|
VHF Broadcast Re-planning |
This report describes work carried out by Aegis Systems Ltd, during February and March 2000 on behalf of the Radiocommunications Agency, the Radio Authority and the BBC.
The study considered the potential for additional sound broadcasting services in the VHF/FM band (Band II) and sought, in particular, to identify allocations that might be used to provide new services in the Leeds and London areas.
A glossary and a summary of the terms of reference of the study are given in Annexes A and B.
This document comprises two major parts:
The UK Band II radio network has evolved in an ad hoc fashion starting (1955) with three BBC services radiated in the band 88-94.6 MHz. The remainder of the band was initially allocated to mobile services, however these were gradually withdrawn, allowing the introduction of additional BBC services and subsequently of commercial local radio services. The current structure of the band is shown in Figure 2.1 (the band 87.5-88.0 MHz is used in the UK for low-power ‘Restricted Service Licences’ (RSLs)).
Figure 2.1 Current UK use of Band II in England
This gradual release of spectrum has meant that the band has never been planned as a whole. Today, for reasons of audience stability and 'brand identity' there exists a disincentive to change the frequencies of existing stations. In the case of commercial services, such a change might be expected to result in a loss of revenue.
The planning of UK Band II broadcast services is undertaken jointly by the Radio Authority and the BBC, with each organisation responsible for planning within its own sub-bands. Historically, formal co-ordination between the two bodies occurred through regular meetings of the Radio Planning Group (RPG), chaired by the Radiocommunications Agency and attended by the Department of Culture, Media and Sport (which holds responsibility for broadcasting policy). During 1998, committee structures were subject to review; this resulted in the Radio Assignment and Coordination Group (RACG) being formed to replace the RPG. The RACG is chaired in alternate years by the BBC and the Radio Authority, which are the responsible planning bodies. The Radiocommunications Agency attends as an observer of the planning process, to ensure technically efficient spectrum planning and provide engineering advice; the Agency will also take agreed proposals forward for co-ordination.
The basis of the current Band II transmitter plan in Europe is the ‘Geneva Plan’ of 1984 (GE84), which built on existing assignment plans and added assignments in the upper part of the band. While this plan still dominates Band II planning considerations, actual network implementations vary quite widely from it. For instance, in the UK, it was originally intended that some of the frequencies above 105 MHz would be made available for the extension of BBC national network services, but this part of the band was eventually used for the provision of Independent Local Radio services.
The number of local radio services has expanded greatly in recent years: for instance, in the London area, there are currently some 15 VHF local radio stations available (including one BBC station). The majority of these (nine) aim to cover the entire Greater London area (i.e. within the M25). A further five national network stations (BBC Radios 1–4 and Classic FM) are also available.
To understand the potential for the introduction of new services in the VHF band it is necessary to appreciate the engineering principles and processes that underpin the planning activity. This section offers a brief overview of the important issues. Further information is in Annexes C, D and E.
The quality of the signal at the listener’s receiver is determined by a variety of technical parameters, of which the most important are the wanted field strength, interference field strength and aerial directivity.
Wanted field strength determines
the subjective quality of signal in the absence of interference. International
recommendations suggest that, to provide acceptable stereophonic reception,
median field strength values of 54 dB
V/m,
66 dBV/m
and 74 dBV/m
are required in rural, urban and dense urban areas respectively. In practice,
services in the UK have generally been planned to provide coverage at 60 dBV/m,
with areas receiving less than 54 dBV/m
being considered unserved.
The field strength received from a given transmitter varies not only with location, but also with time. Atmospheric effects can give rise to enhanced interfering signals for small percentage times, so the unwanted signal generated by a distant transmitter may be undetectable on one day and cause severe interference on the next. It is the need to allow for this potential problem that results in many of the apparently ‘unused’ channels in the FM band. Further technical details can be found in Annex C.
Interference field strength, that is to say the power of an unwanted signal, reduces the overall quality of the signal at the listener’s receiver. In broad terms, the level of interference received will depend upon:
By adjusting these various parameters an appropriate ‘protection ratio’ (that is to say the required difference in field strength between a wanted and an interfering signal, needed to achieve a defined quality of signal) can be achieved. The protection ratio, which is normally expressed in dB, falls as the frequency separation between the two stations increases, and is tabulated in Table 2.1 below.
