Tuesday 28 April, 2009, 07:22 - Radio RandomnessThe threat to short-wave reception caused by PLT (a.k.a. BPL) devices is something that has been covered on Wireless Waffle on numerous previous occasions.
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Whilst it hasn't reached the point of naked protestors parading along the streets of London just yet, a while ago the technical group trying to curb the spread of these devices petitioned the UK Government to do something about it. The Government's response was rather lacklustre:
As with all electrical and electronic products sold in the UK, Power Line Technology (PLT) equipment is required to meet the relevant regulations before it can be placed on the market. In particular, it must comply with the Electromagnetic Compatibility Regulations 2006 (the EMC Regulations) ... and any person who places such products on the market ... must ensure that the products comply and apply the ‘CE’ mark.
The Department for Business Enterprise and Regulatory Reform (BERR) is responsible for the EMC Regulations. Enforcement powers are delegated to local Trading Standards offices, and to Ofcom where there is a radio spectrum protection or management issue. Ofcom estimates there are around 500,000 pieces of PLT equipment in use in the UK. Ofcom have received around 84 individual complaints of interference attributed to PLT equipment. All of these complaints are in the process of being investigated or have been successfully resolved. Each complaint is investigated on its own merits. We do not believe an outright ban of all powerline adaptors is justified.
A lot of buck-passing with the end result that nothing happened. But not to let a roaring lion lie, the good people at UKQRM have submitted a second petition:
We the undersigned petition the Prime Minister to require the relevant regulatory authority namely Ofcom to take active and speedy measures to test samples of all makes and types of PLT device and to remove from the UK market all those devices where the sample is found to be non compliant with the requirements of the Electromagnetic Compatibility Regulations 2006. And to take all practicable and necessary steps to prevent anyone placing non compliant PLT devices on the UK market now and in the future.
Wireless Waffle believes that the spread of PLT devices is something which needs to be checked and that the more cage rattling that is done, the better the chances of some real action being taken.
If you are a UK radio user, listener or someone who depends upon the radio spectrum for your profession or livelihood in the UK, whether you are interested in short-wave or not, we would urge you to sign the petition. The slow march of PLT devices represents what will no doubt be the first of many attacks on the precious raw material which underpins so many UK jobs and with the credit crunch already hitting people's employment, anything which protects future generations has to be good.
Please go and sign the petition at http://petitions.number10.gov.uk/SaveShortwave2/ and add your name and voice to ensure that future voices will be able to hear each other!
Let nation (be able to continue to) speak peace unto nation... as someone once said.
Monday 27 April, 2009, 16:50 - Spectrum ManagementToday, Wireless Waffle's continuing series attempting to explain and simplify the many complex radio technologies, techniques and applications tackles perhaps one of the most complicated spectrum sharing schemes that exists. OFDM or 'Orthogonal Frequency Division Multiplex' to give it its full name is a clever method for sending data across the ether in such a way as to circumvent some specific, commonly occuring, problems. Though many people refer to OFDM as a modulation scheme, it is not! It is more accurately described as a multiplexing or sharing scheme and it can be used as an access scheme to allow the sharing of the spectrum between different users (in which case it becomes known as OFDMA - the 'A' being for 'access').
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Before looking at what OFDM is, let's first consider the problems it aims to address. Chief amongst these are the problem of reflections and one of the upshots of reflections, frequency selective fading. The path between any two points on the radio landscape will rarely be straightforward. The signal may be received directly (i.e. the orange path in the picture below) as well as via reflections from various nearby and distant objects (the purple paths). Reflections from distant objects can commonly be seen on (analogue) television pictures where the main signal is followed by several 'ghosts', each representing the same signal arriving slightly delayed due to the path of the reflected signal being longer than that of the direct one. Where reflections are from nearby objects, the effect is somewhat different and manifests as 'holes' being punched into the received radio spectrum causing some frequencies to be severely attenuated whilst others remain largely unaffected.
