Radio over Fiber Access Networks for Broadband Wireless Communications

Introduction

During the past two decades, the personal communications industry has faced an impressive growth in the number of subscribers worldwide, which make use of the different wireless access 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 voice oriented channel. 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 objective of mobile broadband wireless access, that aims to provide the above mentioned aspects, has been addressed by the IEEE standard (802.15, Wireless Personal Area Networks) (802.16) and by the ITU evolving standard International Mobile Telecommunication- Advanced (ITU-R). The need for efficient transport systems, which could support such bandwidth requirements, are encouraged to exploit the advantages of both, optical fibers and millimeter-wave (mm-wave) frequencies (Cochrane, 1989). Such systems that use an optical distribution network (ODN) for delivering mm-wave radio signals from a Central Station (CS) to many remote Base Stations (BSs) have long been recognized to allow the increase of user capacity, bandwidth, and mobility (Ogawa H., et al., 1992).

A typical RoF system is shown in Figure 1. There is a CS that contains all the data resources, optical transmitters (TX - lasers), optical receivers (RX – photodetectors), interconnection with the Trunk Network, Internet and switching capacity of the network. In the downlink direction the CS up-converts the electrical signal to optical frequency and uses the ODN to communicate with the BSs, in some cases using Remote Nodes (RN) where the optical signal is split or demultiplexed towards the corresponding BS which converts it back to electrical domain and radiates it to the Mobile Terminal (MT) end-user in mm-wave bands (Lim, et al., 2010). In the uplink direction, the BS receives the mm-wave signal from the MT, and depending on the configuration of the BS, this signal can be down-converted to baseband before modulating an electro-optical device to transmit the uplink information via the ODN back to the CS.

Figure 1.

General RoF system architecture, where central station (CS), optical distribution network (ODN), and base stations (BS) are presented

RoF communication systems have several advantages over conventional wireless systems, which are summarized as follows:

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  The distribution of mm-wave frequency signals by the use of optical fibers reduces the attenuation, hence reducing the use of repeaters (amplifiers). This is possible since optical fibers have almost negligible insertion loss for very long distances, something that cannot be achieved in conventional wireless links, where free space loss sets the minimum possible path loss in the link.

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  The architecture of an 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 BSs can be deployed wherever they are required for wireless access, consisting only of an optical-to-electrical converter, RF amplifiers and antennas.

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  Low cost expandability can be easily achieved using RoF, as it is virtually modulation format agnostic.

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  Higher data rates to accommodate future service demands can be transported through RoF systems (larger bandwidth). This is often a severe constraint in a wireless channel, where the use of sophisticated modulation and coding techniques are required to improve channel capacity, and even though, these data rates are no way comparable to those achieved in optical fiber links.

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  Multi-operator and multi-service operation is possible, whereas the same RoF network can be used to distribute traffic from many operators and services, resulting in substantial 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.