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Recent Patents on Electrical Engineering 2011, 4, 114-124
Radio-over-Fiber Systems for Wireless Communications Alejandro Aragón-Zavala1,*, Gerardo Castañón2 and Joaquín Beas3 1
Department of Electronics and Mechatronics, Tecnológico de Monterrey, Campus Querétaro, Epigmenio González 500, Fracc. San Pablo, Querétaro, Qro, CP 76130 México, , 2Department of Electrical and Computer Engineering, Tecnológico de Monterrey, Monterrey, Mexico, 3Motorola, Research and Innovation Technology Park, Monterrey, Mexico Received: September 14, 2010; Accepted: December 8, 2010; Revised: January 15, 2011
Abstract: In recent years, considerable attention has been devoted to the merging of Radio over Fiber (RoF) technologies with millimeter-wave-band signal distribution. This sort of systems have great potential to support secure, cost-effective, and high-capacity vehicular/mobile/wireless access for the future provisioning of broadband, interactive, and multimedia services over wireless media. Current trends in cellular networks are (1) reduction of the cell size to accommodate more users and (2) operation in the microwave/millimeter-wave (mm-wave, 26-100 GHz) frequency bands to avoid spectral congestion in lower frequency bands (2.4 or 5 GHz). The larger radio frequency (RF) propagation losses at mm-wave bands reduce the cell size covered by a single base station (BS) and allow an increased frequency reuse factor to improve the spectrum utilization efficiency. On the other hand, this type of network demands a large number of BSs to cover a service area, and cost effective BSs are the key to success in the market place. Thus, a stable and cost effective BSs should be designed with simple and effective architecture. In general, there are three possible techniques to transport the mmwave wireless signals over the optical fiber 1) RF-over-fiber, 2) Intermediate Frequency (IF)-over-fiber, and 3) Base band-over-fiber. Amongst the schemes, the RF-over-fiber transport scheme has the potential to simplify BS design. However, one of its major drawbacks is the requirement for high-speed optical components and modulation and detection techniques. This requirement has led to the development of system architectures where functions such as signal routing/processing, handover, and frequency allocation are carried out at a centralized station (CS), rather than at the BS. This paper reviews extensively recent patents related to RoF communication systems for wireless communications. It is expected that with this exhaustive review, many researchers and developers will be encouraged to investigate even further and develop newer topologies and systems for the use of RoF for broadband radio, expanding this knowledge in the advent of newer services and applications likely to be deployed in the near future.
Keywords: Millimeter-wave-band signal, radio over fiber, wireless communication. 1. INTRODUCTION During the last couple of decades, the personal communications industry has faced an impressive growth in the number of subscribers worldwide, which make use of the different services and applications offered by service providers. In the early days of mobile radio, only voice was demanded, with small amounts of data, which could be sent over the communications link. This requirement has evolved over the years, as large volumes of information need to be sent from source to destination, even with the additional requirement of on-line connectivity. The need for efficient transport systems, which could convey such bandwidth requirements, encouraged the use of highly spectral-efficient transmission media, such as optical fibers. The use of optical fiber in communication systems brought the benefits of low loss, high bandwidth capability, and immunity to ElectroMagnetic Interference (EMI) and capacity of transporting many channels simultaneously through the same fiber, *Address correspondence to this author at the Department of Electronics and Mechatronics, Tecnológico de Monterrey, Campus Querétaro, Epigmenio González 500, Fracc. San Pablo, Querétaro, Qro, México, CP 76130, Tel.+ 52 442 238 3281; Fax: +52 442 238 3278; E-mail:
[email protected]
1874-4761/11 $100.00+.00
Fig. (1). RoF system based in a central station architecture and base stations.
Wavelength Division Multiplexing (WDM) [1, 2] Thus, optical fibers have replaced copper wire communications in core networks in many countries around the world. Despite the various benefits of the use of optical fibers in communication systems, there was still a need to facilitate mobile wireless access especially for large building areas, which could potentially have high losses if copper cables were used. RoF was created for this purpose, and it refers to © 2011 Bentham Science Publishers
Radio-over-Fiber Systems for Wireless Communications
a technology whereby a radio signal is used to modulate light transmitted through an optical fiber for efficient signal transmission. Although, the technology is used for Cable Television (CATV) as well as in satellite base stations, the term RoF often refers to the use of the technology for wireless access. RoF communication systems have several advantages over conventional wireless systems as follows: •
The transport of RF signals by the use of optical fibers reduces the loss attenuation, hence reducing the use of repeaters.
•
The architecture of a RoF system is often simpler and less expensive since it makes use of the concept of a CS, dealing with signal generation and resource management; and remote BS can be deployed wherever they are required for wireless access, consisting only of an optical-to-electrical converter, amplifiers and antennas.
•
Expandability can be easily achieved with the use of RoF systems since BS can be deployed to extend the network at a low cost and complexity, providing additional flexibility.
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Low cost infrastructure, easy maintenance and low energy consumption.
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Higher data rates to accommodate future service demands can be transported through RoF systems (larger bandwidth).
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Multi-operator and multi-service operation, whereas the same RoF network can be used to distribute traffic from many operators and services, resulting in enormous economic savings.