Into this environment, we now introduce the requirement to transfer large amounts of data. For the sake of argument, let's choose 1 Mbps. If we modulate this data signal onto a radio carrier using basic BPSK (binary phase shift keying - the most basic of digital modulation schemes) the resulting signal has a bandwidth of around 1 MHz and a symbol period (ie the time representing each bit of data) of 1 microSecond. In order to successfully receive this signal, one key factor must hold true: reflections from any delays need to be significantly shorter than 1 microSecond. This is because:
* If a reflected signal arrives at the receiver 1 microSecond later than an undelayed signal, the receiver has finished receiving the bit concerned and has moved onto the next one. Thus the reflection is pure 'interference'. This is equally the case for delays of half a microSecond wherein the delayed signal has equal potential to interfere with the bit we are trying to receive and the one following it.
* A delay of 1 microSecond produces frequency selective fading notches every 1 MHz. As such, if the delay is longer than 1 microSecond, there is every chance that the notch in the frequency spectrum produced by the delay will punch a hole right in the middle of our wanted signal making it unreceivable.
A delay of 1 microSecond represents a reflected path that is 300 metres longer than the unreflected path (the speed of light times 1 microSecond). For a short distance link, this may not be difficult to achieve, but as the length of the link starts to exceed 300 metres, the potential for reflections causing problems increases. With a radio paths over 3 km long, for example, a reflective object which is more than 15 degrees away from the centre line of the path between the two ends will cause such a reflection - clearly a strong likelihood.
One solution to this problem is to minimise the potential for such reflections being caused by focussing the signal carefully between the two ends of the path using highly directional antennas. In this situation, reflections which are 'off-beam' will be heavily attenuated both at the transmit and receive ends of the link. In broadcast situations, however, whilst receiver antennas might be able to be directions, the transmit antenna is, virtually by definition, aiming to send out a signal over as wide an area as possible and in these circumstances reflections are inevitable.
Another solution is OFDM! In OFDM, we take the 1 Mbps of data and break it up into a number of smaller, slower, data streams. For our example, let's break the stream into 100 smaller streams, each which carries only 10 kbps of data. If we modulate one of these streams onto a radio carrier using the same BPSK technique, it now occupies a bandwidth of just 10 kHz and has a symbol period of 100 microSeconds. As such, it can now tolerate delays which are 100 times larger than that the original 1 Mbps conterpart. The problem is that there is only one of them and we need to transmit 100. Normally, when transmitting a 10 kHz wide signal, we would need to leave some space either side of the signal to separate it from its neighbours. A factor of 50% is not unusual meaning that for each 10 kHz signal we might require 15 kHz of spectrum. For our 100 signals, we would therefore require 1.5 MHz of spectrum, making this significantly less efficient in spectrum terms than the single carrier solution. The diagram below shows the spectrum of a single data carrier.
If, however, we modulate each of the adjacent signals intelligently and 'orthogonally' the requirement for space is negated and we can transmit the 100 carriers just 10 kHz apart, putting them back in the 1 MHz of spectrum that the original single carrier solution occupied. Orthogonal implies 'at right angles' and in essence, each adjacent carrier is modulated so that it is 'at spectral right angles' to its neighbour. The diagram below shows the spectrum of multiple orthogonal OFDM carriers. Note that at the centre of each carrier, the signals from all of the adjacent carriers are at a null of zero size.
The upshot of this clever technique is that we can now transmit the data in the same amount of spectrum but in a way in which reflections and delays of much larger extents can be tolerated without effect, using 100 smaller, slower carriers rather that 1 large, fast one. The best non-technical analogy might be the need to transfer 100 bricks across an area of rough land. If we put all 100 bricks in a single wheelbarrow and push it along, it will get bumped and knocked and bricks will fall out. If there is a big enough obstruction the wheelbarrow will get stuck and nothing will make it to the other side of the land. Alternatively, if we put 1 brick in 100 separate wheelbarrows and push these over the land, whilst some may lose their bricks or be blocked, there is a much higher chance that a goodly proportion will make it to the other side.
An additional advantage of OFDM is that if there is interference on some of the spectrum within our 1 MHz channel, the single carrier solution fails, whereas for the OFDM solution only those carriers where the interference is present fail. Thus it is possible to maintain a connection in the presence of certain types of interference with OFDM. Being even cleverer, if we know which of the frequencies are affected we could change the error correction or modulation of the carriers on those frequencies to compensate for the problem, or even just not use them. Whilst all this would reduce the amount of data we could transmit, at least the connection would remain intact.
