Radio-over-fiber networks

1. Radio-over-fiber networks

2. 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

3. 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

4. Radio-over-fiber networks • Fiber-optic microcellular radio

5. 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

6. 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

7. Radio-over-fiber networks • Remote modulation

8. 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

9. Radio-over-fiber networks • Radio-over-SMF networks

10. 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

11. 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

12. 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

13. 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

14. 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

15. Radio-over-fiber networks • RoF & rail track networks