Spectrum Usage Rights - a guide

Executive summary

Introduction

1.1 A key reason for managing spectrum is to avoid interference between different users – if all users were able to access the spectrum without any planning and yet without interfering with each other, then there would be no need for spectrum management. To avoid interference users are given licences which set out in some form their ‘rights’. It is important that neighbours both in geography and in spectrum are given compatible licences – i.e. that they do not cause excessive interference to each other.

1.2 In granting a licence, rights to transmit are capped to limit the risk that significant levels of interference may be caused to others. For example, if users have no restriction on the signal levels they are allowed to transmit outside of their designated bands, they may transmit high levels, to the detriment of neighbouring users.

1.3 However, restrictions tend to reduce flexibility by preventing particular types of deployments or particular technologies. Hence, it is important to find the right balance between preventing interference and restricting alternative uses.

1.4 Licences can either control the power transmitted or the interference caused. Controlling the transmitted power through a “mask” that defines the maximum power levels across a range of frequencies is only a weak control on the interference caused to neighbours. This is because were the licence holder to change the density of deployment of their base stations, perhaps as a result of changing the use that they put the spectrum to, then the interference suffered by the neighbour would increase despite the licence holder remaining within their mask. Significant changes in deployment, for example from a frequency division duplex (FDD) system such as 3G to a time division duplex (TDD) system such as WiMax could have a very severe impact on neighbours even though it might appear that the transmitted power had decreased. This is because in a TDD system interference can come from both base stations and mobiles and, for example, for a neighbouring mobile attempting to listen to its base station, a nearby mobile transmitting in the adjacent band can be a greater source of interference than a base station.

1.5 Where control of interference is important a different form of licensing is needed. An inherently superior approach is a licence that specifies the maximum level of interference that can be caused, rather than the power that can be transmitted since this directly controls the problem rather than indirectly and inaccurately. This form of licensing based on interference is called spectrum usage rights (SURs). SURs place only those restrictions on licence holders that are needed to protect neighbours, and no more, while at the same time they provide certainty to neighbours as to the maximum levels of interference that they can expect.

1.6 In essence, SURs provide much greater certainty for investors that the network they have paid to deploy will not suffer reduced capacity or need expensive re-engineering as a result of neighbours changing their usage, while at the same time ensuring their investment in a licence will not become stranded due to licence restrictions as technology evolves.

Form of an SUR

1.7 There are three ways that one spectrum user (let us term them “A”) can interfere with another (“B”).

1. Geographical interference. In this case both A and B are using the same frequencies but in different locations. If A moves too close to B then signals from A’s transmitters can interfere with reception on the edge of B’s coverage area.
2. Out-of-band interference. In this case, A and B are located in the same geographical area, using separate but nearby frequencies. If A’s transmissions in its own frequency bands “spill out” into neighbouring bands then they can be received by B’s receivers as interference.
3. In-band interference. Again, A and B are located in the same area with nearby frequencies. In this case, B’s receivers are not perfect and also pick up some of the signal A transmits in its own bands causing interference.

1.8 For geographical interference, the signal level generated, specified in terms of aggregate power flux density (PFD), at or beyond the geographical boundary should not exceed a set power level.

1.9 For out of band interference the PFD measured at an agreed height above ground level should not exceed a set power level at more than a certain percentage of locations in a set area. The reason for the percentage of locations is that if just one, or a few, measurements were made they might be close to a neighbour’s base station where the signal level received would be very high. Enough measurements are needed to average over a representative area.

1.10 In-band interference can be specified and measured in an identical manner to out-of-band interference. The only difference is that the set power level would be higher, reflecting the fact that spectrum users are generally allowed to transmit much higher power levels within their bands than outside them.

Setting the initial values

1.11 Determining the numbers to put into these licence terms may not be simple. For example, the 3G specifications set maximum transmitter power but not the distribution of allowed interference. To arrive at the interference distribution requires assumptions about the likely transmit power, the likely base station density and some other factors such as base station antenna gain and height and the use of power control mechanisms. These can then be input to a propagation modelling tool which can be used to estimate the interference distribution and hence arrive at the SUR licence terms. Of these assumptions, the likely base station density is most difficult. For systems still in their early deployment stage, the final base station density may be unclear, even to the operator.

