Radio-over-Fibre (RoF) is an optical communication technique based on the transmission of standard wireless radio signals though optical fibre in their native format. This technique is an enabling step in the deployment of dense fourth generation (4G) cellular and pico-cellular wireless networks. The optical fibre provides a huge bandwidth that can support a variety of wireless systems, regardless of their frequency bands, being protocol-transparent which is reflected in an great network flexibility. Radio-over-fibre techniques enables a high user capacity by frequency reuse, simplifies the network operation as the signals are distribute in their native format, and permits to transfer signal part of the processing power from the base station units to the central control station, thus reducing the overall deployment cost and complexity. The principles of radio-over-fibre are presented in this chapter, including the key transmission impairments and the expected performance. The main application scenarios are discussed. These include the backhaul of 4G or base-stations, addressing 4G and 3G compatibility issues, and distributed-antenna system (DAS). Finally, emerging applications like radio-over-fibre in beyond-3G scenarios and transmission of 60 GHz wireless are also described in this chapter.
TopIntroduction
The huge number of subscribers to the latest media services with multimedia contents increased considerably the demand of broadband communication systems. These users must be able to access multimedia contents at any time and in any place. This bring out the paradigm to achieve high-capacity services always available for the users and pushed for the development of new communication systems using both wireline and wireless technologies (Stuckmann & Zimmermann, 2007). The main benefits of wireless services rely on its intrinsic mobility and fast deployment. For these reasons, wireless architectures have experienced a great development in the last years. But the radio spectrum scarcity has pushed the industry and operators to look for efficient ways of transporting and distributing the radio signals to remote locations, avoiding the use of high-frequency equipment which increases considerably the cost of the operator’s architecture.
Radio-over-fibre (RoF) systems, eventually called microwave-photonics systems (Seeds & Williams, 2006; Capmany & Novak, 2007), offer a cost-effective solution when a dense distribution of the wireless signal is required, like in the cellular coverage of urban areas or when antenna clusters are employed. Using RoF technology, network architectures can be implemented in different scenarios. A description of different applications is shown in Figure 1. The first application scenario consists of distributing wireless services for the operator from the central office (CO) to several base stations (BS), as shown in Figure 1(a) called RoF transport. The second application scenario described in Figure 1(b) is used to interconnect several base stations and provide 4G services to the end costumer, i.e. last-mile access and femtocell networks. Radio-over-fibre provides the benefits of scalability and transparency to the radio service in these applications.
Figure 1. Application scenarios of Radio-over-Fibre (RoF) systems for 4G distribution: (a) RoF for transport and (b) RoF for BS interconnection
TopBackground
Fourth generation (4G) mobile broadband networks employing technologies like WiMAX, LTE (Long Term Evolution), and IMS (IP Multimedia Subsystem) transport high data rate signals to support high quality communication services. Pervasive presence of fourth-generation (4G) wireless requires an efficient, flexible and not bandwidth-limited backhaul. RoF optical technology addresses this need, being one of key enabling technologies for deployment of 4G networks.
In the last years, the migration to IP services has been increased considerable, i.e. WiMax and LTE are all IP, CDMA is migrating, and UMTS 3G is moving to IP via ATM and Ethernet. Ethernet is still the most successful and widely deployed local area network transport technology in the world due to its flexibility and cost-effective networks. On the other hand, wireless networks reduce significantly the backhaul costs. Optical Metro-Ethernet networks are an interesting approach to support 4G bandwidth needs. Metro networks transport digital data, which requires specific modulation/demodulation equipment to convert the digital data to a wireless signal and vice-versa. Nevertheless, RoF transports analog data, i.e. the wireless signal is transported in its final format. This approach is enabled by optics: An extremely large bandwidth is available in the fiber and electromagnetic compatibility is intrinsic to the transmission media. RoF transmission permits flexible, reconfigurable systems with centralized maintenance capable to serve a large number of users. By example, a RoF system was deployed in Sydney Olympics comprising more than 500 remote antenna units to provide multi-standard radio as GSM in 900/1800 MHz bands, 802.11 WLAN, CDMA2000 services demonstrating the RoF reliability and capacity, Henry, P.S. (2007). This system enabled dynamic allocation of the network capacity, providing service to more than 500,000 wireless calls on the opening day of the Olympic Games.