Application of Radio-Over-Fiber (ROF) in mobile communication
Reza Abdolee, Razali Ngah, Vida Vakilian and Tharek A.Rahman
Wireless Communication Centre
Faculty of Electrical Engineering
Universiti Teknologi Malaysia
81310 UTM Skudai Johor
[email protected], [email protected], [email protected], and [email protected]
Abstract - Future generation of mobile
communication system must be capable of serving
high quality and broadband services even in highly
dense populated area. The radio-over-fibre is one of
the promising systems which can be used along the
emerging wireless technologies such as smart
antenna or multiple input multiple output (MIMO)
systems to meet these requirements. In this paper,
the role of ROF technology in next generation of
mobile communication system is presented, and
then the concept is clearly interpreted. Then, base
on previous research and finding, a probable ROF
architecture for present and future wireless mobile
communication is portrayed.
Along this,
challenges and development issues of ROF
technology are presented as well.
Keywords: ROF technology, CBS, RBS, WDM, DWDM
1. Introduction
Base on cellular concept to increase the
capacity of mobile cellular system cell optimization
can be used to take advantage of frequency reuse
scheme. As a result of this scheme, the number of
base station increases in the area. Increasing
number of base stations with current RF technology
especially in millimetre-wave band is very costly.
And technically, using millimetre-wave band signal
is inevitable because the current RF spectrum is
limited. Base on these facts, radio-over-fibre (ROF)
technology is a best choice to apply in cellular
system for cell optimization process since it can
easily be used in millimetre wave band and in
addition it can reduce the system overall cost. [1].
ROF technology gives a lot of advantages such
as; The complicated signal processing is localized
at the central base station (CBS) therefore the
overall system is cost effective, remote base station
(RBS) is very simple, passive and compact
therefore it is transparent to air and its maintenance
is easy. The system is very cost-effective because
of localization of signal processing in CBS and also
simple base station, the reliability of the system is
high due to simple and passive structure of the
RBS. This system can easily serve high dense
populated area such as shopping mall and airports,
dead-zone area and highways can be covered
efficiently and economically, the system can
support multiple wireless standards [2]. Because of
high bandwidth nature of optical fiber, Broadband
services are more feasible using this technology.
The centralized CBS can decrease the number of
handover within the cells [2]. Radiated power from
antenna is very low due to Pico and Micro cells
structure. Low radiated power of antenna make the
system more immune for human body and also
reduces the power consumption of mobile battery.
The co-channel interference can greatly be reduced
due to low radiation power. In pico and micro cell
structure, Propagation loss is less because of small
cell radius. These are some of the advantages of
this system which make it attractive for future
mobile communication system.
The remained material of paper describes the
ROF technology in 4G, probable future ROF
architecture, ROF structure, ROF future trend and
challenges, and at the end paper concludes.
2. Role of ROF Technology In 4G
The minimum bit rate that 4G would provide
for broadband services is 20 Mb/s for indoor and 2
Mb/s for outdoor application even with high
relative mobility. Figure 1 shows the trend of
mobile communication system [3]. As candidate
technologies for future systems, 4G-cellular and
Intelligent Transport System (ITSs), have been
attracting much interest in the mobile
communication field [4]. Both of these
technologies take advantage of ROF technique. The
concept in 4G cellular system is cell optimization.
In ITS system, the key technology in road vehicle
communication system is again ROF, in which
many base stations are equipped along the trunk
road in order to communicate with vehicles, and
several control base stations manage these base
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stations.
needs the optical and electrical component which
has a low noise and distortion in RF frequency. The
most dominant source of noise and distortion in
ROF link is laser source. Therefore by finding the
suitable laser source which has right balance
between cost and performance, the indoor ROF
technology simply can be implemented.
3.2 ROF Technology Structure for Outdoor
Mobile Communication
Figure 1: Trend of mobile communication
system
.
3. An Overview on ROF Mobile Technology
At Architecture Level
By using ROF technology, any type of radio
and millimetre wave signals can be transmitted
through optical fibres. The structure of ROF system
in indoor and outdoor application is slightly
different. Therefore, it should be explained in two
different sections.
