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Radio over fiber access network architecture employing reflective

semiconductor optical amplifiers

Conference Paper · January 2008

DOI: 10.1109/ICTONMW.2007.4446986 · Source: IEEE Xplore

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ICTON-MW'07 Sa2.1

Radio over Fiber Access Network Architecture Employing

Reflective Semiconductor Optical Amplifiers
M.C.R. Medeiros(1), R. Avó(1), P. Laurêncio(1), N.S. Correia(1), A. Barradas(1), H.J.A. da Silva(2)
I. Darwazeh(3), J.E. Mitchell(3) and P.M.N. Monteiro(4)
(1)
Center for Electronic, Optoelectronic and Telecommunications, University of Algarve, 8000 Faro, Portugal
(2)
Instituto de Telecomunicações, Departamento de Egenharia Electrotécnica e de Computadores, Faculdade de
Ciências e Tecnologia, Universidade de Coimbra, Portugal
(3)
Telecommunications Research Group, Department of Electronic and Electrical Engineering
University College London, Torrington Place, London, WC1E 7JE, UK
(4)
Nokia Siemens Networks Portugal S.A., Instituto de Telecomunicações, Universidade de Aveiro
R. Irmãos Siemens 1, 2720-093 Amadora, Portugal
Tel: +351 289 800900; Fax:351 289 819403Tel; e-mail: cmedeiro@ualg.pt

ABSTRACT
This paper introduces the RoFnet-Reconfigurable Radio over Fiber network, which is a project supported by the
Portuguese Foundation for Science and Technology. This project proposes an innovative radio over fiber optical
access network architecture, which combines a low cost Base Station (BS) design, incorporating reflective
semiconductor optical amplifiers, with fiber dispersion mitigation provided by optical single sideband
modulation techniques. Optical wavelength division multiplexing (WDM) techniques are used to simplify the
access network architecture allowing for different Base Stations to be fed by a common fiber. Different
wavelength channels can be allocated to different BSs depending on user requirements. Additionally, in order to
improve radio coverage within a cell, it is considered a sectorized antenna interface. The combination of
subcarrier multiplexing (SCM) with WDM, further simplifies the network architecture, by using a specific
wavelength channel to feed an individual BS and different subcarriers to drive the individual antenna sectors
within the BS.
Keywords: radio over fiber, reflective semiconductor optical amplifiers, optical single side band, optical access
networks, wavelength division multiplexing.

1. INTRODUCTION
Internet technologies are now considered as the universal communication platform. Broadband access
communication is rapidly becoming widely available, e.g. ADSL in now available in almost all parts of Europe.
In parallel to the growth of the Internet, wireless and mobile network technologies have witnessed a great
development. Mobile phone penetration exceeds that of fixed phones in most developed countries. Wireless
communications are entering a new phase where the focus is shifting from voice to multimedia services. Present
mobile network users want to be able to use their mobile terminals and enjoy the same user experience as they
do while connected to their fixed network either at work or at home. WLAN hot-spots based on IEEE802.11 are
a reality, and many consumer devices (Laptop PCs, mobile telephones, PDAs, etc) have Bluetooth enabling
them to establish a wireless personal area network. In this context, third generation of wireless networks (3G)
have already adopted IP (Internet Protocol) as the core network protocol in their data subsystems, as well as
promising guaranteed quality for multimedia service, in both access and core networks. However, the services
offered by wired LAN connections require a broadband network with capacity even higher than 7 Mbit/s per
radio channel. Unfortunately, the actual wireless telecommunication network only provides narrowband
communications when compared to wired LAN connections and thus a radio interface capable of supporting
very high data rates, has to be developed. Over the last decade the mm-wave frequency band (26 to 100 GHz)
has been pointed out as the best spectral region to provide broadband access to wireless networks [1]. However,
the limited propagation characteristics of these high frequencies lead to small cell sizes. As a consequence, a
large number of remote antenna base stations (BSs) is necessary to cover an operational geographical area. The
multiple BSs providing wireless connectivity to users via millimeter-wave radio links are connected with a
central office (CO) via an optical fiber access network. The CO performs the switching and routing
functionalities.
The RoFnet-Reconfigurable Radio over Fiber network project, introduced here, uses optical wavelength
division multiplexing (WDM) techniques to simplify the network architecture allowing different BSs to be fed
by a common fiber, with different WDM channels feeding different BSs. Additionally, in order to improve radio
coverage within a cell, utilization of sectorized antenna interfaces is considered. Each sector of the antenna

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This work was supported by the Foundation for Science and Technology within the project PTDC/EEA-
TEL/71678/2006.