1.12 The approach we follow is for Ofcom to make an estimate as to the initial values based on our knowledge and experience. We then publish this estimate along with details of how we arrived at our conclusions and consult on whether this is reasonable. Any interested party is then able to understand our process and suggest changes if they think necessary. We take all responses into account before coming to our conclusions. However, as explained below, getting these values right is preferable but not essential as there are opportunities to modify them after the issue of the licence.

Verifying that licence holders operate within their SURs

1.13 Perhaps more complex than setting the values is the process of ensuring that licence holders adhere to them. With the mask approach this is relatively simple – it is just a matter of checking whether transmitter powers are below a threshold. However, such a check is also of relatively little value, since, as already explained, transmitter powers are only weakly linked to the interference caused. With SURs, the actual interference caused, or some approximation to it, must be assessed in order to verify compliance. This section explains how we will do this.

1.14 One key decision we faced was whether to verify by measurement or modelling. Measurement would involve using radio receivers to make actual measurements in a number of locations. It is accurate, if done correctly, because it does measure the actual interference caused, but could be expensive and time-consuming to undertake, especially where signal levels vary over time and so substantial averaging is needed. Modelling estimates the level of interference using propagation modelling tools. It approximates to the interference caused, because no models are perfect, but can be performed quickly and at much lower cost than measurements. Also, because it accords to the manner that networks are planned, it makes it simpler for licence holders to verify as they plan their network that it will not exceed its licence terms. Conversely, with measurement there is some risk that they deploy a network based on their modelling tools only to find that in practice it exceeds their licence terms.

1.15 Clearly there are benefits from either approach – measurement is more accurate but modelling is simpler and cheaper. It would be possible to adopt different approaches in different frequency bands and the approach could be changed over time if needed. For the moment, after consultation, workshops and discussion, we have concluded that modelling fits better with the needs of licence holders and propose to use this as our preferred approach. This then leads to a need to specify clearly how the modelling will be done so that all stakeholders end up with the same results for a given network deployment. Defining the modelling falls into two parts – firstly specification of the propagation models and necessary databases and secondly specification of the process and assumptions that need to be made.

1.16 There are many propagation models available. Some are standardised, predominantly by the ITU, others proprietary. Different models often apply for different frequency bands and sometimes for different services. Our preferred approach is to consider each band separately and select the ITU model that fits most closely to the likely usage and frequency range of that band. Because models often have options or variable parameters, we will then specify in detail exactly how the model is to be implemented so that all parties will get identical results when using it. Models make use of geographical databases (which show terrain height) and clutter databases (which divide areas into different usage types such as urban, forest, etc). All licence holders need to use identical databases so we will specify the supplier and version number of these in the licence. Finally, as a check, we will model a hypothetical network ourselves and publish the results. Others can then model the same network to confirm that their model delivers the same results.

1.17 The next stage in the process is to define the area over which the SURs will be verified. There is a balance here. If the verification area is made very large (eg the whole country) then very high levels of interference could be caused in some areas and low interference levels in others. The averaging process would then allow the licence holder to be within the terms of their SUR but this would not provide any real protection to neighbours. If the verification area is made very small, so that, say it only include a part of the coverage area of a single base station, then placing this measurement area near the centre of the cell would deliver very different results from placing it near the edge of the cell. This would lead to excessive variability in results which is not desirable. The solution is to find an area somewhere between these extremes. Our modelling suggests that if the area includes of the order of 5-10 cells then there will be little variability regardless of where it is placed but equally this should avoid the averaging effect.

1.18 Unfortunately, the size of cells varies, both with geography and with time as operators split cells. So it is not possible to define a verification area in terms of its dimensions. Instead, we suggest a process whereby the modelling software tracks across the whole of the licence area (generally the whole country). At each intersection point of the OS 1km grid square lines it forms a square and expands the size of this square in discrete steps until it includes at least ten base stations. It then processes the square as described below before moving onto the next grid square intersection.

