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Spectrum Commons Classes for Licence-Exemption

Executive Summary

1.1 This consultation advances the aims set out in the Licence Exempt Framework Review (LEFR) to develop a framework for the regulation of licence-exempt devices in the specific area of determining which applications should share licence-exempt bands and how they should behave with respect to each other.

1.2 The ideas presented here are for discussion. Subject to the responses it is our intention to introduce these ideas to inform debate within the international bodies such as the relevant European entities considering licence-exempt issues. These ideas are for future licence-exempt decisions and we are not planning to implement them in the UK in the short term.

1.3 The scheme we propose is to divide licence exempt applications into three classes of low, medium and high interference potential. The division would be performed on the basis of an Interference Indicator value calculated according to the likelihood of an application in causing interference and based on its technical parameters of bandwidth, duty cycle, range and expected deployment density. As a result, only applications with like interference potential would share spectrum. Within each class we propose that applications minimise their transmissions where possible and share the resource equitably through the use of polite protocols.

Background

Ofcom’s approach to management of spectrum

1.4 The Spectrum Framework Review[^(-1-)] (SFR) sets out Ofcom’s overall strategy for the management of spectrum through a market-based approach. It also outlines, at a high level, our understanding to when spectrum use should be licensed or licence-exempt.

1.5 The SFR suggests that spectrum use should be licence-exempt if the value that is expected to be derived from the use under such an approach is predicted to be greater than if spectrum use were licensed. It also notes that where harmful interference is unlikely (e.g. where the demand for spectrum in a given frequency band is less than the supply), then licensing may present an unnecessary overhead and a licence-exempt model may be more appropriate.

1.6 The main practical benefit of licence-exempt usage of spectrum is the easier and faster access to spectrum that comes with licence-exemption as compared to with licensing. On the other hand, the less detailed control of interference is the biggest disadvantage associated with the licence-exempt usage of spectrum, and can result in a reduction in value.

Ofcom’s approach to licence-exemption

1.7 The Licence-Exemption Framework Review (LEFR) further developed our approach to the management of licence-exempt use. One aspect addressed by the LEFR was the issue of spectrum commons vs. application specific spectrum allocations[^(-2-)]. Ofcom believes that, in general, application-specific spectrum allocations for licence-exempt devices result in inefficient utilisation and fragmentation of spectrum.

1.8 The LEFR identified a number of aspects where further regulatory work was envisaged, including how flexible politeness rules for licence-exempt use might be defined and enforced in practice.

European activities in the licence-exemption area

1.9 A well known instance of licence exempt use of the spectrum is Short Range Devices (SRD). SRDs are regulated by the European Commission Decision 2006/771/EC and the national regulations based on ECC Rec.70-30[^(-3-)]. The trend in ECC is towards a generic allocation instead of band allocations specific to technologies or applications. In parallel, the EC has recently requested studies on the benefits, economic value and ways of implementation of “Collective Use of Spectrum” (CUS).

Application of the concepts in this consultation

1.10 This consultation contains a proposal for a spectrum management mechanism based on classes of spectrum commons, and a proposal for regulatory requirements for politeness rules. It continues the work of the LEFR in the area of politeness rules and protocols and it seeks to align with the work of ECC and the EC on CUS.

1.11 We are not currently intending to retrospectively apply the principles set out in this consultation to existing licence-exempt devices. Instead, our proposal is that work in this area at national and international level and future licence exemptions made should be guided by the principles in this document.

Spectrum Commons Classes and requirements for an Interference Indicator

1.12 The LEFR showed that the benefits of spectrum commons are maximized when the technologies in a given frequency band are similar in terms of their technical parameters. To achieve this we propose the adoption of multiple “classes” of spectrum commons. Within each class applications would have broadly similar interference generating characteristics, which we will capture with a metric we term “Interference Indicator”.

1.13 The technical and operating characteristics of an application determine its Interference Indicator, and a class is defined as a range of Indicator values. A key element of a class-based spectrum commons is then how the Interference Indicator is defined and calculated. The Indicator represents the interference potential of a technology, hence the factors that contribute to interference have to be taken into account, namely: bandwidth, duty cycle, coverage and density of transmitters.

1.14 In addition, we believe that the Indicator should be: technology-neutral, independent of the victim device, and applicable to all systems.

1.15 The Indicator provides the means to compare the interference potential of applications. It does not have absolute meaning.

The Interference Indicator

1.16 Interference occurs when undesired RF signal appears at the spatial location of a receiver, in its receiver channel frequency, at the time the desired signal is present, and with a power level high enough so that the reception of the desired signal is disturbed. This definition covers the three domains where concurrence is required for interference to appear: geographic or spatial, time and frequency domain. We propose to gauge the interference potential of a technology in each of the three domains separately, and then combine the results into the Interference Indicator.

1.17 We will evaluate each technology in a given scenario. We will select scenarios where the technology usage is busy, yet realistic. The scenario will define the application using the technology and determine factors such as traffic and density of transmitters.

Frequency domain

1.18 A transmitter whose channel occupies a large fraction of a shared band will have a high probability of overlap with a victim receiver within the band. We propose to take the ratio of channel bandwidth to shared bandwidth as an Indicator of interference potential: BW Interferer / BW SharedBand This implies that a particular technology will not have a single Interference Indicator, but one that will vary depending on the frequency band considered for its use.

Time domain

1.19 A transmitter using the channel frequently will have a high probability of interfering with other users in the same channel. We take the duty cycle of a system as an Indicator of its interference potential in the time domain. We consider the duty cycle at the busy hour, and we acknowledge that it depends on the traffic for a majority of technologies. We propose to derive the traffic from the applications used in the scenario.

