What Is Fiber-wireless?

Fiber-wireless is a transmission approach that conveys radio frequency signals over optical fiber links to extend wireless coverage or increase wireless network capacity, then converts the optical signal back to radio for delivery through antennas. The core technique, known as radio over fiber (RoF), modulates a laser source with a radio frequency or microwave signal, transmits the modulated light through a single-mode fiber, and recovers the original RF signal with a photodetector at a remote antenna site. By shifting the complexity of signal processing away from the antenna unit to a central station, fiber-wireless systems make it practical to deploy many geographically dispersed antenna points from a single location.

The concept emerged from research in analog optical links during the 1980s and advanced rapidly with improvements in directly modulated and externally modulated laser sources. It sits at the intersection of microwave photonics, optical communications, and wireless systems engineering.

Radio Over Fiber Architecture

In a radio over fiber system, a central station generates and processes the radio frequency signal in the electrical domain, modulates it onto an optical carrier, and distributes it over fiber to one or more remote antenna units (RAUs). The RAUs perform optical-to-electrical conversion, amplify the signal, and feed it to an antenna for over-the-air transmission. Uplink signals captured by the antenna travel the reverse path back to the central station. Because the RAU requires only a photodetector, RF amplifier, and antenna, deployment costs fall substantially compared with architectures that place full baseband processing at each antenna site. The RP Photonics encyclopedia entry on radio and microwave over fiber describes the signal chain and the optical modulation methods commonly used.

Transmission Characteristics

The principal advantages of the fiber-wireless approach are the low attenuation of single-mode fiber (approximately 0.2 dB/km at 1,550 nm), the large instantaneous bandwidth that optical links offer, and the inherent immunity of the fiber path to electromagnetic interference. These properties allow a central station to serve antenna units located several kilometers away without the cable losses that coaxial distribution systems would incur at the same frequencies. Millimeter-wave bands above 30 GHz, where coaxial cable loss is prohibitive, are particularly well suited to fiber-based distribution. IEEE Xplore publications on radio-over-fiber system design document the tradeoffs between analog and digitized fronthaul variants and their respective effects on signal fidelity and capacity.

Nonlinear distortion from the laser modulation characteristic and laser relative intensity noise are the primary impairments in analog RoF links. Operating the laser at low modulation depth reduces distortion but also reduces link gain, so system designers must balance these competing constraints. Digital RoF, in which the signal is digitized before optical transport, avoids analog impairments at the cost of much higher optical bandwidth per channel.

Analog and Digital Variants

Analog fiber-wireless transports a continuous radio waveform on the optical carrier without prior digitization. It preserves all waveform characteristics and requires no analog-to-digital conversion at the RAU, keeping the remote site simple. Digital fiber-wireless, sometimes called digitized or compressed RoF, samples the signal at the central station and transmits a serial digital stream. The De Gruyter review of RoF-based integrated optical-wireless networks surveys how these two approaches are being adapted for 4G LTE and 5G deployments.

Applications

Fiber-wireless has applications in a wide range of disciplines, including:

• Mobile network fronthaul linking baseband units to remote radio heads in 4G and 5G networks
• Distributed antenna systems for in-building wireless coverage in airports, stadiums, and hospitals
• Microwave backhaul for cellular base stations in dense urban deployments
• Cable television hybrid fiber-coax networks distributing broadcast and broadband signals
• Defense and radar systems requiring remote antenna placement with centralized processing