Managing Spectrum Licensing in Australia
Michael Whittaker
Helen Yang
Abstract
Australia is leading the world in implementing a new form of radiocommunications licensing called spectrum licensing. Under spectrum licensing, licensees are provided with access to a defined parcel of spectrum space through an auction process. After the auction, licensees may trade spectrum space at a fine level in both frequency and area dimensions. The space of each licence is managed by maintaining emission buffer zones along the frequency and area boundaries of a licence to keep the space free of encroachment by neighbouring licensees and ensure that devices operated under a licence only use the spectrum space owned by the licensee. Quality assurance of industry operatives who certify the operation of devices under these licences must also be managed.
An ArcInfo based, Spectrum Licensing application has been designed and implemented to provide visualisation of spectrum licensing management activities and calculate the extent of spectrum use by devices in the VHF, UHF, and microwave bands.
A spectrum map grid layer covering the whole of Australia has been derived. The size of the grid cells in the layer was established by considering the population distribution. Based on the map grid layer, the user can define the boundaries of auction lots and licensing areas. The boundaries of spectrum space used by a transmitter may be calculated from predefined algorithms for different frequency bands by considering radiated power in a number of radial directions, the device's antenna height and surrounding topography. Spectrum space used by a receiver may also be calculated according to its level of protection. Any violations in the device technical specifications can be detected by visually comparing the device space boundaries with the licensed area. Interfaces to other computer systems that manage the basic data are also described.
The paper introduces the principle of spectrum licensing methodology. It emphasises the integration of spectrum licensing with existing GIS technology, the advantage in using spatial analysis tools and the challenges in adapting the tools to spectrum management.
Spectrum Licensing Basics
A spectrum licence provides a licensee with access to a parcel of spectrum space defined in terms of a frequency band and a geographic or licence area. The spectrum space is initially defined as spectrum lots for sale in an auction process. These lots become licences and one licensee can own more than one spectrum licence.
Different conditions may be applied to different licences. The spectrum space may be used for any type of radiocommunications device as long as its emission levels comply with the conditions of the licence. The conditions create emission buffer zones along both the frequency and area boundaries of the licence that act to reserve the total spectrum space free from encroachment by neighbouring licensees. These conditions form part of a complete interference management framework. The complete framework governs the operation of transmitters, the level of protection for receivers and responsibilities for interference settlement.
In Australia, the interference management framework for spectrum licensing is a fully defined baseline structure that allows the operation of different types of services. The framework operates at a broad management level to protect adjacent licensees, the detailed management of the spectrum being left to each licensee who plans the use of their devices within the broad design criteria provided by the framework. The devices must be registered with the communications authority before their use becomes authorised.
STUs and The Spectrum Map Grid
Spectrum spaces owned by licensees are represented in aggregated units called Standard Trading Units (STUs). An STU has bandwidth and geographic dimensions that cannot be further divided. Therefore, the minimum frequency band for any spectrum licence would have a width of one STU bandwidth. In some bands this bandwidth is as small as 0.0125 MHz. The minimum geographic area for an STU is a single cell of a Spectrum Map Grid. A cell on this grid is a curvilinear trapezoid with a side measured in degrees by reference to a spheroid coordinate system. The Spectrum Map Grid covering Australia is shown in Figure 1, and consists of cells of varying resolution depending on their location.
Different cell sizes are used depending on the levels of population. Larger cells are defined in rural areas. Small cells are defined in population density areas, such as cities, towns and their suburban areas. During the definition process, a population distribution based on Census Districts (CD) was considered and an average of 500 people in a cell was used as the criterion to determine when the smallest cell resolution should be used. Cell sizes of 3 degrees, 1 degree and 5 minutes resolution were used in the initial map grid. If necessary, finer resolution cells, that is 1 minute cells, can be achieved by subdividing the existing map grid cells via the application software. The size of a cell in kilometres varies with latitude and a cell of 5 minutes may have a size of between 7 and 9 kilometres. This distance sets the resolution for calculations involving the propagation of emissions over distance.
After definition of the map grid cells, the estimated population for each map grid cell was derived from the CD areas and associated Census data. Population for map grid cells is stored with the map grid layer and the population density is assumed to be evenly distributed within the cell. The population amount can be updated when new Census data is released. The population levels are used to distribute annual spectrum maintenance fees between licensees as well as delineating areas where radiocommunication services will be required.
