In the new digital era of terrestrial TV, there are digital multiplexes across the UK, these multiplexes use different channels, so neighbouring transmitters don't interfere with each other, which means there is a lot of potentially unused spectrum in a particular area. Multiplex sit in the UHF band which covers 470 - 790 MHz.
In order to avoid interference with existing (licensed) spectrum users, devices will need to communicate with databases which apply rules, set by Ofcom, to put limits on the power levels and frequencies at which devices can operate. There is also a 'kill switch' function whereby the database can tell a device to stop operating completely if interference is found to be occurring.
The UHF TV band is currently allocated for use by Digital Terrestrial Television (DTT) broadcasting and Programme Making and Special Events (PMSE). Currently, Freeview TV channels are broadcast using up to ten multiplexes. Each multiplex requires an 8 MHz channel. Multiplexes are transmitted at different frequency channels across the country in the frequency range 470 to 790MHz.
Whilst a total of 32 channels each 8 MHz wide are reserved for DTT in the UK, normally only one channel per multiplex is used at any given location. In other words, the majority of channels are unused for DTT transmission at any given location. This is required because high-power TV broadcasts using the same frequency need geographic separation between their coverage areas to avoid interference.
The channels that are not used by DTT at any given location can be used by lower- power devices on an opportunistic basis. This opportunistic access to interleaved spectrum is not new. Programme making and special events (PMSE) equipment such as radio microphones and audio devices have been exploiting the interleaved spectrum for a number of years, and Ofcom issues more than 50,000 assignments annually for this type of use.
Ofcom refer to the spectrum that is left over by DTT (including local TV) and PMSE use as TV White Spaces (TVWS). By this we mean the combination of locations and frequencies in the UHF TV band that can be used by new users, operating in accordance with technical parameters that ensure that there is a low probability of harmful interference to DTT reception, PMSE usage or services above and below the band.
The following organisations have signed contracts and completed qualification to run the white space databases (WSDB): -
The 'master' devices that talk to the databases should report their height, if they don't the database will use a use conservative default values for the purpose of calculation of operational parameters i.e. it will use height values that would result in operational parameters that are equal or more restrictive than they would be had the device reported its height.
Though the regulations do not specify an update time (for master devices to communicate to the databases), Ofcom has stated a maximum time of 15 minutes which strikes an appropriate balance between the need to be able to act quickly in the event of interference and limiting the practical burden on databases of maintaining frequent communications with potentially large numbers of devices. This may be revised if found to be unsuitable.
The WSD Regulations apply to the United Kingdom and the Isle of Man. They do not extend to the Channel Islands.
A master device is a device which is capable of communicating with and obtaining operational parameters from a database for the purpose of transmitting within the frequency band 470 MHz to 790 MHz.
A slave device is a device which is capable of transmitting within the frequency band 470 MHz to 790 MHz after receiving slave operational parameters from a master device.
Type A equipment as equipment which has an integral antenna, a dedicated antenna or an external antenna11 and is intended for fixed location use only.
Type B equipment as equipment which has a dedicated antenna or an integral antenna and is not intended for fixed location use.
WSDs must not be used airborne.
WSDs must be configured in such a way that a user is unable to input, reconfigure or alter any technical or operational settings or features of a device in a way which (i) would alter the technical characteristics of the device which are communicated to a database (this includes the master and slave device characteristics), or (ii) would cause the device to operate other than in accordance with master operational parameters or slave operational parameters, as applicable. An example of (ii) would be the antenna gain. If this parameter is set to be smaller than the actual gain of the antenna, then the device could radiate at a higher level than the limit communicated by the WSDB.
A master device:
- must be able to determine its location
- must provide device parameters (defined now as its ‘master device characteristics’) to a database, in order to obtain operational parameters from the database. The device parameters include the location and the technical characteristics of the device listed below. The operational parameters indicate to the device the channels and power levels that it can use, together with other constraints.
- must only transmit in the UHF TV band after requesting and receiving operational parameters from, and in accordance with, operational parameters provided by a database
- must apply the simultaneous operation power restriction (described at paragraph 3.23 above), if it operates on more than one DTT channel simultaneously and the master operational parameters indicate that this restriction applies
- must report back to the database the channels and powers that the WSD intends to use – the channel usage parameters – and operate within those channels and powers.
- a database instructs the master device that the parameters are not valid
- a master device cannot verify, according to the update procedure, that the operational parameters are valid.
In order to support more WSDBs Ofcom also intend to publish on our website a machine-readable version of that list on a website hosted by Ofcom so that it can be selected by a WSD through a process known as “database discovery”. Ofcom would expect that list to include those database operators which have informed Ofcom that they are ready to start providing services to white space devices.
It is interesting that Sony is moving into this space, which probably means they will start producing equipment that uses white space technology for short range communication, such as say a PS4 to its peripherals.