Signal Analysis Research (SAR) Group - RNet - Ryerson University

RADIO OVER MULTIMODE FIBER FOR WIRELESS ACCESS
Roland Yuen Xavier N. Fernando Sridhar Krishnan
Ryerson University, Toronto, Ontario, Canada
r yuenaee. ryerson. ca xavier Qieee. org krishnanQee. ryerson. ca
Abstract
A radio over fiber link is a promising technology for
antenna remoting applications. Typically, the radio
over fiber link employs a single mode fiber. But, the
signal power at the remote antenna is very small. The
main reason is large power loss in the E/O and O/E
convertor. But, the coupling efficiency of a E/O con-
vertor can be improved with multimode fiber (MMF),
so we propose to use a ROF link with a vertical-cavity
surface-emitting laser with a graded index MMF to
transport optical signals. A multimode fiber has a larger
core radius compared to a SMF. A larger core radius
allows more optical power coupled into a fiber. With
simple butt-coupling techniques, the coupling eficiency
can be 90% and simplicity leads to reduction in cost of
the link. Normally, the MMF is used in short distance
digital applications with a bandwidth distance product
of about 500 MHDkm, so it is good for local area pic-
ocells. Our approach as to transmit passband signals
such as QPSK and FSK through the ROF link. Our
simulation shows that a 900 MHz carrier can transport
through a link of 1.22 km long. In this paper, we in-
vestigate the feasibility of using a MMF for antenna
remoting in local area picocells and compare the trade-
08 between coupling efficiency and bandwidth.
Keywords: Radio over fiber; multimode fiber; remote
antenna; coupling eficiency.
1. INTRODUCTION
Radio over fiber (ROF) link is used in remote an-
tenna applications to distribute signals for microcell
or picocell base station (BS). In the remote antenna
application, the downlink RF signals are distributed
from a central base station (CBS) to many BS known
as radio access point (RAP) through fibers. The up-
link signals received at the RAPs are sent back to the
CBS for any signal processing. A RAP is much more
cost effective to deploy than a normal BS because it
is mostly consisted of simple devices, which includes a
E/O convertor, a O/E convertor, and an amplifier. The
cost of signal processing in a CBS is shared amounts of
CCECE 2004 - CCGEI 2004, Niagara Falls, May/mai 2004
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many RAPs. Additional to the lower cost advantage, a
smaller cell size coverage reduces the near fax effect and
relaxes the battery requirement on mobile receivers.
Although the fiber is a reliable medium with low
attenuation (0.5 dB/km at 1550 nm), challenge still
exists in large loss due to E/O and O/E conversion [l].
In this paper, we propose to employ multimode fiber
(MMF) to increase the coupling efficiency, which re-
duces the E/O conversion loss. However, MMF has
limited bandwidth largely due to modal dispersion.
In this paper, we will be discussing two topics. The
downlink architecture of the ROF link. The tradeoff
between power and bandwidth in remote antenna ap-
plication.
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2. THE RADIO OVER FIBER LINK
Figure 1: Radio over fiber link in remote antenna ap-
plication i
The radio over fiber (ROF) links in remote antenna
application is illustrated in Figure 1. The central base
station (CBS) and the radio access points (RAPs) are
connected through two fibers, which transport the up-
link and downlink signal. The RAPs act as remote
antenna that receives and transmits signals to mobile
users, whereas the CBS collects signals from the RAPs
for processing and distribute signals to all the RAPs.
The downlink of the ROF can be divided into an op-
tical channel and a wireless channel denoted by ROF
and Air respectively in Figure 2. When a signal s(t)
goes through the optical channel, it is attenuated by
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