jensen, D. Zibar, and I. Tafur Monroy, “Reconfigurable
digital coherent receiver for metro-access networks supporting mixed
modulation formats and bit-rates,” in Proc. Optical Fiber Communi-
cation Conference and Exposition, OFC/NFOEC, Los Angeles, CA,
2011, paper OMW7.
PAPER 8 D. Zibar, R. Sambaraju, R. Alemany, A. Caballero, J. Herrera,
and I. Tafur Monroy, “Radio-frequency transparent demodulation for
broadband hybrid wireless-optical links,” IEEE Photon. Technol.
Lett., vol. 22, no. 11, pp. 784–786, 2010.
PAPER 9 R. Sambaraju, D. Zibar, A. Caballero, I. Tafur Monroy, R. Ale-
many, and J. Herrera, “100-GHz wireless-over-fibre links with up to
16 Gb/s QPSK modulation using optical heterodyne generation and
digital coherent detection,” IEEE Photon. Technol. Lett., vol. 22,
no. 22, pp. 1650–1652, 2010.
PAPER 10 D. Zibar, R. Sambaraju, A. Caballero, J. Herrera, U. West-
ergren, A. Walber, J. B. Jensen, J. Marti, and I. Tafur Monroy,
“High-capacity wireless signal generation and demodulation in 75- to
110-GHz band employing all-optical OFDM,” IEEE Photon. Tech-
nol. Lett., vol. 23, no. 12, pp. 810 –812, 2011.
PAPER 11 A. Caballero, D. Zibar, R. Sambaraju, J. Marti and I. Tafur
Monroy, “High-capacity 60 GHz and 75-110 GHz band links employ-
ing all-optical OFDM generation and digital coherent detection,”
IEEE/OSA J. Lightw. Technol.. accepted for publication, 2011.
PAPER 12 A. Caballero, D. Zibar, R. Sambaraju, N. Guerrero Gonza-
lez and I. Tafur Monroy, “Engineering Rules for Optical Generation
and Detection of High Speed Wireless Millimeter-wave Band Sig-
nals,” in Proc. 37th European Conference on Optical Communica-
tion, ECOC’11, Geneva, Switzerland, 2011, paper We.10.P1.115.
x Summary of Original Work
Other scientific reports associated with the project:
[PAPER 13] D. Zibar, A. Caballero, N. Guerrero Gonzalez, and I. Tafur
Monroy, “Hybrid optical/wireless link with software defined receiver
for simultaneous baseband and wireless signal detection,” in Proc.
36th European Conference on Optical Communication, ECOC’10,
Torino, Italy, 2010, paper P6.09
[PAPER 14] D. Zibar, A. Caballero, N. Guerrero Gonzalez, C. Schaeffer,
and I. Tafur Monroy, “Digital coherent receiver employing photonic
downconversion for phase modulated radio-over-fibre links,” in Proc.
IEEE International Microwave Symposium Digest, MTT’09, Boston,
MA, 2009, pp. 365–368.
[PAPER 15] A. Caballero, D. Zibar, and I. Tafur Monroy, “Digital cohe-
rent detection of multi-gigabit 40 GHz carrier frequency radio-over-
fiber signals using photonic downconversion,” in Proc. 1. Annual
Workshop on Photonic Technologies for Access and Interconnects,
Stanford, CA, 2010.
[PAPER 16] A. Caballero, I. Tafur Monroy, D. Zibar, J. A. Lazaro, J. Prat,
C. Kazmierski, P. Chanclou, I. Tomkos, E. Tangdiongga, X. Qiu,
A. Teixwira, R. Solila, P. Poggiolini, R. Sambaraju, K. Langer,
D. Erasme, E. Kehayas, and H. Avramopoulos, “Subsystems for
future access networks : Euro-fos project,” in Proc.
Workshop on Photonic Technologies for Access and Interconnects,
Stanford, CA, 2010.
[PAPER 17] N. Guerrero Gonzalez, D. Zibar, A. Caballero, and I. Tafur
Monroy, “Experimental 2.5 Gb/s QPSK WDM phase-modulated
radio-over-fiber link with digital demodulation by a K-means algo-
rithm,” IEEE Photon. Technol. Lett., vol. 22, no. 5, pp. 335–337,
[PAPER 18] K. Prince, J. B. Jensen, A. Caballero, X. Yu, T. B. Gibbon,
D. Zibar, N. Guerrero Gonzalez, A. V. Osadchiy, and I. Tafur Mon-
roy, “Converged wireline and wireless access over a 78-km deployed
fiber long-reach WDM PON,” IEEE Photon. Technol. Lett., vol. 21,
no. 17, pp. 1274–1276, 2009.
[PAPER 19] R. Sambaraju, D. Zibar, A. Caballero, I. Tafur Monroy, R. Ale-
many, and J. Herrera, “GHz wireless on-off-keying link employing
all photonic RF carrier generation and digital coherent detection,”
in Proc. Access Networks and In-House Communications, ANIC’10,
Karlsruhe, Germany, 2010, paper ATHA4.
[PAPER 20] I. Tafur Monroy, N. Guerrero Gonzalez, A. Caballero, K. Prince,
D. Zibar, T. B. Gibbon, X. Yu, and J. B. Jensen, “Convergencia de
sistemas de comunicacion opticos e inalambricos (converged wireless
and optical communication systems),” Optica Pura y Aplicada, in-
vited, no. 2, pp. 83–90, 2009.
[PAPER 21] D. Zibar, R. Sambaraju, A. Caballero, I. Tafur Monroy, R. Ale-
many, and J. Herrera, “16 Gb/s QPSK wireless-over-fibre link in 75-
110 GHz band with photonic generation and coherent detection,” in
Proc. 36th European Conference on Optical Communication, ECOC’10,
Torino, Italy, 2010, paper Th.9.B.6.
[PAPER 22] R. Sambaraju, D. Zibar, A. Caballero, J. Herrera, I. Tafur
Monroy, J. B. Jensen, A. Walber, U. Westergren, and J. Marti, “Up
to 40 Gb/s wireless signal generation and demodulation in 75-110
GHz band using photonic techniques,” in Proc. Microwave Photon-
ics, MWP’11, Toronto, Canada, 2011, Post-deadline Paper PDP1.
xii Summary of Original Work
Other scientific reports:
[C1] A. Caballero, R. Rodes, J. B. Jensen, and I. Tafur Monroy, “Im-
pulse radio ultra wide-band over multi-mode fiber for in-home signal
distribution,” in Proc. Microwave Photonics, MWP’09., Valencia,
Spain, 2009, pp. 1–3.
[C2] A. Caballero, J. B. Jensen, X. Yu and I. T. Monroy,, “5 GHz 200
Mbit/s radio over polymer fibre link with envelope detection at 650
nm wavelength,” Electronic Letters, vol. 44, no. 25, pp. 1479–1480,
[C3] T. B. Gibbon, X. Yu, R. Gamatham, N. Guerrero, R. Rodes, J. B.
Jensen, A. Caballero, and I. Tafur Monroy, “3.125 Gb/s impulse
radio ultra-wideband photonic generation and distribution over a
50 km fiber with wireless transmission,” IEEE Microw. and Wireless
Comp. Lett., vol. 20, no. 2, pp. 127–129, 2010.
[C4] J. B. Jensen, R. Rodes, A. Caballero, X. Yu, T. B. Gibbon, and
I. Tafur Monroy, “4 Gbps impulse radio (IR) ultra-wideband (UWB)
transmission over 100 meters multi mode fiber with 4 meters wireless
transmission,” Opt. Express, vol. 17, no. 19, pp. 16898–16903, 2009.
[C5] R. Rodes, A. Caballero, X. Yu, T. B. Gibbon, J. B. Jensen, and
I. Tafur Monroy, “A comparison of electrical and photonic pulse gen-
eration for IR-UWB on fiber links,” IEEE Photon. Technol. Lett.,
vol. 22, no. 5, pp. 263–265, 2010.
[C6] R. Rodes, X. Yu, A. Caballero, J. B. Jensen, T. B. Gibbon, N. Guer-
rero Gonzalez, and I. Tafur Monroy, “Range extension and channel
capacity increase in impulse-radio ultra-wideband communications,”
Tsinghua Science & Technology, Invited, vol. 15, no. 2, pp. 169–173,
Resum´ e iii
Summary of Original Work vii
1.1High capacity wireless links . . . . . . . . . . . . . . . . . .
1.2 Photonic technologies for wireless signal generation and de-
tection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.2.1 Photonic generation techniques . . . . . . . . . . . .
1.2.2 Intensity modulated versus Phase modulated uplink
radio-over-fiber links . . . . . . . . . . . . . . . . . .
