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05/19/2008Interference Issues: Is the 2.4-GHz Band Reaching Supersaturation?
"[The] number of 'conversations' (voice or data) that can theoretically be conducted over a given area in all of the useful radio spectrum... has doubled every two-and-a-half years for the past 104 years."  That remarkable claim comes from mobile phone pioneer Martin Cooper. Since it sounds a bit like Moore's Law, it has been dubbed Cooper's Law.
Cooper's Law shows that the communications capacity of the radio spectrum is not fixed - it depends on technology. In fact, there are many ways to increase the carrying capacity of spectrum. The most important, according to Cooper, is frequency re-use. Limiting a signal's coverage area makes that possible, and nothing shows the success of this approach better than the short-range devices (SRDs) operating in license exempt bands. There are now vastly more SRDs than licensed radio systems, even though licensed systems have been allocated vastly more bandwidth. The Bluetooth SIG says there are about 2 billion devices in the world equipped with their technology. IC Insights predicts sales of additional Bluetooth devices could reach 1.6 billion per year by 2010. Meanwhile, the Wi-Fi Alliance says 325 million electronic devices now have Wi-Fi and annual sales could reach 500 million globally by 2009. These predicted rates of growth exceed Cooper's Law. So are they plausible? Can that many more signals fit into the 2.4 GHz band? Are we close enough to saturating 2.4 GHz that municipal network overlays can degrade the performance of private WLANs while trying to connect the unconnected? "We have definitely seen a negative impact from these systems... we just haven't been able to quantify that impact," Craig Mathias told Wired magazine last summer. There's remarkably little agreement on how many Wi-Fi nodes can fit comfortably within a given area. In 2004, Aegis Systems Ltd. and Transfinite Systems Ltd. wrote a 131-page analysis for the UK Office of Communications (Ofcom). They said 24.79 Wi-Fi access points in one square kilometer constitutes "full occupancy" of the 2.4 GHz band - if no other equipment uses those frequencies. If microwave ovens and Bluetooth devices are factored in at realistic densities, then as few as 4 Wi-Fi nodes per square kilometre could constitute "full occupancy." Add in cordless phones and the situation gets even worse.
Municipal "hot zones" typically have about 20 nodes per square kilometer – more if meshed. So if Aegis/Transfinite are right, adding a municipal network to a neighborhood which already has private Wi-Fi systems could result in mutual interference. And yet a "haphazard" survey in January 2007 by Peter Cochrane found nearly 200 nodes per square kilometer in the blocks around London's "Liverpool Street" station. RSA's 2007 Wireless Security Survey found 7130 public access and office Wi-Fi nets in about 1300 hectares of east/central London - that's about 530 per square kilometer - not including residential networks. Such high densities aren't surprising, given that London is the "the world capital of Wi-Fi." But how to explain this: a "surprising result" of the 2.4 GHz band occupancy measurements commissioned by Ofcom was "the relative lack of Wireless Local Area Network (WLAN) activity, especially in central London... The average [band] utilisation was 5.7% in 2003 rising to 14.3% in 2006." What's going on here? Is the capacity of the 2.4 GHz band much greater than experts tell us - or are current methods of measuring band utilization inadequate? Either way, the implications are troubling. A known defect in current spectrum management practices is that regulators have to decide how much bandwidth to set aside for license exempt applications: there is no agreement on a good way to let the market decide how much open spectrum is enough. Plus, the rate at which re-allocation decisions are implemented is much slower than the rate at which radio equipment evolves. Slowness of response - which is exaggerated in Europe by the need to harmonize allocation changes across sovereign jurisdictions - means regulators must anticipate problems; they need to monitor the free bands to see if they are getting congested. Problem is, the very short signal range of the devices in these bands makes accurate monitoring difficult and costly.  Elsewhere I suggested that Ofcom might use low- and slow-flying miniature helicopters (like the MicroDrone MD4-200, shown at right) to measure occupancy of the license exempt bands. So it was gratifying to see this in their recent announcement of research projects slated for funding in 2008-9:
"Capture of spectrum utilisation information using moving vehicles - The use of spectrum in different parts of the country and across different frequency bands can vary quite dramatically. It is important that we are aware of such variations to enable us, for example, to determine whether additional licence exempt spectrum is needed...
"Licence-exempt spectrum is important for delivering applications that generate significant consumer value, such as Bluetooth and WiFi. However, as the number of devices that support these technologies increases, so does the possibility of congestion.... Understanding the actual levels of congestion and the associated trends will assist us in making better-informed spectrum management decisions. Unfortunately, getting a good understanding of congestion is very difficult – congestion may only occur in a small area, such as in the centre of a shopping centre but not at the periphery, or it may only occur at certain peak periods of the day. We propose a study to seek out congested areas and measure levels of congestion." Despite the difficulties, there is a need for such surveys in every country with SRDs, so it might be a good idea to organize an international conference to share ideas and identify "best practices" in measuring license exempt band occupancy before congestion reaches crisis levels. 
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