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Dynamic resource allocation, since functions such as switching, modulation and others are performed at a CS, being able to allocate capacity dynamically.
Amongst the applications for which RoF have been used throughout the years, the following examples are given: •
Elimination of dead zones, having lack of wireless coverage in areas for which the signal could not penetrate or are difficult to reach (tunnels, secluded places, etc.). This can be seen as a repeater based on optical transportation system.
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Fiber-to-The-Antenna (FTTA) systems, using fiber optics for the link from the BS to the antenna, replacing the use of copper-wire links.
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Capacity enhancement for cellular micro cell and picocell (in-building) systems, as capacity can be expanded by extending the coverage and reducing cell size at a minimum cost.
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Distribution of wireless signals for both mobile and data communication systems inside buildings.
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Fixed Wireless Access Systems (FWA), where RoF may be used to optically transport signals over long distances bringing the remote stations (RS) closer to the end user, achieving broadband first/last mile access in a costeffective way.
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Since RoF involves the use of analogue modulation and detection of light, signal impairments such as noise and distortion can affect the quality of the RoF link, especially by limiting its noise figure and dynamic range. RoF systems are the subject of many research and development activities, as well as are part of many commercial products which are currently in use; e.g. active distributed antenna systems. This paper is organized as follows: Section II describes the RoF system architecture, section III, IV and V present an overview of the most recent developments of RoF systems especially focusing on the patents published for Central Station (CS), Transport Media and Base Station (BS), respectively. Finally, current and future developments are presented in section VI. We consider that this exhaustive review will greatly contribute to researchers and developers interested in the next generation of wireless communication systems to visualize technology opportunities for future RoF systems. 2. RADIO OVER FIBER SYSTEM ARCHITECTURE The concept of a RoF system is comprised of many elements, as shown in Fig. (1). A brief description of each element is given as follows. There is a CS that contains all the data resources, transmitters, receivers, interconnection with the Public Switched Telephone Network (PSTN), Internet and switching capacity of the network. The CS uses the transport fiber optics network to communicate with the BSs. Functions such as signal routing/processing, handover and frequency allocation are carrier out at the CS, rather than at the BS. Furthermore, such a centralized configuration allows sesitive equipment to be located in a safer enviroment and enables the cost of expensive components to be shared among several BSs. The larger RF propagation losses at mm-wave bands reduce the cell size covered by a single BS and allow an increased frequency reuse factor to improve the spectrum utilization efficiency. Without proper security and reliability mechanisms [3, 4], networks will be confined to limited, controlled environments, not fulfilling much of the promise they hold. The limited ability of vehicular/mobile/wireless/ computer networks to thwart failure or attacks makes ensuring network availability more important and difficult. Consideration of security, reliability, and availability at the design stage is the best way to ensure successful network deployment. On the other hand, this type of networks demands a large number of BSs, as depicted in Fig. (2), to cover a service area, and costeffective BSs are key to success in the market. An attractive alternative for linking a CS with several BSs in such radio network is via an optical fiber network, since fiber has low loss, is immune to EMI and has broad bandwidth. As shown in Fig. (2), fiber network topologies can be used for RoF services as well as for fiber to the home and fiber to the building services. Due to the vast bandwidth of the fiber, both wireless and wired services in a single fiber access network can coexist. However, this will require network planning to distribute the bandwidth for both services according to the traffic and demand requirements. The transmission of radio signals over fiber, with simple optical to electrical conversion, followed by radiation at remote antennas, which are connected to a CS, has been
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Fig. (2). The envisioned RoF network in urban areas. RoF for vehicular, mobile, broadband wireless computer communications, and also network infrastructure can be used for fiber to the building and fiber to the home services.
proposed as a method for minimizing costs. This cost reduction can be brought about due to the following three advantages. Firstly, the remote antenna BS or radio distribution point needs to perform only fairly simple functions, having small size and low in cost. Secondly, these simple antennas can be installed on the traffic lights or on electric poles avoiding the expensive right-of-way costs of private owners of buildings where nowadays antennas need to be installed. Thirdly, the resources provided by the CS can be shared among many antenna BSs. Another clear advantage in the use of small cells is the increase in battery lifetime of mobile/wireless terminals, due to the low RF transmit power at the BSs. 3. CENTRALIZED STATION (CS) 3.1. High Speed Lasers for Direct Modulation Intensity Modulation - Direct Detection (IM-DD) is a conventional direct modulation on a semiconductor laser to transport the RF signal over the optical fiber as depicted on Fig. (3). However, the direct modulation bandwidth of a semicondutor laser is limited and, since most of the mmwave applications require a narrow bandwidth centered at a high frequency, several methods have been proposed and demonstrated to exceed this limitation. One of this methods is the active mode-locking of the semiconductor laser coupled to an external cavity to create a resonantly enhanced transmission window at the cavity round-trip frequency. This frequency lies in the mm-wave range. Thus, by cleaving the laser to the appropriate length, transmission of signals at frequencies up to 100GHz is possible [5]. Using resonant
Fig. (3). High speed lasers for direct modulation.
modulation of a conventional (direct modulation bandwidth