3.1 ROF Technology Structure in Indoor
Mobile Communication
In indoor communication and for frequency
below mm-wave, firstly, optical wave from a laser
diode (LD) is modulated directly by the radio
signal at the CBS and transmitted through the
optical fiber. In remote base station (RBS), RF
modulated optical signal detected by photodiode is
transmitted into the wireless link by RBS antenna.
However in indoor application three possible
structures can be considered to implement. In the
first structure, RF signal usually in the range of 800 2200MHz is transmitted over single mode fibre,
second structure IF signal is transmitted over
multimode optical link and third structure the
signal is firstly digitized at CBS and then
transmitted over optical link. This has the
advantages of digital transmission means that no
impairments due to noise and distortion but at the
expense of further complexity [5].
The most common of these architectures is the
first, RF over single mode fiber, because it is the
simplest to design and the lowest in cost.
Consequently it put most stringent requirement on
the optical component. It means that the system
In outdoor mobile communication, the
frequency band which is considered for future
mobile communication is in the range of 60 GHz
known as mm-wave band frequency. Direct
modulation just functions for below mm-wave
frequency [6], therefore for generating mm-wave
frequency, external modulation or heterodyning
techniques must be used. In external modulation
RF signal and optical carrier are optical modulated
using a Mach-zehnder modulator or an electroabsorption modulator. The drawback of this system
is chromatic dispersion due to double-sideband
transmission. Therefore, the most popular
technique can be heterodyning technique. There are
different structures for heterodyning modulation.
In the first technique, two optical sources
(DFB laser) in the operating range of 1550 nm as a
master slave arrangement to produce RF signal are
used in CBS. This RF signal is modulated with the
data. The modulating RF signal can be detected by
photodiode then it is transmitted by RBS antenna
without any electrical mixer [7]. The main
drawback of this method is that it is not amendable
to all modulation format or modulation bandwidth
and also chromatic dispersion is created in the
signal because of mixing optical carriers with RF
signal at the CBS.
In the second structure, the RF carrier using
two master slaves DFB is generated similar to first
method. However, in this technique data is not
mixed with the RF signal and upconversion with
the data is taken place in the RBS using
conventional electrical mixer or in simpler
arrangement using optoelectronic mixer. Therefore,
separate pair of laser source and photodiode is
needed to transmit the data and carriers [6].
Third technique is exactly the same as
technique number two. The only difference is in
signal detection in the RBS where instead of
photodiode an Electro Absorption Modulator
(EAM) is used [6]. This device can operate as a
modulator in return path. Therefore, the laser
source in the RBS can be eliminated.
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3.3
Perspective
Communication
of
ROF
Mobile
Integrated structure of indoor and outdoor ROF
mobile technology might have the structure which
portrayed in Figure-2. This system may use in the
future with some modification in topology. For
example, RBS can be connected to central RBS
using BUS topology. By applying this topology,
system cost is reduced by saving in applied optical
cable in the system. As it can be seen from the
Figure-2 by using ROF repeater which is proposed
in [9] more than 300 base stations in cellular
system can be multiplexed by DWDM and
transmitted over SMF (Single Mode Fiber) to the
CBS. In this structure, every base station uses one
single wavelength to connect to the CBS. WDM or
DWDM is the core of this system. In Every RBS,
single set of antennas can be used to receive
various RF signal in ITS communication [2]. By
using ROF repeater, pre-existing FTTH fiber optic
can be used for 3G applications, even without any
change in EVM (Error Vector Magnitude) [9]. One
of the components in Figure-2 is distributed
antenna system (DAS) which is used for indoor
communication and proposed in [3]. DAS takes
advantage of optical switches which has a lot of
advantages in compared with RF switches. High
level of isolation and very low cross talk are the
significant advantages.
3.4 Remote Base Stations antenna (RBS)
Figure-3 shows the available Equipments in
RBS which can be listed as: one EAM
(Eletroabsorbtion modulator), one remote antenna,
diplexer, high power amplifier, low noise amplifier
and Power supply. However, the trend of ROF
system is simplifying the RBS and changing it to
passive unit.
issue is to simplify the RBS as much as possible.