Table 2.1 Protection ratios required for stereophonic service in relation to steady interference
These protection ratios have been determined on the basis of subjective listening tests and of objective measurements carried out on samples of typical consumer receivers. These tests and measurements were undertaken some years ago, and may not represent current conditions. Further details may be found in Annex C. Where the protection ratio is positive, it will not be possible for transmitters to serve the same area, unless antenna directivity at the receiver provides some additional protection. For co-area coverage by transmitters that are not collocated, a minimum separation of 400 kHz is normally required.
Smaller separations may be achieved with careful planning. In 1995, the BBC brought into operation a 'filler' station at Crystal Palace to improve BBC National Network coverage in South London, with special techniques being used to allow the use of an otherwise 'taboo' channel. Further information can be found in Annex C, Section C.2.
Aerial directivity affects the susceptibility of the receiver to unwanted signals. The planning of modern analogue FM services is complicated by the fact that the planning criteria used in the past assumed the use of fixed receivers with directional, rooftop, antennas. The majority of the contemporary audience, however, is likely to use portable or car receivers, with little directivity, and therefore little discrimination against interference. However, modern receivers may perform better than is assumed, and interference may be less perceptible to a listener in a car or to a portable. These considerations appear to balance out, and the existing field strength limits are generally found to be appropriate in practice.
The concept of ‘protected field
strength’ (pfs) is widely used in the planning of radio services. This
represents the field strength that must be exceeded by a wanted service to overcome
interference. For example, if a distant transmitter produces an interfering
field strength of 30 dBV/m
in a town, a local transmitter operating on the same frequency will need to
produce a wanted field strength of 75 dBV/m
to give the required protection ratio (i.e. 30 dBV/m
+ 45 dB). If, however, the distant transmitter is operating at a frequency
offset of 100 kHz, the wanted field strength need only be 63 dBV/m
(i.e. 30 dBV/m
+ 33 dB).
The pfs predicted on a given channel
in a given area is a useful indication of the ‘usability’ of the channel.
A wanted field strength of some 54 - 60 dBV/m
is normally sufficient to give good stereo reception in the absence of interference.
If the pfs lies below this range, the service is said to be ‘noise-limited’
(i.e. the service area boundary is determined by the point at which the wanted
signal becomes too weak). In the example above, a pfs of 75 dBV/m
indicates that the service is severely ‘interference-limited’, and the boundary
of the service area is determined by the point at which the incoming interference
becomes too strong. It follows that the interference-limited service area
is always smaller than the noise-limited service area. Further information
can be found in Annex C.
The use, in the planning process, of appropriate protection ratios is critical in achieving a balance between quality of service and spectrum efficiency. Current planning assumptions are described in Annex C.
If it could be shown that current protection ratios are unnecessarily severe, it would be possible to re-use the spectrum more intensively within a given area than is currently the case.
A suggestion has been made by one European administration that protection ratios for FM radio planning in Band II might be relaxed: one proposal is shown in Table 2.2 below (for ‘steady’ interference).
To illustrate the impact of such a change, the current protection ratios were assumed to be relaxed by 5 dB (i.e. to 40, 28, 2, -2 and -25 dB), and the impact of one of the changes (the introduction of a new London service on a frequency of 102.6 MHz) described in Annex G re-examined. Under current planning assumptions, a loss of coverage of around 25% was suffered by the transmitters at Bakers Wood and Reigate. With the relaxed protection ratios, the figures become:
|
Before |
Changes |
After |
Changes |
|
|
Protection ratios |
Current |
Relaxed |
Current |
Relaxed |
|
Bakers Wood |
209,568 |
386,246 |
151,008 |
189,944 |
|
Reigate |
322,712 |
373,854 |
239,516 |
273,205 |
Table 2.3 Impact of relaxation of protection ratios (1)
It can be seen that, in all cases, the coverage is increased if the relaxed protection ratios are assumed. It should be borne in mind, of course, that the service experienced by listeners is unchanged - this is only an exercise in accounting!
