Dispersion tolerant radio-over-fibre transmission of 16 and 64 QAM radio signals at 40 GHz

Introduction: Radio-over-fibre (RoF) techniques are attracting much attention from manufacturers and radio network operators for the deployment of flexible and cost-effective radio access systems. For the emerging broadband wireless standards, operating at carrier frequencies beyond 5 GHz, one of the main challenges of RoF techniques is the generation and remote delivery of microwave signals to the antenna stations, while maintaining the link simplicity. Heterodyning techniques have been proposed to generate microwave carriers alloptically, but they make use of several laser sources per link in complex optical injection locking schemes [1]. Techniques based on the harmonics generation by frequency modulation to intensity modulation (FM-IM) conversion in dispersive optical fibre links have the advantage of generating high microwave carriers with the use of one single laser source and low frequency electronics to produce the optical FM signal [2]. The FM-IM conversion can also be implemented with a periodic bandpass filter (e.g. a Mach-Zehnder interferometer) at the transmitter side to make the system independent of fibre length variations. This method has been mainly applied to 1310 nm systems with fibre lengths and RF carriers below 18 GHz [3], where the carrier suppression effects of chromatic dispersion in the direct transmission of microwave carriers are not an issue (considering typical dispersion values of 2–3 ps=nm km of standard singlemode fibre (SSMF) at 1310 nm [4]). In this Letter, we present the generation of a 39.9 GHz microwave carrying 16 and 64 QAM radio signals up to 20 MS=s employing this method. The generated signals were transmitted over different fibre lengths to investigate the dispersion tolerance of the approach.