1.2.3 Hybrid wireless-optical links for next generation ac-
cess networks . . . . . . . . . . . . . . . . . . . . . .
1.3 Photonic digital coherent detection for wireless signals . . .
1.3.1 Phase modulated radio-over-fiber links . . . . . . . .
1.3.2 mm-wave intensity modulated radio-over-fiber links .
1.4 State-of-the-art . . . . . . . . . . . . . . . . . . . . . . . . .
1.5 Beyond state-of-the-art . . . . . . . . . . . . . . . . . . . . .
1.6 Main contribution and outline of the thesis
17. . . . . . . . .
2 Description of papers
2.1 Phase modulated radio-over-fiber links assisted with cohe-
rent detection . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 mm-wave photonic signal generation and detection . . . . .
3.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1 Phase modulated RoF links assisted with coherent
detection . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2mm-wave photonic signal generation and detection .
3.2 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Technologies for ultra high capacity RoF links
3.2.2 Towards 100 Gbps wireless communication links
. . .
Paper 1: Photonic downconversion for coherent phase-modulated radio-
over-fiber links using free-running local oscillator29
Paper 2: Digital coherent detection of multi-gigabit 16-QAM signals
at 40 GHz carrier frequency using photonic downconversion35
Paper 3: Digital coherent detection of multi-gigabit 40 GHz carrier
frequency radio-over-fibre signals using photonic downconversion 39
Paper 4: Performance evaluation of digital coherent receivers for phase
modulated radio-over-fiber links43
Paper 5: Long reach and enhanced power budget DWDM radio-over-
fibre link supported by Raman amplification and coherent detection 57
Paper 6: Distributed MIMO antenna architecture for wireless-over-
fiber backhaul with multicarrier optical phase modulation 61
Paper 7: Reconfigurable digital coherent receiver for metro-access net-
works supporting mixed modulation formats and bit-rates67
Paper 8: Radio-frequency transparent demodulation for broadband
hybrid wireless-optical links73
Paper 9: 100-GHz wireless-over-fibre links with up to 16 Gb/s QPSK
modulation using optical heterodyne generation and digital coherent
Paper 10: High-capacity wireless signal generation and demodulation
in 75- to 110-GHz band employing all-optical OFDM85
Paper 11: High-capacity 60 GHz and 75-110 GHz band links employ-
ing all-optical OFDM generation and digital coherent detection91
Paper 12: Engineering rules for optical generation and detection of
high speed wireless millimeter-wave band signals103
List of Acronyms 117
1.1 High capacity wireless links
The emerging services that end users are demanding, such as High-Definition
video streaming, video-calls and cloud computing, have put severe pressure
on the telecommunication network infrastructure to provide high capacity
links, capable to support diverse service requirements, in different customer-
premises environments and at a low cost. This demand has resulted in an
evolution of short-range and wired access networks: from the copper-based
transmission systems, with limited coverage and bandwidth, to the use of
photonic technologies, achieving high capacity and long reach links [1,2].
Regardless copper or optical fiber is used as a media for wired data
transmission, delivery of data wirelessly to the end-user has advantages
related to flexibility in the placement of transponders and broadcasting
capabilities. However, opposed to wired media, wireless transmission com-
monly exhibits lower reach, caused by the high propagation losses of the
signal through the air, and it is more prone to channel interferences during
propagation. These characteristics result in a received signal after wireless
transmission with strong time-varying properties. Wireless communication
systems operating at low frequencies, below 5 GHz, have limited capac-
ity, as there is scarce of RF spectrum available for broadband operation
at such frequencies. Moreover, this RF spectrum needs to be shared with
different services and users. For example, in the Global System for Mobile
Communications (GSM) bands (900 MHz and 1.8 GHz) only 100 MHz of
bandwidth is available. In order to achieve high capacity wireless links, one
possibility is to move towards higher RF frequencies, where higher band-
width is available. An example is the 5 GHz band, with about 500 MHz
of bandwidth available. For gigabit-per-second data links, mm-wave fre-
quencies (above 60 GHz) can be used, as there are GHz of spectrum avail-
able [3,4]. A different approach is to increase the bite rate over a given
bandwidth, thus increasing the spectral efficiency. Spectral efficiencies over
2 bit/s/Hz can be achieved by using complex modulation formats, such as
Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM),
as well as multiplexing schemes, such as subcarrier multiplexing or Or-
thogonal Frequency-Division Multiplexing (OFDM), as opposed to On-Off
Keying (OOK) that achieves only 1 bit/s/Hz. However, at high RF carrier
frequencies, the high capacity wireless link encounters high propagation
losses. The stringent requirements of high modulation formats in terms
of Signal-to-Noise Ratio (SNR) together with the propagation losses result
in limited coverage. In spite of that, high capacity wireless links can be
achieved, although over short distances.
The application of wireless link for access networks takes place in the
last mile segment of the data transport, providing high capacity links to
the end-user. One type of wireless signal distribution concerns cellular net-
works, where a certain geographical area needs wireless coverage, provided
by a number of antenna Base Station (BS). For next generation cellular
networks with increased capacity, the reach of the wireless link will be short,
as a consequence of the higher RF carriers and the use higher modulation
formats needed to achieve high capacity links. To provide proper coverage,
the density of antennas per area needs to be increased, resulting in a large
number of nodes to be distributed and controlled by the network. The new
architecture shall support high number of BS, with possibly high number
of antennas per BS, to support Multi-Input Multi-Output (MIMO). The
transport of all the aggregated data, to/from the different antennas from/to
a Central Station (CS), where the control is done, requires a high-capacity
backhaul wired infrastructure, commonly optical fiber [26,44]. As a CS can
be placed some tens of kilometers away from the BSs, the fiber link is the
most suitable connection between CS and BSs, benefiting from the high
capacity and low attenuation of the optical fiber as a transmission media.
Traditionally in Radio-over-Fiber (RoF), the baseband to RF up and
downconversion has been done at the BS, resulting in digital transport of
the information through the fiber backhaul. By using optical fiber for the
analogue transport of wireless signals, between BS and CS in a wireless-
over-fiber architecture, it is possible to transport the wireless signal trans-
parently through the fiber infrastructure, while providing the high capacity
required for next generation wireless access networks [5,6].
1.2 Photonic technologies for wireless signal generation and detection3
Figure 1.1: Schematic description of the two types of RoF link: uplink (from wireless
user to BS to CS) and downlink (CS to BS to wireless user).
The basic diagram of a wireless-over-fiber architecture, where the wire-
less signal is transported transparently through the fiber infrastructure, is
shown in Fig. 1.1. The detected wireless signal can be optically transported
from the BS to the CS (uplink) or generated in the CS, transported to the
BS, resulting in passive wireless generation in the BS (downlink) [5, 6].
The centralization of complex equipment at the CS, wireless demodula-
tion, mixers, RF oscillators and network control, results in a simplified BS,
with important cost savings and simple communication between the differ-
ent cells. The requirements for downlink to uplink transmission links are
different. For downlink transmission, the link should be able to generate
a signal with low distortion, low noise and provide high RF power. On
the other hand, the design of an uplink transmission systems has stringent
requirements on dynamic range, as the users signals arrive to the BS after
wireless transmission, with variable RF power caused by fading and poten-
tially with high noise and interferences from the different users sharing the
1.2 Photonic technologies for wireless signal
generation and detection
The generation and detection of wireless signals have been widely studied
within the topic of Microwave Photonics (MWP). The generic microwave
photonic link, called RoF, consists of the transport of an analogue signal
through an optical fiber, by performing electrical to optical conversion at
the transmitter and optical to electrical conversion at the receiver. Yet the
term MWP goes beyond that, covering for example the photonic generation
and processing of microwave signals at high carrier frequencies, optically
controlled phased array antennas and photonic Analogue-to-Digital Con-
verter (A/D) conversion [5,7–9].
1.2.1Photonic generation techniques
The generation of wireless signal using optics has the advantage of the
high bandwidth that photonic components can achieve [7,10,11], currently
with over 100 GHz. An added advantage of photonics is the low attenuation
and low non-linear distortion that optical fiber offers for signal propagation,
as opposed to the electrical transport by using waveguide techniques. A
common technique for the generation of a microwave signal is the beating at
a Photodiode (PD) of two optical carriers, which are coherent and separated
by the desired RF frequency. There are different ways to generate these
optical carriers, mostly based on external electro-optical modulation via a
reference RF frequency synthesizer [12–17]. By using these techniques it
is possible to obtain RF carrier frequencies which are multiples of the one
provided by the frequency synthesizer [12,13]. Based on this approach, RF
frequencies exceeding 100 GHz can be generated supporting bitrates over
20 Gbps [14–17].
A different approach is using optical heterodyning, with free running
lasers, first proposed in , which can provide high bitrate wireless signals
[10, 19]. A baseband signal is generated and optically combined with a
second free running laser to create in a PD the electrical signal.
method avoids the need of an electrical RF source, but requires lasers with
low phase noise and a robust receiver to compensate for the free running
beating of the two optical sources.