The important criterion is the percentage change in coverage implied by the different assumptions regarding protection ratios.
|
Current protection ratios |
Relaxed protection ratios |
|
|
Bakers Wood |
-28% |
-51% |
|
Reigate |
-26% |
-27% |
Table 2.4 Impact of relaxation of protection ratios (2)
The relaxation in assumed protection ratios has actually increased the impact of the changes. As both the existing service areas are already limited by interference, the relaxation in criteria causes the ‘before changes’ population coverage figures to increase, and this increase is proportionally greater than the additional population nominally served following the changes.
If new criteria were to be introduced, these might be applied either to new services only, or to the entire network.
In the former case, the impact of the new service on existing services would be unchanged. There might, however, be more flexibility in identifying new frequencies for the new service, as it will be permitted to suffer higher levels of incoming interference.
If the new planning criteria are to be applied to the entire network, this would imply a wholesale re-planning of the spectrum, allowing a greater geographical and/or spectral density of transmitters. To quantify the increase in capacity that would result from such a change would require a substantial modelling exercise, beyond the scope of this study. It might be noted that, under a regime in which the network had been re-planned with relaxed protection criteria, there would be a smaller difference between the technical coverage area of a service, and that defined for marketing purposes.
It is not possible to determine the ‘efficiency’ with which spectrum has been planned without considering the objectives of the planners. In the UK, several objectives have been defined: some services will require broadly national coverage, while others may be targeted at a small community.
In the case of the BBC National Network services, universality of coverage has been sought, as all licence fee payers should expect an equivalent standard of service. This had led to a fairly uniform network of high, medium and low-power transmitting stations, achieving a population coverage of around 99%. It is particularly important to note that, as the following graph illustrates, the majority of the population can be covered using a small number of higher-power transmitters, with coverage of the remaining population requiring a much greater number of lower-power sites.
Figure 2.2 Relationship between transmitter numbers and network coverage
A necessary consequence of this universal coverage is that it is often possible to hear the same programme on several frequencies, and this is sometimes cited as evidence of inefficient planning. Such a criticism fails to distinguish between receiving a service (i.e. acceptable quality and reliability), as opposed simply to receiving an audible transmission, which may become inaudible around the next corner, or which may experience interference for a substantial period of time.
The overall coverage of the sole independent national radio (INR) network (Classic FM) is similar to that of the BBC networks, and most of the same high-power transmitter sites are used. Commercial considerations, and the form of the ‘coverage versus transmitters’ curve shown above, have led to the use of a much smaller number (35) of transmitters than in the BBC case. This, in turn, has made possible the accommodation of a number of local radio services within the nominal INR sub-band. For example, no relay is used at Guildford, allowing the use of a nominally INR sub-band frequency for a London-wide local service on 100.0 MHz.
Where coverage of a service is provided by several transmitters, it is necessary to distinguish between the net and gross coverage from any given site. This is particularly true for national networks, where the overall coverage in an area may be determined by contributions from, perhaps, five or six transmitters. Even in the case of local radio, many services are provided using more than one transmitter, and a substantial overlap may exist.
Where the gross coverage of a transmitter is much greater than the net coverage, the spectrum is, by definition, being used inefficiently. Considerations of terrain, and practical constraints on the choice of transmitter sites, often impose the need to accept substantial coverage overlaps.
Unless otherwise stated, all population figures in this report refer to gross coverage.
A major problem encountered in undertaking this study related to the establishment of an accurate transmitter database. The problem is that a given transmitter may be represented with different characteristics in different databases.
The Geneva Plan, and the various updates thereof, forms the basis for the international frequency co-ordination of Band II broadcast stations. In order to allow scope for changes within national networks, without having to undertake further lengthy co-ordination, the parameters in this database often overestimate the power used at a given site, and may not reflect the actual aerial pattern in use. The plan may also include allocations that have been ‘cleared’ internationally, but do not actually exist.
Within the UK planning process, it is also sometimes necessary for the planning authorities to build some ‘slack’ into the network, to allow for future modifications. For instance, the Radio Authority must recognise that it may sometimes be necessary for an operator to change transmission site, perhaps on the expiry of a lease, or due to building development. It is necessary, therefore, that the formally ‘cleared’ parameters for a site allow for modest changes to the actual transmission characteristics, without incurring the need for re-planning in other parts of the network.