978-1-4244-1639-4/07/$25.00 ©2007 IEEE 1

ICTON-MW'07 Sa2.1

should be driven by an individual signal. The combination of subcarrier multiplexing (SCM) with WDM,
simplifies the network architecture, since a specific WDM channel is fed to an individual BS and different SCM
channels carried on the WDM wavelength channel are used to drive the individual antenna sectors within the
BS.
For the downlink the mm-wave signals are transmitted directly over the fiber. This approach has the advantage
of a simplified BS design but is susceptible to fiber chromatic dispersion that severely limits the transmission
distance [2]. The RoFnet will employ optical single sideband (OSSB) modulation techniques to overcome fiber
dispersion effects.
For the realization of the uplink of a RoF system using WDM, the BSs must incorporate an optical source,
which is modulated by the mm-wave uplink radio signals. This approach suffers from several disadvantages.
Namely, each BSs requires a WDM optical sources and a high-speed external modulator, resulting in a high cost
BS. The RoFnet proposes a novel low cost uplink configuration, which eliminates the need for an expensive
WDM source and the optical external modulator at the BS. This is accomplished by using a Reflective
Semiconductor Optical Amplifier (RSOA) in the BS which replaces the high cost WDM source and the high-
speed external modulator. This approach offers several advantages. First, it avoids the need of stabilized a laser
at each BS. Second, the RSOA can be used as a modulator which accomplishes both modulation and
amplification functions. Moreover, this amplification function gives additional gain enabling the possibility of
avoiding the use of an EDFA in the system. For the first time, this uplink configuration exploits the capabilities
of using RSOAs in RoF systems and this alternative may be applied to other wireless networks such as 3G
mobile communication systems.

2. NETWORK DESCRIPTION
Fig. 1 shows the schematic of the RoFnet architecture concept, where N base stations (BSs) provide the wireless
connectivity to users via mm-wave radio links. The BSs are connected with a central office (CO) via an optical
fiber access network employing WDM technology. Each BS incorporates an antenna with L sectors. Such
solution provides a simple topology, leading to easier network management and increases the capacity by
allocating different wavelengths to individual remote nodes [3]. This solution is widely accepted and the
necessary WDM technology is available. However, if this approach is to be implemented a cost effective
implementation needs to be found. A dynamic wavelength allocation scheme for WDM RoF systems using
a novel add-drop multiplexer was demonstrated in [4]. An important feature of this scheme is the possibility of
dynamic network reconfiguration when needed, namely through the wavelength reassignment to different base
stations. Flexible wavelength allocation is an elegant strategy for dealing with traffic fluctuations since it allows
efficient allocation of network resources by adaptively adjusting to the offered load. However, the
implementation presented in [4] requires a new expensive device. The RoFnet architecture, by using a RSOA in
the BS, eliminates the need of expensive devices. RSOAs are presently considered key devices for the future
high-speed passive access optical networks (PONs) [5]. A RSOA can be used with both modulation and
amplification functions. Moreover, a RSOA operated in the gain saturation region can reduce the intensity noise
of the optical signal. The 3dB electrical bandwidth of commercially available devices is up do 1.5 GHz in the
long wavelength bands in the range of 50 to 100 nm. The wavelength range mainly depends on the RSOA
manufacturing process.

2
ICTON-MW'07 Sa2.1

BS1

BS2 λ1, λ2, λ3,..., λM

Central
Office
BS3

#
Feeder WDM network, star,
BSN ring, mesh, …

BS – Base Station

Figure 1. Overall RoFnet architecture.

In the RoFnet architecture, the CO, as well as performing all the switching, routing and frequency management,
also generates the M optical WDM carriers required for uplink operation of the N BSs of the RoF network.
Each BS, as represented in Fig.2, is equipped with a fixed optical filter, and thus operates only with a unique
specific wavelength λj.

2.1 Downlink Operation

BS j receives the downlink mm-wave signal on wavelength channel λj.. The downlink mm-wave signal is
composed by L multiplexed sub-carriers combined with a set of un-modulated RF carriers, as shown in Figure 3.
The L SCM channels feed the L antenna sectors, and the set of un-modulated RF carriers are used in the uplink
operation. The un-modulated RF carriers and the downlink signals are generated at the CO and modulate an
optical carrier using optical single side band modulation (OSSB). OSSB modulation is used in order to minimize
the fiber dispersion effects and to improve spectral efficiency. We note that OSSB modulation is required only at
the CO, and thus it does not increase the cost of the BS. At the BS, the downlink optical signal in wavelength
channel λj is split. One part is directed to the RSOA, and the other part is detected by a high bandwidth receiver.
The detected signal consists of the downlink mm-wave signal and the un-modulated RF carriers.

