Wireless Waffle - A whole spectrum of radio related rubbish
Going Underground...signal strength
Sunday 20 October, 2013, 13:49 - Radio Randomness
Posted by Administrator
In major cities around the World it is now unusual to find yourself in a place without a WiFi connection or two going for free. Coffee shops, bars, department stores and many other places now offer free WiFi to their customers. Normally all that is required is to enter an e-mail address and accept a set of terms and conditions and you are surfing gratis. For anyone roaming and facing extortionate fees for their data connection, these WiFi connections can be a godsend.

But there are still some places around the world, even in major cities, where WiFi connections are few and far between and top of the list are the various underground metro systems. To make matters worse, many metros are not only WiFi free zones but are lacking mobile phone coverage too. Thankfully this situation is becoming less frequent as operators (whether transport or mobile) decide to provide the infrastructure necessary to enable mobile coverage underground. Covering underground tunnels and stations is not difficult, but it can be expensive. It requires the use of special antennas and in many cases the laying of leaky feeder - a coax cable specially designed to 'leak' signals along its length thereby acting as a long antenna.

The London underground has long been an area devoid of any kind of broadband internet connectivity, but for the 2012 Olympic Games many stations (not the tunnels connecting them) were fitted with free WiFi by internet service provider Virgin Media. Since the games, the WiFi has no longer been free (except for existing Virgin Media customers) but coverage is slowly being extended.

If you want to know which stations have coverage, you could visit Virgin Media's station guide, or alternatively 'there's an app for that'... The Station Masterstation master app app (unfortunately only for the iPhone at the moment) will let you know exactly which stations have WiFi connectivity and is updated regularly with information from people using the underground network and so often it shows stations that have got WiFi but which have not made it to the official station guide yet. As if to prove a point, at the moment Station Master is showing 123 stations with a WiFi service whereas the Virgin Media web-page only shows 120!

london underground wifi map

Not only does it show you where there's WiFi on the London underground but it also has 3D plans of nearly all of the central London stations (so you know which stairwell is quickest and easiest for getting from one line to another) and also will tell you exactly which door to stand at to get on the train so that you are in the right place to get to that stairwell or walkway when you get off the other end. For anyone who uses the London tube to get around, the app is a must have (even at GBP1.99). Of course you'll have to have a WiFi or mobile connection first to be able to download it in the first place!
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TP-Link WR702N Wireless Nano Routersignal strength
Monday 14 October, 2013, 18:42 - Radio Randomness, Equipment Reviews
Posted by Administrator
Whilst Wireless Waffle is branching out into equipment reviews the time seems right to give a plug to the snappily titled TP-Link WR702N wireless N nano router.

What is it? It's a small (hand-sized) wireless router that can be configured to do a number of jobs. In essence it's a wired network connection and a wireless network connection in a single box that can be configured in a number of ways. For example:
  1. It can add a WiFi connection to a normal wired network or internet connection.
  2. It can share an existing wired internet connection with multiple devices or users.
  3. It can extend the range of an existing WiFi network.
  4. It can act as a WiFi client, to give a wireless connection to things that don't have one (e.g. TV set-top-boxes).
And all this in a case no larger than a deck of playing cards and at an amazingly low price.

The device deserves a special mention because of both its flexibility in being able to do a number of different tasks, but most importantly because of its usefulness. The only downside is that it can be fiddly to set-up (see the CNET review of the device for more info on this).

hotel girl laptopImagine, if you can, being in an overseas hotel where there is a free wired internet connection but no WiFi and all you have to connect to the internet are your phone, a tablet and a laptop that's so slim and sexy that it doesn't have a wired network socket. You could turn on data roaming, but we all know how extortionate that is. You could get high, get moving, get naked and get clear to see if you can snag a signal from a rogue, unencrypted WiFi connection. You could wander down the street to the nearest coffee shop and hope they have WiFi. Or... you could have brought your TP-Link WR702N with you and plug it into the wired connection. Before you can say 'hey espresso', you have a ready made WiFi connection and both your phone and tablet are ready to update your Facebook profile!

Imagine moving into a new house to find that the WiFi connection covers every room except the kitchen. How will you read the morning e-newspaper over your coffee without WiFi in the kitchen? Simple, get your TP-Link WR702N and place it somewhere between the kitchen and your existing WiFi hub and before you can say 'special-k presto' you're back online.

tp link wr702nImagine discovering that you've missed the latest thrilling installement of 'Britain's Got Strictly Celebrity Antique Hunter On Ice'. Your satellite set-top-box has the option to run 'i-TV-Player-on-Demand' but it's only got a wired internet connection and you only have WiFi at home. Just break out the trusty TP-Link WR702N and before you can say 'tres fresco' you're streaming over-the-top video like a pro.

