There is currently a great deal of interest in photonic generation of carrier frequencies around the atmospheric windows at the E-band (60 GHz to 90 GHz) and the F-band (90 GHz to 140 GHz) for the development of high capacity wireless systems in future femto-base stations. The number of operators offering FD-LTE (Frequency division -Long term evolution) and WiMAX based femtocells is expected to grow from zero to 25% by 2012. This is fuelled by recent market reports suggesting that the demand for higher data rates will continue to increase, eventually pushing the carrier frequencies towards the F-band. Wireless transmission in the E-band based on optoelectronic technology has already demonstrated data rates up to 12.5 Gb/s with On-Off modulation schemes, and reaching up to 27 Gb/s with spectrally efficient modulation techniques.
One of the most promising carrier generation methods is photomixing two optical wavelengths on a high speed photodiode, which in combination with injection and phase locking techniques can generate tuneable, high power, narrow linewidth millimetre carrier wave signals. The photodiode is a key component in these systems, being the element where the optical signals are converted to the electrical domain. Uni-Traveling Carrier Photodiodes (UTC-PDs) have been the dominant type of photodiodes for these applications due to their wide bandwidth and high millimetre-wave output power levels.
Fig. 1: The 120GHz dual photodiode.
The research conducted by the consortium of the European project iPHOS - www.iphos-project.eu -has achieved significant advances in this area. The project's objective is to develop wireless communication systems for the licensed 71-76 GHz E-band frequency spectrum as well as for the license-exempt frequency range around 120 GHz. Recent results report the development and integration of high speed photodiodes for both frequency bands.
Worldwide, regulatory bodies have opened-up the frequency ranges 71-76 GHz and 81-86 GHz for enabling licenced broadband wireless communications. For such E-band Radio-over-Fiber systems, the consortium has developed compact and packaged photonic