To impose a modulation onto the wireless carrier, it is possible to
use optical baseband modulation, instead of electrical waveform genera-
tion, as optics can provide high spectral-efficient modulation formats, such
as Quadrature Phase Shift Keying (QPSK) or QAM.
Optical OFDM (O-OFDM) [20,21] has been demonstrated to enable highly
spectral-efficient optical channels, while decreasing the requirements of the
This thesis reports on the generation of mm-wave frequency signals
using optical heterodyning combined with digital coherent detection. In
PAPER 9, up to 16 Gbps in W-band (75-110 GHz) was reported on a
single carrier QPSK signal.For PAPER 10, O-OFDM baseband was
used to generate and detect bitrates exceeding 20 Gbps in the 60 GHz and
75-110 GHz bands.
The use of all-
1.2 Photonic technologies for wireless signal generation and detection5
Modulated and Direct Detected (IM-DD) with external optical modulation (top) and
Phase-Modulated assisted with digital Coherent detection (PM-Coh) (bottom).
Schematic description of two different types of RoF link:Intensity-
1.2.2Intensity modulated versus Phase modulated uplink
Traditionally RoF links are based on Intensity Modulated with Direct De-
tection (IM-DD) links [8,22], since they rely on already well established
technology and can thereby provide low cost implementation. This kind
of links have been studied in detail from the microwave design point of
view and showed their capacity for the transport of broadband wireless
signals, especially for the downlink. However, it might not be the best so-
lution for the uplink, due to the stringent requirements in dynamic range.
In Fig. 1.2 (top) a basic IM-DD link is illustrated, where the microwave
signal modulates the intensity of an optical carrier. After fiber transmis-
sion the detection is performed using a single PD. In this case, an external
modulator is used, typically a Mach-Zehnder Modulator (MZM) or Electro-
Absorption Modulator (EAM), but also direct current modulation of the
semiconductor light source is possible. After photodetection, the signal is
translated to an Intermediate Frequency (IF) by performing RF mixing
with an electrical Local Oscillator (LO). The IF signals falls within the
bandwidth of the A/D and it is demodulated in the digital domain using
Digital Signal Processing (DSP) algorithms.
IM-DD RoF links require high received optical power in order to in-
crease the link gain.The non-linear intrinsic response of the intensity
modulators  results in a limited dynamic range. They also suffer from
periodic RF power fading, due to chromatic dispersion after optical fiber
propagation. More sophisticated variations of the IM-DD basic scheme have
been proposed, such as Single-Side Band (SSB) or Double-Side Band (DSB)
modulation, concatenated MZM, predistortion etc. [7,24–26], in order to
achieve better performance in terms of link gain and linearity, at the ex-
pense of a complex optical transmitter or receiver scheme or lower electro-
As an alternative, optical phase modulated RoF links have been pro-
posed and demonstrated to offer low distortion and higher linearity than
unlinearized IM-DD links for the transport of high speed wireless signals
[23,27–33]. To recover the phase of the optical signal, several techniques
have been proposed, such as phase-tracking receiver [23,28,30], interfero-
metric detection [27,29] or careful design of the link in terms of chromatic
dispersion . The use of these techniques has been proven to achieve
better performance than IM-DD links in terms of dynamic range and lin-
earity [23,27,34]. However, these improvements are typically over a limited
bandwidth range and require low phase-noise optical sources [23,27].
When moving to higher RF frequencies, it is interesting to explore pho-
tonic technologies, that allows the transports and detection of the RoF sig-
nal at a lower RF carrier frequency. The direct transport of the microwave
signals in the optical domain might not be the best solution, due to the
high bandwidth of the components, such as optical modulator, photodiodes
and digitizers, needed at the receiver and the higher influence of chromatic
dispersion. Also, electrical mixers presents high losses, limited bandwidth
and low linearity at high RF frequencies. By using photonic technologies
it is possible to perform downconversion in the optical domain, usually
called Photonic Downconversion (PDC). The RoF signal is shifted to a
lower RF frequency by using, for example, external modulators. An added
advantage of PDC is that photonic sources can generate pulses with very
high repetition rate, low jitter and high stability, something complex to be
achieved in the electrical domain with the current technology . Several
schemes has been proposed [35–40], performing photonic downconversion
of RF frequencies over 40 GHz RoF to IF below 5 GHz with low penalty.
1.3 Photonic digital coherent detection for wireless signals7
1.2.3 Hybrid wireless-optical links for next generation
Next generation broadband access networks will provide heterogeneous ser-
vices, wired and wireless. The inclusion of wireless, in a form of RoF, into
access networks needs to be compatible with existing access architectures
and coexist with the baseband signals. The most promising architecture
for optical access networks is Passive Optical Network (PON) because of
low cost, simple maintenance and operation, and high-bandwidth provi-
sion [1,54]. Several architectures has been proposed in order to include
optical wireless signal distribution in PON, called hybrid PONs. Among
the different multiplexing techniques for PON, WDM is the most promis-
ing solution for future broadband access networks, as it can accommodate
exponential traffic growth, support different broadband services and allow
fast network reconfiguration due to the flexibility in wavelength alloca-
The use of PM-Coh links in hybrid PON can offer advantages over the
traditional IM-DD based PON systems reported in the literature [24,26,
44,54,55,58], due to the higher linearity and superior receiver sensitivity.
PM-Coh links provide also easy integration with Wavelength Division Mul-
tiplexing (WDM) PON networks, by the flexibility in wavelength selection
given by coherent detection. In terms of cost, despite PM-Coh is a more
complex architecture than IM-DD or other phase modulated RoF archi-
tectures, the centralization of the complex equipment in the CS and the
absence of a bias control of PM to operate in the linear regime, as opposed
to IM, makes it suitable for simple BS and allows future upgradeability.
1.3 Photonic digital coherent detection for
The use of digital coherent detection for wireless links relies on the capa-
bility of digital signal processing for the compensation of the various link
distortions occurring both in the wireless as well as in the wired link (op-
tical or electrical signal impairments). The principle of coherent detection
in optical fiber communications started in the 90s  allowing full recon-
struction of the optical received field by beating it with a reference optical
LO in a 90◦optical hybrid. By detecting the In-phase (I) and Quadra-
ture (Q) components with photodiodes, it is possible to recover the optical
field of the detected signal, relative to the LO. This is done by digitizing the
photocurrents using fast A/D converters and performing signal demodula-
tion in the digital domain. By using DSP it is possible to compensate the
phase and frequency offset due to the free-running laser beating and other
impairments, such as chromatic dispersion, imbalances in the transmitter
and receiver, etc. Coherent detection thus allows great flexibility in the
use of different modulation formats, as information can be recovered from
the encoded amplitude, phase and polarization state of an optical carrier
allowing high capacity optical links .
In this thesis, the capabilities of coherent receivers to recover the optical
field has been studied in two different ways. First, in phase-modulated
links, to recover the phase information of the optical source, where the
wireless signal has been imposed. This is named Phase Modulated RoF link
assisted with Coherent Detection (PM-Coh). Furthermore, for intensity
modulated RoF links, by optical filtering one of the modulation side-bands,
resulting in a baseband optical signal. In order to recover the in-phase and
quadrature components, it is necessary to demodulate it using a reference
signal, which is given by the LO in a coherent receiver. In this thesis, this
architecture is named Intensity Modulated RoF link assisted with Coherent
Detection (IM-Coh), as opposite to the PM-Coh.
1.3.1 Phase modulated radio-over-fiber links
The combination of digital coherent detection and phase modulated link for
analogue transport was first reported in  using self homodyne detection
from the same laser source as LO. It offers advantages in dynamic range
and digital demodulation by recovering the optical field using a coherent
receiver, thus the phase information. The detection of high RF frequencies,
exceeding the bandwidth of the A/D converter, is possible by performing
photonic downconversion with a second phase modulator and an RF acting
as an electrical LO . This architecture can be suitable for short links
requiring high electrical gain and dynamic range, however it is not suitable
for access networks, where the BS to CS link is implemented on a single
optical fiber, with lengths of few kilometers.
The solution that is presented in this thesis for the integration of phase
modulated link into next generation hybrid optical-wireless networks is
based on intradyne coherent detection.
