Wireless Waffle - A whole spectrum of radio related rubbish
Chill Over and Outsignal strength
Tuesday 1 December, 2009, 01:21 - Spectrum Management
Posted by Administrator
Some of the biggest brains in Europe, as well as hundreds of millions of Euros of public money are being poured into a concept which has the catchy name of the 'Internet of Things'. The concept in itself is a fairly straightforward one - that as well as people being connected together via the Internet, machines and sensors and all sorts of other electrical and mechanical devices will be connected together as well. So it would be possible for your fridge to talk to your lawnmower, and your kettle to have a chat with your central heating system.

Actually, this is nowhere near as silly as it sounds. From the perspective of saving energy and hence carbon, one of the main problems facing electricity generators is dealing with the peak load. In the USA this occurs on the hottest day of the year when air conditioning units are working overtime, and in the UK typically occurs mid-winter when heating units and lots of TVs are turned on, especially during commercial breaks when everyone gets up to make a cup of tea and turns their kettle on. So at these moments, if the kettle could chat with your freezer, for example, and tell it to stop freezing for a few minutes whilst it boils, no-one would be any the wiser and the net result would be a reduction in peak electricity consumption.

This is all fine and dandy and there are plans for 'smart cities' where lots of devices communicate with each other to the benefit of energy consumption, safety and for lots of other good reasons. But there is a limit to how effective such communication can be. Imagine the following discussion:

Kettle to Fridge Please stop freezing for a bit as I need to boil the water to make a cup of tea.
Fridge to Kettle Sorry, no can do. I've already put freezing on hold for a bit to help the tumble dryer out. Perhaps you could speak with it.
Kettle to Tumble Dryer Please could you stop drying for a bit as I need to boil the water to make a cup of tea?
Tumble Dryer to Kettle You must be joking! I've already had to stop 4 times to let the floodlights come on outside and if I don't get these clothes dry soon, there'll be trouble.
Kettle to TV Hey, TV. Any chance you could turn yourself off for a few minutes whilst I boil some water to make a cup of tea?
TV to Fridge Can you believe it? The Kettle has asked ME, ME the TV to turn off so that it can boil some water, who does it think it is?
Fridge to TV I know. Always trying to steal all the power. Nearly as bad as the iron which is on and off like a faulty switch.
chatty fridgeTV to Fridge You are so right. I just pretend Eastenders is on when the iron asks me to turn of, it knows that I couldn't possibly interrupt that programme.
Microwave oven to Fridge and TV Can you two pipe down a bit, I'm waiting for an important message from the vacuum cleaner about who is sharing the power tomorrow morning.

And so on...

So there you have it. Several billion Euros of investment brought down by a neurotic TV and an overly chatty fridge. Not to mention the fruit bowl and the salt pot who block the airwaves with their inane chatter about whether sweet or savoury is best. And therein lies the problem: all this communication needs bandwidth, and given the nature of the devices, they will need wireless bandwidth. A European Commission white-paper on the subject addresses the issue several times in statements such as:
[the internet of things] requires truly ubiquitous wireless capacity that can handle several magnitudes more data.
Communication infrastructure should provide ubiquitous connectivity in the presence of significantly increased traffic load and should be very efficient so as to reduce the cost per bit... Many of the local connections are naturally wireless.
It goes on to state
Spectrum must be valued: Radio spectrum is one of the most valuable resources of the digital age. As more and more devices and objects become wireless enabled ... spectrum is becoming a key bottleneck. We have to find ways to manage the spectrum more efficiently so as to maximise data throughput and minimise interference.

The report suggests that one possible solution would be to develop a real-time local market in radio spectrum. What does this mean? It means that when you go to make a call from your mobile (or you fridge wants to open a discussion with the vacuum cleaner) it first interrogates the 'spectrum stock market' and chooses the piece of spectrum which offers the right level of connectivity at the appropriate price. Of course the question remains as to how it does this without, in the process, also using a wireless connection.

What is certain, however, is that we have only started to see the beginning of the squeeze on the radio spectrum and that if it seems congested now, compared to the future it is still a wide open space of nothingness. And like oil as it begins to become rare, it is likely we shall see an increase in the value of spectrum too. We here at Wireless Waffle wonder whether there will eventually be unit trusts and other investment wagons on the stock market that invest in spectrum for a profitable return in the same way as they do in gold, silver, oil, crops and other limited resources. If there are, then as prices spiral, it might just shut the fridge up for a bit.
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Ofcom Sanctions Free Radiosignal strength
Thursday 24 September, 2009, 14:12 - Spectrum Management
Posted by Administrator
Well who would have thought it! According to many pages on the subject across the internet, hang gliders have a special arrangement with Ofcom to allow them easy access to various radio frequencies without needing a licence! Yes, apparently a chap called Rod Buck, the then radio officer of the British Hang Gliding and Parachute Association (BHPA) reached an 'agreement' with the Radiocommunications Agency some years ago (must have been quite some years as the Agency was disbanded in 2003) that they could use a set of radio frequencies for air to air and air to ground communications and as long as they stuck to them the Agency would 'turn a blind eye'.