2.2 Uplink Operation

The downlink optical carrier travels through the RSOA, where it is amplified and modulated by the uplink data,
which has been down converted to an Intermediate Frequency (IF). The un-modulated RF carriers act as local
oscillators (LO) and are used to down-convert the uplink data to an Intermediate Frequency (IF), within the
electrical bandwidth of the RSOA (1.2 GHz). The RSOA is directly modulated by the SCM uplink signals, and
thus the optical carrier is double side band modulated, as represented in Fig.3.
Using this technique, the uplink optical signal is generated by recovering a portion of the optical carrier used in
the downlink transmission. Although optical frequency reuse techniques previously used eliminate the need for
a WDM optical source at each BS, they require a high-speed external modulator at the BS. However, the
necessary bandwidth of the external optical modulator can be reduced if the uplink signal is down converted by
mixing it with a local oscillator (LO). The generation of a LO at the BS increases its complexity and therefore
should be avoided. The solution adopted in RoFnet is the remote delivery of the LO, as implemented in [6].

3
ICTON-MW'07 Sa2.1

From / To
the network Downlink1

λ1, λ2, …, λM
Downlink…

Optical filter
DownlinkL

λj fD1 … fDL
Electrical
Band Pass
Filters
High bandwidth
LO1 … LOL
receiver

Uplink1
RSOA

Uplink…

UplinkL
MUX
BPF

Figure 2. Schematic diagram of a Base Station.

Wavelength , λj

Uplink channels Downlink channels

Unmodulated RF carriers
WDM carrier

Figure 3. Optical spectrum around wavelength carrier λj.

3. DYNAMIC WAVELENGTH ALLOCATION

Traffic in wireless networks is highly dynamic. Therefore the access network should be reconfigurable
depending on the traffic scenario. The network architecture defined in the previous section is reconfigurable
and therefore can dynamically change its state according to the traffic needs. The RoFnet network is operated
with M optical WDM sources and N BSs. Each BS is equipped with a fixed optical filter and therefore operates
only with a unique specific wavelength λj. We consider M<N, i.e. the number of optical carriers present in the
network is less than the number of served BSs. Such assumption means that rather than providing fixed capacity
tailored to the ‘busy-hour’ across the network, the optical carriers are allocated to BSs depending on their needs.
The optical carriers are generated by fixed lasers as well as by tunable lasers. The fixed optical carriers are
allocated to BSs which should be always on use and the tunable carriers are allocated to BSs that might be out of
use during some time. Therefore, besides demonstrating the feasibility of the RoFnet architecture another

4
 ICTON-MW'07 Sa2.1

objective is to exploit its capabilities namely by developing wavelength allocation algorithms able to allocate
network resources depending on varying user demand and QoS.

4. CONCLUSION
This paper introduces a novel network architectures suited for radio over fiber networks, combining low
complexity with flexibility and cost effectiveness. Its two main characteristics are: (1) low cost BS based on
RSOAs; (2) use of optical single sideband modulation to improve the system immunity to chromatic dispersion,
as well as the spectral efficiency. Additionally, an important advantage of RoFnet is the generation and
management of the optical carriers at the CO. As well as facilitating wavelength monitoring and control this
approach provides flexible wavelength allocation for the BSs depending on user requirements.

ACKNOWLEDGEMENTS
This work was supported by the Foundation for Science and Technology within the project PTDC/EEA-
TEL/71678/2006.

REFERENCES
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pp:2297-2308, 2000.
[2] H. Schmuck: Comparison of optical millimeter-wave system concepts with regard to chromatic dispersion,
Electron. Lett., vol. 31, pp. 1848-1849, 1995.
[3] H. Bolcskei, A.J. Paulraj, K.V.S. Hari, and R.U. Nabar: Fixed broadband wireless access: State of the art,
challenges, and future directions, IEEE Commun. Mag., vol. 39, no.1, pp.100-108, Jan. 2001.
[4] Wen-Piao Lin: A robust fiber-radio architecture for wavelength-division-multiplexing ring-access
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[5] A. Borghesani: Optoelectronic components for WDM-PON, in Proc. ICTON 2007, pp. 305-308, Rome,
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[6] C. Lim, A. Nirmalathas, D. Novak and R. Waterhouse: Millimeter-wave broad-band fiber-wireless system
incorporating baseband data transmission over fiber remote LO delivery, J. Lightwave Technol., vol. 18,
no. 10, pp. 1355-1363, 2004.

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