Figure 1.
Spectrum Map Grid With Australian State Boundaries
Selecting Auction Lots
Auction lots of spectrum space are defined for sale. An auction lot area is defined by reference to the spectrum map grid. The auction lot areas are defined to cover the total area available from each band release and with no overlap of areas. Auction lots areas are created by a process that aggregates map grid cells. The process takes account of the valuable populated areas, the incumbent services and the requirements of technical framework itself, for example, the size of the emission buffer zone. The boundaries of adjacent map grid cells are then dissolved to define the auction lot areas.
- population in each STU
The population in each cell of the Spectrum Map Grid is examined so that attention can be focussed on delineating economically active areas where radiocommunication coverage will be required. Existing cellular coverage areas often help in this exercise.
- determining which incumbent services to remove
Next, the device boundaries (see below) of the incumbent services are plotted to show which services should be either included in the geographic area so that they will be either subject to the short period of operation during a 're-allocation period' of about two years or left outside because they do not significantly inhibit use of the spectrum and do not need to be removed.
- emission buffer zone
Then the worst case emission buffer zone required by the technical framework for typical systems is used to expand the area around the dense area of population.
Spectrum Space Aggregation and Trading
Lots bought from an auction process can be used to form the geographic areas of licences. Depending upon the requirements of a licensee, the available frequency bandwidth and time period, auction lots can be aggregated in different ways to form a number of licences. In some cases, it is possible for a geographic area of a licence to have an area missing within the outer boundary of the licence, that is, a geographic area can have an inner boundary.
Sometimes, lots may be reserved and the license is tagged accordingly. Status of a licensed area can be current, renewed, pending and expired. Trades between licensees can take place resulting in further spectrum space aggregation or division.
When a trade is made, traded STUs may be aggregated into the buyer's existing spectrum space to form new licence areas. The usual order for aggregating spectrum space is area, frequency and time but a licensee may specify another order.
The level of spectrum space division that is allowed, affects the conditions of the licence necessary for managing interference. For example, intermodulation interference (where interference is transferred across frequency boundaries) may be managed in a simple manner by stipulating a minimum aggregation of STU bandwidths (several MHz) for trading in dense areas. In that case, the probability of intermodulation interference between adjacent licences is minimised by the small number of frequency boundaries that actually exist. If trading at a fine level is provided (down to .0125 MHz) then deployment constraints must be used to keep frequency adjacent transmitters and receivers apart in order to manage intermodulation interference between devices operated by adjacent licensees. Deployment constraints limit the radiated power for transmitters and protection for receivers as a function of antenna height. They are designed to keep transmitters and receivers that operate in the same band, apart from each other for the management of receiver intermodulation and other out-of-band interference. The constraints are established by considering economically viable antenna placement scenarios.
Treatment of Incumbents
When spectrum is sold it is often encumbered in that it is already used by radiocommunication services. In Australia, spectrum may be sold encumbered. These services have a period to continue operation (a re-allocation period of about 2 years) after which they must either cease operation or negotiate with the new spectrum licensee to continue. In addition, services outside the geographic area of a spectrum licence may effectively use space within it because either their levels of emission or their levels of receiver protection inhibit use of the spectrum within the geographic area. Services outside the area are not removed and continue to affect use of the licence throughout its life. Path profiles are used to check that receivers operated under other licensing arrangements, that is, not spectrum licensing (where the device boundary criterion - see below - is sufficient protection), are provided with other specified levels of protection from transmitters operated under a spectrum licence.
- path profiles
Path profiles are a graph of the terrain elevation plus the bulging due to earth curvature plotted against the distance between the locations of a transmitter and a receiver. Receiver protection is provided by any terrain shielding between the two locations. There are different ways in which shielding can occur:
The amount of shielding depends on how much the first Fresnel zone between the transmitter and the receiver is obstructed. The first Fresnel zone for a path profile is based on the wavelength or frequency of the radio emission. A path profile is shown in Figure 2. There are various rules, based on theoretical models of propagation loss by diffraction over obstacles, for estimating the receiver protection caused by a given level of obstruction of the first Fresnel zone. There are also complex algorithms available, specially designed for path profiles constructed from DEMs or some other source where smoothing has been applied, that treat the profile in detail and calculate a total loss of all the contributions caused by different shielding mechanisms. The path profile may be exported for this purpose.