What are these frequencies? 143.750 to 143.950 MHz in 25 kHz steps. If you don't believe me, take a look here. The top of this frequency range, 143.950 MHz, is the unofficial calling channel and from the Wireless Waffle HQ it is alive most days with chitter chatter from enthusiasts dodging in and out of planes, talking about the weather and checking out possible landing sites.

hang glider interferenceNow it's true to say that the use of radio when airborne presents lots of safety benefits, and it's clear from much of the communication that goes on that the guys dipping and diving around are helping each other out. But there are official frequencies for this purpose. The problem with these official frequencies is that the equipment required to use them is expensive and a licence must be obtained (albeit at just 75 per year), whereas the unofficial frequencies come at no charge and equipment can be had from certain on-line auction sites for less than 50 all sold.

Notwithstanding the safety benefits though, the use of these frequencies is, to all intents and purposes, illegal. There is plenty of illegal frequency usage around, from pirate broadcasters to Brazilian satellite hijackers but in all these cases, if the user suffers from interference due to a legitimate user then there is really no harm done as the user's use of radio is not in any way safety related (this is not to say that the legitimate user does not suffer, just that the suffering of the illegitimate user is largely inconsequential). In the case of hang gliders, however, the situation is very different. If they suffer from interference then the implication is that air safety (and possibly even safety of life) is compromised which is quite a big deal when you think about it.

One forum post states:
These frequencies are not currently used or allocated elsewhere, so you won't interfere with anyone else.

radiation causes mutationThat's not strictly true. The frequencies are actually allocated, in the UK, for 'Land Mobile' services, though at an international (ITU) level they are allocated for Off-Route (eg Military) Aeronautical Mobile use. According to the UK frequency allocation table (FAT) the band 143 to 144 MHz is set-aside for emergency service use. In terms of actual frequencies assignments, it is fair to say that they do seem to be few and far between in this frequency range though there is some evidence to suggest that the US Air Force as well as the Metropolitan Police in London use the frequencies, and that they may well be some of the emptier frequencies being considered to alleviate demand for spectrum during the 2012 London Olympics.

Anyway, given the agreement that these users are supposed to have have reached with the regulatory authorities, we here at Wireless Waffle feel that there is plenty of scope to apply the same approach to some other areas of regulation too.

colourful wiring* Allow electric cars to use either side of the road, as long as they keep their lights turned off and aren't painted a bright colour.
* Let children cross railway lines (including level crossings) at any time if they are standing near nettles, or being chased by bees, wasps or other stingy things.
* Permit short people to set fire to whatever they like but only if the device used to start the fire can be hidden if anyone approaches.
* Encourage demolition crews to trigger explosions more straightforwardly by simply shining a green torch at the detonator.
* Allow mains wiring in all new houses to be any colour the electricial likes, as long as it fits with the painter's colour scheme.
* Make sure that all knives sold to people weighing under 154 lb (70 kg), of whatever age, are longer than 18 inches and lethally sharp.
* Inform aircraft to keep from crashing into each other by communicating using semaphore and old tin-cans.

Oh, hang on, apparently the Radiocommunications Agency have secretly agreed the last of these with the Civil Aviation Authority. We await the small print of the manifestos of the various political parties at the next election with great interest to see if any other of our other ideas come to fruition or what else the good folk of the UK will be allowed to get away with. Actually, it's pretty clear why aircraft and hang gliders are allowed to act illegaly and not get prosecuted: they are above the law!
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High Frequency Funsignal strength
Friday 11 September, 2009, 08:08 - Spectrum Management
Posted by Administrator
It seems there is nothing that visitors to the Wireless Waffle web-site like better than a list of interesting radio frequencies to have a go at listening to. Previous articles in subjects such as frequencies for London airports, tuning into US military satellites and unusual UHF activity are some of the more popular pages according to the various logs kept by our web-server.

So with that in mind, here are a few 'interesting' HF (short-wave) frequencies that might be worth a listen. Any frequency where recent activity that is most likely caused by the indicated source has been noted (either by Wireless Waffle or on other useful sources) is marked with an asterisk.