1.19 The processing of each measurement square varies depending on whether the spectrum is being used for downlink (base station transmit) or uplink (mobile transmit).

1.20 If it is a downlink then the first step is to input into the model the location of the base stations, their height, transmit power and antenna patterns. Then the modelling tool needs to assess each “pixel” within the measurement square. A pixel might be say a 50m x 50m square. For each pixel the tool determines the signal strength from each base station using the agreed propagation algorithm and adds them to form the total predicted PFD for the pixel. Once this has been done for all pixels it can be determined whether the licence terms have been exceeded - this occurs if there are more than a given percentage of the pixels (eg 10%) with a modelled PFD above the agreed limit.

1.21 If it is an uplink then more assumptions need to be made since we cannot know where all the mobiles are and whether they are transmitting at any given time. What we do know is the number of base stations operating in receive mode on that frequency and we have some understanding of the uplink capacity of each base station. Knowing this sets a limit on the number of mobiles that can transmit simultaneously for any given data rate. How we proceed from this point depends on the balance that stakeholders prefer between accuracy and simplicity of modelling.

1.22 A relatively simple approach that we currently propose is to select the data rate for a likely representative service (eg voice for cellular) and then set an assumed maximum number of mobiles that could simultaneously access a cell assuming a fully loaded network in the vicinity. This will, of course, be an approximation to reality where mobiles use power control, change data rate and experience fading. Each mobile would then be distributed evenly around the cell and its power set according to its distance from the base station. The next steps depend on whether the neighbouring service which is concerned with the interference caused is also an uplink service or a downlink service.

1.23 If it is an uplink then we are interested in the interference caused into the base stations. So for each pixel in the measurement square we can assess what level of interference a base station placed in that pixel would experience by summing the interference from all the mobiles across the measurement square (although in practice only those few mobiles nearest the pixel would make any difference). We can then repeat for each pixel and determine whether the SUR criteria have been met.

1.24 If it is a downlink then we are interesting in the interference caused into mobiles. In this case, the interference will typically depend on the probability of a transmitting mobile and receiving mobile being in close proximity. We know the density of transmitting mobiles so we can determine the probability of a receiving mobile being within any given distance and the interference they will receive as a result. This interference probability can be pre-calculated by Ofcom for the assumed transmit power and for any given density of transmit mobiles (typically it will be stated in terms of a simple equation where the input is the mobile density). Then for each pixel in the measurement square the number of mobiles can be added up and the equation used to derive the interference levels. Compliance can then be assessed across the measurement square.

1.25 These procedures can be readily automated using modelling tools and macros such that it is a simple process to derive compliance information in any situation.

Changing an SUR

1.26 If a spectrum user wishes to change the technology that they are going to deploy then the in-band, out-of-band or geographical limits might become inappropriate for two neighbouring spectrum users. The solution to this is for these neighbours mutually to agree changed limits. Practically, it seems likely that if, for example, a spectrum user wanted to increase its out-of-band emissions, causing increased interference to a neighbour, it might make some payment to the neighbour to compensate them for the loss of value, or accept a similar increase in interference from the neighbour into its bands. The neighbours would then approach Ofcom to request a change to their licenses. As long as Ofcom was satisfied that all potentially affected licence holders had given their approval it would normally make the requested change.

Summary

1.27 We believe that SURs offer a number of advantages over other licensing methods in that they directly control the interference caused, rather than indirectly as is currently achieved. This provides greater certainty to investors that the networks they deploy will not suffer interference problems in the future while still providing the flexibility to change usage. However, as a result, they are somewhat more complex to set and verify than the current mask-based approach.

1.28 We have shown how we might initially set the SUR terms, how we would verify that these were being met in practice and how they might be modified by licence holders if needed. While there is inevitably a degree of complexity associated with these processes, it seems that many can be automated within modelling tools such that validation can readily and quickly occur.

1.29 SURs were used in the L-Band auction in May 2008. We will offer them as an alternative in forthcoming auctions. We will also allow licence holders with conventional licences to change the terms of their licence to an SUR if they wish to do so.

1.30 The remainder of this document describes all these issues in more detail.