By looking at the latest electronic communication devices that have emerged over the past few years, it's clear that the trend of smaller, portable devices is strong and expected to continue. Yet while all these notebooks, netbooks, and tablet PCs are becoming more and more popular, their explosive growth also poses a problem: these Wireless devices are hogging the already congested lower microwave frequency region of the wireless spectrum.
This congestion problem was not unanticipated by electrical engineers, who, for the past two decades, have been developing new wireless technologies that use different parts of the electromagnetic spectrum. Specifically, these wireless technologies are exploiting the large, unused bandwidths of extremely high frequency (EHF) microwaves in the millimeter-wave (mm-wave) frequency region. One particular area of interest is the unlicensed 60 GHz frequency band, which has 5-mm wavelengths. (In contrast, the heavily burdened lower microwave regions have frequencies of 2-4 GHz, corresponding to wavelengths of 7.5-15 cm.)

However, the 60 GHz frequency band is not without challenges, either. Since wireless signals at 60 GHz frequencies have inherently high propagation losses, they are targeted toward short-range, in-building, high-speed applications. To maintain strong incoming wireless signals for buildings, many antenna base stations must be built near customers. These base stations, in turn, would receive broadband signals from a smaller number of distant central offices. The signals between central offices and base stations would be transmitted through long-range optical fibers. Since such a system uses both optical fibers and mm-wave wireless transmission, the technology is called “fiber-wireless” (Fi-Wi).

The advantage of bimodal Fi-Wi systems is that they can enjoy the strengths of both optical and wireless technologies - specifically, the inherently large bandwidth of optical fiber and the large, unused bandwidth in the mm-wave wireless spectrum. For this reason, a hybrid system has the potential to provide very high data transmission rates with minimal time delay.

Recently, a team of electrical engineers working on fiber-wireless technologies has analyzed the progress made in this field over the past two decades. In a paper published in the Journal of Lightwave Technology, Christina Lim, from the University of Melbourne, and her coauthors have presented an overview of the many different techniques proposed to optically transport mm-wave wireless signals and overcome some of the challenges involved.