Without close co-operation and discussion between the various authorities, planning inefficiencies may result from the assumption of a more interference-limited environment than actually exists. While the need for a distinction between ‘cleared’ and actual transmitter parameters is unavoidable, it does impose an obligation of close co-operation and negotiation on planning engineers.
If two transmitters serving the same area operate at frequencies differing by around 10.7 MHz (e.g. station A at 88.2 MHz, station B at 98.9 MHz) it has been found that many receivers tuned to the lower frequency will suffer interference. It has therefore been normal in the planning process to avoid 10.6–10.8 MHz frequency spacings for transmitters with overlapping coverage. Further information can be found in Annex D.
The frequency band immediately above Band II (i.e. above 108 MHz) is allocated for use by aeronautical services such as Instrument Landing Systems. These must be protected from interference caused by broadcast radio and this can impose a substantial restriction on the use of Band II frequencies. In particular, it has been Radio Authority policy to allocate only low-power services in the upper 1 MHz or so of the band.
Details on this issue, including calculations for the city-wide London plan, can be found in Annex E.
In order to maximise the useful findings that might be generated in a two month period, the work adopted a case study approach, focussing on Band II spectrum use in the Leeds and London areas. It sought to identify allocations that might be used to provide additional radio services. The results of these two casestudies are described in Section 5. The method used to undertake the analysis is briefly outlined below.
For each of the two cases, work started with a review of protected field strengths across Band II in the area. This provided an indication of the existing pattern of spectrum use. The potential for accommodating new services within this pattern was explored. The work then moved on to develop a series of possible re-planning scenarios in which the characteristics of existing stations of power below 20 kW might be changed (NB stations of 20 kW and above would be difficult to co-ordinate again, on different frequencies, and so were considered non-movable). The impact of relaxing the 10.7 MHz criterionwas also considered.
For each scenario, the impact upon listener numbers for existing stations and new stations was then determined. All modelling work was undertaken using ASSET, the Aegis radio systems modelling software.
It should be reiterated that this study was not a comprehensive planning exercise. It has, for instance, not been possible to follow through all the consequences of the frequency changes postulated. Planning considerations such as aeronautical interference, continental interference and intermodulation products have been only touched upon, where addressed at all. The results provide an indication of the possibilities that might exist for accommodating new services and should not be interpreted in a more definite fashion.
Any change to the existing pattern of local radio coverage is likely to have an impact upon listeners, in terms of the type and diversity of programming available, and upon existing stations, in terms of the size and competitiveness of the market. This section provides a brief review of some of the important issues to be taken into account when considering the desirability, or otherwise, of the various scenarios examined by the study. It starts with a brief overview of the existing UK radio market.
Local radio is an ever more competitive market place: the number of independent radio stations in the UK grew from 143 in 1992, to 246 in 1999. London has become a particularly vibrant market, with well over a dozen city-wide ILR services already available, which, combined with BBC and other national stations, offer a diverse range of programme choices, from mass appeal through to special-interest content.
Figure 4.1 London supports a diverse and competitive market
In the Leeds area, the market is somewhat different, with listeners enjoying less diversity of choice, but with existing stations facing less intense competition. Although there are now eight ILR stations in the region as a whole, the geographical separation of the stations means that listeners will tend to receive only two or three ILRs in any given location. Reach varies from around 15% to over 40%, with the average being 30%.
What then is the likely impact of new services upon incumbent services? The evidence indicates that an increase in the overall number of stations on-air can help to increase the total reach of radio, that is to say the overall size of the market, benefiting all those involved. However, existing providers are also likely to experience some loss of market share because of increased diversity of service provision and strengthened competition. As the following short case illustrates, whilst the reach of incumbent stations may not be significantly affected by increased competition, their weekly listener hours, and hence revenue, may be.
The impact of growing competition upon existing stations can be seen by briefly reviewing the experience of Capital Radio. Using RAJAR figures for the final quarter 1999, it can be seen that Capital FM (with a 30% reach) is the dominant ILR station in the London region.
