Getting started with TV White Space – Part 2: Understanding Availability
This post talks in detail about the availability of TV White Space (TVWS), and how it must be taken into account when planning a TVWS network. Part 1 of this post covered high-level considerations of using TVWS to solve problems.
What is TVWS Availability?
TV White Space is spectrum left unused by primary users – specifically Digital Terrestrial Television (DTT) and Program Makers and Special Events (PMSE – e.g. theatres and outside broadcasters). TVWS users can transmit at power levels which will not cause interference to any primary users. These permitted power levels in each channel constitute the TVWS Availability. Devices will be given different availability based on their location, antenna height, and out-of-channel emission mask.
Different permitted power levels in each channel at two locations (Class 1 devices); approximate coverage in channel 22
TVWS Availability adds complexity to the RF planning of a TVWS network, as each device on the network will have different sets of channels available at different power levels.
Protecting Digital Terrestrial Television (DTT)
Each DTT receiver in the UK – i.e. every household – is considered to be a primary user. Any DTT channels which might be used by a household are protected, even if the household can receive the same multiplex in several channels from several transmitters. digitaluk.co.uk is a handy tool for finding protected channels at a given location. Any TVWS device will be unable to use any of the protected channels of any household within several kilometres, even if those channels are unprotected at the device’s own location.
Channels adjacent to protected channels are somewhat restricted, based on the device’s out-of-channel emission mask, particularly if the device lies in between a household and a DTT transmitter. In general, Class 1 devices (with the best emission mask) have considerably more availability than Class 5 devices.
DTT protection is discretised on a 100m grid, so if two locations lie either side of a pixel boundary, even 1m apart, they can be granted quite different availability. Any device in the same 100m pixel as a household (very likely if TVWS is providing domestic broadband) receives additional restrictions, particularly if it is at rooftop height.
Protecting PMSE users
In the majority of cases PMSEs will have no effect on TVWS availability. However, if the TVWS device is within a few kilometres of a theatre or outdoor event location (e.g. a racecourse, or a golf course during The Open) then it will be restricted in the used channels (and to a lesser extent adjacent channels) to prevent interference.
Bidirectional Links & RF Planning
TVWS Availability is based on a single device in isolation. In practice, however, at least two devices must be able to receive each other’s transmissions. To establish a link between a Master in one location and a Slave in another, each with their own TVWS availability, a channel must be available to both devices at a high enough power level to be receive each other’s transmissions. This is a classic RF link budget problem, but with the additional complication of permuting through the different power levels at each location in each channel.
Link Budgets in channels 21-29, green/red denoting bidirectional link is feasible/infeasible. Path loss calculated with the terrain-aware Longley-Rice point-to-point model
Nominet have dedicated tools for planning TVWS networks, combining the features of traditional RF planning tools with TVWS availability, and other complex issues which arise. If you are considering deploying a TVWS network we are able to assist you with your planning, either by our R&D group providing a free feasibility study, or by providing availability data for your own tools.
1st September 2016