Transmitting and receiving OFDM is not straightforward and this is one of the reasons why it has not been used for mobile phones. Transmitters have a high peak-to-average power ratio such that an OFDM transmitter with an average output power of 1 Watt, may produce a peak output of 50 Watts or more, which is not efficient nor would batteries in handsets last long. Decoding the complex OFDM waveform is processor intensive and until recently, the processor power required would also drain batteries pretty pronto. Nonetheless, OFDM offers a number of advantages and many of the proposed fourth generation (4G) mobile standards will adopt it.
OFDM is used in many technologies including the DVB set of digital terrestrial broadcasting standards; for DAB and DRM radio; in some WiFi and WiMAX systems; and in various military and defence links. In these systems the number of carriers differs as does the modulation scheme which each carrier uses (which varies from BPSK to 64QAM) to adapt to the circumstances which are likely to be encountered.
OFDM is not an easy concept to grasp but we, at Wireless Waffle are always keen to try and debunk and demystify difficult radio ideas - we hope we have succeeded.
Wednesday 1 April, 2009, 05:30 - Radio RandomnessFor some time, there has been software available on the internet which would allow anyone with enough brains and patience to hack into a 'WEP' encrypted WiFi link. 'WPA' encrypted links are more secure but even they are open to hacking. The basic problem with such devices is that they transmit the data freely across the ether and if a miscreant within range has the right equipment and software they can intercept the radio signal and decode it. Be sure though that it takes a lot of effort, someone would really have to be serious in order to bother having a go at WPA and WPA2.
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But regardless of the encryption technology used, the key problem with any WiFi network is that the signal is purposefully transmitted over a wide area. Obviously running a direct wired connection between two points is much more secure. Surprise, therefore, may be expressed at the realisation that even the radiation from computer keyboards can be sufficient to allow 'snooping' on your computing activities from a distance.
Two Swiss scientists have proven that this can be done, even through a wall, despite the fact that the levels of radiation coming from the keyboard are very small indeed.
But what about the new PLT (power line telecoms) or BPL (broadband over power line) technologies. These devices send your precious data over electrical cables which, any number of studies have shown, leak the signal hither and thither, causing both radio interference over a wide area and opening up the opportunity for someone to intercept the signal.
Some PLT/BPL devices have been received at over 500 metres from the building in which they are installed, which is, in most cases, further away than it would be possible to receive an equivalent WiFi signal. Wireless Waffle therefore decided to follow in the footsteps of the hitherto mentioned Swiss scientists and see whether or not it was possible to intercept and decode emissions from these devices in order to try and ascertain how secure they are or aren't.
The devices which seem to send out the greatest signal are those manufactured by a company called Comtrend, and which use the chipset from another company, DS2. The first thing to do, therefore, was to get hold of a Comtrend device and modify the circuitry to make a seperate antenna input rather than the device looking for the signal on the mains cable to which it is attached.
A suitable Comtrend device was purchased from the web's best know outlet of all things slightly dodgy which was then dismantled to see where the signal input is. It turns out that the device sniffs the signal from the mains through a couple of high voltage capacitors. It is a straightforward job, therefore, to lift these capacitors from the circuit board and attach an alternative signal feed.
Making a wideband antenna capable of receiving the whole HF frequency range (2 - 28 MHz) used by these devices is not necessarily straightforward, however a short whip (1m or so long) connected directly to the input of a high-impedance FET amplifier does a pretty good job and whilst the response isn't necessarily flat across the HF range it does a reasonable job of receiving something at all frequencies. And, let's face it, the frequency response of the mains cabling to which the devices are normally connected is not flat either so a bit of loss here and there shouldn't be anything to worry about.
So, armed with an inverter (to provide the Comtrend device with 240V from the DC power outlet in a car which was felt easier than supplying it with the various DC voltages it needed), a laptop with which to connect to the modified device and a whip antenna, the intrepid Wireless Waffle team set off to see whether or not it is possible to intercept data being sent over electrical mains wiring and thereby spy on local internet activity.
