An approach to DAB coverage planning

29 June 2010

1.1 The Government has published a Digital Radio Action Plan (DRAP) the purpose of which is 'to provide the information to allow for a well-informed decision by Government on whether to proceed with a radio switchover'. Ofcom has been asked to chair a DAB coverage and spectrum planning group to 'determine the current level of FM coverage and develop a range of options to increase DAB coverage to match FM'.

1.2 This consultation sets out our proposed approach to that task. In particular:

  • Defining the areas within which we aim to replicate on DAB, as far as practicable, the editorial coverage of existing FM radio services (we call these 'editorial areas');
  • The underlying technical assumptions used to predict acceptable levels of FM and DAB coverage for indoor portable and in-vehicle radio reception;
  • The extent of existing FM coverage within each editorial area, for indoor portable radios and for in-vehicle radios on major roads; and
  • A study investigating the feasibility of different radio switchover scenarios illustrating, from a broadcast network perspective, how increasing levels of coverage can be achieved using increasing numbers of transmitters.

1.3 This document is not a policy consultation on a regulatory decision by Ofcom. The decisions that may follow this work are primarily to be taken by multiplex operators and by Government. Rather, we are consulting to give a wider public and stakeholder audience the opportunity to express any views on the issues in this document, before we submit a final report to Government.

1.5 These options do not constitute a definitive or final view on any post-switchover DAB network but will inform the Government's decision about whether to proceed with digital radio switchover. Further technical work on network planning, and public policy decisions by Government on the issues raised, will be necessary.

1.6 National FM radio services seek to target the whole of the UK. Each local FM service has its own unique editorial area, determined roughly by its transmitter location and power. In each local area covered by DAB there is a single 'multiplex' which carries a number of local services, all of which have identical coverage.

1.7 Because of this, we have defined a set of local editorial areas, based on discussions with broadcasters about the areas that stations aim to serve. They take into account both the largest local commercial service and the relevant BBC local or nations' service, to provide a composite editorial area, within which listeners could reasonably expect to be served by one or both of these services. We have not sought to replicate coverage outside these editorial areas (e.g. BBC Radio Manchester can be heard clearly in parts of Liverpool but the service is not intended for those listeners).

1.8 Such editorial areas already form the basis of some of the existing local DAB multiplex areas, which are fixed in licences that confer a right to operate. For multiplexes already broadcasting, transmission infrastructure is in place, and the limited number of DAB frequencies available restricts the opportunities for change.

1.9 However, these areas do not cover the whole of the UK and so for planning purposes, we have either extended existing areas or created new ones where necessary, based on discussions we have held with the BBC and the large commercial operators, taking into account existing FM editorial areas as far as possible.

1.11 The BBC's nations' services are expected to be carried on all of the local multiplexes in each nation, together covering the whole of the relevant nation.

1.12 This consultation is about the principles of DAB planning and not about the boundaries of editorial areas. Any changes to existing areas can only be made at the request of multiplex operators, and we must publicly consult on each one.

Defining FM coverage

1.13 Defining FM coverage is not simple. FM radio signals degrade over distance such that it is still possible to receive some kind of signal that some listeners may regard as acceptable over longer distances.

1.14 The current internationally-agreed method of predicting FM service coverage dates from the 1950s, and is based on an assumption that listeners receive their radio services using a directional rooftop aerial pointing towards the transmitter (like television aerials). These aerials have the effect of boosting the reception of the wanted signal, whilst rejecting unwanted signals (interference) received from other directions.

1.15 Today the vast majority of listeners instead receive their radio services on portable indoor and in-vehicle receivers. Receiver performance has also evolved and some modern FM receivers are more sensitive than receivers were 50 years ago. They also usually include techniques which conceal reception problems, enabling listeners to listen to weaker signals than used to be the case.

1.16 In developing a DAB coverage plan that reflects the level of FM coverage actually experienced, we need to know whether our computer predictions are representative of the FM coverage actually achieved on modern receivers.

1.17 We commissioned a review of the factors affecting portable reception of FM services. This produced what is known as a link budget, modelling every stage in the journey from the signal leaving the transmitter to the sound coming out of the speaker.

1.18 It concluded that the current assumed level of rooftop field strength (54 dBV/m) provides a good way of predicting good, indoor mono FM reception on modern portable receivers. However, many receivers will produce what some listeners might regard as an acceptable service at a field strength of 48 dBV/m, and so this could be used to define variable, indoor portable FM reception. It also supported the view that variable in-vehicle mono reception can be achieved using a lower field strength of 42 dBV/m. We welcome views as to which of these levels we should use to define the FM coverage that DAB should match.

1.19 Within each local editorial area we then predicted FM coverage of the existing BBC local station (e.g. BBC Radio Manchester); the largest commercial station (e.g. Key 103 in Manchester); and the composite coverage. For UK-wide services, we have calculated the BBC and commercial coverage separately. For all, we have produced FM coverage maps and household and road coverage figures.

Defining existing DAB coverage

1.20 Defining the coverage of DAB radio services raises similar technical questions to FM. Here, however, the challenge has been that either a good digital signal is received or none at all. We commissioned research to establish the planned field strength necessary to provide reliable indoor and in-vehicle reception.

