Post by kolton0 on Apr 10, 2013 16:59:24 GMT 1
Although there are seemingly large gaps between the frequency bands for 2G, 3G and now 4G services, the spectrum is a busy place and available slots are rare.
When terrestrial analogue tv was switched off last year, some slots were left around 800MHz and these will be used by 4G services, as will two further chunks at 1.8GHz and 2.6GHz.
Despite raising barely a tenth of the revenues generated by the 3G auction and falling £1billion short of the forecast, the 4G spectrum auction still raised £2.3bn and five of the seven bidders came away with a share of the spoils.
The higher frequency slice at 2.6GHz has been split between Vodafone, Niche Spectrum Ventures (a BT subsidiary) and EE (Everything Everywhere). This is the frequency range that will be most suitable for delivering HD movies and games to smartphones. It is not clear how BT can hit the ground running with 4G services, but having the bandwidth to carry them is a start at least.
EE already had use of the 1.8GHz frequency, part of which it had sold to Three (Hutchinson 3G), as a result of using the 3G capacity of Orange and T-mobile – thus allowing it to launch its 4G service ahead of everyone else and ensuring that Kevin Bacon seemed to be all-the-time, everywhere!
This leaves the 800MHz segment, which will be suitable for providing country-wide coverage due to its greater propagation distance. Because this frequency will require fewer base stations and less infrastructure, this segment is seen as the jewel in the 4G spectrum crown and has been split between EE, Vodafone, Telefonica UK (O2) and Three. And it is this bandwidth that could prove troublesome to Freeview viewers when 4G services are rolled out. The higher frequencies at 1.8GHz and 2.6GHz are not considered a problem in this respect.
Digital Terrestrial Television (DTT), or Freeview, has had its share of technical problems since the analogue switch-off started five years ago. One was the '8k mode issue', which meant that while some old equipment worked fine when the signal was split into 1705 sub-signals (the so called 2k mode), it didn't once the signals were split into 8817 sub-signals (8k mode). With the seemingly rock bottom price of Freeview-enabled tvs and the insatiable consumer desire to replace household electronics, this was only a temporary problem.
Another issue was 'digital area overlap', where signals from more than one transmitter were being received, causing out of area programming. Again, with accurate tuning, this was a short-lived issue.
More significantly followed the paradox of an apparently weak signal being caused by a signal that was too strong. When the analogue signal was turned off, most transmitters increased the power behind digital signals. This had little effect on most homes and meant those further away from the transmitter could receive stable signals.
However, those close to a transmitter, or where high-gain aerials or signal amplifiers had been used, effectively had 'too much signal' – a signal overload. Because of protection circuitry in the receiver, this appeared as a weak signal. The general wisdom being that signal amplifiers, although useful for the lower power digital signals before the switchover, tended to amplify the interference, rather than improve signal quality.
The way to deal with this was to remove signal boosting devices. If that didn't work, then a single or variable attenuator needed to be added. The best result is a signal strength of around 75%, strong enough that it doesn't overload the receiver.
Will 4G swamp the signal?
Now the dust has settled after the analogue to digital switchover, it could be that similar problems will be created by the new tranche of 4G services at the 800MHz frequency. While they do not overlap directly with the Freeview frequencies – which are spread over 31 channels – they rub shoulders closely enough to potentially present signal overload problems once more.
Top end Freeview signals (on channels 59 and 60) are transmitted at 794MHz and Freeview receivers can be sensitive to within 72MHz (nine channels) of that, potentially causing an overload.
Another potential issue relates to the receivers used in some, but increasingly few, Freeview TVs. These superheterodyne, or superhet, receivers are accused of potentially picking up interference.
Interference in the 4G/Freeview world comes in two flavours.
One problem is adjacent channel interference (ACI), caused simply by the proximity of the LTE or mobile signal to the tv bands. While this is true of any receiver, the other problem – image channel interference – is more closely associated with superhets. This can cause a spike of interference in a channel well removed from the signal's frequency.
It is also possible that receivers may not be able to decode the transmission and therefore provide the necessary reception, due to the Signal Interference Noise Ratio caused by 4G signals. As the only 4G signals to date have been transmitted by EE at 1.8GHz, this has not been an issue.
