Broadcast Technology Jammed Tomorrow (Part 2)

4/16/2013 9:25:17 AM

Personal encounter

Probably the simplest way to illustrate the kind of interference problems that millions of viewers may encounter as a result of the 4G plan is to use a specific example – my own!

The map shows where I live relative to the two closest DTTV transmitter sites. At present I get adequate signals from both transmitters. Although there is no local 4G coverage as yet, the closet existing mobile base station to my house is about 400 meters away, in the same direction as Angus.

The documents published over the last year or two regarding 4G propose base station EIRPs from 59 dBm/10MHz up to 64 dBm/10MHz. for the sake of example I chose a value of 0-5 kW for each 5 MHz 4G channel. In practice it is now set at just over 1kW for each 5MHz 4G base station channel.

Personal encounter

Personal encounter

By the end of October 2013 the ‘4G clearance’ should be complete. At that point any DTTV broadcasts that were previously being transmitted above ch60 will have been shifted down to lower frequencies. For Angus the result I can expect using a fairly typical TV aerial will look like the spectrum shown below. Note that the BBCA multiplex is planned to be adjacent to the 4G base station transmissions and use ‘Ch60-’. This multiplex is important as it carries all the main BBC stations – BBC1, BBC2, etc.

The TV multiplexes from Angus and Durris each give me signal levels on the antenna down-lead somewhere between about 3 mV and 6 mV. Whereas a 4G base station placed at the local mobile phone site would produce about 78 mV for each 4G downlink channel.i.e. each 4G channels is over 20dB more powerful than the wanted DTTV multiplexes. Given six 4G downlink channels the actual combined voltage they will produce on average will be around 200 mV!

Anyone familiar with the design and behavior of domestic TV sets, boxes, etc, will realize that this level of input is alarmingly high. It is likely to overload the distribution amplifier I use, and probably stop me watching DTTV unless I can cut down the 4G interference level. From this example I can expect many people within 500 meters of a base station to lose DTTV entirely!

Spectrum be the end of 2013

Spectrum be the end of 2013

Filter effects

The Ofcom report “Technical Analysis of Interference form Mobile Phone Base Station in the 800 MHz Band to DTTV – Further Modeling” (published February 23, 2012) examines four different filter designs for reducing 4G interference. Two of the filters were aimed at being suitable for use with normal domestic DTTV receivers. The other two were aimed at Communal Antenna Systems (CAS) before the preamplifier/distribution systems.

The graphs below show the gain-frequency profilers taken from the two filters designed for use with household DTTV sets and set-top boxes. Two filters were considered because the planned continued use of ch60 poses a serious challenge to making satisfactory filters that can be afforded. Strictly speaking, the intent is that DTTV in this channel may actually be nudged down to ‘ch60-’. However this only represents a downshift of 166 kHz. It seems doubtful this minor tweak will provide much improvement in 4G rejection.

Domestic DTTV RX filters

Domestic DTTV RX filters

The root of the problem by continued use of ch60/60- is that it demands a filter that can transit from passing signals to rejecting them in a very narrow transition range. The graphs below illustrate the difficulties when we compare the two designs. The ‘ch60 filter’ is aimed at situations where ch60 is being used. This means it has to pass the ch60 DTTV signals with low loss and avoid excess tilt in amplitude or phase across the channel that would, itself, degrade the receiver’s ability to operate. Since the LF end of the 4G band is only about 1 MHz away, the result is poor 4G rejection for frequencies in the lowest few MHz of the 4G band. It also leads to relatively poor rejection across the rest of the 4G band. Even well into the 4G band the rejection is only in the range from 25dB to 30dB. The example used earlier as an illustration of the situation that may be common, produced a 4G level from the domestic DTTV antenna in the range between 100 mV and 200 mV. In such cases the ‘ch60’ would reduce this to being 3 to 10 mV. Given a domestic DTTV RX that rejects efficiently, a DTTV output from the domestic antenna around 3-5 mV may allow satisfactory reception. However, it is worth reminding ourselves that this example is one where the DTTV RX system is 400 meters from the 4G base station radiating the (assumed) challenge.

If you happen to be living in a location where DTTV does not require the use of ch60/60- the situation improves. This can be seen by considering the ‘ch59 filter’ which is a design aimed at being implemented in such circumstances.

Because this filter does not need to ensure that it passes ch60/60- it can employ a wider transition and achieves around 35dB of rejection, being around 25dB even at the LF edge of the 4G band. This would effectively cut the above down to around 1.5 – 3 mV, which may well be a useful improvement in many cases.

CAS filters

CAS filters

The CAS filters graph (below, right) shows the results for the second type of filter considered. This is aimed at communal systems. It has a markedly better performance than the ‘domestic’ type. I assume that argument here is that a communal system can be more costly because the price is spread across a number of households. However it is worth noting at this point that many ‘non-communal’ households these days employ a distribution amplifier. This is typically somewhere like in the loft. These add an extra layer that may require protection form 4G signals. Otherwise they may generate intermodulation that makes it harder for the following RX to read the wanted DTTV modulation. Given this, it may be worth wondering if filters to the above ‘CAS’ standard may be required in some or many single-household dwellings. As with the domestic filters. Though, it can be seen that being able to sacrifice ch60 provides a clear improvement in filter performance.

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