Can white space deliver reliable connections?
January 22, 2012The orthodoxy of spectrum usage has been that networks are built using licensed spectrum while local transmissions use unlicensed spectrum. Nobody, it was presumed, would spend billions of dollars to build a network in spectrum that they did not fully own because of the risk that the value of their investment could be undermined by others using the spectrum and causing congestion. If this were true it would suggest that transmissions in licensed spectrum were more likely to succeed than those in unlicensed spectrum.
But the likelihood of a successful transmission is not just whether the operator has spectrum. There has to be coverage and capacity as well. Many will have had the experience of not being able to make a mobile call either because of a lack of coverage or due to “network busy” effects. Capacity itself is linked to spectrum availability but includes other factors such as number of cells and policies around sharing resources. The experience of many is that their home Wi-Fi, using unlicensed spectrum, is typically more reliable than cellular networks. And Wi-Fi networks are increasingly being built – and relied upon by cellular operators for data offload from their networks. So there is much reason to challenge the orthodoxy that reliable communications requires licensed spectrum.
So what about white space? It provides excellent propagation, so coverage-related problems should be fewer. There is also, on average, a large amount of available spectrum – of the order of 120MHz. This is equivalent to the entire 3G band, shared among typically four cellular operators. If there were no interference from other users this should result in reliability much greater than can typically be achieved with a cellular network. The imponderable is how much interference will occur. This depends on how many other users try to access white space, how powerful their transmissions are, what technologies they use and whether there is any coordination amongst users. Simplistically, if there were four systems trying to access the spectrum then this would lead on average to each having the same amount of spectrum as a 3G operator and hence reliability might be expected to be similar. But there is much more to the reliability question than this.
System design is a key issue. There is much that can be done to make a system more reliable. At the radio level techniques such as frequency hopping and dynamically variable spreading can ameliorate the impact of interference. At the resourcing level, resources can be reserved or prioritised towards more important messages such that congestion does not impact high-priority signals. Weightless makes use of all these and more.
Co-planning can also be valuable. Where the other users of the spectrum also have networks, then it makes much sense for the operators to work together to share the spectrum in a way that minimises the interference they cause each other – cellular operators are familiar with doing this in border regions. It might also be expected that as the spectrum became more heavily used it would dissuade new operators from building networks because the risk of interference-related problems would grow. Hence, there would not be a “tragedy of the commons” where the spectrum became so congested that nobody could sensibly use it. This is less effective where the other users are local, such as individuals running home Wi-Fi systems, although the impact of their interference is correspondingly more localised. Actually, we currently do not expect non-networked systems to use white space because of the overhead associated with the “geolocation” process which adds cost to these systems.
Ultimately there is no certainty in wireless transmissions. Cellular systems can be congested, suffer from malicious interference or have equipment failure. White space systems may have less certainty concerning interference but more spectrum leading to less congestion. It is possible to envisage a scenario in a geography with little white space available where multiple networks result in serious interference and even prioritised messages suffer. Our assessment of the likelihood of this happening is that it is no worse than congestion occurring on cellular networks. But there are so many unknowns that it will never be possible to be definitive about reliability on any wireless network.
If absolute reliability is essential use a wire. If a wire is not practical then wireless, even without absolute reliability, is likely better than nothing at all.
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