**2. Radio-over-Fibre systems**

Radio-over-Fibre (RoF) technology [Seeds] offers a perspective solution to the demand for wireless connection to the costumer ("last or first mile problem"). It entails the use of optical fibre links to distribute RF signals from a central location to Remote Antenna Units (RAUs). It combines the properties of the microwave and photonics approaches. In narrowband communication systems and WLANs (wireless local area networks), RF signal processing functions such as frequency up-conversion, carrier modulation, and multiplexing, are performed at the radio base station. RoF makes possible to centralise the RF signal processing functions in one shared location, and then to use optical fibre, which offers low

Multi-Functional SOAs in Microwave Photonic Systems 87

The Point-to-point topology is the simplest method; it is a permanent link between two endpoints. In a star topology, each RAU is connected to a central unit with a point-to-point connection. The star topology is considered the easiest topology to design and implement. Adding additional nodes is simple, but the central unit represents a single point of failure. Fig. 2. represents the star topology of RoF system. Bi-directional data transmission can be achieved over duplex optical fibre. On the other hand the uplink and downlink can be separated by optical circulator. Anyhow, the RAU consist optical source for electrical-to-

RAU

Optical fiber

λ2 CW

**Laser Source1**

mm wave oscillator

**Laser Source2**

**Detector** Data output

CW

λ1 mod

**Modulator** mod Data input

Central Station

mod

Fig. 3. Point-to-point topology RoF system applying SOA transceiver and centralised laser

RAU

Fig. 2. Star topology, RoF system with traditional Electrical-to-Optical (E/O) and Optical-to-

For more simplified RAU structure, the laser source of uplink optical carrier can be moved to the central unit and the continuous wave (CW) optical signal is transmitted over the optical fibre to the RAU. So, there are no injection of light is added at the RAU for the uplink transmission. The optical carrier is attenuated; hence some amplification should be required, in order to arrive to the maximum number of users and larger distances. Bidirectional amplification is possible by EDFAs (erbium doped fiber amplifiers) but the price is high and an additional high pump power is required. In this way, SOAs are suitable to be positioned at the simplified RAU. They not only carry out the modulation and detection but also offer

O/E

Transmitter

Receiver

E/O

Downlink optical fiber

Downlink

Uplink optical fiber

λ2 mod

**SOA Transceiver**  uplink

downlink

λ2 CW λ1

Uplink Radio Access Unit

**2.1 Point-to-point and star topology** 

Central Unit

optical conversion.

Electrical (O/E) converters

an additional optical gain.

mm wave oscillator

**LPF**

sources

signal loss (0.3 dB/km for 1550 nm, and 0.5 dB/km for 1310 nm wavelengths) to distribute the RF signals to the RAUs. This way the RAUs are simplified significantly, as they only need to perform optoelectronic conversion and amplification functions. For broadband services the frequencies are in the millimetre wave range (like 60 GHz fibre radio link). Such a concept is based upon an optical link between the central station and the RAU in a picocellular structure [Ng'oma]. Some of the advantages and benefits of the RoF technology compared with electronic signal distribution are the following: low attenuation loss, large bandwidth, immunity to radio frequency interference, easy installation and maintenance, multi-operator and multi-service operation, and dynamic resource allocation. These benefits can translate into major system installation and operational savings, especially in widecoverage broadband wireless communication systems, where a high density of RAUs is necessary.

The SOA is a potential candidate for an electro-optic transceiver (transmitter and receiver) in a RoF network. The SOA operates as a modulator to add a new channel, as a detector to drop the needed channel and as an in-line amplifier to amplify the other channels, simultaneously. It realizes a compact, small size and cost-effective radio repeater for signal distribution. Fig.1. shows a simplified system setup representing the SOA based RAU. The output power of the laser source is modulated with the 1st information channel applying a SOA-modulator. The SOA transceiver detects the 1st channel and adds the 2nd channel in the 1st stage. In the 2nd stage the SOA transceiver adds the 3rd channel, but it can detect the 1st or 2nd channel selecting by tuneable filter. Finally the receiver side every information channels can be extracted. The information channels and the bias of the SOA are separated by bias tee circuit. The separation of add and drop channels can be achieved in the electrical regime by an electrical branching filter (1st stage) or an electrical circulator with bandpass filter (2nd stage). In the first case the realisation of a reconfigurable add/drop multiplexer is difficult, in the second case an electrical filter is needed also. In the drop branch a high power RF amplifier provides the suitable power at the antenna output, and a low noise amplifier is necessary in the add branch.

Other advanced devices like electro-absorption transceiver for signal remodulation or polarization rotation remodulator have been designed though only modulation is executed, whilst with semiconductor amplifiers it's possible to perform amplification, modulation and detection with the same optical device. The signal distribution of RoF systems can be realised by point-to-point, point-to-multipoint, bus, ring and open loop topologies.

Fig. 1. Simplified system setup with SOA transceivers