In the period 1990 to 1999, the number of stations in London almost doubled, yet despite this Capital FM’s reach has remained fairly consistent at around 28% to 30%. However, the station’s weekly listener hours dropped by around 25% – a significant reduction. Capital’s experience demonstrates that whilst an increase in the number of stations competing for listener attention may not substantially affect the number of people tuning in to a station, it does have the potential to affect overall market share, as described by listener hours.
More information is in Annex F.
Perhaps the greatest detrimental impact that existing stations might experience as a consequence of Band II re-planning, is the need to move frequency. Such a move will inevitably expose the station to a potential loss of listeners. The precise impact of such a move upon the resulting size of the listenership and hence revenue for commercial stations, will depend upon a variety of factors, including, but not limited to:
There exists little empirical evidence to suggest the likely loss of listeners that might be anticipated arising from frequency change. It has not been possible, in the timescales available, to obtain data relating to the consequences of the previous re-arrangement of allocations (in the mid-eighties).
When considering these costs and negative consequences, it should be recalled that under some scenarios stations may acquire an increase in total coverage, and hence TSA and ultimately revenue.
The preceding sections have concentrated on the potential commercial ramifications of change upon the independent radio sector. However, BBC radio is also a vital part of the social infrastructure of the UK, with 65% of the entire population listening at some point during the week. It is therefore important to recognise that whilst a change to the frequency of an existing BBC station might not be easily measured in terms of lost revenue, it would nonetheless have associated ‘costs’. Listeners experiencing difficulties in locating the new frequency, and particularly listeners in an area where coverage is lost, will clearly suffer a reduction in the quality and diversity of programming choice – which the BBC is intended to provide.
It should also be recalled that the public funding that the BBC receives through the licence fee is generally deemed to establish an implicit contract between the Corporation and the public. As part of this implicit contract there exists an expectation that the BBC will provide universal coverage. The benefits arising from any re-planning that causes the reduction or loss of BBC services must therefore be balanced against the possible perception that such a change would constitute a breach of the BBC’s moral contract.
Further details regarding all aspects of the market can be found in Annex F.
This section provides an overview of the two case study areas and highlights the major findings of the modelling activity.
VHF/FM services were first provided in the London area from the BBC high-power station at Wrotham, in Kent, which went into full service in 1955, providing the (then) three national services in the band 88.0–94.6 MHz. Reception problems in South London subsequently led to the addition of a ‘filler station’ at Crystal Palace.
BBC local radio was introduced in London in 1970, in the sub-band that had just been released at 94.6–97.6 MHz. Independent radio followed with the opening of Capital and LBC (later News Direct) in 1973 in the same sub-band.
Independent services expanded rapidly in the early 90s, and today there are eight London-wide ILR stations transmitting from either Croydon or Crystal Palace. In addition, six lower-power stations have been introduced, serving limited areas within London. Finally, Classic FM is transmitted from both Wrotham and Crystal Palace, with the same parameters as the BBC services.
The plot below shows a ‘pfs versus frequency’ scan for London (NGR: TQ300800 – Charing Cross) calculated for each channel within Band II. Calculations are for 50%-time, and are based on the Radio Authority transmitter database, thus including some proposed and Geneva Plan stations in addition to existing services.
Figure 5.1 pfs in London
As a ‘rule of thumb’, a pfs of below
around 60 dBV/m
would indicate a frequency able to provide a good channel with little interference.
The graph therefore indicates that, without re-planning, there appear to be
no likely slots for a new station in the London area.
Scenario 0: assumed that only minimal changes to the existing network were permissible. Any new services would have to be accommodated within the existing pattern of spectrum use. There is very little possibility of accommodating new city-wide services in the London area under the existing pattern of use.
An attempt was made to accommodate London-wide services on a total of eleven frequencies, on a 400 kHz raster designed to ‘mesh’ with as many existing allocations as possible. This ‘scenario’ should not be interpreted as a potential plan, but only as a tool with which to examine the possibility of accommodating further services. It should also be borne in mind that the short timescale of this study has allowed the investigation of only a few of the possible re-arrangements of spectrum use.
It should be noted, however, that this study was not required to take account of continental interference – if such interference is considered, it may be found that there is less scope available for the introduction of further services. Nevertheless, it appears that there is sufficient likelihood that new services might be accommodated as to warrant further detailed investigation.