Test and Development Frequencies

2806, 5750, 7556, 9071, 10438, 11117, 16014, 18990, 20990, 24135 and 26218 kHz

According to the 2007 UK frequency allocation table (Annex I) these frequencies are the preferred HF frequencies for test and development use (interestingly they have disappeared in the 2008 version. Anyone with a test and development licence is allowed to use powers of up to 100 Watts on these frequencies with a bandwidth of up to 25 kHz. Some years ago, whilst working for a manufacturer of military HF radio equipment, tests of the equipment 'on-air' were witnessed by the Wireless Waffle team using one of the rare 'G9***' UK callsign (the exact call letters of which have been forgotten) and it's fairly certain that these were the frequencies that were used. May be worth a tune around to see what else goes on here.

air cadet logoAir Cadet Frequencies

2368, 2490, 2848.5, 3236*, 3306, 3343, 3678, 3715 and 3752 kHz
(Channel designations Lima 1 through Lima 9 respectively)

5245, 4925 and 5088 kHz
(Channel designations November 1 through November 3)

5770*, 5795, 7740, 6775 and 7721 kHz
(Channel designations Hotel 1 and 2, and 5 through 7)

8120, 13545, 10843.5, 14500, 13445 and 13965 kHz
(Channel designations Oscar 1 through 6)

Air cadets are permitted to use these frequencies with powers of up to 350 Watts (a few are restricted to 100 Watts) for communication. Voice and data can both be used (some channels are data only). Checking a few of the channels revealed occasional strong blasts of data. Note that some of the Lima channels are in the 80 metre amateur band!

sea cadet logoSea Cadet Frequencies

2122.5, 2695, 3660, 6805, 6875, 6992.5* and 8187.5 kHz
(Channel designations SCO01, 02 and 11 to 15)

Sea cadets are permitted to use between 25 and 100 Watts on these frequencies. Again note the channel (in this case 3660 kHz) in the middle of the 80 meter amateur radio band.

army cadet logoArmy Cadet Frequencies

2275*, 2415*, 2770*, 3850*, 3865*, 4920, 4922.5*, 4955, 5330, 5345, 6915, 7710 and 7753 kHz

Note that for these frequencies (and those for the other cadet forces), the listed frequencies are supposed to represent the centre of the channel. As such, if listening to an SSB signal, it is necessary to tune 2 kHz lower than the listed frequency. So if you want to listen to 5330 kHz (supposedly a common calling channel for the army cadets, tune initially to 5328 kHz). Also, operation is allowed within 5 kHz of these frequencies so listening plus or minus a bit may yield something interesting.

The sea cadet and RAF cadet frequencies are taken from official publications, whereas the army cadet frequencies are based on an extensive search and cross-check of internet sources but these may not be as accurate as the earlier two!

raf strike command logoRAF Flight Watch (TASCOMM)

4742*, 5702*, 9031*, 11247, 13257 and 18018 kHz

TASCOMM is part of the Defence High Frequency Communication Service (DHFCS) which is managed from the network control centre at Forest Moor. They are the frequencies used by the RAF. DHFCS also managed the HF frequencies for the Army and Navy too. Transmitters are often high power and are located both in the UK and at some overseas military bases (such as Cyprus and Ascension). The net controller uses the callsign 'XSS'.

Note that those frequencies market with an asterisk (*) are ones on which either activity has recently (ie in the past 5 years) been reported somewhere on-line, or where the Wireless Waffle receiver has noted activity.

Now it would be good to try and make something of this list and identify lots of 'secret military HF bands' where there were hotspots of activity, however no such hotspots really emerge, instead there are a few warm damp patches which might be worth exploring. Where there is any commonality between these frequencies it is in the ranges {the ITU allocation to these bands is shown in curly brackets}:
  • 3660 - 3865 kHz (6 assignments) {Mobile/Land Mobile};
  • 4742 - 5088 kHz (6 assignments) {Mobile/Land Mobile/Aeronautical Mobile (OR)};
  • 5702 - 5795 kHz (4 assignments) {Land Mobile/Aeronautical Mobile (OR)};
  • 6775 - 6992.5 kHz (5 assignments) {Mobile};
  • 7556 - 7753 kHz (5 assignments) {Mobile};
  • 8120 - 8187.5 kHz (2 assignments which is not exactly conclusive) {Maritime Mobile}; and
  • 9031 - 9071 kHz (again 2 assignments) {Aeronautical Mobile/Fixed}.
up holding the sanc tit yIn almost all of these cases, the frequencies concerned fall within the 'Mobile', 'Land Mobile', in the case of TASCOMM, in the 'Aeronautical mobile (OR)' and in the case of the Sea Cadets the 'Maritime Mobile' services of the ITU and as such are almost exactly where you would expect to find them had you decided to look there in the first place. The only oddity is the test and development frequency at 9071 kHz which is in a 'Fixed' band, but then again, this is not a military frequency and it's quite possible that someone somewhere might want to test an HF fixed link.

So, sadly, no secret service special frequency bands have been identified but at least it seems that the United Kingdom's armed forces are doing their bit to uphold the sanctity of the ITU frequency allocation table.
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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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