Figure 2.
Path Profile From A Transmitter to A Receiver - the Earth Bulge and a First Fresnel Zone Are also Shown
Registering Devices
Devices that operate under a spectrum licence must be registered with the communications authority in order to have their use authorised. An important part of this process is the calculation of a device boundary. The device boundary must be located within the geographic area of the spectrum licence under which it is operated for it to be authorised.
- Device Boundaries for Transmitters
A device boundary is calculated to check that the spectrum space used by the device fits within the space of the spectrum licence under which it operates. The calculation of a device boundary for a transmitter involves establishing the distance, along radials originating at the device, that is required for emission levels to drop below a benchmark level of protection for receivers (usually based on the noise floor of the receiver). The distance along each radial is calculated using a propagation model. The propagation model is usually a high site, low site path loss model. The high site, low site propagation path is needed to achieve:
The model takes account of the terrain in a simple manner by adjusting the antenna height of a device according to its height above average terrain (called effective antenna height) for each distance increment (usually the STU cell size) along each radial. The effective antenna height is calculated from the antenna height above ground and the effective site height.
- effective site height
The effective site height is calculated by averaging the terrain in the individual areas of fan shapes (see Figure 3.) surrounding transmitters or receivers. The difference between the elevation of the site and the averaged areas is the effective site height. An area average is appropriate because it provides a modelled emission level along a boundary as opposed to a profile which establishes a level at one point. The averaged areas are in the shape of segments of sectors. Effective site height is measured with respect to the average terrain height in a number of 5 minute segments (maximum of 30), for 2.5 degree sectors from the location of the antenna.. .
The segments are defined by a series of concentric circles from the transmitter at five minute intervals. The inner most circle is segment one. The sectors are a series of lines leading from the transmitter (radial lines), at 2.5 degree azimuth intervals, starting at 0 degrees as the first sector. The overlap between sectors and segments create fan shaped sector/segment intersection areas. The segment and sector definition are shown in the following diagram: .
The method of calculating effective site heights utilises the statistical and zonal functions in GRID to obtain the average terrain height for each sector/segment intersection area. The resulting effective site height data is stored in the RDBMS database.
- effective antenna height
The effective antenna height is calculated from the effective site height. The 2.5 degree sector segments may be further averaged for frequency bands that do not require a small resolution. For example, spectrum licensing at 500 MHz averages 4 sector segments to give a resolution of 10 degrees because antenna beamwidths are not less than 10 degrees at 500 MHz. Spectrum licensing at 1.8 GHz uses a 2.5 degree resolution. The effective antenna height is calculated by adding the antenna height above ground to the effective site height for every sector/segment - see Figure 4.
Figure 3. Example of Effective Site Height Sector/Segment Areas
Figure 4.
Calculation of Effective Antenna Height
where
- example of a device boundary
A device boundary for an omnidirectional transmitter with an EIRP of 49.2 dBm located at Mt Lofty, Toowoomba, Queensland is shown in figure 5. The device boundary is not to be viewed as a service area for the transmitter, instead, it establishes the spectrum space used by the transmitter under the high site, low site framework. The site is located on a mountain at the edge of a plateau that overlooks the east. There are even higher sites to the north and south. The calculated space takes in the low sites (but not the higher sites) in accord with the high site, low site framework.
Figure 5.
A Device Boundary For A Transmitter
- device boundaries for receivers
When trading of spectrum at a fine level is allowed in the frequency dimension, it is necessary to introduce a Level of Protection (LOP) for receivers. This is necessary because if a licensee is allowed to own a spectrum space with a bandwidth of, for example, 12.5 kHz, the total spectrum may be made useless by a situation where the propagation from a device operated by a neighbouring licensee is such that it is well outside the likelihood on which the framework is based. In that case, the licensee cannot shift frequency to avoid the interference and the only solution is to reduce the radiated power. Therefore, when trading at a fine level is allowed, the communications authority must impose licence conditions that enable it to make further reductions in radiated power even when a transmitter already satisfies the device boundary conditions. The communications authority can make those reductions through the provision of an LOP.
The level of protection for a receiver is a mean power level in units of dBm for a specified bandwidth (normally the STU bandwidth):
(a) that causes the device boundary of the receiver to be as near as possible to the boundary of the geographic area of the licence while remaining within that geographic area; and
(b) is never less than a benchmark level of protection.