Finally, imagine trying to build a tower of plastic boxes that reaches the moon, to find you're a centimetre short. Just grab your TP-Link WR702N and put it on top and 'pay tesco' you've made it to the lunar surface.

Seriously though, for anyone who travels a lot and stays in hotels with no WiFi, who has patchy WiFi coverage at home, or wants to connect their blu-ray player to the internet to use Netflix, this little beauty does a great job and comes with the Wireless Waffle, 'Recommended Useful Broad Band Internet Signal Heighthener' seal of approvalness. Oops, that was supposed to be serious and look what happened. Maybe Wireless Waffle should just stick to being facetious.
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Topping TP20 Mark II - A Noisy Noise Annoys (Part 4)signal strength
Wednesday 25 September, 2013, 11:35 - Radio Randomness, Spectrum Management, Equipment Reviews
Posted by Administrator
In the past, Wireless Waffle has discussed various things that cause radio interference but which are not supposed to including, for example Power Line Telecommunications devices. This time around it's the turn of a Class T audio amplifier to come under the spotlight.

topping tp20 spotlight

Class T amplifiers are really Class D amplifiers but are supposedly more efficient. Any clearer? No, probably not. The idea behind these types of audio amplifiers (noting that the Class D principal is also used in some radio transmitters too) is that instead of amplifying an analogue signal in an analogue way, such that the output voltage is just an amplified version of the input voltage, they switch the output voltage on and off at a frequency higher than the audio signal, and then use a filter on the output to smooth the square wave that they produce back into a nice analogue signal. This method is known as pulse width modulation.


pulse width modulationThis switching technique is exactly the same one that is used in the majority of modern power supplies (SMPS) and has the prime advantage that as the transistors that do the switching are either turned on or off, they are never in some intermediate state where they would have to act as a resistor and in doing so dissipate power and heat. So they are highly efficient and it is possible to generate audio with over 90% efficiency meaning that more of the power is converted to sound and less is lost as heat, which is, after all, a very admirable quality.

As with switch mode power supplies a good filter is critical in ensuring that none of the original square waves find their way to the output. Square waves are very good at producing harmonics and therefore are equally good at generating radio signals and, of course, radio interference. There have been many cases of switch mode power supplies causing such radio interference and their use in, for example, LED lighting, means that the number of possible sources of interference is ever increasing.

The main problem is that, in many cases, the device will work without the filter fitted - if (and only if) the device that it is powering is not too fussy about all those square waves (e.g. an LED) or has a method of smoothing them out itself (e.g. a loudspeaker). A loudspeaker is basically a large inductor, which is what the filters in switching amplifiers also comprise. Feeding the nasty square waves on the output of the switcher directly into a loudspeaker will not result in a noticeable loss in fidelity (assuming the switching frequency is well above the audible frequency range), nor any particular loss of efficiency. So why fit the filter? To stop radio interference, that's why.

topping tp20 examination

So step up to the examination table, the Topping TP20-MK2 Class T Digital Mini Amplifier. One of these was recently purchased for the Wireless Waffle office, so that we could listen to the oidar through a bigger set of speakers. Being compact, and efficient, and coming in a shiny silver case, it ticked all the right boxes. But ouch, what noise from yonder shiny case breaks? As soon as the amplifier was turned on, reception of radio signals on just about any frequency was wiped out by noise. Even an FM tuner sat receiving a strong local transmission which was previously a perfectly quieting signal, was sent into oblivion by the amplifier. Obviously, the filter on the output of the Topping amplifier is completely inadequate for the purposes of curbing radio interference.

In cases such as these, there is little that can be done. Other than taking the device apart and replacing the filter components with better ones (an idea that is not as daft as it sounds), the solution is to junk the device and use a traditional linear amplifier instead. Which is what has been done. Bye bye trendy, offendy Topping, hello dusty, trusty Sony.
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700 MHz - The Heat Is Onsignal strength
Wednesday 4 September, 2013, 09:46 - Spectrum Management
Posted by Administrator
The new 700 MHz mobile band (703 - 748 paired with 758 - 803 MHz) is a hot topic amongst spectrum aficionados around the world. It raises a number of technical and political issues which are far from being fully resolved. On the political side, the main battle is between broadcasters (who currently occupy the band) and the mobile community who are keen to brush the broadcasters aside to clear the spectrum for more mobile broadband services. Broadcasters argue that they need more spectrum to cope with high definition and other developments whilst the mobile operators believe that the spectrum has greater value if used for extending broadband capacity and especially, given the good propagation characteristics at 700 MHz, the coverage, of their networks.

