th
The 5 Student Conference on Research and Development –SCOReD 2007
11-12 December 2007, Malaysia
Optical Front-end Receiver Design for Radio
over Fiber System
H. Harun, S. M. Idrus, and A. B. Mohammad
Abstract-- Mobile communication system faces serious
challenges serve high quality broadband service in any situation
even in highly dense populated area and high mobility users.
Radio over Fiber (RoF) technology is proposed as a solution for
reducing cost and providing highly reliable communication
services. The RoF system is very cost-effective because the
localization of signal processing in central station and also use a
simple base station. In this paper, two methods of RoF system are
investigated for transmitting the radio signal over the optical
links with intensity modulation. There are RF signal or IF signal
transmission over optical fiber. This paper focus on the RoF
optical front-end receiver configuration especially optical
heterodyne employing an optoelectronic mixer. Thorough review
and comparison of previous works, the HBT is the best
performance as an optoelectronic mixer and frequency
conversion to millimeter wave.
free-space radio path as the final drop to the end-users
provides flexibility since the end-users do not have to be fixed
in location. Such systems are important in a number of
applications, including mobile communications, wireless local
area networks (WLANs), and wireless local loop, etc. rapid
developments in both lightwave and microwave enabling
technologies have fuelled an intense effort into the research
and development of these networks [2-4].By using RoF
technology, any type of radio signal (Table 1) can be
transmitted through optical fiber.
Index Terms--Radio over Fiber system, optoelectronic mixer,
millimeter wave, frequency upconversion, optical receiver.
I. INTRODUCTION
F
OR many years, mobile radio communications have been
the predominant form of communication technique.
Recently, radio communications have become problematic, as
the radio spectrum becomes increasingly crowded. Figure 1
shows the trend of mobile communication system [1]. In
comparison, optical communication systems are expanding,
and offer much greater potential bandwidths, both in fiber and
also for free-space applications, the latter in direct competition
with microwave links. An attractive alternative approach to
demand for costly fixed equipment is to explore ways of
delivering radio frequency (RF) signals from a number of base
stations (BS) to centralized based station equipment, using
optical fiber as the transmission medium.
Radio over fiber (RoF) systems is characterized by having
both a fiber optic link and free-space radio path. The use of
This work was supported in part by the Ministry of Science, Technology
and Innovation Malaysia under EScience funding 01-01-06SF0064 and
Research Management Centre (RMC), Universiti Teknologi Malaysia (UTM)
under Grant 79026..
H. Harun is with Photonic Technology Centre, Faculty of Electrical
Engineering,
Universiti Teknologi Malaysia, Malaysia. (e-mail:
[email protected]).
S. M. Idrus is with Photonic Technology Centre, Faculty of Electrical
Engineering,
Universiti Teknologi Malaysia, Malaysia. (e-mail:
[email protected]).
A. B. Mohammad is with Photonic Technology Centre, Faculty of
Electrical Engineering, Universiti Teknologi Malaysia, Malaysia. (e-mail:
[email protected]).
Fig. 1. Trend of mobile communication system.
TABLE I
DIFFERENT RADIO SIGNAL APPLICATION
Frequency band
0.8 GHz
1.8 – 1.9 GHz
2 GHz
2.4 GHz
2.6 GHz
3.4 GHz
5 GHz
18/19 GHz
28 GHz
38 GHz
58 GHz
62 – 66 GHz
Service type
Cellular, 2G system
2G system
UMTS/3G system
Wireless LANs (IEEE 802.11 b/g)
S-DBM
4G system (TBD)
Wireless LANs (IEEE 802.11 a)
Indoor Wireless LANs
Fixed wireless Access (LMDS)
Fixed wireless Access Pico Cellular
Indoor Wireless LANs
Mobile
With smaller cells and increased frequency re-use, this also
means an increase in the number of required BS, which is a
rather complex and expensive piece of hardware. A solution to
this is to use small, low power transceiver antennas in each
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microcell, connected to a central station (CS) that deals with
all the signal processing. Figure 2 shows an integrated
structured for indoor and outdoor RoF mobile technology.
(a)
(b)
Fig. 2. Perspective of RoF technology in mobile communication.
Having a distributed antenna system (DAS) such as this has
many benefits, most of which come from having small and
low power antennas. Firstly there is the reduced physical
environmental impact due to the reduction in the amount of
bulky BS needed, with the general disruption that go with their
installation. More important, though, is the fact that, as the
distance between the mobile handset an antenna is reduced, so
the power radiated by both can also be reduced. The first
benefit of this is that more frequency re-use within microcells
is possible, because interference is reduced, thus creating
picocells. Analog optical links using RoF are in use today in
many DAS installations around the world. The trend is for
greater penetration of RoF as more large buildings are
provided with DAS; a recent market research report by ABI
Research [5] forecasts that over half of all in-building wireless
deployments worldwide will use RoF by 2009.