LOPs are used to specify the maximum level of interference that a licensee may have to accept when operating a receiver. Receivers are required to accept higher levels of interference as they move closer to the boundary of the geographic area of the spectrum licence under which they are operated. In addition, receivers in these situations are required to accept still higher levels of interference as their effective antenna height increases. This effectively prevents licensees from using spectrum space outside the space of their licence.
In any case, a receiver may be operated without regard to its LOP because actual levels of interference depend upon the existence of any nearby interfering transmitters. Licensees are allowed to manage their own spectrum by weighing up the likelihood of interference. For example, in remote areas where the likelihood of interference is low, a receiver with a poor level of protection under the framework may still operate quite successfully.
- mobile sites
The interference management framework may need to take special account of the interference potential of mobile transmitters when high transmit powers are allowed (above about 25W). In order to simplify the management of mobile transmitters, groups of identical transmitters are treated as one logical device and that one logical device may operate in a number of effective locations. Effective mobile locations are defined as a point location with an effective radius and are based on either the position and size of built-up areas in major towns or sections of major roads that across with the spectrum map grids. Reference lists of effective mobile locations are established and their effective radii are used to further expand the device boundary in a manner that takes account of the roaming of a mobile transmitter.
Auditing Accredited Persons
Persons may be accredited by the communications authority to perform the work associated with calculating device boundaries and other coordination procedures and then issue Interference Impact Certificates (IIC) for the registration of devices using their own tools and software based on the spatial data provided by the communications authority. This work is periodically assessed to ensure that it is being performed satisfactorily. All the devices registered by one accredited person for one licence and that are near the boundary of the licence are found by retrieving all transmitters that are associated with a receiver whose LOP is reduced. The device boundaries of these devices are then recalculated and shown in relation to the licence geographic area so that errors may be detected.
In addition to the above quality assurance work the outcomes of interference settlement are also reviewed. The outcomes of the review process may be either:
In the last case, the number of reported cases of that type is checked to ensure that they do not go above a target rate established for all accredited persons.
The task of auditing the accredited persons is performed using the application software. Accredited persons perform tasks using their own software tools, therefore, any errors in their processing needs to be found.
Application Software and Related Data
The application software that performs spectrum licensing management activities is RADCOM. The RADCOM software is specially designed for the communication authority organisation (Australian Communications Authority in Australia). It provides functions and facilities to plan spectrum usage, managing spectrum licence holders information, and hold spectrum device data.
The RADCOM is a client/server with CA-INGRES supporting RDBMS and ArcInfo with ARCSTORM to manage spatial data. The user interface is designed in CA-OpenRoad and AML software.
The central RDBMS server and the ARCSTORM server are located centrally on UNIX machines. Client applications can be run either on PCs or UNIX workstations. Network software enables local office staff access the RDBMS data located in the central office. Local copies of stable spatial data are used for increased performance and reduction of network traffic. Changes on the dynamic spatial data are made from the central data store where a replication daemon is triggerd to update the data in the local copies.
Spatial data used by RADCOM covers whole of Australia. Census, topographic, postcode, gazetteer data, and digital chart of world data were obtained from the Federal government and other resources. The Digital Elevation Model (RadDEM) was developed based on the topographic frame data. Licence areas, device sites and other linear objects and areas were generated for the management activities.
Security of the RADCOM application software is controlled by predefined rules which are stored in the RDBMS. In particular, the GIS software security is controlled by the same rules in the RDBMS. Spatial data security is controlled by ARCSTORM database management which allows the changes to be made via the application. RadDEM and Census layer are set up so that only GIS DBA role can perform modification tasks. No application update is allowed.
Audit of the spatial data is part of the RADCOM audit process which uses both the facilities provided by the RDBMS application and specially designed functionality.
Authors:
Michael Whittaker
Senior Officer, Australian Communications Authority
Benjamin Office, Chan Street, Belconnen ACT, 2616, Australia
Tel: 61 6 256 5497
Fax: 61 6 256 5231
email: mwhittak@sma.gov.au
Helen Yang
GIS Consultant, BHP Engineering Ltd, BHP Service Companies
26 Atchison Street, Wollongong, NSW, 2500, Australia
Tel: 61 42 280 411
Fax: 61 42 280 893
email: yang.helen.hy@bhp.com.au