Fig. 1.2 (bottom). The microwave signal modulates the phase of the optical
carrier, using an optical phase modulator at BS, and transmitted through
the fiber to the CS. In the detection side, an optical coherent receiver is
used to recover the optical field. The implementation of the PM-Coh link
The basic scheme is shown in
1.3 Photonic digital coherent detection for wireless signals9
Figure 1.3: Hybrid Wireless-Optical Broadband-Access Network scenario consisting on
point-to-multipoint RoF links with WDM.
uses two independent free-running lasers . It avoids the use of a second
fiber for the transport of the reference signal, by using a free running laser
at the CS acting as LO. This architecture presents benefits for the deploy-
ment, centralizing the more complex LO lasers at the CS, leaving a simple
antenna BS. The recovered signal is then processed in the digital domain us-
ing DSP algorithms, which can compensate transmission impairments, such
as fiber chromatic dispersion or beating noise from the two laser sources.
Furthermore, PDC can be applied in a more flexible way, by creating a
pulsed optical LO source, independent from the fiber transmission link.
The report of the experimental linearity measurements of the PM-Coh
link is included in PAPER 1. The experimental demonstration of high
speed wireless detection is reported in PAPER 2 and PAPER 3, for up
to 3.2 Gbps at 40 GHz RF frequencies. A detailed theoretical description
of the PM-Coh link with ultimate performance evaluation is reported in
A proposed scenario for next generation hybrid wireless-optical net-
works, using PM-Coh for wireless transport, is shown in Fig. 1.3. A num-
ber of antennas that provides coverage to a given geographical area are
connected to the CS through an optical fiber link. Each of the anten-
nas has assigned a different wavelength for the optical transport. Thus,
the control of the BS is centralized, offering high flexibility in terms of
reconfiguration management. The integration with existing WDM access
networks is direct, as the same optical infrastructure can also handle base-
band and RF data. It also allows for higher RF frequency reuse within
the same CS coverage, as multiplexation is done in the optical domain.
This thesis includes other networking scenarios where PM-Coh links can
be applied.For example, PAPER 5 describes a network architecture
based on ring topology. The use of fiber distributed Raman amplification
can compensate the losses from the fiber connections through the ring.
The application of PM-Coh link for next generation cellular networks, such
as Long Term Evolution (LTE), takes place when Distributed Antenna
System (DAS) is used to give extended coverage, resulting in a increased
number of wireless channels, shared frequency allocation and high capacity
wireless links. PAPER 6 presents an architecture, based on subcarrier
multiplexing, for the transparent transport to the CO of all the detected
signals from the same BS. The integration of PM link with next gener-
ation coherent access networks is reported in PAPER 7. Moreover, a
single reconfigurable photonic receiver is proposed in this paper and demon-
strated for multiple services. By using the same digital coherent receiver,
it is possible to detect different type of signals, such as baseband (QPSK,
Vertical-Cavity Surface-Emitting laser (VCSEL) based OOK) or wireless
(Ultra-Wide Band (UWB), OFDM).
1.3.2mm-wave intensity modulated radio-over-fiber links
The second approach for the use of digital coherent detection in RoF link
reported in this thesis is for IM-Coh links, namely, for the detection of high
speed microwave signals in the mm-wave frequency range, 60 GHz and 75-
110 GHz. It is based on the use of optical re-modulation of the mm-wave
wireless signals, using a high speed electro-optical modulator following SSB
modulation. The generated SSB signal results in a baseband signal that
can be detected using digital coherent detection.
For the generation, high spectral-efficient modulation formats can be
achievable by optical modulators using baseband electronics and detected
using digital coherent detection, such as QPSK, M-QAM or O-OFDM [20,
45,46]. By using optical heterodyning generation and IM-Coh detection, it
is possible to generate and detect a wireless signal using traditional optical
baseband technologies. Based on the robustness of the receiver, it is possible
to compensate for link impairments, such as laser beating from the optical
heterodyning RF generation and coherent detection or non-ideal response
of the electro-optical components.
The block diagram of a IM-Coh link with O-OFDM signal generation is
shown in Fig. 1.4. For the generation of the high-speed wireless signal, at
the Central Office (CO) are placed a multicarrier optical generator, an O-
OFDM modulator consisting on independent I/Q optical modulators and
1.3 Photonic digital coherent detection for wireless signals11
Transmitter Central Office
Receiver Central Office
Figure 1.4: Block diagram for the generation and detection of mm-wave carrier fre-
quency signals using Optical-OFDM (O-OFDM) baseband generation and optical recep-
tion using SSB modulation combined with digital coherent detection. (a) Multicarrier
generation, (b) O-OFDM baseband signal, (c) Optical signals for RF optical heterodyn-
ing, (d) mm-wave RF signal generated, (e) Optical modulated received RF signal, (f)
SSB baseband signal containing the transmitted O-OFDM signal.
the beating laser source. The optical signal is transported by optical fiber
to the transmitter antenna base station, consisting on a high-speed PD
and the transmitter antenna. For the detection of the wireless signal, at
the receiver antenna base station is placed a SSB optical modulator, which
impose the received wireless signal into an optical carrier and transported
to the CO. At the CO, there is an optical digital coherent receiver that
performs optical signal detection and demodulation.
In this thesis, PAPER 8 presents the principle of IM-Coh and demon-
strates the detection and demodulation of 2.5 Gbps QPSK at 40 GHz.
PAPER 9 reports the generation and detection of 16 Gbaud QPSK in
the W-band (75-110 GHz). Optical heterodyning is used for the generation
and IM-Coh is performed for optical detection. PAPER 10 reports the
use of O-OFDM to increase the capacity using baseband optics, achieving
over 20 Gbps in the 60 GHz and also in the 75-110 GHz. PAPER 11
deals with the theoretical description of the link, photonic generation and
detection, to establish the ultimate performance and the algorithms needed
for the demodulation. PAPER 12 analyzes the extension from QPSK to
16QAM baseband generation, towards 40 Gbps in a single RF carrier.
The efforts in the design of RoF links have been in obtaining high linear
links, quantified in terms of Spurious-Free Dynamic Range (SFDR) with
low noise figure. Conventional and most studied analogueue links are those
based on IM-DD using a MZM modulator biased in the linear regime ,
which can achieve 110 dB/Hz2/3of SFDR. The performance of this con-
figuration has some limitations, due to the intrinsic non-linear response of
the MZM. Also the optical fiber chromatic dispersion tolerance is an is-
sue when moving to high RF frequencies, causing periodic power fading
at the output of the PD. Various linearization techniques has been pro-
posed to improve these issues [7,9,22] based on concatenated MZMs and
predistortion. Despite obtaining higher SFDR, up to 132 dB/Hz2/3, they
require a precise bias control and have narrow frequency band operation.
All these technologies can achieve high performance at frequencies below
10 GHz [22,47], but achieving those values over 10 GHz becomes a chal-
lenge for the architecture and component design. For high RF frequencies,
a linearized IM-DD link at 18 GHz has been demonstrated , achieving
an SFDR of 129 dB/Hz2/3and 114 dB/Hz2/3after PDC.
PM-DD links have been theoretically proven to offer higher linearity
than un-linearized IM-DD links [23,27], however they present challenges in
the phase tracking in order to recover the phase information. The use of
Optical Phase-Locked Loop (PLL) has been proposed for phase tracking
in the receiver, capable to obtain high SFDR at low RF frequencies, with
134 dB/Hz2/3SFDR at 100 MHz  and 122 dB/Hz2/3at 300 MHz .
1.4 State-of-the-art 13
The use of DPLL can improve the tracking performance, due to the null
loop delay. By using a digital coherent receiver and DPLL, 126.8 dB/Hz2/3
was achieved at 900 MHz bandwidth . PDC at 3 GHz and 10 GHz with
107 dB/Hz2/3of SFDR was also demonstrated. A different approach is
the reception of the PM signal using interferometric detection [27,29]. The
theoretical analysis shows that it performs better than IM-DD links in terms
of SFDR, noise figure and RF gain, but within a limited bandwidth .
Linearization using dual wavelength PM has been proven to achieve an
SFDR of 127 dB/Hz2/3at 5 GHz. However, the approaches in [23,27,29,
30,32,48] result in complex and careful matched receivers with short fiber
lengths (below 1 km), making it difficult to integrate in access networks.
There are also efforts to increase the transmission length of analogueue
links , with performance for distances of 40 km and over, proving that
100 km links up to 18 GHz are feasible with current technology, with SFDR
values above 100 dB/Hz2/3and low noise figure (below 20 dB) but perhaps
not simultaneously and neither over a broad band.