1.21 In undertaking our research our aim was to make the most cautious assumptions possible as a starting position for our analysis. This was based on an assumed level of receiver performance, and so a key part of the work was the testing of existing DAB receivers in the market to determine their sensitivity. These tests found a very wide range of receiver performance but many receivers met the standard assumed in the coverage planning model and so this represents a practically achievable target.

1.22 We anticipate that the level of receiver sensitivity performance assumed in the planning model will form part of the receiver specifications being developed by the DRAP's Technology and Equipment Group (TEG). We note the importance of these specifications having an associated product logo or kite mark to enable consumers to identify receivers that come with an assurance that they will reliably operate in their planned DAB coverage area.

1.23 Given our cautious approach, the field strengths we propose using to predict indoor reception of DAB are significantly higher than previously used for DAB planning. (The previous value was 58dBV/m; we used 69dBV/m for robust indoor reception in most areas, rising to 77dBV/m in dense urban areas, but to retain 58dBV/m for in-vehicle reception.) We believe planning to these field strengths will provide consumers with a better, more robust listening experience than that available at present.

1.4 In addition to planning for higher field strength, for in-vehicle listening we have planned for reception in 99% of locations for 99% of the time. This is a deliberately cautious approach at this stage which, in practice, means we are planning coverage so that a listener would only lose reception in marginal coverage locations if they happened to be sat in stationary traffic during certain atmospheric conditions.

1.25 The robustness of our DAB planning criteria carries a cost in terms of the number of transmitters that need to be built. We will need to do further work to determine if this is necessary or appropriate.

1.26 We have produced DAB existing coverage maps and household and road coverage figures both for national coverage and for every editorial area within the UK.

Our proposed approach to increasing DAB coverage

1.27 For the BBC UK national multiplex, the BBC has carried out its own planning, based on the same criteria as we are using for local coverage. This planning so far consists of three phases: existing coverage, coverage by the end of 2011, and 97% population as required by the current licence fee settlement. Further transmitters may be required to match fully existing FM coverage, both variable indoor and in-vehicle.

1.28 The operator of the national commercial multiplex, Digital One, has provided a plan to match the coverage of Classic FM.

1.29 For the local multiplexes we have planned for increasing DAB coverage within each editorial area in four stages for both indoor and road coverage:

  • Existing coverage;
  • Modifying existing transmitters to improve coverage (Scenario 1);
  • Adding transmitters in diminishing order of coverage benefit, to a point which approximates existing FM coverage (Scenario 2); and
  • Continuing to a point where additional transmitters add negligible amounts of coverage and become potentially uneconomic to build (Scenario 3).

1.30 In aggregate, the set of area plans will present Government and service providers with a range of options for extending coverage, and will enable a cost-benefit analysis to inform the political decision of how much coverage to provide.

1.31 The limited number of frequencies available for DAB means that they must be re-used in different local areas around the UK. Interference can be caused by the use of the same frequency in other distant areas. Our plans assume some limited frequency re-allocation, to reduce interference by increasing the distance between the re-use of each frequency, but we have tried to minimise the cost and disruption to services. Any frequency change is subject to international agreement, which can only be negotiated following specific requests. It is likely that interference considerations and frequency availability will constrain the extent of local DAB build-out.

Interim conclusions

1.32 While we have tried to match the consumer experience, FM and DAB are different in kind. We consider both good and variable FM coverage but only good DAB coverage. However, the levels are not directly comparable between FM and DAB. Because of the way FM reception fades gradually compared to DAB, the standard we have set for DAB coverage is far higher than for existing FM. For example, the criterion we have used for road coverage for FM is that any 100m square is regarded as served if reception is available at 50% of locations within that square for 95% of the time. For DAB we have used 99% of locations for 99% of the time.

1.33 Our plans suggest that good DAB indoor coverage can be built to match good FM coverage, even using our very cautious DAB planning assumptions. For roads, our strict measure of good DAB coverage shows lower coverage than good FM, but there are indications that successful in-vehicle DAB reception may not require coverage planned for such a high percentage of locations. Our estimates suggest that even a minor relaxation in the assumptions (to 95% of locations for 99% of the time) would increase coverage by up to ten percentage points. This, together with other possible changes we suggest below, such as further frequency changes, give us confidence that DAB road coverage can be built to match FM.

1.34 This consultation marks only the end of the first stage in the planning process. Ofcom will conduct further research and planning and engage with industry to more fully explore these issues in our final report, including an examination of the implications for DAB coverage and the consumer experience of:

  • Varying our technical assumptions, in particular whether road coverage needs to be planned for 99% of the time and for 99% of locations, other technical characteristics of the DAB broadcast, and the differences between FM and DAB as technologies;
  • The trade-off between adding more transmitters and the increase in interference within the DAB network that brings;
  • The possibility of merging together some areas to make better use of frequencies (this does not mean merging the radio services, just carrying the same local stations over a wider area); and
  • Frequency changes further to those we have already proposed.

1.35 We anticipate both the BBC and Digital One will develop their national build-out plans further, as these issues are examined; their plans presented here are not final.

Next steps

1.36 We welcome responses on the consultation questions in Annex 4, particularly from a technical perspective, or focusing on the consumer experience. Following this consultation, we will provide a final report to Government in Q4 2011.