The research direction aims to design passive RBS
which is needed no maintenance and no service.
Compactness is another issue for RBS which is
important in indoor mobile communication.
To simplify the RBS a lot of researches have
been done. One solution is powerless RBS which is
proposed in [9]. Since this system can operate
without electric power supply, it can be used in
various situations. In some researches the
elimination of the optical source in the RBS using
EAM [11], [6] is suggested. In this method EAM
act as a photodiode in downlink and operate as a
modulator for the uplink. Optical carrier which is
need for the uplink is fed to the RBS from CBS
remotely. One of the proposed ideas in order to
centralize the signal processing is predistortion in
downlink and postdistortion in uplink to
compensate the effect of laser and fiber
nonlinearity [12].
One of the drawbacks of ROF system is costly
CBS due to signal processing in RF frequency
because of costly RF devices. Therefore some
researches are going on to substitute the electrical
processing with optical one. PDC (photonic down
conversion) is the solution to eliminate the use of
electrical RF component in the CBS [13]. Using
this technique the problem of chromatic dispersion
in the fiber also can be resolved [11],[13].
Another research direction in order to reduce
the cost of ROF system is usage of Multi-Mode
Fiber (MMF) instead of SMF. Some researches
have been done to use of pre-existing graded index
fiber in the old urban area to reduce the cost [14].
Also FFTH can be used by applying WDM. In this
structure proposed in [16] the use of OEO (OpticalElectrical-Optical) repeater and WDM for
multiplexing different signal coming from different
base station has been suggested. In contrast, in new
urban area which there is no communication
infrastructure the system cost can be reduced by
sharing the optical fiber from the RBS’s to CBS
using Dense Wave Length Division Multiplexing
(DWDM) over single mode fiber [15]. By this
technique 300 base stations can be connected to
one CBS. In indoor application to flexibility
support the user and giving various services to the
user the software controlled optical switch such as
3MEM has been proposed [3].
5. Some Challenges of ROF Technology
Figure 3: The block diagram of remote base
station (RBS)
4. Future Trend of ROF Mobile Technology
Several issues for future ROF mobile
communication have to be addressed. The first
In order to design the ROF system with high
transmission rate and high capacity to pervade in
future mobile communication, there are a lot of
technical problem which need to be resolved. Some
of theses challenges are summarized in Table 2.
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Table-2: ROF challenges for mobile communication system
Challenges
Some proposed solutions
Modulation technique,
Direct modulation is exist just for below mm-wave frequency. External
modulation, optical heterodyne technique and EAM modulator can be solutions
Chromatic dispersion
Single Sideband Modulation (SSB) modulation, photonic downconversion
Phase distortion[1]
1.Phase noise cancellation method 2.Side band injection locking 3.error
correcting coding 4.Optical Phase Locked Loop (OPLL)
laser and optical fibre nonlinearity
Pre-distortion and post-distortion technique [6]
Noise
Characterization
and
Cancellation for combination of
Optical and wireless noise
Characterize the optical-wireless noise by making relation between optical link
property and optical devices with wireless noise.
multi-user detection in Non linear
optical-wireless Estimation
Optical wireless detection using correlation property of PN code.
RBS compactness and cost
Centralizing the signal processing at CBS , elimination of laser source and
electrical mixer in RBS, Elimination of power supply in RBS,
High data rate wireless link as a
complimentary part of ROF
Using MIMO-OFDM-CDMA and smart antenna system
Expensive and complex uplink
Using photonic down conversion, electrical signal processing can be done in IF
frequency(reduce the cost)
Non availability of Opto-electrical
interfaces
The researches are going on to provide a small size and low cost opto-electrical
interfaces
Optical switch design
3DMEM technology with low price and all optical switches for ultra fast
switching
Figure 2: Perspective of ROF technology in mobile communication
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6. Conclusion
In this paper, the application of ROF technology in
mobile communication is explained. Recent finding
and issues in ROF mobile communication are
discussed. The feature of future ROF technology in
mobile communication is presented. Some of the
challenges are listed down. Also the ROF
technology architecture for future mobile
communication system is portrayed.
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