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Around 10 years ago, the Defense Advanced Research Projects Agency’s Next Generation Communications program constructed a prototype cognitive radio system, which utilized dynamic spectrum access for its communications. By identifying unused sections of spectrum in the area in which it was operating, it was hoped that up to 10 times more spectrum would be available for transmissions. This highlighted a growing interest in the defense community in dynamic spectrum access techniques developed with the challenges of battle-space spectrum in mind, but also apparently had applicability in commercial environments in terms of making more efficient use of valuable spectrum resources and potentially leading the way to spectrum trading. The XG program was one of the largest cognitive radio projects at the time, but interest in Cognitive radio was by no means limited to the U.S.
Martin Cave’s audit of public-sector bands in 2005, which highlighted just how much more efficiently U.K. defense spectrum could be utilized, provoked interest in the U.K. This was produced alongside Ofcom’s Spectrum Framework Review, which set out ambitious targets for a general move from the traditional “command and control” approach to spectrum licensing to a more dynamic approach based on “market mechanisms” with the overall ambition of realizing better value from spectrum for the U.K.
With the switchover to digital television and release of TV white space, a debate was ignited over whether DSA could be applied to these civilian bands too. The obvious example of this has been the activity around TV white space, although the Federal Communications Commission discussion on 3.5 GHz is also significant.
However, the digital TV switchover was six years ago and the commercial rollout of white space devices is still fairly limited due to the complications of deploying these devices in practice. Concerns over the so-called “hidden node” issue – interference provoked by the failure of one device to detect the presence of all other devices – and how devices with different spectral views would liaise with each other have meant that the regulation of these white space devices has taken some time to agree.
In attempting to overcome these limitations, regulators gradually shied away from a pure spectrum-sensing approach, toward the introduction of beacon signals to identify usage, before settling on the use of a centralized database of white spaces in each location that is used in addition to spectrum sensing.
But even then, the practical use of TV white spaces has continued to be fairly limited. Vendors and operators have struggled to find an application that suits the availability of white spaces, as well as handling the lack of guaranteed spectrum.
This same philosophy is being proposed for 3.5 GHz in the U.S., where some locations have other users (e.g. marine radar), but the combination of database and sensing could allow this band to be used. This is especially important as 3.5 GHz is one of the few LTE bands that is supported globally, so there is a clear commercial imperative.
Enter 5G
At the recent 5G Huddle, rethinking how existing technologies make use of spectrum was a key topic of discussion, with spectrum sharing a major part of this.
There are some strong arguments for why this would be sensible:
• We’re starting to reach the limits of what we can achieve through higher-order modulation schemes, with any gains insufficient to keep pace with demand.
• We may still be making some gains with regard to multiple-input, multiple-output and CoMP, but again, not at the same rate that demand is increasing.
• Small cells, which are increasing in usage, and network densification, levels of which are also increasing, both lend themselves well to spectrum sharing.
• The last variable available to us in our attempts to increase capacity is spectrum and, (at least in theory) DSA maximizes availability and efficiency of spectrum across all operators
On that last point, this is of course only if it is deployed correctly, with polite protocols for communications.
However, introducing dynamic spectrum sharing to “5G” would surely result in 5G suffering from the same technical issues that cognitive radio has encountered before.
After all, one of the key differentiators of cellular over many other wireless technologies, such as Wi-Fi, is the guaranteed quality of service. Indeed, we have previously examined how exclusively licensed spectrum loses value as the sharing arrangements increase uncertainty for operators.
Wouldn’t 5G lose this edge if spectrum access became dynamic and without guarantees?
At present, you would be correct, but it is unlikely anyone would be satisfied introducing such a glaring problem into 5G. Rather, the key difference between earlier cognitive radios and 5G is that, as demonstrated with the discussions at the 5G Huddle, major commercial vendors and operators are putting significant research time and investment behind the technology.
Perhaps this time around, with the full weight of the industry behind it, and with an appropriate understanding of what operators need from spectrum-sharing conditions to offer high-quality services, cognitive radio and DSA can really come of age.
Julie Bradford has a broad and deep background in a wide range of wireless technologies. As communications engineer at QinetiQ, Bradford developed bespoke modems on software-defined radio platforms and was technical lead on a year-long study on the regulatory implications of cognitive radio. At PA Consulting, Bradford worked on a variety of studies – with several involving LTE – including evaluation of LTE-related patents, design of an LTE system architecture and examination of capacity limitations in cellular networks. Most recently she acted as senior systems engineer for Airvana’s UMTS femtocell product range. Bradford graduated with a 1st Class MEng degree in electronic engineering with business management from the University of York and was awarded the YES prize for sustained excellence throughout her degree program. She has also earned a postgraduate Certificate in Business Management with a distinction from Bath University and is a Chartered Engineer.