The first test was to set up a couple of devices in a known configuration and then put the 'interception' kit inside the house in which the devices were installed. This gives the set-up the maximum possible chance of receiving the data as the signal received on the antenna within the house as pretty much as strong as it is on the mains wiring itself!
Not surprisingly, in such an 'ideal' test set-up it was a piece of cake to read the data passing over the mains cabling.
Next, the interceptor was moved to a car parked outside the house with a suitably covert antenna placed secretly on the roof. Again, it was easy to receive and read the data being sent over the mains cabling. If it were me using these devices in my house, this is the point that I would begin to realise that the devices are not even as secure as WiFi, and would get rather nervous. The car was then driven 100 metres away from the house under test whilst keeping the system turned-on. At this distance, the signal from the house had fallen significantly (though was still perfectly audible on a test receiver).
At this distance, the simple interceptor spy-tool-device struggled to read the signal, however with some judicious placing of the receiving aerial, some of the data could be read. With such a simple set-up, not a great deal was really expected, however the tests proved PLT/BPL devices to be significantly less secure than WiFi being easy to intercept at distances of up to 100 metres from a house in which they are installed using very simple equipment.
Unlike WiFi, however, it is not as easy to make a 2-way connection: whilst intercepting or spying on data is possible, completely hacking the connection and being able to use it, for example to connect to the internet or into a home network, is much more difficult. Generating enough transmitter power to put a strong signal on the internal mains wiring from 100 metres away would be no mean feat. That doesn't mean that it's not worth trying though...
Thursday 12 March, 2009, 09:00 - LicensedHave you ever tuned into your local radio station and heard the travel news being read out from the 'eye in the sky' - a presenter checking out the traffic from an aircraft high over the area concerned? Have you ever stopped to think how that is done? Well Wireless Waffle is here to help explain it all.
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There is no denying that it would be very easy for any radio station to pretend to have a traffic plane or helicopter by playing sound effects in the background whilst the travel news was read out. However, there are some real benefits about doing it properly, not least it is possible to find out how the traffic is flowing as and when problems occur instead of waiting for listeners to phone in news (which can be unreliable) or for the local police or traffic department to let you know what's happening. But that doesn't mean that the aircraft in question can necessarily see all the problems in an area and in some cases, it is not possible to fly over certain areas due to airspace restrictions (for example, it would not make sense for a 'traffic plane' to be buzzing around a major airport, stopping commercial airliners from landing!)
What happens, therefore, is that there is someone on the ground who collates traffic information in the normal way (eg through listeners or the police) and then relays this information to the man in the aircraft. The plane (or chopper) can then visit some of the travel hotspots and see what is happening and if, along the way, they see other problems that haven't been reported, they can update the person on the ground. This means that, in general, travel news from an aircraft is more accurate and up-to-date than travel news from a regular travel studio.
From the technology perspective, there is lots of radio used (hence the Wireless Waffle interest). For starters, the pilot will be communicating with various air traffic controllers on the VHF aeronautical band (117.975 to 137.000 MHz).
Next, there is a need for the person on the ground, including the presenter in the radio studio, to be able to communicate with the presenter in the aircraft - the 'uplink'. Typically this is done via a simple VHF or UHF PMR frequency (in the UK try listening around 141.000 to 141.500 MHz and 455.000 to 455.500 MHz). As well as passing travel news to the airborne presenter, this frequency is also often used as the 'cue', providing a live feed of the station on which the travel news is to be broadcast so that the airborne presented knows when to start reading the news.
Finally there is a the link from the airborne presenter to the ground - the 'downlink'. This is usually (but not always) a slightly higher quality link than the uplink as the audio is going to be broadcast. In the UK, these links are usually at UHF (try between 467.250 and 469.900 MHz). As they are transmitted from the aircraft, despite being low power, they can often be heard over a wide area.
If the aircraft is providing travel news for a wide area, more than one up and/or downlink might be used for the different areas, depending on whether or not frequencies which can be used over a wide area are available.
In some countries, the presenter uplink and downlink are also in the aeronautical VHF band (this is the case, for example, in Malta), and the frequencies use do vary significantly between countries. If you are in an area where the local radion station has a travel plane or helicopter, why not have a tune around and see what you can find and post a comment to let us all know.