The second application presented in this thesis is the generation of wire-
less signals exceeding 10 Gbps using photonic technologies . Bitrates
over 30 Gbps in the 60 GHz band, within the 7 GHz of available bandwidth,
can be achieved optically based on OFDM RF generation and photonic up-
conversion [50,51]. Concerning the W-band, 75-110 GHz, OOK wireless
systems has been demonstrated employing optical generation and electrical
envelope detection at 10 Gbps  and 20 Gbps . More spectral efficient
links have been demonstrated using PSK modulation, up to 16 Gbps in the
70-80 GHz band  on a single chip all-electrical transceiver. A 1.25 Gbps
link was achieved at 105 GHz RF frequency using IF optical upconver-
sion . The use of all-optical transmitter was reported in  using a
DQPSK optical modulator, with up to 4.6 Gbps at 92 GHz RF frequency.
For gigabit generation beyond 100 GHz, 8 Gbps was achieved at 250 GHz
by optical heterodyning . Recently, a 20 Gbps W-band wireless link
has been demonstrated based on baseband generation and frequency qua-
drupling . The reception was done using a broadband electrical mixer
rather than with photonic technologies. The reported wireless transmission
of only 3 cm (reactive near-field zone) demonstrates the feasibility of wire-
less transmission but reveals that several challenges need to be overcome
to reach longer transmission distances .
With regard to the application of RoF to access networks, several sce-
narios has been proposed to combine PON and distribution of wireless
signals using optical fibers. WDM PON has been the preference due to the
high capacity and flexibility in wavelength allocation [26,54,55]. The inte-
gration into access networks of hybrid wireless and baseband data delivery,
called wireless-optical broadband-access network (WOBAN) scenario, as a
way to extend the reach of existing PON [56,57] is a promising architec-
ture that can save on deployment cost as fibers are not required to reach
each end-user. Moreover, it can be integrated into existing Time Division
Multiplexing (TDM) and WDM PON architectures.
The inclusion of RoF for future hybrid optical-access networks has been
mostly based on IM-DD links using MZM, and revealing that chromatic dis-
persion is the major source of performance degradation of such fiber radio
links [24,44]. The use of SSB and DSB modulation can avoid chromatic dis-
persion fading, thus allowing longer fiber lengths. 155 Mbps data at 35 GHz
carrier frequency distribution was reported using these techniques .
Wavelength reuse has been proposed, by using Reflective Semiconductor
Optical Amplifier (RSOA), for bi-directional links to keep BS simple by
avoiding the need of a local optical source [44,55,58]. The combination of
IM-DD links and wavelength regeneration allows bi-directional transport
for high capacity signals, for instance, up to 2.5 Gbps at 40 GHz  and
1.25 Gbps baseband and wireless data at 60 GHz . The application of
RoF links in future LTE networks, to support distributed antenna systems,
has been analyzed, using IF signal transport [6,59], or combined with gi-
gabit wireless in the 60 GHz band, to create multiservice distribution for
in-building environments . The first demonstration of PM-Coh applied
to WDM access networks was reported in . 50 Mbaud BPSK and QPSK
signals at 5 GHz were detected, using a PM-Coh link with 3 WDM chan-
nels spaced 12.5 GHz; however, no BER performance was reported. The
demonstration of a converged wireless-optical access networks, support-
ing the deliver of different wired and wireless services, was experimentally
demonstrated in . A combined signal transport over a 78.8 km field-
installed fiber of multiple baseband and wireless signal was reported :
differential quadrature phase shift keying (DQPSK) baseband access at
21.4 Gbps per channel, 250 Mbps OFDM PM-Coh at 5 GHz, impulse-radio
UWB at 3.2 Gbps and a 256QAM WiMAX signal at 12 MBaud.
At the start of this Ph.D. project, there were few application of dig-
ital coherent receivers for RoF link: analogue applications (radar) using
PM links  and , which is the first experimental demonstration of
wireless signal transport using PM-Coh link, at a low bitrate. Despite the
described advantage of a PM link for analogue optical transport , it
was also the only work on this topic to the author knowledge. Further-
1.5 Beyond state-of-the-art 15
Table 1.1: Main experimental contributions to the state-of-the-art reported in this
Bitrate Mod. formatCarrier freq. Fiber link
more, there was no reported detailed analysis of the photonic components
requirements, such as linewidth, optical power, modulation index, etc. for
transport of high capacity wireless signals. Neither there was experimental
demonstration of wireless detection of signal at bitrates exceeding 1 Gbps.
The application of PM link for hybrid access networks was not reported
either, as so far was focus on IM links [13,44,54–58]. The use of PDC had
been studied for IM analogue links and also applied to various scenarios of
access networks [35–37,39]; for PM had only been reported for analogue
links [38,40]. However, no work was reported on the application of PDC
in PM links for access networks. Neither the combination of PDC and dig-
ital coherent receivers for RoF link as a way to overcome the bandwidth
limitation of A/D converters.
1.5 Beyond state-of-the-art
The work presented in this thesis has significantly extended the state-of-the-
art in the area of detection of wireless signal using photonic technologies,
by using novel techniques based on the use of digital coherent receivers
and PDC with independent free-running optical LO. Table 1.1 gathers the
main experimental results achieved during this Ph.D. project, financed by
the Danish Research Agency for Technology and Production (FTP) project
OPSCODER, OPtically Sampled COherent DEtection Receiver. It shows
the achieved bitrates, modulation format, RF carrier frequencies and fiber
transmission length. The research performed during my Ph.D. has led
to the demonstration of IM and PM links assisted with digital coherent
detection achieving bitrates and in operating RF carrier frequencies that
are part of today’s state-of-the-art.
In the area of PM-Coh links, my work extended previous results [33,43]
from ∼Mbps at 5 GHz with BPSK to 4 Gbps at 40 GHz using 16QAM.
These achievements are presented in PAPER 2 and PAPER 3. Regard-
ing mm-wave photonic generation and detection, this project first reported
the use of IM-Coh in PAPER 8, achieving 2.5 Gbps at 40 GHz. Further-
more, this project increased the previously reported performance to over
20 Gbps at 100 GHz using all-optical OFDM, PAPER 10.
I have extended the stat-of-the-art of PM RoF links assisted with cohe-
rent detection, expanding previous work towards high bitrates and high fre-
quencies and providing designing engineering rules. I have also introduced
the use of PDC for the detection of high RF frequencies signals, without
the need of high speed electronics at the receiver, thus avoiding electrical
downconversion. The first experimental demonstration of PM-Coh with
PDC in the framework of this Ph.D. project is reported in PAPER 1, con-
sisting of a 50 Mbit/s BPSK received signal, downconverted from 5 GHz
to 300 MHz, including linearity measurements. Through this process, the
PM-Coh link was evaluated for its application in different scenarios for
next generation broadband access networks. PAPER 5 reports the use
of WDM Ring architecture with fiber distributed Raman amplification for
RoF transport at 250 Mbps and 5 GHz. In collaboration with Stanford uni-
versity, we reported in PAPER 6 the use of PM-Coh for high capacity IF
over fiber for next generation cellular network backhaul using distributed
antenna systems, for total aggregated bitrate of 4.8 Gbps. Higher bitrates,
2.5 Gbps QPSK at 5 GHz, presented in PAPER 18, were detected using
k-means clustering method. The most advanced PM-Coh link was reported
in PAPER 2 and PAPER 3, in which up to 3.2 Gbps in 40 GHz RF of
16-QAM modulated PM-Coh link was achieved. Another relevant applica-
tion of PM-Coh was reported in PAPER 7, where up to 500 Mbps OFDM
modulated at 5 GHz was transmitted through 78 km of installed fiber, in an
heterogeneous hybrid wireless-wired access network, using a single reconfig-
urable digital coherent receiver. Finally, I developed a theoretical model of
the PM-Coh link, presented in PAPER 4, and compared the experimental
1.6 Main contribution and outline of the thesis 17
results with computer simulations to define the engineering rules.
Elaborating further on the application of digital coherent receivers to
RoF links, I collaborated under the framework of the EuroFos project of
the European Commission, towards generation and detection of bitrates
over 10 Gbps in mm-wave frequencies using intensity modulated links. The
application of digital coherent detection for intensity modulated links, IM-
Coh, is presented in PAPER 8. The key technology employed was the
conversion of an intensity modulated RoF signal into baseband by optical
filtering. Thus, with standard baseband coherent detection, it was possible
to demodulate the resulting signal. A link exceeding 10 Gbps is reported in
PAPER 9 for QPSK modulation format, with up to 16 Gbps generation
and detection in the 75-110 GHz band. Furthermore, an extension of the
work is presented in PAPER 10 using O-OFDM generation, achieving up
to 24 Gbps and capable of working in 60 GHz and 75-110 GHz bands. Based
on the experimental results, I performed theoretical analysis and computer
simulation, presented in PAPER 11, to determine the ultimate require-
ments of this type of links on laser linewidth, amplifiers linearity and DSP
algorithms. In PAPER 12 I extended the analysis on the requirements
of the electrical and optical components to achieve 40 Gbps at 100 GHz
RF frequencies using optical baseband 16-QAM modulation. The overall
scientific results and technical achievements presented in this Ph.D. thesis
have significantly contributed to current state-of-the-art.