This paper organized in five sections. Section one was an
introduction of delivery RF signal throw the optical fiber.
Section two gives a technique to transmit radio waveform over
the fiber. Section three gives an overview of the RoF
architecture from the previous work. RoF optical front-end
receiver using upconversion techniques to be implementing
are discussed in section four. Finally the conclusion is given in
section five.
II. ROF SYSTEM
RoF not like the conventional optical direct or network is
analog transmission system because it distributes the radio
waveform, directly at the radio carrier frequency, from a CS to
a BS. This radio waveform is transmitted over the optical fiber
can be either intermediate frequency (IF) signal or RF signal,
but for the transmission of IF signal, the additional technique
and hardware for upconverting it to RF band is required at the
BS.
Fig. 3. Representative RoF links configuration. (a) RF modulated signal
through fiber, (b) IF modulated signal through fiber.
At the optical transmitter part, the signal can be imposed on
the optical carrier by using direct or external modulation of the
laser light. However, there have non-linearity and frequency
response limits in the laser and modulation of the laser and
modulation device as well as dispersion in the fiber where
must be consider. The RoF systems which transmit the analog
signals place certain requirements on the linearity and
dynamic range of the optical link. These demands are
difference and more exact than requirements on digital
transmission system [6].
III. ROF ARCHITECTURE
Recently, a lot of researchers have been study to develop
millimeter wave (mm wave) generation and transport
technique using RoF systems. Several states of the art
techniques that have been investigated and classified into the
four categories: external modulation [7–10], optical
heterodyning [11–16], optical transceiver [4, 17–20] and upand down-conversion [21–25].
External modulation is done by a high speed external
modulator such as electro-absorption modulator (EAM). Its
configuration is simple, but it has some disadvantages such as
fiber dispersion effect and high insertion loss. To reduce such
dispersion effects, optical single sideband (SSB) is widely
used [8–10]. Specially designed EAM was developed and
experimented at 60 GHz band RoF system in [9] and [10],
respectively, to produce SSB optical modulation.
In optical self-heterodyning technique, two or more optical
signals are simultaneously transmitted and are heterodyned in
the receiver. One or more of the heterodyning products is the
required RF signal.
An optical transceiver is the simplest BS when implement
an electro-absorption transceiver (EAT). It serves both as an
opto-electronic (O/E) converter for downlink and electro-optic
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(E/O) for the uplink. Two wavelengths are transmitted over an
optical fiber from CS to BS, where one of them is modulated
by user data for downlink transmission and other one is
unmodulated for the uplink transmission. This unmodulated
wavelength is modulated by uplink data at the BS and returns
to the CS. This technique is capable for full duplex operation
in several experiments in mm wave bands [4, 17–19].
fiber and at the end of the fiber optic link, this IF signal is
photodetected and up-converted to millimeter-wave
frequencies before being radiated into free-space and delivered
to the end-users, then the losses due to the optoelectronic
components and fiber dispersion can be greatly reduced. To
realize the BS, millimeter wave range electrical mixers and
local oscillator are required, which make BS complex and
expensive. In order to overcome these problems, photonic upconversion techniques have been proposed, which utilize
optically generated LO signal and optical component such as
photoparametric amplifier (PPA) optoelectronic mixer [26],
the electro-absorption modulator-transceiver [20], and a
photonics up-conversion method using cross-gain modulation
of the semiconductor optical amplifier [27]. A photonics
frequency conversion (PFC) with electrically pumped at base
station is particularly suited for such an application and is
illustrated in Figure 5.
(a)
Fig. 5. Block diagram for RoF optoelectronic up-conversion.
(b)
In the up-conversion mode, the base band is translated to a
higher frequency, which is equal to the sum of baseband plus
pump signal (RFUSB) that will be selected, as shown in Figure
6. In this technique, the mixing element will be performing by
metal semiconductor metal photodetector (MSM-PD), HEMT,
Heterojunction
bipolar
phototransistor
(HPT),
and
heterojunction bipolar transistor (HBT).
Fig. 4. Uplink and downlink RoF architecture using up- and down-conversion
technique. (a) Electrical conversion, (b) optical conversion.
Up- and down-conversion is the technique where the IF
signals is transmitted over optical fiber instead of RF signal.
The transmitted of IF-band optical signal is almost free from
the fiber dispersion effect. There are two techniques either in
electrical conversion or optical conversion domain. Figure 4
shows the difference of these two techniques. However, they
are required additional cost to the BS.
