Radio spectrum is the essential ingredient in every type of wireless service, from satellites and radars to broadcasting and mobile communications. But not all spectrum is equal. Certain frequencies are highly sought after and are consequently in very short supply.
The upcoming release of UK spectrum in the switchover from analogue to digital terrestrial TV (DTT) is one of the largest in the spectrum ‘sweet spot’ since terrestrial TV spectrum was originally allocated in the late 1950s.
The spectrum being released by the digital switchover (DSO) is in the UHF frequency band of 470-862MHz. It is the sweet spot because these frequencies are low enough to provide good coverage with relatively few transmitters, while accommodating bandwidths large enough to provide capacity for mass market services, such as mobile communications.
There are three types of UHF spectrum being considered:
- cleared spectrum: 112MHz of spectrum that will become available as a result of DSO.
- channel 36 and 69: these are used predominantly for airport radar and wireless microphones and have the potential to be cleared, making up 16MHz of spectrum.
- interleaved spectrum: the ‘white space’ that exists geographically between the TV transmitters to prevent interference.
Who is interested?
DSO offers broadcasters real opportunities for wireless innovation, for example, high quality video and interactive media service delivered to handheld devices, more DTT channels and HDTV (high definition TV) on terrestrial.
Naturally these possibilities have attracted the attention, not just of broadcasters, but also Internet service providers (ISPs), mobile network operators (MNOs) and telcos, all of whom want to ensure their fingers are firmly planted in the spectrum pie.
MNOs are attracted by the allure of lower cost network rollout for existing services - resulting from the low frequencies at stake - and the possibility of new spectrum for emerging services such as mobile TV and GSM/3G femto cells in the home.
Ofcom’s initial plan was to auction off the entire range of newly released spectrum. This was based on its research which showed users thought an increased number of DTT channels was more important than having HDTV, mobile Internet or mobile TV.
However, many parties were not happy with the purely market-led approach.
Among the most vociferous were public service broadcasters (PSBs) and the group HDforAll, who argued that HD would be the new broadcasting standard so it must be available on the DTT platform. An auction would not realise this as MNOs, with far superior access to financial resources, would easily outbid PSBs.
The PSBs and HDforAll asked Ofcom to reserve between five and ten channels for HD on DTT. Ofcom is due to decide late this year whether it can do this, or whether the market-led approach will naturally lead to the best possible use of frequency.
Whatever the outcome, it is clear that DSO will bring sweeping changes. And, aside from the battle to win frequency, there are a number of technical issues afoot.
Technical issues
Whatever Ofcom’s final decision, the uncertainty over which mobile TV standard will prevail was reduced when the European Commission recently backed proposals to adopt DVB-H as the common standard in Europe.
However, integrating DVB-H with 900MHz GSM in a handset could be problematic as the GSM transmissions can block or desensitise DVB-H reception, unless there is stringent filtering in the terminal front-end. Successful filtering is clearly aided if the mobile TV receiver operates in a band well removed from the spectrum used by GSM 900.
Issues also arise from out-of-band emissions from GSM basestations interfering with DVB-H.
These problems will be accentuated if some of the released UHF spectrum is used to accommodate GSM or WiMAX.
With various channels reserved for astronomy, emergency services and special events, for example, as well as the sheer number of potential bidders for the spectrum released by DSO, the suitable spectrum available for DVB-H may be limited.
Ofcom’s initial proposals said cleared spectrum remains primarily for DTT operators, and any other service wishing to operate within its frequency would have to protect the coverage and reception of these networks.
Nonetheless, broadcasters asked for further assurances regarding interference and Ofcom commissioned research showed that mobile TV was likely to have a hole-punching effect on broadcast signals.
In order to successfully implement wide scale mobile TV, MNOs will have to accommodate the broadcasters’ one-way transmission with their own two-way transmission in terms of radio system management and network management.
Recent harmonisation recommendations produced by the European Communications Committee reserve the 798-862MHz range for mobile services so as to restrict the possibility of interference with existing services in the UHF band.
DTT and HD
Whilst Ofcom has given HD a lower priority following its consumer research, HDTV sets continue to sell well in the shops.
After Ofcom’s initial proposal for a market-led approach, the BBC suggested that PSBs be given additional spectrum for HD on Freeview while a migration from MPEG2 to MPEG4 takes place. Operating solely on this standard, says the BBC, will free up substantial spectrum which can then be ‘handed back’.
But most of the millions of HDTVs sold to date are compatible only with MPEG2.
In addition, rooftop aerials are generally intended to cover a limited range of frequency channels to improve their efficiency. Current TV transmissions from each site fall within an ‘aerial group’. If a new entrant were to make use of an out-of-group DTT frequency at a site then a significant part of the potential audience would be unable to receive the transmissions without installing a new aerial. So it seems that the not-too-distant future could necessitate substantial upgrading.
It is still unclear exactly how the digital spectrum will be divided. But with the new opportunities which the DSO brings, and the inevitable disruption it will cause, now is definitely the time to be preparing for its consequences.