unlit bonfireWhilst the political issues are like a pile of tinder dry kindling just waiting for a spark to set them alight, there are technical problems to address as well: The way in which the band is paired means that mobile handsets will be transmitting in the frequencies immediately adjacent to television broadcasts. Television broadcasts using channel 48 (in the Region 1 8 MHz raster) will use frequencies up to 694 MHz. Mobile handsets will use frequencies as low as 703 MHz. This leaves a 9 MHz gap between the two.

Whilst it might seem that the likelihood of a low power (50 milliWatt) transmission from a mobile phone causing interference to reception of a high power (100 kiloWatt) television service is small given the vast (2 million times) difference in transmitter power, the reality is that the mobile handset could be within only a few metres of the television receiver whereas the television transmitter might be tens of kilometers away. The strength of interference from the handset could therefore be orders of magnitude higher than the incoming television signal. Using free space path loss:
  • A 50 mW (23 dBm) mobile transmitter 3 metres away produces a field strength of 91 dBuV/m
  • A 100 kW (80 dBm) television transmitter 30 kilometers away produces a field strength of 68 dBuV/m
So the signal caused by the mobile would be 23 dB (or 200 times) bigger than the television signal. In reality the digital signal would be much smaller than this as the free space calculation takes no account of terrain or other topographical factors and thus the difference would be even bigger.

guard band girlThe 9 MHz gap between the mobile transmissions and the television reception is known as a guard-band and is there to give a chance for the television receiver to filter out the unwanted mobile transmissions. There is plenty of work going on to check that this is the case but it will depend on a number of factors that are outside the control of the mobile operators or television broadcasters such as:
  • The quality of the television receiver. Those made to a low price may not perform as well in this regard as more expensive receivers.
  • The quality of the receiver installation. An old antenna may receive less signal and poor coax will allow more mobile signal to leak into the receiver exacerbating the problems.
  • The proximity of the mobile transmitter to the television antenna. In the case where television reception is through a 'rabbit ears' antenna on top of the TV, the distance between the antenna and the mobile could be far less than 3 metres, or even 1 metre.
  • The distance of the receiver from the television transmitter. Those close to the transmitter are less likely to suffer interference but those in areas of fringe reception are at much greater risk.
  • The use of television signal amplifiers. Such amplifiers can easily overload and stop working when presented with a strong nearby mobile signal.
There are other factors too. There are also things that can be done to mitigate against these problems such as replacing aging installations and fitting filters.

The rules of spectrum use state that new users should implement their transmitters in such a way as to protect existing, incumbent users from interference and in that respect, the work to ensure that the new mobile services do not cause harmful interference to television services is totally appropriate. But there is another technical issue that needs to be considered, that of interference from the television transmitter into the mobile network.

Consider a mobile base station that is 1 kilometer away from the television transmitter, trying to receive a signal from a mobile handset that is 500 metres away. Let's run the free space path loss equations again:
  • A 50 mW (23 dBm) mobile transmitter 500 metres away produces a field strength of 47 dBuV/m
  • A 100 kW (80 dBm) television transmitter 1 kilometer away produces a field strength of 98 dBuV/m
streichholzerIn this case, the mobile signal would be 51 dB (or 125,000 times) weaker than the television transmitter. Again the 9 MHz guard-band is there to allow the mobile base station to filter out the television transmission but the job is much (much!) more difficult. Move the mobile handset further away from the base station, or increase the transmitter power of the television signal, or move the base station closer to the television transmitter and the situation gets even worse. In terms of who needs to protect who from interference, interference caused by the television signal into the mobile networks is secondary to ensuring the protection of television reception but is potentially an equally burning problem.

Whilst it is possible to develop filters that can provide 50 dB, 60 dB or even more rejection of the television signals, they are costly. Of course not every site requires such an expensive filter: only those close to television transmitters on channel 48 will need them. But whilst television frequency use is not normally very dynamic, as re-planning of networks takes a lot of co-ordination, they do change channel from time-to-time and so knowing which site to fit the filters to cannot be done with complete certainty.

So if the political issues form tinder dry kindling just waiting to be lit, the problems of interference from mobile handsets into television receivers are a bucket of petrol poured on that kindling. It may therefore be that the problems of interference from television transmitters into the mobile network are the spark that gets the fire burning!

midsummer bonfire
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