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Best of both worlds

Reducing cost and improving quality of service. By Eric Russell.

The convergence of rf transmission and fibre optics has created RF over fibre (RoF), a service that may well reduce the costs of mobile telephony systems and improve the quality of service.

RoF uses fibre optic cable to carry rf signals. Fibre provides greater transmission bandwidth with less signal attenuation at rf, contains emi and rfi, is lighter and is unaffected by lightning. RF signals can also be superimposed on existing fibre systems.

By modulating the rf signal on a light wave, high fidelity transport of signals can be achieved over many kilometres without repeaters or optical amplifiers.

This benefits mobile telephony systems by enabling the electronics to be taken out of each radio base station and replaced by a single central transmitter, reducing equipment costs. This central station feeds rf over fibre to all its base stations, which then only require a basic duplex optical transceiver between antenna and fibre.

Costly functions – such as rf modulation and demodulation, frequency assignment, spectrum delivery switching and signal processing – are centralised; reducing equipment and maintenance costs.
Where fibre replaces existing copper links, bandwidth is increased. This means more subscribers can be connected through any given antenna at any one time. This will help mobile 'phone systems handle the extra traffic expected in the next few years.

In addition, compact local antennae can be placed near sources of high traffic and in previous dead spots, improving quality of service. These distributed antennae radiate low power and enable smaller and lower power handsets, which reduces the impact of the health issue on mobile 'phone usage. Localised antennae enable microcells – with radii of several hundred metres – and picocells, which are effective just within a building.
Away from mobile telephony, RoF can be used to drive leaky feeders – a type of coaxial cable that emits radiation along its length – often used for communications in railway tunnels and mines.

Nevertheless, RoF is not a new technology: its first generation appeared some ten years ago. There are a number of RoF installations around the UK: one of the largest is at the Bluewater shopping centre in Kent. Here, all four mobile 'phone service providers use common fibre optic cabling and antennae around the site. Interference is avoided because each company has its own frequency band.

However, 'near to production' research has ramped up in the last five years through a number of initiatives. This volume of research should mean more systems coming on line before long. Companies such as Vodafone and One2One are sponsoring further research into the technology with a view to incorporating it in future mobile 'phone systems.

The driver behind RoF is the need to take cost out of mobile communications systems. This will reduce 'phone prices, encourage greater use and increase the profitability of equipment manufacturers and service providers. Development of RoF has largely been a learning curve, establishing how to use fibre optic cable instead of copper coax or free air to carry radio frequency signals.

Professor Alwyn Seeds, head of the optoelectronics and networking group at University College London, says the university has probably the greatest number of live RoF research projects of any in the UK.

Amongst these, his group is looking – in collaboration with Nortel Networks and BT Exact, with EPSRC funding – at providing 40GHz broadband over fibre at 100Mbit/s. Nearer rollout is the Passive Integrated Picocells project that is developing a fibre to antenna module that draws its power from the energy in the light beam, meeting the HiperLAN specification for broadband radio transmission and wireless networking technology.

But closest to production is FRIDAY, the Fibre Radio for In building Distributed Antenna sYstems. This aims to reduce the cost of systems by using standard vcsels and multimode fibre. This is in collaboration with Agilent, Remec Airtouch and SpectraSite Transco, a provider of infrastructure solutions for the wireless communications industry in the UK.

Low cost components tend to be less linear and more noisy than high performance versions, resulting in intermodulation distortion of signals. But Prof Seeds is using a special modulation technology and launch to fibre conditions to overcome this.

If laser diodes are significantly non linear, they can cause mixing of the frequency division multiplexed subcarriers and create new frequencies, creating an additional noise source known as intermodulation distortion.
Low cost lasers also generate a high noise level and this constitutes a major source of interference and noise in radio over fibre systems. This noise is known to be signal level dependent and signal to noise ratio is a critical parameter in mobile telephony specifications.

Senior lecturer Dr Hamed Al-Raweshidy is in charge of RoF projects at the University of Kent. He is currently working with Vodafone and One2One on 4G mobile telephony, with 5G just over the horizon. He says his research shows an optical fibre microcellular system will outperform a full wireless system.

Dr Al-Raweshidy is also a director of IIT, a commercial company operating within the university which has developed a full simulation software package for RoF research. Dr Al-Raweshidy says the software is unique in that it combines both rf and fibre optic modelling capability. All others, he says, cover one sector or the other and their results need combining manually when investigating a RoF system.

Anacom Systems (www.anacominc.com) has developed an RF on Fibre system where the rf signal modulates the bias current of a laser diode directly. Internal optical feedback monitors the laser'scondition and adjusts the bias quiescent point for maximum dynamic range. The receiver is based on a high speed linear photo detector.

There are several sources of loss in fibre optic systems: in the fibre itself, at the connector, and at the splitter. Fibre has an rf loss of less than 1dBe/km; connectors typically have less than 1dB loss; and optical splitters have losses dependent on their configuration: a 1 x 2 optical splitter, for example, has an optical loss of 3.5dBo (7dBe rf). There is a two to one relationship between optical loss and electronic loss – 1dBo (dB optical) corresponds to 2dBe (dB electronic).

As with rf copper systems, reflections are an issue. Optical reflections back into the laser diode cause a disturbance in the laser's gain cavity creating noise and distortion.
The main sources of optical reflections are connector interfaces: angle polished connectors reduce the problem. The tip of the connector is polished at an angle of 8o, the optimum angle to minimise reflections and to direct them out of the fibre.

Pacer (www.pacer.co.uk) offers a number of devices for RoF, including Mitsubishi's FU-68SDF-V802MxxB, a 1550nm DFB-LD module with single mode fibre pigtail suited for dwdm systems. The 6mW maximum optical output, directly modulated, light source is for use in 2.5Gbit/s digital optical communications systems. Pacer says it will drive fibre up to 200km. The module includes an optical isolator, thermal electric cooler and a photodiode optical output monitor.

Distributed feedback lasers are injection laser diodes with a Bragg reflection grating in the active region. This suppresses multiple longitudinal modes and enhances a single longitudinal mode.

Author
Graham Pitcher

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