1.6Main contribution and outline of the thesis
The main contributions of this thesis are in the area of the detection of
high speed wireless signals using optics and digital coherent receivers.
First, this thesis proposes, studies and experimentally demonstrates the
use of PM-Coh links combined with photonic downconversion to allow the
detection and demodulation of high speed wireless signals with low speed
electrical components. Detection of 3.2 Gbps at 40 GHz are reported with
a A/D receiver bandwidth of only 3 GHz. Secondly, it contributes on the
mm-wave signal detection using photonic technologies, with a transparent
architecture in terms of carrier frequency, allowing the detection of high
bitrate signals, over 20 Gbps, at RF frequencies from 60 GHz to 100 GHz,
which could not be demodulated before using previous photonic technolo-
This thesis is structured as follows: Chapter 1 introduces the context
of the main research papers included. It provides a short overview on the
topic of wireless signal generation and detection using photonic technologies
and the impact that digital coherent receivers to extend the performance
to analogue optical fiber links. Chapter 2 describes the novelty of the
main contributions of the thesis. To conclude, chapter 3 summarizes the
main achievements of this thesis and provides an outlook on the prospects
of photonics technologies for the generation, distribution and detection of
high speed wireless signals.
Description of papers
This thesis is based on a set of articles already published or submitted for
publication in peer-reviewed journals and conference proceedings. These
articles present the results obtained during the course of my doctoral stud-
ies on the detection of high speed wireless signals using photonic meth-
ods, combining theoretical analysis, simulation and experimental results.
The papers are grouped in two categories, dealing with the use of digi-
tal coherent receivers for phase modulated or intensity modulated radio-
over-fiber links. Phase Modulated RoF link assisted with Coherent De-
tection (PM-Coh) links are studied and demonstrated experimentally in
PAPER 1 to PAPER 7. PAPER 8 to PAPER 12 present the theo-
retical analysis and experimental results for mm-wave generation and de-
tection using Intensity Modulated RoF link assisted with Coherent Detec-
tion (IM-Coh) links.
2.1 Phase modulated radio-over-fiber links
assisted with coherent detection
I present in PAPER 1 the first reported experimental results for Photonic
Downconversion (PDC) applied to PM-Coh links. The main novelty of
this paper is the use of a pulsed and free-running laser source as Local
Oscillator (LO) to realize PDC for a PM-Coh link. PAPER 1 includes
evaluation of the linearity of the link, in terms of Spurious-Free Dynamic
Range (SFDR), for high RF carrier frequencies (5 GHz) with Continuous
Wave (CW) operation and with PDC. As a proof of concept, a 50 Mbit/s
BPSK signal at 5 GHz RF carrier frequency was phase modulated and
20Description of papers
transmitted through 40 km of Single Mode Fibre (SMF). In the detection
side, PDC was employed at the coherent receiver to transfer the signal
down to 300 MHz, employing an A/D converter with a bandwidth of only
PAPER 2 presents the first demonstration of the detection of a high bi-
trate wireless signals, with complex modulation formats, using digital pho-
tonic receivers. By using a PM-Coh link with PDC, up to 3.2 Gbps 16QAM
at 40 GHz could be demodulated after 40 km of SMF transmission. This
was the highest RoF uplink experimental demonstration reported to that
date. This achievement is relevant because of the high bitrates and RF
carrier frequencies employed and the fact that the lasers employed were
not locked to each other. This free running behaviour was compensated
afterwards using Digital Signal Processing (DSP) algorithms. PAPER 2
was accepted as a post-deadline contribution in the 35thEuropean Confer-
ence on Optical Communication (ECOC’09). A more detail presentation
of these results is reported in PAPER 3.
PAPER 4 presents a theoretical model and computer simulation results for
a PM-Coh link with CW and PDC. The ultimate performance is evaluated
for the transport of wireless links with capacities in the order of hundreds
of Mbps. The analysis proves that PM-Coh link can provide low end-to-
end distortion for the transport of high capacity wireless signals through a
fiber link. It concludes that laser linewidth and modulation index are the
key factors in the link design. By using commercially available lasers, with
linewidth values in the 100 kHz range and low Vπmodulators, below 7 V,
it is possible to transport 1 Gbaud 16QAM signal with an induced Error
Vector Magnitude (EVM) of only 7%. It also includes the experimental
results from PAPER 3 and PAPER 6, compared with the computer
simulation results, showing a good agreement between the model and the
PAPER 5 presents a novel networking architecture for hybrid wireless-
optical networks, consisting of a fiber ring assisted with distributed Raman
amplification for detection and transport of high speed wireless signals from
multiple BS to a single CO. Raman amplification for wireless access scenar-
ios is a novel approach for extended optical fiber reach and high capacity
networks. 5 channels at 250 Mbps and 5 GHz carrier frequency were opti-
cally transported through 60 km of fiber with Wavelength Division Multi-
2.1 Phase modulated radio-over-fiber links assisted with coherent
plexing (WDM) channels spaced only 12.5 GHz In the system a 2 m wireless
link was also included.
PAPER 6 presents a networking architecture for transparent transport of
multiple wireless channels employing a single laser source for Multi-Input
Multi-Output (MIMO)-Distributed Antenna System (DAS). It is capable
to support multiple users with high individual bandwidth requirements,
making this architecture suitable for next generation cellular networks.
This paper includes computer simulation and experimental demonstration
of a cellular coverage of 12 x 400 Mbit/s (3 cells x 4 antennas, 100 Mbaud
16QAM) and 6 x 800 Mbit/s (3 cells x 2 antennas, 200 Mbaud 16QAM) on
a single optical carrier, proving the potential integration of the proposed
architecture into the next generation cellular networks.
PAPER 7 reports on the application of a reconfigurable digital coherent
receiver for unified heterogeneous metro-access networks. 4 signals from dif-
ferent optical access technologies were WDM multiplexed onto the same op-
tical fiber, transported through 78 km of installed optical fiber and detected
using a single reconfigurable digital coherent receiver. The heterogeneous
metro access network was composed of the following subsystems: 1) 5 Gbps
directly modulated Vertical-Cavity Surface-Emitting laser (VCSEL). 2)
Baseband 20 Gbps non return-to-zero (NRZ)-Quadrature Phase Shift Key-
ing (QPSK). 3) Optically phase-modulated 2 Gbps Impulse Radio (IR)
Ultra-Wide Band (UWB) and 4) PM-Coh link with 500 Mbps OFDM at
5 GHz RF frequency.
22Description of papers
2.2 mm-wave photonic signal generation and
PAPER 8 proposes and experimentally demonstrates the use of IM-Coh
link for for the detection of high speed wireless signals. A QPSK at 2.5 Gbps
and 40 GHz was modulated in the optical domain using a MZM. The RF
signal is converted to baseband in the optical domain by optically filtering
one of the side-lobes, which is called Single-Side Band (SSB). After that,
standard digital coherent detection was used for demodulation.
PAPER 9 presents a novel technique for all-photonic millimeter-wave wire-
less signal generation and digital coherent detection. For signal generation,
direct conversion of an optical baseband QPSK signal to a millimeter-wave
wireless signal using optical heterodyning was used. An optical baseband
QPSK signal was mixed in a photodiode with a free-running unmodulated
laser separated 100 GHz in frequency, generating a high capacity RF elec-
trical signal. For demodulation, the technique presented in PAPER 8 was
used. 5 Gb/s amplitude-shift keying and up to a 16 Gb/s QPSK wireless
signal in the band of 75–110 GHz was generated and successfully demodu-
PAPER 10 presents an extension of PAPER 9 by the use of all-Optical
OFDM (O-OFDM) for increased spectral efficiency. Up to 3 orthogonal
subcarriers were generated to create an all-optical OFDM up to 40 Gbps,
with only 10 Gbps electronics. In order to demonstrate the RF frequency
scalability and bit-rate transparency, the system was tested in the 60 GHz
and 75-110 GHz bands at the baud rates of 5 and 10 Gbaud. The proposed
system was experimentally tested up to 40 Gb/s. Additionally, a novel
digital carrier phase/frequency recovery structure was employed to enable
robust phase and frequency tracking between the beating lasers.
PAPER 11 presents theoretical and simulation results of the generation
and detection of mm-wave wireless signals using baseband optics.
technique analyzed in PAPER 11 is the same presented in PAPER 8,
PAPER 9 and PAPER 10. PAPER 11 identifies the main signal im-
pairments in the link, electrical and optical, and assesses their impact in the
overall link performance. From this analysis, we draw engineering rules for
the link design. The algorithms needed for the demodulation are proposed
and studied by computer simulations and validated with experimental data.
