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 reza.ab@ieee.org, razalin@fke.utm.my, v.vakilian@gmail.com, and tharek@fke.utm.my 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 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 1-4244-1435-0/07/$25.00©2007 IEEE 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 Role of ROF Technology In 4G The minimum bit rate that 4G would provide for broadband services is 20 Mb/s for indoor and Mb/s for outdoor application even with high relative mobility Figure 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 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 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 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] Some Challenges of ROF Technology Figure 3: The block diagram of remote base station (RBS) 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 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 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 References [1] Braun, R.P.; Grosskopf, G.; Rohde, D.”Optical millimeter-wave generation and transmission technologies for mobile communications, an overview”Microwave Systems Conference, 1995 Conference Proceedings., IEEE NTC '95 17-19 May 1995 Page(s):239 – 242 [2] Tang, P.K.; Ong, L.C.; Luo, B.; Alphones, A.; Fujise, M.; “Transmission of multiple wireless standards over a radio-over-fiber network” Microwave Symposium Digest, 2004 IEEE MTT-S International Volume 3, 6-11 June 2004 Page(s):2051 - 2054 Vol.3 [3] M Fujise, K Sato and H Harada, “New RoadVehicle 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technology in mobile communication Conclusion In this paper, the application of ROF technology in mobile communication is explained Recent finding and... 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... source in the RBS can be eliminated 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