Novel cost-efficient techniques for microwave signal delivery in fibre-wireless networks

Abstract

The Optical Frequency Multiplying technique feeds very pure microwave signals to simple radio antenna stations through single-mode and multimode fibre networks. It supports operation at multiple radio standards, and dynamic capacity allocation by optical routing. Introduction Wireless communication services are steadily increasing their share of the telecommunication market. Next to their prime feature, mobility, they are offering growing bandwidths to the end users. This entails also an increase of the radio carrier frequencies, which leads to smaller radio cell coverage due to the increased propagation losses and line-of-sight needs. Wireless LANs in the 2.4 GHz range according to the IEEE 802.11b standard carry up to 11 Mbit/s, evolving up to 54 Mbit/s in the IEEE 802.11g standard. The IEEE 802.11a and the HIPERLAN/2 standard provide up to 54 Mbit/s in the 5.4 GHz range. Research is ongoing in systems that may deliver more than 100 Mbit/s in the radio frequency range well above 10 GHz (e.g., LMDS at 28 GHz, HyperAccess at 17 GHz and 42 GHz, MVDS at 40 GHz, MBS at 60 GHz, etc.). Due to the shrinkage of radio cells at higher radio frequencies, ever more antenna sites are needed to cover a certain area such as the rooms in an office building, in a hospital, the departure lounges of an airport, etc. Thus, it becomes increasingly important to simplify the antenna stations and to consolidate the signal processing in a centralised site. Carrying radio signals over fibre is an interesting solution to achieve this; it exploits the basic virtues of optical fibre, its large bandwidth and low loss, in supporting cheap ubiquitous broadband mobility. Single-mode fibre has adequate characteristics for efficient distribution of microwave signals, e.g. by means of optical heterodyning to remotely generate microwave carriers [1]. However, its tiny core necessitates delicate handling in installation, requiring highly skilled personnel, which results in relatively high installation costs. In highly cost-sensitive areas, such as in-building networks, multimode fibre is an interesting alternative. Its large core facilitates splicing, easier light injection at the source, and avoids non-linearities due to reduced light intensity. Polymer multimode optical fibre (POF) is even more easy to install, due to its flexibility and ductility; it allows e.g. connectorisation by just cramping a metal ferrule on the fibre, without cracks as would occur with silica fibre. Moreover, multimode fibre is already widely accepted for short-range data communications in broadband LANs, benefiting from low-cost transceiver modules and the installation easiness. Several Ethernet standards have been established using multimode fibre: 100 Mbit/s Fast Ethernet IEEE 802.3u standard 100BASE-SX for up to 2 km multimode fibre at 850 nm wavelength, and 100BASE-FX up to 2 km at 1310 nm; Gigabit Ethernet (line rate 1.25 Gbit/s) IEEE 802.3z standard 1000BASE-SX up to 550 m multimode silica fibre at 850 nm, and 1000BASE-LX up to 550 m at 1310 nm; even 10 Gigabit Ethernet (line rate 10.31 Gbit/s) up to 300 m multimode fibre, at 850 nm wavelength. Striving for convergence of in-building networks for reasons of service integration, upgradability, and economy of installation and maintenance, an attractive scenario would be to build radio-over-fibre systems on top of (already installed) multimode fibre data networks such as the Ethernet-based ones mentioned above. Such a multimode fibre-based integrated-services in-building network is exemplified in Fig. 1. Coaxial cable network FD MD POF Twisted pair network