2.2 mm-wave photonic signal generation and detection23
PAPER 12 evaluates the requirements of the electrical and optical com-
ponents for the generation and detection of microwave signals with bitrates
approaching 40 Gbps. The performance for the case of 10 Gbaud QPSK
links at 100 GHz was studied and proposed the extension steps towards
a more complex modulation format, optically generated 16QAM, identify-
ing the requirements for this modulation format. The conclusion is that a
matched RF passband design of the electro-optical components, to have a
linear response in the 100 GHz band, is required to move from QPSK to
16QAM links, as 16QAM modulation format has stringent requirements in
the linearity and requires higher SNR than QPSK.
This thesis addresses the design and performance evaluation of high speed
wireless signals generation and detection using optics. The focus of the
thesis is on the use of photonic digital coherent receivers as a technique for
wireless signal detection and compensation for transmission impairments
by using Digital Signal Processing (DSP) algorithms. The research re-
sults presented in this thesis are pioneering in two main areas: firstly, in
the distribution and detection of high-speed wireless signals using Phase
Modulated RoF link assisted with Coherent Detection (PM-Coh) architec-
tures, combined with Photonic Downconversion (PDC). Detection of up to
3.2 Gbps at 40 GHz was experimentally demonstrated using this approach.
Secondly, in the generation and detection of over 20 Gbps wireless signals
in the mm-wave bands (60 GHz and 75-100 GHz), using baseband photonic
for wireless generation and Intensity Modulated RoF link assisted with Co-
herent Detection (IM-Coh) links for the detection. These achievements
fulfilled the requirements on transmission robustness and high capacity of
next generation hybrid optical fibre-wireless networks.
3.1.1 Phase modulated RoF links assisted with coherent
PM-Coh links are shown in this thesis to be a prospective alterative to
traditional Intensity Modulated with Direct Detection (IM-DD) links for
the transport of wireless signals over the fiber infrastructure. The results
in PAPER 2, PAPER 3 and PAPER 6 provide experimental demon-
stration of the capabilities of PM-Coh link for the transport high bitrate
signals, at high RF frequencies and with low end-to-end distortion. The
presented technology can be used with PDC to enable detection of high
RF signals exceeding the Analogue-to-Digital Converter (A/D) bandwidth,
while maintaining the linearity of the link. This conclusion is based on the
results shown in PAPER 1, reporting experimental measurements of the
linearity in terms of Spurious-Free Dynamic Range (SFDR), and also in
PAPER 2 and PAPER 3, for the case of high bitrate signal detection.
The integration of PM-Coh link into different scenarios of next genera-
tion optical access networks, was reported in PAPER 5 ,PAPER 6 and
PAPER 7. The integration of PM-Coh RoF links with baseband access
networks was also demonstrated in PAPER 1, PAPER 5, PAPER 7
and PAPER 13 to be inherent due to the use of digital coherent receiver.
3.1.2mm-wave photonic signal generation and detection
The use of photonic digital coherent receivers for the detection of wireless
signals open new possibilities for novel networking architectures in high ca-
pacity hybrid wired-wireless access links. The use of photonics enables the
transport and detection of wireless signal. It also allows the centralization
of complex signal processing for the compensation of impairments caused
by wireless and fiber transmission. A method for optical transport and
detection of wireless signals using digital coherent receivers is presented in
PAPER 8 using Single-Side Band (SSB) modulation and IM-Coh link.
Combining the transparency of direct electro-optical transmission and de-
tection, given by IM-Coh, with the capabilities of the DSP to support high
bitrates and to compensate impairments, it is possible to use high RF fre-
quencies, in the mm-wave range, to realize high capacity wireless signals.
The new possibilities that optics brings to create high spectral efficient
modulation formats, by using IQ optical modulation, are used to generate
high capacity wireless signals by optical heterodyning. PAPER 9 reports
the generation of 16 Gbps capacity link at 100 GHz carrier frequency. To
achieve higher spectral efficiency, all-optical Orthogonal Frequency-Division
Multiplexing (OFDM) was used in PAPER 10. The system was tested at
bitrates up to 40 Gbps at 60 GHz and 100 GHz RF frequencies.
3.2 Future work 27
In this section I would like to provide a view of the future work that could
be pursued for the generation and detection of high speed wireless signals
using the optical technologies presented in this thesis, from the system ar-
chitecture to the devices. Additionally, I would like to consider a more
concrete target, 100 Gbps wireless transmission and envision on the appli-
3.2.1 Technologies for ultra high capacity RoF links
New photonic devices with extended operation bandwidth and matched
RF design will allow the implementation of higher capacity links beyond
the work presented in this thesis. For wireless signal modulation in the
optical domain, high bandwidth modulators have been reported in the lit-
erature, with bandwidths over 100 GHz in the form of Mach-Zehnder Mod-
ulator (MZM) and Electro-Absorption Modulator (EAM) [22,62]. How-
ever, they are mostly design for broadband operation; a passband design
on the frequency band of interest will results in improved efficiency. For the
photonic generation, high speed photodetectors, with 300 GHz bandwidth
have also been reported . The generation of high repetition rate signals
can be performed using pulsed optical light sources, such as mode-locked
lasers [11,41,63], with repetition rates over 100 GHz, offering high stability
and low jitter. The combination of pulsed sources with frequency doubling
or quadrupling techniques [14,17] will allow the generation of subterahertz
RF sources without the need of a high frequency electrical reference source
or RF mixer. With regard to digital coherent receivers, current challenges
are related to the A/D convertors limited bandwidth and effective num-
ber of bits and the real time implementation of DSP algorithms at the
3.2.2 Towards 100 Gbps wireless communication links
The high bitrates that fiber optics can provide is creating an imbalance
between wired and wireless data delivery. Single carrier optical transmitters
and receivers exceeding 100 Gbps are commercially available , however
wireless links are still limited to some Gbps. The application scenarios for
100 Gbps wireless links are numerous. For instance, the delivery of the new
High-Definition 3D video systems, including multi-view systems, requires
high bitrate in order to support uncompressed video transmission so as to
avoid latency and decrease codec power consumption. High data rates will
be needed for cloud computing, enabling the highly hardware processing
to be moved out from the end-user to the network provider. In order to
provide real time access to a large number of cloud services, high speed
wireless connection will be needed.
To rise to the same level the capacities of optical fiber and wireless links,
it is necessary to move to high RF carriers, beyond 100 GHz [3,4]. The use
of photonic generation and detection, as proposed in this thesis, can provide
high capacity links at high RF carriers. Links exceeding 100 Gbps could
be achieved with RF carrier frequencies in the low Terahertz (THz) region,
from 100 GHz to 2000 GHz. For example, there is a THz transmission
window with a center frequency around 240 GHz that offers about 100 GHz
of bandwidth .
The challenges for wireless generation and detection at these high fre-
quencies are many. The need of high power THz RF sources is a critical
issue due to the high attenuation and low efficiency of a broadband RF
range of operation. A careful design of components in the THz bandwidth
will be needed to obtain maximum efficiency. Simple On-Off Keying (OOK)
modulation might not be sufficient to achieve 100 Gbps due to the difficulty
in broadband design of the components and the frequency dependent wire-
less channel properties. A more efficient use of the RF spectrum, by using
advanced modulation formats, will decrease the requirements of the com-
ponents for broadband operation, yet increase in terms of linearity. The
capability of generating advanced modulation formats, in order to achieve
spectral efficient wireless links, is an added challenge for the THz compo-
nent design to provide also sufficient linearity.
Paper 1: Photonic
downconversion for coherent
links using free-running local
A. Caballero, D. Zibar, C. G. Sch¨ affer, and I. Tafur Monroy, “Photonic
downconversion for coherent phase-modulated radio-over-fiber links using
free-running local oscillator,” Optical Fiber Technology, 17, pp. 263—266,
Photonic downconversion for coherent phase-modulated radio-over-fiber links
using free-running local oscillator
Antonio Caballeroa,⇑, Darko Zibara, Christian G. Schäfferb, Idelfonso Tafur Monroya
aDTU Fotonik, Technical University of Denmark, Oersteds Plads, Building 345v, DK-2800 Kgs-Lyngby, Denmark
bHelmut Schmidt University Hamburg, Holstenhofweg 85, 22043 Hamburg, Germany
a r t i c l ei n f o
Received 24 August 2010
Revised 14 February 2011
Available online 29 April 2011
a b s t r a c t
A digital coherent receiver employing photonic downconversion is presented and experimentally demon-
strated for phase-modulated radio-over-fiber optical links. Photonic downconversion adds additional
advantages to optical phase modulated links by allowing demodulation of signals with RF carrier fre-
quencies exceeding the bandwidth of electrical analog-to-digital converter. High spurious-free dynamic
range is observed for RF carriers at 5 GHz photonically downconverted to 1 GHz.
