Radio-over-fiber networks
Radio-over-fiber networks
• RoF networks
– Optical fiber is medium of choice in wide, metro, access, and
local area (wired) networks
– PONs might be viewed as final frontier of optical wired
networks interfacing with a number of wireless technologies
– One interesting approach to integrate optical fiber
networks & wireless networks are so-called radio-over-fiber
(RoF) networks
– In RoF networks, radiofrequencies (RFs) are carried over
optical fiber links to support various wireless applications
Radio-over-fiber networks
• Fiber-optic microcellular radio
– To increase frequency reuse & thereby support growing
number of mobile users in cellular radio networks, cells may
be subdivided into smaller units called microcells
– Beside increased capacity, microcells also reduce power
consumption & size of handset devices
– Distributed antenna system connected to base station via
optical fibers avoids base station antenna with high-power
radiation => fiber optic microcellular radio system
• Radio signals in each microcell are transmitted & received to &
from mobile users by using a separate small canister attached
to base station via optical fiber
• Each canister is equipped with optical-to-RF & RF-to-optical
converters, laser, and optical receiver
• Subcarrier multiplexed radio signals directly modulate laser
• Radio signals are recovered from optical signal by means of
direct detection
Radio-over-fiber networks
• Fiber-optic microcellular radio
Radio-over-fiber networks
• Dynamic channel assignment
– Spectrum delivery scheme (SDS) is a centralized dynamic
channel assignment applied at central station
– SDS dynamically assigns one or more subcarriers to any base
station according to current traffic demands
– SDS helps improve flexibility of fiber optic microcellular
radio networks by assigning more subcarriers to heavily
loaded base stations & fewer subcarriers to lightly loaded
base stations
– As a result, SDS effectively reduces call blocking
probability in fiber optic microcellular radio networks whose
traffic loads vary over time
Radio-over-fiber networks
• Remote modulation
– Remote modulation avoids equipping each radio port with a
laser & associated circuit to control laser parameters such
as temperature, output power, and linearity
– Remote modulation allows design of cost-effective radio
port architecture for fiber optic microcellular radio
networks using a single high-power laser at base station that
is shared among many microcells
Radio-over-fiber networks
• Remote
modulation
Radio-over-fiber networks
• Radio-over-SMF networks
– Apart from microcellular radio signals, optical fibers can be
used to support wide variety of other radio signals
– RoF networks provide transparency against modulation
techniques & support various digital formats and wireless
standards in cost-effective manner
– Experimental demonstration of RoF network able to
simultaneously transmit following four wireless standards in
downstream direction using a single antenna
• WCDMA
• IEEE 802.11 WLAN
• PHS
• GSM
– Electroabsorption modulator (EAM) based method used to
combine various radio signals onto common single-mode fiber
(SMF) => radio-over-SMF networks
Radio-over-fiber networks
• Radio-over-SMF networks
Radio-over-fiber networks
• Radio-over-MMF networks
– Many buildings have preinstalled multimode fiber (MMF)
cables rather than SMF links => radio-over-MMF networks
– Cost-effective MMF-based networks can be realized by
deploying low-cost vertical-cavity surface-emitting lasers
(VCSELs) operating in 850-nm transmission window
– Experimental demonstration of indoor radio-over-MMF
networks using different kinds of MMF in conjunction with
commercial off-the-shelf (COTS) components for in-building
coverage of following four wireless standards
• GSM
• UMTS
• IEEE 802.11 WLAN
• DECT PRS
Radio-over-fiber networks
• WDM RoF networks
– Introduction of wavelength dimension not only increases
capacity of WDM RoF networks but also increases number
of base stations serviced by a single central station
– Experimental demonstration of WDM RoF ring network
based on ROADMs
• WDM fiber loop connects multiple remote nodes with central
office
• Each remote node deploys array of tunable FBGs
• A remote node is able to locally drop one or more wavelengths
by tuning its FBGs accordingly
• Several so-called radio access units (RAUs) are attached to
each remote node
• Each RAU may serve one or more mobile users
• ROADMs used at remote nodes allow add-drop wavelengths to
be dynamically assigned to remote nodes & attached RAUs in
response to given traffic loads
Radio-over-fiber networks
• RoF & FTTH networks
– Future multiservice access networks can be realized by
integrating RoF systems with existing optical access
networks, (e.g., FTTH networks)
– To achieve this, both wireless RF & wired-line (FTTH)
baseband signals should be simultaneously modulated &
transmitted on a single wavelength over a single fiber
Radio-over-fiber networks
• RoF & WDM PON networks
– Given that WDM PONs become rapidly mature, it is
desirable to integrate WDM PONs with RoF systems
– Experimental demonstration of seamless integration of eight
2.5 Gb/s WDM signals with RoF system
• Simultaneous frequency upconversion of the eight WDM signals
was done all-optically by means of FWM
• FWM is independent of signal bit rate & modulation format
=> FWM can be used for simultaneous frequency upconversion
of different optical WDM signals
Radio-over-fiber networks
• RoF & rail track networks
– Fast-moving users (e.g., train passengers) suffer from
frequent hand-overs in cellular networks => numerous packet
losses & significantly decreased network throughput
– So-called moving cell concept solves this problem
• RoF network is installed along rail tracks
• High-capacity wireless services provided to high-speed-train
passengers by using hierarchical approach
– Wireless link between railway & train using RoF network
– Separate wireless link between train & users deploying one
or more WLAN access points in each train carriage
• Concept of moving cells lets cell pattern move together with
passing train => train communicates on same RFs during whole
connection without requiring hand-overs
• Moving cells implemented at central station
• Central station is able to track location of train based on
received upstream RF signals
Radio-over-fiber networks
• RoF & rail track networks