BbRof

 

Broadband Radio-over-Fibre
(BRoF)

C.P. Liu, L.A. Johansson

Partners
University of Kent, University of Canterbury

Funding
Nortel Networks, BT Adastral Park, EPSRC

 

Context

The project aims at developing low cost technologies for remote optical delivery of millimetre-wave radio. Millimetre-waves can provide the required spectral bandwidth for broadband wireless applications such as high-definition video. As a consequence of the transmission characteristics of millimetre-waves (i.e. limited propagation range and line-of-sight transmission), a large number of antenna units are required to cover a given area. Furthermore, well-defined cell boundaries with a high degree of frequency reuse are possible.

This requires cost and complexity reduction of the antenna units. Fibre-radio allows for the relocation of a range of functions including modulation/demodulation and frequency allocation, away from the antenna units to a more conveniently located central station, thereby allowing the sharing of equipment and the implementation of simple, robust and inexpensive antenna units.

 

Results

We implemented an Optical Injection Phase-Lock Loop (OIPLL) for generating single sideband millimetre-wave modulated optical signals using only standard off-the-shelf optical and electrical components. This method is based on optical heterodyning, a combination of Optical Injection Locking (OIL) and Optical Phase-Lock Loop (OPLL) techniques.

A 36 GHz signal is produced with high modulation depth, thereby providing a very efficient signal generation. With the combination of OIL and OPLL, the difficulty of designing a wideband OPLL to get wideband phase noise suppression has been overcome, a phase noise level of <-90 dBc/Hz has been achieved at 10 kHz offset with a narrowband phase lock loop. Also the limited locking range of OIL techniques has been overcome, a locking range > 30 GHz has been achieved with the OIPLL, compared to a locking range < 3 GHz provided by OIL only.

These results confirm that the OIPLL delivers adequate signal purity and stability for millimetre-wave over fibre applications. Furthermore, The OIPLL uses two commercially available Distributed FeedBack (DFB) lasers, without the need for precision temperature control. The fact that the OIPLL can be constructed from established technologies ensures that the OIPLL is cost effective.

The OIPLL has been used for two transmission experiments.

The first experiment was based on external modulation at the output of the OIPLL. An electro-absorption modulator was used to switch the optical signal on and off at data baseband, thus generating a 140 Mbit/s ASK modulated 36 GHz signal. Error-free (BER<10-10) fibre-radio transmission was demonstrated over 12.6 km of fibre with a dispersion induced penalty < 1 dB.

The second experiment used a modulated microwave reference for the locking of the two lasers. With this method, no external optical modulated needed to be used, thereby eliminating the modulator insertion loss and minimising the required number of optical components. Moreover, error free (BER<10-9) fibre-radio transmission of a 68 Mbit/s modulated 36 GHz signal was demonstrated over 12.6 km and 25 km of fibre with a dispersion induced penalty < 0.5 dB.