But, with the auction out of the way it is expected that 4G services at the lower frequency will be introduced imminently. 4G-ready Sony, Samsung, Blackberry, HTC and Nokia phones are already available from Vodaphone and, having spent more than any of the other operators (£790million) on securing the licences, it will be keen to roll out 4G this year.
The company set up to deal with this problem is Digital Mobile Spectrum (DMSL), owned by the four 800MHz licence holders and funded by £180m of revenue from the auction. Ofcom estimates that 2.3m households that rely on DTT will be near a 4G base station and may be affected.
It will be DMSL's job to support these households to resolve reception issues. Originally, it was suggested that only houses which use DTT as their prime method of receiving tv – around 900,000 – would be eligible for support. But difficulties in determining who these people are means blanket support will be available for all affected households.
Dealing with the effects of 800MHz signals
Reuben Braddock is a technical advisor for DMSL, a company that has relaunched itself for public consumption as AT800, to reflect that its remit is solely to deal with the 800MHz band and its effect on DTT. Braddock believes some of the attention has been diverted from the core problem. "I think the superheterodyne problem has been overplayed a bit – ACI is more of an issue from an interference point of view.
However, the one that drives the numbers and makes it a UK-wide problem, rather than a geographically limited problem, is the blocking or overload problem which is just the power of the LTE signal being received at the TV receiver."
The simplest remedy to this will be filters, one of which will supplied free to each affected household. Braddock said: "DTT receiver filters work by allowing the tv signal through, while attenuating the power received from mobile base stations in the 800MHz band. In this way, the filter effectively enhances the frequency selectivity of the tv receiver."
These filters will be sent out in advance of 4G's arrival. By plugging them in without 4G signals around, users can make sure the filters don't affect the DTT signal. Customers in weak DTT signal areas, for example, might find the attenuation caused by adding the filter could affect reception. In this instance, DMSL may provide a different sort of filter or find alternative solutions – the most extreme of which is transferring the customer to satellite or cable television free of charge.
Five companies have been awarded contracts to supply the filters, each design fulfilling a slightly different technical requirement, depending on location and therefore signal strengths. One thing worth noting at the moment is that because all tv transmitters are in place, DTT signal strength can be assessed for any given place. However, none of the 4G companies have announced where their base stations will be and, until they do so, filtering requirements will not be clear.
The filters will be designed either to remove 4G signals from above Channel 59 or above Channel 60, depending on the area, and to filter out different strengths of signal, depending on how far they are away from the base stations. Link Microtek will be supplying what it describes as 'the premium' Channel 59 filter (those to be used closest to the base station). The company's sales director Stuart Hendry commented: "DMSL has specified different levels of insertion loss and rejection, so some are higher performing than others.
Our Lynx filter is an air cavity filter and there are a number of poles inside that give us our rejection and insertion loss performance. Not all of the filters will work this way. Some filters are from companies who work in the television world. We work in the rf and microwave world and do things differently, coming at it from the interference mitigation point of view, which is why you end up with the higher performing filters at higher cost. There are companies coming at it from a low cost filter television point of view."
UK sets example for other European countries
The filters plug into the aerial socket at the back of the tv, so are simple to fit. Braddock added: "As an organisation, we also have to cater for people who have issues with rooftop amplifiers, who will need to have an aerial installer, and communal properties, where a signal may need to be boosted for a number of flats in the building. The filters, according to Ofcom research, should solve the vast majority of the problems."
DMSL's remit goes no further than ensuring DTT remains unaffected by 4G base station signals – a function, incidentally, that Braddock believes is not being undertaken proactively elsewhere in Europe, even though many countries are going through the same 4G roll-out process.
But could other issues emerge? Hendry commented: "I don't think anyone really knows. DMSL has only been set up to look at the LTE base station issue with Freeview and nothing else; if anything else happens, it doesn't have the licence or the approval to do anything about that."
One particular issue may be the effect the handset has when transmitting in close proximity to a Freeview tv. "Handsets transmit at a slightly higher frequency, but still in the 800MHz band. The adjacent channel is the downlink for the base stations and the handsets operate at a slightly higher frequency than the base stations.
But, of course, you will have the phone in the same house as your television and antenna and everything else, so there could be an issue with overload, rather than interference. I think that is something that needs to be looked at and it is not going to be covered by these filters," Hendry concluded, although the filters should still attenuate signals going through the aerial, just not those picked up directly by the TV itself.
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