IV. UPCONVERSION TECHNIQUE
Presently, the highest modulation bandwidth for a
semiconductors laser reported is around 30 GHz and as a
result direct modulation of the laser beyond this limit is not yet
practical. The bandwidths of most commercially available,
packaged lasers are even lower. Although external modulators
can be used and fast external modulators with a 3dB electrical
bandwidth of 75 GHz have been demonstrated in research
laboratories, they usually require excessive RF drive power
(+18 to +24 dBm) to operate. Fiber dispersion further limits
the link performance at such high frequencies. If only the low
frequency IF input signal needs to be transmitted over the
Fig. 6. Frequency spectrum across the output of the optical RF converter
GaAs MSM-PD and MESFET preamplifier have been
demonstrated a monolithic integrated optoelectronic mixing
receiver circuit where the measured conversion loss is 14 dB
and signal-to-noise ratios are 43 dB and 49 dB at dc bias of 0V
and -10 V, respectively [28].
The InAlAs/InGaAs metamorphic HEMT on GaAs
substrate have been demonstrate as a 60 GHz harmonic
optoelectronic upconversion and at optimum bias conditions
make it possible to enhance desire harmonic optoelectronic
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mixing component and are believe can be useful in fiber optic
mm wave transmission system [29]. Based on the
photodetection mechanism in the InP HEMT, internal
conversion gain of 18 dB was obtained and can be useful in
simplifying BS architecture in 60 GHz RoF system [30, 31].
The upconverted signal at 30 GHz band of an InP/InGaAs
three terminal HPT shown that a detected power level of -37.6
dBm with conversion loss of 8.8 dB [32]. InP HPT can be use
for mm wave band bi-directional RoF system and it provides
not only optoelectronic mixing function but also the
possibility of one chip integration with other circuitries, which
can greatly simplify BS architecture [33, 34].
The HBT has high conversion efficiency, an unlimited LO
frequency as long as high-speed photodiode is available and
wide wavelength range. The HBT up-converter again reduces
the complexity of an optical receiver at the base station by
simultaneously performing photodetection, amplification and
mixing in the single device, which would otherwise be carried
out by separate components [35-37].
V. CONCLUSION
Several system architecture of mm wave RoF; external
modulation, optical heterodyning, optical transceiver and upand down-conversion are discussed. Fiber dispersion are limits
the link performance at such high frequencies. The previous
work shows that PPA, HBT and HPT can be performing as a
photodetector, optoelectronic mixer and preamplifier
simultaneously. Those methods provide IF frequency
conversion for RoF system that enable for development of the
photodetection with photonics frequency upconversion for
generation of RF up to mm wave band
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Universiti Teknologi Malaysia. He is also actively involved in various
committees at National level, which include Chairman of the Technical
Evaluation Committee for Photonics Technology, Ministry of Science
Technology and Innovation. He is also the member of the evaluation
committee for Prioritized (PR) and Strategic Research (SR) on Planar Optical
Waveguides and Optical Crystal. He was also an EXCO member in the
National Photonics Project (NPP) and the head for the photonics switch
research group within NPP. He has conducted research in various field of
electrical engineering especially in telecommunications. He is an author for
more than 50 technical papers published in journals and submitted at
international and national conferences. He owns patent for four products. His
field of research includes MEMS, photonic communications system design,
photonics switching and photonics devices
VII. BIOGRAPHIES
Hilman Harun was born in Kelantan,
Malaysia, on October 26, 1983. He graduated
(2006) and now studied Master in Universiti
Teknologi Malaysia, Malaysia.
All his research has been conducted in
Photonics Technology Centre, Faculty of
Electrical Engineering, Universiti Teknologi
Malaysia since December 2006 until now
(October 2007). Radio over Fiber with
heterodyning technique is his specialty.
Dr Sevia M. Idrus is the Deputy Director of
the Photonics Technology Centre, Universiti
Teknologi Malaysia. She received B. Eng
degree in Electrical and Master in
Engineering both from the Universiti
Teknologi Malaysia. She obtained her PhD
in Optical Communication System from
University Of Warwick in 2004. She served
the Universiti Teknologi Malaysia since
1998 both as the academic and
administrative staff. Her main research areas
are the characterization, modeling, and
design of optoelectronic devices, radio over
fiber system, optical transceiver design and
optical wireless communication technology.
ABU BAKAR BIN MOHAMMAD was born
in Kuala Lumpur in 1960. He enrolled at
Universiti Teknologi Malaysia (UTM) and
graduated successfully with a Diploma in
Electrical
Engineering
majoring
in
Telecommunication in 1981. After acquiring
substantial industrial exposure at Intel, he
decided to join UTM as an academic staff in
1982. He obtained his Bachelor's degree from
University of Strathclyde, Scotland in 1985,
Master's degree from Hatfield Polytechnic
(now known as Hertfordshire University) and
Ph.D. from Bradford University in 1995.
Presently he is a Professor and Director of the
Photonics
Technology
Centre
(PTC),
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