Wireless Waffle - A whole spectrum of radio related rubbish
Wireless and the London Olympicssignal strength
Wednesday 6 May, 2009, 22:45 - Broadcasting
Posted by Administrator
There is currently much ongoing debate, and some might suggest ensuing debacle, taking place to ensure that there is sufficient radio spectrum available for the London Olympic Games to be held in 2012. However, Wireless Waffle has uncovered the official Government plans for the use of the radio spectrum for the last Olympic Games held in London in 1948. Interestingly, these were before the main legislation relating to the use of the spectrum, the Wireless Telegraphy Act of 1949 were brought into power and thus predate any previous attempt to specifically control radio use in the UK.

1948 olympic games londonLet us step back to 1948... Much of London was still in ruins and rubble strewn streets were not uncommon. The budget for the games was £600,000, a figure which today would just about pay the salary of the organising committee for about a month. Food was still being rationed (indeed rationing did not end until 1954), King George VI was on the throne and Frankie Laine, Perry Como, Al Jolson and the Andrews Sisters topped the charts.

1948 bush televisionThe Olympic games were televised using the EMI 405-line black and white system selected by the BBC before World War II as its preferred technology (having beaten the Baird 240 line system in trials). Only one transmitter in London was operative but an estimated 80,000 people had television receivers and were able to view the 64 hours of coverage that was broadcast. Radio coverage was also provided by the BBC and broadcast around the world using short wave.

olympic wireless decree 194So, with that in mind, here is the previously unpublished Government document which details usage of the spectrum. The original is not that easy to read (click on the image on the right to see it in full size) in pictorial format so here is the actual text.
Be it hereby enacted by the King's Most Excellent Majesty, by and with the advice and consent of the Lords Spiritual and Temporal, and Commons, in this present Parliament assembled, and by the commandment of the same aforementioned, as follows:-

(1) No sporting personage, radio announcer or visual televiser shall establish or use any station for wireless replication of Olympic events or install or use any apparatus for wireless tomfoolery except under the authority of a licence in that behalf granted by the Postmaster General hereinunder purported to, and any person who establishes or uses any station for wirelessness of any nature or installs or uses any apparatus for wireless purposes except under and in accordance with such a licence shall be guilty of an offence of the most serious nature which shall be punishable by flogging, booting or in any such manner as is seen fit by His Most Excellent Majesty or his appointed Government torturer or executioner.

(2) A licence granted under this section (hereafter in this Decree referred to as an olympic wireless licence or 'owl') may be issued subject to such terms, provisions and commandments as the Postmaster General may think befitting, including in particular and in peculiar in the case of a licence to establish an olympic wireless station, limitations as to the positionality and nature of the station, the purposes for which, the circumstances in which, and the persons by whom the station may be used, and the manufactory of the apparatus which may be installed or used therein therefor, and, in the case of any other licence, limitations and hampering as to the apparatus which may be installed, operated or used, and the places or locations where, the purposes for which, the circumstances in which and the person or persons by whom the apparatus may be used for such purposes thereafter.

(3) Nothing or no item in this section shall authorise or approve the inclusion, in any olympic wireless licence relating solely to apparatus not designed or adapted for emission or transmission (as opposed to reception), of any term, item or provision requiring any person to concede any form of right of entry into any private dwellinghouse, manufactory, residence or abode.

(4) Through jurisdiction of this Decree, the commandment of the use of those radio wavelengths perporting to emissions authorised herinunderafter by an olympic wireless licence shall be designated for usage and utility as per the identification and categorisation indicated in the ensuing remainder of this document.

(5) Notwithstanding these categorisations and registrations of wireless lengths subscribed to by His Majesty's Government at the International Telecommunications Union 1947 Atlantic City Plenipotentiary Conference howsoever agreed, and in cognisance of the need to control and restrict miscreant emissions of wireless stations to other wireless stations, all emissive equipment of a nature requiring a licence under this Decree should be designed, manufactured, construed and operated in ways in which other wireless stations shall be safe from explosion and other maleficent discreation.

Wavelengths of below 1 metre (over 300 Mega Cycles per second) are reserved exclusively for secret Government use. The use of these wavelenghts shall remain secret and the fact that such wavelengths exist and the fact that the use of them is secret is also a secret and should be treated accordingly.

Wavelengths of between 3 metres and 1 metre (between 100 and 300 Mega Cycles per second) may be used for televisual distribution of motion or stationary pictorial information between private wireless stations which are fixed in location and fortitude.

Wavelengths of between 10 metres and 3 metres (between 30 and 100 Mega Cycles per second) may be used for televisual distribution of motion or stationary pictorial information between a public wireless station and domestic or official wireless receptors.

Wavelengths of between 100 metres and 10 metres (between 3 and 30 Mega Cycles per second) may be used for audible distribution of broadcast material to His Majesty's Overseas Territories and other fuzzy wuzzy lands to whom the English language is comprehensible. The use of some wavelengths within this range are reserved for secret Government use, the provisions of which are secret as described above therein.

Wavelengths larger than 100 metres (below 3 Mega Cycles) may be used for audible distribution of broadcast material to the United Kingdom of this marvellousness nation hereover. Some wavelengths within this range may also be used for the exchange of message between shipping and shipping or between shipping and port establishments for the perfunction of safety and maritime information exchange and for other safety matters as may be permitted in the olympic wireless licence therein permittivised. The use of some wavelengths within this range are reserved for secret Government use, the provisions of which are secret as described above therein.
What stands out from this is:

* There is a clear division between different bands and their uses - this no doubt stems from the work ongoing at the ITU at the time in establishing the international frequency registration board.
* The highest frequencies were reserved for Government use. At the time, this was largely for radar and navigation tools that had proven invaluable in the success of the war effort.
* Some bands are shared between users (though the wording of the document would imply that within any given band, frequencies were only assigned to one use or another and that no true 'sharing' as we recognise it today was authorised).
* The operation of a wireless transmitting device permitted the authorities to access your property.
* There was nothing much on television tonight and so this drivel got written.

Note: one or more of the above may be substantially or materially inaccurate.

This document (erstwhile the 'Olympic Wireless Station Commandment of Licence Neccessity and Specifity Decree, 1948') clearly sets the framework for what became the WT Act of 1949 the following year and established the groundwork for spectrum regulation in the UK that still exists today and which will continue to apply until the London Olympics of 2012 and even after that. Perhaps.
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Don't Wave Goodbye to Short-Wavesignal strength
Tuesday 28 April, 2009, 07:22 - Radio Randomness
Posted by Administrator
wave goodbye to shortwaveThe 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.

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.
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OFDM: How does that work then?signal strength
Monday 27 April, 2009, 16:50 - Spectrum Management
Posted by Administrator
Today, 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').

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.

radio reflections

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.

ofdm sinc function

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.

ofdm orthogonal carriers

wheelbarrow of bricksThe 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.
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Comtrend-ulations and Jubliationssignal strength
Wednesday 1 April, 2009, 05:30 - Radio Randomness
Posted by Administrator
For 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.

trypingBut 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.

devil slop 2The 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.

inconspicuous antennaNext, 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).

covert spy girlAt 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...
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