The scope for use of the BBC National Network sub-bands was limited by the large number of high-power transmitters in this part of the spectrum. One frequency was identified, which might be used for a further London-wide service at the expense of some 100,000 potential BBC Radio 4 listeners in Surrey.
Scenario B: addressed services with a radius of around 7–10 km, roughly a quarter of the greater London area. For the purposes of investigation, hypothetical services were assumed to cover the north-east, the north-west, south-east and south-west quadrants of the M25 area.
It had been hoped that it would be possible to identify frequencies for use by these services, on which interference existed only in a part of Greater London. In the event, this did not prove possibleand the tentative assumption has been made that the introduction of services of this type would require significant re-planning.
Scenario C: attempted to accommodate a number of ‘community’ type services, with service area radii of some 3–5 km, limited primarily by interference. Results suggest that it will be possible to accommodate a number of these stationsby making use of frequencies that could not be used for wide-area coverage, but could accommodate interference-limited services of a small size. These services will have aninsignificant effect on existing networks (particularly if accommodated within National Network bands) and will not require any existing services to change frequency.
The Leeds/Bradford conurbation was selected to allow examination of an area that is more typical of the UK than the densely-populated London area. The terrain of the Leeds/Bradford area prevents a single transmitter from serving the whole area, as can be done in London. However, the shielding provided by the terrain allows transmitters to be located quite close to each other without interference.
In this area, therefore, it is appropriate to serve each town separately using many smaller transmitters rather than attempt to use a single, large transmitter. The same frequency can be re-used in more places due to the lower power of each transmitter. This approach also introduces flexibility to the network planning, because planners can offer either individual stations for each town or the same station to multiple towns.
The area receives services from the BBC, Classic FM, and six ILR stations; one ILR service covers the whole region, and the other ILRs cover smaller, and sometimes overlapping, areas.
The plot below shows the protected field strengths predicted to exist in the centre of Leeds (NGR: SE300330).
Figure 5.2 pfs in Leeds
Comparing this with the London plots, it can be seen that there are fewer channels on which the field strength is high, and that the remaining channels are less uniform than is the case in London. This reflects the greater number of transmitters surrounding London, and the less rugged nature of the terrain.
Scenario 0: examined a minimum-change scenario and identified three frequencies that seemed capable of supporting a new service. It has since been determined that one of these frequencies will shortly be licensed by the Radio Authority while a second is earmarked to rectify an existing frequency relationship which is not allowed under current planning standards. The third frequency was initially made available to the Classic FM network, but has not been used and the company’s entitlement has now expired. It may be noted that this situation may occur elsewhere around the UK where Classic FM has found it uneconomical to use similar in-fill transmitters
There appears to be no advantage in relaxing the 10.7 MHz constraint within the existing frequency plan. Inserting stations 10.7 MHz below an existing service in Leeds or Bradford results in mutual interference with existing services on adjacent frequencies. It may be noted that a 10.8 MHz relationship has existed for some years between the ILR service in Bradford and a BBC service from Holme Moss.
Scenario A: assumes a near ‘blank sheet’ with the area effectively being ‘planned’ from scratch.
The greatest scope for re-planning was found in the lower part of Band II, between BBC Radio 4 and BBC Radio 1, which is currently used by both BBC local and ILR services. By taking advantage of the shielding provided by the Pennines, combined with local geography, it seems possible to introduce two new services to the area. However, this does require all three existing local services in this lower part of the band to change frequency.
The upper part of Band II appears to offer fewer options. It did prove possible to introduce three services in the Leeds and Bradford areas. However, this resulted in a loss of coverage for many of the existing stations in the local area, and perhaps most noticeably, required that a high-power service in York be closed down.
Scenario B had been intended to explore the provision of additional services with a radius of approximately 7–10 km. It was found, however, that the topography of the area did not allow a useful distinction to be made between such services and those considered in Scenario A.
Scenario C: examined the potential for introducing stations with a 3–5 km radius. Here, the results suggest that considerable potential for new stations may exist. Although these smaller transmitters operate at low power, and have consequentially limited coverage compared to the more powerful transmitters, in flat areas they can nonetheless radiate a signal over a substantial area. However, in the Leeds area, the local geography should ensure that the signal from a transmitter placed at a suitable site in the centre of its service area, on top of a tall building for example, will not spread too far beyond the intended coverage. This characteristic should enable community services to be located close in frequency to existing, out-of-area, high-power transmitters without causing interference. It seems possible that a few of these small community stations might potentially be introduced to each city in the area.