? 2011 Elsevier Inc. All rights reserved.
Radio-over-fiber (RoF) is a promising technology capable to pro-
vide simple antenna front ends, increased capacity and increased
wireless access coverage . RoF system has relied on already well
established technology for baseband signals, based on intensity
modulated and direct detection links, which can provide a very
low cost of implementation . However, baseband optical com-
munication is moving towards coherent detection with digital sig-
nal processing (DSP), thus allowing for advanced modulation
formats to be used . A similar trend may follow for RoF systems.
The use of coherent detection will enable the information to be
carried either in amplitude, phase or in different states of the
polarization of the optical field. Additionally, the selectivity of
coherent receivers is very well suited for wavelength division
multiplexing (WDM) access networks, where no optical filters
are then needed in the receiver.
The proposed scenario is shown in Fig. 1, where a group of
antennas, base stations (BS), providing coverage to a given geo-
graphical area, are connected to the central station (CS) through
an optical fiber link, each of them using a different wavelength.
Thus, the control of the BS is centralized, offering high flexibility
in terms of reconfiguration. Furthermore, the same infrastructure
can also handle baseband and RF data. It also allows of frequency
reuse within the same CS coverage, as multiplexation is done in
the optical domain.
Recently, there has been a lot of effort on coherent radio-over-
fiber optical links using either analog or digital demodulation
techniques [4–8]. Both analog and digital demodulation tech-
niques have their advantages and disadvantages, however, when
it comes to flexibility, implementation of different functionalities
and impairment compensation, digital demodulation schemes are
advantageous , for the use of phase modulation offers the man-
datory high linearity needed for the transparent transport of high
speed wireless signals at high carrier frequencies and complex
modulation formats [4,5]. Optical phase modulated links assisted
with coherent detection (PM-Coh) combine the advantageous fea-
tures of both technologies to realize a transparent transport of
wireless signal through the optical fiber link.
One of the challenges associated with digital demodulation
techniques are the high sampling rates and bandwidth limitations
of analog-to-digital (A/D) converters, which limits the operating
range of coherent receivers using digital demodulation techniques
up to few GHz [9,10].
One way to extend the frequency range of digital coherent
receivers beyond few GHz in RoF systems is using photonic down-
conversion (PDC) [9–12]. In the electrical domain, downconversion
is commonly done by using high frequency mixers, which presents
high losses, limited bandwidth and low linearity at high RF fre-
quencies. In contrast, PDC, by performing the downconversion in
the optical domain, has been shown to offer low distortion while
keeping required electronic to baudrate operation speed [7,9].
PDC is performed in the optical receiver front by employing a
pulsed optical local oscillator (LO). With today technology, optical
pulses can be generated with very high repetition rate, low jitter
and high stability, making a universal optical front-end very
1068-5200/$ - see front matter ? 2011 Elsevier Inc. All rights reserved.
⇑Corresponding author. Address: Oersteds Plads B. 343 R.211, 2800 Kgs. Lyngby,
E-mail address: email@example.com (A. Caballero).
Optical Fiber Technology 17 (2011) 263–266
Contents lists available at ScienceDirect
Optical Fiber Technology
suitable for PDC . These advantageous performance metrics are Download full-text
difficult to be achieved in the electrical domain with the current
As it is shown in Fig. 2, the LO in the receiver is a pulse source
with a repetition rate FLO. After the coherent receiver, the RF signal,
with a carrier frequency FRF, is transferred to an intermediate fre-
quency (IF), FIF. The sum of the maximum estimated frequency
deviation of the LO from the signal wavelength (Foffset) and the IF
is designed to fall within the operating range of A/D converters.
The demonstration of PDC in combination with digital coherent
demodulation for phase modulated RoF links has been reported
in , using homodyne detection with a single laser source. How-
ever, by taking advantage of digital signal processing, it is possible
to use an independent and free-running laser in the receiver side
and compensate the frequency offset beating from the two lasers
by using a Digital Phase-Locked Loop (DPLL).
In this paper, we propose and experimentally demonstrate the
use of PM-Coh combined with PDC for high speed wireless trans-
port over fiber. The main novelty of our approach is the use of a
free running pulsed laser source as LO to realize PDC, instead of
high speed electrical downconversion. We evaluate the perfor-
mance of the system in terms of linearity of the PM-Coh link with
and without PDC for high RF carrier frequencies (5 GHz) by realiz-
ing two-tone measurements. Thereafter, PM-Coh with PDC is em-
ployed in order to transfer a 50 Mbit/s BPSK signal at 5 GHz RF
carrier frequency down to 300 MHz with A/D converter BW of only
1 GHz. We report on successful signal demodulation and data
recovery for back-to-back and after 40 km of Single Mode Fiber
(SMF) transmission. In the system presented in this paper, trans-
mitter and LO laser are free-running, i.e. intradyne system. This
means that the system is fairly simple since it does not require
complex analog phase-locked loop for laser synchronization. Laser
synchronization is performed in software as explained later.
2. Setup description
A schematic of the experimental setup is shown in Fig. 3. Two
independent tones, with common reference clock, are generated
and added to drive the Phase Modulator (UM). The frequencies
are set to 5 GHz and 5.01 GHz. The two RF paths are isolated to en-
sure that any spurious intermodulation products are suppressed
by >70 dB. The optical source (k0) is a distributed feedback laser
(DFB) with ?1 MHz of linewidth at a wavelength of 1550 nm.
The pulsed optical LO is composed of a tunable laser source,
?1 MHz linewidth, followed by an electro absorption modulator
(EAM) driven by a +15 dBm RF power sinusoidal signal at 4 GHz.
This results in an optical comb with a duty cycle of approximately
30%. The resultant optical LO signal is amplified using an erbium
doped fiber amplifier (EDFA) and a 1 nm optical band-pass filter
is used to reject out of band ASE noise. The wavelength of the LO
(kLO) is tuned to be close to k0, leading to a typical frequency mis-
match of ?300 MHz. In the receiver side, the incoming optical sig-
nal and LO are passed through polarization beam splitters (PBS) to
assure detection of the same linear polarization. In a more practical
approach, a polarization tracking system could be used to follow
the polarization drifts of the received signal.
The coherent receiver is a 90? optical hybrid that has integrated
balanced photodiodes with a 3 dB cut-off frequency of 7.5 GHz. The
input optical power level is ?2 dBm for the signal and ?8 dBm for
the LO. The LO average power is low to avoid saturating the photo-
diodes, due to high peak power of sampling pulses. The detected
in-phase (I) and quadrature (Q) photocurrents are stored using a
digital sampling oscilloscope (DSO) performing A/D conversion at
40 GSamples/s for offline (DSP).
For the validation of data transmission, a 50 Mbps BPSK signal
at 5 GHz drives the UM with 8 dBm input power. The local oscilla-
tor frequency is set to 4.7 GHz. The incoming optical signal power
is ?18 dBm after 40 km of SMF transmission. The post-processing
of the digitized signals consists of carrier-recovery DPLL, linear sig-
nal demodulation and RF demodulation (residual frequency esti-
mation unit, RF carrier phase recovery and matched filtering) .
The carrier-recovery DPLL is used to remove optical frequency
and phase difference between the transmitter and LO laser.
In this section, we investigate the linearity of our RoF link oper-
ating at 5 GHz. Two-tone measurements have been done to assess
the SFDR with respect to the third order distortion (TOD), which
are the first order intermodulation components laying within the
signal bandwidth. Fig. 4 shows the measured results for continuous
wave operation and photonic downconversion (PDC) with an opti-
cal sampling frequency of 4 GHz. Similarly, the spectra of the
recovered two tones for 9 dBm at CW and 12 dBm at PDC are also
included in each graph. The noise level of both schemes can be
determined by the minimum distinguishable third order distortion
(TOD) from the resulting spectrum. The number of quantification
Fig. 1. Proposed scenario for RoF link with photonic downconversion. The base stations (BS) are connected to the central station (CS) through optical fiber, each of them using
Fig. 2. Schematic description of photonic downconversion. The frequency of the RF
signal (FRF) is transferred to an intermediate frequency (FIF) due to the repetition
rate of the local oscillator (FLO). The value of the IF is equal to the difference of the
incoming RF and LO repetition rate. As the light sources are not frequency locked,
there is still a remaining frequency offset (Foffset) that is removed at the DSP
A. Caballero et al./Optical Fiber Technology 17 (2011) 263–266