Geographic domain

1.20 For a victim operating at the same frequency and time as a transmitter, interference will only happen if the victim is physically located within reach of the transmissions. Two factors determine this:

Interference coverage of the transmitter. This is the area where the power level of the signal from the transmitter is higher than a certain threshold. The coverage area is determined by the output power of the transmitter, the propagation conditions, the antenna pattern and the victim’s sensitivity to interference.
A victim will suffer interference if the level of the unwanted signal at its receiver is higher than a threshold, but this threshold is different for each receiver technology and implementation. Since we are seeking an Interference Indicator that is independent of the victim, we need to select a typical threshold. Based on the current performance of popular licence-exempt devices we have selected –80 dBm/MHz.

Density of victims. Density, expressed in terms of interfering transmitters per area unit, can be used together with the coverage calculation above to give a measure of the usage of the space resource. For two technologies with the same coverage area per transmitter, the more ubiquitous one will result in a higher value of interference.
The number of licence exempt units in any given scenario can only be estimated since there is no single licensee that controls them. Furthermore, technologies will normally be evaluated at their development phase, so density estimates will be based on sales projections and expected uses. Typically, we would work with interested parties to reach a consensus on this factor.

Construction of the Interference Indicator

1.21 We have defined and calculated the four factors that provide the level of occupancy of the resources in the frequency (I f = BW Interferer / BW SharedBand), time (I t = Duty Cycle) and geographic domains (coverage & density). These factors are combined as follows to yield a single figure Interference Indicator:

The Interference Indicator of existing technologies

1.22 As an example of how the Indicator can be calculated in real life, we have looked at four existing licence-exempt technologies and a fifth one under development. We have calculated the Indicator for each technology in its own operating band, and in a hypothetical case where all would use the 2.4GHz ISM band.

	RFID	IEEE 802.11b	Bluetooth	Home automation	60 GHz WPAN
Normal allocation	1.1788	0.1641	0.1607	0.2008	0.0131
Allocation to the 83.5MHz wide 2.4GHz ISM band	0.0282	0.1641	0.1607	0.0014	1.0963

Table 1. The Interference Indicator of existing technologies

Spectrum Commons Classes

1.23 Classes are defined as ranges of Interference Indicator values. We believe that a scheme with three classes is the right compromise; fewer classes would mean that technologies with very different interference potential could be grouped together, and more than three could result in inefficient use of spectrum as bands tended towards becoming application-specific.

1.24 Prior to assigning a new band for licence-exempt use, Ofcom will have to decide what class (or classes) would be allowed in it. We propose that the decision is made fundamentally on the basis of the class predicted to generate the greatest economic value.

Politeness rules and protocols

1.25 Although the application of classes will ensure that dissimilar applications are in different bands, there is still a possibility of interference. In the LEFR we suggested that this possibility be reduced through the application of so-called “politeness rules” that require devices to take account of other users and act responsibly. However, a regulatory requirement for a particular polite protocol would steer developers towards a particular technical solution. This would be against current European regulations and hinder innovation. Instead, we will simply require that devices make a fair use of the resources and comply with a few high level rules towards interference mitigation. We think a fair wireless user is one that

shares the resources equitably with other users, and
behaves appropriately according to its needs.

1.26 We consider that the key capability for equitable sharing is to have some information about other users. In a decentralised licence-exempt environment we believe this can only be gained through sensing other use in the band. However, for low-interference devices we do not believe that a requirement for sensing would be justified.

1.27 We propose that in order to share equitably technologies belonging to medium and high interference class should

Implement a method to become aware of other users of the same resources.
Not monopolize the resources so that other users cannot access them.
Implement a method to reduce its channel occupancy when there is congestion

1.28 We consider appropriate behaviour to be that where resource usage is kept to the minimum within the limits of their applications and technologies. For example, this might include transmit power control and a reduction in data rate when high rates are not required.

Impact on stakeholders

1.29 The impact of these proposals will be felt by future users of spectrum. Ofcom anticipates that this impact will be beneficial because the proposals strive to optimise the efficiency and value of the licence-exempt uses of spectrum.

1.30 In addition, we believe that these proposals help to create an environment in which industrial stakeholders are made aware of the likely directions of licence-exemption policy development, and find it easier to invest as a result.

Citizens and consumers

1.31 We believe that the proposals set out in this document will deliver benefits to citizens and consumers for two main reasons:

1.32 A spectrum management strategy based on classes of spectrum commons guarantees better interference conditions, and thus an environment that bring benefits to consumers and citizens in terms of the ability to use more licence-exempt applications.

1.33 Secondly, it is Ofcom’s goal to impose as few technology restrictions as possible. This will let the market and the users decide on the best solutions and hence maximise innovation.

Next steps

1.34 This consultation, published on 6 May 2008 lasts for 10 weeks. The closing date for responses is 15 July 2008. We expect to release a statement on this consultation around summer 2008, having taken into account any stakeholder responses to our proposals. Based on the results of this consultation, we will seek opportunities to present these proposals to the relevant European bodies.

Footnotes:

1.- “Spectrum Framework Review: A consultation on Ofcom’s views as to how spectrum should be managed,” Ofcom, November 2005. See: http://www.ofcom.org.uk/consult/condocs/sfr/ .

2.- In application-specific spectrum, frequencies are reserved for exclusive licence-exempt use by a single application (e.g. spectrum used by DECT cordless phones). Spectrum commons allow for multiple wireless applications to operate on a co-channel basis.

3.- Electronic Communications Committee Recommendation 70-30

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