It is suggested that the following work might be undertaken, further to inform any decision regarding the re-planning of UK VHF/FM spectrum.
The overall intention of this brief study has been to investigate the efficiency with which the Band II spectrum has been planned in the UK, and to estimate the scope for the expansion of services within the band.
Opportunities
As the preceding sections have shown, the study indicates that there is a sufficient likelihood of being able to introduce a number of new FM radio services within Band II to merit further investigation. A detailed, investigative planning exercise does appear to be justified.
Modelling results from both the atypical London area, and the arguably more representative Leeds/Bradford areas, indicate that it may prove possible to accommodate a variety of city-wide services and smaller stations with service area radii of approximately 7-10 km. Some small-scale services, with a radius of 3-5 km, might be introduced without an impact on existing services. The numbers of stations involved, together with some tentative frequency re-planning scenarios, are detailed in Section 5, Annex G and Annex H of this report.
Wide-area services
In neither area was there found to be any scope for the introduction of further wide-area coverage services within the existing pattern of use.
In both the London and Leeds areas, it was found that there appeared to be some opportunity for the accommodation of further city-wide services, if the need for some re-planning of the network is accepted. For a small number of such additional services it may be that the ‘cost’ of the re-planning would be limited to changes in the frequencies of surrounding services and a relatively small loss of coverage by a few surrounding services. It should be noted that this study was not required to consider the constraints imposed by continental interference. If this is taken into account there will be less scope for additional services, particularly in the London area.
The scope for use of the BBC National Network sub-bands was limited by the large number of high-power transmitters in this part of the spectrum. One frequency was identified, however, which might be used to provide additional London-wide service at the expense of current BBC coverage.
The reasons for the greater opportunity apparently available in the local radio sub-bands are discussed earlier in this report – in particular, it is noted that the ‘spare’ spectrum that is sometimes said to exist in the BBC sub-bands is a consequence of the obligation of the Corporation to provide universal coverage. If this obligation were to be relaxed, it is possible that some additional local services might be introduced, though to quantify the gains and losses involved would require a further study of national, rather than local, scope.
Smaller-scale services
In the London area it was not possible to identify any frequencies that would be specifically suitable for services of around 7-10 km radius, although such services might use those frequencies already identified for city-wide services. In the Leeds/Bradford case, the local terrain does not allow any useful distinction to be made between city-wide and 7-10 km radius services.
It appears that it will be possible to accommodate small, 3–5 km radius, services without an impact on existing services. Such services would be interference-limited by high-power, out-of-area transmitters. These services would be most easily accommodated within, but need not be restricted to, the National Network bands (both BBC and INR).
Relaxation of the 10.7 MHz constraint
The 10.7 MHz constraints that are currently applied in the planning of Band II services were examined to see whether their relaxation might permit additional stations to operate in the band.
It was found that this seemed not to be the case in the two areas considered – such ‘taboo’ channels were generally found to accommodate services in surrounding areas that would be displaced if the 10.7 MHz constraint were relaxed. Therefore, while the constraint may impose an additional planning burden, it probably does not limit overall capacity in a network that serves a fairly evenly-distributed population (as is generally the case in Europe as opposed to, for example, the USA or Australia).
Efficiency of current and past planning
No evidence has been found, given existing UK policy, ITU-R planning standards and the history of ad-hoc development of services within Band II, that this spectrum could have been planned significantly more efficiently. Annex I makes some comparisons with the situation in the USA, noting that the spectrum there is, in many ways, less intensively used.
Planning standards
It is suggested that, before any comprehensive re-planning of the UK Band II spectrum is undertaken, it might be wise to review the current planning standards: the current standards work well in practice, but they do not appear to reflect the way in which the majority of listening is done, and may be unnecessarily conservative.
It has also been suggested that if some new services were to radiate in mono only, there might be some scope for additional stations. However, this issue is beyond the scope of the current study and would require further investigation.
| Previous |
 |
